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signature.go
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signature.go
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// Copyright 2011 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 packet
import (
"code.google.com/p/go.crypto/openpgp/errors"
"code.google.com/p/go.crypto/openpgp/s2k"
"crypto"
"crypto/dsa"
"crypto/rsa"
"encoding/binary"
"hash"
"io"
"strconv"
"time"
)
// Signature represents a signature. See RFC 4880, section 5.2.
type Signature struct {
SigType SignatureType
PubKeyAlgo PublicKeyAlgorithm
Hash crypto.Hash
// HashSuffix is extra data that is hashed in after the signed data.
HashSuffix []byte
// HashTag contains the first two bytes of the hash for fast rejection
// of bad signed data.
HashTag [2]byte
CreationTime time.Time
RSASignature parsedMPI
DSASigR, DSASigS parsedMPI
ECDSASigR, ECDSASigS parsedMPI
// rawSubpackets contains the unparsed subpackets, in order.
rawSubpackets []outputSubpacket
// The following are optional so are nil when not included in the
// signature.
SigLifetimeSecs, KeyLifetimeSecs *uint32
PreferredSymmetric, PreferredHash, PreferredCompression []uint8
IssuerKeyId *uint64
IsPrimaryId *bool
// FlagsValid is set if any flags were given. See RFC 4880, section
// 5.2.3.21 for details.
FlagsValid bool
FlagCertify, FlagSign, FlagEncryptCommunications, FlagEncryptStorage bool
outSubpackets []outputSubpacket
}
func (sig *Signature) parse(r io.Reader) (err error) {
// RFC 4880, section 5.2.3
var buf [5]byte
_, err = readFull(r, buf[:1])
if err != nil {
return
}
if buf[0] != 4 {
err = errors.UnsupportedError("signature packet version " + strconv.Itoa(int(buf[0])))
return
}
_, err = readFull(r, buf[:5])
if err != nil {
return
}
sig.SigType = SignatureType(buf[0])
sig.PubKeyAlgo = PublicKeyAlgorithm(buf[1])
switch sig.PubKeyAlgo {
case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly, PubKeyAlgoDSA, PubKeyAlgoECDSA:
default:
err = errors.UnsupportedError("public key algorithm " + strconv.Itoa(int(sig.PubKeyAlgo)))
return
}
var ok bool
sig.Hash, ok = s2k.HashIdToHash(buf[2])
if !ok {
return errors.UnsupportedError("hash function " + strconv.Itoa(int(buf[2])))
}
hashedSubpacketsLength := int(buf[3])<<8 | int(buf[4])
l := 6 + hashedSubpacketsLength
sig.HashSuffix = make([]byte, l+6)
sig.HashSuffix[0] = 4
copy(sig.HashSuffix[1:], buf[:5])
hashedSubpackets := sig.HashSuffix[6:l]
_, err = readFull(r, hashedSubpackets)
if err != nil {
return
}
// See RFC 4880, section 5.2.4
trailer := sig.HashSuffix[l:]
trailer[0] = 4
trailer[1] = 0xff
trailer[2] = uint8(l >> 24)
trailer[3] = uint8(l >> 16)
trailer[4] = uint8(l >> 8)
trailer[5] = uint8(l)
err = parseSignatureSubpackets(sig, hashedSubpackets, true)
if err != nil {
return
}
_, err = readFull(r, buf[:2])
if err != nil {
return
}
unhashedSubpacketsLength := int(buf[0])<<8 | int(buf[1])
unhashedSubpackets := make([]byte, unhashedSubpacketsLength)
_, err = readFull(r, unhashedSubpackets)
if err != nil {
return
}
err = parseSignatureSubpackets(sig, unhashedSubpackets, false)
if err != nil {
return
}
_, err = readFull(r, sig.HashTag[:2])
if err != nil {
return
}
switch sig.PubKeyAlgo {
case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
sig.RSASignature.bytes, sig.RSASignature.bitLength, err = readMPI(r)
case PubKeyAlgoDSA:
sig.DSASigR.bytes, sig.DSASigR.bitLength, err = readMPI(r)
if err == nil {
sig.DSASigS.bytes, sig.DSASigS.bitLength, err = readMPI(r)
}
case PubKeyAlgoECDSA:
sig.ECDSASigR.bytes, sig.ECDSASigR.bitLength, err = readMPI(r)
if err == nil {
sig.ECDSASigS.bytes, sig.ECDSASigS.bitLength, err = readMPI(r)
}
default:
panic("unreachable")
}
return
}
// parseSignatureSubpackets parses subpackets of the main signature packet. See
// RFC 4880, section 5.2.3.1.
