# Dictionary Definition

cryptanalysis n : the science of analyzing and
deciphering codes and ciphers and cryptograms [syn: cryptanalytics, cryptography, cryptology]

# User Contributed Dictionary

#### Related terms

# Extensive Definition

Cryptanalysis (from the Greek
kryptós, "hidden", and analýein, "to loosen" or "to untie") is the
study of methods for obtaining the meaning of encrypted information, without
access to the secret
information which is normally required to do so. Typically, this
involves finding a secret key. In
non-technical language, this is the practice of codebreaking or
cracking the code, although these phrases also have a specialised
technical meaning (see code).

"Cryptanalysis" is also used to refer to any
attempt to circumvent the security of other types of cryptographic algorithms and protocols
in general, and not just encryption. However,
cryptanalysis usually excludes methods of attack that do not
primarily target weaknesses in the actual cryptography, such as
bribery, physical
coercion, burglary,
keystroke
logging, and social
engineering, although these types of attack are an important
concern and are often more effective than traditional
cryptanalysis.

Even though the goal has been the same, the
methods and techniques of cryptanalysis have changed drastically
through the history of cryptography, adapting to increasing
cryptographic complexity, ranging from the pen-and-paper methods of
the past, through machines like Enigma in
World
War II, to the computer-based schemes of the present. The
results of cryptanalysis have also changed — it is no longer
possible to have unlimited success in codebreaking, and there is a
hierarchical classification of what constitutes a rare practical
attack. In the mid-1970s, a new class of cryptography was
introduced: asymmetric
cryptography. Methods for breaking these cryptosystems are typically
radically different from before, and usually involve solving
carefully-constructed problems in pure
mathematics, the best-known being integer
factorization.

## History of cryptanalysis

Cryptanalysis has coevolved together with
cryptography, and the contest can be traced through the history
of cryptography—new ciphers being designed to replace
old broken designs, and new cryptanalytic techniques invented to
crack the improved schemes . In practice, they are viewed as two
sides of the same coin: in order to create secure cryptography, you
have to design against possible cryptanalysis.

### Classical cryptanalysis

Although the actual word "cryptanalysis" is
relatively recent (it was coined by William
Friedman in 1920), methods for breaking codes
and ciphers are much
older. The first known recorded explanation of cryptanalysis was
given by 9th century
Arabian
polymath
Abu Yusuf Yaqub ibn Ishaq al-Sabbah Al-Kindi in A Manuscript on
Deciphering Cryptographic Messages. This treatise includes a
description of the method of frequency
analysis (Ibrahim
Al-Kadi, 1992- ref-3).

Frequency
analysis is the basic tool for breaking most classical
ciphers. In natural languages, certain letters of the alphabet appear more frequently
than others; in English,
"E" is likely to
be the most common letter in any sample of plaintext. Similarly, the
digraph
"TH" is the most likely pair of letters in English, and so on.
Frequency analysis relies on a cipher failing to hide these
statistics. For
example, in a simple
substitution cipher (where each letter is simply replaced with
another), the most frequent letter in the ciphertext would be a
likely candidate for "E".

In practice, frequency analysis relies as much on
linguistic knowledge
as it does on statistics, but as ciphers
became more complex, mathematics became more
important in cryptanalysis. This change was particularly evident
during World War
II, where efforts to crack Axis ciphers
required new levels of mathematical sophistication. Moreover,
automation was first applied to cryptanalysis in that era with the
Polish Bomba
device, use of punched card
equipment, and in the Colossus
— one of the earliest computers (arguably the first programmable
electronic digital computer).

### Modern cryptanalysis

Even though computation was used to great effect
in cryptanalysis in World War II, it also made possible new methods
of cryptography orders
of magnitude more complex than ever before. Taken as a whole,
modern cryptography has become much more impervious to
cryptanalysis than the pen-and-paper systems of the past, and now
seems to have the upper hand against pure cryptanalysis. The
historian David Kahn
notes, ''"Many are the cryptosystems offered by the hundreds of
commercial vendors today that cannot be broken by any known methods
of cryptanalysis. Indeed, in such systems even a chosen plaintext
attack, in which a selected plaintext is matched against its
ciphertext, cannot yield the key that unlock other messages. In a
sense, then, cryptanalysis is dead. But that is not the end of the
story. Cryptanalysis may be dead, but there is - to mix my
metaphors - more than one way to skin a cat."''.

