Add patched chunker

Referenced in https://forum.restic.net/t/restic-slice-bounds-out-of-range/2617/20
2020-04-24 20:56:45 +02:00 · 2020-04-24 20:56:45 +02:00 · 7e72d638df
commit 7e72d638df
parent 070d43e290
11 changed files with 1650 additions and 0 deletions
--- a/chunker/.travis.yml
+++ b/chunker/.travis.yml
@ -0,0 +1,25 @@
+language: go
+sudo: false
+
+matrix:
+  include:
+    - os: linux
+      go: "1.9.x"
+    - os: linux
+      go: "1.10.x"
+    - os: linux
+      go: "tip"
+    - os: osx
+      go: "1.10.x"
+
+install:
+  - go get -t ./...
+  - go get -u golang.org/x/lint/golint
+  - go get -u golang.org/x/tools/cmd/goimports
+
+script:
+  - go vet ./...
+  - go test -v -cpu=2 ./...
+  - go test -v -cpu=1,2,4 -short -race ./...
+  - diff -au <(goimports -d .) <(printf "")
+  - diff -au <(golint ./...) <(printf "")
--- a/chunker/LICENSE
+++ b/chunker/LICENSE
@ -0,0 +1,23 @@
+Copyright (c) 2014, Alexander Neumann <alexander@bumpern.de>
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
+ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
+WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
--- a/chunker/README.md
+++ b/chunker/README.md
@ -0,0 +1,12 @@
+[![GoDoc](https://godoc.org/github.com/restic/chunker?status.svg)](http://godoc.org/github.com/restic/chunker)
+[![Build Status](https://travis-ci.com/restic/chunker.svg?branch=master)](https://travis-ci.com/restic/chunker)
+
+The package `chunker` implements content-defined-chunking (CDC) based on a
+rolling Rabin Hash. The library is part of the [restic backup
+program](https://github.com/restic/restic).
+
+An introduction to Content Defined Chunking can be found in the restic blog
+post [Foundation - Introducing Content Defined Chunking (CDC)](https://restic.github.io/blog/2015-09-12/restic-foundation1-cdc).
+
+You can find the API documentation at
+https://godoc.org/github.com/restic/chunker
--- a/chunker/chunker.go
+++ b/chunker/chunker.go
@ -0,0 +1,383 @@
+package chunker
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"sync"
+)
+
+const (
+	kiB = 1024
+	miB = 1024 * kiB
+
+	// WindowSize is the size of the sliding window.
+	windowSize = 64
+
+	// MinSize is the default minimal size of a chunk.
+	MinSize = 512 * kiB
+	// MaxSize is the default maximal size of a chunk.
+	MaxSize = 8 * miB
+
+	chunkerBufSize = 512 * kiB
+)
+
+type tables struct {
+	out [256]Pol
+	mod [256]Pol
+}
+
+// cache precomputed tables, these are read-only anyway
+var cache struct {
+	entries map[Pol]tables
+	sync.Mutex
+}
+
+func init() {
+	cache.entries = make(map[Pol]tables)
+}
+
+// Chunk is one content-dependent chunk of bytes whose end was cut when the
+// Rabin Fingerprint had the value stored in Cut.
+type Chunk struct {
+	Start  uint
+	Length uint
+	Cut    uint64
+	Data   []byte
+}
+
+type chunkerState struct {
+	window [windowSize]byte
+	wpos   uint
+
+	buf  []byte
+	bpos uint
+	bmax uint
+
+	start uint
+	count uint
+	pos   uint
+
+	pre uint // wait for this many bytes before start calculating an new chunk
+
+	digest uint64
+}
+
+type chunkerConfig struct {
+	MinSize, MaxSize uint
+
+	pol               Pol
+	polShift          uint
+	tables            tables
+	tablesInitialized bool
+	splitmask         uint64
+
+	rd     io.Reader
+	closed bool
+}
+
+// Chunker splits content with Rabin Fingerprints.
+type Chunker struct {
+	chunkerConfig
+	chunkerState
+}
+
+// SetAverageBits allows to control the frequency of chunk discovery:
+// the lower averageBits, the higher amount of chunks will be identified.
+// The default value is 20 bits, so chunks will be of 1MiB size on average.
+func (c *Chunker) SetAverageBits(averageBits int) {
+	c.splitmask = (1 << uint64(averageBits)) - 1
+}
+
+// New returns a new Chunker based on polynomial p that reads from rd.
+func New(rd io.Reader, pol Pol) *Chunker {
+	return NewWithBoundaries(rd, pol, MinSize, MaxSize)
+}
+
+// NewWithBoundaries returns a new Chunker based on polynomial p that reads from
+// rd and custom min and max size boundaries.
+func NewWithBoundaries(rd io.Reader, pol Pol, min, max uint) *Chunker {
+	c := &Chunker{
+		chunkerState: chunkerState{
+			buf: make([]byte, chunkerBufSize),
+		},
+		chunkerConfig: chunkerConfig{
+			pol:       pol,
+			rd:        rd,
+			MinSize:   min,
+			MaxSize:   max,
+			splitmask: (1 << 20) - 1, // aim to create chunks of 20 bits or about 1MiB on average.
+		},
+	}
+
+	c.reset()
+
+	return c
+}
+
+// Reset reinitializes the chunker with a new reader and polynomial.
+func (c *Chunker) Reset(rd io.Reader, pol Pol) {
+	c.ResetWithBoundaries(rd, pol, MinSize, MaxSize)
+}
+
+// ResetWithBoundaries reinitializes the chunker with a new reader, polynomial
+// and custom min and max size boundaries.
+func (c *Chunker) ResetWithBoundaries(rd io.Reader, pol Pol, min, max uint) {
+	*c = Chunker{
+		chunkerState: chunkerState{
+			buf: c.buf,
+		},
+		chunkerConfig: chunkerConfig{
+			pol:       pol,
+			rd:        rd,
+			MinSize:   min,
+			MaxSize:   max,
+			splitmask: (1 << 20) - 1,
+		},
+	}
+
+	c.reset()
+}
+
+func (c *Chunker) reset() {
+	c.polShift = uint(c.pol.Deg() - 8)
+	c.fillTables()
+
+	for i := 0; i < windowSize; i++ {
+		c.window[i] = 0
+	}
+
+	c.closed = false
+	c.digest = 0
+	c.wpos = 0
+	c.count = 0
+	c.digest = c.slide(c.digest, 1)
+	c.start = c.pos
+
+	// do not start a new chunk unless at least MinSize bytes have been read
+	c.pre = c.MinSize - windowSize
+}
+
+// fillTables calculates out_table and mod_table for optimization. This
+// implementation uses a cache in the global variable cache.
+func (c *Chunker) fillTables() {
+	// if polynomial hasn't been specified, do not compute anything for now
+	if c.pol == 0 {
+		return
+	}
+
+	c.tablesInitialized = true
+
+	// test if the tables are cached for this polynomial
+	cache.Lock()
+	defer cache.Unlock()
+	if t, ok := cache.entries[c.pol]; ok {
+		c.tables = t
+		return
+	}
+
+	// calculate table for sliding out bytes. The byte to slide out is used as
+	// the index for the table, the value contains the following:
+	// out_table[b] = Hash(b || 0 ||        ...        || 0)
+	//                          \ windowsize-1 zero bytes /
+	// To slide out byte b_0 for window size w with known hash
+	// H := H(b_0 || ... || b_w), it is sufficient to add out_table[b_0]:
+	//    H(b_0 || ... || b_w) + H(b_0 || 0 || ... || 0)
+	//  = H(b_0 + b_0 || b_1 + 0 || ... || b_w + 0)
+	//  = H(    0     || b_1 || ...     || b_w)
+	//
+	// Afterwards a new byte can be shifted in.
