In draft-ietf-tls-tls13-20 Appendix B we find that:
This section describes protocol types and constants. Values listed
as _RESERVED were used in previous versions of TLS and are listed
here for completeness. TLS 1.3 implementations MUST NOT send them
but might receive them from older TLS implementations.
Similarly, in section 4.2.3 we see:
Legacy algorithms Indicates algorithms which are being deprecated
because they use algorithms with known weaknesses, specifically
SHA-1 which is used in this context with either with RSA using
RSASSA-PKCS1-v1_5 or ECDSA. These values refer solely to
signatures which appear in certificates (see Section 4.4.2.2) and
are not defined for use in signed TLS handshake messages.
Endpoints SHOULD NOT negotiate these algorithms but are permitted
to do so solely for backward compatibility. Clients offering
these values MUST list them as the lowest priority (listed after
all other algorithms in SignatureSchemeList). TLS 1.3 servers
MUST NOT offer a SHA-1 signed certificate unless no valid
certificate chain can be produced without it (see
Section 4.4.2.2).
However, we are currently sending the SHA2-based DSA signature schemes
and many SHA1-based schemes, which is in contradiction with the specification.
Because TLS 1.3 support will appear in OpenSSL 1.1, we are bound by
stability requirements to continue to offer the DSA signature schemes
and the deprecated hash algorithms. at least until OpenSSL 1.2.
However, for pure TLS 1.3 clients that do not offer lower TLS versions,
we can be compliant. Do so, and leave a note to revisit the issue when
we are permitted to break with sacred historical tradition.
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3326)
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