While this timing difference may be more significant in some languages or libraries than others, it's now believed that this is a practical threat for all languages and libraries when the application's response to failure is taken into account. Modern computer networks are of such high quality that an attacker can detect very small (less than 0.1 ms) differences in execution time on remote systems. Applications that are assuming that a successful decryption can only happen when the data wasn't tampered with may be vulnerable to attack from tools that are designed to observe differences in successful and unsuccessful decryption.
From this response, the attacker can decrypt the message byte by byte.
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CBC introduces an initial random block, known as the Initialization Vector (IV), and combines the previous block with the result of static encryption to make it such that encrypting the same message with the same key doesn't always produce the same encrypted output.Īn attacker can use a padding oracle, in combination with how CBC data is structured, to send slightly changed messages to the code that exposes the oracle, and keep sending data until the oracle tells them the data is correct. One of the most commonly used modes is CBC. The oracle could be something as simple as returning a value that says "Invalid padding" or something more complicated like taking a measurably different time to process a valid block as opposed to an invalid block.īlock-based ciphers have another property, called the mode, which determines the relationship of data in the first block to the data in the second block, and so on.
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Putting the two things together, a software implementation with a padding oracle reveals whether decrypted data has valid padding.
This allows the padding to always be safely removed upon decryption. Many forms of padding require that padding to always be present, even if the original input was of the right size. If the data you want to encrypt isn't the right size to fill the blocks, your data is padded until it does. Some ciphers, which are the algorithms used to encrypt your data, work on blocks of data where each block is a fixed size. Padding is a specific cryptographic term. You, as the opponent, can use this oracle to plan your next move appropriately. When their face lights up with a big smile because they think they're about to make a good move, that's an oracle. Imagine playing a board or card game with a child. IntroductionĪ padding oracle attack is a type of attack against encrypted data that allows the attacker to decrypt the contents of the data, without knowing the key.Īn oracle refers to a "tell" which gives an attacker information about whether the action they're executing is correct or not. This judgement is based on currently known cryptographic research. Microsoft believes that it's no longer safe to decrypt data encrypted with the Cipher-Block-Chaining (CBC) mode of symmetric encryption when verifiable padding has been applied without first ensuring the integrity of the ciphertext, except for very specific circumstances.
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