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The GNU Privacy Handbook

1. 1 4 2 Detached signatures s i ece ee ee ee 2 Concepts 2 1 Symmetric Ciphers aos ee ee ed baa ded 2 2 Publiczkey ciphers eo Hee raid opo gae Re 2 3 Hybrid ciphers ss a ere 2 4 Digital signatures s 6 4 ooo Se EEE os 3 Key Management 3 1 Managing your own keypair Skid Eeyanbegrity esd aa dare alee lee nop oe REGE oe eet mA 3 1 2 Adding and deleting key 3 1 3 Revoking key 3 1 4 Updating key s expiration time 3 2 Validating other keys on your public 3 2 1 Trustin akey ssOWner i sa gie edera Ree 3 2 2 Using trust to validate 3 3 i Distributing Keys sod he toe e MURIS RH Oa e e n d 4 Daily use of GnuPG 4 Defining your security needs eA 4 1 1 Choosinga key size ees 4 1 2 Protecting your private key 4 1 3 Selecting expiration dates and using subkeys 4 1 4 Managing your web of trust 4 2 Building your web of trust kx uide ed RE Redes 4 3 Using GnuPG legally eu
2. A 2 APPLICABILITY AND DEFINITIONS This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License The Document below refers to any such manual or work Any member of the public is a licensee and is addressed as you A Modified Version of the Document means any work containing the Document or a portion of it either copied verbatim or with modifications and or translated into another language A Secondary Section is a named appendix or a front matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Docu ment s overall subject or to related matters and contains nothing that could fall directly within that overall subject For example if the Document is in part a textbook of mathematics a Sec ondary Section may not explain any mathematics The relationship could be a matter of historical connection with the subject or with related matters or of legal commercial philosophical ethical or political position regarding them The Invariant Sections are certain Secondary Sections whose titles are designated as being those of Invariant Sections in the notice that says that the Document is released under this 33 34 APPENDIX A GNU FREE DOCUMENTATION LICENSE License The Cover Texts are certain short passages of text that are li
3. To solve this problem public key servers are used to collect and distribute public keys A public key received by the server is either added to the server s database or merged with the existing key if already present When a key request comes to the server the server consults its database and returns the requested public key if found A keyserver is also valuable when many people are frequently signing other people s keys Without a keyserver when Blake sign s Alice s key then Blake would send Alice a copy of her public key signed by him so that Alice could add the updated key to her ring as well as distribute it to all of her correspondents Going through this effort fulfills Alice s and Blake s responsibility to the community at large in building tight webs of trust and thus improving the security of PGP It is nevertheless a nuisance if key signing is frequent Using a keyserver makes the process somewhat easier When Blake signs Alice s key he sends the signed key to the key server The key server adds Blake s signature to its copy of Alice s key Individuals interested in updating their copy of Alice s key then consult the keyserver on their own initiative to retrieve the updated key Alice need never be involved with distribution and can retrieve signatures on her key simply by querying a keyserver One or more keys may be sent to a keyserver using the command line option send keys The option takes one or more key specifier
4. Finally as of 1999 laws regarding digital encryption and in particular whether or not using GnuPG is legal vary from country to country and is currently being debated by many national governments This chapter addresses these issues It gives practical advice on how to use GnuPG to meet your security needs It also suggests ways to promote the use of GnuPG for secure communication between yourself and your colleagues when your colleagues are not currently using GnuPG Finally the legal status of GnuPG is outlined given the current status of encryption laws in the world 4 1 Defining your security needs GnuPG is a tool you use to protect your privacy Your privacy is protected if you can correspond with others without eavesdroppers reading those messages How you should use GnuPG depends on the determination and resourcefulness of those who might want to read your encrypted messages An eavesdropper may be an unscrupulous system administrator casually scanning your mail it might be an industrial spy trying to collect your company s secrets or it might be a law enforcement agency trying to prosecute you Using GnuPG to protect against casual eavesdropping is going to be different than using GnuPG to protect against a determined adversary Your goal ultimately is to make it more expensive to recover the unencrypted data than that data is worth Customizing your use of GnuPG revolves around four issues e choosing the key size of your public
5. agree the sender encrypts a message using the key sends it to the receiver and the receiver decrypts the message using the key As an example the German Enigma is a symmetric cipher and daily keys were distributed as code books Each day a sending or receiving radio operator would consult his copy of the code book to find the day s key Radio traffic for that day was then encrypted and decrypted using the day s key Modern examples of symmetric ciphers include 3DES Blowfish and IDEA A good cipher puts all the security in the key and none in the algorithm In other words it should be no help to an attacker if he knows which cipher is being used Only if he obtains the key would knowledge of the algorithm be needed The ciphers used in GnuPG have this property Since all the security is in the key then it is important that it be very difficult to guess the key In other words the set of possible keys i e the key space needs to be large While at Los Alamos Richard Feynman was famous for his ability to crack safes To encourage the mystique he even carried around a set of tools including an old stethoscope In reality he used a variety of tricks to reduce the number of combinations he had to try to a small number and then simply guessed until he found the right combination In other words he reduced the size of the key space Britain used machines to guess keys during World War 2 The German Enigma had a very large key space but the B
6. you could store them on a floppy and hide it in your house Whatever you do they should be put on media that is safe to store for as long as you expect to keep the key and you should store them more carefully than the copy of your private key you use daily To help safeguard your key GnuPG does not store your raw private key on disk Instead it encrypts it using a symmetric encryption algorithm That is why you need a passphrase to access the key Thus there are two barriers an attacker must cross to access your private key 1 he must actually acquire the key and 2 he must get past the encryption Safely storing your private key is important but there is a cost Ideally you would keep the private key on a removable write protected disk such as a floppy disk and you would use it on a single user machine not connected to a network This may be inconvenient or impossible for you to do For example you may not own your own machine and must use a computer at work or school or it may mean you have to physically disconnect your computer from your cable modem every time you want to use GnuPG This does not mean you cannot or should not use GnuPG It means only that you have decided that the data you are protecting is important enough to encrypt but not so important as to take extra steps to make the first barrier stronger It is your choice A good passphrase is absolutely critical when using GnuPG Any attacker who gains access to your private ke
7. 36 APPENDIX A GNU FREE DOCUMENTATION LICENSE 12 Preserve all the Invariant Sections of the Document unaltered in their text and in their titles Section numbers or the equivalent are not considered part of the section titles 13 Delete any section entitled Endorsements Such a section may not be included in the Modified Version 14 Do not retitle any existing section as Endorsements or to conflict in title with any Invariant Section If the Modified Version includes new front matter sections or appendices that qualify as Sec ondary Sections and contain no material copied from the Document you may at your option designate some or all of these sections as invariant To do this add their titles to the list of Invariant Sections in the Modified Version s license notice These titles must be distinct from any other section titles You may add a section entitled Endorsements provided it contains nothing but endorsements of your Modified Version by various parties for example statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard You may add a passage of up to five words as a Front Cover Text and a passage of up to 25 words as a Back Cover Text to the end of the list of Cover Texts in the Modified Version Only one passage of Front Cover Text and one of Back Cover Text may be added by or through arrangements made by any one entity If the Document a
8. by hashing it and the hash value is the signature Another person can check the signature by also hashing their copy of the document and comparing the hash value they get with the hash value of the original document If they match it is almost certain that the documents are identical Of course the problem now is using a hash function for digital signatures without permitting an attacker to interfere with signature checking If the document and signature are sent unen crypted an attacker could modify the document and generate a corresponding signature without the recipient s knowledge If only the document is encrypted an attacker could tamper with the signature and cause a signature check to fail A third option is to use a hybrid public key encryp tion to encrypt both the signature and document The signer uses his private key and anybody can use his public key to check the signature and document This sounds good but is actually nonsense If this algorithm truly secured the document it would also secure it from tampering and there would be no need for the signature The more serious problem however is that this does not protect either the signature or document from tampering With this algorithm only the session key for the symmetric cipher is encrypted using the signer s private key Anybody can use the public key to recover the session key Therefore it is straightforward for an attacker to recover the session key and use it to encrypt su
9. chapter covers miscellaneous topics that do not fit elsewhere in the user manual As topics are added they may be collected and factored into chapters that stand on their own If you would like to see a particular topic covered please suggest it Even better volunteer to write a first draft covering your suggested topic 5 1 Writing user interfaces Alma Whitten and Doug Tygar have done a study on NAI s PGP 5 0 user interface and came to the conclusion that novice users find PGP confusing and frustrating In their human factors study only four out of twelve test subjects managed to correctly send encrypted email to their team members and three out of twelve emailed the secret without encryption Furthermore half of the test subjects had a technical background These results are not surprising PGP 5 0 has a nice user interface that is excellent if you already understand how public key encryption works and are familiar with the web of trust key management model specified by OpenPGP Unfortunately novice users understand neither public key encryption nor key management and the user interface does little to help You should certainly read Whitten and Tygar s report if you are writing a user interface It gives specific comments from each of the test subjects and those details are enlightening For example it would appear that many of subjects believed that a message being sent to other people should be encrypted to the test subject s ow
