diff --git a/src/cmd/cgo/main.go b/src/cmd/cgo/main.go
index 338fef391d..2749ca74f2 100644
--- a/src/cmd/cgo/main.go
+++ b/src/cmd/cgo/main.go
@@ -189,7 +189,7 @@ func main() {
 		io.Copy(h, f)
 		f.Close()
 	}
-	cPrefix = fmt.Sprintf("_%x", h.Sum()[0:6])
+	cPrefix = fmt.Sprintf("_%x", h.Sum(nil)[0:6])
 
 	fs := make([]*File, len(goFiles))
 	for i, input := range goFiles {
diff --git a/src/cmd/gofix/typecheck.go b/src/cmd/gofix/typecheck.go
index a6c7eac634..1614a90d33 100644
--- a/src/cmd/gofix/typecheck.go
+++ b/src/cmd/gofix/typecheck.go
@@ -97,7 +97,7 @@ func (cfg *TypeConfig) typeof(name string) string {
 // looked for in the Embed list.
 type Type struct {
 	Field  map[string]string // map field name to type
-	Method map[string]string // map method name to comma-separated return types
+	Method map[string]string // map method name to comma-separated return types (should start with "func ")
 	Embed  []string          // list of types this type embeds (for extra methods)
 	Def    string            // definition of named type
 }
diff --git a/src/pkg/archive/tar/reader_test.go b/src/pkg/archive/tar/reader_test.go
index 794cedb2d7..5ca4212ae7 100644
--- a/src/pkg/archive/tar/reader_test.go
+++ b/src/pkg/archive/tar/reader_test.go
@@ -222,7 +222,7 @@ func TestIncrementalRead(t *testing.T) {
 			h.Write(rdbuf[0:nr])
 		}
 		// verify checksum
-		have := fmt.Sprintf("%x", h.Sum())
+		have := fmt.Sprintf("%x", h.Sum(nil))
 		want := cksums[nread]
 		if want != have {
 			t.Errorf("Bad checksum on file %s:\nhave %+v\nwant %+v", hdr.Name, have, want)
diff --git a/src/pkg/crypto/ecdsa/ecdsa_test.go b/src/pkg/crypto/ecdsa/ecdsa_test.go
index 22360b5708..45433e1020 100644
--- a/src/pkg/crypto/ecdsa/ecdsa_test.go
+++ b/src/pkg/crypto/ecdsa/ecdsa_test.go
@@ -214,7 +214,7 @@ func TestVectors(t *testing.T) {
 		msg, _ := hex.DecodeString(test.msg)
 		sha.Reset()
 		sha.Write(msg)
-		hashed := sha.Sum()
+		hashed := sha.Sum(nil)
 		r := fromHex(test.r)
 		s := fromHex(test.s)
 		if Verify(&pub, hashed, r, s) != test.ok {
diff --git a/src/pkg/crypto/hmac/hmac.go b/src/pkg/crypto/hmac/hmac.go
index 6a17bbd44f..deaceafb26 100644
--- a/src/pkg/crypto/hmac/hmac.go
+++ b/src/pkg/crypto/hmac/hmac.go
@@ -48,15 +48,15 @@ func (h *hmac) tmpPad(xor byte) {
 	}
 }
 
-func (h *hmac) Sum() []byte {
-	sum := h.inner.Sum()
+func (h *hmac) Sum(in []byte) []byte {
+	sum := h.inner.Sum(nil)
 	h.tmpPad(0x5c)
 	for i, b := range sum {
 		h.tmp[padSize+i] = b
 	}
 	h.outer.Reset()
 	h.outer.Write(h.tmp)
-	return h.outer.Sum()
+	return h.outer.Sum(in)
 }
 
 func (h *hmac) Write(p []byte) (n int, err error) {
@@ -81,7 +81,7 @@ func New(h func() hash.Hash, key []byte) hash.Hash {
 	if len(key) > padSize {
 		// If key is too big, hash it.
 		hm.outer.Write(key)
-		key = hm.outer.Sum()
+		key = hm.outer.Sum(nil)
 	}
 	hm.key = make([]byte, len(key))
 	copy(hm.key, key)
diff --git a/src/pkg/crypto/hmac/hmac_test.go b/src/pkg/crypto/hmac/hmac_test.go
index 03431c92f7..eac254b9d1 100644
--- a/src/pkg/crypto/hmac/hmac_test.go
+++ b/src/pkg/crypto/hmac/hmac_test.go
@@ -192,7 +192,7 @@ func TestHMAC(t *testing.T) {
 
 			// Repetitive Sum() calls should return the same value
 			for k := 0; k < 2; k++ {
-				sum := fmt.Sprintf("%x", h.Sum())
+				sum := fmt.Sprintf("%x", h.Sum(nil))
 				if sum != tt.out {
 					t.Errorf("test %d.%d.%d: have %s want %s\n", i, j, k, sum, tt.out)
 				}
diff --git a/src/pkg/crypto/md4/md4.go b/src/pkg/crypto/md4/md4.go
index f21cc51a21..e51e8bee50 100644
--- a/src/pkg/crypto/md4/md4.go
+++ b/src/pkg/crypto/md4/md4.go
@@ -77,7 +77,7 @@ func (d *digest) Write(p []byte) (nn int, err error) {
 	return
 }
 
-func (d0 *digest) Sum() []byte {
+func (d0 *digest) Sum(in []byte) []byte {
 	// Make a copy of d0, so that caller can keep writing and summing.
 	d := new(digest)
 	*d = *d0
@@ -103,14 +103,11 @@ func (d0 *digest) Sum() []byte {
 		panic("d.nx != 0")
 	}
 
-	p := make([]byte, 16)
-	j := 0
 	for _, s := range d.s {
-		p[j+0] = byte(s >> 0)
-		p[j+1] = byte(s >> 8)
-		p[j+2] = byte(s >> 16)
-		p[j+3] = byte(s >> 24)
-		j += 4
+		in = append(in, byte(s>>0))
+		in = append(in, byte(s>>8))
+		in = append(in, byte(s>>16))
+		in = append(in, byte(s>>24))
 	}
-	return p
+	return in
 }
diff --git a/src/pkg/crypto/md4/md4_test.go b/src/pkg/crypto/md4/md4_test.go
index 721bd4cbcc..b56edd7875 100644
--- a/src/pkg/crypto/md4/md4_test.go
+++ b/src/pkg/crypto/md4/md4_test.go
@@ -58,10 +58,10 @@ func TestGolden(t *testing.T) {
 				io.WriteString(c, g.in)
 			} else {
 				io.WriteString(c, g.in[0:len(g.in)/2])
-				c.Sum()
+				c.Sum(nil)
 				io.WriteString(c, g.in[len(g.in)/2:])
 			}
-			s := fmt.Sprintf("%x", c.Sum())
+			s := fmt.Sprintf("%x", c.Sum(nil))
 			if s != g.out {
 				t.Fatalf("md4[%d](%s) = %s want %s", j, g.in, s, g.out)
 			}
diff --git a/src/pkg/crypto/md5/md5.go b/src/pkg/crypto/md5/md5.go
index 20f3a1b6f7..182cfb8537 100644
--- a/src/pkg/crypto/md5/md5.go
+++ b/src/pkg/crypto/md5/md5.go
@@ -77,7 +77,7 @@ func (d *digest) Write(p []byte) (nn int, err error) {
 	return
 }
 