func parseSignatureSubpackets(sig *Signature, subpackets []byte, isHashed bool) (err error) {
for len(subpackets) > 0 {
subpackets, err = parseSignatureSubpacket(sig, subpackets, isHashed)
if err != nil {
return
}
}
if sig.CreationTime.IsZero() {
err = errors.StructuralError("no creation time in signature")
}
return
}
type signatureSubpacketType uint8
const (
creationTimeSubpacket signatureSubpacketType = 2
signatureExpirationSubpacket signatureSubpacketType = 3
keyExpirationSubpacket signatureSubpacketType = 9
prefSymmetricAlgosSubpacket signatureSubpacketType = 11
issuerSubpacket signatureSubpacketType = 16
prefHashAlgosSubpacket signatureSubpacketType = 21
prefCompressionSubpacket signatureSubpacketType = 22
primaryUserIdSubpacket signatureSubpacketType = 25
keyFlagsSubpacket signatureSubpacketType = 27
)
// parseSignatureSubpacket parses a single subpacket. len(subpacket) is >= 1.
func parseSignatureSubpacket(sig *Signature, subpacket []byte, isHashed bool) (rest []byte, err error) {
// RFC 4880, section 5.2.3.1
var (
length uint32
packetType signatureSubpacketType
isCritical bool
)
switch {
case subpacket[0] < 192:
length = uint32(subpacket[0])
subpacket = subpacket[1:]
case subpacket[0] < 255:
if len(subpacket) < 2 {
goto Truncated
}
length = uint32(subpacket[0]-192)<<8 + uint32(subpacket[1]) + 192
subpacket = subpacket[2:]
default:
if len(subpacket) < 5 {
goto Truncated
}
length = uint32(subpacket[1])<<24 |
uint32(subpacket[2])<<16 |
uint32(subpacket[3])<<8 |
uint32(subpacket[4])
subpacket = subpacket[5:]
}
if length > uint32(len(subpacket)) {
goto Truncated
}
rest = subpacket[length:]
subpacket = subpacket[:length]
if len(subpacket) == 0 {
err = errors.StructuralError("zero length signature subpacket")
return
}
packetType = signatureSubpacketType(subpacket[0] & 0x7f)
isCritical = subpacket[0]&0x80 == 0x80
subpacket = subpacket[1:]
sig.rawSubpackets = append(sig.rawSubpackets, outputSubpacket{isHashed, packetType, isCritical, subpacket})
switch packetType {
case creationTimeSubpacket:
if !isHashed {
err = errors.StructuralError("signature creation time in non-hashed area")
return
}
if len(subpacket) != 4 {
err = errors.StructuralError("signature creation time not four bytes")
return
}
t := binary.BigEndian.Uint32(subpacket)
sig.CreationTime = time.Unix(int64(t), 0)
case signatureExpirationSubpacket:
// Signature expiration time, section 5.2.3.10
if !isHashed {
return
}
if len(subpacket) != 4 {
err = errors.StructuralError("expiration subpacket with bad length")
return
}
sig.SigLifetimeSecs = new(uint32)
*sig.SigLifetimeSecs = binary.BigEndian.Uint32(subpacket)
case keyExpirationSubpacket:
// Key expiration time, section 5.2.3.6
if !isHashed {
return
}
if len(subpacket) != 4 {
err = errors.StructuralError("key expiration subpacket with bad length")
return
}
sig.KeyLifetimeSecs = new(uint32)
*sig.KeyLifetimeSecs = binary.BigEndian.Uint32(subpacket)
case prefSymmetricAlgosSubpacket:
// Preferred symmetric algorithms, section 5.2.3.7
if !isHashed {
return
}
sig.PreferredSymmetric = make([]byte, len(subpacket))
copy(sig.PreferredSymmetric, subpacket)
case issuerSubpacket:
// Issuer, section 5.2.3.5
if len(subpacket) != 8 {