10. in key signing parties If you are going to a conference look ahead of time for a key signing party and if you do not see one being held offer to hold one You can also be more passive and carry your fingerprint with you for impromptu key exchanges In such a situation the person to whom you gave the fingerprint would verify it and sign your public key once he returned home lIn this section GnuPG refers to the GnuPG implementation of OpenPGP as well as other implementations such as NAI s PGP product http www herrons com kb2nsx keysign html 4 3 USING GNUPG LEGALLY 29 Keep in mind though that this is optional You have no obligation to either publicly advertise your key or sign other people s keys The power of GnuPG is that it is flexible enough to adapt to your security needs whatever they may be The social reality however is that you will need to take the initiative if you want to grow your web of trust and use GnuPG for as much of your communication as possible 4 3 Using GnuPG legally The legal status of encryption software varies from country to country and law regarding encryp tion software is rapidly evolving Bert Japp Koops has an excellent Crypto Law Survey to which you should refer for the legal status of encryption software in your country 3http cwis kub nl frw people koops bertjaap htm http cwis kub nl frw people koops lawsurvy htm 30 CHAPTER 4 DAILY USE OF GNUPG Chapter 5 Topics This
11. in your initial web of trust you may want to communicate securely with others who are also using GnuPG Doing so however can be awkward for two reasons 1 you do not always know when someone uses or is willing to use GnuPG and 2 if you do know of someone who uses it you may still have trouble validating their key The first reason occurs because people do not always advertise that they use GnuPG The way to change this behavior is to set the example and advertise that you use GnuPG There are at least three ways to do this you can sign messages you mail to others or post to message boards you can put your public key on your web page or if you put your key on a keyserver you can put your key ID in your email signature If you advertise your key then you make it that much more acceptable for others to advertise their keys Furthermore you make it easier for others to start communicating with you securely since you have taken the initiative and made it clear that you use GnuPG Key validation is more difficult If you do not personally know the person whose key you want to sign then it is not possible to sign the key yourself You must rely on the signatures of others and hope to find a chain of signatures leading from the key in question back to your own To have any chance of finding a chain you must take the initiative and get your key signed by others outside of your initial web of trust An effective way to accomplish this is to participate
12. info see http www gnupg org Many command line options that are frequently used can also be set in a configuration file 8 CHAPTER 1 GETTING STARTED 1 2 2 Importing a public key A public key may be added to your public keyring with the import option alice gpg import blake gpg gpg key 9E98BC16 public key imported gpg Total number processed 1 gpg imported 1 alice gpg list keys users alice gnupg pubring gpg pub 1024D BB7576AC 1999 06 04 Alice Judge lt alice cyb org gt sub 1024g 78E9A8FA 1999 06 04 pub 1024D 9E98BC16 1999 06 04 Blake Executioner lt blake cyb org gt sub 1024g 5C8CBD41 1999 06 04 Once a key is imported it should be validated GnuPG uses a powerful and flexible trust model that does not require you to personally validate each key you import Some keys may need to be personally validated however A key is validated by verifying the key s fingerprint and then signing the key to certify it as a valid key A key s fingerprint can be quickly viewed with the fingerprint command line option but in order to certify the key you must edit it alice gpg edit key blake cyb org pub 1024D 9E98BC16 created 1999 06 04 expires never trust q sub 1024g 5C8CBD41 created 1999 06 04 expires never 1 Blake Executioner lt blake cyb org gt Command gt fpr pub 1024D 9E98BC16 1999 06 04 Blake Executioner lt blake cyb org gt Fingerprint 268F 448F CCD7 AF34 183E 52D8 9BDE 1A08 9E98 BC16
13. key is also considered valid The root is Alice s key which is axiomatically assumed to be valid 3 2 1 Trust in a key s owner In practice trust is subjective For example Blake s key is valid to Alice since she signed it but she may not trust Blake to properly validate keys that he signs In that case she would not take Chloe s and Dharma s key as valid based on Blake s signatures alone The web of trust model accounts for this by associating with each public key on your keyring an indication of how much you trust the key s owner There are four trust levels 22 CHAPTER 3 KEY MANAGEMENT unknown Nothing is known about the owner s judgment in key signing Keys on your public keyring that you do not own initially have this trust level none The owner is known to improperly sign other keys marginal The owner understands the implications of key signing and properly validates keys before signing them full The owner has an excellent understanding of key signing and his signature on a key would be as good as your own A key s trust level is something that you alone assign to the key and it is considered private information It is not packaged with the key when it is exported it is even stored separately from your keyrings in a separate database The GnuPG key editor may be used to adjust your trust in a key s owner The command is trust In this example Alice edits her trust in Blake and then updates the trust datab
14. key rings Appendix A GNU Free Documentation License Version 1 1 March 2000 Copyright C 2000 Free Software Foundation Inc 59 Temple Place Suite 330 Boston MA 02111 1307 USA Everyone is permitted to copy and distribute verbatim copies of this license document but chang ing it is not allowed 1 PREAMBLE The purpose of this License is to make a manual textbook or other written document free in the sense of freedom to assure everyone the effective freedom to copy and redistribute it with or without modifying it either commercially or noncommercially Secondarily this License preserves for the author and publisher a way to get credit for their work while not being considered responsible for modifications made by others This License is a kind of copyleft which means that derivative works of the document must themselves be free in the same sense It complements the GNU General Public License which is a copyleft license designed for free software We have designed this License in order to use it for manuals for free software because free software needs free documentation a free program should come with manuals providing the same freedoms that the software does But this License is not limited to software manuals it can be used for any textual work regardless of subject matter or whether it is published as a printed book We recommend this License principally for works whose purpose is instruction or reference
15. of the certificate to store somewhere safe such as your safe deposit box The certificate should not be stored where others can access it since anybody can publish the revocation certificate and render the corresponding public key useless 1 2 Exchanging keys To communicate with others you must exchange public keys To list the keys on your public keyring use the command line option list keys alice gpg list keys users alice gnupg pubring gpg pub 1024D BB7576AC 1999 06 04 Alice Judge lt alice cyb org gt sub 1024g 78E9A8FA 1999 06 04 1 2 1 Exporting a public key To send your public key to a correspondent you must first export it The command line option export is used to do this It takes an additional argument identifying the public key to export As with the gen revoke option either the key ID or any part of the user ID may be used to identify the key to export alice gpg output alice gpg export alice cyb org The key is exported in a binary format but this can be inconvenient when the key is to be sent though email or published on a web page GnuPG therefore supports a command line option armor that causes output to be generated in an ASCII armored format similar to uuencoded documents In general any output from GnuPG e g keys encrypted documents and signatures can be ASCII armored by adding the armor option alice gpg armor export alice cyb org Version GnuPG v0 9 7 GNU Linux Comment For
16. pages as are needed to hold legibly the material this License requires to appear in the title page For works in formats which do not have any title page as such Title Page means the text near the most prominent appearance of the work s title preceding the beginning of the body of the text A 3 VERBATIM COPYING You may copy and distribute the Document in any medium either commercially or noncommer cially provided that this License the copyright notices and the license notice saying this License applies to the Document are reproduced in all copies and that you add no other conditions what soever to those of this License You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute However you may accept com pensation in exchange for copies If you distribute a large enough number of copies you must also follow the conditions in section A 4 You may also lend copies under the same conditions stated above and you may publicly display copies A 4 COPYING IN QUANTITY If you publish printed copies of the Document numbering more than 100 and the Document s license notice requires Cover Texts you must enclose the copies in covers that carry clearly and legibly all these Cover Texts Front Cover Texts on the front cover and Back Cover Texts on the back cover Both covers must also clearly and legibly identify you as the publisher of these copies The front cover m
17. private key is one of the weakest points in GnuPG and other public key encryption systems as well since it is the only protection you have if another individual gets your private key Ideally the passphrase should not use words from a dictionary and should mix the case of alphabetic characters as well as use non alphabetic characters A good passphrase is crucial to the secure use of GnuPG 1 2 EXCHANGING KEYS i 1 1 1 Generating a revocation certificate After your keypair is created you should immediately generate a revocation certificate for the primary public key using the option gen revoke If you forget your passphrase or if your private key is compromised or lost this revocation certificate may be published to notify others that the public key should no longer be used A revoked public key can still be used to verify signatures made by you in the past but it cannot be used to encrypt future messages to you It also does not affect your ability to decrypt messages sent to you in the past if you still do have access to the private key alice gpg output revoke asc gen revoke mykey The argument mykey must be a key specifier either the key ID of your primary keypair or any part of a user ID that identifies your keypair The generated certificate will be left in the file revoke asc If the output option is omitted the result will be placed on standard output Since the certificate is short you may wish to print a hardcopy
