crypto/openpgp: add support for symmetrically encrypting files.

This mostly adds the infrastructure for writing various forms of
packets as well as reading them. Adding symmetric encryption support
was simply an easy motivation.

There's also one brown-paper-bag fix in here. Previously I had the
conditional for the MDC hash check backwards: the code was checking
that the hash was *incorrect*. This was neatly counteracted by another
bug: it was hashing the ciphertext of the OCFB prefix, not the
plaintext.

R=bradfitz
CC=golang-dev
https://golang.org/cl/4564046

12 files changed, 492 insertions, 12 deletions

diff --git a/src/pkg/crypto/cipher/ocfb.go b/src/pkg/crypto/cipher/ocfb.go
index b2d8775911..031e74a9dc 100644
--- a/src/pkg/crypto/cipher/ocfb.go
+++ b/src/pkg/crypto/cipher/ocfb.go
@@ -80,9 +80,10 @@ type ocfbDecrypter struct {
 // NewOCFBDecrypter returns a Stream which decrypts data with OpenPGP's cipher
 // feedback mode using the given Block. Prefix must be the first blockSize + 2
 // bytes of the ciphertext, where blockSize is the Block's block size. If an
-// incorrect key is detected then nil is returned. Resync determines if the
-// "resynchronization step" from RFC 4880, 13.9 step 7 is performed. Different
-// parts of OpenPGP vary on this point.
+// incorrect key is detected then nil is returned. On successful exit,
+// blockSize+2 bytes of decrypted data are written into prefix. Resync
+// determines if the "resynchronization step" from RFC 4880, 13.9 step 7 is
+// performed. Different parts of OpenPGP vary on this point.
 func NewOCFBDecrypter(block Block, prefix []byte, resync OCFBResyncOption) Stream {
 	blockSize := block.BlockSize()
 	if len(prefix) != blockSize+2 {
@@ -118,6 +119,7 @@ func NewOCFBDecrypter(block Block, prefix []byte, resync OCFBResyncOption) Strea
 		x.fre[1] = prefix[blockSize+1]
 		x.outUsed = 2
 	}
+	copy(prefix, prefixCopy)
 	return x
 }
 
diff --git a/src/pkg/crypto/openpgp/packet/literal.go b/src/pkg/crypto/openpgp/packet/literal.go
index 04f50e53e1..9411572d7c 100644
--- a/src/pkg/crypto/openpgp/packet/literal.go
+++ b/src/pkg/crypto/openpgp/packet/literal.go
@@ -51,3 +51,40 @@ func (l *LiteralData) parse(r io.Reader) (err os.Error) {
 	l.Body = r
 	return
 }
+
+// SerializeLiteral serializes a literal data packet to w and returns a
+// WriteCloser to which the data itself can be written and which MUST be closed
+// on completion. The fileName is truncated to 255 bytes.
+func SerializeLiteral(w io.WriteCloser, isBinary bool, fileName string, time uint32) (plaintext io.WriteCloser, err os.Error) {
+	var buf [4]byte
+	buf[0] = 't'
+	if isBinary {
+		buf[0] = 'b'
+	}
+	if len(fileName) > 255 {
+		fileName = fileName[:255]
+	}
+	buf[1] = byte(len(fileName))
+
+	inner, err := serializeStreamHeader(w, packetTypeLiteralData)
+	if err != nil {
+		return
+	}
+
+	_, err = inner.Write(buf[:2])
+	if err != nil {
+		return
+	}
+	_, err = inner.Write([]byte(fileName))
+	if err != nil {
+		return
+	}
+	binary.BigEndian.PutUint32(buf[:], time)
+	_, err = inner.Write(buf[:])
+	if err != nil {
+		return
+	}
+
+	plaintext = inner
+	return
+}
diff --git a/src/pkg/crypto/openpgp/packet/packet.go b/src/pkg/crypto/openpgp/packet/packet.go
index e583670fb2..640a5b76f3 100644
--- a/src/pkg/crypto/openpgp/packet/packet.go
+++ b/src/pkg/crypto/openpgp/packet/packet.go
@@ -92,6 +92,46 @@ func (r *partialLengthReader) Read(p []byte) (n int, err os.Error) {
 	return
 }
 
