In this section, we will see how padding works in the PKCS # 7 system and then show you a system with the PADDING ERROR message. Plus, we'll also deal with the padding oracle attack, which makes it possible to craft ciphertext that will decode 20 plaintext we want.
Here is the encryption routine:
We'll have three blocks of data, each 16-bytes long. We'll encrypt the data with AES in CBC mode, so in comes the initialization vector and the key. You produce three blocks of ciphertext, and each one of the blocks after the first uses the output of the previous encryption routine as an initialization vector to XOR with the plaintext.
Here's how PKCS#7 padding works:
- If one byte of padding is needed, use
01 - If two bytes of padding are needed, use
0202 - If three bytes of padding are needed, use
030303 - And so on...
If we have a message here that is only 47-bytes long, then we can't fill the last block, so we have to add a byte of padding. You could use a variety of numbers as the padding...