RFC 7635, "Session Traversal Utilities for NAT (STUN) Extension for Third-Party Authorization", August 2015

Errata ID: 5059
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Taylor Brandstetter
Date Reported: 2017-07-05

Section 6.2 says:

   key_length:  Length of the session key in octets.  The key length of
      160 bits MUST be supported (i.e., only the 160-bit key is used by
      HMAC-SHA-1 for message integrity of STUN messages).  The key
      length facilitates the hash agility plan discussed in Section 16.3
      of [RFC5389].

It should say:

   key_length:  Length of the session key in octets.

Notes:

RFC2104 section 2 states:

The authentication key K can be of any length up to B, the
block length of the hash function. Applications that use keys longer
than B bytes will first hash the key using H and then use the
resultant L byte string as the actual key to HMAC.

Meaning any key length is allowed. The fact that the hash output is 20 bytes doesn't mean the key needs to be 20 bytes as well.

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