crypto/tls: Add support for ECDHE-ECDSA

Add support for ECDHE-ECDSA (RFC4492), which uses an ephemeral server
key pair to perform ECDH with ECDSA signatures. Like ECDHE-RSA,
ECDHE-ECDSA also provides PFS.

R=agl
CC=golang-dev
https://golang.org/cl/7006047

src/pkg/crypto/crypto.go

@@ -77,5 +77,8 @@ func RegisterHash(h Hash, f func() hash.Hash) {
 	hashes[h] = f
 }
 
+// PublicKey represents a public key using an unspecified algorithm.
+type PublicKey interface{}
+
 // PrivateKey represents a private key using an unspecified algorithm.
 type PrivateKey interface{}

src/pkg/crypto/tls/cipher_suites.go

@@ -55,8 +55,11 @@ var cipherSuites = []*cipherSuite{
 	// Ciphersuite order is chosen so that ECDHE comes before plain RSA
 	// and RC4 comes before AES (because of the Lucky13 attack).
 	{TLS_ECDHE_RSA_WITH_RC4_128_SHA, 16, 20, 0, ecdheRSAKA, true, cipherRC4, macSHA1},
+	{TLS_ECDHE_ECDSA_WITH_RC4_128_SHA, 16, 20, 0, ecdheECDSAKA, true, cipherRC4, macSHA1},
 	{TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA, 16, 20, 16, ecdheRSAKA, true, cipherAES, macSHA1},
+	{TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA, 16, 20, 16, ecdheECDSAKA, true, cipherAES, macSHA1},
 	{TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA, 32, 20, 16, ecdheRSAKA, true, cipherAES, macSHA1},
+	{TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA, 32, 20, 16, ecdheECDSAKA, true, cipherAES, macSHA1},
 	{TLS_RSA_WITH_RC4_128_SHA, 16, 20, 0, rsaKA, false, cipherRC4, macSHA1},
 	{TLS_RSA_WITH_AES_128_CBC_SHA, 16, 20, 16, rsaKA, false, cipherAES, macSHA1},
 	{TLS_RSA_WITH_AES_256_CBC_SHA, 32, 20, 16, rsaKA, false, cipherAES, macSHA1},
@@ -161,8 +164,16 @@ func rsaKA(version uint16) keyAgreement {
 	return rsaKeyAgreement{}
 }
 
+func ecdheECDSAKA(version uint16) keyAgreement {
+	return &ecdheKeyAgreement{
+		sigType: signatureECDSA,
+		version: version,
+	}
+}
+
 func ecdheRSAKA(version uint16) keyAgreement {
-	return &ecdheRSAKeyAgreement{
+	return &ecdheKeyAgreement{
+		sigType: signatureRSA,
 		version: version,
 	}
 }
@@ -186,12 +197,15 @@ func mutualCipherSuite(have []uint16, want uint16) *cipherSuite {
 // A list of the possible cipher suite ids. Taken from
 // http://www.iana.org/assignments/tls-parameters/tls-parameters.xml
 const (
-	TLS_RSA_WITH_RC4_128_SHA            uint16 = 0x0005
-	TLS_RSA_WITH_3DES_EDE_CBC_SHA       uint16 = 0x000a
-	TLS_RSA_WITH_AES_128_CBC_SHA        uint16 = 0x002f
-	TLS_RSA_WITH_AES_256_CBC_SHA        uint16 = 0x0035
-	TLS_ECDHE_RSA_WITH_RC4_128_SHA      uint16 = 0xc011
-	TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA uint16 = 0xc012
-	TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA  uint16 = 0xc013
-	TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA  uint16 = 0xc014
+	TLS_RSA_WITH_RC4_128_SHA             uint16 = 0x0005
+	TLS_RSA_WITH_3DES_EDE_CBC_SHA        uint16 = 0x000a
+	TLS_RSA_WITH_AES_128_CBC_SHA         uint16 = 0x002f
+	TLS_RSA_WITH_AES_256_CBC_SHA         uint16 = 0x0035
+	TLS_ECDHE_ECDSA_WITH_RC4_128_SHA     uint16 = 0xc007
+	TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA uint16 = 0xc009
+	TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA uint16 = 0xc00a
+	TLS_ECDHE_RSA_WITH_RC4_128_SHA       uint16 = 0xc011
+	TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA  uint16 = 0xc012
+	TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA   uint16 = 0xc013
+	TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA   uint16 = 0xc014
 )

