1. Using GPG - the GNU Privacy Guard Why does a security professional need this skill ? 3 GPG HOWTO's Use by checking Correspondents' Keys Directly Using a TTP as a Certification Authority Using GPG for Symmetric encryption Security features of GPG PGP or GPG ? Conclusions Further Reading

2. Why does a security professional need this skill ? Many unattributable and bogus security alerts are distributed by chain email. Supposing a bank needs to reset account passwords and wants account holders to believe them ? Because privacy is normal. Do you use envelopes around your snail mail or prefer postcards ? Confirming that the developer whose program you are about to install is the maintainer for the package recognised by the distribution team. (Value of free software ecosystem $40G/annum by 2010 (estimate by IDC).

3. Using GPG where users are checking each others' keys directly The following set of slides present an experiment using GPG where users rich and bob establish and communicate keys directly and send a secret message, without using trusted third party certification.

4. Use of GPG checking keys directly Creating a key pair 1 rich@saturn:~/gpg$ gpg --gen-key gpg (GnuPG) 1.4.3; Copyright (C) 2006 Free Software Foundation, Inc. gpg: keyring `/home/rich/.gnupg/secring.gpg' created Please select what kind of key you want: (1) DSA and Elgamal (default)‏ (2) DSA (sign only)‏ (5) RSA (sign only)‏ Your selection? 1 DSA keypair will have 1024 bits. ELG-E keys may be between 1024 and 4096 bits long. What keysize do you want? (2048)‏ Requested keysize is 2048 bits Please specify how long the key should be valid. 0 = key does not expire = key expires in n days w = key expires in n weeks m = key expires in n months Key is valid for? (0) 52w Key expires at Tue 29 Jan 2008 19:10:37 GMT Is this correct? (y/N) y

5. Use of GPG checking keys directly Creating a key pair 2 You need a user ID to identify your key; the software constructs the user ID from the Real Name, Comment and E-mail Address in this form: "Heinrich Heine (Der Dichter) " Real name: Richard Kay E-mail address: rich@example.net Comment: Experimental Purposes Only You selected this USER-ID: "Richard Kay (Experimental Purposes Only) " Change (N)ame, (C)omment, (E)-mail or (O)kay/(Q)uit? O You need a Passphrase to protect your secret key. It didn't echo We need to generate a lot of random bytes. It is a good idea to perform some other action (type on the keyboard, move the mouse, use the disks) during the prime generation; this gives the random number generator a better chance to gain enough entropy. ++++++++++.+++++++++++++++++++++++++.+++++++++++++ gpg: /home/rich/.gnupg/trustdb.gpg: trustdb created gpg: key EBEF27FB marked as ultimately trusted public and secret key created and signed.

6. Use of GPG checking keys directly Creating a key pair 3 gpg: checking the trustdb gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u gpg: next trustdb check due at 2008-01-29 pub 1024D/EBEF27FB 2007-01-30 [expires: 2008-01-29] Key fingerprint = CDA4 E092 B12A 99EA B093 689F 8AE7 E694 EBEF 27FB uid Richard Kay (Experimental Purposes Only) sub 2048g/9F119F7F 2007-01-30 [expires: 2008-01-29] The above proceedure was repeated for user bob resulting in the following information about bob's key: pub 1024D/357B2A4F 2007-01-30 [expires: 2008-01-29] Key fingerprint = 1BD0 6E5E 7A7D 1D0B 24E7 9A80 F8DF 8B17 357B 2A4F uid Bob User (Im a Tester) sub 2048g/F2B63464 2007-01-30 [expires: 2008-01-29]

7. Use of GPG checking keys directly Exporting keys As rich: rich@saturn:~/gpg$ gpg -a --export rich > richpub As bob: bob@saturn:~$ gpg -a --export bob > bobpub Use of the -a flag exported the public key in ascii- armoured format, suitable for sending by email.

8. What does a public key look like ? -----BEGIN PGP PUBLIC KEY BLOCK----- Version: GnuPG v1.4.3 (GNU/Linux)‏ mQGiBEW/m4sRBADT839C4QkJuglzTFy3GZRIYZD2vXdDrOwnDwrODryqXfUO5s1I x8v2AHKtbn8YttaZtVxdg5cXr1aeFH9VyAWqXukdFuqFBMNIM1qdVKOiXl2CO9bj XPb0LGT+9X9lL7Q7pflHppnPSgcwMKvqb8OzqRLwRwnuWEVPL/ZCyJyujwCghii5 20 lines skipped AgAPBQJFv5uUAhsMBQkB3+IAAAoJEPjfixc1eypPB9gAn2pI6NjFugtRWZftxX8h 4argxIQxAJ0ZH6PD+Li7yimSqCQhuiE+6AJWuA== =XY9V -----END PGP PUBLIC KEY BLOCK-----