+// partialLengthWriter writes a stream of data using OpenPGP partial lengths.
+// See RFC 4880, section 4.2.2.4.
+type partialLengthWriter struct {
+	w          io.WriteCloser
+	lengthByte [1]byte
+}
+
+func (w *partialLengthWriter) Write(p []byte) (n int, err os.Error) {
+	for len(p) > 0 {
+		for power := uint(14); power < 32; power-- {
+			l := 1 << power
+			if len(p) >= l {
+				w.lengthByte[0] = 224 + uint8(power)
+				_, err = w.w.Write(w.lengthByte[:])
+				if err != nil {
+					return
+				}
+				var m int
+				m, err = w.w.Write(p[:l])
+				n += m
+				if err != nil {
+					return
+				}
+				p = p[l:]
+				break
+			}
+		}
+	}
+	return
+}
+
+func (w *partialLengthWriter) Close() os.Error {
+	w.lengthByte[0] = 0
+	_, err := w.w.Write(w.lengthByte[:])
+	if err != nil {
+		return err
+	}
+	return w.w.Close()
+}
+
 // A spanReader is an io.LimitReader, but it returns ErrUnexpectedEOF if the
 // underlying Reader returns EOF before the limit has been reached.
 type spanReader struct {
@@ -195,6 +235,20 @@ func serializeHeader(w io.Writer, ptype packetType, length int) (err os.Error) {
 	return
 }
 
+// serializeStreamHeader writes an OpenPGP packet header to w where the
+// length of the packet is unknown. It returns a io.WriteCloser which can be
+// used to write the contents of the packet. See RFC 4880, section 4.2.
+func serializeStreamHeader(w io.WriteCloser, ptype packetType) (out io.WriteCloser, err os.Error) {
+	var buf [1]byte
+	buf[0] = 0x80 | 0x40 | byte(ptype)
+	_, err = w.Write(buf[:])
+	if err != nil {
+		return
+	}
+	out = &partialLengthWriter{w: w}
+	return
+}
+
 // Packet represents an OpenPGP packet. Users are expected to try casting
 // instances of this interface to specific packet types.
 type Packet interface {
@@ -327,10 +381,10 @@ const (
 type CipherFunction uint8
 
 const (
-	CipherCAST5  = 3
-	CipherAES128 = 7
-	CipherAES192 = 8
-	CipherAES256 = 9
+	CipherCAST5  CipherFunction = 3
+	CipherAES128 CipherFunction = 7
+	CipherAES192 CipherFunction = 8
+	CipherAES256 CipherFunction = 9
 )
 
 // keySize returns the key size, in bytes, of cipher.
diff --git a/src/pkg/crypto/openpgp/packet/packet_test.go b/src/pkg/crypto/openpgp/packet/packet_test.go
index 1a4692cd4f..23d9978ae1 100644
--- a/src/pkg/crypto/openpgp/packet/packet_test.go
+++ b/src/pkg/crypto/openpgp/packet/packet_test.go
@@ -210,3 +210,47 @@ func TestSerializeHeader(t *testing.T) {
 		}
 	}
 }
+
+func TestPartialLengths(t *testing.T) {
+	buf := bytes.NewBuffer(nil)
+	w := new(partialLengthWriter)
+	w.w = noOpCloser{buf}
+
+	const maxChunkSize = 64
+
+	var b [maxChunkSize]byte
+	var n uint8
+	for l := 1; l <= maxChunkSize; l++ {
+		for i := 0; i < l; i++ {
+			b[i] = n
+			n++
+		}
+		m, err := w.Write(b[:l])
+		if m != l {
+			t.Errorf("short write got: %d want: %d", m, l)
+		}
+		if err != nil {
+			t.Errorf("error from write: %s", err)
+		}
+	}
+	w.Close()
+
+	want := (maxChunkSize * (maxChunkSize + 1)) / 2
+	copyBuf := bytes.NewBuffer(nil)
+	r := &partialLengthReader{buf, 0, true}
+	m, err := io.Copy(copyBuf, r)
+	if m != int64(want) {
+		t.Errorf("short copy got: %d want: %d", m, want)
+	}
+	if err != nil {
+		t.Errorf("error from copy: %s", err)
+	}
+
+	copyBytes := copyBuf.Bytes()
+	for i := 0; i < want; i++ {
+		if copyBytes[i] != uint8(i) {
+			t.Errorf("bad pattern in copy at %d", i)
+			break
+		}
+	}
+}
diff --git a/src/pkg/crypto/openpgp/packet/symmetric_key_encrypted.go b/src/pkg/crypto/openpgp/packet/symmetric_key_encrypted.go
index d9010f88a3..25d264acf9 100644
--- a/src/pkg/crypto/openpgp/packet/symmetric_key_encrypted.go
+++ b/src/pkg/crypto/openpgp/packet/symmetric_key_encrypted.go
@@ -5,6 +5,7 @@
 package packet
 
 import (
+	"bytes"
 	"crypto/cipher"
 	"crypto/openpgp/error"
 	"crypto/openpgp/s2k"
@@ -27,6 +28,8 @@ type SymmetricKeyEncrypted struct {
 	encryptedKey []byte
 }
 
