brian m. carlson sandals at
Sun Apr 29 02:28:28 CEST 2012

On Sat, Apr 28, 2012 at 10:21:52PM +0100, michael crane wrote:
> what is the reasoning for attaching the key ID to the end of the
> fingerprint string ?

That's the way the key ID is derived for v4 keys.  v4 keys use the low
64 bits (or 32 bits for short key IDs) as the key ID.  v3 keys used the
low 64 bits (respectively 32 bits) of the RSA modulus.  However, this
posed two problems.  One is that the low bit is always one (multiplying
two large primes together does that).  The other is that originally v4
keys were all DSA or Elgamal.  Those algorithms don't have a modulus in
the same way[0], so a different technique had to be used to derive a
unique fingerprint.

[0] Basically, the one (for Elgamal) or two (for DSA) primes that are
use as moduli can be shared securely among many keys, so using them as
the sole basis for a key ID means arbitrarily many keys can have the
same key ID, which kinda defeats the purpose.

brian m. carlson / brian with sandals: Houston, Texas, US
+1 832 623 2791 | | My opinion only
OpenPGP: RSA v4 4096b: 88AC E9B2 9196 305B A994 7552 F1BA 225C 0223 B187
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