- Block ciphers only work on fixed size blocks.
- If the message isn’t of the right block size we need to pad the message.
- But the receiver needs to tell the difference between the padding and message.
PKCS 5/7
- If there is 1 byte of space write 01
- If there are 2 bytes write 0202.
PKCS 7 : 16 byte block.
PKCS 5: 8 byte block.