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.
Asymmetric cryptography In 1998, Daniel Bleichenbacher published a seminal paper on what became known as Bleichenbacher's attack (also known as "million message attack"). The attack uses a padding oracle against RSA with PKCS #1 v1.5 padding, but it does not include the term. Later authors have classified his attack as a padding oracle attack. Manger (2001) reports an attack on the replacement for PKCS #1 v1.5 padding, PKCS #1 v2.0 "OAEP".
Symmetric cryptography In symmetric cryptography, the padding oracle attack can be applied to the CBC mode of operation. Leaked data on padding validity can allow attackers to decrypt (and sometimes encrypt) messages through the oracle using the oracle's key, without knowing the encryption key. Compared to Bleichenbacher's attack on RSA with PKCS #1 v1.5, Vaudenay's attack on CBC is much more efficient. Both attacks target crypto systems commonly used for the time: CBC is the original mode used in Secure Sockets Layer (SSL) and had continued to be supported in TLS. A number of mitigations have been performed to prevent the decryption software from acting as an oracle, but newer attacks based on timing have repeatedly revived this oracle. TLS 1.2 introduces a number of authenticated encryption with additional data modes that do not rely on CBC.
Padding oracle attack on CBC encryption The standard implementation of CBC decryption in block ciphers is to decrypt all ciphertext blocks, validate the padding, remove the PKCS7 padding, and return the message's plaintext. If the server allows an attacker to hand over arbitrary data, the attacker can use the server as a padding oracle to decrypt (and sometimes encrypt) messages. The only way to prevent this attack is to make sure that the attacker cannot manipulate the data handed over to the algorithm, e.g. by only accepting messages which are secured with a digital signature or message authentication code which the attacker cannot forge.
The mathematical formula for CBC decryption is
P i = D K ( C i ) ⊕ C i − 1 , if i ∈ { 1 , N } {\displaystyle P_{i}=D_{K}(C_{i})\oplus C_{i-1},{\text{ if }}i\in \{1,N\}}
P 0 = I V ⊕ D K ( C 0 ) . {\displaystyle P_{0}=IV\oplus D_{K}(C_{0}).}
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