How are the GPG usage flags defined in the key details listing?

Question

When I list the details of key I get output like this:

$ gpg --edit-key SOMEID
pub [..] created: [..] expires: [..]   usage:SC
[..]
sub [..] created: [..] expires: [..]   usage: E

Or even usage: SCA on another key (the master-key part).

What does these abbreviation in the usage field mean?

I can derive that:

S -> for signing
E -> for encrypting

But what about C and A?

And are there more?

And where to look stuff like this up?

Answer 1

Ok, the gpg manual does not seem to mention these abbreviations. Thus, one has to look at the source.

For example under Debian/Ubuntu:

$ apt-get source gnupg2
$ cd gnupg2-2.0.17
$ cscope -bR
$ grep 'usage: %' . -r --exclude '*po*'
$ vim g10/keyedit.c
jump to usage: %
jump to definition of `usagestr_from_pk`

From the code one can derive following table:

━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━
Constant              Character
───────────────────────────────
PUBKEY_USAGE_SIG      S
PUBKEY_USAGE_CERT     C
PUBKEY_USAGE_ENC      E
PUBKEY_USAGE_AUTH     A
━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━

Thus, for example, usage: SCA means that the sub-key can be used for signing, for creating a certificate and authentication purposes.