Ciphertext-based Attacks

Ciphertext-based Attacks

The ciphertext-only attack is one of the most difficult because the attacker has so little information with which to start.

All the attacker starts with is some unintelligible data — ciphertext — that the attacker suspects may be an important encrypted message. The attack becomes simpler when the attacker is able to gather several pieces of ciphertext and thereby look for patterns or trends or statistical data that would help in the attack. In many cases, frequency analysis can be helpful in ciphertext-only attacks. Frequency analysis is defined as knowing linguistic patterns of letters and word frequency in certain alphabets and languages.

Frequency Analysis

This attack works closely with several other types of attacks. It is especially useful when attacking a substitution cipher where the statistics of the plaintext language are known, for example in a ciphertext-only attack.

In the English language, for example, some letters will appear more often than others will, allowing an attacker to assume that those letters may represent an "e" or "s." The "e" is the most commonly used letter in the English alphabet. Another example is the most commonly used three-letter word in the English language, which is "the." Knowing language statistics may be very helpful in conducting certain cryptanalysis attacks.

Chosen Ciphertext

This is similar to the chosen plaintext attack in that the attacker has access to the decryption device or software and is attempting to defeat the cryptographic protection by decrypting chosen pieces of ciphertext to see what the corresponding plaintext is to discover the key.

An adaptive chosen ciphertext would be the same, except that the attacker can repeatedly modify the ciphertext prior to putting it through the algorithm.