crypto/tls: refactor certificate and signature algorithm logic
This refactors a lot of the certificate support logic to make it cleaner and reusable where possible. These changes will make the following CLs much simpler. In particular, the heavily overloaded pickSignatureAlgorithm is gone. That function used to cover both signing and verifying side, would work both for pre-signature_algorithms TLS 1.0/1.1 and TLS 1.2, and returned sigalg, type and hash. Now, TLS 1.0/1.1 and 1.2 are differentiated at the caller, as they have effectively completely different logic. TLS 1.0/1.1 simply use legacyTypeAndHashFromPublicKey as they employ a fixed hash function and signature algorithm for each public key type. TLS 1.2 is instead routed through selectSignatureScheme (on the signing side) or isSupportedSignatureAlgorithm (on the verifying side) and typeAndHashFromSignatureScheme, like TLS 1.3. On the signing side, signatureSchemesForCertificate was already version aware (for PKCS#1 v1.5 vs PSS support), so selectSignatureScheme just had to learn the Section 7.4.1.4.1 defaults for a missing signature_algorithms to replace pickSignatureAlgorithm. On the verifying side, pickSignatureAlgorithm was also checking the public key type, while isSupportedSignatureAlgorithm + typeAndHashFromSignatureScheme are not, but that check was redundant with the one in verifyHandshakeSignature. There should be no major change in behavior so far. A few minor changes came from the refactor: we now correctly require signature_algorithms in TLS 1.3 when using a certificate; we won't use Ed25519 in TLS 1.2 if the client didn't send signature_algorithms; and we don't send ec_points_format in the ServerHello (a compatibility measure) if we are not doing ECDHE anyway because there are no mutually supported curves. The tests also got simpler because they test simpler functions. The caller logic switching between TLS 1.0/1.1 and 1.2 is tested by the transcript tests. Updates #32426 Change-Id: Ice9dcaea78d204718f661f8d60efdb408ba41577 Reviewed-on: https://go-review.googlesource.com/c/go/+/205061Reviewed-by: Katie Hockman <katie@golang.org>
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