+const symmetricKeyEncryptedVersion = 4
+
 func (ske *SymmetricKeyEncrypted) parse(r io.Reader) (err os.Error) {
 	// RFC 4880, section 5.3.
 	var buf [2]byte
@@ -34,7 +37,7 @@ func (ske *SymmetricKeyEncrypted) parse(r io.Reader) (err os.Error) {
 	if err != nil {
 		return
 	}
-	if buf[0] != 4 {
+	if buf[0] != symmetricKeyEncryptedVersion {
 		return error.UnsupportedError("SymmetricKeyEncrypted version")
 	}
 	ske.CipherFunc = CipherFunction(buf[1])
@@ -100,3 +103,60 @@ func (ske *SymmetricKeyEncrypted) Decrypt(passphrase []byte) os.Error {
 	ske.Encrypted = false
 	return nil
 }
+
+// SerializeSymmetricKeyEncrypted serializes a symmetric key packet to w. The
+// packet contains a random session key, encrypted by a key derived from the
+// given passphrase. The session key is returned and must be passed to
+// SerializeSymmetricallyEncrypted.
+func SerializeSymmetricKeyEncrypted(w io.Writer, rand io.Reader, passphrase []byte, cipherFunc CipherFunction) (key []byte, err os.Error) {
+	keySize := cipherFunc.keySize()
+	if keySize == 0 {
+		return nil, error.UnsupportedError("unknown cipher: " + strconv.Itoa(int(cipherFunc)))
+	}
+
+	s2kBuf := new(bytes.Buffer)
+	keyEncryptingKey := make([]byte, keySize)
+	// s2k.Serialize salts and stretches the passphrase, and writes the
+	// resulting key to keyEncryptingKey and the s2k descriptor to s2kBuf.
+	err = s2k.Serialize(s2kBuf, keyEncryptingKey, rand, passphrase)
+	if err != nil {
+		return
+	}
+	s2kBytes := s2kBuf.Bytes()
+
+	packetLength := 2 /* header */ + len(s2kBytes) + 1 /* cipher type */ + keySize
+	err = serializeHeader(w, packetTypeSymmetricKeyEncrypted, packetLength)
+	if err != nil {
+		return
+	}
+
+	var buf [2]byte
+	buf[0] = symmetricKeyEncryptedVersion
+	buf[1] = byte(cipherFunc)
+	_, err = w.Write(buf[:])
+	if err != nil {
+		return
+	}
+	_, err = w.Write(s2kBytes)
+	if err != nil {
+		return
+	}
+
+	sessionKey := make([]byte, keySize)
+	_, err = io.ReadFull(rand, sessionKey)
+	if err != nil {
+		return
+	}
+	iv := make([]byte, cipherFunc.blockSize())
+	c := cipher.NewCFBEncrypter(cipherFunc.new(keyEncryptingKey), iv)
+	encryptedCipherAndKey := make([]byte, keySize+1)
+	c.XORKeyStream(encryptedCipherAndKey, buf[1:])
+	c.XORKeyStream(encryptedCipherAndKey[1:], sessionKey)
+	_, err = w.Write(encryptedCipherAndKey)
+	if err != nil {
+		return
+	}
+
+	key = sessionKey
+	return
+}
diff --git a/src/pkg/crypto/openpgp/packet/symmetric_key_encrypted_test.go b/src/pkg/crypto/openpgp/packet/symmetric_key_encrypted_test.go
index 717c8ffa6d..823ec400d4 100644
--- a/src/pkg/crypto/openpgp/packet/symmetric_key_encrypted_test.go
+++ b/src/pkg/crypto/openpgp/packet/symmetric_key_encrypted_test.go
@@ -6,6 +6,7 @@ package packet
 
 import (
 	"bytes"
+	"crypto/rand"
 	"encoding/hex"
 	"io/ioutil"
 	"os"
@@ -60,3 +61,41 @@ func TestSymmetricKeyEncrypted(t *testing.T) {
 