src/pkg/crypto/tls/common.go

@@ -9,6 +9,7 @@ import (
 	"crypto/rand"
 	"crypto/x509"
 	"io"
+	"math/big"
 	"strings"
 	"sync"
 	"time"
@@ -98,6 +99,12 @@ const (
 	certTypeDSSSign    = 2 // A certificate containing a DSA key
 	certTypeRSAFixedDH = 3 // A certificate containing a static DH key
 	certTypeDSSFixedDH = 4 // A certificate containing a static DH key
+
+	// See RFC4492 sections 3 and 5.5.
+	certTypeECDSASign      = 64 // A certificate containing an ECDSA-capable public key, signed with ECDSA.
+	certTypeRSAFixedECDH   = 65 // A certificate containing an ECDH-capable public key, signed with RSA.
+	certTypeECDSAFixedECDH = 66 // A certificate containing an ECDH-capable public key, signed with ECDSA.
+
 	// Rest of these are reserved by the TLS spec
 )
 
@@ -120,10 +127,11 @@ type signatureAndHash struct {
 }
 
 // supportedSignatureAlgorithms contains the signature and hash algorithms that
-// the code will adverse as supported both in a TLS 1.2 ClientHello and
+// the code can advertise as supported both in a TLS 1.2 ClientHello and
 // CertificateRequest.
 var supportedSignatureAlgorithms = []signatureAndHash{
 	{hashSHA256, signatureRSA},
+	{hashSHA256, signatureECDSA},
 }
 
 // ConnectionState records basic TLS details about the connection.
@@ -372,7 +380,7 @@ func (c *Config) BuildNameToCertificate() {
 // A Certificate is a chain of one or more certificates, leaf first.
 type Certificate struct {
 	Certificate [][]byte
-	PrivateKey  crypto.PrivateKey // supported types: *rsa.PrivateKey
+	PrivateKey  crypto.PrivateKey // supported types: *rsa.PrivateKey, *ecdsa.PrivateKey
 	// OCSPStaple contains an optional OCSP response which will be served
 	// to clients that request it.
 	OCSPStaple []byte
@@ -395,6 +403,13 @@ type handshakeMessage interface {
 	unmarshal([]byte) bool
 }
 
+// TODO(jsing): Make these available to both crypto/x509 and crypto/tls.
+type dsaSignature struct {
+	R, S *big.Int
+}
+
+type ecdsaSignature dsaSignature
+
 var emptyConfig Config
 
 func defaultConfig() *Config {

src/pkg/crypto/tls/handshake_client.go

@@ -6,9 +6,11 @@ package tls
 
 import (
 	"bytes"
+	"crypto/ecdsa"
 	"crypto/rsa"
 	"crypto/subtle"
 	"crypto/x509"
+	"encoding/asn1"
 	"errors"
 	"io"
 	"strconv"
@@ -124,7 +126,10 @@ func (c *Conn) clientHandshake() error {
 		}
 	}
 
-	if _, ok := certs[0].PublicKey.(*rsa.PublicKey); !ok {
+	switch certs[0].PublicKey.(type) {
+	case *rsa.PublicKey, *ecdsa.PublicKey:
+		break
+	default:
 		return c.sendAlert(alertUnsupportedCertificate)
 	}
 
@@ -187,12 +192,13 @@ func (c *Conn) clientHandshake() error {
 
 		finishedHash.Write(certReq.marshal())
 
-		// For now, we only know how to sign challenges with RSA
-		rsaAvail := false
+		var rsaAvail, ecdsaAvail bool
 		for _, certType := range certReq.certificateTypes {
-			if certType == certTypeRSASign {
+			switch certType {
+			case certTypeRSASign:
 				rsaAvail = true
-				break
+			case certTypeECDSASign:
+				ecdsaAvail = true
 			}
 		}
 
