decryption failed: weak key

[Hasnain Mujtaba]

Hi,

I saw this message when decrypting a file encrypted with a 1536 ElGamal
key, 3DES symmetric cipher:

gpg: WARNING: message was encrypted with a weak key in the symmetric
cipher.
gpg: decryption failed: weak key.

Looks like GPG is checking for randomness property of the session key
after decrypting it out of the Public-key Encrypted Session key packet.
Correct? And does the RFC2440 specify how this check should be
performed, i.e is it a MUST have feature?  

Thanks,

Hasnain.