Comprender Padding Schemes

问题

Your bloque cifrador requires el entrada a ser un multiple de el bloque tamaño, pero your datos es no, así que you necesitar a pad it correctly.

Requisitos previos

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

Pasos

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.

   entrada

   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.

   entrada

   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.

   entrada

   'test' + 0x0C × 12

   输出

   'test' (4 bytes)

Resultado esperado

Correctly padded plaintext ese encrypts a un válido bloque-aligned ciphertext y unambiguously unpads después descifrado.

Errores comunes

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.

Solución de problemas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.

Límites de seguridad

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