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NSA Type 2 cryptographic algorithm

Skipjack is an encryption algorithm for the transmission of information (voice data in particular), developed by the National Security Agency (NSA) in the USA. It uses the Diffie-Hellman key exchange algorithm for the distribution of the cryptographic session keys between peers.

The Skipjack algorithm was classified as an NSA Type 2 encryption product. It was intended for voice data networks (telephone) and was initially developed for the NSA's ill-fated Clipper Chip project, shown in the image on the right [1]. In 1994, (then) AT&T researcher Matt Blaze, found a serious weakness in the Escrowed Encryption System (EES), which allowed a malicious party to bypass the clipper chip's escrow capability [4].

The algorithm was initially classified as SECRET, so that it could not be examined in the usual manner by the encryption research community.

After much debate, the Skipjack algorithm was finally declassified and published by the NSA on 24 June 1998 [2]. It used an 80-bit key and a symmetric cipher algorithm, similar to DES.

Data is encrypted in blocks of 64 bits, using an unbalanced Feistel network with 32 rounds [3]. The algorithm was initially developed for use in secure telephones, such as the AT&T TSD-3600. It was also used in the first Fortezza Crypto Card. Both systems are now defunct.

 Download a full description of the algorithm

SKIPJACK-based products on this website
Clipper Chip (used for key escrow)
AT&T TSD-3600-E (using the Clipper Chip)
Fortezza Crypto Card
The SKIPJACK algorithm was implemented inthe following products [4]:

Cipher modes
  • ECB
    Electronic Code Book
  • CBC
    Cipher Block Chaining
  • CFB
    Cipher Feed Back
  • OFB
    Output Feed Back
  1. Wikipedia, Clipper chip
    Retrieved February 2011.

  2. Wikipedia, Skipjack (cipher)
    Retrieved February 2011.

  3. NSA, SKIPJACK and KEA Algorithm Specifications
    Version 2.0, 29 May 1998.

  4. Matt Blaze, Protocol Failure in the Escrowed Encryption Standard
    AT&T Bell Laboratories. 20 August 1994.
Further information
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Crypto Museum. Created: Wednesday 12 February 2014. Last changed: Wednesday, 20 December 2023 - 15:24 CET.
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