I am newbie to the crypto world, but my question is, If block cipher BLOCK satisfies the following relation (where ENC is encrypt and "+" means XOR): For ever pair of blocks of input A and B, ENC(A+B) = ENC(A) + ENC(B).
I was wondering what would be the security of BLOCK? Assuming the key is of fixed size and that you have unfetterd access to the ENC function. In other words, how hard is it to decrypt a given ciphertext without knowing the details of BLOCK.