Skipjack (cipher)
Encyclopedia
In cryptography
, Skipjack is a block cipher
—an algorithm
for encryption
—developed by the U.S.
National Security Agency
(NSA). Initially classified
, it was originally intended for use in the controversial Clipper chip
. Subsequently, the algorithm was declassified and now provides a unique insight into the cipher
designs of a government intelligence agency.
, and the cipher
was provided for use in the Clipper chip
, implemented in tamperproof
hardware. Skipjack is used only for encryption; the key escrow is achieved through the use of a separate mechanism known as the Law Enforcement Access Field (LEAF).
The design was initially secret, and was regarded with considerable suspicion by many in the public cryptography community for that reason. It was declassified
on 24 June 1998.
To ensure public confidence in the algorithm, several academic researchers from outside the government were called in to evaluate the algorithm (Brickell et al., 1993). The researchers found no problems with either the algorithm itself or the evaluation process. Moreover, their report gave some insight into the (classified) history and development of Skipjack:
key
to encrypt or decrypt 64-bit
data blocks. It is an unbalanced Feistel network with 32 rounds.
It was designed to be used in secured phones.
and Adi Shamir
discovered an attack against 16 of the 32 rounds within one day of declassification, and (with Alex Biryukov
) extended this to 31 of the 32 rounds (but with an attack only slightly faster than exhaustive search) within months using impossible differential cryptanalysis
.
A truncated differential attack was also published against 28 rounds of Skipjack cipher. This was later complemented by a slide attack
on all 32 rounds.
Biham, Shamir and Biryukov's attack continues to be the best cryptanalysis of Skipjack known to the public.
to Dan Brown
's 1998 novel Digital Fortress
. In Brown's novel, Skipjack is proposed as the new public-key encryption standard, along with a back door secretly inserted by the NSA ("a few lines of cunning programming") which would have allowed them to decrypt Skipjack using a secret password and thereby "read the world's email". When details of the cipher are publicly released, programmer Greg Hale discovers and announces details of the backdoor. This is arguably similar to the Dual EC DRBG
NSA controversy.
Additionally, in the Half-Life 2
modification Dystopia
, the "encryption" program used in cyberspace apparently uses both Skipjack and Blowfish
algorithms.
Skipjack is also mentioned in the novels Hostile Intent and Shock Warning, both by author Michael Walsh
.
Cryptography
Cryptography is the practice and study of techniques for secure communication in the presence of third parties...
, Skipjack is a block cipher
Block cipher
In cryptography, a block cipher is a symmetric key cipher operating on fixed-length groups of bits, called blocks, with an unvarying transformation. A block cipher encryption algorithm might take a 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext...
—an algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
for encryption
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
—developed by the U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
National Security Agency
National Security Agency
The National Security Agency/Central Security Service is a cryptologic intelligence agency of the United States Department of Defense responsible for the collection and analysis of foreign communications and foreign signals intelligence, as well as protecting U.S...
(NSA). Initially classified
Classified information
Classified information is sensitive information to which access is restricted by law or regulation to particular groups of persons. A formal security clearance is required to handle classified documents or access classified data. The clearance process requires a satisfactory background investigation...
, it was originally intended for use in the controversial Clipper chip
Clipper chip
The Clipper chip was a chipset that was developed and promoted by the U.S. National Security Agency as an encryption device to be adopted by telecommunications companies for voice transmission...
. Subsequently, the algorithm was declassified and now provides a unique insight into the cipher
Cipher
In cryptography, a cipher 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. In non-technical usage, a “cipher” is the same thing as a “code”; however, the concepts...
designs of a government intelligence agency.
History of Skipjack
Skipjack was proposed as the encryption algorithm in a US government-sponsored scheme of key escrowKey escrow
Key escrow is an arrangement in which the keys needed to decrypt encrypted data are held in escrow so that, under certain circumstances, an authorized third party may gain access to those keys...
