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DVP   Vulcan
Digital Voice Protection

Digital Voice Protection, abbreviated DVP, 1 is a proprietary self-synchronising algorithm for the encryption and decryption of voice communication, developed in the 1970s by Motorola Inc. in Chicago (Illinois, USA). The algorithm was internally known at Motorola by the codename VULCAN [1]. The name 'DVP' refers to both the VULCAN algorithm and the equipment that incorporates it.

DVP (VULCAN) typically works with a 12 kb/s bitstream from a continuous variable slope delta modulation (CVSD) digitiser. The cipher is basically a Ciphertext Auto Key (CTAK), also known as Cipher Feedback (CFB) or autoclave. Although it has nonlinear properties, the shift-register-based algorithm is basically a linear one. Encryption and decryption takes place in a custom chip marked SC76807. For encoding and decoding CVSD, Motorola used a custom CMOS integrated circuit (IC), codenamed BUTTERSCOTCH [1].

DVP was succeeded in the late 1980s by DVP-XL, which is not based on the VULCAN algorithm. Instead it uses the improved LINUS algorithm plus a clever technique — internally known as REX 2 — which converts the 1-bit CTAK block cipher into a kind of counter addressing mode. The addition of REX — denoted by the suffix 'XL' — makes the device less prone to error propagation, especially near the edge of the device's range. It is described in US Patent 4,893,339 [3].

  1. Registered trademark of Motorola Inc.
  2. REX is a diminutive of Range Extension.

DVP equipment on this website
Motorola MX-300 portable radio with optional DVP or DES encryption
Motorola Saber II secure portable radio
Motorola Key Variable Loader for DVP
In 2014, Cornelius, Jenkins and Riddler reverse engineered the SC76807 custom-developed cryptographic chip and the firmware of the T-3010 key loader, and came to the conclusion that the algorithm is notoriously weak, even by the standards of the 1970s. They managed to simulate the algorithm in software and proved that it could be broken in realtime with simple methods. It seems likely that the algorithm was deliberately weakened in oder to obtain an export licence.

 Description and analysis of the VULCAN algorithm

  1. Cornelius Jenkins Riddler, Vulcan: A Proprietary Cipher of the 1970s
    September 2014. Obtained via Cryptome.

  2. US Patent 4,167,700 - Digital voice protection system and method
    Description of Motorola's proprietary encryption algorithm DVP. Filed 2 May 1977.

  3. US Patent 4,893,339 - Secure Communication System
    Michael Bright et al. Filed 30 September 1986.
Further information
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Crypto Museum. Created: Monday 22 August 2022. Last changed: Monday, 22 August 2022 - 14:03 CET.
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