9. Use of GPG checking keys directly Importing a key Users bob and rich sent their exported public key files to each, one by email, the other through a file copy. rich used the Mutt email client to read mail. Here the ? help key within the attachment menu displayed: ^K extract-keys extract supported public keys So pressing and together displayed: gpg: key 357B2A4F: public key "Bob User (Im a Tester) " imported gpg: Total number processed: 1 gpg: imported: 1 Press any key to continue... User bob read the gpg manpage and used the following command to import rich's key as a file: bob@saturn:~$ gpg --import /tmp/richpub gpg: key EBEF27FB: public key "Richard Kay (Experimental Purposes Only) " imported gpg: Total number processed: 1 gpg: imported: 1

10. Use of GPG checking keys directly Editing the trust level on received keys 1 As bob: bob@saturn:~$ gpg --edit-key rich gpg (GnuPG) 1.4.3; Copyright (C) 2006 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. pub 1024D/EBEF27FB created: 2007-01-30 expires: 2008-01-29 usage: SC trust: unknown validity: unknown sub 2048g/9F119F7F created: 2007-01-30 expires: 2008-01-29 usage: E [ unknown] (1). Richard Kay (Experimental Purposes Only) Command> sign pub 1024D/EBEF27FB created: 2007-01-30 expires: 2008-01-29 usage: SC trust: unknown validity: unknown Primary key fingerprint: CDA4 E092 B12A 99EA B093 689F 8AE7 E694 EBEF 27FB Richard Kay (Experimental Purposes Only)

11. Use of GPG checking keys directly Editing the trust level on received keys 2 As bob, continued: This key is due to expire on 2008-01-29. Are you sure that you want to sign this key with your key "Bob User (Im A Tester) " (0C86136D)‏ Really sign? (y/N) y You need a passphrase to unlock the secret key for user: "Bob User (Im A Tester) " 1024-bit DSA key, ID 0C86136D, created 2007-02-02 Command> q Save changes? (y/N) y

12. Use of GPG checking keys directly Signing and encryptin g a message A message was created and stored in the file: secret rich@saturn:~/gpg$ cat secret This is a secret message. The following GPG actions and flags were used: -s to sign -e to encrypt -a ASCII armoured output -r userid -o name of output file rich@saturn:~/gpg$ gpg -r bob -o secret.asc -sea secret You need a passphrase to unlock the secret key for user: "Richard Kay (Experimental Purposes Only) " 1024-bit DSA key, ID EBEF27FB, created 2007-01-30

13. Signed and encrypted output rich@saturn:~/gpg$ cat secret.asc -----BEGIN PGP MESSAGE----- Version: GnuPG v1.4.3 (GNU/Linux)‏ hQIOAxVuAHHdtcplEAf/ZrUQpN7ClwSAa/ZX+nOd+mG2vRiCx 3jp9D/Y8M3DY0jC mA2H774ZeJNl0++hefGCTbxeGCGnjsh3t1xBM1x9sxKy9Wu0 eRSLOAB5PS6ivEO6 10 lines skipped cYeP880shJSZMBqg/fTElSHUxJgGfTOFcKyM1DDemk0/51Wh I2b1zdMcwNKV9dap spEdrBmY2qXKtvjVvBXNSVT9IHATcjoB6i2kpjqC/jc4TlXC352v 1JNKwvzVDAvY T5SLsO8tbz4k3r6VjKyCyaAyi1k= =muEX -----END PGP MESSAGE-----

14. Use of GPG checking keys directly Decryptin g the secret message bob@saturn:~$ gpg -o secret -d secret.asc You need a passphrase to unlock the secret key for user: "Bob User (Im A Tester) " 2048-bit ELG-E key, ID DDB5CA65, created 2007-02-02 (main key ID 0C86136D)‏ gpg: encrypted with 2048-bit ELG-E key, ID DDB5CA65, created 2007- 02-02 "Bob User (Im A Tester) " gpg: Signature made Fri 02 Feb 2007 15:33:41 GMT using DSA key ID EBEF27FB gpg: Good signature from "Richard Kay (Experimental Purposes Only) " bob@saturn:~$ cat secret This is a secret message.

15. Using a Trusted Third Party as a Certification Authority In the following set of slides, users rich and bob use the services of dave to act as a certification authority. Dave will need to sign rich and bob's public keys having confirmed their identities. Dave's signatures on the others' keys will enable rich and bob to communicate securely.