-func (d0 *digest) Sum() []byte {
+func (d0 *digest) Sum(in []byte) []byte {
 	// Make a copy of d0 so that caller can keep writing and summing.
 	d := new(digest)
 	*d = *d0
@@ -103,14 +103,11 @@ func (d0 *digest) Sum() []byte {
 		panic("d.nx != 0")
 	}
 
-	p := make([]byte, 16)
-	j := 0
 	for _, s := range d.s {
-		p[j+0] = byte(s >> 0)
-		p[j+1] = byte(s >> 8)
-		p[j+2] = byte(s >> 16)
-		p[j+3] = byte(s >> 24)
-		j += 4
+		in = append(in, byte(s>>0))
+		in = append(in, byte(s>>8))
+		in = append(in, byte(s>>16))
+		in = append(in, byte(s>>24))
 	}
-	return p
+	return in
 }
diff --git a/src/pkg/crypto/md5/md5_test.go b/src/pkg/crypto/md5/md5_test.go
index 857002b701..b15e4668c3 100644
--- a/src/pkg/crypto/md5/md5_test.go
+++ b/src/pkg/crypto/md5/md5_test.go
@@ -58,10 +58,10 @@ func TestGolden(t *testing.T) {
 				io.WriteString(c, g.in)
 			} else {
 				io.WriteString(c, g.in[0:len(g.in)/2])
-				c.Sum()
+				c.Sum(nil)
 				io.WriteString(c, g.in[len(g.in)/2:])
 			}
-			s := fmt.Sprintf("%x", c.Sum())
+			s := fmt.Sprintf("%x", c.Sum(nil))
 			if s != g.out {
 				t.Fatalf("md5[%d](%s) = %s want %s", j, g.in, s, g.out)
 			}
diff --git a/src/pkg/crypto/ocsp/ocsp.go b/src/pkg/crypto/ocsp/ocsp.go
index aff7913b2f..b9dfdf94e3 100644
--- a/src/pkg/crypto/ocsp/ocsp.go
+++ b/src/pkg/crypto/ocsp/ocsp.go
@@ -161,7 +161,7 @@ func ParseResponse(bytes []byte) (*Response, error) {
 
 	pub := ret.Certificate.PublicKey.(*rsa.PublicKey)
 	h.Write(basicResp.TBSResponseData.Raw)
-	digest := h.Sum()
+	digest := h.Sum(nil)
 	signature := basicResp.Signature.RightAlign()
 
 	if rsa.VerifyPKCS1v15(pub, hashType, digest, signature) != nil {
diff --git a/src/pkg/crypto/openpgp/canonical_text.go b/src/pkg/crypto/openpgp/canonical_text.go
index fe4557aafc..98cee5e75a 100644
--- a/src/pkg/crypto/openpgp/canonical_text.go
+++ b/src/pkg/crypto/openpgp/canonical_text.go
@@ -41,8 +41,8 @@ func (cth *canonicalTextHash) Write(buf []byte) (int, error) {
 	return len(buf), nil
 }
 
-func (cth *canonicalTextHash) Sum() []byte {
-	return cth.h.Sum()
+func (cth *canonicalTextHash) Sum(in []byte) []byte {
+	return cth.h.Sum(in)
 }
 
 func (cth *canonicalTextHash) Reset() {
diff --git a/src/pkg/crypto/openpgp/canonical_text_test.go b/src/pkg/crypto/openpgp/canonical_text_test.go
index ae54f8c83e..841475f80c 100644
--- a/src/pkg/crypto/openpgp/canonical_text_test.go
+++ b/src/pkg/crypto/openpgp/canonical_text_test.go
@@ -17,8 +17,8 @@ func (r recordingHash) Write(b []byte) (n int, err error) {
 	return r.buf.Write(b)
 }
 
-func (r recordingHash) Sum() []byte {
-	return r.buf.Bytes()
+func (r recordingHash) Sum(in []byte) []byte {
+	return append(in, r.buf.Bytes()...)
 }
 
 func (r recordingHash) Reset() {
@@ -33,7 +33,7 @@ func testCanonicalText(t *testing.T, input, expected string) {
 	r := recordingHash{bytes.NewBuffer(nil)}
 	c := NewCanonicalTextHash(r)
 	c.Write([]byte(input))
-	result := c.Sum()
+	result := c.Sum(nil)
 	if expected != string(result) {
 		t.Errorf("input: %x got: %x want: %x", input, result, expected)
 	}
diff --git a/src/pkg/crypto/openpgp/packet/private_key.go b/src/pkg/crypto/openpgp/packet/private_key.go
index 729e88d686..d67e968861 100644
--- a/src/pkg/crypto/openpgp/packet/private_key.go
+++ b/src/pkg/crypto/openpgp/packet/private_key.go
@@ -192,7 +192,7 @@ func (pk *PrivateKey) Decrypt(passphrase []byte) error {
 		}
 		h := sha1.New()
 		h.Write(data[:len(data)-sha1.Size])
-		sum := h.Sum()
+		sum := h.Sum(nil)
 		if !bytes.Equal(sum, data[len(data)-sha1.Size:]) {
 			return error_.StructuralError("private key checksum failure")
 		}
diff --git a/src/pkg/crypto/openpgp/packet/public_key.go b/src/pkg/crypto/openpgp/packet/public_key.go
index 865313e597..9aa30e0c15 100644
--- a/src/pkg/crypto/openpgp/packet/public_key.go
+++ b/src/pkg/crypto/openpgp/packet/public_key.go
@@ -88,7 +88,7 @@ func (pk *PublicKey) setFingerPrintAndKeyId() {
 	fingerPrint := sha1.New()
 	pk.SerializeSignaturePrefix(fingerPrint)
 	pk.serializeWithoutHeaders(fingerPrint)
-	copy(pk.Fingerprint[:], fingerPrint.Sum())
+	copy(pk.Fingerprint[:], fingerPrint.Sum(nil))
 	pk.KeyId = binary.BigEndian.Uint64(pk.Fingerprint[12:20])
 }
 
