# Dictionary Definition

cryptography

### Noun

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

2 act of writing in code or cipher [syn: coding, secret
writing]

# User Contributed Dictionary

### Noun

- The discipline
concerned with communication security (eg, confidentiality of
messages, integrity of
messages, sender authentication,
non-repudiation of
messages, and many other related issues).
- 1658:, Sir Thomas
Browne (first use in English),
- We might abate...the strange cryptography of Gaffarell in his Starrie Booke of Heaven.

- 1658:, Sir Thomas
Browne (first use in English),

#### Usage notes

Cryptography is not limited to pencil and paper, nor to computing, but to the general field and possible solutions to such issues. Subfields include encoding, decoding, cryptanalysis, codes, ciphers, etc.In many languages, though less so in English,
cognates to "cryptology" are also used with the meaning given
above, and even preferred. Cryptography is very closely connected
with information
theory as the only adequate theory for it is based on work done
in that field, beginning around WWII at Bell Laboratories.
Cryptography is also somewhat related to steganography which is
concerned with concealing the very existence of a message. Contrast
this with cryptography's indifference, from a confidentiality
viewpoint, to whether a message is copied by some attacker.

#### Related terms

#### Derived terms

#### Translations

discipline concerned with communication security

- Bulgarian: криптография
- Chinese: 密码学 名词
- Czech: kryptografie
- Danish: kryptografi
- Dutch: cryptografie , geheimschrift
- Finnish: kryptografia, salaustekniikka
- French: cryptographie
- German: Kryptographie
- Japanese: 暗号論 (angōron)
- Portuguese: criptografia

# Extensive Definition

Cryptography (or cryptology; derived from
Greek
κρύπτω krýpto "hidden" and the verb γράφω gráfo "to write" or
λέγειν legein "to speak") is the practice and study of hiding
information. In modern times, cryptography is considered to be a
branch of both mathematics and computer
science, and is affiliated closely with information
theory, computer
security, and engineering. Cryptography is
used in applications present in technologically advanced societies;
examples include the security of ATM
cards, computer
passwords, and electronic
commerce, which all depend on cryptography.

## Terminology

Until modern times, cryptography referred almost exclusively to encryption, the process of converting ordinary information (plaintext) into unintelligible gibberish (i.e., ciphertext).The study of characteristics of languages which
have some application in cryptology, i.e. frequency data, letter
combinations, universal patterns, etc. is called Cryptolinguistics.

## History of cryptography and cryptanalysis

Before the modern era, cryptography was concerned solely with message confidentiality (i.e., encryption) — conversion of messages from a comprehensible form into an incomprehensible one, and back again at the other end, rendering it unreadable by interceptors or eavesdroppers without secret knowledge (namely, the key needed for decryption of that message). In recent decades, the field has expanded beyond confidentiality concerns to include techniques for message integrity checking, sender/receiver identity authentication, digital signatures, interactive proofs, and secure computation, amongst others.The earliest forms of secret writing required
little more than local pen and paper analogs, as most people could
not read. More literacy, or opponent literacy, required actual
cryptography. The main classical cipher types are transposition
ciphers, which rearrange the order of letters in a message
(e.g., 'help me' becomes 'ehpl em' in a trivially simple
rearrangement scheme), and substitution
ciphers, which systematically replace letters or groups of
letters with other letters or groups of letters (e.g., 'fly at
once' becomes 'gmz bu podf' by replacing each letter with the one
following it in the English alphabet). Simple versions of either
offered little confidentiality from enterprising opponents, and
still don't. An early substitution cipher was the Caesar
cipher, in which each letter in the plaintext was replaced by a
letter some fixed number of positions further down the alphabet. It
was named after Julius
Caesar who is reported to have used it, with a shift of 3, to
communicate with his generals during his military campaigns, just
like EXCESS-3 code in
boolean algebra.