Kahn goes on to mention increased opportunities
for interception, bugging, side
channel attacks and quantum
computers as replacements for the traditional means of
cryptanalysis.

Kahn may have been premature in his cryptanalysis
postmortem; weak ciphers are not yet extinct, and cryptanalytic
methods employed by intelligence agencies remain unpublished. In
academia, new designs
are regularly presented, and are also frequently broken: the 1984
block
cipher Madryga was found
to be susceptible to ciphertext-only
attacks in 1998; FEAL-4, proposed as a
replacement for the DES
standard encryption algorithm, was demolished by a spate of attacks
from the academic community, many of which are entirely practical.
In industry, too,
ciphers are not free from flaws: for example, the A5/1, A5/2 and CMEA
algorithms, used in mobile phone
technology, can all be broken in hours, minutes or even in
real-time using widely-available computing equipment. In 2001,
Wired
Equivalent Privacy (WEP), a protocol used to secure Wi-Fi wireless
networks, was shown to be susceptible to a practical related-key
attack.

### The results of cryptanalysis

Successful cryptanalysis has undoubtedly
influenced history; the ability to read the presumed-secret
thoughts and plans of others can be a decisive advantage, and never
more so than during wartime. For example, in World War
I, the breaking of the Zimmermann
Telegram was instrumental in bringing the United States into
the war. In World War
II, the cryptanalysis of the German ciphers — including the
Enigma
machine and the Lorenz
cipher — has been credited with everything between shortening
the end of the European war by a few months to determining the
eventual result (see ULTRA). The United
States also benefited from the cryptanalysis of the Japanese
PURPLE code
(see MAGIC).

Governments have long recognised the potential
benefits of cryptanalysis for intelligence,
both military and diplomatic, and established dedicated
organisations devoted to breaking the codes and ciphers of other
nations, for example, GCHQ and the NSA, organisations
which are still very active today. In 2004, it was reported that
the United States had broken Iranian ciphers. (It
is unknown, however, whether this was pure cryptanalysis, or
whether other factors were involved: http://news.bbc.co.uk/1/hi/technology/3804895.stm).

## Characterising attacks

Cryptanalytic attacks vary in potency and how
much of a threat they pose to real-world cryptosystems. A
certificational weakness is a theoretical attack that is unlikely
to be applicable in any real-world situation; the majority of
results found in modern cryptanalytic research are of this type.
Essentially, the practical importance of an attack is dependent on
the answers to the following three questions:

- What knowledge and capabilities are needed as a prerequisite?
- How much additional secret information is deduced?
- How much effort is required? (What is the computational complexity?)

### Prior knowledge: scenarios for cryptanalysis

Cryptanalysis can be performed under a number of
assumptions about how much can be observed or found out about the
system under attack. As a basic starting point it is normally
assumed that, for the purposes of analysis, the general algorithm is known; this is
Kerckhoffs'
principle of "the enemy knows the system". This is a reasonable
assumption in practice — throughout history, there are countless
examples of secret algorithms falling into wider knowledge,
variously through espionage, betrayal and reverse
engineering. (On occasion, ciphers have been reconstructed
through pure deduction; for example, the German Lorenz
cipher and the Japanese Purple code,
and a variety of classical schemes).

Other assumptions include:

- Ciphertext-only: the cryptanalyst has access only to a collection of ciphertexts or codetexts.
- Known-plaintext: the attacker has a set of ciphertexts to which he knows the corresponding plaintext.
- Chosen-plaintext (chosen-ciphertext): the attacker can obtain the ciphertexts (plaintexts) corresponding to an arbitrary set of plaintexts (ciphertexts) of his own choosing.
- Adaptive chosen-plaintext: like a chosen-plaintext attack, except the attacker can choose subsequent plaintexts based on information learned from previous encryptions. Similarly Adaptive chosen ciphertext attack.
- Related-key attack: Like a chosen-plaintext attack, except the attacker can obtain ciphertexts encrypted under two different keys. The keys are unknown, but the relationship between them is known; for example, two keys that differ in the one bit.