@@ -271,7 +271,7 @@ func (pk *PublicKey) VerifySignature(signed hash.Hash, sig *Signature) (err erro
 	}
 
 	signed.Write(sig.HashSuffix)
-	hashBytes := signed.Sum()
+	hashBytes := signed.Sum(nil)
 
 	if hashBytes[0] != sig.HashTag[0] || hashBytes[1] != sig.HashTag[1] {
 		return error_.SignatureError("hash tag doesn't match")
diff --git a/src/pkg/crypto/openpgp/packet/signature.go b/src/pkg/crypto/openpgp/packet/signature.go
index f5bc8e86ad..1cdc1ee0f0 100644
--- a/src/pkg/crypto/openpgp/packet/signature.go
+++ b/src/pkg/crypto/openpgp/packet/signature.go
@@ -423,7 +423,7 @@ func (sig *Signature) signPrepareHash(h hash.Hash) (digest []byte, err error) {
 	}
 
 	h.Write(sig.HashSuffix)
-	digest = h.Sum()
+	digest = h.Sum(nil)
 	copy(sig.HashTag[:], digest)
 	return
 }
diff --git a/src/pkg/crypto/openpgp/packet/symmetrically_encrypted.go b/src/pkg/crypto/openpgp/packet/symmetrically_encrypted.go
index 8225db6d2f..dff776e3eb 100644
--- a/src/pkg/crypto/openpgp/packet/symmetrically_encrypted.go
+++ b/src/pkg/crypto/openpgp/packet/symmetrically_encrypted.go
@@ -201,7 +201,7 @@ func (ser *seMDCReader) Close() error {
 	}
 	ser.h.Write(ser.trailer[:2])
 
-	final := ser.h.Sum()
+	final := ser.h.Sum(nil)
 	if subtle.ConstantTimeCompare(final, ser.trailer[2:]) != 1 {
 		return error_.SignatureError("hash mismatch")
 	}
@@ -227,7 +227,7 @@ func (w *seMDCWriter) Close() (err error) {
 	buf[0] = mdcPacketTagByte
 	buf[1] = sha1.Size
 	w.h.Write(buf[:2])
-	digest := w.h.Sum()
+	digest := w.h.Sum(nil)
 	copy(buf[2:], digest)
 
 	_, err = w.w.Write(buf[:])
diff --git a/src/pkg/crypto/openpgp/s2k/s2k.go b/src/pkg/crypto/openpgp/s2k/s2k.go
index 2a753db16b..83673e1733 100644
--- a/src/pkg/crypto/openpgp/s2k/s2k.go
+++ b/src/pkg/crypto/openpgp/s2k/s2k.go
@@ -34,7 +34,7 @@ func Salted(out []byte, h hash.Hash, in []byte, salt []byte) {
 		}
 		h.Write(salt)
 		h.Write(in)
-		n := copy(out[done:], h.Sum())
+		n := copy(out[done:], h.Sum(nil))
 		done += n
 	}
 }
@@ -68,7 +68,7 @@ func Iterated(out []byte, h hash.Hash, in []byte, salt []byte, count int) {
 				written += len(combined)
 			}
 		}
-		n := copy(out[done:], h.Sum())
+		n := copy(out[done:], h.Sum(nil))
 		done += n
 	}
 }
diff --git a/src/pkg/crypto/ripemd160/ripemd160.go b/src/pkg/crypto/ripemd160/ripemd160.go
index 6ccfe875f5..c128ee445a 100644
--- a/src/pkg/crypto/ripemd160/ripemd160.go
+++ b/src/pkg/crypto/ripemd160/ripemd160.go
@@ -81,7 +81,7 @@ func (d *digest) Write(p []byte) (nn int, err error) {
 	return
 }
 
-func (d0 *digest) Sum() []byte {
+func (d0 *digest) Sum(in []byte) []byte {
 	// Make a copy of d0 so that caller can keep writing and summing.
 	d := new(digest)
 	*d = *d0
@@ -107,11 +107,11 @@ func (d0 *digest) Sum() []byte {
 		panic("d.nx != 0")
 	}
 
-	p := make([]byte, 20)
-	j := 0
 	for _, s := range d.s {
-		p[j], p[j+1], p[j+2], p[j+3] = byte(s), byte(s>>8), byte(s>>16), byte(s>>24)
-		j += 4
+		in = append(in, byte(s))
+		in = append(in, byte(s>>8))
+		in = append(in, byte(s>>16))
+		in = append(in, byte(s>>24))
 	}
-	return p
+	return in
 }
diff --git a/src/pkg/crypto/ripemd160/ripemd160_test.go b/src/pkg/crypto/ripemd160/ripemd160_test.go
index f4135f5cf6..5df1b2593d 100644
--- a/src/pkg/crypto/ripemd160/ripemd160_test.go
+++ b/src/pkg/crypto/ripemd160/ripemd160_test.go
@@ -38,10 +38,10 @@ func TestVectors(t *testing.T) {
 				io.WriteString(md, tv.in)
 			} else {
 				io.WriteString(md, tv.in[0:len(tv.in)/2])
-				md.Sum()
+				md.Sum(nil)
 				io.WriteString(md, tv.in[len(tv.in)/2:])
 			}
-			s := fmt.Sprintf("%x", md.Sum())
+			s := fmt.Sprintf("%x", md.Sum(nil))
 			if s != tv.out {
 				t.Fatalf("RIPEMD-160[%d](%s) = %s, expected %s", j, tv.in, s, tv.out)
 			}
@@ -56,7 +56,7 @@ func TestMillionA(t *testing.T) {
 		io.WriteString(md, "aaaaaaaaaa")
 	}
 	out := "52783243c1697bdbe16d37f97f68f08325dc1528"
-	s := fmt.Sprintf("%x", md.Sum())
+	s := fmt.Sprintf("%x", md.Sum(nil))
 	if s != out {
 		t.Fatalf("RIPEMD-160 (1 million 'a') = %s, expected %s", s, out)
 	}
diff --git a/src/pkg/crypto/rsa/pkcs1v15_test.go b/src/pkg/crypto/rsa/pkcs1v15_test.go
index 66188ac10e..58d5fda197 100644
--- a/src/pkg/crypto/rsa/pkcs1v15_test.go
+++ b/src/pkg/crypto/rsa/pkcs1v15_test.go
@@ -168,7 +168,7 @@ func TestSignPKCS1v15(t *testing.T) {
 	for i, test := range signPKCS1v15Tests {
 		h := sha1.New()
 		h.Write([]byte(test.in))
-		digest := h.Sum()
+		digest := h.Sum(nil)
 
 		s, err := SignPKCS1v15(nil, rsaPrivateKey, crypto.SHA1, digest)
 		if err != nil {
@@ -186,7 +186,7 @@ func TestVerifyPKCS1v15(t *testing.T) {
 	for i, test := range signPKCS1v15Tests {
 		h := sha1.New()
 		h.Write([]byte(test.in))
-		digest := h.Sum()
+		digest := h.Sum(nil)
 
 		sig, _ := hex.DecodeString(test.out)
 
diff --git a/src/pkg/crypto/rsa/rsa.go b/src/pkg/crypto/rsa/rsa.go
index 27ccf61c4f..f74525c103 100644
--- a/src/pkg/crypto/rsa/rsa.go
+++ b/src/pkg/crypto/rsa/rsa.go
@@ -194,7 +194,7 @@ func mgf1XOR(out []byte, hash hash.Hash, seed []byte) {
 	for done < len(out) {
 		hash.Write(seed)
 		hash.Write(counter[0:4])
-		digest := hash.Sum()
+		digest := hash.Sum(nil)
 		hash.Reset()
 
 		for i := 0; i < len(digest) && done < len(out); i++ {
@@ -231,7 +231,7 @@ func EncryptOAEP(hash hash.Hash, random io.Reader, pub *PublicKey, msg []byte, l
 	}
 
 	hash.Write(label)
-	lHash := hash.Sum()
+	lHash := hash.Sum(nil)
 	hash.Reset()
 
 	em := make([]byte, k)
@@ -428,7 +428,7 @@ func DecryptOAEP(hash hash.Hash, random io.Reader, priv *PrivateKey, ciphertext
 	}
 
 	hash.Write(label)
-	lHash := hash.Sum()
+	lHash := hash.Sum(nil)
 	hash.Reset()
 
