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326 lines
8.2 KiB
326 lines
8.2 KiB
// Copyright 2011 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Package s2k implements the various OpenPGP string-to-key transforms as
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// specified in RFC 4800 section 3.7.1.
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package s2k // import "github.com/keybase/go-crypto/openpgp/s2k"
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import (
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"crypto"
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"hash"
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"io"
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"strconv"
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"github.com/keybase/go-crypto/openpgp/errors"
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)
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// Config collects configuration parameters for s2k key-stretching
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// transformatioms. A nil *Config is valid and results in all default
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// values. Currently, Config is used only by the Serialize function in
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// this package.
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type Config struct {
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// Hash is the default hash function to be used. If
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// nil, SHA1 is used.
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Hash crypto.Hash
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// S2KCount is only used for symmetric encryption. It
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// determines the strength of the passphrase stretching when
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// the said passphrase is hashed to produce a key. S2KCount
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// should be between 1024 and 65011712, inclusive. If Config
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// is nil or S2KCount is 0, the value 65536 used. Not all
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// values in the above range can be represented. S2KCount will
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// be rounded up to the next representable value if it cannot
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// be encoded exactly. When set, it is strongly encrouraged to
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// use a value that is at least 65536. See RFC 4880 Section
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// 3.7.1.3.
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S2KCount int
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}
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func (c *Config) hash() crypto.Hash {
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if c == nil || uint(c.Hash) == 0 {
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// SHA1 is the historical default in this package.
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return crypto.SHA1
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}
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return c.Hash
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}
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func (c *Config) encodedCount() uint8 {
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if c == nil || c.S2KCount == 0 {
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return 96 // The common case. Correspoding to 65536
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}
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i := c.S2KCount
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switch {
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// Behave like GPG. Should we make 65536 the lowest value used?
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case i < 1024:
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i = 1024
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case i > 65011712:
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i = 65011712
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}
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return encodeCount(i)
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}
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// encodeCount converts an iterative "count" in the range 1024 to
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// 65011712, inclusive, to an encoded count. The return value is the
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// octet that is actually stored in the GPG file. encodeCount panics
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// if i is not in the above range (encodedCount above takes care to
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// pass i in the correct range). See RFC 4880 Section 3.7.7.1.
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func encodeCount(i int) uint8 {
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if i < 1024 || i > 65011712 {
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panic("count arg i outside the required range")
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}
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for encoded := 0; encoded < 256; encoded++ {
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count := decodeCount(uint8(encoded))
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if count >= i {
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return uint8(encoded)
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}
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}
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return 255
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}
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// decodeCount returns the s2k mode 3 iterative "count" corresponding to
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// the encoded octet c.
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func decodeCount(c uint8) int {
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return (16 + int(c&15)) << (uint32(c>>4) + 6)
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}
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// Simple writes to out the result of computing the Simple S2K function (RFC
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// 4880, section 3.7.1.1) using the given hash and input passphrase.
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func Simple(out []byte, h hash.Hash, in []byte) {
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Salted(out, h, in, nil)
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}
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var zero [1]byte
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// Salted writes to out the result of computing the Salted S2K function (RFC
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// 4880, section 3.7.1.2) using the given hash, input passphrase and salt.
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func Salted(out []byte, h hash.Hash, in []byte, salt []byte) {
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done := 0
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var digest []byte
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for i := 0; done < len(out); i++ {
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h.Reset()
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for j := 0; j < i; j++ {
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h.Write(zero[:])
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}
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h.Write(salt)
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h.Write(in)
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digest = h.Sum(digest[:0])
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n := copy(out[done:], digest)
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done += n
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}
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}
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// Iterated writes to out the result of computing the Iterated and Salted S2K
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// function (RFC 4880, section 3.7.1.3) using the given hash, input passphrase,
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// salt and iteration count.
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func Iterated(out []byte, h hash.Hash, in []byte, salt []byte, count int) {
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combined := make([]byte, len(in)+len(salt))
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copy(combined, salt)
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copy(combined[len(salt):], in)
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if count < len(combined) {
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count = len(combined)
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}
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done := 0
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var digest []byte
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for i := 0; done < len(out); i++ {
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h.Reset()
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for j := 0; j < i; j++ {
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h.Write(zero[:])
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}
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written := 0
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for written < count {
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if written+len(combined) > count {
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todo := count - written
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h.Write(combined[:todo])
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written = count
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} else {
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h.Write(combined)
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written += len(combined)
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}
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}
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digest = h.Sum(digest[:0])
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n := copy(out[done:], digest)
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done += n
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}
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}
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func parseGNUExtensions(r io.Reader) (f func(out, in []byte), err error) {
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var buf [9]byte
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// A three-byte string identifier
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_, err = io.ReadFull(r, buf[:3])
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if err != nil {
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return
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}
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gnuExt := string(buf[:3])
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if gnuExt != "GNU" {
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return nil, errors.UnsupportedError("Malformed GNU extension: " + gnuExt)
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}
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_, err = io.ReadFull(r, buf[:1])
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if err != nil {
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return
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}
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gnuExtType := int(buf[0])
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switch gnuExtType {
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case 1:
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return nil, nil
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case 2:
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// Read a serial number, which is prefixed by a 1-byte length.
