diff --git a/src/crypto/tls/cipher_suites.go b/src/crypto/tls/cipher_suites.go
index ea16ef9..9a35675 100644
--- a/src/crypto/tls/cipher_suites.go
+++ b/src/crypto/tls/cipher_suites.go
@@ -160,7 +160,7 @@
 	// flags is a bitmask of the suite* values, above.
 	flags  int
 	cipher func(key, iv []byte, isRead bool) interface{}
-	mac    func(version uint16, macKey []byte) macFunction
+	mac    func(key []byte) hash.Hash
 	aead   func(key, fixedNonce []byte) aead
 }
 
@@ -247,24 +247,15 @@
 	return cipher.NewCBCEncrypter(block, iv)
 }
 
-// macSHA1 returns a macFunction for the given protocol version.
-func macSHA1(version uint16, key []byte) macFunction {
-	return tls10MAC{h: hmac.New(newConstantTimeHash(sha1.New), key)}
+// macSHA1 returns a SHA-1 based constant time MAC.
+func macSHA1(key []byte) hash.Hash {
+	return hmac.New(newConstantTimeHash(sha1.New), key)
 }
 
-// macSHA256 returns a SHA-256 based MAC. These are only supported in TLS 1.2
-// so the given version is ignored.
-func macSHA256(version uint16, key []byte) macFunction {
-	return tls10MAC{h: hmac.New(sha256.New, key)}
-}
-
-type macFunction interface {
-	// Size returns the length of the MAC.
-	Size() int
-	// MAC appends the MAC of (seq, header, data) to out. The extra data is fed
-	// into the MAC after obtaining the result to normalize timing. The result
-	// is only valid until the next invocation of MAC as the buffer is reused.
-	MAC(seq, header, data, extra []byte) []byte
+// macSHA256 returns a SHA-256 based MAC. This is only supported in TLS 1.2 and
+// is currently only used in disabled-by-default cipher suites.
+func macSHA256(key []byte) hash.Hash {
+	return hmac.New(sha256.New, key)
 }
 
 type aead interface {
@@ -412,26 +403,14 @@
 }
 
 // tls10MAC implements the TLS 1.0 MAC function. RFC 2246, Section 6.2.3.
-type tls10MAC struct {
-	h   hash.Hash
-	buf []byte
-}
-
-func (s tls10MAC) Size() int {
-	return s.h.Size()
-}
-
-// MAC is guaranteed to take constant time, as long as
-// len(seq)+len(header)+len(data)+len(extra) is constant. extra is not fed into
-// the MAC, but is only provided to make the timing profile constant.
-func (s tls10MAC) MAC(seq, header, data, extra []byte) []byte {
-	s.h.Reset()
-	s.h.Write(seq)
-	s.h.Write(header)
-	s.h.Write(data)
-	res := s.h.Sum(s.buf[:0])
+func tls10MAC(h hash.Hash, out, seq, header, data, extra []byte) []byte {
+	h.Reset()
+	h.Write(seq)
+	h.Write(header)
+	h.Write(data)
+	res := h.Sum(out)
 	if extra != nil {
-		s.h.Write(extra)
+		h.Write(extra)
 	}
 	return res
 }
diff --git a/src/crypto/tls/conn.go b/src/crypto/tls/conn.go
index 2f5d430..2788c3c 100644
--- a/src/crypto/tls/conn.go
+++ b/src/crypto/tls/conn.go
@@ -14,6 +14,7 @@
 	"crypto/x509"
 	"errors"
 	"fmt"
+	"hash"
 	"io"
 	"net"
 	"sync"
@@ -155,15 +156,16 @@
 type halfConn struct {
 	sync.Mutex
 