18. would probably be much easier for an attacker to work to intercept the key than it is to try all the keys in the key space Another problem is the number of keys needed If there are n people who need to communicate then n n 1 2 keys are needed for each pair of people to communicate privately This may be OK for a small number of people but quickly becomes unwieldy for large groups of people Public key ciphers were invented to avoid the key exchange problem entirely A public key cipher uses a pair of keys for sending messages The two keys belong to the person receiving the message One key is a public key and may be given to anybody The other key is a private key and is kept secret by the owner A sender encrypts a message using the public key and once encrypted only the private key may be used to decrypt it This protocol solves the key exchange problem inherent with symmetric ciphers There is no need for the sender and receiver to agree upon a key All that is required is that some time before secret communication the sender gets a copy of the receiver s public key Furthermore the one public key can be used by anybody wishing to communicate with the receiver So only n keypairs are needed for n people to communicate secretly with one another Public key ciphers are based on one way trapdoor functions A one way function is a function that is easy to compute but the inverse is hard to compute For example it is easy to multiply two prime nu
19. A key s fingerprint is verified with the key s owner This may be done in person or over the phone or through any other means as long as you can guarantee that you are communicating with the key s true owner If the fingerprint you get is the same as the fingerprint the key s owner gets then you can be sure that you have a correct copy of the key After checking the fingerprint you may sign the key to validate it Since key verification is a weak point in public key cryptography you should be extremely careful and always check a key s fingerprint with the owner before signing the key Command gt sign pub 1024D 9E98BC16 created 1999 06 04 expires never trust q Fingerprint 268F 448F CCD7 AF34 183E 52D8 9BDE 1A08 9E98 BC16 Blake Executioner lt blake cyb org gt Are you really sure that you want to sign this key with your key Alice Judge lt alice cyb org gt Really sign Once signed you can check the key to list the signatures on it and see the signature that you have added Every user ID on the key will have one or more self signatures as well as a signature for each user that has validated the key 1 3 ENCRYPTING AND DECRYPTING DOCUMENTS 9 Command gt check uid Blake Executioner lt blake cyb org gt sig 9E98BC16 1999 06 04 self signature sig BB7576AC 1999 06 04 Alice Judge lt alice cyb org gt 1 8 Encrypting and decrypting documents A public and private key each have a specific role when encr
20. EMENT can then be used to fully validate Francis s key and marginally validate Elena s key Lastly when Blake Chloe and Elena are fully trusted this is still insufficient to validate Geoff s key since the maximum certification path is three but the path length from Geoff back to Alice is four The web of trust model is a flexible approach to the problem of safe public key exchange It permits you to tune GnuPG to reflect how you use it At one extreme you may insist on multiple short paths from your key to another key K in order to trust it On the other hand you may be satisfied with longer paths and perhaps as little as one path from your key to the other key Requiring multiple short paths is a strong guarantee that K belongs to whom your think it does The price of course is that it is more difficult to validate keys since you must personally sign more keys than if you accepted fewer and longer paths 3 3 Distributing keys Ideally you distribute your key by personally giving it to your correspondents In practice how ever keys are often distributed by email or some other electronic communication medium Distri bution by email is good practice when you have only a few correspondents and even if you have many correspondents you can use an alternative means such as posting your public key on your World Wide Web homepage This is unacceptable however if people who need your public key do not know where to find it on the Web
21. TURE REVISIONS OF THIS LICENSE The Free Software Foundation may publish new revised versions of the GNU Free Documentation License from time to time Such new versions will be similar in spirit to the present version but may differ in detail to address new problems or concerns See http www gnu org copyleft Each version of the License is given a distinguishing version number If the Document specifies that a particular numbered version of this License or any later version applies to it you have the option of following the terms and conditions either of that specified version or of any later version that has been published not as a draft by the Free Software Foundation If the Document does not specify a version number of this License you may choose any version ever published not as a draft by the Free Software Foundation A 12 How to use this License for your documents To use this License in a document you have written include a copy of the License in the document and put the following copyright and license notices just after the title page Copyright c YEAR YOUR NAME Permission is granted to copy distribute and or modify this document under the terms of the GNU Free Documentation License Version 1 1 or any later version published by the Free Software Foundation with the Invariant Sections being LIST THEIR TITLES with the Front Cover Texts being LIST and with the Back Cover Texts being LIST A copy of the license is include
22. The GNU Privacy Handbook The GNU Privacy Handbook 1999 The Free Software Foundation Permission is granted to copy distribute and or modify this document under the terms of the GNU Free Documentation License Version 1 1 or any later version published by the Free Software Foundation with no Invariant Sections no Front Cover Texts and no Back Cover Texts A copy of the license is included in the section entitled GNU Free Documentation License Please direct questions bug reports or suggestions concerning this manual to the maintainer Mike Ashley lt jashley acm org gt When referring to the manual please specify which version of the manual you have by using this version string v1_1 Contributors to this manual include Matthew Copeland Joergen Grahn and David A Wheeler J Horacio MG has translated the manual to Spanish Typeset by Roman Pavlik lt rp tns cz gt with the ATEX 22 Documentation System Contents 1 Getting Started 1 1 Generating new keypair 1 1 1 Generating a revocation certificate 12 Exchanging keys a VIR a ee 1 21 Exporting a publie key oo s s a e esaeo 12 29 Importinga publie key a a ow ek eee RE oe Se gs 1 3 Encrypting and decrypting documents 1 4 Making and verifying 5 1 4 1 Clearsigned
23. a signed document you can either check the signature or check the signature and recover the original document To check the signature use the verify option To verify the signature and extract the document use the decrypt option The signed document to verify and recover is input and the recovered document is output blake gpg output doc decrypt doc sig gpg Signature made Fri Jun 4 12 02 38 1999 CDT using DSA key ID BB7576AC gpg Good signature from Alice Judge lt alice cyb org gt 1 4 1 Clearsigned documents A common use of digital signatures is to sign usenet postings or email messages In such situations it is undesirable to compress the document while signing it The option clearsign causes the document to be wrapped in an signature but otherwise does not modify the document alice gpg clearsign doc You need a passphrase to unlock the secret key for user Alice Judge lt alice cyb org gt 1024 bit DSA key ID BB7576AC created 1999 06 04 Version GnuPG v0 9 7 GNU Linux 1 4 MAKING AND VERIFYING SIGNATURES 11 Comment For info see http www gnupg org iEYEARECAAYFAjaYCQoACgkQJ9S6ULt1dqz6IwCfQ7wP6i i8HhbcOSKFAELyQB1 oCoAoDuqpRqEzr4k kQqHRLE Db8 Rw2k y6kj 1 4 2 Detached signatures A signed document has limited usefulness Other users must recover the original document from the signed version and even with clearsigned documents the signed document must be edited to recover th
24. ase to recompute which keys are valid based on her new trust in Blake alice gpg edit key blake pub 1024D 8B927C8A created 1999 07 02 expires never trust q f sub 1024g C19EA233 created 1999 07 02 expires never 1 Blake Executioner lt blake cyb org gt Command gt trust pub 1024D 8B927C8A created 1999 07 02 expires never trust q f sub 1024g C19EA233 created 1999 07 02 expires never 1 Blake Executioner lt blake cyb org gt Please decide how far you trust this user to correctly verify other users keys by looking at passports checking fingerprints from different sources Don t know I do NOT trust I trust marginally I trust fully please show me more information Ba back to the main menu Your decision 3 pub 1024D 8B927C8A created 1999 07 02 expires never trust m f sub 1024g C19EA233 created 1999 07 02 expires never 1 Blake Executioner lt blake cyb org gt Command gt quit Trust in the key s owner and the key s validity are indicated to the right when the key is displayed Trust in the owner is displayed first and the key s validity is second 1GnuPG overloads the word trust by using it to mean trust in an owner and trust in a key This can be confusing Sometimes trust in an owner is referred to as owner trust to distinguish it from trust in a key Throughout this manual however trust is used to mean trust in a key s owner an
25. be revoked by revoking the self signature on the user ID For the security reasons described previously correspondents will not trust a user ID with no valid self signature A signature is revoked by using the command revsig Since you may have signed any number of user IDs the user interface prompts you to decide for each signature whether or not to revoke it Command revsig You have signed these user IDs Chloe Jester lt chloe cyb org gt signed by B87DBA93 at 1999 06 28 Chloe Plebian lt chloe tel net gt signed by B87DBA93 at 1999 06 28 user ID Chloe Jester lt chloe cyb org gt signed with your key B87DBA93 at 1999 06 28 Create a revocation certificate for this signature y N n user ID Chloe Plebian lt chloe tel net gt signed with your key B87DBA93 at 1999 06 28 Create a revocation certificate for this signature y N y You are about to revoke these signatures Chloe Plebian lt chloe tel net gt signed by B87DBA93 at 1999 06 28 Really create the revocation certificates y N y You need a passphrase to unlock the secret key for user Chloe Jester lt chloe cyb org gt 1024 bit DSA key ID B87DBA93 created 1999 06 28 pub 1024D B87DBA93 created 1999 06 28 expires never trust u sub 2048g B7934539 created 1999 06 28 expires never sub 1792G 4E3160AD created 1999 06 29 expires 2000 06 28 rev subkey has been revoked 1999 06 29 sub 960D E1F56448 created 1999 06 29 expires 2000 06 28 1 Chloe J
