[Pidgin] #4381: Add HMAC digest to libpurple

[Pidgin]

#4381: Add HMAC digest to libpurple
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 Reporter:  QuLogic       |       Type:  patch
   Status:  new           |   Priority:  minor
Component:  libpurple     |    Version:  2.3.1
 Keywords:  HMAC, digest  |    Pending:  0    
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 So khc wanted to take a look at the MSNP15 stuff, so I'm posting these
 tickets.

 This patch adds the HMAC digest to libpurple. A few points:
  * The underlying hash function can be set via
 `purple_cipher_context_set_option(cipher, "hash", hash)`. The blocksize
 isn't really available, so I just assumed a size of 64 bytes. Fortunately,
 the supplied hashes all have a blocksize of 64 bytes, but this will need
 to be fixed if a different hash is added.
  * The key can be set with `purple_cipher_context_set_key`. Because of the
 way it's implemented, this acts almost like a reset (but without changing
 the hash).
  * Since the key can be of any length, perhaps with NULLs, and `_set_key`
 doesn't accept a length, I've had to add
 `purple_cipher_context_set_option(cipher, "keyLen", key_len)` so that
 things would work properly.
  * Finally, I've added some tests with both MD5 and SHA1 as underlying
 hashes from RFC2202.

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Ticket URL: <http://developer.pidgin.im/ticket/4381>
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