Problems for PGP5 users to use my GPG-key.

[J. Michael Ashley]

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On 3 Nov 1999, Jonas Steverud wrote:
> A friend of mine uses PGP 5.0 and we want to encrypt our emails. He can
> decrypt my mails to him but when he tries to encrypt with my GPG 1.0.0
> key he gets "No algorithm available that all keys support.".

When your friend encrypts, a symmetric encryption algorithm must be found
that all the public keys to which he is encrypting is willing to support.
The algorithms a key is willing to support is encoded in the key.

For example,

  [ct39416-a] DING! ~/gph> gpg --edit-key jashley@acm.org

  pub  1024D/8904C8E2  created: 1999-03-03 expires: 2003-03-14 trust: -/u
  sub  1024g/2840C21F  created: 1999-03-03 expires: 2000-03-14
  (1)  J. Michael Ashley <jashley@acm.org>

  Command> pref
  pub  1024D/8904C8E2  created: 1999-03-03 expires: 2003-03-14 trust: -/u
  (1)  J. Michael Ashley <jashley@acm.org>
       S10 S3 H3 H2 Z2 Z1

  Command> 

In this case I'm willing to support twofish and cast5.  (The H and Z are
for hash and compression algorithms, respectively.)  Unfortunately, the
command-line interface does not list the algorithms by name, but I found
the encryption algorithms in include/cipher.h in the source.

  #define CIPHER_ALGO_NONE         0
  #define CIPHER_ALGO_IDEA         1
  #define CIPHER_ALGO_3DES         2
  #define CIPHER_ALGO_CAST5        3
  #define CIPHER_ALGO_BLOWFISH     4  /* blowfish 128 bit key */
  #define CIPHER_ALGO_SAFER_SK128  5
  #define CIPHER_ALGO_DES_SK       6
  #define CIPHER_ALGO_TWOFISH     10  /* twofish 256 bit */
  #define CIPHER_ALGO_SKIPJACK   101  /* experimental: skipjack */
  #define CIPHER_ALGO_TWOFISH_OLD 102 /* experimental: twofish 128 bit */
  #define CIPHER_ALGO_DUMMY      110  /* no encryption at all */

The problem could be that your friend is encrypting the message to
himself as well, and his key does not have cast5 or twofish in his
preferences.

I'm not sure how to change one's algorithm preferences in GnuPG.  

As ana aside, I'm a bit surprised that 3DES is not in a GnuPG-generated
key's preferences by default.

I've put on the todo list to add a section about this to the manual.

Mike

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