+	for b := 0; b < 256; b++ {
+		var h Pol
+
+		h = appendByte(h, byte(b), c.pol)
+		for i := 0; i < windowSize-1; i++ {
+			h = appendByte(h, 0, c.pol)
+		}
+		c.tables.out[b] = h
+	}
+
+	// calculate table for reduction mod Polynomial
+	k := c.pol.Deg()
+	for b := 0; b < 256; b++ {
+		// mod_table[b] = A | B, where A = (b(x) * x^k mod pol) and  B = b(x) * x^k
+		//
+		// The 8 bits above deg(Polynomial) determine what happens next and so
+		// these bits are used as a lookup to this table. The value is split in
+		// two parts: Part A contains the result of the modulus operation, part
+		// B is used to cancel out the 8 top bits so that one XOR operation is
+		// enough to reduce modulo Polynomial
+		c.tables.mod[b] = Pol(uint64(b)<<uint(k)).Mod(c.pol) | (Pol(b) << uint(k))
+	}
+
+	cache.entries[c.pol] = c.tables
+}
+
+// Next returns the position and length of the next chunk of data. If an error
+// occurs while reading, the error is returned. Afterwards, the state of the
+// current chunk is undefined. When the last chunk has been returned, all
+// subsequent calls yield an io.EOF error.
+func (c *Chunker) Next(data []byte) (Chunk, error) {
+	data = data[:0]
+	if !c.tablesInitialized {
+		return Chunk{}, errors.New("tables for polynomial computation not initialized")
+	}
+
+	tabout := c.tables.out
+	tabmod := c.tables.mod
+	polShift := c.polShift
+	// go guarantees the expected behavior for bit shifts even for shift counts
+	// larger than the value width. Bounding the value of polShift allows the compiler
+	// to optimize the code for 'digest >> polShift'
+	if polShift > 53-8 {
+		return Chunk{}, errors.New("the polynomial must have a degree less than or equal 53")
+	}
+	minSize := c.MinSize
+	maxSize := c.MaxSize
+	buf := c.buf
+	for {
+		if c.bpos >= c.bmax {
+			n, err := io.ReadFull(c.rd, buf[:])
+
+			if err == io.ErrUnexpectedEOF {
+				err = nil
+			}
+
+			// io.ReadFull only returns io.EOF when no bytes could be read. If
+			// this is the case and we're in this branch, there are no more
+			// bytes to buffer, so this was the last chunk. If a different
+			// error has occurred, return that error and abandon the current
+			// chunk.
+			if err == io.EOF && !c.closed {
+				c.closed = true
+
+				// return current chunk, if any bytes have been processed
+				if c.count > 0 {
+					return Chunk{
+						Start:  c.start,
+						Length: c.count,
+						Cut:    c.digest,
+						Data:   data,
+					}, nil
+				}
+			}
+
+			if err != nil {
+				return Chunk{}, err
+			}
+
+			if n < 0 {
+				return Chunk{}, fmt.Errorf("ReadFull returned negative number of bytes read: %v", n)
+			}
+
+			c.bpos = 0
+			c.bmax = uint(n)
+		}
+
+		// check if bytes have to be dismissed before starting a new chunk
+		if c.pre > 0 {
+			n := c.bmax - c.bpos
+			if c.pre > uint(n) {
+				c.pre -= uint(n)
+				data = append(data, buf[c.bpos:c.bmax]...)
+
+				c.count += uint(n)
+				c.pos += uint(n)
+				c.bpos = c.bmax
+
+				continue
+			}
+
+			data = append(data, buf[c.bpos:c.bpos+c.pre]...)
+
+			c.bpos += c.pre
+			c.count += c.pre
+			c.pos += c.pre
+			c.pre = 0
+		}
+
+		add := c.count
+		digest := c.digest
+		win := c.window
+		wpos := c.wpos
+		for _, b := range buf[c.bpos:c.bmax] {
+			// slide(b)
+			// limit wpos before to elide array bound checks
+			wpos = wpos % windowSize
+			out := win[wpos]
+			win[wpos] = b
+			digest ^= uint64(tabout[out])
+			wpos++
+
+			// updateDigest
+			index := byte(digest >> polShift)
+			digest <<= 8
+			digest |= uint64(b)
+
+			digest ^= uint64(tabmod[index])
+			// end manual inline
+
+			add++
+			if add < minSize {
+				continue
+			}
+
+			if (digest&c.splitmask) == 0 || add >= maxSize {
+				i := add - c.count - 1
+				data = append(data, c.buf[c.bpos:c.bpos+uint(i)+1]...)
+				c.count = add
+				c.pos += uint(i) + 1
+				c.bpos += uint(i) + 1
+				c.buf = buf
+
+				chunk := Chunk{
+					Start:  c.start,
+					Length: c.count,
+					Cut:    digest,
+					Data:   data,
+				}
+
+				c.reset()
+
+				return chunk, nil
+			}
+		}
+		c.digest = digest
+		c.window = win
+		c.wpos = wpos % windowSize
+
+		steps := c.bmax - c.bpos
+		if steps > 0 {
+			data = append(data, c.buf[c.bpos:c.bpos+steps]...)