 	// Converting the plaintext number to bytes will strip any
diff --git a/src/pkg/crypto/sha1/sha1.go b/src/pkg/crypto/sha1/sha1.go
index 4cdf5b2e98..f41cdb5b02 100644
--- a/src/pkg/crypto/sha1/sha1.go
+++ b/src/pkg/crypto/sha1/sha1.go
@@ -79,7 +79,7 @@ func (d *digest) Write(p []byte) (nn int, err error) {
 	return
 }
 
-func (d0 *digest) Sum() []byte {
+func (d0 *digest) Sum(in []byte) []byte {
 	// Make a copy of d0 so that caller can keep writing and summing.
 	d := new(digest)
 	*d = *d0
@@ -105,14 +105,11 @@ func (d0 *digest) Sum() []byte {
 		panic("d.nx != 0")
 	}
 
-	p := make([]byte, 20)
-	j := 0
 	for _, s := range d.h {
-		p[j+0] = byte(s >> 24)
-		p[j+1] = byte(s >> 16)
-		p[j+2] = byte(s >> 8)
-		p[j+3] = byte(s >> 0)
-		j += 4
+		in = append(in, byte(s>>24))
+		in = append(in, byte(s>>16))
+		in = append(in, byte(s>>8))
+		in = append(in, byte(s))
 	}
-	return p
+	return in
 }
diff --git a/src/pkg/crypto/sha1/sha1_test.go b/src/pkg/crypto/sha1/sha1_test.go
index 2712fe35ea..c23df6c41e 100644
--- a/src/pkg/crypto/sha1/sha1_test.go
+++ b/src/pkg/crypto/sha1/sha1_test.go
@@ -60,10 +60,10 @@ func TestGolden(t *testing.T) {
 				io.WriteString(c, g.in)
 			} else {
 				io.WriteString(c, g.in[0:len(g.in)/2])
-				c.Sum()
+				c.Sum(nil)
 				io.WriteString(c, g.in[len(g.in)/2:])
 			}
-			s := fmt.Sprintf("%x", c.Sum())
+			s := fmt.Sprintf("%x", c.Sum(nil))
 			if s != g.out {
 				t.Fatalf("sha1[%d](%s) = %s want %s", j, g.in, s, g.out)
 			}
diff --git a/src/pkg/crypto/sha256/sha256.go b/src/pkg/crypto/sha256/sha256.go
index 14b8cfc7ec..34861f6cf4 100644
--- a/src/pkg/crypto/sha256/sha256.go
+++ b/src/pkg/crypto/sha256/sha256.go
@@ -123,7 +123,7 @@ func (d *digest) Write(p []byte) (nn int, err error) {
 	return
 }
 
-func (d0 *digest) Sum() []byte {
+func (d0 *digest) Sum(in []byte) []byte {
 	// Make a copy of d0 so that caller can keep writing and summing.
 	d := new(digest)
 	*d = *d0
@@ -149,17 +149,15 @@ func (d0 *digest) Sum() []byte {
 		panic("d.nx != 0")
 	}
 
-	p := make([]byte, 32)
-	j := 0
-	for _, s := range d.h {
-		p[j+0] = byte(s >> 24)
-		p[j+1] = byte(s >> 16)
-		p[j+2] = byte(s >> 8)
-		p[j+3] = byte(s >> 0)
-		j += 4
-	}
+	h := d.h[:]
 	if d.is224 {
-		return p[0:28]
+		h = d.h[:7]
 	}
-	return p
+	for _, s := range h {
+		in = append(in, byte(s>>24))
+		in = append(in, byte(s>>16))
+		in = append(in, byte(s>>8))
+		in = append(in, byte(s))
+	}
+	return in
 }
diff --git a/src/pkg/crypto/sha256/sha256_test.go b/src/pkg/crypto/sha256/sha256_test.go
index 42a3fa7a01..a6efb37545 100644
--- a/src/pkg/crypto/sha256/sha256_test.go
+++ b/src/pkg/crypto/sha256/sha256_test.go
@@ -94,10 +94,10 @@ func TestGolden(t *testing.T) {
 				io.WriteString(c, g.in)
 			} else {
 				io.WriteString(c, g.in[0:len(g.in)/2])
-				c.Sum()
+				c.Sum(nil)
 				io.WriteString(c, g.in[len(g.in)/2:])
 			}
-			s := fmt.Sprintf("%x", c.Sum())
+			s := fmt.Sprintf("%x", c.Sum(nil))
 			if s != g.out {
 				t.Fatalf("sha256[%d](%s) = %s want %s", j, g.in, s, g.out)
 			}
@@ -112,10 +112,10 @@ func TestGolden(t *testing.T) {
 				io.WriteString(c, g.in)
 			} else {
 				io.WriteString(c, g.in[0:len(g.in)/2])
-				c.Sum()
+				c.Sum(nil)
 				io.WriteString(c, g.in[len(g.in)/2:])
 			}
-			s := fmt.Sprintf("%x", c.Sum())
+			s := fmt.Sprintf("%x", c.Sum(nil))
 			if s != g.out {
 				t.Fatalf("sha224[%d](%s) = %s want %s", j, g.in, s, g.out)
 			}
diff --git a/src/pkg/crypto/sha512/sha512.go b/src/pkg/crypto/sha512/sha512.go
index 1bd27982bb..3cf65cbe7c 100644
--- a/src/pkg/crypto/sha512/sha512.go
+++ b/src/pkg/crypto/sha512/sha512.go
@@ -123,7 +123,7 @@ func (d *digest) Write(p []byte) (nn int, err error) {
 	return
 }
 
-func (d0 *digest) Sum() []byte {
+func (d0 *digest) Sum(in []byte) []byte {
 	// Make a copy of d0 so that caller can keep writing and summing.
 	d := new(digest)
 	*d = *d0
@@ -149,21 +149,19 @@ func (d0 *digest) Sum() []byte {
 		panic("d.nx != 0")
 	}
 
