RFC 2631, "Diffie-Hellman Key Agreement Method", June 1999

Errata ID: 5954
Status: Held for Document Update
Type: Technical
Publication Format(s) : TEXT
Reported By: Paul Janson
Date Reported: 2020-01-02
Held for Document Update by: Benjamin Kaduk
Date Held: 2020-01-13

Section 2.1.5. says:

     1. Verify that y lies within the interval [2,p-1]. If it does not,
        the key is invalid.
     2. Compute y^q mod p. If the result == 1, the key is valid.
        Otherwise the key is invalid.

It should say:

     1. Verify that y lies within the interval [2,p-1]. If it does not,
        the key is invalid.
     2. Compute y^q mod p. If the result == 1, the key is valid.
        Otherwise the key is invalid.
     3. Verify that y does not match g.

Notes:

Validating that (g == received y) needs to be an additional exclusion to the valid range [2,p-1]. If party 'a' accepts received public key 'yb' matching 'g', then ZZ matches public key 'ya'. i.e. if yb = 2, then xb = 1, therefore ZZ = ya^1 = ya

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