, and the cipher
Cipher
In cryptography, a cipher 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. In non-technical usage, a “cipher” is the same thing as a “code”; however, the concepts...
was provided for use in the Clipper chip
Clipper chip
The Clipper chip was a chipset that was developed and promoted by the U.S. National Security Agency as an encryption device to be adopted by telecommunications companies for voice transmission...
, implemented in tamperproof
Tamper resistance
Tamper resistance is resistance to tampering by either the normal users of a product, package, or system or others with physical access to it. There are many reasons for employing tamper resistance....
hardware. Skipjack is used only for encryption; the key escrow is achieved through the use of a separate mechanism known as the Law Enforcement Access Field (LEAF).
The design was initially secret, and was regarded with considerable suspicion by many in the public cryptography community for that reason. It was declassified
Classified information
Classified information is sensitive information to which access is restricted by law or regulation to particular groups of persons. A formal security clearance is required to handle classified documents or access classified data. The clearance process requires a satisfactory background investigation...
on 24 June 1998.
To ensure public confidence in the algorithm, several academic researchers from outside the government were called in to evaluate the algorithm (Brickell et al., 1993). The researchers found no problems with either the algorithm itself or the evaluation process. Moreover, their report gave some insight into the (classified) history and development of Skipjack:
- [Skipjack] is representative of a family of encryption algorithms developed in 1980 as part of the NSA suite of "Type I" algorithms... Skipjack was designed using building blocks and techniques that date back more than forty years. Many of the techniques are related to work that was evaluated by some of the world's most accomplished and famous experts in combinatoricsCombinatoricsCombinatorics is a branch of mathematics concerning the study of finite or countable discrete structures. Aspects of combinatorics include counting the structures of a given kind and size , deciding when certain criteria can be met, and constructing and analyzing objects meeting the criteria ,...
and abstract algebraAbstract algebraAbstract algebra is the subject area of mathematics that studies algebraic structures, such as groups, rings, fields, modules, vector spaces, and algebras...
. Skipjack's more immediate heritage dates to around 1980, and its initial design to 1987...The specific structures included in Skipjack have a long evaluation history, and the cryptographic properties of those structures had many prior years of intense study before the formal process began in 1987.
Description
Skipjack uses an 80-bitKey size
In cryptography, key size or key length is the size measured in bits of the key used in a cryptographic algorithm . An algorithm's key length is distinct from its cryptographic security, which is a logarithmic measure of the fastest known computational attack on the algorithm, also measured in bits...
key
Key (cryptography)
In cryptography, a key is a piece of information that determines the functional output of a cryptographic algorithm or cipher. Without a key, the algorithm would produce no useful result. In encryption, a key specifies the particular transformation of plaintext into ciphertext, or vice versa...
to encrypt or decrypt 64-bit
Block size (cryptography)
In modern cryptography, symmetric key ciphers are generally divided into stream ciphers and block ciphers. Block ciphers operate on a fixed length string of bits. The length of this bit string is the block size...
data blocks. It is an unbalanced Feistel network with 32 rounds.
It was designed to be used in secured phones.
Cryptanalysis
Eli BihamEli Biham
Eli Biham is an Israeli cryptographer and cryptanalyst, currently a professor at the Technion Israeli Institute of Technology Computer Science department. Starting from October 2008, Biham is the dean of the Technion Computer Science department, after serving for two years as chief of CS graduate...
and Adi Shamir
Adi Shamir
Adi Shamir is an Israeli cryptographer. He is a co-inventor of the RSA algorithm , a co-inventor of the Feige–Fiat–Shamir identification scheme , one of the inventors of differential cryptanalysis and has made numerous contributions to the fields of cryptography and computer...
discovered an attack against 16 of the 32 rounds within one day of declassification, and (with Alex Biryukov
Alex Biryukov
Alex Biryukov is a cryptographer, currently an assistant professor at the University of Luxembourg. His notable work includes the design of the stream cipher LEX, as well as the cryptanalysis of numerous cryptographic primitives. In 1998, he developed impossible differential cryptanalysis together...