@@ -201,7 +207,7 @@ func (c *Conn) clientHandshake() error {
 		// certReq.certificateAuthorities
 	findCert:
 		for i, chain := range c.config.Certificates {
-			if !rsaAvail {
+			if !rsaAvail && !ecdsaAvail {
 				continue
 			}
 
@@ -216,7 +222,10 @@ func (c *Conn) clientHandshake() error {
 					}
 				}
 
-				if x509Cert.PublicKeyAlgorithm != x509.RSA {
+				switch {
+				case rsaAvail && x509Cert.PublicKeyAlgorithm == x509.RSA:
+				case ecdsaAvail && x509Cert.PublicKeyAlgorithm == x509.ECDSA:
+				default:
 					continue findCert
 				}
 
@@ -271,9 +280,21 @@ func (c *Conn) clientHandshake() error {
 	}
 
 	if chainToSend != nil {
+		var signed []byte
 		certVerify := new(certificateVerifyMsg)
-		digest, hashFunc := finishedHash.hashForClientCertificate()
-		signed, err := rsa.SignPKCS1v15(c.config.rand(), c.config.Certificates[0].PrivateKey.(*rsa.PrivateKey), hashFunc, digest)
+		switch key := c.config.Certificates[0].PrivateKey.(type) {
+		case *ecdsa.PrivateKey:
+			digest, _ := finishedHash.hashForClientCertificate(signatureECDSA)
+			r, s, err := ecdsa.Sign(c.config.rand(), key, digest)
+			if err == nil {
+				signed, err = asn1.Marshal(ecdsaSignature{r, s})
+			}
+		case *rsa.PrivateKey:
+			digest, hashFunc := finishedHash.hashForClientCertificate(signatureRSA)
+			signed, err = rsa.SignPKCS1v15(c.config.rand(), key, hashFunc, digest)
+		default:
+			err = errors.New("unknown private key type")
+		}
 		if err != nil {
 			return c.sendAlert(alertInternalError)
 		}

src/pkg/crypto/tls/handshake_server.go

@@ -5,9 +5,12 @@
 package tls
 
 import (
+	"crypto"
+	"crypto/ecdsa"
 	"crypto/rsa"
 	"crypto/subtle"
 	"crypto/x509"
+	"encoding/asn1"
 	"errors"
 	"io"
 )
@@ -305,7 +308,10 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 	if config.ClientAuth >= RequestClientCert {
 		// Request a client certificate
 		certReq := new(certificateRequestMsg)
-		certReq.certificateTypes = []byte{certTypeRSASign}
+		certReq.certificateTypes = []byte{
+			byte(certTypeRSASign),
+			byte(certTypeECDSASign),
+		}
 		if c.vers >= VersionTLS12 {
 			certReq.hasSignatureAndHash = true
 			certReq.signatureAndHashes = supportedSignatureAlgorithms
@@ -327,7 +333,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 	hs.finishedHash.Write(helloDone.marshal())
 	c.writeRecord(recordTypeHandshake, helloDone.marshal())
 
-	var pub *rsa.PublicKey // public key for client auth, if any
+	var pub crypto.PublicKey // public key for client auth, if any
 
 	msg, err := c.readHandshake()
 	if err != nil {
@@ -372,7 +378,7 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 
 	// If we received a client cert in response to our certificate request message,
 	// the client will send us a certificateVerifyMsg immediately after the
-	// clientKeyExchangeMsg.  This message is a MD5SHA1 digest of all preceding
+	// clientKeyExchangeMsg.  This message is a digest of all preceding
 	// handshake-layer messages that is signed using the private key corresponding
 	// to the client's certificate. This allows us to verify that the client is in
 	// possession of the private key of the certificate.
@@ -386,8 +392,25 @@ func (hs *serverHandshakeState) doFullHandshake() error {
 			return c.sendAlert(alertUnexpectedMessage)
 		}
 