-	p := make([]byte, 64)
-	j := 0
-	for _, s := range d.h {
-		p[j+0] = byte(s >> 56)
-		p[j+1] = byte(s >> 48)
-		p[j+2] = byte(s >> 40)
-		p[j+3] = byte(s >> 32)
-		p[j+4] = byte(s >> 24)
-		p[j+5] = byte(s >> 16)
-		p[j+6] = byte(s >> 8)
-		p[j+7] = byte(s >> 0)
-		j += 8
-	}
+	h := d.h[:]
 	if d.is384 {
-		return p[0:48]
+		h = d.h[:6]
 	}
-	return p
+	for _, s := range h {
+		in = append(in, byte(s>>56))
+		in = append(in, byte(s>>48))
+		in = append(in, byte(s>>40))
+		in = append(in, byte(s>>32))
+		in = append(in, byte(s>>24))
+		in = append(in, byte(s>>16))
+		in = append(in, byte(s>>8))
+		in = append(in, byte(s))
+	}
+	return in
 }
diff --git a/src/pkg/crypto/sha512/sha512_test.go b/src/pkg/crypto/sha512/sha512_test.go
index dd116dc17b..a70f7c54e3 100644
--- a/src/pkg/crypto/sha512/sha512_test.go
+++ b/src/pkg/crypto/sha512/sha512_test.go
@@ -94,10 +94,10 @@ func TestGolden(t *testing.T) {
 				io.WriteString(c, g.in)
 			} else {
 				io.WriteString(c, g.in[0:len(g.in)/2])
-				c.Sum()
+				c.Sum(nil)
 				io.WriteString(c, g.in[len(g.in)/2:])
 			}
-			s := fmt.Sprintf("%x", c.Sum())
+			s := fmt.Sprintf("%x", c.Sum(nil))
 			if s != g.out {
 				t.Fatalf("sha512[%d](%s) = %s want %s", j, g.in, s, g.out)
 			}
@@ -112,10 +112,10 @@ func TestGolden(t *testing.T) {
 				io.WriteString(c, g.in)
 			} else {
 				io.WriteString(c, g.in[0:len(g.in)/2])
-				c.Sum()
+				c.Sum(nil)
 				io.WriteString(c, g.in[len(g.in)/2:])
 			}
-			s := fmt.Sprintf("%x", c.Sum())
+			s := fmt.Sprintf("%x", c.Sum(nil))
 			if s != g.out {
 				t.Fatalf("sha384[%d](%s) = %s want %s", j, g.in, s, g.out)
 			}
diff --git a/src/pkg/crypto/tls/cipher_suites.go b/src/pkg/crypto/tls/cipher_suites.go
index ff0ff2b09a..c0e8656f79 100644
--- a/src/pkg/crypto/tls/cipher_suites.go
+++ b/src/pkg/crypto/tls/cipher_suites.go
@@ -127,13 +127,13 @@ func (s ssl30MAC) MAC(seq, record []byte) []byte {
 	s.h.Write(record[:1])
 	s.h.Write(record[3:5])
 	s.h.Write(record[recordHeaderLen:])
-	digest := s.h.Sum()
+	digest := s.h.Sum(nil)
 
 	s.h.Reset()
 	s.h.Write(s.key)
 	s.h.Write(ssl30Pad2[:padLength])
 	s.h.Write(digest)
-	return s.h.Sum()
+	return s.h.Sum(nil)
 }
 
 // tls10MAC implements the TLS 1.0 MAC function. RFC 2246, section 6.2.3.
@@ -149,7 +149,7 @@ func (s tls10MAC) MAC(seq, record []byte) []byte {
 	s.h.Reset()
 	s.h.Write(seq)
 	s.h.Write(record)
-	return s.h.Sum()
+	return s.h.Sum(nil)
 }
 
 func rsaKA() keyAgreement {
diff --git a/src/pkg/crypto/tls/handshake_client.go b/src/pkg/crypto/tls/handshake_client.go
index 5559c7a22d..b4337f2aac 100644
--- a/src/pkg/crypto/tls/handshake_client.go
+++ b/src/pkg/crypto/tls/handshake_client.go
@@ -232,8 +232,8 @@ func (c *Conn) clientHandshake() error {
 	if cert != nil {
 		certVerify := new(certificateVerifyMsg)
 		var digest [36]byte
-		copy(digest[0:16], finishedHash.serverMD5.Sum())
-		copy(digest[16:36], finishedHash.serverSHA1.Sum())
+		copy(digest[0:16], finishedHash.serverMD5.Sum(nil))
+		copy(digest[16:36], finishedHash.serverSHA1.Sum(nil))
 		signed, err := rsa.SignPKCS1v15(c.config.rand(), c.config.Certificates[0].PrivateKey, crypto.MD5SHA1, digest[0:])
 		if err != nil {
 			return c.sendAlert(alertInternalError)
diff --git a/src/pkg/crypto/tls/handshake_server.go b/src/pkg/crypto/tls/handshake_server.go
index 11ea500fc7..bbb23c0c9f 100644
--- a/src/pkg/crypto/tls/handshake_server.go
+++ b/src/pkg/crypto/tls/handshake_server.go
@@ -235,8 +235,8 @@ FindCipherSuite:
 		}
 
 		digest := make([]byte, 36)
-		copy(digest[0:16], finishedHash.serverMD5.Sum())
-		copy(digest[16:36], finishedHash.serverSHA1.Sum())
+		copy(digest[0:16], finishedHash.serverMD5.Sum(nil))
+		copy(digest[16:36], finishedHash.serverSHA1.Sum(nil))
 		err = rsa.VerifyPKCS1v15(pub, crypto.MD5SHA1, digest, certVerify.signature)
 		if err != nil {
 			c.sendAlert(alertBadCertificate)
diff --git a/src/pkg/crypto/tls/key_agreement.go b/src/pkg/crypto/tls/key_agreement.go
index 08fb852d66..b531717d84 100644
--- a/src/pkg/crypto/tls/key_agreement.go
+++ b/src/pkg/crypto/tls/key_agreement.go
@@ -90,13 +90,13 @@ func md5SHA1Hash(slices ...[]byte) []byte {
 	for _, slice := range slices {
 		hmd5.Write(slice)
 	}
-	copy(md5sha1, hmd5.Sum())
+	copy(md5sha1, hmd5.Sum(nil))
 
 	hsha1 := sha1.New()
 	for _, slice := range slices {
 		hsha1.Write(slice)
 	}
-	copy(md5sha1[md5.Size:], hsha1.Sum())
+	copy(md5sha1[md5.Size:], hsha1.Sum(nil))
 	return md5sha1
 }
 
diff --git a/src/pkg/crypto/tls/prf.go b/src/pkg/crypto/tls/prf.go
index d758f21aa8..637ef03e2d 100644
--- a/src/pkg/crypto/tls/prf.go
+++ b/src/pkg/crypto/tls/prf.go
@@ -22,14 +22,14 @@ func splitPreMasterSecret(secret []byte) (s1, s2 []byte) {
 func pHash(result, secret, seed []byte, hash func() hash.Hash) {
 	h := hmac.New(hash, secret)
 	h.Write(seed)
-	a := h.Sum()
+	a := h.Sum(nil)
 
 	j := 0
 	for j < len(result) {
 		h.Reset()
 		h.Write(a)
 		h.Write(seed)
-		b := h.Sum()
+		b := h.Sum(nil)
 		todo := len(b)
 		if j+todo > len(result) {
 			todo = len(result) - j
@@ -39,7 +39,7 @@ func pHash(result, secret, seed []byte, hash func() hash.Hash) {
 
 		h.Reset()
 		h.Write(a)
-		a = h.Sum()
+		a = h.Sum(nil)
 	}
 }
 
@@ -84,13 +84,13 @@ func pRF30(result, secret, label, seed []byte) {
 		hashSHA1.Write(b[:i+1])
 		hashSHA1.Write(secret)
 		hashSHA1.Write(seed)
-		digest := hashSHA1.Sum()
+		digest := hashSHA1.Sum(nil)
 
 		hashMD5.Reset()
 		hashMD5.Write(secret)
 		hashMD5.Write(digest)
 
