# Using a round function intended for an SP network in a Feistel network?

If I use the round function from a secure SP network (such as AES) and use it in a Feistel netwok, is this a good starting point for the second cipher?

My thought is "yes" because:

• it already has good diffusion and confusion
• it doubles the block size, so the number of rounds can be doubled at no perf cost.
• Yes, most feistel functions are some kind of SP network. – LightBit May 22 '16 at 8:58
• I already did this, 1 round of AES is not enough, you need 2 for complete mixing of the input – Richie Frame May 22 '16 at 20:55
• @RichieFrame how many Feistel rounds did you use? What was the key size and schedule? Was the cipher published anywhere? – Demi May 22 '16 at 23:06

If you meant to use that $n$-bit secure SPN (such as Rijndael) in your Feistel $2n$-block-cipher, then you will have to make a sufficient number of rounds (at least 4 to get full diffusion : see p. 41). Therefore compared to a parallel CTR encryption method just using that $n$-SPN, you are 2 times slower and you don't really have a gain in security in term of key as the space is the same.