// 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 s2k implements the various OpenPGP string-to-key transforms as
// specified in RFC 4800 section 3.7.1.
package s2k // import "golang.org/x/crypto/openpgp/s2k"
import (
"crypto"
"hash"
"io"
"strconv"
"golang.org/x/crypto/openpgp/errors"
)
// Config collects configuration parameters for s2k key-stretching
// transformatioms. A nil *Config is valid and results in all default
// values. Currently, Config is used only by the Serialize function in
// this package.
type Config struct {
// Hash is the default hash function to be used. If
// nil, SHA1 is used.
Hash crypto.Hash
// S2KCount is only used for symmetric encryption. It
// determines the strength of the passphrase stretching when
// the said passphrase is hashed to produce a key. S2KCount
// should be between 1024 and 65011712, inclusive. If Config
// is nil or S2KCount is 0, the value 65536 used. Not all
// values in the above range can be represented. S2KCount will
// be rounded up to the next representable value if it cannot
// be encoded exactly. When set, it is strongly encrouraged to
// use a value that is at least 65536. See RFC 4880 Section
// 3.7.1.3.
S2KCount int
}
func (c *Config) hash() crypto.Hash {
if c == nil || uint(c.Hash) == 0 {
// SHA1 is the historical default in this package.
return crypto.SHA1
}
return c.Hash
}
func (c *Config) encodedCount() uint8 {
if c == nil || c.S2KCount == 0 {
return 96 // The common case. Correspoding to 65536
}
i := c.S2KCount
switch {
// Behave like GPG. Should we make 65536 the lowest value used?
case i < 1024:
i = 1024
case i > 65011712:
i = 65011712
}
return encodeCount(i)
}
// encodeCount converts an iterative "count" in the range 1024 to
// 65011712, inclusive, to an encoded count. The return value is the
// octet that is actually stored in the GPG file. encodeCount panics
// if i is not in the above range (encodedCount above takes care to
// pass i in the correct range). See RFC 4880 Section 3.7.7.1.
func encodeCount(i int) uint8 {
if i < 1024 || i > 65011712 {
panic("count arg i outside the required range")
}
for encoded := 0; encoded < 256; encoded++ {
count := decodeCount(uint8(encoded))
if count >= i {
return uint8(encoded)
}
}
return 255
}
// decodeCount returns the s2k mode 3 iterative "count" corresponding to
// the encoded octet c.
func decodeCount(c uint8) int {
return (16 + int(c&15)) << (uint32(c>>4) + 6)
}
// Simple writes to out the result of computing the Simple S2K function (RFC
// 4880, section 3.7.1.1) using the given hash and input passphrase.
func Simple(out []byte, h hash.Hash, in []byte) {
Salted(out, h, in, nil)
}
var zero [1]byte
// Salted writes to out the result of computing the Salted S2K function (RFC
// 4880, section 3.7.1.2) using the given hash, input passphrase and salt.
func Salted(out []byte, h hash.Hash, in []byte, salt []byte) {
done := 0
var digest []byte
for i := 0; done < len(out); i++ {
h.Reset()
for j := 0; j < i; j++ {
h.Write(zero[:])
}
h.Write(salt)
h.Write(in)
digest = h.Sum(digest[:0])
n := copy(out[done:], digest)
done += n
}
}
// Iterated writes to out the result of computing the Iterated and Salted S2K
// function (RFC 4880, section 3.7.1.3) using the given hash, input passphrase,
// salt and iteration count.
func Iterated(out []byte, h hash.Hash, in []byte, salt []byte, count int) {
combined := make([]byte, len(in)+len(salt))
copy(combined, salt)
copy(combined[len(salt):], in)
if count < len(combined) {
count = len(combined)
}
done := 0
var digest []byte
for i := 0; done < len(out); i++ {
h.Reset()
for j := 0; j < i; j++ {
h.Write(zero[:])
}
written := 0
for written < count {
if written+len(combined) > count {
todo := count - written
h.Write(combined[:todo])
written = count
} else {
h.Write(combined)
written += len(combined)
}
}
digest = h.Sum(digest[:0])
n := copy(out[done:], digest)
done += n
}
}
// Parse reads a binary specification for a string-to-key transformation from r
// and returns a function which performs that transform.
func Parse(r io.Reader) (f func(out, in []byte), err error) {
var buf [9]byte
_, err = io.ReadFull(r, buf[:2])
if err != nil {
return
}
hash, ok := HashIdToHash(buf[1])
if !ok {
return nil, errors.UnsupportedError("hash for S2K function: " + strconv.Itoa(int(buf[1])))
}
if !hash.Available() {
return nil, errors.UnsupportedError("hash not available: " + strconv.Itoa(int(hash)))
}
h := hash.New()
switch buf[0] {
case 0:
f := func(out, in []byte) {
Simple(out, h, in)
}
return f, nil
case 1:
_, err = io.ReadFull(r, buf[:8])
if err != nil {
return
}
f := func(out, in []byte) {
Salted(out, h, in, buf[:8])
}
return f, nil
case 3:
_, err = io.ReadFull(r, buf[:9])
if err != nil {
return
}
count := decodeCount(buf[8])
f := func(out, in []byte) {
Iterated(out, h, in, buf[:8], count)
}
return f, nil
}
return nil, errors.UnsupportedError("S2K function")
}
// Serialize salts and stretches the given passphrase and writes the
// resulting key into key. It also serializes an S2K descriptor to
// w. The key stretching can be configured with c, which may be
// nil. In that case, sensible defaults will be used.
func Serialize(w io.Writer, key []byte, rand io.Reader, passphrase []byte, c *Config) error {
var buf [11]byte
buf[0] = 3 /* iterated and salted */
buf[1], _ = HashToHashId(c.hash())
salt := buf[2:10]
if _, err := io.ReadFull(rand, salt); err != nil {
return err
}
encodedCount := c.encodedCount()
count := decodeCount(encodedCount)
buf[10] = encodedCount
if _, err := w.Write(buf[:]); err != nil {
return err
}
Iterated(key, c.hash().New(), passphrase, salt, count)
return nil
}
// hashToHashIdMapping contains pairs relating OpenPGP's hash identifier with
// Go's crypto.Hash type. See RFC 4880, section 9.4.
var hashToHashIdMapping = []struct {
id byte
hash crypto.Hash
name string
}{
{1, crypto.MD5, "MD5"},
{2, crypto.SHA1, "SHA1"},
{3, crypto.RIPEMD160, "RIPEMD160"},
{8, crypto.SHA256, "SHA256"},
{9, crypto.SHA384, "SHA384"},
{10, crypto.SHA512, "SHA512"},
{11, crypto.SHA224, "SHA224"},
}
// HashIdToHash returns a crypto.Hash which corresponds to the given OpenPGP
// hash id.
func HashIdToHash(id byte) (h crypto.Hash, ok bool) {
for _, m := range hashToHashIdMapping {
if m.id == id {
return m.hash, true
}
}
return 0, false
}
// HashIdToString returns the name of the hash function corresponding to the
// given OpenPGP hash id.
func HashIdToString(id byte) (name string, ok bool) {
for _, m := range hashToHashIdMapping {
if m.id == id {
return m.name, true
}
}
return "", false
}
// HashIdToHash returns an OpenPGP hash id which corresponds the given Hash.
func HashToHashId(h crypto.Hash) (id byte, ok bool) {
for _, m := range hashToHashIdMapping {
if m.hash == h {
return m.id, true
}
}
return 0, false
}
