Padding Schemes verstehen

问题

Your Block Chiffre requires der Eingabe in sein ein multiple von der Block Größe, aber your Daten ist nicht, also you brauchen in pad it correctly.

Voraussetzungen

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

Schritte

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.

   Eingabe

   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.

   Eingabe

   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.

   Eingabe

   'test' + 0x0C × 12

   输出

   'test' (4 bytes)

Erwartetes Ergebnis

Correctly padded plaintext das encrypts in ein gültig Block-aligned ciphertext und unambiguously unpads nach Entschlüsselung.

Häufige Fehler

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.

Sicherheitsgrenzen

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