Xor-based crypto modes: OFB and CTR stream encryption.

R=r
DELTA=643  (643 added, 0 deleted, 0 changed)
OCL=29017
CL=29047

src/lib/crypto/block/ctr.go

+// Copyright 2009 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.
+
+// Counter (CTR) mode.
+
+// CTR converts a block cipher into a stream cipher by
+// repeatedly encrypting an incrementing counter and
+// xoring the resulting stream of data with the input.
+
+// See NIST SP 800-38A, pp 13-15
+
+package block
+
+import (
+	"crypto/block";
+	"io";
+)
+
+type ctrStream struct {
+	c Cipher;
+	ctr []byte;
+	out []byte;
+}
+
+func newCTRStream(c Cipher, ctr []byte) *ctrStream {
+	x := new(ctrStream);
+	x.c = c;
+	x.ctr = copy(ctr);
+	x.out = make([]byte, len(ctr));
+	return x;
+}
+
+func (x *ctrStream) Next() []byte {
+	// Next block is encryption of counter.
+	x.c.Encrypt(x.ctr, x.out);
+
+	// Increment counter
+	for i := len(x.ctr) - 1; i >= 0; i-- {
+		x.ctr[i]++;
+		if x.ctr[i] != 0 {
+			break;
+		}
+	}
+
+	return x.out;
+}
+
+// NewCTRReader returns a reader that reads data from r, decrypts (or encrypts)
+// it using c in counter (CTR) mode with the initialization vector iv.
+// The returned Reader does not buffer and has no block size.
+// In CTR mode, encryption and decryption are the same operation:
+// a CTR reader applied to an encrypted stream produces a decrypted
+// stream and vice versa.
+func NewCTRReader(c Cipher, iv []byte, r io.Reader) io.Reader {
+	return NewXorReader(newCTRStream(c, iv), r);
+}
+
+// NewCTRWriter returns a writer that encrypts (or decrypts) data using c
+// in counter (CTR) mode with the initialization vector iv
+// and writes the encrypted data to w.
+// The returned Writer does not buffer and has no block size.
+// In CTR mode, encryption and decryption are the same operation:
+// a CTR writer applied to an decrypted stream produces an encrypted
+// stream and vice versa.
+func NewCTRWriter(c Cipher, iv []byte, w io.Writer) io.Writer {
+	return NewXorWriter(newCTRStream(c, iv), w);
+}
+