-		done += copy(result[done:], hashMD5.Sum())
+		done += copy(result[done:], hashMD5.Sum(nil))
 		i++
 	}
 }
@@ -182,24 +182,24 @@ func finishedSum30(md5, sha1 hash.Hash, masterSecret []byte, magic [4]byte) []by
 	md5.Write(magic[:])
 	md5.Write(masterSecret)
 	md5.Write(ssl30Pad1[:])
-	md5Digest := md5.Sum()
+	md5Digest := md5.Sum(nil)
 
 	md5.Reset()
 	md5.Write(masterSecret)
 	md5.Write(ssl30Pad2[:])
 	md5.Write(md5Digest)
-	md5Digest = md5.Sum()
+	md5Digest = md5.Sum(nil)
 
 	sha1.Write(magic[:])
 	sha1.Write(masterSecret)
 	sha1.Write(ssl30Pad1[:40])
-	sha1Digest := sha1.Sum()
+	sha1Digest := sha1.Sum(nil)
 
 	sha1.Reset()
 	sha1.Write(masterSecret)
 	sha1.Write(ssl30Pad2[:40])
 	sha1.Write(sha1Digest)
-	sha1Digest = sha1.Sum()
+	sha1Digest = sha1.Sum(nil)
 
 	ret := make([]byte, len(md5Digest)+len(sha1Digest))
 	copy(ret, md5Digest)
@@ -217,8 +217,8 @@ func (h finishedHash) clientSum(masterSecret []byte) []byte {
 		return finishedSum30(h.clientMD5, h.clientSHA1, masterSecret, ssl3ClientFinishedMagic)
 	}
 
-	md5 := h.clientMD5.Sum()
-	sha1 := h.clientSHA1.Sum()
+	md5 := h.clientMD5.Sum(nil)
+	sha1 := h.clientSHA1.Sum(nil)
 	return finishedSum10(md5, sha1, clientFinishedLabel, masterSecret)
 }
 
@@ -229,7 +229,7 @@ func (h finishedHash) serverSum(masterSecret []byte) []byte {
 		return finishedSum30(h.serverMD5, h.serverSHA1, masterSecret, ssl3ServerFinishedMagic)
 	}
 
-	md5 := h.serverMD5.Sum()
-	sha1 := h.serverSHA1.Sum()
+	md5 := h.serverMD5.Sum(nil)
+	sha1 := h.serverSHA1.Sum(nil)
 	return finishedSum10(md5, sha1, serverFinishedLabel, masterSecret)
 }
diff --git a/src/pkg/crypto/x509/x509.go b/src/pkg/crypto/x509/x509.go
index d64723a156..7e6b5c96f5 100644
--- a/src/pkg/crypto/x509/x509.go
+++ b/src/pkg/crypto/x509/x509.go
@@ -398,7 +398,7 @@ func (c *Certificate) CheckSignature(algo SignatureAlgorithm, signed, signature
 	}
 
 	h.Write(signed)
-	digest := h.Sum()
+	digest := h.Sum(nil)
 
 	switch pub := c.PublicKey.(type) {
 	case *rsa.PublicKey:
@@ -957,7 +957,7 @@ func CreateCertificate(rand io.Reader, template, parent *Certificate, pub *rsa.P
 
 	h := sha1.New()
 	h.Write(tbsCertContents)
-	digest := h.Sum()
+	digest := h.Sum(nil)
 
 	signature, err := rsa.SignPKCS1v15(rand, priv, crypto.SHA1, digest)
 	if err != nil {
@@ -1024,7 +1024,7 @@ func (c *Certificate) CreateCRL(rand io.Reader, priv *rsa.PrivateKey, revokedCer
 
 	h := sha1.New()
 	h.Write(tbsCertListContents)
-	digest := h.Sum()
+	digest := h.Sum(nil)
 
 	signature, err := rsa.SignPKCS1v15(rand, priv, crypto.SHA1, digest)
 	if err != nil {
diff --git a/src/pkg/exp/ssh/client.go b/src/pkg/exp/ssh/client.go
index 7f0515806f..429dee975b 100644
--- a/src/pkg/exp/ssh/client.go
+++ b/src/pkg/exp/ssh/client.go
@@ -172,7 +172,7 @@ func (c *ClientConn) kexDH(group *dhGroup, hashFunc crypto.Hash, magics *handsha
 	marshalInt(K, kInt)
 	h.Write(K)
 
-	H := h.Sum()
+	H := h.Sum(nil)
 
 	return H, K, nil
 }
diff --git a/src/pkg/exp/ssh/client_auth_test.go b/src/pkg/exp/ssh/client_auth_test.go
index 6467f57835..4ef9213a9c 100644
--- a/src/pkg/exp/ssh/client_auth_test.go
+++ b/src/pkg/exp/ssh/client_auth_test.go
@@ -70,7 +70,7 @@ func (k *keychain) Sign(i int, rand io.Reader, data []byte) (sig []byte, err err
 	hashFunc := crypto.SHA1
 	h := hashFunc.New()
 	h.Write(data)
-	digest := h.Sum()
+	digest := h.Sum(nil)
 	return rsa.SignPKCS1v15(rand, k.keys[i], hashFunc, digest)
 }
 
diff --git a/src/pkg/exp/ssh/server.go b/src/pkg/exp/ssh/server.go
index 428a747e1e..1eee9a4a97 100644
--- a/src/pkg/exp/ssh/server.go
+++ b/src/pkg/exp/ssh/server.go
@@ -207,11 +207,11 @@ func (s *ServerConn) kexDH(group *dhGroup, hashFunc crypto.Hash, magics *handsha
 	marshalInt(K, kInt)
 	h.Write(K)
 
-	H = h.Sum()
+	H = h.Sum(nil)
 
 	h.Reset()
 	h.Write(H)
-	hh := h.Sum()
+	hh := h.Sum(nil)
 
 	var sig []byte
 	switch hostKeyAlgo {
@@ -478,7 +478,7 @@ userAuthLoop:
 					hashFunc := crypto.SHA1
 					h := hashFunc.New()
 					h.Write(signedData)
-					digest := h.Sum()
+					digest := h.Sum(nil)
 					rsaKey, ok := parseRSA(pubKey)
 					if !ok {
 						return ParseError{msgUserAuthRequest}
diff --git a/src/pkg/exp/ssh/transport.go b/src/pkg/exp/ssh/transport.go
index b8cb2c319d..bcd073e7ce 100644
--- a/src/pkg/exp/ssh/transport.go
+++ b/src/pkg/exp/ssh/transport.go
@@ -123,7 +123,7 @@ func (r *reader) readOnePacket() ([]byte, error) {
 
 	if r.mac != nil {
 		r.mac.Write(packet[:length-1])
-		if subtle.ConstantTimeCompare(r.mac.Sum(), mac) != 1 {
+		if subtle.ConstantTimeCompare(r.mac.Sum(nil), mac) != 1 {
 			return nil, errors.New("ssh: MAC failure")
 		}
 	}
@@ -201,7 +201,7 @@ func (w *writer) writePacket(packet []byte) error {
 	}
 
 	if w.mac != nil {
-		if _, err := w.Write(w.mac.Sum()); err != nil {
+		if _, err := w.Write(w.mac.Sum(nil)); err != nil {
 			return err
 		}
 	}
@@ -297,7 +297,7 @@ func generateKeyMaterial(out, tag []byte, K, H, sessionId []byte, h hash.Hash) {
 			h.Write(digestsSoFar)
 		}
 