+		}
+		c.count += steps
+		c.pos += steps
+		c.bpos = c.bmax
+	}
+}
+
+func updateDigest(digest uint64, polShift uint, tab tables, b byte) (newDigest uint64) {
+	index := digest >> polShift
+	digest <<= 8
+	digest |= uint64(b)
+
+	digest ^= uint64(tab.mod[index])
+	return digest
+}
+
+func (c *Chunker) slide(digest uint64, b byte) (newDigest uint64) {
+	out := c.window[c.wpos]
+	c.window[c.wpos] = b
+	digest ^= uint64(c.tables.out[out])
+	c.wpos = (c.wpos + 1) % windowSize
+
+	digest = updateDigest(digest, c.polShift, c.tables, b)
+	return digest
+}
+
+func appendByte(hash Pol, b byte, pol Pol) Pol {
+	hash <<= 8
+	hash |= Pol(b)
+
+	return hash.Mod(pol)
+}
--- a/chunker/chunker_test.go
+++ b/chunker/chunker_test.go
@ -0,0 +1,347 @@
+package chunker
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
+	"io"
+	"math/rand"
+	"testing"
+	"time"
+)
+
+func parseDigest(s string) []byte {
+	d, err := hex.DecodeString(s)
+	if err != nil {
+		panic(err)
+	}
+
+	return d
+}
+
+type chunk struct {
+	Length uint
+	CutFP  uint64
+	Digest []byte
+}
+
+// polynomial used for all the tests below
+const testPol = Pol(0x3DA3358B4DC173)
+
+// created for 32MB of random data out of math/rand's Uint32() seeded by
+// constant 23
+//
+// chunking configuration:
+// window size 64, avg chunksize 1<<20, min chunksize 1<<19, max chunksize 1<<23
+// polynom 0x3DA3358B4DC173
+var chunks1 = []chunk{
+	chunk{2163460, 0x000b98d4cdf00000, parseDigest("4b94cb2cf293855ea43bf766731c74969b91aa6bf3c078719aabdd19860d590d")},
+	chunk{643703, 0x000d4e8364d00000, parseDigest("5727a63c0964f365ab8ed2ccf604912f2ea7be29759a2b53ede4d6841e397407")},
+	chunk{1528956, 0x0015a25c2ef00000, parseDigest("a73759636a1e7a2758767791c69e81b69fb49236c6929e5d1b654e06e37674ba")},
+	chunk{1955808, 0x00102a8242e00000, parseDigest("c955fb059409b25f07e5ae09defbbc2aadf117c97a3724e06ad4abd2787e6824")},
+	chunk{2222372, 0x00045da878000000, parseDigest("6ba5e9f7e1b310722be3627716cf469be941f7f3e39a4c3bcefea492ec31ee56")},
+	chunk{2538687, 0x00198a8179900000, parseDigest("8687937412f654b5cfe4a82b08f28393a0c040f77c6f95e26742c2fc4254bfde")},
+	chunk{609606, 0x001d4e8d17100000, parseDigest("5da820742ff5feb3369112938d3095785487456f65a8efc4b96dac4be7ebb259")},
+	chunk{1205738, 0x000a7204dd600000, parseDigest("cc70d8fad5472beb031b1aca356bcab86c7368f40faa24fe5f8922c6c268c299")},
+	chunk{959742, 0x00183e71e1400000, parseDigest("4065bdd778f95676c92b38ac265d361f81bff17d76e5d9452cf985a2ea5a4e39")},
+	chunk{4036109, 0x001fec043c700000, parseDigest("b9cf166e75200eb4993fc9b6e22300a6790c75e6b0fc8f3f29b68a752d42f275")},
+	chunk{1525894, 0x000b1574b1500000, parseDigest("2f238180e4ca1f7520a05f3d6059233926341090f9236ce677690c1823eccab3")},
+	chunk{1352720, 0x00018965f2e00000, parseDigest("afd12f13286a3901430de816e62b85cc62468c059295ce5888b76b3af9028d84")},
+	chunk{811884, 0x00155628aa100000, parseDigest("42d0cdb1ee7c48e552705d18e061abb70ae7957027db8ae8db37ec756472a70a")},
+	chunk{1282314, 0x001909a0a1400000, parseDigest("819721c2457426eb4f4c7565050c44c32076a56fa9b4515a1c7796441730eb58")},
+	chunk{1318021, 0x001cceb980000000, parseDigest("842eb53543db55bacac5e25cb91e43cc2e310fe5f9acc1aee86bdf5e91389374")},
+	chunk{948640, 0x0011f7a470a00000, parseDigest("b8e36bf7019bb96ac3fb7867659d2167d9d3b3148c09fe0de45850b8fe577185")},
+	chunk{645464, 0x00030ce2d9400000, parseDigest("5584bd27982191c3329f01ed846bfd266e96548dfa87018f745c33cfc240211d")},
+	chunk{533758, 0x0004435c53c00000, parseDigest("4da778a25b72a9a0d53529eccfe2e5865a789116cb1800f470d8df685a8ab05d")},
+	chunk{1128303, 0x0000c48517800000, parseDigest("08c6b0b38095b348d80300f0be4c5184d2744a17147c2cba5cc4315abf4c048f")},
+	chunk{800374, 0x000968473f900000, parseDigest("820284d2c8fd243429674c996d8eb8d3450cbc32421f43113e980f516282c7bf")},
+	chunk{2453512, 0x001e197c92600000, parseDigest("5fa870ed107c67704258e5e50abe67509fb73562caf77caa843b5f243425d853")},
+	chunk{2651975, 0x000ae6c868000000, parseDigest("181347d2bbec32bef77ad5e9001e6af80f6abcf3576549384d334ee00c1988d8")},
+	chunk{237392, 0x0000000000000001, parseDigest("fcd567f5d866357a8e299fd5b2359bb2c8157c30395229c4e9b0a353944a7978")},
+}
+
+// test if nullbytes are correctly split, even if length is a multiple of MinSize.
+var chunks2 = []chunk{
+	chunk{MinSize, 0, parseDigest("07854d2fef297a06ba81685e660c332de36d5d18d546927d30daad6d7fda1541")},
+	chunk{MinSize, 0, parseDigest("07854d2fef297a06ba81685e660c332de36d5d18d546927d30daad6d7fda1541")},
+	chunk{MinSize, 0, parseDigest("07854d2fef297a06ba81685e660c332de36d5d18d546927d30daad6d7fda1541")},
+	chunk{MinSize, 0, parseDigest("07854d2fef297a06ba81685e660c332de36d5d18d546927d30daad6d7fda1541")},
+}
+
+// the same as chunks1, but avg chunksize is 1<<19
+var chunks3 = []chunk{
+	chunk{1491586, 0x00023e586ea80000, parseDigest("4c008237df602048039287427171cef568a6cb965d1b5ca28dc80504a24bb061")},
+	chunk{671874, 0x000b98d4cdf00000, parseDigest("fa8a42321b90c3d4ce9dd850562b2fd0c0fe4bdd26cf01a24f22046a224225d3")},
+	chunk{643703, 0x000d4e8364d00000, parseDigest("5727a63c0964f365ab8ed2ccf604912f2ea7be29759a2b53ede4d6841e397407")},
+	chunk{1284146, 0x0012b527e4780000, parseDigest("16d04cafecbeae9eaedd49da14c7ad7cdc2b1cc8569e5c16c32c9fb045aa899a")},
+	chunk{823366, 0x000d1d6752180000, parseDigest("48662c118514817825ad4761e8e2e5f28f9bd8281b07e95dcafc6d02e0aa45c3")},
+	chunk{810134, 0x0016071b6e180000, parseDigest("f629581aa05562f97f2c359890734c8574c5575da32f9289c5ba70bfd05f3f46")},