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// The maximum length is 16.
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var lenBuf [1]byte
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_, err = io.ReadFull(r, lenBuf[:])
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if err != nil {
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return
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}
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maxLen := 16
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ivLen := int(lenBuf[0])
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if ivLen > maxLen {
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ivLen = maxLen
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}
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ivBuf := make([]byte, ivLen)
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// For now we simply discard the IV
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_, err = io.ReadFull(r, ivBuf)
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if err != nil {
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return
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}
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return nil, nil
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default:
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return nil, errors.UnsupportedError("unknown S2K GNU protection mode: " + strconv.Itoa(int(gnuExtType)))
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}
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}
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// Parse reads a binary specification for a string-to-key transformation from r
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// and returns a function which performs that transform.
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func Parse(r io.Reader) (f func(out, in []byte), err error) {
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var buf [9]byte
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_, err = io.ReadFull(r, buf[:2])
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if err != nil {
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return
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}
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// GNU Extensions; handle them before we try to look for a hash, which won't
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// be needed in most cases anyway.
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if buf[0] == 101 {
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return parseGNUExtensions(r)
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}
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hash, ok := HashIdToHash(buf[1])
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if !ok {
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return nil, errors.UnsupportedError("hash for S2K function: " + strconv.Itoa(int(buf[1])))
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}
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if !hash.Available() {
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return nil, errors.UnsupportedError("hash not available: " + strconv.Itoa(int(hash)))
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}
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h := hash.New()
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switch buf[0] {
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case 0:
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f := func(out, in []byte) {
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Simple(out, h, in)
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}
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return f, nil
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case 1:
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_, err = io.ReadFull(r, buf[:8])
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if err != nil {
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return
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}
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f := func(out, in []byte) {
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Salted(out, h, in, buf[:8])
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}
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return f, nil
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case 3:
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_, err = io.ReadFull(r, buf[:9])
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if err != nil {
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return
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}
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count := decodeCount(buf[8])
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f := func(out, in []byte) {
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Iterated(out, h, in, buf[:8], count)
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}
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return f, nil
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}
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return nil, errors.UnsupportedError("S2K function")
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}
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// Serialize salts and stretches the given passphrase and writes the
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// resulting key into key. It also serializes an S2K descriptor to
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// w. The key stretching can be configured with c, which may be
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// nil. In that case, sensible defaults will be used.
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func Serialize(w io.Writer, key []byte, rand io.Reader, passphrase []byte, c *Config) error {
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var buf [11]byte
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buf[0] = 3 /* iterated and salted */
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buf[1], _ = HashToHashId(c.hash())
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salt := buf[2:10]
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if _, err := io.ReadFull(rand, salt); err != nil {
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return err
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}
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encodedCount := c.encodedCount()
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count := decodeCount(encodedCount)
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buf[10] = encodedCount
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if _, err := w.Write(buf[:]); err != nil {
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return err
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}
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Iterated(key, c.hash().New(), passphrase, salt, count)
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return nil
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}
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// hashToHashIdMapping contains pairs relating OpenPGP's hash identifier with
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// Go's crypto.Hash type. See RFC 4880, section 9.4.
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var hashToHashIdMapping = []struct {
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id byte
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hash crypto.Hash
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name string
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}{
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{1, crypto.MD5, "MD5"},
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{2, crypto.SHA1, "SHA1"},
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{3, crypto.RIPEMD160, "RIPEMD160"},
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{8, crypto.SHA256, "SHA256"},
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{9, crypto.SHA384, "SHA384"},
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{10, crypto.SHA512, "SHA512"},
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{11, crypto.SHA224, "SHA224"},
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}
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// HashIdToHash returns a crypto.Hash which corresponds to the given OpenPGP
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// hash id.
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func HashIdToHash(id byte) (h crypto.Hash, ok bool) {
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for _, m := range hashToHashIdMapping {
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if m.id == id {
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return m.hash, true
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}
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}
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return 0, false
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}
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// HashIdToString returns the name of the hash function corresponding to the
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// given OpenPGP hash id, or panics if id is unknown.
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func HashIdToString(id byte) (name string, ok bool) {
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for _, m := range hashToHashIdMapping {
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if m.id == id {
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return m.name, true
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}
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}
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return "", false
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}
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// HashIdToHash returns an OpenPGP hash id which corresponds the given Hash.
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func HashToHashId(h crypto.Hash) (id byte, ok bool) {
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for _, m := range hashToHashIdMapping {
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if m.hash == h {
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return m.id, true
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}
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}
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return 0, false
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}
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