In cryptography , a ciphertext-only attack COA or known ciphertext attack is an attack model for cryptanalysis where the attacker is assumed to have access only to a set of ciphertexts.
While the attacker has no channel providing access to the plaintext prior to encryption, in all practical ciphertext-only attacks, the attacker still has some knowledge of the plaintext.
For instance, the attacker might know the language in which the plaintext is written or the expected statistical distribution of characters in the plaintext. Standard protocol data and messages are commonly part of the plaintext in many deployed systems and can usually be guessed or known efficiently as part of a ciphertext-only attack on these systems.
The attack is completely successful if the corresponding plaintexts can be deduced, or even better, the key. The ability to obtain any information at all about the underlying plaintext beyond what was pre-known to the attacker is still considered a success.
For example, if an adversary is sending ciphertext continuously to maintain traffic-flow security , it would be very useful to be able to distinguish real messages from nulls. Even making an informed guess of the existence of real messages would facilitate traffic analysis.
In the history of cryptography , early ciphers, implemented using pen-and-paper, were routinely broken using ciphertexts alone. Cryptographers developed statistical techniques for attacking ciphertext, such as frequency analysis.
Mechanical encryption devices such as Enigma made these attacks much more difficult although, historically, Polish cryptographers were able to mount a successful ciphertext-only cryptanalysis of the Enigma by exploiting an insecure protocol for indicating the message settings. More advanced ciphertext-only attacks on the Enigma were mounted in Bletchley Park during World War II , by intelligently guessing plaintexts corresponding to intercepted ciphertexts. Every modern cipher attempts to provide protection against ciphertext-only attacks.
The vetting process for a new cipher design standard usually takes many years and includes exhaustive testing of large quantities of ciphertext for any statistical departure from random noise. See: Advanced Encryption Standard process.
Also, the field of steganography evolved, in part, to develop methods like mimic functions that allow one piece of data to adopt the statistical profile of another. Nonetheless poor cipher usage or reliance on home-grown proprietary algorithms that have not been subject to thorough scrutiny has resulted in many computer-age encryption systems that are still subject to ciphertext-only attack.
Examples include:. From Wikipedia, the free encyclopedia.
This article has multiple issues. Please help improve it or discuss these issues on the talk page.
Learn how and when to remove these template messages. This article includes a list of references , related reading or external links , but its sources remain unclear because it lacks inline citations.
Known Plaintext Attack -- Chosen Plaintext Attack
Please help to improve this article by introducing more precise citations. May Learn how and when to remove this template message. This article needs additional citations for verification.
Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
Attack models in cryptanalysis. Ciphertext-only attack Chosen-plaintext attack adaptive. Categories : Cryptographic attacks. Hidden categories: Articles lacking in-text citations from May All articles lacking in-text citations Articles needing additional references from May All articles needing additional references Articles with multiple maintenance issues.