Block Cipher Padding

When you encrypt using a modern block cipher (e.g. AES) the last plaintext block is often not exactly of block size bytes. To still allow the block cipher to encrypt it, we have to apply “padding”.

Padding adds data to the end of a message prior to the encryption. There is bit padding (which adds bits) and byte padding (which works on complete bytes). Here, we focus on byte padding.

Some examples for (byte) padding (modes) also implemented in CrypTool 2 are:
– “None” –> no padding at all
– 0-Padding –> adds zeros to the end of the block
– 1-0-Padding –> adds a one and then zeros to the end of the block
– ANSI X9.23 Padding –> adds zeros and the last byte is the number of padded bytes
– ISO 10126 Padding –> adds random bytes and last byte is the number of padded bytes
– PKCS#7 Padding –> adds the value n of padded bytes n-times to the end of block

In the video below of my “Cryptography for everybody” YouTube channel, I discuss what padding is, show all mentioned different padding modes, and also analyze these using CrypTool 2.

My Video about Block Cipher Padding

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