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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. [1] The term "padding oracle" appeared in literature in 2002, [2] after Serge Vaudenay's attack on the CBC mode decryption used within symmetric block ciphers. [3]
For CBC ciphertext stealing, there is a clever (but opaque) method of implementing the described ciphertext stealing process using a standard CBC interface. Using this method imposes a performance penalty in the decryption stage of one extra block decryption operation over what would be necessary using a dedicated implementation.
The attacker can then combine the oracle with a systematic search of the problem space to complete their attack. [1] The padding oracle attack, and compression oracle attacks such as BREACH, are examples of oracle attacks, as was the practice of "crib-dragging" in the cryptanalysis of the Enigma machine. An oracle need not be 100% accurate ...
It makes some of the plaintext structure visible in the ciphertext. Selecting other modes, such as using a sequential counter over the block prior to encryption (i.e., CTR mode) and removing it after decryption avoids this problem. Another mode, Cipher Block Chaining (CBC) is one of the most commonly used modes of AES due to its use in TLS. CBC ...
Also like CBC, decryption can be parallelized. CFB, OFB and CTR share two advantages over CBC mode: the block cipher is only ever used in the encrypting direction, and the message does not need to be padded to a multiple of the cipher block size (though ciphertext stealing can also be used for CBC mode to make padding unnecessary).
The oracle returns the bitwise exclusive-or of the key with the string of zeroes. The string returned by the oracle is the secret key. While the one-time pad is used as an example of an information-theoretically secure cryptosystem, this security only holds under security definitions weaker than CPA security.
In the non-adaptive case (IND-CCA1), the adversary may not make further calls to the decryption oracle. In the adaptive case (IND-CCA2), the adversary may make further calls to the decryption oracle, but may not submit the challenge ciphertext C. Finally, the adversary outputs a guess for the value of b.
The term "lunchtime attack" refers to the idea that a user's computer, with the ability to decrypt, is available to an attacker while the user is out to lunch. This form of the attack was the first one commonly discussed: obviously, if the attacker has the ability to make adaptive chosen ciphertext queries, no encrypted message would be safe ...