In
cryptography
Cryptography, or cryptology (from grc, , translit=kryptós "hidden, secret"; and ''graphein'', "to write", or ''-logia'', "study", respectively), is the practice and study of techniques for secure communication in the presence of adver ...
, a ciphertext-only attack (COA) or known ciphertext attack is an
attack model for
cryptanalysis
Cryptanalysis (from the Greek ''kryptós'', "hidden", and ''analýein'', "to analyze") refers to the process of analyzing information systems in order to understand hidden aspects of the systems. Cryptanalysis is used to breach cryptographic sec ...
where the attacker is assumed to have access only to a set of
ciphertext
In cryptography, ciphertext or cyphertext is the result of encryption performed on plaintext using an algorithm, called a cipher. Ciphertext is also known as encrypted or encoded information because it contains a form of the original plaintext ...
s. 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.
Attack
The attack is completely successful if the corresponding
plaintext
In cryptography, plaintext usually means unencrypted information pending input into cryptographic algorithms, usually encryption algorithms. This usually refers to data that is transmitted or stored unencrypted.
Overview
With the advent of comp ...
s can be deduced, or even better, the
key
Key or The Key may refer to:
Common meanings
* Key (cryptography), a piece of information that controls the operation of a cryptography algorithm
* Key (lock), device used to control access to places or facilities restricted by a lock
* Key (map ...
. 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
Traffic analysis is the process of intercepting and examining messages in order to deduce information from patterns in communication, it can be performed even when the messages are encrypted. In general, the greater the number of messages observed ...
, 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
Traffic analysis is the process of intercepting and examining messages in order to deduce information from patterns in communication, it can be performed even when the messages are encrypted. In general, the greater the number of messages observed ...
.
In the
history of cryptography
Cryptography, the use of codes and ciphers to protect secrets, began thousands of years ago. Until recent decades, it has been the story of what might be called classical cryptography — that is, of methods of encryption that use pen and paper, ...
, early ciphers, implemented using pen-and-paper, were routinely broken using ciphertexts alone. Cryptographers developed statistical techniques for attacking ciphertext, such as
frequency analysis
In cryptanalysis, frequency analysis (also known as counting letters) is the study of the frequency of letters or groups of letters in a ciphertext. The method is used as an aid to breaking classical ciphers.
Frequency analysis is based on t ...
. Mechanical encryption devices such as
Enigma
Enigma may refer to:
*Riddle, someone or something that is mysterious or puzzling
Biology
*ENIGMA, a class of gene in the LIM domain
Computing and technology
* Enigma (company), a New York-based data-technology startup
* Enigma machine, a family ...
made these attacks much more difficult (although, historically, Polish cryptographers were able to mount a successful ciphertext-only
cryptanalysis of the Enigma
Cryptanalysis of the Enigma ciphering system enabled the western Allies in World War II to read substantial amounts of Morse-coded radio communications of the Axis powers that had been enciphered using Enigma machines. This yielded military in ...
by exploiting an insecure protocol for indicating the message settings). More advanced ciphertext-only attacks on the Enigma were mounted in
Bletchley Park
Bletchley Park is an English country house and estate in Bletchley, Milton Keynes ( Buckinghamshire) that became the principal centre of Allied code-breaking during the Second World War. The mansion was constructed during the years following ...
during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, by intelligently guessing plaintexts corresponding to intercepted ciphertexts.
Modern
Every modern
cipher
In cryptography, a cipher (or cypher) is an algorithm for performing encryption or decryption—a series of well-defined steps that can be followed as a procedure. An alternative, less common term is ''encipherment''. To encipher or encode i ...
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
The Advanced Encryption Standard (AES), the symmetric block cipher ratified as a standard by National Institute of Standards and Technology of the United States (NIST), was chosen using a process lasting from 1997 to 2000 that was markedly more ...
. Also, the field of
steganography
Steganography ( ) is the practice of representing information within another message or physical object, in such a manner that the presence of the information is not evident to human inspection. In computing/electronic contexts, a computer file, ...
evolved, in part, to develop methods like
mimic function
A mimic function changes a file A so it assumes the statistical properties of another file B. That is, if p(t,A) is the probability of some substring t occurring in A, then a mimic function f, recodes A so that p(t,f(A)) approximates p(t,B) for al ...
s 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:
Examples
*Early versions of
Microsoft
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
's
PPTP
The Point-to-Point Tunneling Protocol (PPTP) is an obsolete method for implementing virtual private networks. PPTP has many well known security issues.
