Padding oracle attack

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In cryptography, a padding oracle attack is an attack which uses the padding validation of a cryptographic message to decrypt the ciphertext. In cryptography, variable-length plaintext messages often have to be padded (expanded) to be compatible with the underlying cryptographic primitive. The attack relies on having a "padding oracle" which freely responds to queries about whether a message is correctly padded or not. The information could be directly given, or leaked through a side-channel. The earliest well-known attack that uses a padding oracle is Bleichenbacher's attack of 1998, which attacks RSA with PKCS #1 v1.5 padding. The term "padding oracle" appeared in literature in 2002, after Serge Vaudenay's attack on the CBC mode decryption used within symmetric block ciphers. Variants of both attacks continue to find success more than one decade after their original publication.