Encryption attempts to ensure secrecy in communications, such
as those of spies, military
leaders, and diplomats.
There is record of several early Hebrew ciphers as well.
Cryptography is recommended in the Kama Sutra as
a way for lovers to communicate without inconvenient discovery.
Steganography
(i.e., hiding even the existence of a message so as to keep it
confidential) was also first developed in ancient times. An early
example, from Herodotus,
concealed a message - a tattoo on a slave's shaved head - under the
regrown hair. More modern examples of steganography include the use
of invisible
ink, microdots, and
digital
watermarks to conceal information.

Ciphertexts produced by classical ciphers (and
some modern ones) always reveal statistical information about the
plaintext, which can often be used to break them. After the
discovery of
frequency analysis (perhaps by the Arab polymath al-Kindi) in the
9th century, nearly all such ciphers became more or less readily
breakable by an informed attacker. Such classical ciphers still
enjoy popularity today, though mostly as puzzles (see cryptogram). Essentially all
ciphers remained vulnerable to cryptanalysis using this technique
until the invention of the polyalphabetic
cipher, most clearly by Leon
Battista Alberti around the year 1467 (though there is some
indication of earlier Arab knowledge of them). Alberti's innovation
was to use different ciphers (i.e., substitution alphabets) for
various parts of a message (perhaps for each successive plaintext
letter in the limit). He also invented what was probably the first
automatic cipher
device, a wheel which implemented a partial realization of his
invention. In the polyalphabetic Vigenère
cipher, encryption uses a key word, which controls letter
substitution depending on which letter of the key word is used. In
the mid 1800s Babbage
showed that polyalphabetic ciphers of this type remained partially
vulnerable to frequency analysis techniques. The ciphers
implemented by better quality examples of these designs brought
about a substantial increase in cryptanalytic difficulty after
WWI.

The development of digital computers and electronics after WWII made
possible much more complex ciphers. Furthermore, computers allowed
for the encryption of any kind of data represented by computers in
any binary format, unlike classical ciphers which only encrypted
written language texts, thus dissolving much of the utility of a
linguistic approach to cryptanalysis. Many computer ciphers can be
characterized by their operation on binary
bit sequences (sometimes in
groups or blocks), unlike classical and mechanical schemes, which
generally manipulate traditional characters (i.e., letters and
digits) directly. However, computers have also assisted
cryptanalysis, which has compensated to some extent for increased
cipher complexity. Nonetheless, good modern ciphers have stayed
ahead of cryptanalysis; it is typically the case that use of a
quality cipher is very efficient (i.e., fast and requiring few
resources), while breaking it requires an effort many orders of
magnitude larger than before, making cryptanalysis so inefficient
and impractical as to be effectively impossible.

Extensive open academic research into
cryptography is relatively recent — it began only in the
mid-1970s with the public specification of DES (the Data
Encryption Standard) by the US Government's National Bureau of
Standards, the Diffie-Hellman
paper, and the public release of the RSA algorithm. Since
then, cryptography has become a widely used tool in communications,
computer
networks, and computer security generally. The present security
level of many modern cryptographic techniques is based on the
difficulty of certain computational problems, such as the integer
factorisation or the discrete
logarithm problems. In many cases, there are proofs that
cryptographic techniques are secure if a certain computational
problem cannot be solved efficiently. With one notable exception -—
the one-time pad
—- these proofs are contingent, and thus not definitive, but are
currently the best available for cryptographic algorithms and
protocols.

As well as being aware of cryptographic history,
cryptographic algorithm and system designers must also sensibly
consider probable future developments in their designs. For
instance, continuous improvements in computer processing power have
increased the scope of brute-force
attacks, thus when specifying key lengths,
the standard is similarly advancing. The potential effects of
quantum
computing are already being considered by some cryptographic
system designers; the announced imminence of small implementations
of these machines is making the need for this preemptive caution
fully explicit.