16. Using a TTP as a certificati on authority Users dave, rich and bob create new keys dave's key pub 1024D/57E0F876 2007-02-16 [expires: 2008-02-15] Key fingerprint = ACF5 7915 4C5E 6F1D 26E0 8662 6637 B994 57E0 F876 uid Dave Trusted (TTP keysigning key) sub 2048g/A6BFD1FD 2007-02-16 [expires: 2008-02-15] rich's key pub 1024D/D224BF4D 2007-02-16 [expires: 2008-02-15] Key fingerprint = 28D5 9340 3329 2ABD F853 3524 1A88 D35B D224 BF4D uid Rich Kay (Demo use of ttp key) sub 2048g/401D9F40 2007-02-16 [expires: 2008-02-15] bob's key pub 1024D/12D2BFBA 2007-02-16 [expires: 2008-02-15] Key fingerprint = C4D9 2D11 FFE9 6B73 3824 64E7 D02F E07B 12D2 BFBA uid Bob Person (bob TTP process) sub 2048g/10C076AE 2007-02-16 [expires: 2008-02-15]

17. Using a TTP as a certificati on authority exporting unsigned keys dave@saturn:~$ gpg -a --export dave > /tmp/davepub bob@saturn:~$ gpg -a --export bob > /tmp/bobpub rich@saturn:~$ gpg -a --export rich > /tmp/richpub bob@saturn:~$ ls -l /tmp/*pub -rw-r--r-- 1 dave dave 1730 2007-02-16 17:47 /tmp/davepub -rw-r--r-- 1 rich rich 1726 2007-02-16 17:49 /tmp/richpub -rw-r--r-- 1 bob bob 1726 2007-02-16 17:48 /tmp/bobpub

18. Using a TTP as a certificati on authority importing unsigned keys dave@saturn:~$ gpg --import /tmp/richpub gpg: key D224BF4D: public key "Rich Kay (Demo use of ttp key) " imported gpg: Total number processed: 1 gpg: imported: 1 dave@saturn:~$ gpg --import /tmp/bobpub gpg: key 12D2BFBA: public key "Bob Person (bob TTP process) " imported gpg: Total number processed: 1 gpg: imported: 1

19. Using a TTP as a certificati on authority Dave signs rich's and bob's keys 1 dave@saturn:~$ gpg --edit-key rich gpg (GnuPG) 1.4.3; Copyright (C) 2006 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. pub 1024D/D224BF4D created: 2007-02-16 expires: 2008-02-15 usage: SC trust: unknown validity: unknown sub 2048g/401D9F40 created: 2007-02-16 expires: 2008-02-15 usage: E [ unknown] (1). Rich Kay (Demo use of ttp key) Command> sign pub 1024D/D224BF4D created: 2007-02-16 expires: 2008-02-15 usage: SC trust: unknown validity: unknown Primary key fingerprint: 28D5 9340 3329 2ABD F853 3524 1A88 D35B D224 BF4D

20. Using a TTP as a certificati on authority Dave signs rich's and bob's keys 2 Rich Kay (Demo use of ttp key) This key is due to expire on 2008-02-15. Are you sure that you want to sign this key with your key "Dave Trusted (TTP keysigning key) " (57E0F876)‏ Really sign? (y/N) y You need a passphrase to unlock the secret key for user: "Dave Trusted (TTP keysigning key) " 1024-bit DSA key, ID 57E0F876, created 2007-02-16

21. Using a TTP as a certificati on authority Dave exports the signed keys dave@saturn:~$ gpg -a --export rich > /tmp/richspub dave@saturn:~$ gpg -a --export bob > /tmp/bobspub

22. Using a TTP as a certificati on authority rich and bob import them rich@saturn:~$ gpg --import /tmp/richspub gpg: key D224BF4D: "Rich Kay (Demo use of ttp key) " 1 new signature gpg: Total number processed: 1 gpg: new signatures: 1 gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u gpg: next trustdb check due at 2008-02-15 rich@saturn:~$ gpg --import /tmp/bobspub gpg: key 12D2BFBA: "Bob Person (bob TTP process) " 1 new signature gpg: Total number processed: 1 gpg: new signatures: 1 gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u gpg: next trustdb check due at 2008-02-15

23. Using a TTP as a certificati on authority bob states he trusts dave to sign other keys 1 Note that bob has to sign dave's key as well as trust it. Signing it means that bob believes dave's key belongs to dave. Trusting it means that bob trusts dave to identify the owners of other keys before signing them. bob@saturn:~$ gpg --edit-key dave gpg (GnuPG) 1.4.3; Copyright (C) 2006 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. pub 1024D/57E0F876 created: 2007-02-16 expires: 2008-02-15 usage: SC trust: unknown validity: unknown sub 2048g/A6BFD1FD created: 2007-02-16 expires: 2008-02-15 usage: E [ unknown] (1). Dave Trusted (TTP keysigning key)