-	err            error       // first permanent error
-	version        uint16      // protocol version
-	cipher         interface{} // cipher algorithm
-	mac            macFunction
-	seq            [8]byte  // 64-bit sequence number
-	additionalData [13]byte // to avoid allocs; interface method args escape
+	err     error       // first permanent error
+	version uint16      // protocol version
+	cipher  interface{} // cipher algorithm
+	mac     hash.Hash
+	seq     [8]byte // 64-bit sequence number
+
+	scratchBuf [13]byte // to avoid allocs; interface method args escape
 
 	nextCipher interface{} // next encryption state
-	nextMac    macFunction // next MAC algorithm
+	nextMac    hash.Hash   // next MAC algorithm
 
 	trafficSecret []byte // current TLS 1.3 traffic secret
 }
@@ -188,7 +190,7 @@
 
 // prepareCipherSpec sets the encryption and MAC states
 // that a subsequent changeCipherSpec will use.
-func (hc *halfConn) prepareCipherSpec(version uint16, cipher interface{}, mac macFunction) {
+func (hc *halfConn) prepareCipherSpec(version uint16, cipher interface{}, mac hash.Hash) {
 	hc.version = version
 	hc.nextCipher = cipher
 	hc.nextMac = mac
@@ -350,15 +352,14 @@
 			}
 			payload = payload[explicitNonceLen:]
 
-			additionalData := hc.additionalData[:]
+			var additionalData []byte
 			if hc.version == VersionTLS13 {
 				additionalData = record[:recordHeaderLen]
 			} else {
-				copy(additionalData, hc.seq[:])
-				copy(additionalData[8:], record[:3])
+				additionalData = append(hc.scratchBuf[:0], hc.seq[:]...)
+				additionalData = append(additionalData, record[:3]...)
 				n := len(payload) - c.Overhead()
-				additionalData[11] = byte(n >> 8)
-				additionalData[12] = byte(n)
+				additionalData = append(additionalData, byte(n>>8), byte(n))
 			}
 
 			var err error
@@ -424,7 +425,7 @@
 		record[3] = byte(n >> 8)
 		record[4] = byte(n)
 		remoteMAC := payload[n : n+macSize]
-		localMAC := hc.mac.MAC(hc.seq[0:], record[:recordHeaderLen], payload[:n], payload[n+macSize:])
+		localMAC := tls10MAC(hc.mac, hc.scratchBuf[:0], hc.seq[:], record[:recordHeaderLen], payload[:n], payload[n+macSize:])
 
 		// This is equivalent to checking the MACs and paddingGood
 		// separately, but in constant-time to prevent distinguishing
@@ -460,7 +461,7 @@
 }
 
 // encrypt encrypts payload, adding the appropriate nonce and/or MAC, and
-// appends it to record, which contains the record header.
+// appends it to record, which must already contain the record header.
 func (hc *halfConn) encrypt(record, payload []byte, rand io.Reader) ([]byte, error) {
 	if hc.cipher == nil {
 		return append(record, payload...), nil
@@ -477,7 +478,7 @@
 			// an 8 bytes nonce but its nonces must be unpredictable (see RFC
 			// 5246, Appendix F.3), forcing us to use randomness. That's not
 			// 3DES' biggest problem anyway because the birthday bound on block
-			// collision is reached first due to its simlarly small block size
+			// collision is reached first due to its similarly small block size
 			// (see the Sweet32 attack).
 			copy(explicitNonce, hc.seq[:])
 		} else {
@@ -487,14 +488,10 @@
 		}
 	}
 
