Cryptanalysis and Attacks on Cryptosystems
Cryptanalysis is the art of deciphering encrypted communications without knowing the proper keys. Some of the more important cryptanalytic techniques are:
Cipher text only attack: This is the situation where the attacker does not know anything about the contents of the message, and must work from cipher text only. In practice it is quite often possible to make guesses about the plain text contents of messages, as many types of messages have fixed-format headers. Even ordinary letters and documents begin in a very predictable way. It may also be possible to guess that some cipher text block contains a common word.
Known plain text attack: The attacker knows or can guess the plain text for some parts of the message. The task is to decrypt the rest of the cipher text blocks using this information. This may be done by determining the key used to encrypt the data, or via some shortcut.
Chosen plain text attack: The attacker is able to have any text he likes encrypted with the unknown key. The task is to determine the key used for encryption. Some encryption methods, particularly RSA, are extremely vulnerable to chosen-plain text attacks. When such algorithms are used, extreme care must be taken to design the entire system so that an attacker can never have chosen plain text encrypted.
Man-in-the-middle attack: This attack is relevant for cryptographic communication and key exchange protocols. When two parties are exchanging keys for secure communications (e.g., using Diffie-Hellman), an adversary puts himself between the parties on the communication line. The adversary then performs a separate key exchange with each party, decrypts communications, and encrypts them again for sending to the other party. The parties think that they are communicating securely, but in fact the adversary is hearing everything.
Man-in-the-middle attacks can be averted if each party computes a cryptographic hash function of the key exchange (or at least the encryption keys), signs it using a digital signature algorithm, and sends the signature to the other. The recipient then verifies that the signature came from the other party, and that the hash in the signature matches the one computed locally.
Timing Attack: This attack is based on repeatedly measuring the exact execution times of modular exponentiation operations. It is relevant to at least RSA, Diffie-Hellman, and Elliptic Curve methods.