crypto/openpgp: add clear-signing support.

Clear signed messages are often seen in email and are formatted so
that the plaintext of the signed message is readable without any
special tools.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5604043

openpgp/clearsign/clearsign.go

@@ -0,0 +1,365 @@
+// Copyright 2012 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 clearsign generates and processes OpenPGP, clear-signed data. See
+// RFC 4880, section 7.
+//
+// Clearsigned messages are cryptographically signed, but the contents of the
+// message are kept in plaintext so that it can be read without special tools.
+package clearsign
+
+import (
+	"bufio"
+	"bytes"
+	"crypto"
+	"hash"
+	"io"
+	"net/textproto"
+	"strconv"
+	"time"
+
+	"code.google.com/p/go.crypto/openpgp/armor"
+	"code.google.com/p/go.crypto/openpgp/errors"
+	"code.google.com/p/go.crypto/openpgp/packet"
+)
+
+// A Block represents a clearsigned message. A signature on a Block can
+// be checked by passing Bytes into openpgp.CheckDetachedSignature.
+type Block struct {
+	Headers          textproto.MIMEHeader // Optional message headers
+	Plaintext        []byte               // The original message text
+	Bytes            []byte               // The signed message
+	ArmoredSignature *armor.Block         // The signature block
+}
+
+// start is the marker which denotes the beginning of a clearsigned message.
+var start = []byte("\n-----BEGIN PGP SIGNED MESSAGE-----")
+
+// dashEscape is prefixed to any lines that begin with a hypen so that they
+// can't be confused with endText.
+var dashEscape = []byte("- ")
+
+// endText is a marker which denotes the end of the message and the start of
+// an armored signature.
+var endText = []byte("-----BEGIN PGP SIGNATURE-----")
+
+// end is a marker which denotes the end of the armored signature.
+var end = []byte("\n-----END PGP SIGNATURE-----")
+
+var crlf = []byte("\r\n")
+var lf = byte('\n')
+
+// getLine returns the first \r\n or \n delineated line from the given byte
+// array. The line does not include the \r\n or \n. The remainder of the byte
+// array (also not including the new line bytes) is also returned and this will
+// always be smaller than the original argument.
+func getLine(data []byte) (line, rest []byte) {
+	i := bytes.Index(data, []byte{'\n'})
+	var j int
+	if i < 0 {
+		i = len(data)
+		j = i
+	} else {
+		j = i + 1
+		if i > 0 && data[i-1] == '\r' {
+			i--
+		}
+	}
+	return data[0:i], data[j:]
+}
+
+// Decode finds the first clearsigned message in data and returns it, as well
+// as the suffix of data which remains after the message.
+func Decode(data []byte) (b *Block, rest []byte) {
+	// start begins with a newline. However, at the very beginning of
+	// the byte array, we'll accept the start string without it.
+	rest = data
+	if bytes.HasPrefix(data, start[1:]) {
+		rest = rest[len(start)-1:]
+	} else if i := bytes.Index(data, start); i >= 0 {
+		rest = rest[i+len(start):]
+	} else {
+		return nil, data
+	}
+
+	// Consume the start line.
+	_, rest = getLine(rest)
+
+	var line []byte
+	b = &Block{
+		Headers: make(textproto.MIMEHeader),
+	}
+
+	// Next come a series of header lines.
+	for {
+		// This loop terminates because getLine's second result is
+		// always smaller than its argument.
+		if len(rest) == 0 {
+			return nil, data
+		}
