Method generateServerKeyExchange

key_agreement.go:168–255

(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg)

}

func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
	var curveID CurveID
	for _, c := range clientHello.supportedCurves {
		if config.supportsCurve(c) {
			curveID = c
			break
		}
	}

	if curveID == 0 {
		return nil, errors.New("tls: no supported elliptic curves offered")
	}
	if _, ok := curveForCurveID(curveID); curveID != X25519 && !ok {
		return nil, errors.New("tls: CurvePreferences includes unsupported curve")
	}

	params, err := generateECDHEParameters(config.rand(), curveID)
	if err != nil {
		return nil, err
	}
	ka.params = params

	// See RFC 4492, Section 5.4.
	ecdhePublic := params.PublicKey()
	serverECDHEParams := make([]byte, 1+2+1+len(ecdhePublic))
	serverECDHEParams[0] = 3 // named curve
	serverECDHEParams[1] = byte(curveID >> 8)
	serverECDHEParams[2] = byte(curveID)
	serverECDHEParams[3] = byte(len(ecdhePublic))
	copy(serverECDHEParams[4:], ecdhePublic)

	priv, ok := cert.PrivateKey.(crypto.Signer)
	if !ok {
		return nil, fmt.Errorf("tls: certificate private key of type %T does not implement crypto.Signer", cert.PrivateKey)
	}

	var signatureAlgorithm SignatureScheme
	var sigType uint8
	var sigHash crypto.Hash
	if ka.version >= VersionTLS12 {
		signatureAlgorithm, err = selectSignatureScheme(ka.version, cert, clientHello.supportedSignatureAlgorithms)
		if err != nil {
			return nil, err
		}
		sigType, sigHash, err = typeAndHashFromSignatureScheme(signatureAlgorithm)
		if err != nil {
			return nil, err
		}
	} else {
		sigType, sigHash, err = legacyTypeAndHashFromPublicKey(priv.Public())
		if err != nil {
			return nil, err
		}
	}
	if (sigType == signaturePKCS1v15 || sigType == signatureRSAPSS) != ka.isRSA {
		return nil, errors.New("tls: certificate cannot be used with the selected cipher suite")
	}