err = errors.StructuralError("issuer subpacket with bad length")
return
}
sig.IssuerKeyId = new(uint64)
*sig.IssuerKeyId = binary.BigEndian.Uint64(subpacket)
case prefHashAlgosSubpacket:
// Preferred hash algorithms, section 5.2.3.8
if !isHashed {
return
}
sig.PreferredHash = make([]byte, len(subpacket))
copy(sig.PreferredHash, subpacket)
case prefCompressionSubpacket:
// Preferred compression algorithms, section 5.2.3.9
if !isHashed {
return
}
sig.PreferredCompression = make([]byte, len(subpacket))
copy(sig.PreferredCompression, subpacket)
case primaryUserIdSubpacket:
// Primary User ID, section 5.2.3.19
if !isHashed {
return
}
if len(subpacket) != 1 {
err = errors.StructuralError("primary user id subpacket with bad length")
return
}
sig.IsPrimaryId = new(bool)
if subpacket[0] > 0 {
*sig.IsPrimaryId = true
}
case keyFlagsSubpacket:
// Key flags, section 5.2.3.21
if !isHashed {
return
}
if len(subpacket) == 0 {
err = errors.StructuralError("empty key flags subpacket")
return
}
sig.FlagsValid = true
if subpacket[0]&1 != 0 {
sig.FlagCertify = true
}
if subpacket[0]&2 != 0 {
sig.FlagSign = true
}
if subpacket[0]&4 != 0 {
sig.FlagEncryptCommunications = true
}
if subpacket[0]&8 != 0 {
sig.FlagEncryptStorage = true
}
default:
if isCritical {
err = errors.UnsupportedError("unknown critical signature subpacket type " + strconv.Itoa(int(packetType)))
return
}
}
return
Truncated:
err = errors.StructuralError("signature subpacket truncated")
return
}
// subpacketLengthLength returns the length, in bytes, of an encoded length value.
func subpacketLengthLength(length int) int {
if length < 192 {
return 1
}
if length < 16320 {
return 2
}
return 5
}
// serializeSubpacketLength marshals the given length into to.
func serializeSubpacketLength(to []byte, length int) int {
// RFC 4880, Section 4.2.2.
if length < 192 {
to[0] = byte(length)
return 1
}
if length < 16320 {
length -= 192
to[0] = byte((length >> 8) + 192)
to[1] = byte(length)
return 2
}
to[0] = 255
to[1] = byte(length >> 24)
to[2] = byte(length >> 16)
to[3] = byte(length >> 8)
to[4] = byte(length)
return 5
}
// subpacketsLength returns the serialized length, in bytes, of the given
// subpackets.
func subpacketsLength(subpackets []outputSubpacket, hashed bool) (length int) {
for _, subpacket := range subpackets {
if subpacket.hashed == hashed {
length += subpacketLengthLength(len(subpacket.contents) + 1)
length += 1 // type byte
length += len(subpacket.contents)
}
}
return
}
// serializeSubpackets marshals the given subpackets into to.
func serializeSubpackets(to []byte, subpackets []outputSubpacket, hashed bool) {
for _, subpacket := range subpackets {
if subpacket.hashed == hashed {
n := serializeSubpacketLength(to, len(subpacket.contents)+1)
to[n] = byte(subpacket.subpacketType)
to = to[1+n:]
n = copy(to, subpacket.contents)
to = to[n:]
}
}
return
}
// KeyExpired returns whether sig is a self-signature of a key that has
// expired.
func (sig *Signature) KeyExpired(currentTime time.Time) bool {
if sig.KeyLifetimeSecs == nil {
return false
}
expiry := sig.CreationTime.Add(time.Duration(*sig.KeyLifetimeSecs) * time.Second)
return currentTime.After(expiry)
}
// buildHashSuffix constructs the HashSuffix member of sig in preparation for signing.