+	chunk{567118, 0x00102a8242e00000, parseDigest("d4f0797c56c60d01bac33bfd49957a4816b6c067fc155b026de8a214cab4d70a")},
+	chunk{821315, 0x001b3e42c8180000, parseDigest("8ebd0fd5db0293bd19140da936eb8b1bbd3cd6ffbec487385b956790014751ca")},
+	chunk{1401057, 0x00045da878000000, parseDigest("001360af59adf4871ef138cfa2bb49007e86edaf5ac2d6f0b3d3014510991848")},
+	chunk{2311122, 0x0005cbd885380000, parseDigest("8276d489b566086d9da95dc5c5fe6fc7d72646dd3308ced6b5b6ddb8595f0aa1")},
+	chunk{608723, 0x001cfcd86f280000, parseDigest("518db33ba6a79d4f3720946f3785c05b9611082586d47ea58390fc2f6de9449e")},
+	chunk{980456, 0x0013edb7a7f80000, parseDigest("0121b1690738395e15fecba1410cd0bf13fde02225160cad148829f77e7b6c99")},
+	chunk{1140278, 0x0001f9f017e80000, parseDigest("28ca7c74804b5075d4f5eeb11f0845d99f62e8ea3a42b9a05c7bd5f2fca619dd")},
+	chunk{2015542, 0x00097bf5d8180000, parseDigest("6fe8291f427d48650a5f0f944305d3a2dbc649bd401d2655fc0bdd42e890ca5a")},
+	chunk{904752, 0x000e1863eff80000, parseDigest("62af1f1eb3f588d18aff28473303cc4731fc3cafcc52ce818fee3c4c2820854d")},
+	chunk{713072, 0x001f3bb1b9b80000, parseDigest("4bda9dc2e3031d004d87a5cc93fe5207c4b0843186481b8f31597dc6ffa1496c")},
+	chunk{675937, 0x001fec043c700000, parseDigest("5299c8c5acec1b90bb020cd75718aab5e12abb9bf66291465fd10e6a823a8b4a")},
+	chunk{1525894, 0x000b1574b1500000, parseDigest("2f238180e4ca1f7520a05f3d6059233926341090f9236ce677690c1823eccab3")},
+	chunk{1352720, 0x00018965f2e00000, parseDigest("afd12f13286a3901430de816e62b85cc62468c059295ce5888b76b3af9028d84")},
+	chunk{811884, 0x00155628aa100000, parseDigest("42d0cdb1ee7c48e552705d18e061abb70ae7957027db8ae8db37ec756472a70a")},
+	chunk{1282314, 0x001909a0a1400000, parseDigest("819721c2457426eb4f4c7565050c44c32076a56fa9b4515a1c7796441730eb58")},
+	chunk{1093738, 0x0017f5d048880000, parseDigest("5dddfa7a241b68f65d267744bdb082ee865f3c2f0d8b946ea0ee47868a01bbff")},
+	chunk{962003, 0x000b921f7ef80000, parseDigest("0cb5c9ebba196b441c715c8d805f6e7143a81cd5b0d2c65c6aacf59ca9124af9")},
+	chunk{856384, 0x00030ce2d9400000, parseDigest("7734b206d46f3f387e8661e81edf5b1a91ea681867beb5831c18aaa86632d7fb")},
+	chunk{533758, 0x0004435c53c00000, parseDigest("4da778a25b72a9a0d53529eccfe2e5865a789116cb1800f470d8df685a8ab05d")},
+	chunk{1128303, 0x0000c48517800000, parseDigest("08c6b0b38095b348d80300f0be4c5184d2744a17147c2cba5cc4315abf4c048f")},
+	chunk{800374, 0x000968473f900000, parseDigest("820284d2c8fd243429674c996d8eb8d3450cbc32421f43113e980f516282c7bf")},
+	chunk{2453512, 0x001e197c92600000, parseDigest("5fa870ed107c67704258e5e50abe67509fb73562caf77caa843b5f243425d853")},
+	chunk{665901, 0x00118c842cb80000, parseDigest("deceec26163842fdef6560311c69bf8a9871a56e16d719e2c4b7e4d668ceb61f")},
+	chunk{1986074, 0x000ae6c868000000, parseDigest("64cd64bf3c3bc389eb20df8310f0427d1c36ab2eaaf09e346bfa7f0453fc1a18")},
+	chunk{237392, 0x0000000000000001, parseDigest("fcd567f5d866357a8e299fd5b2359bb2c8157c30395229c4e9b0a353944a7978")},
+}
+
+func testWithData(t *testing.T, chnker *Chunker, testChunks []chunk, checkDigest bool) []Chunk {
+	chunks := []Chunk{}
+
+	pos := uint(0)
+	for i, chunk := range testChunks {
+		c, err := chnker.Next(nil)
+
+		if err != nil {
+			t.Fatalf("Error returned with chunk %d: %v", i, err)
+		}
+
+		if c.Start != pos {
+			t.Fatalf("Start for chunk %d does not match: expected %d, got %d",
+				i, pos, c.Start)
+		}
+
+		if c.Length != chunk.Length {
+			t.Fatalf("Length for chunk %d does not match: expected %d, got %d",
+				i, chunk.Length, c.Length)
+		}
+
+		if c.Cut != chunk.CutFP {
+			t.Fatalf("Cut fingerprint for chunk %d/%d does not match: expected %016x, got %016x",
+				i, len(chunks)-1, chunk.CutFP, c.Cut)
+		}
+
+		if checkDigest {
+			digest := hashData(c.Data)
+			if !bytes.Equal(chunk.Digest, digest) {
+				t.Fatalf("Digest fingerprint for chunk %d/%d does not match: expected %02x, got %02x",
+					i, len(chunks)-1, chunk.Digest, digest)
+			}
+		}
+
+		pos += c.Length
+		chunks = append(chunks, c)
+	}
+
+	_, err := chnker.Next(nil)
+	if err != io.EOF {
+		t.Fatal("Wrong error returned after last chunk")
+	}
+
+	if len(chunks) != len(testChunks) {
+		t.Fatal("Amounts of test and resulting chunks do not match")
+	}
+
+	return chunks
+}
+
+func getRandom(seed int64, count int) []byte {
+	buf := make([]byte, count)
+
+	rnd := rand.New(rand.NewSource(seed))
+	for i := 0; i < count; i += 4 {
+		r := rnd.Uint32()
+		buf[i] = byte(r)
+		buf[i+1] = byte(r >> 8)
+		buf[i+2] = byte(r >> 16)
+		buf[i+3] = byte(r >> 24)
+	}
+
+	return buf
+}
+
+func hashData(d []byte) []byte {
+	h := sha256.New()
+	h.Write(d)
+	return h.Sum(nil)
+}
+
+func TestChunker(t *testing.T) {
+	// setup data source
+	buf := getRandom(23, 32*1024*1024)
+	ch := New(bytes.NewReader(buf), testPol)
+	testWithData(t, ch, chunks1, true)
+
+	// setup nullbyte data source
+	buf = bytes.Repeat([]byte{0}, len(chunks2)*MinSize)
+	ch = New(bytes.NewReader(buf), testPol)
+
+	testWithData(t, ch, chunks2, true)
+}
+
+func TestChunkerWithCustomAverageBits(t *testing.T) {
+	buf := getRandom(23, 32*1024*1024)
+	ch := New(bytes.NewReader(buf), testPol)
+
+	// sligthly decrease averageBits to get more chunks
+	ch.SetAverageBits(19)
+	testWithData(t, ch, chunks3, true)
+}
+
+func TestChunkerReset(t *testing.T) {
+	buf := getRandom(23, 32*1024*1024)
+	ch := New(bytes.NewReader(buf), testPol)
+	testWithData(t, ch, chunks1, true)
+
+	ch.Reset(bytes.NewReader(buf), testPol)
