Woo–Lam Explained

In cryptography, Woo–Lam refers to various computer network authentication protocols designed by Simon S. Lam and Thomas Woo.^[1] The protocols enable two communicating parties to authenticate each other's identity and to exchange session keys, and involve the use of a trusted key distribution center (KDC) to negotiate between the parties. Both symmetric-key and public-key variants have been described. However, the protocols suffer from various security flaws, and in part have been described as being inefficient compared to alternative authentication protocols.^[2]

Public-key protocol

Notation

The following notation is used to describe the algorithm:

A,B - network nodes.KU _{x} - public key of node x .KR _{x} - private key of x .N _{x} - nonce chosen by x .ID _{x} - unique identifier of x .E _{k} - public-key encryption using key k .S _{k} - digital signature using key k .K - random session key chosen by the KDC.|| - concatenation.

It is assumed that all parties know the KDC's public key.

Message exchange

1 ) A → KDC : ID _{A} || ID _{B} 2 ) KDC → A : S _{KR _{KDC}} [ID _{B||KU} _{B]} 3 ) A → B : E _{KU _{B}} [N _{A||ID} _{A]} 4 ) B → KDC: ID _{B||ID} _{A||E} ^{KU _{KDC}} [N _{A]} 5 ) KDC → B : S _{KR _{KDC}} [ID _{A||KU} _{A]||E} ^{KU _{B}} [S _{KR _{KDC}} [N _{A||K||ID} _{B||ID} _{A]]} 6 ) B → A : E _{KU _{A}} [S _{KR _{KDC}} [N _{A} || K ] || N _{B]} 7 ) A → B : E _{K} [N _{B]}

The original version of the protocol^[3] had the identifier

ID _{A} omitted from lines 5 and 6, which did not account for the fact that N _{A} is unique only among nonces generated by A and not by other parties. The protocol was revised after the authors themselves spotted a flaw in the algorithm.^{[4] [2]}

See also

• Kerberos
• Needham–Schroeder protocol
• Otway–Rees protocol

Notes and References

1. Book: Colin Boyd . Anish Mathuria . Protocols for authentication and key establishment. limited. 2003. Springer. 978-3-540-43107-7. 78 and 99.
2. Book: Stallings, William. Cryptography and Network Security Principles and Practices, Fourth Edition. 2005. Prentice Hall. 978-0-13-187316-2. 387.
3. Thomas Y.C. Woo . Simon S. Lam . Authentication for Distributed Systems. Computer. January 1992. 25. 1. 39–52. 10.1109/2.108052. 10.1.1.38.9374.
4. T.Y.C. Woo . S.S. Lam . Authentication Revisited. Computer. March 1992. 25. 3. 10. 10.1109/2.121502.