Clarify comments and API behaviour for equal-preference for TLS 1.3 (#2873)

The algorithm used to compute the negotiated cipher suite in TLS 1.3 is different than the one used for the older versions. Clarify what this means for equal-preference groups.

Author: torben-hansen

include/openssl/ssl.h

@@ -1769,8 +1769,8 @@ OPENSSL_EXPORT size_t SSL_get_all_standard_cipher_names(const char **out,
 // The |DEFAULT| directive, when appearing at the front of the string, expands
 // to the default ordering of available ciphers.
 //
-// If configuring a server, one may also configure equal-preference groups to
-// partially respect the client's preferences when
+// For TLS < 1.3, if configuring a server, one may also configure
+// equal-preference groups to partially respect the client's preferences when
 // |SSL_OP_CIPHER_SERVER_PREFERENCE| is enabled. Ciphers in an equal-preference
 // group have equal priority and use the client order. This may be used to
 // enforce that AEADs are preferred but select AES-GCM vs. ChaCha20-Poly1305
@@ -1781,10 +1781,6 @@ OPENSSL_EXPORT size_t SSL_get_all_standard_cipher_names(const char **out,
 //
 // Once an equal-preference group is used, future directives must be
 // opcode-less. Inside an equal-preference group, spaces are not allowed.
-//
-// Note: TLS 1.3 ciphersuites are only configurable via
-// |SSL_CTX_set_ciphersuites| or |SSL_set_ciphersuites|. Other setter functions have
-// no impact on TLS 1.3 ciphersuites.
 
 // SSL_DEFAULT_CIPHER_LIST is the default cipher suite configuration. It is
 // substituted when a cipher string starts with 'DEFAULT'.
@@ -1843,6 +1839,8 @@ OPENSSL_EXPORT int SSL_set_ciphersuites(SSL *ssl, const char *str);
 // |ssl->config->tls13_cipher_list| and otherwise falling back to
 // |ssl->ctx->tls13_cipher_list|.
 //
+// Equal-preference groups cannot be configured for TLS 1.3 through these APIs.
+//
 // It returns one on success and zero on failure.
 //
 // Prefer to use |SSL_set_strict_cipher_list|. This function tolerates garbage

ssl/internal.h

@@ -668,7 +668,9 @@ BSSL_NAMESPACE_BEGIN
 // picks the cipher and groups cannot be expressed on the wire. However, for
 // servers, the equal-preference groups allow the client's preferences to be
 // partially respected. (This only has an effect with
-// SSL_OP_CIPHER_SERVER_PREFERENCE).
+// SSL_OP_CIPHER_SERVER_PREFERENCE). Note that the method used to compute the
+// negotiated cipher suite in TLS 1.3 does not use the equal-preference
+// data structure described below.
 //
 // The equal-preference groups are expressed by grouping SSL_CIPHERs together.
 // All elements of a group have the same priority: no ordering is expressed