+	testWithData(t, ch, chunks1, true)
+}
+
+func TestChunkerWithRandomPolynomial(t *testing.T) {
+	// setup data source
+	buf := getRandom(23, 32*1024*1024)
+
+	// generate a new random polynomial
+	start := time.Now()
+	p, err := RandomPolynomial()
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("generating random polynomial took %v", time.Since(start))
+
+	start = time.Now()
+	ch := New(bytes.NewReader(buf), p)
+	t.Logf("creating chunker took %v", time.Since(start))
+
+	// make sure that first chunk is different
+	c, err := ch.Next(nil)
+	if err != nil {
+		t.Fatal(err.Error())
+	}
+
+	if c.Cut == chunks1[0].CutFP {
+		t.Fatal("Cut point is the same")
+	}
+
+	if c.Length == chunks1[0].Length {
+		t.Fatal("Length is the same")
+	}
+
+	if bytes.Equal(hashData(c.Data), chunks1[0].Digest) {
+		t.Fatal("Digest is the same")
+	}
+}
+
+func TestChunkerWithoutHash(t *testing.T) {
+	// setup data source
+	buf := getRandom(23, 32*1024*1024)
+
+	ch := New(bytes.NewReader(buf), testPol)
+	chunks := testWithData(t, ch, chunks1, false)
+
+	// test reader
+	for i, c := range chunks {
+		if uint(len(c.Data)) != chunks1[i].Length {
+			t.Fatalf("reader returned wrong number of bytes: expected %d, got %d",
+				chunks1[i].Length, len(c.Data))
+		}
+
+		if !bytes.Equal(buf[c.Start:c.Start+c.Length], c.Data) {
+			t.Fatalf("invalid data for chunk returned: expected %02x, got %02x",
+				buf[c.Start:c.Start+c.Length], c.Data)
+		}
+	}
+
+	// setup nullbyte data source
+	buf = bytes.Repeat([]byte{0}, len(chunks2)*MinSize)
+	ch = New(bytes.NewReader(buf), testPol)
+
+	testWithData(t, ch, chunks2, false)
+}
+
+func benchmarkChunker(b *testing.B, checkDigest bool) {
+	size := 32 * 1024 * 1024
+	rd := bytes.NewReader(getRandom(23, size))
+	ch := New(rd, testPol)
+	buf := make([]byte, MaxSize)
+
+	b.ResetTimer()
+	b.SetBytes(int64(size))
+
+	var chunks int
+	for i := 0; i < b.N; i++ {
+		chunks = 0
+
+		_, err := rd.Seek(0, 0)
+		if err != nil {
+			b.Fatalf("Seek() return error %v", err)
+		}
+
+		ch.Reset(rd, testPol)
+
+		cur := 0
+		for {
+			chunk, err := ch.Next(buf)
+
+			if err == io.EOF {
+				break
+			}
+
+			if err != nil {
+				b.Fatalf("Unexpected error occurred: %v", err)
+			}
+
+			if chunk.Length != chunks1[cur].Length {
+				b.Errorf("wrong chunk length, want %d, got %d",
+					chunks1[cur].Length, chunk.Length)
+			}
+
+			if chunk.Cut != chunks1[cur].CutFP {
+				b.Errorf("wrong cut fingerprint, want 0x%x, got 0x%x",
+					chunks1[cur].CutFP, chunk.Cut)
+			}
+
+			if checkDigest {
+				h := hashData(chunk.Data)
+				if !bytes.Equal(h, chunks1[cur].Digest) {
+					b.Errorf("wrong digest, want %x, got %x",
+						chunks1[cur].Digest, h)
+				}
+			}
+
+			chunks++
+			cur++
+		}
+	}
+
+	b.Logf("%d chunks, average chunk size: %d bytes", chunks, size/chunks)
+}
+
+func BenchmarkChunkerWithSHA256(b *testing.B) {
+	benchmarkChunker(b, true)
+}
+
+func BenchmarkChunker(b *testing.B) {
+	benchmarkChunker(b, false)
+}
+
+func BenchmarkNewChunker(b *testing.B) {
+	p, err := RandomPolynomial()
+	if err != nil {
+		b.Fatal(err)
+	}
+
+	b.ResetTimer()
+
+	for i := 0; i < b.N; i++ {
+		New(bytes.NewBuffer(nil), p)
+	}
+}
--- a/chunker/doc.go
+++ b/chunker/doc.go
@ -0,0 +1,82 @@
+// Copyright 2014 Alexander Neumann. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+/*
+Package chunker implements Content Defined Chunking (CDC) based on a rolling
+Rabin Checksum.
+
+Choosing a Random Irreducible Polynomial
+
+The function RandomPolynomial() returns a new random polynomial of degree 53
+for use with the chunker. The degree 53 is chosen because it is the largest
+prime below 64-8 = 56, so that the top 8 bits of an uint64 can be used for
+optimising calculations in the chunker.
+
+A random polynomial is chosen selecting 64 random bits, masking away bits
+64..54 and setting bit 53 to one (otherwise the polynomial is not of the
+desired degree) and bit 0 to one (otherwise the polynomial is trivially
+reducible), so that 51 bits are chosen at random.
+
+This process is repeated until Irreducible() returns true, then this
+polynomials is returned. If this doesn't happen after 1 million tries, the
+function returns an error. The probability for selecting an irreducible
+polynomial at random is about 7.5% ( (2^53-2)/53 / 2^51), so the probability
+that no irreducible polynomial has been found after 100 tries is lower than
+0.04%.
+
+Verifying Irreducible Polynomials
+
+During development the results have been verified using the computational
+discrete algebra system GAP, which can be obtained from the website at
+http://www.gap-system.org/.
+
+For filtering a given list of polynomials in hexadecimal coefficient notation,
+the following script can be used:
+
+	# create x over F_2 = GF(2)
+	x := Indeterminate(GF(2), "x");
+
+	# test if polynomial is irreducible, i.e. the number of factors is one
+	IrredPoly := function (poly)
+		return (Length(Factors(poly)) = 1);
+	end;;
+
+	# create a polynomial in x from the hexadecimal representation of the
+	# coefficients
+	Hex2Poly := function (s)
+		return ValuePol(CoefficientsQadic(IntHexString(s), 2), x);
+	end;;
+
+	# list of candidates, in hex
+	candidates := [ "3DA3358B4DC173" ];
+
+	# create real polynomials
+	L := List(candidates, Hex2Poly);
+
+	# filter and display the list of irreducible polynomials contained in L
+	Display(Filtered(L, x -> (IrredPoly(x))));
+
+All irreducible polynomials from the list are written to the output.