Essentially, prior to the early 20th century,
cryptography was chiefly concerned with linguistic patterns. Since then
the emphasis has shifted, and cryptography now makes extensive use
of mathematics, including aspects of information
theory,
computational complexity, statistics, combinatorics, abstract
algebra, and number
theory. Cryptography is also a branch of engineering, but an unusual
one as it deals with active, intelligent, and malevolent opposition
(see cryptographic
engineering and security
engineering); most other kinds of engineering need deal only
with neutral natural forces. There is also active research
examining the relationship between cryptographic problems and
quantum
physics (see quantum
cryptography and quantum
computing).

## Modern cryptography

The modern field of cryptography can be divided into several areas of study. The chief ones are discussed here; see Topics in Cryptography for more.### Symmetric-key Cryptography

Symmetric-key cryptography refers to encryption methods in which both the sender and receiver share the same key (or, less commonly, in which their keys are different, but related in an easily computable way). This was the only kind of encryption publicly known until June 1976. Despite its deprecation as an official standard, DES (especially its still-approved and much more secure triple-DES variant) remains quite popular; it is used across a wide range of applications, from ATM encryption to e-mail privacy and secure remote access. Many other block ciphers have been designed and released, with considerable variation in quality. Many have been thoroughly broken. See Category:Block ciphers.Stream ciphers, in contrast to the 'block' type,
create an arbitrarily long stream of key material, which is
combined with the plaintext bit-by-bit or character-by-character,
somewhat like the one-time
pad. In a stream cipher, the output stream is created based on
an internal state which changes as the cipher operates. That state
change is controlled by the key, and, in some stream ciphers, by
the plaintext stream as well. RC4 is an example of a
well-known, and widely used, stream cipher; see Category:Stream
ciphers. A public key system is so constructed that calculation
of one key (the 'private key') is computationally infeasible from
the other (the 'public key'), even though they are necessarily
related. Instead, both keys are generated secretly, as an
interrelated pair. The historian David Kahn
described public-key cryptography as "the most revolutionary new
concept in the field since polyalphabetic substitution emerged in
the Renaissance".

In public-key cryptosystems, the public key may
be freely distributed, while its paired private key must remain
secret. The public key is typically used for encryption, while the
private or secret key is used for decryption. Diffie and Hellman
showed that public-key cryptography was possible by presenting the
Diffie-Hellman
key exchange protocol.

In 1997, it finally became publicly known that
asymmetric key cryptography had been invented by James H.
Ellis at GCHQ, a British
intelligence organization, and that, in the early 1970s, both the
Diffie-Hellman and RSA algorithms had been previously developed (by
Malcolm
J. Williamson and Clifford
Cocks, respectively).

The Diffie-Hellman and RSA algorithms, in
addition to being the first publicly known examples of high quality
public-key algorithms, have been among the most widely used. Others
include the Cramer-Shoup
cryptosystem, ElGamal
encryption, and various
elliptic curve techniques. See
Category:Asymmetric-key cryptosystems.

## References

## Further reading

- Handbook of Applied Cryptography by A. J. Menezes, P. C. van Oorschot, and S. A. Vanstone CRC Press, (PDF download available), somewhat more mathematical than Schneier's Applied Cryptography.
- Introduction to Modern Cryptography by Jonathan Katz and Yehuda Lindell. http://www.cs.umd.edu/~jkatz/imc.html.
- Introduction to Modern Cryptography by Phillip Rogaway and Mihir Bellare, a mathematical introduction to theoretical cryptography including reduction-based security proofs. PDF download.
- Stealing Secrets, Telling Lies: How Spies and Codebreakers Helped Shape the Twentieth Century, by James Gannon.
- Cryptonomicon by Neal Stephenson (novel, WW2 Enigma cryptanalysis figures into the story, though not always realistically).
- Alvin's Secret Code by Clifford B. Hicks (children's novel that introduces some basic cryptography and cryptanalysis).
- In Code: A Mathematical Journey by Sarah Flannery (with David Flannery). Popular account of Sarah's award-winning project on public-key cryptography, co-written with her father.
- Cryptography and Mathematics by Bernhard Esslinger, 200 pages, part of the free open-source package Cryptool, http://www.cryptool.com.
- Ibrahim A. Al-Kadi ,"The origins of cryptology: The Arab contributions”, Cryptologia, 16(2) (April 1992) pp. 97–126.
- Andreas Pfitzmann: Security in IT Networks: Multilateral Security in Distributed and by Distributed Systems
- Introduction to Cryptology Excellent coverage of many classical ciphers and cryptograpy concepts and of the "modern" DES and RSA systems.