24. Using a TTP as a certificati on authority bob states he trusts dave to sign other keys 2 Command> help sign quit quit this menu save save and quit help show this help fpr show key fingerprint list list key and user IDs uid select user ID N key select subkey N check check signatures sign sign selected user IDs [* see below for related commands] lsign sign selected user IDs locally tsign sign selected user IDs with a trust signature nrsign sign selected user IDs with a non-revocable signature deluid delete selected user IDs delkey delete selected subkeys delsig delete signatures from the selected user IDs pref list preferences (expert)‏ showpref list preferences (verbose)‏ trust change the ownertrust revsig revoke signatures on the selected user IDs enable enable key disable disable key showphoto show selected photo IDs

25. Using a TTP as a certificati on authority bob states he trusts dave to sign other keys 3 Command> trust pub 1024D/57E0F876 created: 2007-02-16 expires: 2008-02-15 usage: SC trust: unknown validity: unknown sub 2048g/A6BFD1FD created: 2007-02-16 expires: 2008-02-15 usage: E [ unknown] (1). Dave Trusted (TTP keysigning key) Please decide how far you trust this user to correctly verify other users' keys (by looking at passports, checking fingerprints from different sources, etc.)‏ 1 = I don't know or won't say 2 = I do NOT trust 3 = I trust marginally 4 = I trust fully 5 = I trust ultimately m = back to the main menu Your decision? 4

26. Using a TTP as a certificati on authority bob signs dave's key to say it belongs to dave Command> sign pub 1024D/57E0F876 created: 2007-02-16 expires: 2008-02-15 usage: SC trust: full validity: unknown Primary key fingerprint: ACF5 7915 4C5E 6F1D 26E0 8662 6637 B994 57E0 Dave Trusted (TTP keysigning key) This key is due to expire on 2008-02-15. Are you sure that you want to sign this key with your key "Bob Person (bob TTP process) " (12D2BFBA)‏ Really sign? (y/N) y You need a passphrase to unlock the secret key for user: "Bob Person (bob TTP process) " 1024-bit DSA key, ID 12D2BFBA, created 2007-02-16 Command> quit Save changes? (y/N) y Rich does the same with dave's key

27. Using a TTP as a certificati on authority rich encrypts a message to bob's key and signs it rich@saturn:~/gpg$ cat secret This is a secret message sent by rich to bob, after both rich and bob have trusted dave to sign each others keys. rich@saturn:~/gpg$ gpg -r bob -o secret.asc -sea secret You need a passphrase to unlock the secret key for user: "Rich Kay (Demo use of ttp key) " 1024-bit DSA key, ID D224BF4D, created 2007-02-16 gpg: checking the trustdb gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model gpg: depth: 0 valid: 1 signed: 1 trust: 0-, 0q, 0n, 0m, 0f, 1u gpg: depth: 1 valid: 1 signed: 1 trust: 0-, 0q, 0n, 0m, 1f, 0u gpg: depth: 2 valid: 1 signed: 0 trust: 1-, 0q, 0n, 0m, 0f, 0u gpg: next trustdb check due at 2008-02-15 rich@saturn:~/gpg$ cat secret.asc -----BEGIN PGP MESSAGE----- encrypted message not shown

28. Using a TTP as a certificati on authority Rich sends and bob decrypts the message. bob@saturn:~$ gpg -o secret -d secret.asc You need a passphrase to unlock the secret key for user: "bob Person (bob TTP process) " 2048-bit ELG-E key, ID 10C076AE, created 2007-02-16 (main key ID 12D2BFBA)‏ gpg: encrypted with 2048-bit ELG-E key, ID 10C076AE, created 2007-02- 16 "Bob Person (bob TTP process) " gpg: Signature made Sat 17 Feb 2007 16:04:24 GMT using DSA key ID D224BF4D gpg: checking the trustdb gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model trust data cut for sake of brevity gpg: Good signature from "Rich Kay (Demo use of ttp key) " bob@saturn:~$ cat secret This is a secret message sent by rich to bob, after both rich and bob have trusted dave to sign each others keys.

29. Using GPG for Symmetric Cryptography The following slides demonstrate use of GPG with a single shared secret. A passphrase is used to generate a session key which is used to encrypt the data. The decryption process involves using the passphrase to regenerate the session key which is then used to decrypt the data.