src/lib/crypto/block/ctr_aes_test.go

+// Copyright 2009 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.
+
+// CTR AES test vectors.
+
+// See U.S. National Institute of Standards and Technology (NIST)
+// Special Publication 800-38A, ``Recommendation for Block Cipher
+// Modes of Operation,'' 2001 Edition, pp. 55-58.
+
+package block
+
+import (
+	"crypto/aes";
+	"crypto/block";
+	"io";
+	"os";
+	"testing";
+
+	"./ecb_aes_test";
+)
+
+type ctrTest struct {
+	name string;
+	key []byte;
+	iv []byte;
+	in []byte;
+	out []byte;
+}
+
+var commonCounter = []byte {
+	0xf0, 0xf1, 0xf2, 0xf3, 0xf4, 0xf5, 0xf6, 0xf7, 0xf8, 0xf9, 0xfa, 0xfb, 0xfc, 0xfd, 0xfe, 0xff,
+}
+
+var ctrAESTests = []ctrTest {
+	// NIST SP 800-38A pp 55-58
+	ctrTest {
+		"CTR-AES128",
+		commonKey128,
+		commonCounter,
+		commonInput,
+		[]byte {
+			0x87, 0x4d, 0x61, 0x91, 0xb6, 0x20, 0xe3, 0x26, 0x1b, 0xef, 0x68, 0x64, 0x99, 0x0d, 0xb6, 0xce,
+			0x98, 0x06, 0xf6, 0x6b, 0x79, 0x70, 0xfd, 0xff, 0x86, 0x17, 0x18, 0x7b, 0xb9, 0xff, 0xfd, 0xff,
+			0x5a, 0xe4, 0xdf, 0x3e, 0xdb, 0xd5, 0xd3, 0x5e, 0x5b, 0x4f, 0x09, 0x02, 0x0d, 0xb0, 0x3e, 0xab,
+			0x1e, 0x03, 0x1d, 0xda, 0x2f, 0xbe, 0x03, 0xd1, 0x79, 0x21, 0x70, 0xa0, 0xf3, 0x00, 0x9c, 0xee,
+		},
+	},
+	ctrTest {
+		"CTR-AES192",
+		commonKey192,
+		commonCounter,
+		commonInput,
+		[]byte {
+			0x1a, 0xbc, 0x93, 0x24, 0x17, 0x52, 0x1c, 0xa2, 0x4f, 0x2b, 0x04, 0x59, 0xfe, 0x7e, 0x6e, 0x0b,
+			0x09, 0x03, 0x39, 0xec, 0x0a, 0xa6, 0xfa, 0xef, 0xd5, 0xcc, 0xc2, 0xc6, 0xf4, 0xce, 0x8e, 0x94,
+			0x1e, 0x36, 0xb2, 0x6b, 0xd1, 0xeb, 0xc6, 0x70, 0xd1, 0xbd, 0x1d, 0x66, 0x56, 0x20, 0xab, 0xf7,
+			0x4f, 0x78, 0xa7, 0xf6, 0xd2, 0x98, 0x09, 0x58, 0x5a, 0x97, 0xda, 0xec, 0x58, 0xc6, 0xb0, 0x50,
+		},
+	},
+	ctrTest {
+		"CTR-AES256",
+		commonKey256,
+		commonCounter,
+		commonInput,
+		[]byte {
+			0x60, 0x1e, 0xc3, 0x13, 0x77, 0x57, 0x89, 0xa5, 0xb7, 0xa7, 0xf5, 0x04, 0xbb, 0xf3, 0xd2, 0x28,
+			0xf4, 0x43, 0xe3, 0xca, 0x4d, 0x62, 0xb5, 0x9a, 0xca, 0x84, 0xe9, 0x90, 0xca, 0xca, 0xf5, 0xc5,
+			0x2b, 0x09, 0x30, 0xda, 0xa2, 0x3d, 0xe9, 0x4c, 0xe8, 0x70, 0x17, 0xba, 0x2d, 0x84, 0x98, 0x8d,
+			0xdf, 0xc9, 0xc5, 0x8d, 0xb6, 0x7a, 0xad, 0xa6, 0x13, 0xc2, 0xdd, 0x08, 0x45, 0x79, 0x41, 0xa6,
+		}
+	},
+}
+
+func TestCTR_AES(t *testing.T) {
+	for i, tt := range ctrAESTests {
+		test := tt.name;
+
+		c, err := aes.NewCipher(tt.key);
+		if err != nil {
+			t.Errorf("%s: NewCipher(%d bytes) = %s", test, len(tt.key), err);
+			continue;
+		}
+
+		for j := 0; j <= 5; j += 5 {
+			var crypt io.ByteBuffer;
+			in := tt.in[0:len(tt.in) - j];
+			w := block.NewCTRWriter(c, tt.iv, &crypt);
+			var r io.Reader = io.NewByteReader(in);
+			n, err := io.Copy(r, w);
+			if n != int64(len(in)) || err != nil {
+				t.Errorf("%s/%d: CTRWriter io.Copy = %d, %v want %d, nil", test, len(in), n, err, len(in));
+			} else if d, out := crypt.Data(), tt.out[0:len(in)]; !same(out, d) {
+				t.Errorf("%s/%d: CTRWriter\ninpt %x\nhave %x\nwant %x", test, len(in), in, d, out);
+			}
+		}
+
+		for j := 0; j <= 7; j += 7 {
+			var plain io.ByteBuffer;
+			out := tt.out[0:len(tt.out) - j];
+			r := block.NewCTRReader(c, tt.iv, io.NewByteReader(out));
+			w := &plain;
+			n, err := io.Copy(r, w);
+			if n != int64(len(out)) || err != nil {
+				t.Errorf("%s/%d: CTRReader io.Copy = %d, %v want %d, nil", test, len(out), n, err, len(out));
+			} else if d, in := plain.Data(), tt.in[0:len(out)]; !same(in, d) {
+				t.Errorf("%s/%d: CTRReader\nhave %x\nwant %x", test, len(out), d, in);
+			}
+		}
+
+		if t.Failed() {
+			break;
+		}
+	}
+}

