paddingattacker

Padding Oracle Attack is a type of cryptanalytic attack that targets block ciphers in specific encryption modes, most notably cipher block chaining (CBC) mode, when they are used with padding. The attack exploits how decryption errors related to padding are handled by the system.

Block cipher encryption operates on fixed-size blocks of data. If the plaintext message is not a multiple of the block size, padding must be added to the last block to fill it out. A padding oracle is a server or system that, when presented with a ciphertext, will reveal whether the padding of the decrypted plaintext is valid or invalid. This seemingly small piece of information, the validity of the padding, is enough for the attacker to deduce information about the original plaintext.

The attacker typically works by intercepting a ciphertext. They then systematically modify parts of the ciphertext and send it to the padding oracle. By observing whether the oracle returns a padding error or not, the attacker can iteratively determine the bytes of the plaintext, block by block. This is achieved by manipulating the last byte of a block and observing how it affects the padding validation. Once enough information is gathered, the attacker can reconstruct the original plaintext without knowing the encryption key. Padding oracle attacks are particularly effective against older or improperly implemented cryptographic systems.