Add a signature package and stsigtool CLI utility

cmd/stsigtool/main.go

@@ -0,0 +1,102 @@
+// Copyright (C) 2015 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package main
+
+import (
+	"flag"
+	"io/ioutil"
+	"log"
+	"os"
+
+	"github.com/syncthing/syncthing/lib/signature"
+)
+
+func main() {
+	log.SetFlags(0)
+	log.SetOutput(os.Stdout)
+
+	flag.Parse()
+
+	if flag.NArg() < 1 {
+		log.Println(`Usage:
+	stsigtool <command>
+
+Where command is one of:
+
+	gen
+		- generate a new key pair
+
+	sign <privkeyfile> <datafile>
+		- sign a file
+
+	verify <pubkeyfile> <signaturefile> <datafile>
+		- verify a signature
+`)
+	}
+
+	switch flag.Arg(0) {
+	case "gen":
+		gen()
+	case "sign":
+		sign(flag.Arg(1), flag.Arg(2))
+	case "verify":
+		verify(flag.Arg(1), flag.Arg(2), flag.Arg(3))
+	}
+}
+
+func gen() {
+	priv, pub, err := signature.GenerateKeys()
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	os.Stdout.Write(priv)
+	os.Stdout.Write(pub)
+}
+
+func sign(keyname, dataname string) {
+	privkey, err := ioutil.ReadFile(keyname)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fd, err := os.Open(dataname)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer fd.Close()
+
+	sig, err := signature.Sign(privkey, fd)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	os.Stdout.Write(sig)
+}
+
+func verify(keyname, signame, dataname string) {
+	pubkey, err := ioutil.ReadFile(keyname)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	sig, err := ioutil.ReadFile(signame)
+	if err != nil {
+		log.Fatal(err)
+	}
+
+	fd, err := os.Open(dataname)
+	if err != nil {
+		log.Fatal(err)
+	}
+	defer fd.Close()
+
+	err = signature.Verify(pubkey, sig, fd)
+	if err != nil {
+		log.Fatal(err)
+	}
+}

lib/signature/signature.go

@@ -0,0 +1,195 @@
+// Copyright (C) 2015 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+// Package signature provides simple methods to create and verify signatures
+// in PEM format.
+package signature
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/rand"
+	"crypto/sha256"
+	"crypto/x509"
+	"encoding/asn1"
+	"encoding/pem"
+	"errors"
+	"fmt"
+	"io"
+	"math/big"
+)
+
+// GenerateKeys returns a new key pair, with the private and public key
+// encoded in PEM format.
+func GenerateKeys() (privKey []byte, pubKey []byte, err error) {
+	// Generate a new key pair
+	key, err := ecdsa.GenerateKey(elliptic.P521(), rand.Reader)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Marshal the private key
+	bs, err := x509.MarshalECPrivateKey(key)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Encode it in PEM format
+	privKey = pem.EncodeToMemory(&pem.Block{
+		Type:  "EC PRIVATE KEY",
+		Bytes: bs,
+	})
+
+	// Marshal the public key
+	bs, err = x509.MarshalPKIXPublicKey(key.Public())
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Encode it in PEM format
+	pubKey = pem.EncodeToMemory(&pem.Block{
+		Type:  "EC PUBLIC KEY",
+		Bytes: bs,
+	})
+
+	return
+}
+
+// Sign computes the hash of data and signs it with the private key, returning
+// a signature in PEM format.
+func Sign(privKeyPEM []byte, data io.Reader) ([]byte, error) {
+	// Parse the private key
+	key, err := loadPrivateKey(privKeyPEM)
+	if err != nil {
+		return nil, err
+	}
+
+	// Hash the reader data
+	hash, err := hashReader(data)
+	if err != nil {
+		return nil, err
+	}
+
+	// Sign the hash
+	r, s, err := ecdsa.Sign(rand.Reader, key, hash)
+	if err != nil {
+		return nil, err
+	}
+
+	// Marshal the signature using ASN.1
+	sig, err := marshalSignature(r, s)
+	if err != nil {
+		return nil, err
+	}
+
+	// Encode it in a PEM block
+	bs := pem.EncodeToMemory(&pem.Block{
+		Type:  "SIGNATURE",
+		Bytes: sig,
+	})
+
+	return bs, nil
+}
+
+// Verify computes the hash of data and compares it to the signature using the
+// given public key. Returns nil if the signature is correct.
+func Verify(pubKeyPEM []byte, signature []byte, data io.Reader) error {
+	// Parse the public key
+	key, err := loadPublicKey(pubKeyPEM)
+	if err != nil {
+		return err
+	}
+
+	// Parse the signature
+	block, _ := pem.Decode(signature)
+	r, s, err := unmarshalSignature(block.Bytes)
+	if err != nil {
+		return err
+	}
+
+	// Compute the hash of the data
+	hash, err := hashReader(data)
+	if err != nil {
+		return err
+	}
+
+	// Verify the signature
+	if !ecdsa.Verify(key, hash, r, s) {
+		return errors.New("incorrect signature")
+	}
+
+	return nil
+}
+
+// hashReader returns the SHA256 hash of the reader
+func hashReader(r io.Reader) ([]byte, error) {
+	h := sha256.New()
+	if _, err := io.Copy(h, r); err != nil {
+		return nil, err
+	}
+	hash := []byte(fmt.Sprintf("%x", h.Sum(nil)))
+	return hash, nil
+}
+
+// loadPrivateKey returns the ECDSA private key structure for the given PEM
+// data.
+func loadPrivateKey(bs []byte) (*ecdsa.PrivateKey, error) {
+	block, _ := pem.Decode(bs)
+	return x509.ParseECPrivateKey(block.Bytes)
+}
+
+// loadPublicKey returns the ECDSA public key structure for the given PEM
+// data.
+func loadPublicKey(bs []byte) (*ecdsa.PublicKey, error) {
+	// Decode and parse the public key PEM block
+	block, _ := pem.Decode(bs)
+	intf, err := x509.ParsePKIXPublicKey(block.Bytes)
+	if err != nil {
+		return nil, err
+	}
+
+	// It should be an ECDSA public key
+	pk, ok := intf.(*ecdsa.PublicKey)
+	if !ok {
+		return nil, errors.New("unsupported public key format")
+	}
+
+	return pk, nil
+}
+
+// A wrapper around the signature integers so that we can marshal and
+// unmarshal them.
+type signature struct {
+	R, S *big.Int
+}
+
+// marhalSignature returns ASN.1 encoded bytes for the given integers,
+// suitable for PEM encoding.
+func marshalSignature(r, s *big.Int) ([]byte, error) {
+	sig := signature{
+		R: r,
+		S: s,
+	}
+
+	bs, err := asn1.Marshal(sig)
+	if err != nil {
+		return nil, err
+	}
+
+	return bs, nil
+}
+
+// unmarshalSignature returns the R and S integers from the given ASN.1
+// encoded signature.
+func unmarshalSignature(sig []byte) (r *big.Int, s *big.Int, err error) {
+	var ts signature
+	_, err = asn1.Unmarshal(sig, &ts)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return ts.R, ts.S, nil
+}

