Size of subkey array in Blowfish

SykesTheLord

12/14/23, 11:23 PM

I'm writing a school paper about Blowfish encryption, and couldn't seem to find out if the size of the subkey array for Blowfish changes based on the length of the key. I would assume that it doesn't effect the size of the subkey array but I'm relatively new to Blowfish encryption.

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blowfish

key-schedule

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Crypto

Daniel S

12/15/23, 12:44 AM

In block ciphers, the size of the subkey array (sometimes referred to as round keys; in the case of Blowfish, sometimes called the $P$-array) can change according to two parameters: the block size (which determines the size of the elements of the array) and the number of rounds (which determines the number of elements of the array).

In the case of Blowfish, the block size is fixed at 64-bits for all key sizes and the number of rounds is also fixed at 16 for all key sizes. In other words, you are correct in your assumption that the size of the array does not change in the case of Blowfish.

This is not true of other block ciphers (e.g. AES where the number of rounds varies with key size).

Your report should also note that in Blowfish the key is also used to determine $S$-box entries (again, this is not the case for all block ciphers). The size of the $S$-box also does not change for different key sizes.

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