src/lib/crypto/block/ofb.go

+// Copyright 2009 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.
+
+// Output feedback (OFB) mode.
+
+// OFB converts a block cipher into a stream cipher by
+// repeatedly encrypting an initialization vector and
+// xoring the resulting stream of data with the input.
+
+// See NIST SP 800-38A, pp 13-15
+
+package block
+
+import (
+	"crypto/block";
+	"io";
+)
+
+type ofbStream struct {
+	c Cipher;
+	iv []byte;
+}
+
+func newOFBStream(c Cipher, iv []byte) *ofbStream {
+	x := new(ofbStream);
+	x.c = c;
+	n := len(iv);
+	if n != c.BlockSize() {
+		panicln("crypto/block: newOFBStream: invalid iv size", n, "!=", c.BlockSize());
+	}
+	x.iv = copy(iv);
+	return x;
+}
+
+func (x *ofbStream) Next() []byte {
+	x.c.Encrypt(x.iv, x.iv);
+	return x.iv;
+}
+
+// NewOFBReader returns a reader that reads data from r, decrypts (or encrypts)
+// it using c in output feedback (OFB) mode with the initialization vector iv.
+// The returned Reader does not buffer and has no block size.
+// In OFB mode, encryption and decryption are the same operation:
+// an OFB reader applied to an encrypted stream produces a decrypted
+// stream and vice versa.
+func NewOFBReader(c Cipher, iv []byte, r io.Reader) io.Reader {
+	return NewXorReader(newOFBStream(c, iv), r);
+}
+
+// NewOFBWriter returns a writer that encrypts (or decrypts) data using c
+// in cipher feedback (OFB) mode with the initialization vector iv
+// and writes the encrypted data to w.
+// The returned Writer does not buffer and has no block size.
+// In OFB mode, encryption and decryption are the same operation:
+// an OFB writer applied to an decrypted stream produces an encrypted
+// stream and vice versa.
+func NewOFBWriter(c Cipher, iv []byte, w io.Writer) io.Writer {
+	return NewXorWriter(newOFBStream(c, iv), w);
+}
+

