Encryption Bit Size

[Amakuza]

Hello,

I was wondering how you determine the bit size of an encryption algorithm. IE: Blowfish is 128 bit, how is that figure determined? Thanks.

- Ama

[Chris Coble]

It's determined by the algorithm and what it supports. Blowfish actually supports a 448 bit key, I use that length in my program. At the moment, anything over 128 is pretty much overkill though. Typically, if applications need to chop down the bitlength of the security (for international applications assuming you live in the US), they just pad the rightmost bits with 0's. So, if you had a blowfish algorithm that took a 128 bit key, and you wanted to make it legal for international shipment, you would force the key to 56 bits (7 bytes) and 0 out the remaining 9 bytes of your key input. Now, you don't have to do that with BlowFish because that particular cipher can take any lengh key. But other algorithms may only accept certain length keys... (ie. DES)

There are a number of encryption algorithms out now competing for the Advanced Encryption Standard, it's required that they run at 128 bits.

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Chris Coble
chrisc@novastor.com
NovaStor Corporation