crypto/tls: implement (*ClientHelloInfo).SupportsCertificate

We'll also use this function for a better selection logic from
Config.Certificates in a later CL.

Updates #32426

Change-Id: Ie239574d02eb7fd2cf025ec36721c8c7e082d0bc
Reviewed-on: https://go-review.googlesource.com/c/go/+/205057
Reviewed-by: Katie Hockman <katie@golang.org>

common.go

@@ -7,7 +7,11 @@ package tls
 import (
 	"container/list"
 	"crypto"
+	"crypto/ecdsa"
+	"crypto/ed25519"
+	"crypto/elliptic"
 	"crypto/rand"
+	"crypto/rsa"
 	"crypto/sha512"
 	"crypto/x509"
 	"errors"
@@ -384,6 +388,10 @@ type ClientHelloInfo struct {
 	// from, or write to, this connection; that will cause the TLS
 	// connection to fail.
 	Conn net.Conn
+
+	// config is embedded by the GetCertificate or GetConfigForClient caller,
+	// for use with SupportsCertificate.
+	config *Config
 }
 
 // CertificateRequestInfo contains information from a server's
@@ -901,6 +909,167 @@ func (c *Config) getCertificate(clientHello *ClientHelloInfo) (*Certificate, err
 	return &c.Certificates[0], nil
 }
 
+// SupportsCertificate returns nil if the provided certificate is supported by
+// the client that sent the ClientHello. Otherwise, it returns an error
+// describing the reason for the incompatibility.
+//
+// If this ClientHelloInfo was passed to a GetConfigForClient or GetCertificate
+// callback, this method will take into account the associated Config. Note that
+// if GetConfigForClient returns a different Config, the change can't be
+// accounted for by this method.
+//
+// This function will call x509.ParseCertificate unless c.Leaf is set, which can
+// incur a significant performance cost.
+func (chi *ClientHelloInfo) SupportsCertificate(c *Certificate) error {
+	// Note we don't currently support certificate_authorities nor
+	// signature_algorithms_cert, and don't check the algorithms of the
+	// signatures on the chain (which anyway are a SHOULD, see RFC 8446,
+	// Section 4.4.2.2).
+
+	config := chi.config
+	if config == nil {
+		config = &Config{}
+	}
+	vers, ok := config.mutualVersion(chi.SupportedVersions)
+	if !ok {
+		return errors.New("no mutually supported protocol versions")
+	}
+
+	// If the client specified the name they are trying to connect to, the
+	// certificate needs to be valid for it.
+	if chi.ServerName != "" {
+		x509Cert, err := c.leaf()
+		if err != nil {
+			return fmt.Errorf("failed to parse certificate: %w", err)
+		}
+		if err := x509Cert.VerifyHostname(chi.ServerName); err != nil {
+			return fmt.Errorf("certificate is not valid for requested server name: %w", err)
+		}
+	}
+
+	// supportsRSAFallback returns nil if the certificate and connection support
+	// the static RSA key exchange, and unsupported otherwise. The logic for
+	// supporting static RSA is completely disjoint from the logic for
+	// supporting signed key exchanges, so we just check it as a fallback.
+	supportsRSAFallback := func(unsupported error) error {
+		// TLS 1.3 dropped support for the static RSA key exchange.
+		if vers == VersionTLS13 {
+			return unsupported
+		}
+		// The static RSA key exchange works by decrypting a challenge with the
+		// RSA private key, not by signing, so check the PrivateKey implements
+		// crypto.Decrypter, like *rsa.PrivateKey does.
+		if priv, ok := c.PrivateKey.(crypto.Decrypter); ok {
+			if _, ok := priv.Public().(*rsa.PublicKey); !ok {
+				return unsupported
+			}
+		} else {
+			return unsupported
+		}
+		// Finally, there needs to be a mutual cipher suite that uses the static
+		// RSA key exchange instead of ECDHE.
+		rsaCipherSuite := selectCipherSuite(chi.CipherSuites, config.cipherSuites(), func(c *cipherSuite) bool {
+			if c.flags&suiteECDHE != 0 {
+				return false
+			}
+			if vers < VersionTLS12 && c.flags&suiteTLS12 != 0 {
+				return false
+			}
+			return true
+		})
+		if rsaCipherSuite == nil {
+			return unsupported
+		}
+		return nil
+	}
+
+	// If the client sent the signature_algorithms extension, ensure it supports
+	// schemes we can use with this certificate and TLS version.
+	if len(chi.SignatureSchemes) > 0 {
+		if _, err := selectSignatureScheme(vers, c, chi.SignatureSchemes); err != nil {
+			return supportsRSAFallback(err)
+		}
+	}
+
+	// In TLS 1.3 we are done because supported_groups is only relevant to the
+	// ECDHE computation, point format negotiation is removed, cipher suites are
+	// only relevant to the AEAD choice, and static RSA does not exist.
+	if vers == VersionTLS13 {
+		return nil
+	}
+
+	// The only signed key exchange we support is ECDHE.
+	if !supportsECDHE(config, chi.SupportedCurves, chi.SupportedPoints) {
+		return supportsRSAFallback(errors.New("client doesn't support ECDHE, can only use legacy RSA key exchange"))
+	}
+
+	var ecdsaCipherSuite bool
+	if priv, ok := c.PrivateKey.(crypto.Signer); ok {
+		switch pub := priv.Public().(type) {
+		case *ecdsa.PublicKey:
+			var curve CurveID
+			switch pub.Curve {
+			case elliptic.P256():
+				curve = CurveP256
+			case elliptic.P384():
+				curve = CurveP384
+			case elliptic.P521():
+				curve = CurveP521
+			default:
+				return supportsRSAFallback(unsupportedCertificateError(c))
+			}
+			var curveOk bool
+			for _, c := range chi.SupportedCurves {
+				if c == curve && config.supportsCurve(c) {
+					curveOk = true
+					break
+				}
+			}
+			if !curveOk {
+				return errors.New("client doesn't support certificate curve")
+			}
+			ecdsaCipherSuite = true
+		case ed25519.PublicKey:
+			if vers < VersionTLS12 || len(chi.SignatureSchemes) == 0 {
+				return errors.New("connection doesn't support Ed25519")
+			}
+			ecdsaCipherSuite = true
+		case *rsa.PublicKey:
+		default:
+			return supportsRSAFallback(unsupportedCertificateError(c))
+		}
+	} else {
+		return supportsRSAFallback(unsupportedCertificateError(c))
+	}
+
+	// Make sure that there is a mutually supported cipher suite that works with
+	// this certificate. Cipher suite selection will then apply the logic in
+	// reverse to pick it. See also serverHandshakeState.cipherSuiteOk.
+	cipherSuite := selectCipherSuite(chi.CipherSuites, config.cipherSuites(), func(c *cipherSuite) bool {
+		if c.flags&suiteECDHE == 0 {
+			return false
+		}
+		if c.flags&suiteECSign != 0 {
+			if !ecdsaCipherSuite {
+				return false
+			}
+		} else {
+			if ecdsaCipherSuite {
+				return false
+			}
+		}
+		if vers < VersionTLS12 && c.flags&suiteTLS12 != 0 {
+			return false
+		}
+		return true
+	})
+	if cipherSuite == nil {
+		return supportsRSAFallback(errors.New("client doesn't support any cipher suites compatible with the certificate"))
+	}
+
+	return nil
+}
+
 // BuildNameToCertificate parses c.Certificates and builds c.NameToCertificate
 // from the CommonName and SubjectAlternateName fields of each of the leaf
 // certificates.