src/lib/crypto/block/ofb_aes_test.go

+// Copyright 2009 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.
+
+// OFB AES test vectors.
+
+// See U.S. National Institute of Standards and Technology (NIST)
+// Special Publication 800-38A, ``Recommendation for Block Cipher
+// Modes of Operation,'' 2001 Edition, pp. 52-55.
+
+package block
+
+// gotest: $GC ecb_aes_test.go
+
+import (
+	"crypto/aes";
+	"crypto/block";
+	"io";
+	"os";
+	"testing";
+
+	"./ecb_aes_test";
+)
+
+type ofbTest struct {
+	name string;
+	key []byte;
+	iv []byte;
+	in []byte;
+	out []byte;
+}
+
+var ofbAESTests = []ofbTest {
+	// NIST SP 800-38A pp 52-55
+	ofbTest {
+		"OFB-AES128",
+		commonKey128,
+		commonIV,
+		commonInput,
+		[]byte {
+			0x3b, 0x3f, 0xd9, 0x2e, 0xb7, 0x2d, 0xad, 0x20, 0x33, 0x34, 0x49, 0xf8, 0xe8, 0x3c, 0xfb, 0x4a,
+			0x77, 0x89, 0x50, 0x8d, 0x16, 0x91, 0x8f, 0x03, 0xf5, 0x3c, 0x52, 0xda, 0xc5, 0x4e, 0xd8, 0x25,
+			0x97, 0x40, 0x05, 0x1e, 0x9c, 0x5f, 0xec, 0xf6, 0x43, 0x44, 0xf7, 0xa8, 0x22, 0x60, 0xed, 0xcc,
+			0x30, 0x4c, 0x65, 0x28, 0xf6, 0x59, 0xc7, 0x78, 0x66, 0xa5, 0x10, 0xd9, 0xc1, 0xd6, 0xae, 0x5e,
+		},
+	},
+	ofbTest {
+		"OFB-AES192",
+		commonKey192,
+		commonIV,
+		commonInput,
+		[]byte {
+			0xcd, 0xc8, 0x0d, 0x6f, 0xdd, 0xf1, 0x8c, 0xab, 0x34, 0xc2, 0x59, 0x09, 0xc9, 0x9a, 0x41, 0x74,
+			0xfc, 0xc2, 0x8b, 0x8d, 0x4c, 0x63, 0x83, 0x7c, 0x09, 0xe8, 0x17, 0x00, 0xc1, 0x10, 0x04, 0x01,
+			0x8d, 0x9a, 0x9a, 0xea, 0xc0, 0xf6, 0x59, 0x6f, 0x55, 0x9c, 0x6d, 0x4d, 0xaf, 0x59, 0xa5, 0xf2,
+			0x6d, 0x9f, 0x20, 0x08, 0x57, 0xca, 0x6c, 0x3e, 0x9c, 0xac, 0x52, 0x4b, 0xd9, 0xac, 0xc9, 0x2a,
+		},
+	},
+	ofbTest {
+		"OFB-AES256",
+		commonKey256,
+		commonIV,
+		commonInput,
+		[]byte {
+			0xdc, 0x7e, 0x84, 0xbf, 0xda, 0x79, 0x16, 0x4b, 0x7e, 0xcd, 0x84, 0x86, 0x98, 0x5d, 0x38, 0x60,
+			0x4f, 0xeb, 0xdc, 0x67, 0x40, 0xd2, 0x0b, 0x3a, 0xc8, 0x8f, 0x6a, 0xd8, 0x2a, 0x4f, 0xb0, 0x8d,
+			0x71, 0xab, 0x47, 0xa0, 0x86, 0xe8, 0x6e, 0xed, 0xf3, 0x9d, 0x1c, 0x5b, 0xba, 0x97, 0xc4, 0x08,
+			0x01, 0x26, 0x14, 0x1d, 0x67, 0xf3, 0x7b, 0xe8, 0x53, 0x8f, 0x5a, 0x8b, 0xe7, 0x40, 0xe4, 0x84,
+		}
+	},
+}
+
+func TestOFB_AES(t *testing.T) {
+	for i, tt := range ofbAESTests {
+		test := tt.name;
+
+		c, err := aes.NewCipher(tt.key);
+		if err != nil {
+			t.Errorf("%s: NewCipher(%d bytes) = %s", test, len(tt.key), err);
+			continue;
+		}
+
+		for j := 0; j <= 5; j += 5 {
+			var crypt io.ByteBuffer;
+			in := tt.in[0:len(tt.in) - j];
+			w := NewOFBWriter(c, tt.iv, &crypt);
+			var r io.Reader = io.NewByteReader(in);
+			n, err := io.Copy(r, w);
+			if n != int64(len(in)) || err != nil {
+				t.Errorf("%s/%d: OFBWriter io.Copy = %d, %v want %d, nil", test, len(in), n, err, len(in));
+			} else if d, out := crypt.Data(), tt.out[0:len(in)]; !same(out, d) {
+				t.Errorf("%s/%d: OFBWriter\ninpt %x\nhave %x\nwant %x", test, len(in), in, d, out);
+			}
+		}
+
+		for j := 0; j <= 7; j += 7 {
+			var plain io.ByteBuffer;
+			out := tt.out[0:len(tt.out) - j];
+			r := NewOFBReader(c, tt.iv, io.NewByteReader(out));
+			w := &plain;
+			n, err := io.Copy(r, w);
+			if n != int64(len(out)) || err != nil {
+				t.Errorf("%s/%d: OFBReader io.Copy = %d, %v want %d, nil", test, len(out), n, err, len(out));
+			} else if d, in := plain.Data(), tt.in[0:len(out)]; !same(in, d) {
+				t.Errorf("%s/%d: OFBReader\nhave %x\nwant %x", test, len(out), d, in);
+			}
+		}
+
+		if t.Failed() {
+			break;
+		}
+	}
+}

