Padding oracle attack
|This article is outdated. (February 2014)|
In cryptography, a padding oracle attack is a side channel attack which is performed on the padding of a cryptographic message. The plain text message often has to be padded (expanded) to be compatible with the underlying cryptographic primitive. Leakage of information about the padding may occur mainly during decryption of the ciphertext. Padding oracle attacks are mostly associated with ECB or CBC mode decryption used within block ciphers. Padding modes for asymmetric algorithms such as OAEP may also be vulnerable to padding oracle attacks.
In symmetric cryptography, the padding oracle attack is most commonly applied to the CBC mode of operation, where the "oracle" (usually a server) leaks data about whether the padding of an encrypted message is correct or not. This can allow attackers to decrypt (and sometimes encrypt) messages through the oracle using the oracle's key, without knowing the encryption key.
Attacks using padding oracles
The original attack was published in 2002 by Serge Vaudenay. In 2010 the attack was applied to several web application frameworks, including JavaServer Faces, Ruby on Rails and ASP.NET. In 2012 it was shown to be effective against some hardened security devices.
While these earlier attacks were fixed by most TLS implementors following its public announcement, a new variant, the Lucky Thirteen attack, published in 2013, used a timing side-channel to re-open the vulnerability even in implementations that had previously been fixed. As of February 2013[update], TLS implementors are still working on developing fixes for their TLS code.
To prevent this attack, one could append an HMAC (Hash-based message authentication code) to the ciphertext. Without the key used to generate the HMAC, an attacker won't be able to produce valid ciphertexts. Since the HMAC is checked before the decryption stage, the attacker cannot do the required bit-fiddling and hence cannot discover the plaintext.
- Manger, James. "A Chosen Ciphertext Attack on RSA Optimal Asymmetric Encryption Padding (OAEP) as Standardized in PKCS #1 v2.0". Telstra Research Laboratories.
- Serge Vaudenay (2002). "Security Flaws Induced by CBC Padding Applications to SSL, IPSEC, WTLS...". EUROCRYPT 2002.
- Juliano Rizzo, Thai Duong (2010-05-25). "Practical Padding Oracle Attacks". USENIX WOOT 2010.
- Thai Duong, Juliano Rizzo (2011). "Cryptography in the Web: The Case of Cryptographic Design Flaws in ASP.NET". IEEE Symposium on Security and Privacy 2011.
- Dennis Fisher (2010-09-13). "'Padding Oracle' Crypto Attack Affects Millions of ASP.NET Apps". Threat Post.
- Vlad Azarkhin (2010-09-19). "“Padding Oracle” ASP.NET Vulnerability Explanation".
- Romain Bardou, Riccardo Focardi, Yusuke Kawamoto, Lorenzo Simionato, Graham Steel, Joe-Kai Tsay (2012), "Efficient Padding Oracle Attacks on Cryptographic Hardware"
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