+		// An empty line marks the end of the headers.
+		if line, rest = getLine(rest); len(line) == 0 {
+			break
+		}
+
+		i := bytes.Index(line, []byte{':'})
+		if i == -1 {
+			return nil, data
+		}
+
+		key, val := line[0:i], line[i+1:]
+		key = bytes.TrimSpace(key)
+		val = bytes.TrimSpace(val)
+		b.Headers.Add(string(key), string(val))
+	}
+
+	for {
+		start := rest
+
+		line, rest = getLine(rest)
+		if bytes.Equal(line, endText) {
+			// Back up to the start of the line because armor expects to see the
+			// header line.
+			rest = start
+			break
+		}
+
+		// The final CRLF isn't included in the hash so we don't write it until
+		// we've seen the next line.
+		if len(b.Bytes) > 0 {
+			b.Bytes = append(b.Bytes, crlf...)
+		}
+		if bytes.HasPrefix(line, dashEscape) {
+			line = line[2:]
+		}
+		line = bytes.TrimRight(line, " \t")
+		b.Bytes = append(b.Bytes, line...)
+
+		b.Plaintext = append(b.Plaintext, line...)
+		b.Plaintext = append(b.Plaintext, lf)
+	}
+
+	// We want to find the extent of the armored data (including any newlines at
+	// the end).
+	i := bytes.Index(rest, end)
+	if i == -1 {
+		return nil, data
+	}
+	i += len(end)
+	for i < len(rest) && (rest[i] == '\r' || rest[i] == '\n') {
+		i++
+	}
+	armored := rest[:i]
+	rest = rest[i:]
+
+	var err error
+	b.ArmoredSignature, err = armor.Decode(bytes.NewBuffer(armored))
+	if err != nil {
+		return nil, data
+	}
+
+	return b, rest
+}
+
+// A dashEscaper is an io.WriteCloser which processes the body of a clear-signed
+// message. The clear-signed message is written to buffered and a hash, suitable
+// for signing, is maintained in h.
+//
+// When closed, an armored signature is created and written to complete the
+// message.
+type dashEscaper struct {
+	buffered *bufio.Writer
+	h        hash.Hash
+	hashType crypto.Hash
+
+	atBeginningOfLine bool
+	isFirstLine       bool
+
+	whitespace []byte
+	byteBuf    []byte // a one byte buffer to save allocations
+
+	privateKey  *packet.PrivateKey
+	signingTime time.Time
+	rand        io.Reader
+}
+
+func (d *dashEscaper) Write(data []byte) (n int, err error) {
+	for _, b := range data {
+		d.byteBuf[0] = b
+
+		if d.atBeginningOfLine {
+			// The final CRLF isn't included in the hash so we have to wait
+			// until this point (the start of the next line) before writing it.
+			if !d.isFirstLine {
+				d.h.Write(crlf)
+			}
+			d.isFirstLine = false
+
+			// At the beginning of a line, hyphens have to be escaped.
+			if b == '-' {
+				// The signature isn't calculated over the dash-escaped text so
+				// the escape is only written to buffered.
+				if _, err = d.buffered.Write(dashEscape); err != nil {
+					return
+				}
+				d.h.Write(d.byteBuf)
+				d.atBeginningOfLine = false
+			} else if b == '\n' {
+				// Nothing to do because we dely writing CRLF to the hash.
+			} else {
+				d.h.Write(d.byteBuf)
+				d.atBeginningOfLine = false
+			}
+			if err = d.buffered.WriteByte(b); err != nil {
+				return
+			}
+		} else {
+			// Any whitespace at the end of the line has to be removed so we
+			// buffer it until we find out whether there's more on this line.
+			if b == ' ' || b == '\t' || b == '\r' {
+				d.whitespace = append(d.whitespace, b)
+			} else if b == '\n' {
+				// We got a raw \n. Drop any trailing whitespace and write a
+				// CRLF.
