diff --git a/src/pkg/crypto/sha1/sha1_test.go b/src/pkg/crypto/sha1/sha1_test.go
index 6d2a9f24dc..4a629518b7 100644
--- a/src/pkg/crypto/sha1/sha1_test.go
+++ b/src/pkg/crypto/sha1/sha1_test.go
@@ -7,6 +7,7 @@
 package sha1
 
 import (
+	"crypto/rand"
 	"fmt"
 	"io"
 	"testing"
@@ -90,6 +91,18 @@ func TestBlockSize(t *testing.T) {
 	}
 }
 
+// Tests that blockGeneric (pure Go) and block (in assembly for amd64, 386, arm) match.
+func TestBlockGeneric(t *testing.T) {
+	gen, asm := New().(*digest), New().(*digest)
+	buf := make([]byte, BlockSize*20) // arbitrary factor
+	rand.Read(buf)
+	blockGeneric(gen, buf)
+	block(asm, buf)
+	if *gen != *asm {
+		t.Error("block and blockGeneric resulted in different states")
+	}
+}
+
 var bench = New()
 var buf = make([]byte, 8192)
 
diff --git a/src/pkg/crypto/sha1/sha1block.go b/src/pkg/crypto/sha1/sha1block.go
index 1c264e5771..fde3c981c0 100644
--- a/src/pkg/crypto/sha1/sha1block.go
+++ b/src/pkg/crypto/sha1/sha1block.go
@@ -2,12 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !amd64,!386,!arm
-
-// SHA1 block step.
-// In its own file so that a faster assembly or C version
-// can be substituted easily.
-
 package sha1
 
 const (
@@ -17,7 +11,9 @@ const (
 	_K3 = 0xCA62C1D6
 )
 
-func block(dig *digest, p []byte) {
+// blockGeneric is a portable, pure Go version of the SHA1 block step.
+// It's used by sha1block_generic.go and tests.
+func blockGeneric(dig *digest, p []byte) {
 	var w [16]uint32
 
 	h0, h1, h2, h3, h4 := dig.h[0], dig.h[1], dig.h[2], dig.h[3], dig.h[4]
diff --git a/src/pkg/crypto/sha1/sha1block_generic.go b/src/pkg/crypto/sha1/sha1block_generic.go
new file mode 100644
index 0000000000..2c78683aa4
--- /dev/null
+++ b/src/pkg/crypto/sha1/sha1block_generic.go
@@ -0,0 +1,9 @@
+// Copyright 2014 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !amd64,!386,!arm
+
+package sha1
+
+var block = blockGeneric