26. bstitute documents and signatures to send to others in the sender s name An algorithm that does work is to use a public key algorithm to encrypt only the signature In particular the hash value is encrypted using the signer s private key and anybody can check the signature using the public key The signed document can be sent using any other encryption algorithm including none if it is a public document If the document is modified the signature 3The cipher must have the property that the actual public key or private key could be used by the encryption algorithm as the public key RSA is an example of such an algorithm while ElGamal is not an example 16 CHAPTER 2 CONCEPTS check will fail but this is precisely what the signature check is supposed to catch The Digital Signature Standard DSA is a public key signature algorithm that works as just described DSA is the primary signing algorithm used in GnuPG Chapter 3 Key Management Key tampering is a major security weakness with public key cryptography An eavesdropper may tamper with a user s keyrings or forge a user s public key and post it for others to download and use For example suppose Chloe wants to monitor the messages that Alice sends to Blake She could mount what is called a man in the middle attack In this attack Chloe creates a new public private keypair She replaces Alice s copy of Blake s public key with the new public key She then intercepts the messa
27. d validity is used to mean trust that a key belongs to the human associated with the key ID 3 2 VALIDATING OTHER KEYS ON YOUR PUBLIC KEYRING 23 The four trust validity levels are abbreviated unknown q none n marginal m and full In this case Blake s key is fully valid since Alice signed it herself She initially has an unknown trust in Blake to properly sign other keys but decides to trust him marginally 3 2 2 Using trust to validate keys The web of trust allows a more elaborate algorithm to be used to validate a key Formerly a key was considered valid only if you signed it personally A more flexible algorithm can now be used A key K is considered valid if it meets two conditions 1 it is signed by enough valid keys meaning e you have signed it personally e it has been signed by one fully trusted key or e it has been signed by three marginally trusted keys and 2 the path of signed keys leading from K back to your own key is five steps or shorter The path length number of marginally trusted keys required and number of fully trusted keys required may be adjusted The numbers given above are the default values used by GnuPG Figure 3 1 shows a web of trust rooted at Alice The graph illustrates who has signed who s keys The table shows which keys Alice considers valid based on her trust in the other members of the web This example assumes that two marginally trusted keys or one fully trusted key is need
28. d in the section entitled GNU Free Documentation License 38 APPENDIX A GNU FREE DOCUMENTATION LICENSE If you have no Invariant Sections write with no Invariant Sections instead of saying which ones are invariant If you have no Front Cover Texts write no Front Cover Texts instead of Front Cover Texts being LIST likewise for Back Cover Texts If your document contains nontrivial examples of program code we recommend releasing these examples in parallel under your choice of free software license such as the GNU General Public License to permit their use in free software Index addkey 19 adduid 19 armor 7 certificate 18 check 18 ciphers hybrid 14 public key 14 symmetric 13 clearsign 10 completes needed 28 decrypt 9 10 delkey 19 deluid 19 detach signature 11 edit key 17 encrypt 9 expire 21 export 7 fingerprint 8 gen key 5 gen revoke 7 import 8 key 19 key space 13 keyserver 24 list key 7 man in the middle 17 marginals needed 28 optionsfile 7 output 7 9 private key 5 14 public key 5 14 recipient 9 recv keys 24 39 revkey 19 revocation 26 revsig 20 self signing 18 send keys 24 sign 10 symmetric 9 toggle 18 trust 22 uid 19 verify 10 11 web of trust 17 21
29. deed both components are available Command gt toggle sec 1024D 26B6AAE1 created 1999 06 15 expires never sbb 2048g 0CF8CB7A created 1999 06 15 expires never sbb 1792G 08224617 created 1999 06 15 expires 2002 06 14 sbb 960D B1F423E7 created 1999 06 15 expires 2002 06 14 1 Chloe Jester lt chloe cyb org gt 2 Chloe Plebian lt chloe tel net gt The information provided is similar to the listing for the public key component The key word sec identifies the private master signing key and the keyword sbb identifies the private subordinates keys The user IDs from the public key are also listed for convenience 3 1 1 Key integrity When you distribute your public key you are distributing the public components of your master and subordinate keys as well as the user IDs Distributing this material alone however is a security risk since it is possible for an attacker to tamper with the key The public key can be modified by adding or substituting keys or by adding or changing user IDs By tampering with a user ID the attacker could change the user ID s email address to have email redirected to himself By changing one of the encryption keys the attacker would also be able to decrypt the messages redirected to him Using digital signatures is a solution to this problem When data is signed by a private key the corresponding public key is bound to the signed data In other words only the corresponding public key can be used
30. e keys to have different lifetimes The master signing key is used to make digital signatures and it also collects the signatures of others who have confirmed your identity The encryption key is used only for decrypting encrypted documents sent to you Typically a digital signature has a long lifetime e g forever and you also do not want to lose the signatures on your key that you worked hard to collect On the other hand the encryption subkey may be changed periodically for extra security since if an encryption key is broken the attacker can read all documents encrypted to that key both in the future and from the past It is almost always the case that you will not want the master key to expire There are two reasons why you may choose an expiration date First you may intend for the key to have a limited lifetime For example it is being used for an event such as a political campaign and will no longer be useful after the campaign is over Another reason is that if you lose control of the key and do not have a revocation certificate with which to revoke the key having an expiration date on the master key ensures that the key will eventually fall into disuse Changing encryption subkeys is straightforward but can be inconvenient If you generate a new keypair with an expiration date on the subkey that subkey will eventually expire Shortly before the expiration you will add a new subkey and publish your updated public key Once the subkey expi
31. e latest self signature takes precedence however so all correspondents will unambiguously know the expiration times of your keys 3 2 Validating other keys on your public keyring In Chapter 1 a procedure was given to validate your correspondents public keys a correspondent s key is validated by personally checking his key s fingerprint and then signing his public key with your private key By personally checking the fingerprint you can be sure that the key really does belong to him and since you have signed they key you can be sure to detect any tampering with it in the future Unfortunately this procedure is awkward when either you must validate a large number of keys or communicate with people whom you do not know personally GnuPG addresses this problem with a mechanism popularly known as the web of trust In the web of trust model responsibility for validating public keys is delegated to people you trust For example suppose e Alice has signed Blake s key and e Blake has signed Chloe s key and Dharma s key If Alice trusts Blake to properly validate keys that he signs then Alice can infer that Chloe s and Dharma s keys are valid without having to personally check them She simply uses her validated copy of Blake s public key to check that Blake s signatures on Chloe s and Dharma s are good In general assuming that Alice fully trusts everybody to properly validate keys they sign then any key signed by a valid
32. e original Therefore there is a third method for signing a document that creates a de tached signature which is a separate file A detached signature is created using the detach sig option alice gpg output doc sig detach sig doc You need a passphrase to unlock the secret key for user Alice Judge lt alice cyb org gt 1024 bit DSA key ID BB7576AC created 1999 06 04 Enter passphrase Both the document and detached signature are needed to verify the signature The verify option can be to check the signature blake gpg verify doc sig doc gpg Signature made Fri Jun 4 12 38 46 1999 CDT using DSA key ID BB7576AC gpg Good signature from Alice Judge lt alice cyb org gt 12 CHAPTER 1 GETTING STARTED Chapter 2 Concepts GnuPG makes uses of several cryptographic concepts including symmetric ciphers public key ciphers and one way hashing You can make basic use GnuPG without fully understanding these concepts but in order to use it wisely some understanding of them is necessary This chapter introduces the basic cryptographic concepts used in GnuPG Other books cover these topics in much more detail A good book with which to pursue further study is Bruce Schneier s Applied Cryptography 2 1 Symmetric ciphers A symmetric cipher is a cipher that uses the same key for both encryption and decryption Two parties communicating using a symmetric cipher must agree on the key beforehand Once they
33. ed to validate another key The maximum path length is three Blake Alice Chloe Elena Geoff Dharma Francis trust validity marginal full marginal full Dharma Blake Chloe Dharma Francis Blake Dharma Francis Blake Chloe Dharma Chloe Dharma Chloe Francis Blake Dharma Blake Chloe Dharma Elena Blake Chloe Dharma Francis Blake Chloe Elena Blake Chloe Elena Francis Figure 3 1 A hypothetical web of trust When computing valid keys in the example Blake and Dharma s are always considered fully valid since they were signed directly by Alice The validity of the other keys depends on trust In the first case Dharma is trusted fully which implies that Chloe s and Francis s keys will be considered valid In the second example Blake and Dharma are trusted marginally Since two marginally trusted keys are needed to fully validate a key Chloe s key will be considered fully valid but Francis s key will be considered only marginally valid In the case where Chloe and Dharma are marginally trusted Chloe s key will be marginally valid since Dharma s key is fully valid Francis s key however will also be considered marginally valid since only a fully valid key can be used to validate other keys and Dharma s key is the only fully valid key that has been used to sign Francis s key When marginal trust in Blake is added Chloe s key becomes fully valid and 24 CHAPTER 3 KEY MANAG
34. ently for updating your own key it is better to revoke key components instead of deleting them 3 1 3 Revoking key components To revoke a subkey it must be selected Once selected it may be revoked with the revkey command The key is revoked by adding a revocation selfsignature to the key Unlike the command line option gen revoke the effect of revoking a subkey is immediate Command revkey Do you really want to revoke this key y 20 CHAPTER 3 KEY MANAGEMENT You need a passphrase to unlock the secret key for user Chloe Jester lt chloe cyb org gt 1024 bit DSA key ID B87DBA93 created 1999 06 28 pub 1024D B87DBA93 created 1999 06 28 expires never trust u sub 2048g B7934539 created 1999 06 28 expires never sub 1792G 4E3160AD created 1999 06 29 expires 2000 06 28 rev subkey has been revoked 1999 06 29 sub 960D E1F56448 created 1999 06 29 expires 2000 06 28 1 Chloe Jester lt chloe cyb org gt 2 Chloe Plebian lt chloe tel net gt A user ID is revoked differently Normally a user ID collects signatures that attest that the user ID describes the person who actually owns the associated key In theory a user ID describes a person forever since that person will never change In practice though elements of the user ID such as the email address and comment may change over time thus invalidating the user ID The OpenPGP specification does not support user ID revocation but a user ID can effec tively