30. Using GPG for symmetri c cryptogra phy Encryptin g a message rich@saturn:~/gpg$ gpg -o secret.asc -ca secret rich@saturn:~/gpg$ cat secret.asc -----BEGIN PGP MESSAGE----- Version: GnuPG v1.4.3 (GNU/Linux)‏ jA0EAwMC5xVjg4/8UtRgyTDYJAmJer3Q5bJ/SIHrs5e MNa2TpxQ5cuwyXmMay+L/8CPJ2IOQOoHnCOdHQO 7APi8MEvq -----END PGP MESSAGE----- Here the c option involves use of the default symmetric encryption algorithm CAST5, the a option involves ASCII armouring the output. Any passphrase can be input, but the same will be needed to decrypt the file.

31. Using GPG for symmetri c cryptogra phy Decryptin g the message rich@saturn:~/gpg$ gpg -d secret.asc gpg: CAST5 encrypted data gpg: encrypted with 1 passphrase This is a secret message. gpg: WARNING: message was not integrity protected The message was successfully decrypted. The warning message was investigated. The reasons for this were answered here: http://lists.gnupg.org/pipermail/gnupg-users/2004- October/023500.html and here: http://lwn.net/Articles/7688/ It turned out that in order to obtain backwards compatibility with older versions of PGP and GPG that the CAST5 algorithm is used by default. GPG will always use a MDC (Manipulation Detection Code) with newer algorithms.

32. Using GPG for symmetri c cryptogra phy Avoiding Message Manipulat ion rich@saturn:~/gpg$ gpg --version gpg (GnuPG) 1.4.3 Home: ~/.gnupg Supported algorithms: Pubkey: RSA, RSA-E, RSA-S, ELG-E, DSA Cypher: 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH Hash: MD5, SHA1, RIPEMD160, SHA256, SHA384, SHA512 Compression: Uncompressed, ZIP, ZLIB, BZIP2 rich@saturn:~/gpg$ gpg --cipher-algo AES256 -o secret.asc -ca secret File `secret.asc' exists. Overwrite? (y/N) y rich@saturn:~/gpg$ gpg -d secret.asc gpg: AES256 encrypted data gpg: encrypted with 1 passphrase This is a secret message.

33. Security features of GPG Passphrases are used whenever a security-sensitive event occurs. A copy of the secret key would be difficult or impossible to unlock without knowledge of the passphrase. These events include encryption, decryption and confirming the authenticity of an imported key. GPG has many other design features to improve the security of the processing, e.g. forcing memory used to not be written out to extended memory (swap file or partition). A practical system design involving messages sent between automated systems is likely to have to involve compromising this security to an extent, because all secrets needed to secure communications will need to be stored locally on the relevant systems.

34. GPG or PGP ? Slide 1 PGP stands for Pretty Good Privacy, which is a program designed by Phil Zimmerman and which became available in 1991. At this time cryptographic software was controlled under the same US export restrictions as munitions. By posting this program on the Internet its author was suspected of illegally exporting it and was investigated. However, Phil was never charged, probably due to the degree of support his cause attracted. Eventually the investigation against him was dropped. PGP later became the basis of the RFC 4880 OpenPGP Message Format standard (this updated RFC2440).

35. GPG or PGP ? Slide 2 GPG stands for "GNU Privacy Guard". It was engineered based on RFC2440 (and later RFC4880) in order to be interoperable with PGP. PGP was distributed in source-available form without requiring payment. But PGP was not released based on a software license considered by the Free Software Foundation as constituting free software. In practical terms the licensing restrictions on PGP made it difficult freely and internationally to distribute and maintain it as part of larger packages, e.g. operating system distributions. GPG was developed as a fully open-source product with financial support from the German government.

36. Conclusions Use of trusted third parties (TTPs) in connection with cryptography takes some preparation and planning. Many separate actions have to occur in the correct sequence. All parties need to learn use of the tools and procedures and the rationale for these, before these tools can be used effectively and securely. Some developments involving keys stored in hardware devices and standardisation of automatable PKI protocols are likely to be required before the kind of procedures demonstrated above are likely to be usable automatically and simply without requiring advanced knowledge on the part of the end user.

37. Further reading GNU Privacy Guard Wikipedia Entry http://en.wikipedia.org/wiki/GNU_Privacy_Guard The GNU Privacy Guard Handbook http://www.gnupg.org/gph/en/manual.html Essay by Phil Zimmerman: "Why I wrote PGP" http://www.philzimmermann.com/EN/essays/WhyIWrotePGP. html PGP User Guide, including a good general introduction to cryptography http://www.pgpi.org/doc/guide/7.0/en/