PPTP uses a TCP control channel and a Generic Routing Encapsulation tunnel to encapsulate PP ...
virtual private network
A virtual private network (VPN) extends a private network across a public network and enables users to send and receive data across shared or public networks as if their computing devices were directly connected to the private network. The be ...
software used the same
RC4
In cryptography, RC4 (Rivest Cipher 4, also known as ARC4 or ARCFOUR, meaning Alleged RC4, see below) is a stream cipher. While it is remarkable for its simplicity and speed in software, multiple vulnerabilities have been discovered in RC4, ren ...
key for the sender and the receiver (later versions had other problems). In any case where a stream cipher like RC4 is used twice with the same key it is open to ciphertext-only attack. ''See:''
stream cipher attack
Stream ciphers, where plaintext bits are combined with a cipher bit stream by an exclusive-or operation ( xor), can be very secure if used properly. However, they are vulnerable to attacks if certain precautions are not followed:
*keys must neve ...
*
Wired Equivalent Privacy
Wired Equivalent Privacy (WEP) was a security algorithm for 802.11 wireless networks. Introduced as part of the original IEEE 802.11 standard ratified in 1997, its intention was to provide data confidentiality comparable to that of a traditional wi ...
(WEP), the first security protocol for
Wi-Fi
Wi-Fi () is a family of wireless network protocols, based on the IEEE 802.11 family of standards, which are commonly used for local area networking of devices and Internet access, allowing nearby digital devices to exchange data by radio wave ...
, proved vulnerable to several attacks, most of them ciphertext-only.
* GSM's
A5/1 and
A5/2
A5/2 is a stream cipher used to provide voice privacy in the GSM cellular telephone protocol. It was designed in 1992-1993 (finished March 1993) as a replacement for the relatively stronger (but still weak) A5/1, to allow the GSM standard to b ...
* Some modern cipher designs have later been shown to be vulnerable to ciphertext-only attacks. For example,
Akelarre.
* A cipher whose key space is too small is subject to
brute force attack
In cryptography, a brute-force attack consists of an attacker submitting many passwords or passphrases with the hope of eventually guessing correctly. The attacker systematically checks all possible passwords and passphrases until the correct ...
with access to nothing but ciphertext by simply trying all possible keys. All that is needed is some way to distinguish valid plaintext from random noise, which is easily done for natural languages when the ciphertext is longer than the
unicity distance
In cryptography, unicity distance is the length of an original ciphertext needed to break the cipher by reducing the number of possible spurious keys to zero in a brute force attack. That is, after trying every possible key, there should be just ...
. One example is
DES
Des is a masculine given name, mostly a short form (hypocorism) of Desmond. People named Des include:
People
* Des Buckingham, English football manager
* Des Corcoran, (1928–2004), Australian politician
* Des Dillon (disambiguation), sever ...
, which only has 56-bit keys. All too common current examples are commercial security products that derive keys for otherwise impregnable ciphers like
AES from a user-selected
password
A password, sometimes called a passcode (for example in Apple devices), is secret data, typically a string of characters, usually used to confirm a user's identity. Traditionally, passwords were expected to be memorized, but the large number of ...
. Since users rarely employ passwords with anything close to the
entropy of the cipher's key space, such systems are often quite easy to break in practice using only ciphertext. The 40-bit
CSS
Cascading Style Sheets (CSS) is a style sheet language used for describing the presentation of a document written in a markup language such as HTML or XML (including XML dialects such as SVG, MathML or XHTML). CSS is a cornerstone techno ...
cipher used to encrypt
DVD
The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any kind ...
video discs can always be broken with this method, as all that is needed is to look for
MPEG-2
MPEG-2 (a.k.a. H.222/H.262 as was defined by the ITU) is a standard for "the generic video coding format, coding of moving pictures and associated audio information". It describes a combination of Lossy compression, lossy video compression and ...
video data.
References
*
Alex Biryukov and Eyal Kushilevitz, From Differential Cryptanalysis to Ciphertext-Only Attacks,
CRYPTO
Crypto commonly refers to:
* Cryptocurrency, a type of digital currency secured by cryptography and decentralization
* Cryptography, the practice and study of hiding information
Crypto or Krypto may also refer to:
Cryptography
* Cryptanalysis, ...
1998, pp72–88;
Cryptographic attacks
{{Attack models in cryptanalysis, state=expanded