35. erned that there may be a determined attacker interested in invading your privacy then you should be much less trusting of other signatures and spend more time personally verifying signatures Regardless of your own security needs though you should always be careful when signing other keys It is selfish to sign a key with just enough confidence in the key s validity to satisfy your own security needs Others with more stringent security needs may want to depend on your signature If they cannot depend on you then that weakens the web of trust and makes it more 28 CHAPTER 4 DAILY USE OF GNUPG difficult for all GnuPG users to communicate Use the same care in signing keys that you would like others to use when you depend on their signatures In practice managing your web of trust reduces to assigning trust to others and tuning the options marginals needed and completes needed Any key you personally sign will be considered valid but except for small groups it will not be practical to personally sign the key of every person with whom you communicate You will therefore have to assign trust to others It is probably wise to be accurate when assigning trust and then use the options to tune how careful GnuPG is with key validation As a concrete example you may fully trust a few close friends that you know are careful with key signing and then marginally trust all others on your keyring From there you may set completes needed to 1 and
36. ester lt chloe cyb org gt 2 Chloe Plebian lt chloe tel net gt 3 2 VALIDATING OTHER KEYS ON YOUR PUBLIC KEYRING 21 A revoked user ID is indicated by the revocation signature on the ID when the signatures on the key s user IDs are listed Command gt check uid Chloe Jester lt chloe cyb org gt sig B87DBA93 1999 06 28 self signature uid Chloe Plebian lt chloe tel net gt rev B87DBA93 1999 06 29 revocation sig B87DBA93 1999 06 28 self signature Revoking both subkeys and self signatures on user IDs adds revocation self signatures to the key Since signatures are being added and no material is deleted a revocation will always be visible to others when your updated public key is distributed and merged with older copies of it Revocation therefore guarantees that everybody has a consistent copy of your public key 3 1 4 Updating a key s expiration time The expiration time of a key may be updated with the command expire from the key edit menu If no key is selected the expiration time of the primary key is updated Otherwise the expiration time of the selected subordinate key is updated A key s expiration time is associated with the key s self signature The expiration time is updated by deleting the old self signature and adding a new self signature Since correspondents will not have deleted the old self signature they will see an additional self signature on the key when they update their copy of your key Th
37. f the Document If the Cover Text requirement of section A 4 is applicable to these copies of the Document then if the Document is less than one quarter of the entire aggregate the Document s Cover Texts may be placed on covers that surround only the Document within the aggregate Otherwise they must appear on covers around the whole aggregate A 9 TRANSLATION Translation is considered a kind of modification so you may distribute translations of the Doc ument under the terms of section A 5 Replacing Invariant Sections with translations requires special permission from their copyright holders but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections You may in clude a translation of this License provided that you also include the original English version of this License In case of a disagreement between the translation and the original English version of this License the original English version will prevail A 10 TERMINATION You may not copy modify sublicense or distribute the Document except as expressly provided for under this License Any other attempt to copy modify sublicense or distribute the Document is void and will automatically terminate your rights under this License However parties who have received copies or rights from you under this License will not have their licenses terminated so long as such parties remain in full compliance A 11 FU
38. ges that Alice sends to Blake For each intercept she decrypts it using the new private key reencrypts it using Blake s true public key and forwards the reencrypted message to Blake All messages sent from Alice to Blake can now be read by Chloe Good key management is crucial in order to ensure not just the integrity of your keyrings but the integrity of other users keyrings as well The core of key management in GnuPG is the notion of signing keys Key signing has two main purposes it permits you to detect tampering on your keyring and it allows you to certify that a key truly belongs to the person named by a user ID on the key Key signatures are also used in a scheme known as the web of trust to extend certification to keys not directly signed by you but signed by others you trust Responsible users who practice good key management can defeat key tampering as a practical attack on secure communication with GnuPG 3 1 Managing your own keypair A keypair has a public key and a private key A public key consists of the public portion of the master signing key the public portions of the subordinate signing and encryption subkeys and a set of user IDs used to associate the public key with a real person Each piece has data about itself For a key this data includes its ID when it was created when it will expire etc For a user ID this data includes the name of the real person it identifies an optional comment and an email address The s
39. ing While 128 bits is sufficient for symmetric ciphers given today s factoring technology public keys with 1024 bits are recommended for most purposes 2 3 Hybrid ciphers Public key ciphers are no panacea Many symmetric ciphers are stronger from a security stand point and public key encryption and decryption are more expensive than the corresponding oper ations in symmetric systems Public key ciphers are nevertheless an effective tool for distributing symmetric cipher keys and that is how they are used in hybrid cipher systems A hybrid cipher uses both a symmetric cipher and a public key cipher It works by using 2 4 DIGITAL SIGNATURES 15 a public key cipher to share a key for the symmetric cipher The actual message being sent is then encrypted using the key and sent to the recipient Since symmetric key sharing is secure the symmetric key used is different for each message sent Hence it is sometimes called a session key Both PGP and GnuPG use hybrid ciphers The session key encrypted using the public key cipher and the message being sent encrypted with the symmetric cipher are automatically combined in one package The recipient uses his private key to decrypt the session key and the session key is then used to decrypt the message A hybrid cipher is no stronger than the public key cipher or symmetric cipher it uses whichever is weaker In PGP and GnuPG the public key cipher is probably the weaker of the pair Fortu natel
40. lready includes a cover text for the same cover previously added by you or by arrangement made by the same entity you are acting on behalf of you may not add another but you may replace the old one on explicit permission from the previous publisher that added the old one The author s and publisher s of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version A 6 COMBINING DOCUMENTS You may combine the Document with other documents released under this License under the terms defined in section A 5 above for modified versions provided that you include in the combination all of the Invariant Sections of all of the original documents unmodified and list them all as Invariant Sections of your combined work in its license notice The combined work need only contain one copy of this License and multiple identical Invariant Sections may be replaced with a single copy If there are multiple Invariant Sections with the same name but different contents make the title of each such section unique by adding at the end of it in parentheses the name of the original author or publisher of that section if known or else a unique number Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work In the combination you must combine any sections entitled History in the various original documents fo
41. marginals needed to 2 If you are more concerned with security you might choose values of 1 and 3 or 2 and 3 respectively If you are less concerned with privacy attacks and just want some reasonable confidence about validity set the values to 1 and 1 In general higher numbers for these options imply that more people would be needed to conspire against you in order to have a key validated that does not actually belong to the person whom you think it does 4 2 Building your web of trust Wanting to use GnuPG yourself is not enough In order to use to communicate securely with others you must have a web of trust At first glance however building a web of trust is a daunting task The people with whom you communicate need to use GnuPG and there needs to be enough key signing so that keys can be considered valid These are not technical problems they are social problems Nevertheless you must overcome these problems if you want to use GnuPG When getting started using GnuPG it is important to realize that you need not securely communicate with every one of your correspondents Start with a small circle of people perhaps just yourself and one or two others who also want to exercise their right to privacy Generate your keys and sign each other s public keys This is your initial web of trust By doing this you will appreciate the value of a small robust web of trust and will be more cautious as you grow your web in the future In addition to those
42. mbers together to get a composite but it is difficult to factor a composite into its prime components A one way trapdoor function is similar but it has a trapdoor That is if some piece of information is known it becomes easy to compute the inverse For example if you have a number made of two prime factors then knowing one of the factors makes it easy to compute the second Given a public key cipher based on prime factorization the public key contains a composite number made from two large prime factors and the encryption algorithm uses that composite to encrypt the message The algorithm to decrypt the message requires knowing the prime factors so decryption is easy if you have the private key containing one of the factors but extremely difficult if you do not have it As with good symmetric ciphers with a good public key cipher all of the security rests with the key Therefore key size is a measure of the system s security but one cannot compare the size of a symmetric cipher key and a public key cipher key as a measure of their relative security In a brute force attack on a symmetric cipher with a key size of 80 bits the attacker must enumerate up to 280 keys to find the right key In a brute force attack on a public key cipher with a key size of 512 bits the attacker must factor a composite number encoded in 512 bits up to 155 decimal digits The workload for the attacker is fundamentally different depending on the cipher he is attack
