Comprendre Padding Schemes

问题

Your bloc chiffre requires le entrée en être un multiple de le bloc taille, mais your données est non, donc you besoin en pad it correctly.

Prérequis

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

Étapes

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.

   entrée

   13 bytes of data

   输出

   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.

   entrée

   PKCS#7 on 'test' (4 bytes)

   输出

   '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.

   entrée

   'test' + 0x0C × 12

   输出

   'test' (4 bytes)

Résultat attendu

Correctly padded plaintext ce encrypts en un valide bloc-aligned ciphertext et unambiguously unpads après déchiffrement.

Erreurs courantes

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.

Dépannage指引

• 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.

Limites de sécurité

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