## External links

- AttackPrevention Resource for Cryptography Whitepapers, Tools, Videos, and Podcasts.
- Handbook of Applied Cryptography by A. J. Menezes, P. C. van Oorschot, and S. A. Vanstone (PDF download available), somewhat more mathematical than Schneier's book.
- Cryptography: The Ancient Art of Secret Messages by Monica Pawlan - February 1998
- sci.crypt mini-FAQ
- NSA's CryptoKids.
- RSA Laboratories' Frequently Asked Questions About Today's Cryptography

cryptography in Afrikaans: Kriptografie

cryptography in Arabic: علم التعمية

cryptography in Belarusian (Tarashkevitsa):
Крыптаграфія

cryptography in Bavarian: Kriptografie

cryptography in Catalan: Criptografia

cryptography in Czech: Kryptografie

cryptography in Danish: Kryptografi

cryptography in German: Kryptographie

cryptography in Estonian: Krüptograafia

cryptography in Modern Greek (1453-):
Κρυπτογραφία

cryptography in Spanish: Criptografía

cryptography in Esperanto: Kriptografio

cryptography in Basque: Kriptografia

cryptography in Persian: رمزنگاری

cryptography in French: Cryptographie

cryptography in Galician: Criptografía

cryptography in Georgian: კრიპტოგრაფია

cryptography in Korean: 암호학

cryptography in Hindi: बीज-लेखन

cryptography in Croatian: Kriptografija

cryptography in Indonesian: Kriptografi

cryptography in Italian: Crittografia

cryptography in Hebrew: קריפטוגרפיה

cryptography in Latin: Cryptographia

cryptography in Hungarian: Kriptográfia

cryptography in Malay (macrolanguage):
Kriptografi

cryptography in Dutch: Cryptografie

cryptography in Japanese: 暗号理論

cryptography in Norwegian: Kryptografi

cryptography in Norwegian Nynorsk:
Kryptografi

cryptography in Uzbek: Kriptografiya

cryptography in Polish: Kryptografia

cryptography in Portuguese: Criptografia

cryptography in Romanian: Criptografie

cryptography in Russian: Криптография

cryptography in Albanian: Kriptografia

cryptography in Slovenian: Kriptografija

cryptography in Serbian: Криптографија

cryptography in Finnish: Salaus

cryptography in Swedish: Kryptografi

cryptography in Thai: วิทยาการเข้ารหัสลับ

cryptography in Vietnamese: Mật mã học

cryptography in Turkish: Kriptografi

cryptography in Ukrainian: Криптографія

cryptography in Chinese: 密码学

# Synonyms, Antonyms and Related Words

cipher,
code, coded message, coup
de plume, criticism,
cryptanalysis,
cryptoanalysis,
cryptoanalytics,
cryptogram, cryptograph, cryptographer, cryptology, diagnostics, engrossment, epigraphy, exegetics, hermeneutics, ink spilling,
inkslinging,
inscription,
invisible ink, lettering, lexicography, literary
criticism, macrography, metoposcopy, micrography, oneirology, paleography, pathognomy, pen, pen-and-ink, pencil driving,
physiognomics,
physiognomy,
scrivenery, scrivening, secret language,
secret writing, semeiology, semeiotics, sympathetic ink,
symptomatology,
textual criticism, tropology, typewriting, typing, writing