43. n public key Consider it for a minute and you will see that it is an easy mistake to make In general novice users have difficulty understanding the different roles of the public key and private key when using GnuPG As a user interface designer you should try to make it clear at all times when one of the two keys is being used You could also use wizards or other common GUI techniques for guiding the user through common tasks such as key generation where extra steps such as generating a key revocation certification and making a backup are all but essential for using GnuPG correctly Other comments from the paper include the following e Security is usually a secondary goal people want to send email browse and so on Do not assume users will be motivated to read manuals or go looking for security controls e The security of a networked computer is only as strong as its weakest component Users need to be guided to attend to all aspects of their security not left to proceed through random exploration as they might with a word processor or a spreadsheet e Consistently use the same terms for the same actions Do not alternate between synonyms like encrypt and encipher lhttp www cs cmu edu alma http www cs berkeley edu tygar 3http reports archive adm cs cmu edu anon 1998 abstracts 98 155 html 31 32 CHAPTER 5 TOPICS e For inexperienced users simplify the display Too much information hides the important inf
44. nd 4 GnuPG uses public key cryptography so that users may communicate securely In a public key system each user has a pair of keys consisting of a private key and a public key A user s private key is kept secret it need never be revealed The public key may be given to anyone with whom the user wants to communicate GnuPG uses a somewhat more sophisticated scheme in which a user has a primary keypair and then zero or more additional subordinate keypairs The primary and subordinate keypairs are bundled to facilitate key management and the bundle can often be considered simply as one keypair 1 1 Generating a new keypair The command line option gen key is used to create a new primary keypair 11 gpg gen key gpg GnuPG 0 9 4 Copyright C 1999 Free Software Foundation Inc This program comes with ABSOLUTELY NO WARRANTY This is free software and you are welcome to redistribute it under certain conditions See the file COPYING for details Please select what kind of key you want 1 DSA and ElGamal default 2 DSA sign only 4 ElGamal sign and encrypt Your selection GnuPG is able to create several different types of keypairs but a primary key must be capable of making signatures There are therefore only three options Option 1 actually creates two keypairs A DSA keypair is the primary keypair usable only for making signatures An ElGamal subordinate keypair is also created for encryption Option 2 is similar b
45. nly the data encrypted with the one broken key will be revealed Subkeys and user IDs may also be deleted To delete a subkey or user ID you must first select it using the key or uid commands respectively These commands are toggles For example the command key 2 selects the second subkey and invoking key 2 again deselects it If no extra argument is given all subkeys or user IDs are deselected Once the user IDs to be deleted are selected the command deluid actually deletes the user IDs from your key Similarly the command delkey deletes all selected subkeys from both your public and private keys For local keyring management deleting key components is a good way to trim other people s public keys of unnecessary material Deleting user IDs and subkeys on your own key however is not always wise since it complicates key distribution By default when a user imports your updated public key it will be merged with the old copy of your public key on his ring if it exists The components from both keys are combined in the merge and this effectively restores any components you deleted To properly update the key the user must first delete the old version of your key and then import the new version This puts an extra burden on the people with whom you communicate Furthermore if you send your key to a keyserver the merge will happen regardless and anybody who downloads your key from a keyserver will never see your key with components deleted Consequ
46. ormation An initial display configuration could concentrate on giving the user the correct model of the relationship between public and private keys and a clear understanding of the functions for acquiring and distributing keys Designing an effective user interface for key management is even more difficult The OpenPGP web of trust model is unfortunately quite obtuse For example the specification imposes three arbitrary trust levels onto the user none marginal and complete All degrees of trust felt by the user must be fit into one of those three cubbyholes The key validation algorithm is also difficult for non computer scientists to understand particularly the notions of marginals needed and completes needed Since the web of trust model is well specified and cannot be changed you will have to do your best and design a user interface that helps to clarify it for the user A definite improvement for example would be to generate a diagram of how a key was validated when requested by the user Relevant comments from the paper include the following e Users are likely to be uncertain on how and when to grant accesses e Place a high priority on making sure users understand their security well enough to prevent them from making potentially high cost mistakes Such mistakes include accidentally delet ing the private key accidentally publicizing a key accidentally revoking a key forgetting the pass phrase and failing to back up the
47. private key that complements one of the recipients public keys In particular you cannot decrypt a document encrypted by you unless you included your own public key in the recipient list To decrypt a message the option decrypt is used You need the private key to which the message was encrypted Similar to the encryption process the document to decrypt is input and the decrypted result is output blake gpg output doc decrypt doc gpg You need a passphrase to unlock the secret key for user Blake Executioner lt blake cyb org gt 1024 bit ELG E key ID 5C8CBD41 created 1999 06 04 main key ID 9E98BC16 Enter passphrase Documents may also be encrypted without using public key cryptography Instead you use a symmetric cipher to encrypt the document The key used to drive the symmetric cipher is derived from a passphrase supplied when the document is encrypted and for good security it should not be the same passphrase that you use to protect your private key Symmetric encryption is useful for securing documents when the passphrase does not need to be communicated to others A document can be encrypted with a symmetric cipher by using the symmetric option alice gpg output doc gpg symmetric doc Enter passphrase 10 CHAPTER 1 GETTING STARTED 1 4 Making and verifying signatures A digital signature certifies and timestamps a document If the document is subsequently modified in any way a verification of the signat
48. private keypair e protecting your private key e selecting expiration dates and using subkeys and e managing your web of trust A well chosen key size protects you against brute force attacks on encrypted messages Protect ing your private key prevents an attacker from simply using your private key to decrypt encrypted messages and sign messages in your name Correctly managing your web of trust prevents at tackers from masquerading as people with whom you communicate Ultimately addressing these issues with respect to your own security needs is how you balance the extra work required to use GnuPG with the privacy it gives you 25 26 CHAPTER 4 DAILY USE OF GNUPG 4 1 1 Choosing a key size Selecting a key size depends on the key In OpenPGP a public private keypair usually has multiple keys At the least it has a master signing key and it probably has one or more additional subkeys for encryption Using default key generation parameters with GnuPG the master key will be a DSA key and the subkeys will be ElGamal keys DSA allows a key size up to 1024 bits This is not especially good given today s factoring technology but that is what the standard specifies Without question you should use 1024 bit DSA keys ElGamal keys on the other hand may be of any size Since GnuPG is a hybrid public key system the public key is used to encrypt a 128 bit session key and the private key is used to decrypt it Key size nevertheless affec
49. rea e nass e ose eee RS 5 Topics 5 1 Writing user interfaces 2 eee enn 4 CONTENTS A GNU Free Documentation License 33 Al PREAMBLE tie eset aya ie des Ech Ol dad aes 33 2 APPLICABILITY AND DEFINITIONS 20048 33 VERBATIM COPYING 4e 34 A 4 COPYING IN QUANTITY 34 MODIFICATIONS 4 4 a uoa deu BARA So RD SALA AUS pq tre Dee E et Se 35 A 6 COMBINING DOCUMENTS es 36 COLLECTIONS OF DOCUMENTS 36 A 8 AGGREGATION WITH INDEPENDENT WORKS 3T 9 TREANSDACBIQN E a A ee ce ee de ded A ed 3T A 10 TERMINATION es 37 A 11 FUTURE REVISIONS OF THIS LICENSE 37 A 12 How to use this License for your documents 37 Chapter 1 Getting Started GnuPG is a tool for secure communication This chapter is a quick start guide that covers the core functionality of GnuPG This includes keypair creation exchanging and verifying keys encrypting and decrypting documents and authenticating documents with digital signatures It does not explain in detail the concepts behind public key cryptography encryption and digital signatures This is covered in Chapter 2 It also does not explain how to use GnuPG wisely This is covered in Chapters 3 a
50. res those who wish to correspond with you must find your updated key since they will no longer be able to encrypt to the expired key This may be inconvenient depending on how you distribute the key Fortunately however no extra signatures are necessary since the new subkey will have been signed with your master signing key which presumably has already been validated by your correspondents The inconvenience may or may not be worth the extra security Just as you can an attacker can still read all documents encrypted to an expired subkey Changing subkeys only protects future documents In order to read documents encrypted to the new subkey the attacker would need to mount a new attack using whatever techniques he used against you the first time Finally it only makes sense to have one valid encryption subkey on a keyring There is no additional security gained by having two or more active subkeys There may of course be any number of expired keys on a keyring so that documents encrypted in the past may still be decrypted but only one subkey needs to be active at any given time 4 1 4 Managing your web of trust As with protecting your private key managing your web of trust is another aspect of using GnuPG that requires balancing security against ease of use If you are using GnuPG to protect against casual eavesdropping and forgeries then you can afford to be relatively trusting of other people s signatures On the other hand if you are conc