func (sig *Signature) buildHashSuffix() (err error) {
hashedSubpacketsLen := subpacketsLength(sig.outSubpackets, true)
var ok bool
l := 6 + hashedSubpacketsLen
sig.HashSuffix = make([]byte, l+6)
sig.HashSuffix[0] = 4
sig.HashSuffix[1] = uint8(sig.SigType)
sig.HashSuffix[2] = uint8(sig.PubKeyAlgo)
sig.HashSuffix[3], ok = s2k.HashToHashId(sig.Hash)
if !ok {
sig.HashSuffix = nil
return errors.InvalidArgumentError("hash cannot be represented in OpenPGP: " + strconv.Itoa(int(sig.Hash)))
}
sig.HashSuffix[4] = byte(hashedSubpacketsLen >> 8)
sig.HashSuffix[5] = byte(hashedSubpacketsLen)
serializeSubpackets(sig.HashSuffix[6:l], sig.outSubpackets, true)
trailer := sig.HashSuffix[l:]
trailer[0] = 4
trailer[1] = 0xff
trailer[2] = byte(l >> 24)
trailer[3] = byte(l >> 16)
trailer[4] = byte(l >> 8)
trailer[5] = byte(l)
return
}
func (sig *Signature) signPrepareHash(h hash.Hash) (digest []byte, err error) {
err = sig.buildHashSuffix()
if err != nil {
return
}
h.Write(sig.HashSuffix)
digest = h.Sum(nil)
copy(sig.HashTag[:], digest)
return
}
// Sign signs a message with a private key. The hash, h, must contain
// the hash of the message to be signed and will be mutated by this function.
// On success, the signature is stored in sig. Call Serialize to write it out.
// If config is nil, sensible defaults will be used.
func (sig *Signature) Sign(h hash.Hash, priv *PrivateKey, config *Config) (err error) {
sig.outSubpackets = sig.buildSubpackets()
digest, err := sig.signPrepareHash(h)
if err != nil {
return
}
switch priv.PubKeyAlgo {
case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
sig.RSASignature.bytes, err = rsa.SignPKCS1v15(config.Random(), priv.PrivateKey.(*rsa.PrivateKey), sig.Hash, digest)
sig.RSASignature.bitLength = uint16(8 * len(sig.RSASignature.bytes))
case PubKeyAlgoDSA:
dsaPriv := priv.PrivateKey.(*dsa.PrivateKey)
// Need to truncate hashBytes to match FIPS 186-3 section 4.6.
subgroupSize := (dsaPriv.Q.BitLen() + 7) / 8
if len(digest) > subgroupSize {
digest = digest[:subgroupSize]
}
r, s, err := dsa.Sign(config.Random(), dsaPriv, digest)
if err == nil {
sig.DSASigR.bytes = r.Bytes()
sig.DSASigR.bitLength = uint16(8 * len(sig.DSASigR.bytes))
sig.DSASigS.bytes = s.Bytes()
sig.DSASigS.bitLength = uint16(8 * len(sig.DSASigS.bytes))
}
default:
err = errors.UnsupportedError("public key algorithm: " + strconv.Itoa(int(sig.PubKeyAlgo)))
}
return
}
// SignUserId computes a signature from priv, asserting that pub is a valid
// key for the identity id. On success, the signature is stored in sig. Call
// Serialize to write it out.
// If config is nil, sensible defaults will be used.
func (sig *Signature) SignUserId(id string, pub *PublicKey, priv *PrivateKey, config *Config) error {
h, err := userIdSignatureHash(id, pub, sig.Hash)
if err != nil {
return nil
}
return sig.Sign(h, priv, config)
}
// SignKey computes a signature from priv, asserting that pub is a subkey. On
// success, the signature is stored in sig. Call Serialize to write it out.
// If config is nil, sensible defaults will be used.
func (sig *Signature) SignKey(pub *PublicKey, priv *PrivateKey, config *Config) error {
h, err := keySignatureHash(&priv.PublicKey, pub, sig.Hash)
if err != nil {
return err
}
return sig.Sign(h, priv, config)
}
// Serialize marshals sig to w. Sign, SignUserId or SignKey must have been
// called first.
