crypto/tls: disable RSA-PSS in TLS 1.2

Most of the issues that led to the decision on #30055 were related to
incompatibility with or faulty support for RSA-PSS (#29831, #29779,
v1.5 signatures). RSA-PSS is required by TLS 1.3, but is also available
to be negotiated in TLS 1.2.

Altering TLS 1.2 behavior based on GODEBUG=tls13=1 feels surprising, so
just disable RSA-PSS entirely in TLS 1.2 until TLS 1.3 is on by default,
so breakage happens all at once.

Updates #30055

Change-Id: Iee90454a20ded8895e5302e8bcbcd32e4e3031c2
Reviewed-on: https://go-review.googlesource.com/c/160998
Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/crypto/tls/key_agreement.go b/src/crypto/tls/key_agreement.go
index 628e578e48..05fe77b3e2 100644
--- a/src/crypto/tls/key_agreement.go
+++ b/src/crypto/tls/key_agreement.go
@@ -177,7 +177,7 @@ NextCandidate:
 		return nil, errors.New("tls: certificate private key does not implement crypto.Signer")
 	}
 
-	signatureAlgorithm, sigType, hashFunc, err := pickSignatureAlgorithm(priv.Public(), clientHello.supportedSignatureAlgorithms, supportedSignatureAlgorithms, ka.version)
+	signatureAlgorithm, sigType, hashFunc, err := pickSignatureAlgorithm(priv.Public(), clientHello.supportedSignatureAlgorithms, supportedSignatureAlgorithmsTLS12, ka.version)
 	if err != nil {
 		return nil, err
 	}