51. ritish built specialized computing engines the Bombes to mechanically try keys until the day s key was found This meant that sometimes they found the day s key within hours of the new key s use but it also meant that on some days they never did find the right key The Bombes were not general purpose computers but were precursors to modern day computers Today computers can guess keys very quickly and this is why key size is important in modern cryptosystems The cipher DES uses 56 bit key which means that there are 256 possible keys 256 is 72 057 594 037 927 936 keys This is a lot of keys but a general purpose computer can check the entire key space in a matter of days A specialized computer can check it in hours On the other hand more recently designed ciphers such as 3DES Blowfish and IDEA all use 128 bit lhttp www counterpane com schneier html http www counterpane com applied html 13 14 CHAPTER 2 CONCEPTS keys which means there are 2 28 possible keys This is many many more keys and even if all the computers on the planet cooperated it could still take more time than the age of the universe to find the key 2 2 Public key ciphers The primary problem with symmetric ciphers is not their security but with key exchange Once the sender and receiver have exchanged keys that key can be used to securely communicate but what secure communication channel was used to communicate the key itself In particular it
52. rming one section entitled History likewise combine any sections entitled Ac knowledgements and any sections entitled Dedications You must delete all sections entitled Endorsements COLLECTIONS OF DOCUMENTS You may make a collection consisting of the Document and other documents released under this License and replace the individual copies of this License in the various documents with a single copy that is included in the collection provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects You may extract a single document from such a collection and distribute it individually under this License provided you insert a copy of this License into the extracted document and follow this License in all other respects regarding verbatim copying of that document A 8 AGGREGATION WITH INDEPENDENT WORKS 37 A 8 AGGREGATION WITH INDEPENDENT WORKS A compilation of the Document or its derivatives with other separate and independent documents or works in or on a volume of a storage or distribution medium does not as a whole count as a Modified Version of the Document provided no compilation copyright is claimed for the compilation Such a compilation is called an aggregate and this License does not apply to the other self contained works thus compiled with the Document on account of their being thus compiled if they are not themselves derivative works o
53. s and sends the specified keys to the key server The key server to which to send the keys is specified with the command line option keyserver Similarly the option recv keys is used to retrieve keys from a keyserver but the option recv keys requires a key ID be used to specify the key In the following example Alice updates her public key with new signatures from the keyserver certserver pgp com and then sends her copy of Blake s public key to the same keyserver to contribute any new signatures she may have added alice gpg keyserver certserver pgp com recv key 0xBB7576AC gpg requesting key BB7576AC from certserver pgp com gpg key BB7576AC 1 new signature gpg Total number processed 1 gpg new signatures 1 alice gpg keyserver certserver pgp com send key blake cyb org gpg success sending to certserver pgp com status 200 There are several popular keyservers in use around the world The major keyservers synchronize themselves so it is fine to pick a keyserver close to you on the Internet and then use it regularly for sending and receiving keys Chapter 4 Daily use of GnuPG GnuPG is a complex tool with technical social and legal issues surrounding it Technically it has been designed to be used in situations having drastically different security needs This complicates key management Socially using GnuPG is not strictly a personal decision To use GnuPG effectively both parties communicating must use it
54. se notice giving the public permission to use the Modified Version under the terms of this License in the form shown in the Addendum below 7 Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document s license notice 8 Include an unaltered copy of this License 9 Preserve the section entitled History and its title and add to it an item stating at least the title year new authors and publisher of the Modified Version as given on the Title Page If there is no section entitled History in the Document create one stating the title year authors and publisher of the Document as given on its Title Page then add an item describing the Modified Version as stated in the previous sentence 10 Preserve the network location if any given in the Document for public access to a Trans parent copy of the Document and likewise the network locations given in the Document for previous versions it was based on These may be placed in the History section You may omit a network location for a work that was published at least four years before the Document itself or if the original publisher of the version it refers to gives permission 11 In any section entitled Acknowledgements or Dedications preserve the section s title and preserve in the section all the substance and tone of each of the contributor acknowl edgements and or dedications given therein
55. should be valid key does not expire lt n gt key expires in n days lt n gt w key expires in n weeks lt n gt m key expires in n months lt n gt y key expires in n years Key is valid for 0 For most users a key that does not expire is adequate The expiration time should be chosen with care however since although it is possible to change the expiration date after the key is created it may be difficult to communicate a change to users who have your public key You must provide a user ID in addition to the key parameters The user ID is used to associate the key being created with a real person You need a User ID to identify your key the software constructs the user id from Real Name Comment and Email Address in this form Heinrich Heine Der Dichter lt heinrichh duesseldorf de gt Real name Only one user ID is created when a key is created but it is possible to create additional user IDs if you want to use the key in two or more contexts e g as an employee at work and a political activist on the side A user ID should be created carefully since it cannot be edited after it is created GnuPG needs a passphrase to protect the primary and subordinate private keys that you keep in your possession You need a Passphrase to protect your private key Enter passphrase There is no limit on the length of a passphrase and it should be carefully chosen From the perspective of security the passphrase to unlock the
56. sted as Front Cover Texts or Back Cover Texts in the notice that says that the Document is released under this License A Transparent copy of the Document means a machine readable copy represented in a format whose specification is available to the general public whose contents can be viewed and edited directly and straightforwardly with generic text editors or for images composed of pixels generic paint programs or for drawings some widely available drawing editor and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent A copy that is not Transparent is called Opaque Examples of suitable formats for Transparent copies include plain ASCII without markup Texinfo input format LaTeX input format SGML or XML using a publicly available DTD and standard conforming simple HTML designed for human modification Opaque formats include PostScript PDF proprietary formats that can be read and edited only by proprietary word processors SGML or XML for which the DTD and or processing tools are not generally available and the machine generated HTML produced by some word processors for output purposes only The Title Page means for a printed book the title page itself plus such following
57. to verify the signature and ensure that the data has not been modified A public key can be protected from tampering by using its corresponding private master key to sign the public key components and user IDs thus binding the components to the public master key Signing public key components with the corresponding private master signing key is called self signing and a public key that has self signed user IDs bound to it is called a certificate As an example Chloe has two user IDs and three subkeys The signatures on the user IDs can be checked with the command check from the key edit menu chloe gpg edit key chloe Secret key is available pub 1024D 26B6AAE1 created 1999 06 15 expires never trust u sub 2048g 0CF8CB7A created 1999 06 15 expires never sub 1792G 08224617 created 1999 06 15 expires 2002 06 14 sub 960D B1F423E7 created 1999 06 15 expires 2002 06 14 1 Chloe Jester lt chloe cyb org gt 2 Chloe Plebian lt chloe tel net gt Command check uid Chloe Jester lt chloe cyb org gt 3 1 MANAGING YOUR OWN KEYPAIR 19 sig 26B6AAE1 1999 06 15 self signature uid Chloe Plebian lt chloe tel net gt sig 26B6AAE1 1999 06 15 self signature As expected the signing key for each signature is the master signing key with ID 0x26B6AAE1 The self signatures on the subkeys are present in the public key but they are not shown by the GnuPG interface 3 1 2 Adding and deleting key components Both new subke
58. tributing any large number of copies to give them a chance to provide you with an updated version of the Document A 5 MODIFICATIONS You may copy and distribute a Modified Version of the Document under the conditions of sec tions A 3 and A 4 above provided that you release the Modified Version under precisely this License with the Modified Version filling the role of the Document thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it In addition you must do these things in the Modified Version 1 Use in the Title Page and on the covers if any a title distinct from that of the Document and from those of previous versions which should if there were any be listed in the History section of the Document You may use the same title as a previous version if the original publisher of that version gives permission 2 List on the Title Page as authors one or more persons or entities responsible for authorship of the modifications in the Modified Version together with at least five of the principal authors of the Document all of its principal authors if it has less than five 3 State on the Title page the name of the publisher of the Modified Version as the publisher 4 Preserve all the copyright notices of the Document 5 Add an appropriate copyright notice for your modifications adjacent to the other copyright notices 6 Include immediately after the copyright notices a licen
59. tructure of the private key is similar except that it contains only the private portions of the keys and there is no user ID information The command line option edit key may be used to view a keypair For example chloe gpg edit key chloe cyb org Secret key is available pub 1024D 26B6AAE1 created 1999 06 15 expires never trust u sub 2048g 0CF8CB7A created 1999 06 15 expires never sub 1792G 08224617 created 1999 06 15 expires 2002 06 14 sub 960D B1F423E7 created 1999 06 15 expires 2002 06 14 1 Chloe Jester lt chloe cyb org gt 2 Chloe Plebian lt chloe tel net gt Command 17 18 CHAPTER 3 KEY MANAGEMENT The public key is displayed along with an indication of whether or not the private key is available Information about each component of the public key is then listed The first column indicates the type of the key The keyword pub identifies the public master signing key and the keyword sub identifies a public subordinate key The second column indicates the key s bit length type and ID The type is D for a DSA key g for an encryption only ElGamal key and G for an ElGamal key that may be used for both encryption and signing The creation date and expiration date are given in columns three and four The user IDs are listed following the keys More information about the key can be obtained with interactive commands The command toggle switches between the public and private components of a keypair if in
