On Aug 6, 2013, at 9:22 AM, Kenneth Jones <kenten...@me.com> wrote:

> I'm referring to the information you see for example in the prompt to
> enter your private key when you have received an encrypted message in
> Thunderbird/Enigmail. The window "pinetry" prompts "Please enter the
> pass...2048-bit RSA key, ID DEADBEEF, created ... (main key ID
> ABCD0123)." Notice there are two key ID mentioned in the window, one
> called Main, which is also the public Key ID, (the one I expected, the
> one I remember) and the other for the secret key (which I have Never
> Paid any attention to).

Ah, that clarifies it.  Yes, as a few people have suggested, that's the subkey 
ID.  It's not inherently public or secret, but just another key attached to 
your primary key.  In OpenPGP, "your key" refers to a primary key, plus some 
number of subkeys (occasionally zero, but that's fairly rare).  The primary key 
is the one that the user IDs (email addresses, etc) are attached to, and the 
one that gathers signatures from other people if you get your key signed.

The subkey(s) are keys attached to the primary key, that can be used for 
encryption or signing.  The idea is that since it is difficult to change your 
primary key (you'd need to get it re-signed, and re-print your business cards, 
and the like) you should be able to change the subkey quickly and easily.  A 
common methodology (and in fact the default for many programs) is to use the 
primary key for signing, and a subkey for encryption.  There are interesting 
variations that can be used with this basic design: some people leave their 
primary key offline completely, only taking it out to make new subkeys.  Some 
people use different passphrases on different subkeys.

To answer your original question, though, traditionally the key-as-a-whole is 
referred to by its primary key ID and fingerprint.  The subkeys are effectively 
along for the ride. Some programs make a point of telling you which subkey is 
in use at a particular time.  Some do not.

David


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