+
+Background Literature
+
+An introduction to Rabin Fingerprints/Checksums can be found in the following articles:
+
+Michael O. Rabin (1981): "Fingerprinting by Random Polynomials"
+http://www.xmailserver.org/rabin.pdf
+
+Ross N. Williams (1993): "A Painless Guide to CRC Error Detection Algorithms"
+http://www.zlib.net/crc_v3.txt
+
+Andrei Z. Broder (1993): "Some Applications of Rabin's Fingerprinting Method"
+http://www.xmailserver.org/rabin_apps.pdf
+
+Shuhong Gao and Daniel Panario (1997): "Tests and Constructions of Irreducible Polynomials over Finite Fields"
+http://www.math.clemson.edu/~sgao/papers/GP97a.pdf
+
+Andrew Kadatch, Bob Jenkins (2007): "Everything we know about CRC but afraid to forget"
+http://crcutil.googlecode.com/files/crc-doc.1.0.pdf
+
+*/
+package chunker
--- a/chunker/example_test.go
+++ b/chunker/example_test.go
@ -0,0 +1,39 @@
+package chunker
+
+import (
+	"bytes"
+	"crypto/sha256"
+	"fmt"
+	"io"
+)
+
+func ExampleChunker() {
+	// generate 32MiB of deterministic pseudo-random data
+	data := getRandom(23, 32*1024*1024)
+
+	// create a chunker
+	chunker := New(bytes.NewReader(data), Pol(0x3DA3358B4DC173))
+
+	// reuse this buffer
+	buf := make([]byte, 8*1024*1024)
+
+	for i := 0; i < 5; i++ {
+		chunk, err := chunker.Next(buf)
+		if err == io.EOF {
+			break
+		}
+
+		if err != nil {
+			panic(err)
+		}
+
+		fmt.Printf("%d %02x\n", chunk.Length, sha256.Sum256(chunk.Data))
+	}
+
+	// Output:
+	// 2163460 4b94cb2cf293855ea43bf766731c74969b91aa6bf3c078719aabdd19860d590d
+	// 643703 5727a63c0964f365ab8ed2ccf604912f2ea7be29759a2b53ede4d6841e397407
+	// 1528956 a73759636a1e7a2758767791c69e81b69fb49236c6929e5d1b654e06e37674ba
+	// 1955808 c955fb059409b25f07e5ae09defbbc2aadf117c97a3724e06ad4abd2787e6824
+	// 2222372 6ba5e9f7e1b310722be3627716cf469be941f7f3e39a4c3bcefea492ec31ee56
+}
--- a/chunker/go.mod
+++ b/chunker/go.mod
@ -0,0 +1,3 @@
+module github.com/restic/chunker
+
+go 1.14
--- a/chunker/polynomials.go
+++ b/chunker/polynomials.go
@ -0,0 +1,310 @@
+package chunker
+
+import (
+	"crypto/rand"
+	"encoding/binary"
+	"errors"
+	"fmt"
+	"io"
+	"strconv"
+)
+
+// Pol is a polynomial from F_2[X].
+type Pol uint64
+
+// Add returns x+y.
+func (x Pol) Add(y Pol) Pol {
+	r := Pol(uint64(x) ^ uint64(y))
+	return r
+}
+
+// mulOverflows returns true if the multiplication would overflow uint64.
+// Code by Rob Pike, see
+// https://groups.google.com/d/msg/golang-nuts/h5oSN5t3Au4/KaNQREhZh0QJ
+func mulOverflows(a, b Pol) bool {
+	if a <= 1 || b <= 1 {
+		return false
+	}
+	c := a.mul(b)
+	d := c.Div(b)
+	if d != a {
+		return true
+	}
+
+	return false
+}
+
+func (x Pol) mul(y Pol) Pol {
+	if x == 0 || y == 0 {
+		return 0
+	}
+
+	var res Pol
+	for i := 0; i <= y.Deg(); i++ {
+		if (y & (1 << uint(i))) > 0 {
+			res = res.Add(x << uint(i))
+		}
+	}
+
+	return res
+}
+
+// Mul returns x*y. When an overflow occurs, Mul panics.
+func (x Pol) Mul(y Pol) Pol {
+	if mulOverflows(x, y) {
+		panic("multiplication would overflow uint64")
+	}
+
+	return x.mul(y)
+}
+
+// Deg returns the degree of the polynomial x. If x is zero, -1 is returned.
+func (x Pol) Deg() int {
+	// the degree of 0 is -1
+	if x == 0 {
+		return -1
+	}
+
+	// see https://graphics.stanford.edu/~seander/bithacks.html#IntegerLog
+
+	r := 0
+	if uint64(x)&0xffffffff00000000 > 0 {
+		x >>= 32
+		r |= 32
+	}
+
+	if uint64(x)&0xffff0000 > 0 {
+		x >>= 16
+		r |= 16
+	}
+
+	if uint64(x)&0xff00 > 0 {
+		x >>= 8
+		r |= 8
+	}
+
+	if uint64(x)&0xf0 > 0 {
+		x >>= 4
+		r |= 4
+	}
+
+	if uint64(x)&0xc > 0 {
+		x >>= 2
+		r |= 2
+	}
+
+	if uint64(x)&0x2 > 0 {
+		r |= 1
+	}
+
+	return r
+}
+
+// String returns the coefficients in hex.
+func (x Pol) String() string {
+	return "0x" + strconv.FormatUint(uint64(x), 16)
+}
+
+// Expand returns the string representation of the polynomial x.
+func (x Pol) Expand() string {
+	if x == 0 {
+		return "0"
+	}
+
+	s := ""
+	for i := x.Deg(); i > 1; i-- {
+		if x&(1<<uint(i)) > 0 {
+			s += fmt.Sprintf("+x^%d", i)
+		}
+	}
+
+	if x&2 > 0 {
+		s += "+x"
+	}
+
+	if x&1 > 0 {
+		s += "+1"
+	}
+
+	return s[1:]
+}
+
+// DivMod returns x / d = q, and remainder r,
+// see https://en.wikipedia.org/wiki/Division_algorithm
+func (x Pol) DivMod(d Pol) (Pol, Pol) {
+	if x == 0 {
+		return 0, 0
+	}
+
+	if d == 0 {
+		panic("division by zero")
+	}
+
+	D := d.Deg()
+	diff := x.Deg() - D
+	if diff < 0 {
+		return 0, x
+	}
+
+	var q Pol
+	for diff >= 0 {
+		m := d << uint(diff)
+		q |= (1 << uint(diff))
+		x = x.Add(m)
+
+		diff = x.Deg() - D
+	}
+
+	return q, x
+}
+
+// Div returns the integer division result x / d.
+func (x Pol) Div(d Pol) Pol {
+	q, _ := x.DivMod(d)
+	return q
+}
+
+// Mod returns the remainder of x / d
+func (x Pol) Mod(d Pol) Pol {
+	_, r := x.DivMod(d)
+	return r
+}
+
+// I really dislike having a function that does not terminate, so specify a
+// really large upper bound for finding a new irreducible polynomial, and
+// return an error when no irreducible polynomial has been found within
+// randPolMaxTries.
+const randPolMaxTries = 1e6
+
+// RandomPolynomial returns a new random irreducible polynomial
+// of degree 53 using the default System CSPRNG as source.
+// It is equivalent to calling DerivePolynomial(rand.Reader).
+func RandomPolynomial() (Pol, error) {
+	return DerivePolynomial(rand.Reader)
+}
+
+// DerivePolynomial returns an irreducible polynomial of degree 53
+// (largest prime number below 64-8) by reading bytes from source.
+// There are (2^53-2/53) irreducible polynomials of degree 53 in
+// F_2[X], c.f. Michael O. Rabin (1981): "Fingerprinting by Random
+// Polynomials", page 4. If no polynomial could be found in one
+// million tries, an error is returned.
+func DerivePolynomial(source io.Reader) (Pol, error) {
+	for i := 0; i < randPolMaxTries; i++ {
+		var f Pol
+
+		// choose polynomial at (pseudo)random
+		err := binary.Read(source, binary.LittleEndian, &f)
+		if err != nil {
+			return 0, err
+		}
+
+		// mask away bits above bit 53
+		f &= Pol((1 << 54) - 1)
+
+		// set highest and lowest bit so that the degree is 53 and the
+		// polynomial is not trivially reducible
+		f |= (1 << 53) | 1
+
+		// test if f is irreducible
+		if f.Irreducible() {
+			return f, nil
+		}
+	}
+
+	// If this is reached, we haven't found an irreducible polynomial in
+	// randPolMaxTries. This error is very unlikely to occur.
