I have designed an encryption algorithm. The algorithm will probably use CFB mode. The algorithm can encrypt any block size and this does not change the size of the key. There are always about $20$ rounds and the key is always around $128$-bits.
However, the algorithm works much slower when it works on $128$-bits blocks, than on about $32$-bits blocks. What are the block size limitations for a secure cipher? If the key is $128$-bits, can it be encrypted in about $32$-bit blocks? Is it secure?
The only condition I found is that $n!$ should be greater than $2^{128}$, where $n$ is the block size.
