RSA Laboratories

6.3.5 What are the important patents in cryptography?

Here is a selection of some of the important and well established patents in cryptography, including several expired patents of historical interest. The expiration date for patents used to be 17 years after issuing, but for outstandning patents as of June 8, 1995 (the day the United States ratified the GATT patent treaty), the expiration date is 17 years after the date of issue or 20 years after the date of filing, whichever is later. Today, the expiration date for U.S. patents is 20 years from filing, pursuant to the international standard.

U.S. Patent: 3,962,539
Filed: February 24, 1975
Issued: June 8, 1976
Inventors: Ehrsam et al.
Assignee: IBM

This patent covered the DES cipher and was placed in the public domain by IBM. It is now expired.

U.S. Patent: 4,200,770
Filed: September 6, 1977
Issued: April 29, 1980
Inventors: Hellman, Diffie, and Merkle
Assignee: Stanford University

This is the first patent covering a public-key cryptosystem. It describes Diffie-Hellman key agreement, as well as a means of authentication using long-term Diffie-Hellman public keys. This patent is now expired.

Public-key cryptosystems
U.S. Patent: 4,218,582
Filed: October 6, 1977
Issued: August 19, 1980
Inventors: Hellman and Merkle
Assignee: Stanford University

The Hellman-Merkle patent covers public-key systems based on the knapsack problem and now known to be insecure. Its broader claims cover general methods of public-key encryption and digital signatures using public keys. This patent is expired.

U.S. Patent: 4,405,829
Filed: December 14, 1977
Issued: September 20, 1983
Inventors: Rivest, Shamir, and Adleman
Assignee: MIT

This patent describes the RSA public-key cryptosystem as used for both encryption and signing. It served as the basis for the founding of RSADSI.

Fiat-Shamir identification
U.S. Patent: 4,748,668
Filed: July 9, 1986
Issued: May 31, 1988
Inventors: Shamir and Fiat
Assignee: Yeda Research and Development (Israel)

This patent describes the Fiat-Shamir identification scheme.

Control vectors
U.S. Patent: 4,850,017
Filed: May 29, 1987
Issued: July 18, 1989
Inventors: Matyas, Meyer, and Brachtl
Assignee: IBM

Patent 4,850,017 is the most prominent among a number describing the use of control vectors for key management. This patent describes a method enabling a description of privileges to be bound to a cryptographic key, serving as a deterrent to the key's misuse.

GQ identification
U.S. Patent: 5,140,634
Filed: October 9, 1991
Issued: August 18, 1992
Inventors: Guillou and Quisquater
Assignee: U.S. Phillips Corporation

This patent describes the GQ identification scheme.

U.S. Patent: 5,214,703
Filed: January 7, 1992
Issued: May 25, 1993
Inventors: Lai and Massey
Assignee: Ascom Tech AG (Switzerland)

Patent 5,214,703 covers the IDEA block cipher, an alternative to DES that employs 128-bit keys.

U.S. Patent: 5,231,668
Filed: July 26, 1991
Issued: July 27, 1993
Inventor: Kravitz
Assignee: United States of America

This patent covers the Digital Signature Algorithm (DSA), the algorithm specified in the Digital Signature Standard (DSS) of the U.S. National Institute of Standards (NIST).

Fair cryptosystems
U.S. Patent: 5,315,658
Filed: April 19, 1993
Issued: May 24, 1994
Inventor: Micali
Assignee: none

This patent covers systems in which keys are held in escrow among multiple trustees, only a specified quorum of which can reconstruct these keys.

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