+	return 0, errors.New("unable to find new random irreducible polynomial")
+}
+
+// GCD computes the Greatest Common Divisor x and f.
+func (x Pol) GCD(f Pol) Pol {
+	if f == 0 {
+		return x
+	}
+
+	if x == 0 {
+		return f
+	}
+
+	if x.Deg() < f.Deg() {
+		x, f = f, x
+	}
+
+	return f.GCD(x.Mod(f))
+}
+
+// Irreducible returns true iff x is irreducible over F_2. This function
+// uses Ben Or's reducibility test.
+//
+// For details see "Tests and Constructions of Irreducible Polynomials over
+// Finite Fields".
+func (x Pol) Irreducible() bool {
+	for i := 1; i <= x.Deg()/2; i++ {
+		if x.GCD(qp(uint(i), x)) != 1 {
+			return false
+		}
+	}
+
+	return true
+}
+
+// MulMod computes x*f mod g
+func (x Pol) MulMod(f, g Pol) Pol {
+	if x == 0 || f == 0 {
+		return 0
+	}
+
+	var res Pol
+	for i := 0; i <= f.Deg(); i++ {
+		if (f & (1 << uint(i))) > 0 {
+			a := x
+			for j := 0; j < i; j++ {
+				a = a.Mul(2).Mod(g)
+			}
+			res = res.Add(a).Mod(g)
+		}
+	}
+
+	return res
+}
+
+// qp computes the polynomial (x^(2^p)-x) mod g. This is needed for the
+// reducibility test.
+func qp(p uint, g Pol) Pol {
+	num := (1 << p)
+	i := 1
+
+	// start with x
+	res := Pol(2)
+
+	for i < num {
+		// repeatedly square res
+		res = res.MulMod(res, g)
+		i *= 2
+	}
+
+	// add x
+	return res.Add(2).Mod(g)
+}
+
+// MarshalJSON returns the JSON representation of the Pol.
+func (x Pol) MarshalJSON() ([]byte, error) {
+	buf := strconv.AppendUint([]byte{'"'}, uint64(x), 16)
+	buf = append(buf, '"')
+	return buf, nil
+}
+
+// UnmarshalJSON parses a Pol from the JSON data.
+func (x *Pol) UnmarshalJSON(data []byte) error {
+	if len(data) < 2 {
+		return errors.New("invalid string for polynomial")
+	}
+	n, err := strconv.ParseUint(string(data[1:len(data)-1]), 16, 64)
+	if err != nil {
+		return err
+	}
+	*x = Pol(n)
+
+	return nil
+}
--- a/chunker/polynomials_test.go
+++ b/chunker/polynomials_test.go
@ -0,0 +1,424 @@
+package chunker
+
+import (
+	"strconv"
+	"testing"
+)
+
+var polAddTests = []struct {
+	x, y Pol
+	sum  Pol
+}{
+	{23, 16, 23 ^ 16},
+	{0x9a7e30d1e855e0a0, 0x670102a1f4bcd414, 0xfd7f32701ce934b4},
+	{0x9a7e30d1e855e0a0, 0x9a7e30d1e855e0a0, 0},
+}
+
+func TestPolAdd(t *testing.T) {
+	for i, test := range polAddTests {
+		if test.sum != test.x.Add(test.y) {
+			t.Errorf("test %d failed: sum != x+y", i)
+		}
+
+		if test.sum != test.y.Add(test.x) {
+			t.Errorf("test %d failed: sum != y+x", i)
+		}
+	}
+}
+
+func parseBin(s string) Pol {
+	i, err := strconv.ParseUint(s, 2, 64)
+	if err != nil {
+		panic(err)
+	}
+
+	return Pol(i)
+}
+
+var polMulTests = []struct {
+	x, y Pol
+	res  Pol
+}{
+	{1, 2, 2},
+	{
+		parseBin("1101"),
+		parseBin("10"),
+		parseBin("11010"),
+	},
+	{
+		parseBin("1101"),
+		parseBin("11"),
+		parseBin("10111"),
+	},
+	{
+		0x40000000,
+		0x40000000,
+		0x1000000000000000,
+	},
+	{
+		parseBin("1010"),
+		parseBin("100100"),
+		parseBin("101101000"),
+	},
+	{
+		parseBin("100"),
+		parseBin("11"),
+		parseBin("1100"),
+	},
+	{
+		parseBin("11"),
+		parseBin("110101"),
+		parseBin("1011111"),
+	},
+	{
+		parseBin("10011"),
+		parseBin("110101"),
+		parseBin("1100001111"),
+	},
+}
+
+func TestPolMul(t *testing.T) {
+	for i, test := range polMulTests {
+		m := test.x.Mul(test.y)
+		if test.res != m {
+			t.Errorf("TestPolMul failed for test %d: %v * %v: want %v, got %v",
+				i, test.x, test.y, test.res, m)
+		}
+		m = test.y.Mul(test.x)
+		if test.res != test.y.Mul(test.x) {
+			t.Errorf("TestPolMul failed for %d: %v * %v: want %v, got %v",
+				i, test.x, test.y, test.res, m)
+		}
+	}
+}
+
+func TestPolMulOverflow(t *testing.T) {
+	defer func() {
+		// try to recover overflow error
+		err := recover()
+
+		if e, ok := err.(string); ok && e == "multiplication would overflow uint64" {
+			return
+		}
+
+		t.Logf("invalid error raised: %v", err)
+		// re-raise error if not overflow
+		panic(err)
+	}()
+
+	x := Pol(1 << 63)
+	x.Mul(2)
+	t.Fatal("overflow test did not panic")
+}
+
+var polDivTests = []struct {
+	x, y Pol
+	res  Pol
+}{
+	{10, 50, 0},
+	{0, 1, 0},
+	{
+		parseBin("101101000"), // 0x168
+		parseBin("1010"),      // 0xa
+		parseBin("100100"),    // 0x24
+	},
+	{2, 2, 1},
+	{
+		0x8000000000000000,
+		0x8000000000000000,
+		1,
+	},
+	{
+		parseBin("1100"),
+		parseBin("100"),
+		parseBin("11"),
+	},
+	{
+		parseBin("1100001111"),
+		parseBin("10011"),
+		parseBin("110101"),
+	},
+}
+
+func TestPolDiv(t *testing.T) {
+	for i, test := range polDivTests {
+		m := test.x.Div(test.y)
+		if test.res != m {
+			t.Errorf("TestPolDiv failed for test %d: %v * %v: want %v, got %v",
+				i, test.x, test.y, test.res, m)
+		}
+	}
+}
+
+func TestPolDeg(t *testing.T) {
+	var x Pol
+	if x.Deg() != -1 {
+		t.Errorf("deg(0) is not -1: %v", x.Deg())
+	}
+
+	x = 1
+	if x.Deg() != 0 {
+		t.Errorf("deg(1) is not 0: %v", x.Deg())
+	}
+
+	for i := 0; i < 64; i++ {
+		x = 1 << uint(i)
+		if x.Deg() != i {
