I'm looking into disabling certain ciphers and I'm trying to wrap my head around how the block ciphers are named in Windows. Specifically what the numbers follow the name mean. For example, RC4 40/128, DES 56/56, AES 128/128 or AES 256/256. I thought maybe this was key size/block size but that doesn't make sense. Take AES 256/256. It is my understanding that for it to be AES it needs a block size of 128. Thus the last number can't specify block size. Or take the RC4 ciphers. It would appear that the first number is the key size but it's a stream cipher, so what is the 128? OR DES 56/56. I thought DES had a key size of 56 bits and a block size of 64 bits? Does anyone know the history or the reasoning behind the naming?