SHA-1 collision finder

sha1_attack.go

package main

import (
  "bytes"
	"crypto/rand"
	"crypto/sha1"
	"encoding/hex"
	"flag"
	"fmt"
)

const numBytes = 8

var hashBytes = flag.Int("bytes", 3, "the number of bytes at the beginning of the hash to consider")

var hasher = sha1.New()

func collision(hashBytes int) (attempts int, bytes1, bytes2, collisionHash []byte) {
	hasher.Reset()

	hashes := map[string][]byte{}

	randBytes := make([]byte, numBytes)
	for {
		attempts++

		rand.Read(randBytes)
		hasher.Write(randBytes)
		h := hasher.Sum(nil)[:hashBytes]
		hashString := hex.EncodeToString(h)

		if _randBytes, ok := hashes[hashString]; ok {
			return attempts, _randBytes, randBytes, h
		} else {
			_randBytes := make([]byte, numBytes)
			copy(_randBytes, randBytes)
			hashes[hashString] = _randBytes
		}

		hasher.Reset()
	}
}

func preImage(search []byte) (attempts int, collisionBytes, collisionHash []byte) {
	hasher.Reset()
	hashBytes := len(search)

	randBytes := make([]byte, numBytes)
	for {
		attempts++

		rand.Read(randBytes)
		hasher.Write(randBytes)
		h := hasher.Sum(nil)[:hashBytes]

		if bytes.Equal(h, search) {
			return attempts, randBytes, h
		}

		hasher.Reset()
	}
}

var attackType = flag.String("type", "collision", "the type of attack to perform (collision, preimage, or both)")

func main() {
	flag.Parse()

	if *attackType == "collision" || *attackType == "both" {
		fmt.Println("Finding collision...")
		attempts, bytes1, bytes2, collisionHash := collision(*hashBytes)
		// attempts, _, _, _ := collision(*hashBytes)
		// fmt.Printf("%v,weak,%v\n", *hashBytes*8, attempts)

		fmt.Printf("Found collision in %v attempts: %x and %x both hash to %x\n\n",
			attempts,
			bytes1,
			bytes2,
			collisionHash,
		)
	}
	if *attackType == "preimage" || *attackType == "both" {
		randBytes := make([]byte, numBytes)
		rand.Read(randBytes)
		hasher.Write(randBytes)
		search := hasher.Sum(nil)[:*hashBytes]

		fmt.Printf("Searching for:   %x (%x)\n", search, randBytes)
		attempts, collisionBytes, collisionHash := preImage(search)
		// attempts, _, _ = preImage(search)
		// fmt.Printf("%v,strong,%v\n", *hashBytes*8, attempts)

		fmt.Printf("Found collision in %v attempts: %x (%x)\n",
			attempts,
			collisionHash,
			collisionBytes,
		)
	}
}

USAGE

Use like:

    sha1_attack --bytes 2 --type preimage
    sha1_attack --bytes 4 --type collision
    sha1_attack --bytes 3 --type both

--bytes is the number of bytes to consider (i.e. the number of bytes to which the SHA-1 digest will be truncated)
--type can be preimage, collision, or both