-		digest := h.Sum()
+		digest := h.Sum(nil)
 		n := copy(out, digest)
 		out = out[n:]
 		if len(out) > 0 {
@@ -317,9 +317,9 @@ func (t truncatingMAC) Write(data []byte) (int, error) {
 	return t.hmac.Write(data)
 }
 
-func (t truncatingMAC) Sum() []byte {
-	digest := t.hmac.Sum()
-	return digest[:t.length]
+func (t truncatingMAC) Sum(in []byte) []byte {
+	out := t.hmac.Sum(in)
+	return out[:len(in)+t.length]
 }
 
 func (t truncatingMAC) Reset() {
diff --git a/src/pkg/hash/adler32/adler32.go b/src/pkg/hash/adler32/adler32.go
index 10bed2f05e..8103a89d43 100644
--- a/src/pkg/hash/adler32/adler32.go
+++ b/src/pkg/hash/adler32/adler32.go
@@ -71,14 +71,13 @@ func (d *digest) Write(p []byte) (nn int, err error) {
 
 func (d *digest) Sum32() uint32 { return finish(d.a, d.b) }
 
-func (d *digest) Sum() []byte {
-	p := make([]byte, 4)
+func (d *digest) Sum(in []byte) []byte {
 	s := d.Sum32()
-	p[0] = byte(s >> 24)
-	p[1] = byte(s >> 16)
-	p[2] = byte(s >> 8)
-	p[3] = byte(s)
-	return p
+	in = append(in, byte(s>>24))
+	in = append(in, byte(s>>16))
+	in = append(in, byte(s>>8))
+	in = append(in, byte(s))
+	return in
 }
 
 // Checksum returns the Adler-32 checksum of data.
diff --git a/src/pkg/hash/crc32/crc32.go b/src/pkg/hash/crc32/crc32.go
index 5980ec0dc9..557fab8a52 100644
--- a/src/pkg/hash/crc32/crc32.go
+++ b/src/pkg/hash/crc32/crc32.go
@@ -119,14 +119,13 @@ func (d *digest) Write(p []byte) (n int, err error) {
 
 func (d *digest) Sum32() uint32 { return d.crc }
 
-func (d *digest) Sum() []byte {
-	p := make([]byte, 4)
+func (d *digest) Sum(in []byte) []byte {
 	s := d.Sum32()
-	p[0] = byte(s >> 24)
-	p[1] = byte(s >> 16)
-	p[2] = byte(s >> 8)
-	p[3] = byte(s)
-	return p
+	in = append(in, byte(s>>24))
+	in = append(in, byte(s>>16))
+	in = append(in, byte(s>>8))
+	in = append(in, byte(s))
+	return in
 }
 
 // Checksum returns the CRC-32 checksum of data
diff --git a/src/pkg/hash/crc64/crc64.go b/src/pkg/hash/crc64/crc64.go
index 42e53c3a5b..e5c1db4b3d 100644
--- a/src/pkg/hash/crc64/crc64.go
+++ b/src/pkg/hash/crc64/crc64.go
@@ -75,18 +75,17 @@ func (d *digest) Write(p []byte) (n int, err error) {
 
 func (d *digest) Sum64() uint64 { return d.crc }
 
-func (d *digest) Sum() []byte {
-	p := make([]byte, 8)
+func (d *digest) Sum(in []byte) []byte {
 	s := d.Sum64()
-	p[0] = byte(s >> 56)
-	p[1] = byte(s >> 48)
-	p[2] = byte(s >> 40)
-	p[3] = byte(s >> 32)
-	p[4] = byte(s >> 24)
-	p[5] = byte(s >> 16)
-	p[6] = byte(s >> 8)
-	p[7] = byte(s)
-	return p
+	in = append(in, byte(s>>56))
+	in = append(in, byte(s>>48))
+	in = append(in, byte(s>>40))
+	in = append(in, byte(s>>32))
+	in = append(in, byte(s>>24))
+	in = append(in, byte(s>>16))
+	in = append(in, byte(s>>8))
+	in = append(in, byte(s))
+	return in
 }
 
 // Checksum returns the CRC-64 checksum of data
diff --git a/src/pkg/hash/fnv/fnv.go b/src/pkg/hash/fnv/fnv.go
index ce3ed0d0f4..2c8a25118e 100644
--- a/src/pkg/hash/fnv/fnv.go
+++ b/src/pkg/hash/fnv/fnv.go
@@ -8,7 +8,6 @@
 package fnv
 
 import (
-	"encoding/binary"
 	"hash"
 )
 
@@ -105,26 +104,46 @@ func (s *sum32a) Size() int { return 4 }
 func (s *sum64) Size() int  { return 8 }
 func (s *sum64a) Size() int { return 8 }
 
-func (s *sum32) Sum() []byte {
-	a := make([]byte, 4)
-	binary.BigEndian.PutUint32(a, uint32(*s))
-	return a
+func (s *sum32) Sum(in []byte) []byte {
+	v := uint32(*s)
+	in = append(in, byte(v>>24))
+	in = append(in, byte(v>>16))
+	in = append(in, byte(v>>8))
+	in = append(in, byte(v))
+	return in
 }
 
-func (s *sum32a) Sum() []byte {
-	a := make([]byte, 4)
-	binary.BigEndian.PutUint32(a, uint32(*s))
-	return a
+func (s *sum32a) Sum(in []byte) []byte {
+	v := uint32(*s)
+	in = append(in, byte(v>>24))
+	in = append(in, byte(v>>16))
+	in = append(in, byte(v>>8))
+	in = append(in, byte(v))
+	return in
 }
 
-func (s *sum64) Sum() []byte {
-	a := make([]byte, 8)
-	binary.BigEndian.PutUint64(a, uint64(*s))
-	return a
+func (s *sum64) Sum(in []byte) []byte {
+	v := uint64(*s)
+	in = append(in, byte(v>>56))
+	in = append(in, byte(v>>48))
+	in = append(in, byte(v>>40))
+	in = append(in, byte(v>>32))
+	in = append(in, byte(v>>24))
+	in = append(in, byte(v>>16))
+	in = append(in, byte(v>>8))
+	in = append(in, byte(v))
+	return in
 }
 