+				d.whitespace = d.whitespace[:0]
+				// We dely writing CRLF to the hash until the start of the
+				// next line.
+				if err = d.buffered.WriteByte(b); err != nil {
+					return
+				}
+				d.atBeginningOfLine = true
+			} else {
+				// Any buffered whitespace wasn't at the end of the line so
+				// we need to write it out.
+				if len(d.whitespace) > 0 {
+					d.h.Write(d.whitespace)
+					if _, err = d.buffered.Write(d.whitespace); err != nil {
+						return
+					}
+					d.whitespace = d.whitespace[:0]
+				}
+				d.h.Write(d.byteBuf)
+				if err = d.buffered.WriteByte(b); err != nil {
+					return
+				}
+			}
+		}
+	}
+
+	n = len(data)
+	return
+}
+
+func (d *dashEscaper) Close() (err error) {
+	if !d.atBeginningOfLine {
+		if err = d.buffered.WriteByte(lf); err != nil {
+			return
+		}
+	}
+	sig := new(packet.Signature)
+	sig.SigType = packet.SigTypeText
+	sig.PubKeyAlgo = d.privateKey.PubKeyAlgo
+	sig.Hash = d.hashType
+	sig.CreationTime = d.signingTime
+	sig.IssuerKeyId = &d.privateKey.KeyId
+
+	if err = sig.Sign(d.rand, d.h, d.privateKey); err != nil {
+		return
+	}
+
+	out, err := armor.Encode(d.buffered, "PGP SIGNATURE", nil)
+	if err != nil {
+		return
+	}
+
+	if err = sig.Serialize(out); err != nil {
+		return
+	}
+	if err = out.Close(); err != nil {
+		return
+	}
+	if err = d.buffered.Flush(); err != nil {
+		return
+	}
+	return
+}
+
+// Encode returns a WriteCloser which will clear-sign a message with privateKey
+// and write it to w. If config is nil, sensible defaults are used.
+func Encode(w io.Writer, privateKey *packet.PrivateKey, config *packet.Config) (plaintext io.WriteCloser, err error) {
+	if privateKey.Encrypted {
+		return nil, errors.InvalidArgumentError("signing key is encrypted")
+	}
+
+	hashType := config.Hash()
+	name := nameOfHash(hashType)
+	if len(name) == 0 {
+		return nil, errors.UnsupportedError("unknown hash type: " + strconv.Itoa(int(hashType)))
+	}
+
+	h := hashType.New()
+	if h == nil {
+		return nil, errors.UnsupportedError("unsupported hash type: " + strconv.Itoa(int(hashType)))
+	}
+
+	buffered := bufio.NewWriter(w)
+	// start has a \n at the beginning that we don't want here.
+	if _, err = buffered.Write(start[1:]); err != nil {
+		return
+	}
+	if err = buffered.WriteByte(lf); err != nil {
+		return
+	}
+	if _, err = buffered.WriteString("Hash: "); err != nil {
+		return
+	}
+	if _, err = buffered.WriteString(name); err != nil {
+		return
+	}
+	if err = buffered.WriteByte(lf); err != nil {
+		return
+	}
+	if err = buffered.WriteByte(lf); err != nil {
+		return
+	}
+
+	plaintext = &dashEscaper{
+		buffered: buffered,
+		h:        h,
+		hashType: hashType,
+
+		atBeginningOfLine: true,
+		isFirstLine:       true,
+
+		byteBuf: make([]byte, 1),
+
+		privateKey:  privateKey,
+		signingTime: config.Now(),
+		rand:        config.Random(),
+	}
+
+	return
+}
+
+// nameOfHash returns the OpenPGP name for the given hash, or the empty string
+// if the name isn't known. See RFC 4880, section 9.4.
+func nameOfHash(h crypto.Hash) string {
+	switch h {
+	case crypto.MD5:
+		return "MD5"
+	case crypto.SHA1:
+		return "SHA1"
+	case crypto.RIPEMD160:
+		return "RIPEMD160"
+	case crypto.SHA224:
+		return "SHA224"
+	case crypto.SHA256:
+		return "SHA256"
+	case crypto.SHA384:
+		return "SHA384"
+	case crypto.SHA512:
+		return "SHA512"
+	}
+	return ""
+}