60. ts encryption and decryption speed since the cost of these algorithms is exponential in the size of the key Larger keys also take more time to generate and take more space to store Ultimately there are diminishing returns on the extra security a large key provides you After all if the key is large enough to resist a brute force attack an eavesdropper will merely switch to some other method for obtaining your plaintext data Examples of other methods include robbing your home or office and mugging you 1024 bits is thus the recommended key size If you genuinely need a larger key size then you probably already know this and should be consulting an expert in data security 4 1 2 Protecting your private key Protecting your private key is the most important job you have to use GnuPG correctly If someone obtains your private key then all data encrypted to the private key can be decrypted and signatures can be made in your name If you lose your private key then you will no longer be able to decrypt documents encrypted to you in the future or in the past and you will not be able to make signatures Losing sole possession of your private key is catastrophic Regardless of how you use GnuPG you should store the public key s revocation certificate and a backup of your private key on write protected media in a safe place For example you could burn them on a CD ROM and store them in your safe deposit box at the bank in a sealed envelope Alternatively
61. ure will fail A digital signature can serve the same purpose as a hand written signature with the additional benefit of being tamper resistant The GnuPG source distribution for example is signed so that users can verify that the source code has not been modified since it was packaged Creating and verifying signatures uses the public private keypair in an operation different from encryption and decryption A signature is created using the private key of the signer The signature is verified using the corresponding public key For example Alice would use her own private key to digitally sign her latest submission to the Journal of Inorganic Chemistry The associate editor handling her submission would use Alice s public key to check the signature to verify that the submission indeed came from Alice and that it had not been modified since Alice sent it A consequence of using digital signatures is that it is difficult to deny that you made a digital signature since that would imply your private key had been compromised The command line option sign is used to make a digital signature The document to sign is input and the signed document is output alice gpg output doc sig sign doc You need a passphrase to unlock the private key for user Alice Judge lt alice cyb org gt 1024 bit DSA key ID BB7576AC created 1999 06 04 Enter passphrase The document is compressed before being signed and the output is in binary format Given
62. ust present the full title with all words of the title equally prominent and visible You may add other material on the covers in addition Copying with changes limited to the covers as long as they preserve the title of the Document and satisfy these conditions can be treated as verbatim copying in other respects If the required texts for either cover are too voluminous to fit legibly you should put the first ones listed as many as fit reasonably on the actual cover and continue the rest onto adjacent pages If you publish or distribute Opaque copies of the Document numbering more than 100 you must either include a machine readable Transparent copy along with each Opaque copy or state in or with each Opaque copy a publicly accessible computer network location containing a complete Transparent copy of the Document free of added material which the general network using public has access to download anonymously at no charge using public standard network protocols If you A 5 MODIFICATIONS 35 use the latter option you must take reasonably prudent steps when you begin distribution of Opaque copies in quantity to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy directly or through your agents or retailers of that edition to the public It is requested but not required that you contact the authors of the Document well before redis
63. ut creates only a DSA keypair Option 4 creates a single ElGamal keypair usable for both making signatures and performing encryption In all cases it is possible to later add additional subkeys for encryption and signing For most users the default option is fine 1Option 3 is to generate an ElGamal keypair that is not usable for making signatures 6 CHAPTER 1 GETTING STARTED You must also choose a key size The size of a DSA key must be between 512 and 1024 bits and an ElGamal key may be of any size GnuPG however requires that keys be no smaller than 768 bits Therefore if Option 1 was chosen and you choose a keysize larger than 1024 bits the ElGamal key will have the requested size but the DSA key will be 1024 bits About to generate a new ELG E keypair minimum keysize is 768 bits default keysize is 1024 bits highest suggested keysize is 2048 bits What keysize do you want 1024 The longer the key the more secure it is against brute force attacks but for almost all purposes the default keysize is adequate since it would be cheaper to circumvent the encryption than try to break it Also encryption and decryption will be slower as the key size is increased and a larger keysize may affect signature length Once selected the keysize can never be changed Finally you must choose an expiration date If Option 1 was chosen the expiration date will be used for both the ElGamal and DSA keypairs Please specify how long the key
64. y however if an attacker could decrypt a session key it would only be useful for reading the one message encrypted with that session key The attacker would have to start over and decrypt another session key in order to read any other message 2 4 Digital signatures A hash function is a many to one function that maps its input to a value in a finite set Typically this set is a range of natural numbers A simple hash function is f x 0 for all integers A more interesting hash function is f x x mod 37 which maps to the remainder of dividing x by 37 A document s digital signature is the result of applying a hash function to the document To be useful however the hash function needs to satisfy two important properties First it should be hard to find two documents that hash to the same value Second given a hash value it should be hard to recover the document that produced that value Some public key ciphers could be used to sign documents The signer encrypts the document with his private key Anybody wishing to check the signature and see the document simply uses the signer s public key to decrypt the document This algorithm does satisfy the two properties needed from a good hash function but in practice this algorithm is too slow to be useful An alternative is to use hash functions designed to satisfy these two important properties SHA and 5 are examples of such algorithms Using such an algorithm a document is signed
65. y must bypass the encryption on the private key Instead of brute force guessing the key an attacker will almost certainly instead try to guess the passphrase The motivation for trying passphrases is that most people choose a passphrase that is easier to guess than a random 128 bit key If the passphrase is a word it is much cheaper to try all the words in the dictionaries of the world s languages Even if the word is permuted e g k3wldood it is still easier to try dictionary words with a catalog of permutations The same problem applies to quotations In general passphrases based on natural language utterances are poor passphrases 4 1 DEFINING YOUR SECURITY NEEDS 27 since there is little randomness and lots of redundancy in natural language You should avoid natural language passphrases if you can A good passphrase is one that you can remember but is hard for someone to guess It should include characters from the whole range of printable characters on your keyboard This includes uppercase alphabetics characters numbers and special characters such as and Be creative and spend a little time considering your passphrase a good choice is important to ensure your privacy 4 1 3 Selecting expiration dates and using subkeys By default a DSA master signing key and an ElGamal encryption subkey are generated when you create a new keypair This is convenient because the roles of the two keys are different and you may therefore want th
66. ypting and decrypting documents A public key may be thought of as an open safe When a correspondent encrypts a document using a public key that document is put in the safe the safe shut and the combination lock spun several times The corresponding private key is the combination that can reopen the safe and retrieve the document In other words only the person who holds the private key can recover a document encrypted using the associated public key The procedure for encrypting and decrypting documents is straightforward with this mental model If you want to encrypt a message to Alice you encrypt it using Alice s public key and she decrypts it with her private key If Alice wants to send you a message she encrypts it using your public key and you decrypt it with your private key To encrypt a document the option encrypt is used You must have the public keys of the intended recipients The software expects the name of the document to encrypt as input if omitted it reads standard input The encrypted result is placed on standard output or as specified using the option output The document is compressed for additional security in addition to encrypting it alice gpg output doc gpg encrypt recipient blake cyb org doc The recipient option is used once for each recipient and takes an extra argument specifying the public key to which the document should be encrypted The encrypted document can only be decrypted by someone with a
67. ys and new user IDs may be added to your keypair after it has been created A user ID is added using the command adduid You are prompted for a real name email address and comment just as when you create an initial keypair A subkey is added using the command addkey The interface is similar to the interface used when creating an initial keypair The subkey may be a DSA signing key and encrypt only ElGamal key or a sign and encrypt ElGamal key When a subkey or user ID is generated it is self signed using your master signing key which is why you must supply your passphrase when the key is generated Additional user IDs are useful when you need multiple identities For example you may have an identity for your job and an identity for your work as a political activist Coworkers will know you by your work user ID Coactivists will know you by your activist user ID Since those groups of people may not overlap though each group may not trust the other user ID Both user IDs are therefore necessary Additional subkeys are also useful The user IDs associated with your public master key are validated by the people with whom you communicate and changing the master key therefore requires recertification This may be difficult and time consuming if you communicate with many people On the other hand it is good to periodically change encryption subkeys If a key is broken all the data encrypted with that key will be vulnerable By changing keys however o

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