) extended this to 31 of the 32 rounds (but with an attack only slightly faster than exhaustive search) within months using impossible differential cryptanalysis
Impossible differential cryptanalysis
In cryptography, impossible differential cryptanalysis is a form of differential cryptanalysis for block ciphers. While ordinary differential cryptanalysis tracks differences that propagate through the cipher with greater than expected probability, impossible differential cryptanalysis exploits...
.
A truncated differential attack was also published against 28 rounds of Skipjack cipher. This was later complemented by a slide attack
Slide attack
The slide attack is a form of cryptanalysis designed to deal with the prevailing idea that even weak ciphers can become very strong by increasing the number of rounds, which can ward off a differential attack. The slide attack works in such a way as to make the number of rounds in a cipher irrelevant...
on all 32 rounds.
Biham, Shamir and Biryukov's attack continues to be the best cryptanalysis of Skipjack known to the public.
In pop culture
An algorithm named Skipjack forms part of the back-storyBack-story
A back-story, background story, or backstory is the literary device of a narrative chronologically earlier than, and related to, a narrative of primary interest. Generally, it is the history of characters or other elements that underlie the situation existing at the main narrative's start...
to Dan Brown
Dan Brown
Dan Brown is an American author of thriller fiction, best known for the 2003 bestselling novel, The Da Vinci Code. Brown's novels, which are treasure hunts set in a 24-hour time period, feature the recurring themes of cryptography, keys, symbols, codes, and conspiracy theories...
's 1998 novel Digital Fortress
Digital Fortress
Digital Fortress is a techno-thriller novel written by American author Dan Brown and published in 1998 by St. Martin's Press. The book explores the theme of government surveillance of electronically stored information on the private lives of citizens, and the possible civil liberties and ethical...
. In Brown's novel, Skipjack is proposed as the new public-key encryption standard, along with a back door secretly inserted by the NSA ("a few lines of cunning programming") which would have allowed them to decrypt Skipjack using a secret password and thereby "read the world's email". When details of the cipher are publicly released, programmer Greg Hale discovers and announces details of the backdoor. This is arguably similar to the Dual EC DRBG
Dual EC DRBG
Dual_EC_DRBG or Dual Elliptic Curve Deterministic Random Bit Generator is a controversial pseudorandom number generator designed and published by the National Security Agency. It is based on the elliptic curve discrete logarithm problem and is one of the four PRNGs standardized in the NIST...
NSA controversy.
Additionally, in the Half-Life 2
Half-Life 2
Half-Life 2 , the sequel to Half-Life, is a first-person shooter video game and a signature title in the Half-Life series. It is singleplayer, story-driven, science fiction, and linear...
modification Dystopia
Dystopia (computer game)
Dystopia is a team-based, objective-driven, first-person shooter video game, developed as a total conversion modification on the Valve Corporation's proprietary Source engine. It is based on the cyberpunk literary genre; somewhat based on popular role-playing game Shadowrun, created by an amateur...
, the "encryption" program used in cyberspace apparently uses both Skipjack and Blowfish
Blowfish (cipher)
Blowfish is a keyed, symmetric block cipher, designed in 1993 by Bruce Schneier and included in a large number of cipher suites and encryption products. Blowfish provides a good encryption rate in software and no effective cryptanalysis of it has been found to date...
algorithms.
Skipjack is also mentioned in the novels Hostile Intent and Shock Warning, both by author Michael Walsh
Michael Walsh (author)
Michael A. Walsh is a music critic, author, screenwriter, and media critic. After graduating from the Eastman School of Music in Rochester, New York in 1971, he became a reporter for the Rochester Democrat and Chronicle in February 1972, where he shared the New York State Publishers Association...
.
Further reading
- L.Granboulan, "Flaws in differential cryptanalysis of Skipjack", FSE 2001.
- R.Chung-Wei Phan, "Cryptanalysis of full Skipjack block cipher", Electronics Letters, Volume 38, Issue 2, p. 69—71, 2002.