# Woo–Lam

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In cryptography, Woo–Lam refers to various computer network authentication protocols designed by Simon S. Lam and Thomas Woo.[1][2] 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.[3]

## 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

$1) A \rightarrow KDC : ID_A || ID_B$
$2) KDC \rightarrow : S_{KR_{KDC}}[ID_B||KU_B]$
$3) A \rightarrow B : E_{KU_B}[N_A||ID_A]$
$4) B \rightarrow KDC: ID_B||ID_A||E_{KU_{KDC}}[N_A]$
$5) KDC \rightarrow B : S_{KR_{KDC}}[ID_A||KU_A]||E_{KU_B}[S_{KR_{KDC}}[N_A||K||ID_B||ID_A]]$
$6) B \rightarrow A : E_{KU_A}[S_{KR_{KDC}}[N_A || K || ID_A || ID_B ] || N_B]]$
$7) A \rightarrow B : E_{K}[N_B]$

The original version of the protocol[4] 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.[1][3]

## References

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