These types of attack clearly differ in how
plausible they would be to mount in practice. Although some are
more likely than others, cryptographers will often take a
conservative approach to security and assume the worst-case when
designing algorithms, reasoning that if a scheme is secure even
against unrealistic threats, then it should also resist real-world
cryptanalysis as well.

The assumptions are often more realistic than
they might seem upon first glance. For a known-plaintext attack,
the cryptanalyst might well know or be able to guess at a likely
part of the plaintext, such as an encrypted letter beginning with
"Dear Sir", or a computer session starting with "LOGIN:". A
chosen-plaintext attack is less likely, but it is sometimes
plausible: for example, you could convince someone to forward a
message you have given them, but in encrypted form. Related-key
attacks are mostly theoretical, although they can be realistic in
certain situations, for example, when constructing
cryptographic hash functions using a block
cipher.

### Classifying success in cryptanalysis

The results of cryptanalysis can also vary in
usefulness. For example, cryptographer Lars Knudsen
(1998) classified various types of attack on block
ciphers according to the amount and quality of secret
information that was discovered:

- Total break — the attacker deduces the secret key.
- Global deduction — the attacker discovers a functionally equivalent algorithm for encryption and decryption, but without learning the key.
- Instance (local) deduction — the attacker discovers additional plaintexts (or ciphertexts) not previously known.
- Information deduction — the attacker gains some Shannon information about plaintexts (or ciphertexts) not previously known.
- Distinguishing algorithm — the attacker can distinguish the cipher from a random permutation.

Similar considerations apply to attacks on other
types of cryptographic algorithm.

### Complexity

Attacks can also be characterised by the amount
of resources they require. This can be in the form of:

- Time — the number of "primitive operations" which must be performed. This is quite loose; primitive operations could be basic computer instructions, such as addition, XOR, shift, and so forth, or entire encryption methods.
- Memory — the amount of storage required to perform the attack.
- Data — the quantity of plaintexts and ciphertexts required.

In academic cryptography, a weakness or a break
in a scheme is usually defined quite conservatively. Bruce Schneier
sums up this approach: "''Breaking a cipher simply means finding a
weakness in the cipher that can be exploited with a complexity less
than brute force. Never mind that brute-force might require 2128
encryptions; an attack requiring 2110 encryptions would be
considered a break...simply put, a break can just be a
certificational weakness: evidence that the cipher does not perform
as advertised.''" (Schneier, 2000).

## Cryptanalysis of asymmetric cryptography

Asymmetric
cryptography (or public
key cryptography) is cryptography that relies on using two
keys; one private, and one public. Such ciphers invariably rely on
"hard" mathematical
problems as the basis of their security, so an obvious point of
attack is to develop methods for solving the problem. The security
of two-key cryptography depends on mathematical questions in a way
that single-key cryptography generally does not, and conversely
links cryptanalysis to wider mathematical research in a new
way.

Asymmetric schemes are designed around the
(conjectured)
difficulty of solving various mathematical problems. If an improved
algorithm can be found to solve the problem, then the system is
weakened. For example, the security of the
Diffie-Hellman key exchange scheme depends on the difficulty of
calculating the discrete
logarithm. In 1983, Don
Coppersmith found a faster way to find discrete logarithms (in
certain groups), and thereby requiring cryptographers to use larger
groups (or different types of groups). RSA's security depends (in
part) upon the difficulty of integer
factorization — a breakthrough in factoring would impact the
security of RSA.

In 1980, one could factor a difficult 50-digit
number at an expense of 1012 elementary computer operations. By
1984 the state of the art in factoring algorithms had advanced to a
point where a 75-digit number could be factored in 1012 operations.
Advances in computing technology also meant that the operations
could be performed much faster, too. Moore's law
predicts that computer speeds will continue to increase. Factoring
techniques may continue do so as well, but will most likely depend
on mathematical insight and creativity, neither of which has ever
been successfully predictable. 150-digit numbers of the kind once
used in RSA have been factored. The effort was greater than above,
but was not unreasonable on fast modern computers. By the start of
the 21st century, 150-digit numbers were no longer considered a
large enough key size for
RSA. Numbers with several hundred digits are still considered too
hard to factor in 2005, though methods will probably continue to
improve over time, requiring key size to keep
pace or new algorithms to be used.

