c09.py

def pad(data, size):
    pad_size = size - len(data) % size
    return data + bytes([pad_size] * pad_size)


def main():
    want = b'YELLOW SUBMARINE\x04\x04\x04\x04'
    got = pad(b'YELLOW SUBMARINE', 20)

    assert(want == got)


if __name__ == '__main__':
    main()

'''
Implement PKCS#7 padding
A block cipher transforms a fixed-sized block (usually 8 or 16 bytes) of plaintext into ciphertext. 
But we almost never want to transform a single block; we encrypt irregularly-sized messages.

One way we account for irregularly-sized messages is by padding, creating a plaintext that 
is an even multiple of the blocksize. The most popular padding scheme is called PKCS#7.

So: pad any block to a specific block length, by appending the number of bytes 
of padding to the end of the block. For instance,

"YELLOW SUBMARINE"
... padded to 20 bytes would be:

"YELLOW SUBMARINE\x04\x04\x04\x04"
'''