src/lib/crypto/block/xor.go

+// Copyright 2009 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.
+
+// Encrypt/decrypt data by xor with a pseudo-random data stream.
+
+package block
+
+import (
+	"crypto/block";
+	"io";
+	"os";
+)
+
+type DataStream interface {
+	Next() []byte
+}
+
+type xorReader struct {
+	r io.Reader;
+	rand DataStream;	// pseudo-random
+	buf []byte;		// data available from last call to rand
+}
+
+func NewXorReader(rand DataStream, r io.Reader) io.Reader {
+	x := new(xorReader);
+	x.r = r;
+	x.rand = rand;
+	return x;
+}
+
+func (x *xorReader) Read(p []byte) (n int, err os.Error) {
+	n, err = x.r.Read(p);
+
+	// xor input with stream.
+	bp := 0;
+	buf := x.buf;
+	for i := 0; i < n; i++ {
+		if bp >= len(buf) {
+			buf = x.rand.Next();
+			bp = 0;
+		}
+		p[i] ^= buf[bp];
+		bp++;
+	}
+	x.buf = buf[bp:len(buf)];
+	return n, err;
+}
+
+type xorWriter struct {
+	w io.Writer;
+	rand DataStream;	// pseudo-random
+	buf []byte;	// last buffer returned by rand
+	extra []byte;	// extra random data (use before buf)
+	work []byte;	// work space
+}
+
+func NewXorWriter(rand DataStream, w io.Writer) io.Writer {
+	x := new(xorWriter);
+	x.w = w;
+	x.rand = rand;
+	x.work = make([]byte, 4096);
+	return x;
+}
+
+func (x *xorWriter) Write(p []byte) (n int, err os.Error) {
+	for len(p) > 0 {
+		// Determine next chunk of random data
+		// and xor with p into x.work.
+		var chunk []byte;
+		m := len(p);
+		if nn := len(x.extra); nn > 0 {
+			// extra points into work, so edit directly
+			if m > nn {
+				m = nn;
+			}
+			for i := 0; i < m; i++ {
+				x.extra[i] ^= p[i];
+			}
+			chunk = x.extra[0:m];
+		} else {
+			// xor p ^ buf into work, refreshing buf as needed
+			if nn := len(x.work); m > nn {
+				m = nn;
+			}
+			bp := 0;
+			buf := x.buf;
+			for i := 0; i < m; i++ {
+				if bp >= len(buf) {
+					buf = x.rand.Next();
+					bp = 0;
+				}
+				x.work[i] = buf[bp] ^ p[i];
+				bp++;
+			}
+			x.buf = buf[bp:len(buf)];
+			chunk = x.work[0:m];
+		}
+
+		// Write chunk.
+		var nn int;
+		nn, err = x.w.Write(chunk);
+		if nn != len(chunk) && err == nil {
+			err = io.ErrShortWrite;
+		}
+		if nn < len(chunk) {
+			// Reconstruct the random bits from the unwritten
+			// data and save them for next time.
+			for i := nn; i < m; i++ {
+				chunk[i] ^= p[i];
+			}
+			x.extra = chunk[nn:len(chunk)];
+		}
+		n += nn;
+		if err != nil {
+			return;
+		}
+		p = p[m:len(p)];
+	}
+	return;
+}
+