-func (s *sum64a) Sum() []byte {
-	a := make([]byte, 8)
-	binary.BigEndian.PutUint64(a, uint64(*s))
-	return a
+func (s *sum64a) Sum(in []byte) []byte {
+	v := uint64(*s)
+	in = append(in, byte(v>>56))
+	in = append(in, byte(v>>48))
+	in = append(in, byte(v>>40))
+	in = append(in, byte(v>>32))
+	in = append(in, byte(v>>24))
+	in = append(in, byte(v>>16))
+	in = append(in, byte(v>>8))
+	in = append(in, byte(v))
+	return in
 }
diff --git a/src/pkg/hash/fnv/fnv_test.go b/src/pkg/hash/fnv/fnv_test.go
index 429230c80b..17454deda9 100644
--- a/src/pkg/hash/fnv/fnv_test.go
+++ b/src/pkg/hash/fnv/fnv_test.go
@@ -72,7 +72,7 @@ func testGolden(t *testing.T, hash hash.Hash, gold []golden) {
 		if done != len(g.text) {
 			t.Fatalf("wrote only %d out of %d bytes", done, len(g.text))
 		}
-		if actual := hash.Sum(); !bytes.Equal(g.sum, actual) {
+		if actual := hash.Sum(nil); !bytes.Equal(g.sum, actual) {
 			t.Errorf("hash(%q) = 0x%x want 0x%x", g.text, actual, g.sum)
 		}
 	}
@@ -97,26 +97,26 @@ func TestIntegrity64a(t *testing.T) {
 func testIntegrity(t *testing.T, h hash.Hash) {
 	data := []byte{'1', '2', 3, 4, 5}
 	h.Write(data)
-	sum := h.Sum()
+	sum := h.Sum(nil)
 
 	if size := h.Size(); size != len(sum) {
 		t.Fatalf("Size()=%d but len(Sum())=%d", size, len(sum))
 	}
 
-	if a := h.Sum(); !bytes.Equal(sum, a) {
+	if a := h.Sum(nil); !bytes.Equal(sum, a) {
 		t.Fatalf("first Sum()=0x%x, second Sum()=0x%x", sum, a)
 	}
 
 	h.Reset()
 	h.Write(data)
-	if a := h.Sum(); !bytes.Equal(sum, a) {
+	if a := h.Sum(nil); !bytes.Equal(sum, a) {
 		t.Fatalf("Sum()=0x%x, but after Reset() Sum()=0x%x", sum, a)
 	}
 
 	h.Reset()
 	h.Write(data[:2])
 	h.Write(data[2:])
-	if a := h.Sum(); !bytes.Equal(sum, a) {
+	if a := h.Sum(nil); !bytes.Equal(sum, a) {
 		t.Fatalf("Sum()=0x%x, but with partial writes, Sum()=0x%x", sum, a)
 	}
 
@@ -162,6 +162,6 @@ func benchmark(b *testing.B, h hash.Hash) {
 	for todo := b.N; todo != 0; todo-- {
 		h.Reset()
 		h.Write(data)
-		h.Sum()
+		h.Sum(nil)
 	}
 }
diff --git a/src/pkg/hash/hash.go b/src/pkg/hash/hash.go
index 3536c0b6a6..0d7765dc50 100644
--- a/src/pkg/hash/hash.go
+++ b/src/pkg/hash/hash.go
@@ -13,9 +13,9 @@ type Hash interface {
 	// It never returns an error.
 	io.Writer
 
-	// Sum returns the current hash, without changing the
-	// underlying hash state.
-	Sum() []byte
+	// Sum appends the current hash in the same manner as append(), without
+	// changing the underlying hash state.
+	Sum(in []byte) []byte
 
 	// Reset resets the hash to one with zero bytes written.
 	Reset()
diff --git a/src/pkg/io/multi_test.go b/src/pkg/io/multi_test.go
index 0de5cc312d..eb717f7bc2 100644
--- a/src/pkg/io/multi_test.go
+++ b/src/pkg/io/multi_test.go
@@ -77,7 +77,7 @@ func TestMultiWriter(t *testing.T) {
 		t.Errorf("unexpected error: %v", err)
 	}
 
-	sha1hex := fmt.Sprintf("%x", sha1.Sum())
+	sha1hex := fmt.Sprintf("%x", sha1.Sum(nil))
 	if sha1hex != "01cb303fa8c30a64123067c5aa6284ba7ec2d31b" {
 		t.Error("incorrect sha1 value")
 	}
diff --git a/src/pkg/patch/git.go b/src/pkg/patch/git.go
index 454eadecec..5c233fbaeb 100644
--- a/src/pkg/patch/git.go
+++ b/src/pkg/patch/git.go
@@ -22,7 +22,7 @@ func gitSHA1(data []byte) []byte {
 	h := sha1.New()
 	fmt.Fprintf(h, "blob %d\x00", len(data))
 	h.Write(data)
-	return h.Sum()
+	return h.Sum(nil)
 }
 
 // BUG(rsc): The Git binary delta format is not implemented, only Git binary literals.
diff --git a/src/pkg/websocket/hixie.go b/src/pkg/websocket/hixie.go
index 4d5360ff4b..ec7b7ae0a4 100644
--- a/src/pkg/websocket/hixie.go
+++ b/src/pkg/websocket/hixie.go
@@ -274,7 +274,7 @@ func getChallengeResponse(number1, number2 uint32, key3 []byte) (expected []byte
 	if _, err = h.Write(challenge); err != nil {
 		return
 	}
-	expected = h.Sum()
+	expected = h.Sum(nil)
 	return
 }
 
diff --git a/src/pkg/websocket/hybi.go b/src/pkg/websocket/hybi.go
index b17d9470bb..ff386dc7f2 100644
--- a/src/pkg/websocket/hybi.go
+++ b/src/pkg/websocket/hybi.go
@@ -371,7 +371,7 @@ func getNonceAccept(nonce []byte) (expected []byte, err error) {
 		return
 	}
 	expected = make([]byte, 28)
-	base64.StdEncoding.Encode(expected, h.Sum())
+	base64.StdEncoding.Encode(expected, h.Sum(nil))
 	return
 }
 
diff --git a/test/fixedbugs/bug257.go b/test/fixedbugs/bug257.go
index 713c42481f..1b32475003 100644
--- a/test/fixedbugs/bug257.go
+++ b/test/fixedbugs/bug257.go
@@ -20047,11 +20047,10 @@ var gettysburg = "  Four score and seven years ago our fathers brought forth on\
 	"\n" +
 	"Abraham Lincoln, November 19, 1863, Gettysburg, Pennsylvania\n"
 
-
 func main() {
 	m := md5.New()
 	io.WriteString(m, data)
-	hash := fmt.Sprintf("%x", m.Sum())
+	hash := fmt.Sprintf("%x", m.Sum(nil))
 	if hash != "525f06bc62a65017cd2217d7584e5920" {
 		println("BUG a", hash)
 		return
@@ -20059,7 +20058,7 @@ func main() {
 
 	m = md5.New()
 	io.WriteString(m, gettysburg)
-	hash = fmt.Sprintf("%x", m.Sum())
+	hash = fmt.Sprintf("%x", m.Sum(nil))
 	if hash != "d7ec5d9d47a4d166091e8d9ebd7ea0aa" {
 		println("BUG gettysburg", hash)
 		println(len(gettysburg))