-		digest, hashFunc := hs.finishedHash.hashForClientCertificate()
-		err = rsa.VerifyPKCS1v15(pub, hashFunc, digest, certVerify.signature)
+		switch key := pub.(type) {
+		case *ecdsa.PublicKey:
+			ecdsaSig := new(ecdsaSignature)
+			if _, err = asn1.Unmarshal(certVerify.signature, ecdsaSig); err != nil {
+				break
+			}
+			if ecdsaSig.R.Sign() <= 0 || ecdsaSig.S.Sign() <= 0 {
+				err = errors.New("ECDSA signature contained zero or negative values")
+				break
+			}
+			digest, _ := hs.finishedHash.hashForClientCertificate(signatureECDSA)
+			if !ecdsa.Verify(key, digest, ecdsaSig.R, ecdsaSig.S) {
+				err = errors.New("ECDSA verification failure")
+				break
+			}
+		case *rsa.PublicKey:
+			digest, hashFunc := hs.finishedHash.hashForClientCertificate(signatureRSA)
+			err = rsa.VerifyPKCS1v15(key, hashFunc, digest, certVerify.signature)
+		}
 		if err != nil {
 			c.sendAlert(alertBadCertificate)
 			return errors.New("could not validate signature of connection nonces: " + err.Error())
@@ -507,7 +530,7 @@ func (hs *serverHandshakeState) sendFinished() error {
 // processCertsFromClient takes a chain of client certificates either from a
 // Certificates message or from a sessionState and verifies them. It returns
 // the public key of the leaf certificate.
-func (hs *serverHandshakeState) processCertsFromClient(certificates [][]byte) (*rsa.PublicKey, error) {
+func (hs *serverHandshakeState) processCertsFromClient(certificates [][]byte) (crypto.PublicKey, error) {
 	c := hs.c
 
 	hs.certsFromClient = certificates
@@ -554,8 +577,11 @@ func (hs *serverHandshakeState) processCertsFromClient(certificates [][]byte) (*
 	}
 
 	if len(certs) > 0 {
-		pub, ok := certs[0].PublicKey.(*rsa.PublicKey)
-		if !ok {
+		var pub crypto.PublicKey
+		switch key := certs[0].PublicKey.(type) {
+		case *ecdsa.PublicKey, *rsa.PublicKey:
+			pub = key
+		default:
 			return nil, c.sendAlert(alertUnsupportedCertificate)
 		}
 		c.peerCertificates = certs

src/pkg/crypto/tls/key_agreement.go

@@ -6,12 +6,14 @@ package tls
 
 import (
 	"crypto"
+	"crypto/ecdsa"
 	"crypto/elliptic"
 	"crypto/md5"
 	"crypto/rsa"
 	"crypto/sha1"
 	"crypto/sha256"
 	"crypto/x509"
+	"encoding/asn1"
 	"errors"
 	"io"
 	"math/big"
@@ -83,6 +85,15 @@ func (ka rsaKeyAgreement) generateClientKeyExchange(config *Config, clientHello
 	return preMasterSecret, ckx, nil
 }
 
+// sha1Hash calculates a SHA1 hash over the given byte slices.
+func sha1Hash(slices [][]byte) []byte {
+	hsha1 := sha1.New()
+	for _, slice := range slices {
+		hsha1.Write(slice)
+	}
+	return hsha1.Sum(nil)
+}
+
 // md5SHA1Hash implements TLS 1.0's hybrid hash function which consists of the
 // concatenation of an MD5 and SHA1 hash.
 func md5SHA1Hash(slices [][]byte) []byte {
@@ -92,12 +103,7 @@ func md5SHA1Hash(slices [][]byte) []byte {
 		hmd5.Write(slice)
 	}
 	copy(md5sha1, hmd5.Sum(nil))
-
-	hsha1 := sha1.New()
-	for _, slice := range slices {
-		hsha1.Write(slice)
-	}
-	copy(md5sha1[md5.Size:], hsha1.Sum(nil))
+	copy(md5sha1[md5.Size:], sha1Hash(slices))
 	return md5sha1
 }
 
