# Twofish key length

I'm using twofish. Say you have a 120 bit key, is there any difference in using a 128 bit bit keylength or a 256 bit one?

Since the most significant digits of the key will be allo zero in both cases, does the keylen parameter really matter?

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From a security standpoint, the goal (which, as far as we know, Twofish achieves) is that if you know all but N bits of the key, it still takes about $2^N$ trial decrypts to recover the remaining bits. So, it doesn't matter if you have a 128 bit Twofish key (and give the attacker 8), or if you have a 256 bit Twofish key (and give the attacker 136); he still needs to iterate through the remaining 120 bits.