src/lib/crypto/block/xor_test.go

+// Copyright 2009 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.
+
+package block
+
+import (
+	"crypto/block";
+	"fmt";
+	"io";
+	"testing";
+	"testing/iotest";
+)
+
+// Simple "pseudo-random" stream for testing.
+type incStream struct {
+	buf []byte;
+	n byte;
+}
+
+func newIncStream(blockSize int) *incStream {
+	x := new(incStream);
+	x.buf = make([]byte, blockSize);
+	return x;
+}
+
+func (x *incStream) Next() []byte {
+	x.n++;
+	for i := range x.buf {
+		x.buf[i] = x.n;
+		x.n++;
+	}
+	return x.buf;
+}
+
+func testXorWriter(t *testing.T, maxio int) {
+	var plain, crypt [256]byte;
+	for i := 0; i < len(plain); i++ {
+		plain[i] = byte(i);
+	}
+	b := new(io.ByteBuffer);
+	for block := 1; block <= 64 && block <= maxio; block *= 2 {
+		// compute encrypted version
+		n := byte(0);
+		for i := 0; i < len(crypt); i++ {
+			if i % block == 0 {
+				n++;
+			}
+			crypt[i] = plain[i] ^ n;
+			n++;
+		}
+
+		for frag := 0; frag < 2; frag++ {
+			test := fmt.Sprintf("block=%d frag=%d maxio=%d", block, frag, maxio);
+			b.Reset();
+			r := io.NewByteReader(&plain);
+			s := newIncStream(block);
+			w := NewXorWriter(s, b);
+
+			// copy plain into w in increasingly large chunks: 1, 1, 2, 4, 8, ...
+			// if frag != 0, move the 1 to the end to cause fragmentation.
+			if frag == 0 {
+				nn, err := io.Copyn(r, w, 1);
+				if err != nil {
+					t.Errorf("%s: first Copyn: %s", test, err);
+					continue;
+				}
+			}
+			for n := 1; n <= len(plain)/2; n *= 2 {
+				nn, err := io.Copyn(r, w, int64(n));
+				if err != nil {
+					t.Errorf("%s: Copyn %d: %s", test, n, err);
+				}
+			}
+
+			// check output
+			crypt := crypt[0:len(crypt) - frag];
+			data := b.Data();
+			if len(data) != len(crypt) {
+				t.Errorf("%s: want %d bytes, got %d", test, len(crypt), len(data));
+				continue;
+			}
+
+			if string(data) != string(crypt) {
+				t.Errorf("%s: want %x got %x", test, data, crypt);
+			}
+		}
+	}
+}
+
+
+func TestXorWriter(t *testing.T) {
+	// Do shorter I/O sizes first; they're easier to debug.
+	for n := 1; n <= 256 && !t.Failed(); n *= 2 {
+		testXorWriter(t, n);
+	}
+}
+
+func testXorReader(t *testing.T, maxio int) {
+	var readers = []func(io.Reader) io.Reader {
+		func (r io.Reader) io.Reader { return r },
+		iotest.OneByteReader,
+		iotest.HalfReader,
+	};
+	var plain, crypt [256]byte;
+	for i := 0; i < len(plain); i++ {
+		plain[i] = byte(255 - i);
+	}
+	b := new(io.ByteBuffer);
+	for block := 1; block <= 64 && block <= maxio; block *= 2 {
+		// compute encrypted version
+		n := byte(0);
+		for i := 0; i < len(crypt); i++ {
+			if i % block == 0 {
+				n++;
+			}
+			crypt[i] = plain[i] ^ n;
+			n++;
+		}
+
+		for mode := 0; mode < len(readers); mode++ {
+			for frag := 0; frag < 2; frag++ {
+				test := fmt.Sprintf("block=%d mode=%d frag=%d maxio=%d", block, mode, frag, maxio);
+				s := newIncStream(block);
+				b.Reset();
+				r := NewXorReader(s, readers[mode](io.NewByteReader(crypt[0:maxio])));
+
+				// read from crypt in increasingly large chunks: 1, 1, 2, 4, 8, ...
+				// if frag == 1, move the 1 to the end to cause fragmentation.
+				if frag == 0 {
+					nn, err := io.Copyn(r, b, 1);
+					if err != nil {
+						t.Errorf("%s: first Copyn: %s", test, err);
+						continue;
+					}
+				}
+				for n := 1; n <= maxio/2; n *= 2 {
+					nn, err := io.Copyn(r, b, int64(n));
+					if err != nil {
+						t.Errorf("%s: Copyn %d: %s", test, n, err);
+					}
+				}
+
+				// check output
+				data := b.Data();
+				crypt := crypt[0:maxio - frag];
+				plain := plain[0:maxio - frag];
+				if len(data) != len(plain) {
+					t.Errorf("%s: want %d bytes, got %d", test, len(plain), len(data));
+					continue;
+				}
+
+				if string(data) != string(plain) {
+					t.Errorf("%s: input=%x want %x got %x", test, crypt, plain, data);
+				}
+			}
+		}
+	}
+}
+
+func TestXorReader(t *testing.T) {
+	// Do shorter I/O sizes first; they're easier to debug.
+	for n := 1; n <= 256 && !t.Failed(); n *= 2 {
+		testXorReader(t, n);
+	}
+}
+
+// TODO(rsc): Test handling of writes after write errors.
+