RFC 5280, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", May 2008

Errata ID: 6414
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Rob Stradling
Date Reported: 2021-01-28

Section 4.2.1.12 says:

   id-kp-serverAuth             OBJECT IDENTIFIER ::= { id-kp 1 }
   -- TLS WWW server authentication
   -- Key usage bits that may be consistent: digitalSignature,
   -- keyEncipherment or keyAgreement

It should say:

   id-kp-serverAuth             OBJECT IDENTIFIER ::= { id-kp 1 }
   -- TLS WWW server authentication
   -- Key usage bits that may be consistent: digitalSignature
   -- and/or (keyEncipherment or keyAgreement)

Notes:

In https://github.com/zmap/zlint/issues/553 there's been some disagreement and confusion about how to correctly interpret the "or" in the Original Text. "You can only set one of these three bits" is one interpretation, and it's hard to argue that this interpretation is inconsistent with the Original Text.

However, digitalSignature+keyEncipherment makes sense for an RSA leaf certificate, and digitalSignature+keyAgreement makes sense for an ECC leaf certificate. Both are widely used, to enable ephemeral and non-ephemeral TLS ciphersuites in conjunction with a single server certificate.

Given that RFC5480 section 3 explicitly permits digitalSignature+keyAgreement in an ECC leaf certificate, I think it's likely that my proposed Corrected Text conveys the RFC5280 authors' intended meaning.

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