lib/signature/signature_test.go

@@ -0,0 +1,81 @@
+// Copyright (C) 2015 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at http://mozilla.org/MPL/2.0/.
+
+package signature_test
+
+import (
+	"bytes"
+	"testing"
+
+	"github.com/syncthing/syncthing/lib/signature"
+)
+
+var (
+	// A private key for signing
+	privKey = []byte(`-----BEGIN EC PRIVATE KEY-----
+MIHbAgEBBEFGXB1IgefFF6kSyE17xIAU7fDIn07sPnGf1kLOCVrEZyUbnAmNFk8u
+lUt/knnvo+Gw1i9ucFjmtYtzDevrhSlG5aAHBgUrgQQAI6GBiQOBhgAEASlcbcgJ
+4PN+TSnAYiMlA0I/PRtFrDCgrt27K7hR+U7Afjc4KqW+QYwoRLvxueNh7gUK+zc0
+Aqrk3z+O1epiQTq8ACikHUXsx/bSzEFlPdMygUAAj3hChlgCL6/vOocuRUbtAqc6
+Zr0L9px+J4L0K+uqhyhKya7y6QLJrYPovFq3A7AK
+-----END EC PRIVATE KEY-----`)
+
+	// The matching public key
+	pubKey = []byte(`-----BEGIN EC PUBLIC KEY-----
+MIGbMBAGByqGSM49AgEGBSuBBAAjA4GGAAQBKVxtyAng835NKcBiIyUDQj89G0Ws
+MKCu3bsruFH5TsB+Nzgqpb5BjChEu/G542HuBQr7NzQCquTfP47V6mJBOrwAKKQd
+RezH9tLMQWU90zKBQACPeEKGWAIvr+86hy5FRu0CpzpmvQv2nH4ngvQr66qHKErJ
+rvLpAsmtg+i8WrcDsAo=
+-----END EC PUBLIC KEY-----`)
+
+	// A signature of "this is a string to sign" created with the private key
+	// above
+	exampleSig = []byte(`-----BEGIN SIGNATURE-----
+MIGGAkFdHjdarlFOrtcnCqcb0BX7Mjjq/Sbgp4mopCxBwXmfamtCeRGhZJ5MikyD
+VXScaJ2Dq2Ov7L4/gTcYj9fZwcrWgQJBc7+tcw5fpO0/y8DNq0t3g9bqt2MkmoNm
+eSAM8Fze4usVXHEi+QeMuYM2IKeVPyAR3iyl5gflVul9NRXS3OPAH3A=
+-----END SIGNATURE-----`)
+)
+
+func TestGenerateKeys(t *testing.T) {
+	priv, pub, err := signature.GenerateKeys()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !bytes.Contains(priv, []byte("PRIVATE KEY")) {
+		t.Fatal("should be a private key")
+	}
+	if !bytes.Contains(pub, []byte("PUBLIC KEY")) {
+		t.Fatal("should be a private key")
+	}
+}
+
+func TestSign(t *testing.T) {
+	data := bytes.NewReader([]byte("this is a string to sign"))
+
+	s, err := signature.Sign(privKey, data)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if !bytes.Contains(s, []byte("SIGNATURE")) {
+		t.Error("should be a signature")
+	}
+}
+
+func TestVerify(t *testing.T) {
+	data := bytes.NewReader([]byte("this is a string to sign"))
+	err := signature.Verify(pubKey, exampleSig, data)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	data = bytes.NewReader([]byte("thus is a string to sign"))
+	err = signature.Verify(pubKey, exampleSig, data)
+	if err == nil {
+		t.Fatal("signature should not match")
+	}
+}