|crypto:decrypt cipher source dest &key start1 end1 start2||[Generic Function]|
Decrypt a block from the source vector into the dest vector using the cipher. The source is read from index start1, which defaults to zero, to end1, which defaults to the end of the vector. The decrypted block is written to the dest vector starting at index start2. The size of the block is expected to be a multiple of the cipher block size.