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Type 4 cryptographic engine - this page is a stub

CITADEL is a military-grade cryptographic engine for non-Type 1 applications 1 developed in 1997 by Harris Corporation in Rochester (NY,USA). Approved for export from the USA, it can be embedded in customer's equipment. It provides 3 cryptographic algorithms, based on a mixed-mode arithmetic block cipher, that are said to withstand sophisticated cryptographic attacks.

Citadel is provided as a chip – as shown in the image on the right – that can be incorporated in a customer's equipments. It features a Harris-developed proprietary encryption algorithm, known as MK-128, that is implemented entirely in hardware, and has been designed in such a way that its executes slowly in software [1].

Some aspects of the MK-128 algorithm are explained in a 2009 presentation [1] of Michael Kurdziel, the Chief Cryptographer of Harris Corporation at the time. According to Kurziel, Citadel uses a key length of 64 to 128-bits.
Harris Citadel cryptographic engine

With a key length of 115 bits, it is estimated that the algorithm is strong enough to provide 50 years of security. 2 According to the product leaflet, Citadel uses a Traffic Encryption Key (TEK) and a Key Encryption Key (KEK) with a minimum length of 64-bits each. Citadel was first used in a commercially available product in 1998, and has since become a widely accepted cryptographic engine, both in the US and elsewhere, for example in the Harris Falcon II range of military radios.

Citadel was presented for peer review on the IEEE MILCOM conference in 1998. In 2004, it was succeeded by Citadel II, which offers improved cipher security and is claimed to use 256-bit keys. Note that Citadel was not used by the US Government and by the US Army. Instead, the US (and some of its allies) use the Sierra and Sierra II cryptographic engines, which provide several Type 1 algorithms. Sierra and Sierra II are backward compatible with Citadel and Citadel II respectively.

 Read Citadel datasheet

  1. Type 1 refers to NSA endorsed classified or controlled cryptographic items (CCI) that are approved for the transmission of Classified or Sensitive US Government Information (TOP SECRET, or TS). The fact that no export licence is required for Citadel, means that it is a Type 4 device.  More
  2. 20 years of fielded service and 30 years of legacy.

Harris Citadel cryptographic engine
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Harris Citadel cryptographic engine

Products on this website that use Citadel
RF-5811 secure voice and data unit
Other products that use Citadel
  • RF-5800V
    Falcon II VHF manpack radio
  • RF-5800H
    Falcon II HF manpack radio
  • RF-5800M
    Falcon II multi-band manpack radio
  • RF-5800V-HH
    Falcon II VHF handheld radio
  • RF-5800M-HH
    Falcon II multi-band handheld radio
  • AN/PRC-152(C)
    Falcon III multiband handheld radio
  • Half-duplex encryption/decryption
  • Up to 5 Mbps
  • Serial or parallel clear and encrypted data
  • Key management support
  • Non-Type 1 encryption
  • Configurable key lengths (64 to 128 bits)
  • Secure against differential and linear cryptanalysis
  • Third party verified
  1. Standard Citadel high-grade algorithm
  2. Harris-configured Citadel algorithm (customer unique)
  3. Customer-configurable Citadel algorithm
  • Block Cipher Feedback
  • Self-synchronizing Cipher Feedback (autoclave)
  • Long Cycle or Minimum Error Propagation
  • Codebook (key processing only)
  • Traffic Encryption Key (TEK), minimum length 1.8·1019 (64 bits)
  • Key Encryption Key (KEK), minimum length 1.8·1019 (64 bits)
  • On-chip key storage for KEKs anbd TEKs
  • Key wrapping/unwrapping
  • Key updating
  • Deterministic key generation
  • Non-deterministic key generation
  • Power
    3.3 or 5V
  • Parallel
    8- or 16-bit
  • Serial traffic
    Seperate plaintext and ciphertext ports
  • Serial fill
    Key loading (red)
  • Serial EEPROM
    User configurable
  • Housekeeping
  • Test port
    Test Access Port (TAP) for Boundary Scan
  • 80-pin TQFP (16 x 16 mm
  • 3.3V and 5V CMOS compatible signal levels
  • Temperature -40°C tp +85°C
  • Speed up to 5 Mbps
  1. Harris Citadel Cryptographic Engine, product leaflet
    DS-206G. Harris Corporation November 2015.
  1. Michael Kurdziel, Military threat model and cryptographic response MK-128
    Presentation, February 2009.
Further information
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Crypto Museum. Created: Friday 08 November 2019. Last changed: Sunday, 22 January 2023 - 21:15 CET.
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