[crypto] Consider using larger shared key sizes

[zeroXten]

Current 128 bit random keys are used to match 128 bit security level provided by AES-256. Could increase the shared key size to 256 bits and maybe reduce the ID size from 128 bits to 64 bits.

[zeroXten]

See http://keccak.noekeon.org/on_128bit_security.html for related discussion.