func (sig *Signature) Serialize(w io.Writer) (err error) {
if len(sig.outSubpackets) == 0 {
sig.outSubpackets = sig.rawSubpackets
}
if sig.RSASignature.bytes == nil && sig.DSASigR.bytes == nil && sig.ECDSASigR.bytes == nil {
return errors.InvalidArgumentError("Signature: need to call Sign, SignUserId or SignKey before Serialize")
}
sigLength := 0
switch sig.PubKeyAlgo {
case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
sigLength = 2 + len(sig.RSASignature.bytes)
case PubKeyAlgoDSA:
sigLength = 2 + len(sig.DSASigR.bytes)
sigLength += 2 + len(sig.DSASigS.bytes)
case PubKeyAlgoECDSA:
sigLength = 2 + len(sig.ECDSASigR.bytes)
sigLength += 2 + len(sig.ECDSASigS.bytes)
default:
panic("impossible")
}
unhashedSubpacketsLen := subpacketsLength(sig.outSubpackets, false)
length := len(sig.HashSuffix) - 6 /* trailer not included */ +
2 /* length of unhashed subpackets */ + unhashedSubpacketsLen +
2 /* hash tag */ + sigLength
err = serializeHeader(w, packetTypeSignature, length)
if err != nil {
return
}
_, err = w.Write(sig.HashSuffix[:len(sig.HashSuffix)-6])
if err != nil {
return
}
unhashedSubpackets := make([]byte, 2+unhashedSubpacketsLen)
unhashedSubpackets[0] = byte(unhashedSubpacketsLen >> 8)
unhashedSubpackets[1] = byte(unhashedSubpacketsLen)
serializeSubpackets(unhashedSubpackets[2:], sig.outSubpackets, false)
_, err = w.Write(unhashedSubpackets)
if err != nil {
return
}
_, err = w.Write(sig.HashTag[:])
if err != nil {
return
}
switch sig.PubKeyAlgo {
case PubKeyAlgoRSA, PubKeyAlgoRSASignOnly:
err = writeMPIs(w, sig.RSASignature)
case PubKeyAlgoDSA:
err = writeMPIs(w, sig.DSASigR, sig.DSASigS)
case PubKeyAlgoECDSA:
err = writeMPIs(w, sig.ECDSASigR, sig.ECDSASigS)
default:
panic("impossible")
}
return
}
// outputSubpacket represents a subpacket to be marshaled.
type outputSubpacket struct {
hashed bool // true if this subpacket is in the hashed area.
subpacketType signatureSubpacketType
isCritical bool
contents []byte
}
func (sig *Signature) buildSubpackets() (subpackets []outputSubpacket) {
creationTime := make([]byte, 4)
binary.BigEndian.PutUint32(creationTime, uint32(sig.CreationTime.Unix()))
subpackets = append(subpackets, outputSubpacket{true, creationTimeSubpacket, false, creationTime})
if sig.IssuerKeyId != nil {
keyId := make([]byte, 8)
binary.BigEndian.PutUint64(keyId, *sig.IssuerKeyId)
subpackets = append(subpackets, outputSubpacket{true, issuerSubpacket, false, keyId})
}
if sig.SigLifetimeSecs != nil && *sig.SigLifetimeSecs != 0 {
sigLifetime := make([]byte, 4)
binary.BigEndian.PutUint32(sigLifetime, *sig.SigLifetimeSecs)
subpackets = append(subpackets, outputSubpacket{true, signatureExpirationSubpacket, true, sigLifetime})
}
// Key flags may only appear in self-signatures or certification signatures.
if sig.FlagsValid {
var flags byte
if sig.FlagCertify {
flags |= 1
}
if sig.FlagSign {
flags |= 2
}
if sig.FlagEncryptCommunications {
flags |= 4
}
if sig.FlagEncryptStorage {
flags |= 8
}
subpackets = append(subpackets, outputSubpacket{true, keyFlagsSubpacket, false, []byte{flags}})
}
// The following subpackets may only appear in self-signatures
if sig.KeyLifetimeSecs != nil && *sig.KeyLifetimeSecs != 0 {
keyLifetime := make([]byte, 4)
binary.BigEndian.PutUint32(keyLifetime, *sig.KeyLifetimeSecs)
subpackets = append(subpackets, outputSubpacket{true, keyExpirationSubpacket, true, keyLifetime})
}
if sig.IsPrimaryId != nil && *sig.IsPrimaryId {
subpackets = append(subpackets, outputSubpacket{true, primaryUserIdSubpacket, false, []byte{1}})
}
if len(sig.PreferredSymmetric) > 0 {
subpackets = append(subpackets, outputSubpacket{true, prefSymmetricAlgosSubpacket, false, sig.PreferredSymmetric})
}
if len(sig.PreferredHash) > 0 {
subpackets = append(subpackets, outputSubpacket{true, prefHashAlgosSubpacket, false, sig.PreferredHash})
}
if len(sig.PreferredCompression) > 0 {
subpackets = append(subpackets, outputSubpacket{true, prefCompressionSubpacket, false, sig.PreferredCompression})
}
return
}