@@ -112,24 +118,29 @@ func sha256Hash(slices [][]byte) []byte {
 
 // hashForServerKeyExchange hashes the given slices and returns their digest
 // and the identifier of the hash function used.
-func hashForServerKeyExchange(version uint16, slices ...[]byte) ([]byte, crypto.Hash) {
+func hashForServerKeyExchange(sigType uint8, version uint16, slices ...[]byte) ([]byte, crypto.Hash) {
 	if version >= VersionTLS12 {
 		return sha256Hash(slices), crypto.SHA256
 	}
+	if sigType == signatureECDSA {
+		return sha1Hash(slices), crypto.SHA1
+	}
 	return md5SHA1Hash(slices), crypto.MD5SHA1
 }
 
 // ecdheRSAKeyAgreement implements a TLS key agreement where the server
 // generates a ephemeral EC public/private key pair and signs it. The
-// pre-master secret is then calculated using ECDH.
-type ecdheRSAKeyAgreement struct {
+// pre-master secret is then calculated using ECDH. The signature may
+// either be ECDSA or RSA.
+type ecdheKeyAgreement struct {
 	version    uint16
+	sigType    uint8
 	privateKey []byte
 	curve      elliptic.Curve
 	x, y       *big.Int
 }
 
-func (ka *ecdheRSAKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
+func (ka *ecdheKeyAgreement) generateServerKeyExchange(config *Config, cert *Certificate, clientHello *clientHelloMsg, hello *serverHelloMsg) (*serverKeyExchangeMsg, error) {
 	var curveid uint16
 
 Curve:
@@ -170,10 +181,30 @@ Curve:
 	serverECDHParams[3] = byte(len(ecdhePublic))
 	copy(serverECDHParams[4:], ecdhePublic)
 
-	digest, hashFunc := hashForServerKeyExchange(ka.version, clientHello.random, hello.random, serverECDHParams)
-	sig, err := rsa.SignPKCS1v15(config.rand(), cert.PrivateKey.(*rsa.PrivateKey), hashFunc, digest)
-	if err != nil {
-		return nil, errors.New("failed to sign ECDHE parameters: " + err.Error())
+	digest, hashFunc := hashForServerKeyExchange(ka.sigType, ka.version, clientHello.random, hello.random, serverECDHParams)
+	var sig []byte
+	switch ka.sigType {
+	case signatureECDSA:
+		privKey, ok := cert.PrivateKey.(*ecdsa.PrivateKey)
+		if !ok {
+			return nil, errors.New("ECDHE ECDSA requires an ECDSA server private key")
+		}
+		r, s, err := ecdsa.Sign(config.rand(), privKey, digest)
+		if err != nil {
+			return nil, errors.New("failed to sign ECDHE parameters: " + err.Error())
+		}
+		sig, err = asn1.Marshal(ecdsaSignature{r, s})
+	case signatureRSA:
+		privKey, ok := cert.PrivateKey.(*rsa.PrivateKey)
+		if !ok {
+			return nil, errors.New("ECDHE RSA requires a RSA server private key")
+		}
+		sig, err = rsa.SignPKCS1v15(config.rand(), privKey, hashFunc, digest)
+		if err != nil {
+			return nil, errors.New("failed to sign ECDHE parameters: " + err.Error())
+		}
+	default:
+		return nil, errors.New("unknown ECDHE signature algorithm")
 	}
 
 	skx := new(serverKeyExchangeMsg)
@@ -186,7 +217,7 @@ Curve:
 	k := skx.key[len(serverECDHParams):]
 	if ka.version >= VersionTLS12 {
 		k[0] = hashSHA256
-		k[1] = signatureRSA
+		k[1] = ka.sigType
 		k = k[2:]
 	}
 	k[0] = byte(len(sig) >> 8)
@@ -196,7 +227,7 @@ Curve:
 	return skx, nil
 }
 