-	var mac []byte
-	if hc.mac != nil {
-		mac = hc.mac.MAC(hc.seq[:], record[:recordHeaderLen], payload, nil)
-	}
-
 	var dst []byte
 	switch c := hc.cipher.(type) {
 	case cipher.Stream:
+		mac := tls10MAC(hc.mac, hc.scratchBuf[:0], hc.seq[:], record[:recordHeaderLen], payload, nil)
 		record, dst = sliceForAppend(record, len(payload)+len(mac))
 		c.XORKeyStream(dst[:len(payload)], payload)
 		c.XORKeyStream(dst[len(payload):], mac)
@@ -518,11 +515,12 @@
 			record = c.Seal(record[:recordHeaderLen],
 				nonce, record[recordHeaderLen:], record[:recordHeaderLen])
 		} else {
-			copy(hc.additionalData[:], hc.seq[:])
-			copy(hc.additionalData[8:], record)
-			record = c.Seal(record, nonce, payload, hc.additionalData[:])
+			additionalData := append(hc.scratchBuf[:0], hc.seq[:]...)
+			additionalData = append(additionalData, record[:recordHeaderLen]...)
+			record = c.Seal(record, nonce, payload, additionalData)
 		}
 	case cbcMode:
+		mac := tls10MAC(hc.mac, hc.scratchBuf[:0], hc.seq[:], record[:recordHeaderLen], payload, nil)
 		blockSize := c.BlockSize()
 		plaintextLen := len(payload) + len(mac)
 		paddingLen := blockSize - plaintextLen%blockSize
diff --git a/src/crypto/tls/handshake_client.go b/src/crypto/tls/handshake_client.go
index d09a8c8..123df7b 100644
--- a/src/crypto/tls/handshake_client.go
+++ b/src/crypto/tls/handshake_client.go
@@ -15,6 +15,7 @@
 	"crypto/x509"
 	"errors"
 	"fmt"
+	"hash"
 	"io"
 	"net"
 	"strings"
@@ -651,12 +652,12 @@
 	clientMAC, serverMAC, clientKey, serverKey, clientIV, serverIV :=
 		keysFromMasterSecret(c.vers, hs.suite, hs.masterSecret, hs.hello.random, hs.serverHello.random, hs.suite.macLen, hs.suite.keyLen, hs.suite.ivLen)
 	var clientCipher, serverCipher interface{}
-	var clientHash, serverHash macFunction
+	var clientHash, serverHash hash.Hash
 	if hs.suite.cipher != nil {
 		clientCipher = hs.suite.cipher(clientKey, clientIV, false /* not for reading */)
-		clientHash = hs.suite.mac(c.vers, clientMAC)
+		clientHash = hs.suite.mac(clientMAC)
 		serverCipher = hs.suite.cipher(serverKey, serverIV, true /* for reading */)
-		serverHash = hs.suite.mac(c.vers, serverMAC)
+		serverHash = hs.suite.mac(serverMAC)
 	} else {
 		clientCipher = hs.suite.aead(clientKey, clientIV)
 		serverCipher = hs.suite.aead(serverKey, serverIV)
diff --git a/src/crypto/tls/handshake_server.go b/src/crypto/tls/handshake_server.go
index 1fe026a..73df19d 100644
--- a/src/crypto/tls/handshake_server.go
+++ b/src/crypto/tls/handshake_server.go
@@ -14,6 +14,7 @@
 	"crypto/x509"
 	"errors"
 	"fmt"
+	"hash"
 	"io"
 	"sync/atomic"
 	"time"
@@ -645,13 +646,13 @@
 		keysFromMasterSecret(c.vers, hs.suite, hs.masterSecret, hs.clientHello.random, hs.hello.random, hs.suite.macLen, hs.suite.keyLen, hs.suite.ivLen)
 
 	var clientCipher, serverCipher interface{}
-	var clientHash, serverHash macFunction
+	var clientHash, serverHash hash.Hash
 
 	if hs.suite.aead == nil {
 		clientCipher = hs.suite.cipher(clientKey, clientIV, true /* for reading */)
-		clientHash = hs.suite.mac(c.vers, clientMAC)
+		clientHash = hs.suite.mac(clientMAC)
 		serverCipher = hs.suite.cipher(serverKey, serverIV, false /* not for reading */)
-		serverHash = hs.suite.mac(c.vers, serverMAC)
+		serverHash = hs.suite.mac(serverMAC)
 	} else {
 		clientCipher = hs.suite.aead(clientKey, clientIV)
 		serverCipher = hs.suite.aead(serverKey, serverIV)