+			t.Errorf("deg(1<<%d) is not %d: %v", i, i, x.Deg())
+		}
+	}
+}
+
+var polModTests = []struct {
+	x, y Pol
+	res  Pol
+}{
+	{10, 50, 10},
+	{0, 1, 0},
+	{
+		parseBin("101101001"),
+		parseBin("1010"),
+		parseBin("1"),
+	},
+	{2, 2, 0},
+	{
+		0x8000000000000000,
+		0x8000000000000000,
+		0,
+	},
+	{
+		parseBin("1100"),
+		parseBin("100"),
+		parseBin("0"),
+	},
+	{
+		parseBin("1100001111"),
+		parseBin("10011"),
+		parseBin("0"),
+	},
+}
+
+func TestPolModt(t *testing.T) {
+	for i, test := range polModTests {
+		res := test.x.Mod(test.y)
+		if test.res != res {
+			t.Errorf("test %d failed: want %v, got %v", i, test.res, res)
+		}
+	}
+}
+
+func BenchmarkPolDivMod(t *testing.B) {
+	f := Pol(0x2482734cacca49)
+	g := Pol(0x3af4b284899)
+
+	for i := 0; i < t.N; i++ {
+		g.DivMod(f)
+	}
+}
+
+func BenchmarkPolDiv(t *testing.B) {
+	f := Pol(0x2482734cacca49)
+	g := Pol(0x3af4b284899)
+
+	for i := 0; i < t.N; i++ {
+		g.Div(f)
+	}
+}
+
+func BenchmarkPolMod(t *testing.B) {
+	f := Pol(0x2482734cacca49)
+	g := Pol(0x3af4b284899)
+
+	for i := 0; i < t.N; i++ {
+		g.Mod(f)
+	}
+}
+
+func BenchmarkPolDeg(t *testing.B) {
+	f := Pol(0x3af4b284899)
+	d := f.Deg()
+	if d != 41 {
+		t.Fatalf("BenchmalPolDeg: Wrong degree %d returned, expected %d",
+			d, 41)
+	}
+
+	for i := 0; i < t.N; i++ {
+		f.Deg()
+	}
+}
+
+func TestRandomPolynomial(t *testing.T) {
+	_, err := RandomPolynomial()
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
+func BenchmarkRandomPolynomial(t *testing.B) {
+	for i := 0; i < t.N; i++ {
+		_, err := RandomPolynomial()
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+func TestExpandPolynomial(t *testing.T) {
+	pol := Pol(0x3DA3358B4DC173)
+	s := pol.Expand()
+	if s != "x^53+x^52+x^51+x^50+x^48+x^47+x^45+x^41+x^40+x^37+x^36+x^34+x^32+x^31+x^27+x^25+x^24+x^22+x^19+x^18+x^16+x^15+x^14+x^8+x^6+x^5+x^4+x+1" {
+		t.Fatal("wrong result")
+	}
+}
+
+var polIrredTests = []struct {
+	f     Pol
+	irred bool
+}{
+	{0x38f1e565e288df, false},
+	{0x3DA3358B4DC173, true},
+	{0x30a8295b9d5c91, false},
+	{0x255f4350b962cb, false},
+	{0x267f776110a235, false},
+	{0x2f4dae10d41227, false},
+	{0x2482734cacca49, true},
+	{0x312daf4b284899, false},
+	{0x29dfb6553d01d1, false},
+	{0x3548245eb26257, false},
+	{0x3199e7ef4211b3, false},
+	{0x362f39017dae8b, false},
+	{0x200d57aa6fdacb, false},
+	{0x35e0a4efa1d275, false},
+	{0x2ced55b026577f, false},
+	{0x260b012010893d, false},
+	{0x2df29cbcd59e9d, false},
+	{0x3f2ac7488bd429, false},
+	{0x3e5cb1711669fb, false},
+	{0x226d8de57a9959, false},
+	{0x3c8de80aaf5835, false},
+	{0x2026a59efb219b, false},
+	{0x39dfa4d13fb231, false},
+	{0x3143d0464b3299, false},
+}
+
+func TestPolIrreducible(t *testing.T) {
+	for _, test := range polIrredTests {
+		if test.f.Irreducible() != test.irred {
+			t.Errorf("Irreducibility test for Polynomial %v failed: got %v, wanted %v",
+				test.f, test.f.Irreducible(), test.irred)
+		}
+	}
+}
+
+func BenchmarkPolIrreducible(b *testing.B) {
+	// find first irreducible polynomial
+	var pol Pol
+	for _, test := range polIrredTests {
+		if test.irred {
+			pol = test.f
+			break
+		}
+	}
+
+	for i := 0; i < b.N; i++ {
+		if !pol.Irreducible() {
+			b.Errorf("Irreducibility test for Polynomial %v failed", pol)
+		}
+	}
+}
+
+var polGCDTests = []struct {
+	f1  Pol
+	f2  Pol
+	gcd Pol
+}{
+	{10, 50, 2},
+	{0, 1, 1},
+	{
+		parseBin("101101001"),
+		parseBin("1010"),
+		parseBin("1"),
+	},
+	{2, 2, 2},
+	{
+		parseBin("1010"),
+		parseBin("11"),
+		parseBin("11"),
+	},
+	{
+		0x8000000000000000,
+		0x8000000000000000,
+		0x8000000000000000,
+	},
+	{
+		parseBin("1100"),
+		parseBin("101"),
+		parseBin("11"),
+	},
+	{
+		parseBin("1100001111"),
+		parseBin("10011"),
+		parseBin("10011"),
+	},
+	{
+		0x3DA3358B4DC173,
+		0x3DA3358B4DC173,
+		0x3DA3358B4DC173,
+	},
+	{
+		0x3DA3358B4DC173,
+		0x230d2259defd,
+		1,
+	},
+	{
+		0x230d2259defd,
+		0x51b492b3eff2,
+		parseBin("10011"),
+	},
+}
+
+func TestPolGCD(t *testing.T) {
+	for i, test := range polGCDTests {
+		gcd := test.f1.GCD(test.f2)
+		if test.gcd != gcd {
+			t.Errorf("GCD test %d (%+v) failed: got %v, wanted %v",
+				i, test, gcd, test.gcd)
+		}
+
+		gcd = test.f2.GCD(test.f1)
+		if test.gcd != gcd {
+			t.Errorf("GCD test %d (%+v) failed: got %v, wanted %v",
+				i, test, gcd, test.gcd)
+		}
+	}
+}
+
+var polMulModTests = []struct {
+	f1  Pol
+	f2  Pol
+	g   Pol
+	mod Pol
+}{
+	{
+		0x1230,
+		0x230,
+		0x55,
+		0x22,
+	},
+	{
+		0x0eae8c07dbbb3026,
+		0xd5d6db9de04771de,
+		0xdd2bda3b77c9,
+		0x425ae8595b7a,
+	},
+}
+
+func TestPolMulMod(t *testing.T) {
+	for i, test := range polMulModTests {
+		mod := test.f1.MulMod(test.f2, test.g)
+		if mod != test.mod {
+			t.Errorf("MulMod test %d (%+v) failed: got %v, wanted %v",
+				i, test, mod, test.mod)
+		}
+	}
+}
--- a/go.mod
+++ b/go.mod
@ -49,3 +49,5 @@ require (
 )

 go 1.13
+
+replace github.com/restic/chunker => ./chunker