openpgp/clearsign/clearsign_test.go

@@ -0,0 +1,161 @@
+// Copyright 2012 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 clearsign
+
+import (
+	"bytes"
+	"code.google.com/p/go.crypto/openpgp"
+	"testing"
+)
+
+func TestParse(t *testing.T) {
+	b, rest := Decode(clearsignInput)
+	if b == nil {
+		t.Fatal("failed to decode clearsign message")
+	}
+	if !bytes.Equal(rest, []byte("trailing")) {
+		t.Errorf("unexpected remaining bytes returned: %s", string(rest))
+	}
+	if b.ArmoredSignature.Type != "PGP SIGNATURE" {
+		t.Errorf("bad armor type, got:%s, want:PGP SIGNATURE", b.ArmoredSignature.Type)
+	}
+	expected := []byte("Hello world\r\nline 2")
+	if !bytes.Equal(b.Bytes, expected) {
+		t.Errorf("bad body, got:%x want:%x", b.Bytes, expected)
+	}
+
+	expected = []byte("Hello world\nline 2\n")
+	if !bytes.Equal(b.Plaintext, expected) {
+		t.Errorf("bad plaintext, got:%x want:%x", b.Plaintext, expected)
+	}
+
+	keyring, err := openpgp.ReadArmoredKeyRing(bytes.NewBufferString(signingKey))
+	if err != nil {
+		t.Errorf("failed to parse public key: %s", err)
+	}
+
+	if _, err := openpgp.CheckDetachedSignature(keyring, bytes.NewBuffer(b.Bytes), b.ArmoredSignature.Body); err != nil {
+		t.Errorf("failed to check signature: %s", err)
+	}
+}
+
+func TestParseWithNoNewlineAtEnd(t *testing.T) {
+	input := clearsignInput
+	input = input[:len(input)-len("trailing")-1]
+	b, rest := Decode(input)
+	if b == nil {
+		t.Fatal("failed to decode clearsign message")
+	}
+	if len(rest) > 0 {
+		t.Errorf("unexpected remaining bytes returned: %s", string(rest))
+	}
+}
+
+var signingTests = []struct {
+	in, signed, plaintext string
+}{
+	{"", "", ""},
+	{"a", "a", "a\n"},
+	{"a\n", "a", "a\n"},
+	{"-a\n", "-a", "-a\n"},
+	{"--a\nb", "--a\r\nb", "--a\nb\n"},
+}
+
+func TestSigning(t *testing.T) {
+	keyring, err := openpgp.ReadArmoredKeyRing(bytes.NewBufferString(signingKey))
+	if err != nil {
+		t.Errorf("failed to parse public key: %s", err)
+	}
+
+	for i, test := range signingTests {
+		var buf bytes.Buffer
+
+		plaintext, err := Encode(&buf, keyring[0].PrivateKey, nil)
+		if err != nil {
+			t.Errorf("#%d: error from Encode: %s", i, err)
+			continue
+		}
+		if _, err := plaintext.Write([]byte(test.in)); err != nil {
+			t.Errorf("#%d: error from Write: %s", i, err)
+			continue
+		}
+		if err := plaintext.Close(); err != nil {
+			t.Fatalf("#%d: error from Close: %s", i, err)
+			continue
+		}
+
+		b, _ := Decode(buf.Bytes())
+		if b == nil {
+			t.Errorf("#%d: failed to decode clearsign message", i)
+			continue
+		}
+		if !bytes.Equal(b.Bytes, []byte(test.signed)) {
+			t.Errorf("#%d: bad result, got:%x, want:%x", i, b.Bytes, test.signed)
+			continue
+		}
+		if !bytes.Equal(b.Plaintext, []byte(test.plaintext)) {
+			t.Errorf("#%d: bad result, got:%x, want:%x", i, b.Plaintext, test.plaintext)
+			continue
+		}
+
+		if _, err := openpgp.CheckDetachedSignature(keyring, bytes.NewBuffer(b.Bytes), b.ArmoredSignature.Body); err != nil {
+			t.Errorf("#%d: failed to check signature: %s", i, err)
+		}
+	}
+}
+
+var clearsignInput = []byte(`
+;lasjlkfdsa
+
+-----BEGIN PGP SIGNED MESSAGE-----
+Hash: SHA1
+
+Hello world
+line 2
+-----BEGIN PGP SIGNATURE-----
+Version: GnuPG v1.4.10 (GNU/Linux)
+
+iJwEAQECAAYFAk8kMuEACgkQO9o98PRieSpMsAQAhmY/vwmNpflrPgmfWsYhk5O8
+pjnBUzZwqTDoDeINjZEoPDSpQAHGhjFjgaDx/Gj4fAl0dM4D0wuUEBb6QOrwflog
+2A2k9kfSOMOtk0IH/H5VuFN1Mie9L/erYXjTQIptv9t9J7NoRBMU0QOOaFU0JaO9
+MyTpno24AjIAGb+mH1U=
+=hIJ6
+-----END PGP SIGNATURE-----
+trailing`)
+
+var signingKey = `-----BEGIN PGP PRIVATE KEY BLOCK-----
+Version: GnuPG v1.4.10 (GNU/Linux)
+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+=zNCn
+-----END PGP PRIVATE KEY BLOCK-----
+`

openpgp/packet/config.go

@@ -0,0 +1,58 @@
+// Copyright 2012 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 (
+	"crypto"
+	"crypto/rand"
+	"io"
+	"time"
+)
+
+// Config collects a number of parameters along with sensible defaults.
+
+// A nil *Config is valid and produces all default values.
+type Config struct {
+	// Rand provides the source of entropy.
+	// If nil, the crypto/rand Reader is used.
+	Rand io.Reader
+	// DefaultHash is the default hash function to be used.
+	// If zero, SHA-256 is used.
+	DefaultHash crypto.Hash
+	// DefaultCipher is the cipher to be used.
+	// If zero, AES-128 is used.
+	DefaultCipher CipherFunction
+	// Time returns the current time as the number of seconds since the
+	// epoch. If Time is nil, time.Now is used.
+	Time func() time.Time
+}
+
+func (c *Config) Random() io.Reader {
+	if c == nil || c.Rand == nil {
+		return rand.Reader
+	}
+	return c.Rand
+}
+
+func (c *Config) Hash() crypto.Hash {
+	if c == nil || uint(c.DefaultHash) == 0 {
+		return crypto.SHA256
+	}
+	return c.DefaultHash
+}
+
+func (c *Config) Cipher() CipherFunction {
+	if c == nil || uint8(c.DefaultCipher) == 0 {
+		return CipherAES128
+	}
+	return c.DefaultCipher
+}
+
+func (c *Config) Now() time.Time {
+	if c == nil || c.Time == nil {
+		return time.Now()
+	}
+	return c.Time()
+}