Understanding Padding Schemes

Problem

Your block cipher requires the input to be a multiple of the block size, but your data is not, so you need to pad it correctly.

Prerequisites

• Understanding of block ciphers
• Knowledge of block size (16 bytes for AES)

Steps

1. 1. Understand why padding is needed

   Block ciphers like AES process data in fixed-size blocks (16 bytes). If the plaintext is not a multiple of the block size, the last block is incomplete. Padding adds bytes to fill the block and must be removable without ambiguity.

   Input

   13 bytes of data

   Output

   pad to 16 bytes

2. 2. Choose a padding scheme

   PKCS#7 (the standard for AES) appends N bytes each with value N, where N is the number of padding bytes. If the data is already block-aligned, a full block of padding is added. Zero padding appends zeros but is ambiguous if the data ends with zeros. ISO 10126 uses random bytes with the last byte indicating the count.

   Input

   PKCS#7 on 'test' (4 bytes)

   Output

   'test' + 0x0C × 12

3. 3. Remove padding after decryption

   After decryption, strip the padding to recover the original. For PKCS#7, read the last byte's value N and remove N bytes, verifying they all equal N. If verification fails, the padding is invalid, indicating a wrong key or tampering.

   Input

   'test' + 0x0C × 12

   Output

   'test' (4 bytes)

Expected result

Correctly padded plaintext that encrypts to a valid block-aligned ciphertext and unambiguously unpads after decryption.

Common errors

Using zero padding on data that ends with zeros

Zero padding is ambiguous. Use PKCS#7, which always adds a padding block and is unambiguous.

Not adding padding when data is block-aligned

PKCS#7 always adds at least one byte. Without this, you cannot distinguish padded from unpadded data.

Troubleshooting

• If unpadding fails with 'invalid padding', the key is likely wrong or the ciphertext was corrupted.
• Use GCM or CTR mode to avoid padding entirely if possible.

Security limits

• Padding oracle attacks exploit padding validation errors. Use authenticated encryption (GCM) to prevent them.