-func (ka *ecdheRSAKeyAgreement) processClientKeyExchange(config *Config, cert *Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
+func (ka *ecdheKeyAgreement) processClientKeyExchange(config *Config, cert *Certificate, ckx *clientKeyExchangeMsg, version uint16) ([]byte, error) {
 	if len(ckx.ciphertext) == 0 || int(ckx.ciphertext[0]) != len(ckx.ciphertext)-1 {
 		return nil, errors.New("bad ClientKeyExchange")
 	}
@@ -214,7 +245,7 @@ func (ka *ecdheRSAKeyAgreement) processClientKeyExchange(config *Config, cert *C
 
 var errServerKeyExchange = errors.New("invalid ServerKeyExchange")
 
-func (ka *ecdheRSAKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, cert *x509.Certificate, skx *serverKeyExchangeMsg) error {
+func (ka *ecdheKeyAgreement) processServerKeyExchange(config *Config, clientHello *clientHelloMsg, serverHello *serverHelloMsg, cert *x509.Certificate, skx *serverKeyExchangeMsg) error {
 	if len(skx.key) < 4 {
 		return errServerKeyExchange
 	}
@@ -261,11 +292,39 @@ func (ka *ecdheRSAKeyAgreement) processServerKeyExchange(config *Config, clientH
 	}
 	sig = sig[2:]
 
-	digest, hashFunc := hashForServerKeyExchange(ka.version, clientHello.random, serverHello.random, serverECDHParams)
-	return rsa.VerifyPKCS1v15(cert.PublicKey.(*rsa.PublicKey), hashFunc, digest, sig)
+	digest, hashFunc := hashForServerKeyExchange(ka.sigType, ka.version, clientHello.random, serverHello.random, serverECDHParams)
+	switch ka.sigType {
+	case signatureECDSA:
+		pubKey, ok := cert.PublicKey.(*ecdsa.PublicKey)
+		if !ok {
+			return errors.New("ECDHE ECDSA requires a ECDSA server public key")
+		}
+		ecdsaSig := new(ecdsaSignature)
+		if _, err := asn1.Unmarshal(sig, ecdsaSig); err != nil {
+			return err
+		}
+		if ecdsaSig.R.Sign() <= 0 || ecdsaSig.S.Sign() <= 0 {
+			return errors.New("ECDSA signature contained zero or negative values")
+		}
+		if !ecdsa.Verify(pubKey, digest, ecdsaSig.R, ecdsaSig.S) {
+			return errors.New("ECDSA verification failure")
+		}
+	case signatureRSA:
+		pubKey, ok := cert.PublicKey.(*rsa.PublicKey)
+		if !ok {
+			return errors.New("ECDHE RSA requires a RSA server public key")
+		}
+		if err := rsa.VerifyPKCS1v15(pubKey, hashFunc, digest, sig); err != nil {
+			return err
+		}
+	default:
+		return errors.New("unknown ECDHE signature algorithm")
+	}
+
+	return nil
 }
 
-func (ka *ecdheRSAKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
+func (ka *ecdheKeyAgreement) generateClientKeyExchange(config *Config, clientHello *clientHelloMsg, cert *x509.Certificate) ([]byte, *clientKeyExchangeMsg, error) {
 	if ka.curve == nil {
 		return nil, nil, errors.New("missing ServerKeyExchange message")
 	}

src/pkg/crypto/tls/prf.go

@@ -274,11 +274,15 @@ func (h finishedHash) serverSum(masterSecret []byte) []byte {
 
 // hashForClientCertificate returns a digest and hash function identifier
 // suitable for signing by a TLS client certificate.
-func (h finishedHash) hashForClientCertificate() ([]byte, crypto.Hash) {
+func (h finishedHash) hashForClientCertificate(sigType uint8) ([]byte, crypto.Hash) {
 	if h.version >= VersionTLS12 {
 		digest := h.server.Sum(nil)
 		return digest, crypto.SHA256
 	}
+	if sigType == signatureECDSA {
+		digest := h.server.Sum(nil)
+		return digest, crypto.SHA1
+	}
 
 	digest := make([]byte, 0, 36)
 	digest = h.serverMD5.Sum(digest)