Another distinguishing feature of asymmetric
schemes is that, unlike attacks on symmetric cryptosystems, any
cryptanalysis has the opportunity to make use of knowledge gained
from the public
key.

## Quantum computing applications for cryptanalysis

Quantum
computers, which are still in the early phases of development,
have potential use in cryptanalysis. For example, Shor's
Algorithm could factor large numbers in polynomial
time, in effect breaking some commonly used forms of public-key
encryption.

By using Grover's
algorithm on a quantum computer, brute-force key search can be
made quadratically faster. However, this could be countered by
increasing the key length.

## Methods of cryptanalysis

Classical cryptanalysis:Symmetric algorithms:

Hash functions:

Network attacks:

External attacks:

## See also

### General

### National

## External links

- Basic Cryptanalysis (files contain 5 line header, that has to be removed first)
- Simon Singh's crypto corner
- Distributed Computing Projects
- UltraAnvil tool for attacking simple substitution ciphers
- A lot of real encrypted messages on newsgroups
- A javascript codesystems solver for many types of ciphers, with examples
- The National Museum of Computing Cipher Challenge

## References

- Helen Fouché Gaines, "Cryptanalysis", 1939, Dover. ISBN 0-486-20097-3
- Abraham Sinkov, Elementary Cryptanalysis: A Mathematical Approach, Mathematical Association of America, 1966. ISBN 0-88385-622-0
- Ibrahim A. Al-Kadi ,"The origins of cryptology: The Arab contributions”, Cryptologia, 16(2) (April 1992) pp. 97–126.
- David Kahn, "The Codebreakers - The Story of Secret Writing", 1967. ISBN 0-684-83130-9
- Lars R. Knudsen: Contemporary Block Ciphers. Lectures on Data Security 1998: 105-126
- Bruce Schneier, "Self-Study Course in Block Cipher Cryptanalysis", Cryptologia, 24(1) (January 2000), pp. 18–34.
- Friedrich L. Bauer: "Decrypted Secrets". Springer 2002. ISBN 3-540-42674-4
- Friedman, William F., Military Cryptanalysis, Part I, ISBN 0-89412-044-1
- Friedman, William F., Military Cryptanalysis, Part II, ISBN 0-89412-064-6
- Friedman, William F., Military Cryptanalysis, Part III, Simpler Varieties of Aperiodic Substitution Systems, ISBN 0-89412-196-0
- Friedman, William F., Military Cryptanalysis, Part IV, Transposition and Fractionating Systems, ISBN 0-89412-198-7
- Friedman, William F. and Lambros D. Callimahos, Military Cryptanalytics, Part I, Volume 1, ISBN 0-89412-073-5
- Friedman, William F. and Lambros D. Callimahos, Military Cryptanalytics, Part I, Volume 2, ISBN 0-89412-074-3
- Friedman, William F. and Lambros D. Callimahos, Military Cryptanalytics, Part II, Volume 1, ISBN 0-89412-075-1
- Friedman, William F. and Lambros D. Callimahos, Military Cryptanalytics, Part II, Volume 2, ISBN 0-89412-076-X

cryptanalysis in Czech: Kryptoanalýza

cryptanalysis in Danish: Kryptoanalyse

cryptanalysis in German: Kryptoanalyse

cryptanalysis in Modern Greek (1453-):
Κρυπτανάλυση

cryptanalysis in Spanish: Criptoanálisis

cryptanalysis in French: Cryptanalyse

cryptanalysis in Galician: Criptoanalítica

cryptanalysis in Indonesian:
Kriptoanalisis

cryptanalysis in Italian: Crittoanalisi

cryptanalysis in Dutch: Cryptoanalyse

cryptanalysis in Japanese: 暗号解読

cryptanalysis in Norwegian: Kryptoanalyse

cryptanalysis in Portuguese: Criptoanálise

cryptanalysis in Russian: Криптоанализ

cryptanalysis in Simple English:
Cryptanalysis

cryptanalysis in Finnish: Kryptoanalyysi

cryptanalysis in Swedish: Kryptoanalys

cryptanalysis in Ukrainian: Криптоаналіз

cryptanalysis in Chinese: 密码分析