key capabilities usage meanings

[archimedes at infinito.it]

What is the meaning of usage/capabilities listings for 
keys(shown, for
example, during edit-keys interactive sessions)?
S -> sign
E -> encrypt
C -> ?
A -> ?
looking at doc/DETAILS I found
C -> certification
A -> authentication

But I dont' understand the difference between 
certification,
authentication and signing.
I have different keys, each for a different internet 
"personality",
and I noticed that one primary key is listed as CSA and 
another
CS. The two keys were generated with the same options (DSA 
for signing
+ElGamal subkey for pubkey encryption), so why this 
difference?

Another question: I read in manpage that MDC is enabled by 
default
with newer ciphers(blocksize>64bit) and with CAST5. So why 
when you
decipher a symmetrically encrypted message you get 
"WARNING: message
was not integrity protected" and only with --force-mdc the 
warning
goes away?

-- 
Archimedes
OpenPGP public key available trough keyservers, ID: 
0x58D14EB3
Key fingerprint: 00B9 3E17 630F F2A7 FF96 DA6B AEE0 EC27 
58D1 4EB3 Always check key fingerprints!
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