 const symmetricallyEncryptedHex = "8c0d04030302371a0b38d884f02060c91cf97c9973b8e58e028e9501708ccfe618fb92afef7fa2d80ddadd93cf"
 const symmetricallyEncryptedContentsHex = "cb1062004d14c4df636f6e74656e74732e0a"
+
+func TestSerializeSymmetricKeyEncrypted(t *testing.T) {
+	buf := bytes.NewBuffer(nil)
+	passphrase := []byte("testing")
+	cipherFunc := CipherAES128
+
+	key, err := SerializeSymmetricKeyEncrypted(buf, rand.Reader, passphrase, cipherFunc)
+	if err != nil {
+		t.Errorf("failed to serialize: %s", err)
+		return
+	}
+
+	p, err := Read(buf)
+	if err != nil {
+		t.Errorf("failed to reparse: %s", err)
+		return
+	}
+	ske, ok := p.(*SymmetricKeyEncrypted)
+	if !ok {
+		t.Errorf("parsed a different packet type: %#v", p)
+		return
+	}
+
+	if !ske.Encrypted {
+		t.Errorf("SKE not encrypted but should be")
+	}
+	if ske.CipherFunc != cipherFunc {
+		t.Errorf("SKE cipher function is %d (expected %d)", ske.CipherFunc, cipherFunc)
+	}
+	err = ske.Decrypt(passphrase)
+	if err != nil {
+		t.Errorf("failed to decrypt reparsed SKE: %s", err)
+		return
+	}
+	if !bytes.Equal(key, ske.Key) {
+		t.Errorf("keys don't match after Decrpyt: %x (original) vs %x (parsed)", key, ske.Key)
+	}
+}
diff --git a/src/pkg/crypto/openpgp/packet/symmetrically_encrypted.go b/src/pkg/crypto/openpgp/packet/symmetrically_encrypted.go
index fc19ffe809..236c367745 100644
--- a/src/pkg/crypto/openpgp/packet/symmetrically_encrypted.go
+++ b/src/pkg/crypto/openpgp/packet/symmetrically_encrypted.go
@@ -7,6 +7,7 @@ package packet
 import (
 	"crypto/cipher"
 	"crypto/openpgp/error"
+	"crypto/rand"
 	"crypto/sha1"
 	"crypto/subtle"
 	"hash"
@@ -24,6 +25,8 @@ type SymmetricallyEncrypted struct {
 	prefix   []byte
 }
 
+const symmetricallyEncryptedVersion = 1
+
 func (se *SymmetricallyEncrypted) parse(r io.Reader) os.Error {
 	if se.MDC {
 		// See RFC 4880, section 5.13.
@@ -32,7 +35,7 @@ func (se *SymmetricallyEncrypted) parse(r io.Reader) os.Error {
 		if err != nil {
 			return err
 		}
-		if buf[0] != 1 {
+		if buf[0] != symmetricallyEncryptedVersion {
 			return error.UnsupportedError("unknown SymmetricallyEncrypted version")
 		}
 	}
@@ -174,6 +177,9 @@ func (ser *seMDCReader) Read(buf []byte) (n int, err os.Error) {
 	return
 }
 
+// This is a new-format packet tag byte for a type 19 (MDC) packet.
+const mdcPacketTagByte = byte(0x80) | 0x40 | 19
+
 func (ser *seMDCReader) Close() os.Error {
 	if ser.error {
 		return error.SignatureError("error during reading")
@@ -191,16 +197,95 @@ func (ser *seMDCReader) Close() os.Error {
 		}
 	}
 
-	// This is a new-format packet tag byte for a type 19 (MDC) packet.
-	const mdcPacketTagByte = byte(0x80) | 0x40 | 19
 	if ser.trailer[0] != mdcPacketTagByte || ser.trailer[1] != sha1.Size {
 		return error.SignatureError("MDC packet not found")
 	}
 	ser.h.Write(ser.trailer[:2])
 
 	final := ser.h.Sum()
-	if subtle.ConstantTimeCompare(final, ser.trailer[2:]) == 1 {
+	if subtle.ConstantTimeCompare(final, ser.trailer[2:]) != 1 {
 		return error.SignatureError("hash mismatch")
 	}
 	return nil
 }
+
+// An seMDCWriter writes through to an io.WriteCloser while maintains a running
+// hash of the data written. On close, it emits an MDC packet containing the
+// running hash.
+type seMDCWriter struct {
+	w io.WriteCloser
+	h hash.Hash
+}
+
+func (w *seMDCWriter) Write(buf []byte) (n int, err os.Error) {
+	w.h.Write(buf)
+	return w.w.Write(buf)
+}
+
+func (w *seMDCWriter) Close() (err os.Error) {
+	var buf [mdcTrailerSize]byte
+
+	buf[0] = mdcPacketTagByte
+	buf[1] = sha1.Size
+	w.h.Write(buf[:2])
+	digest := w.h.Sum()
+	copy(buf[2:], digest)
+
+	_, err = w.w.Write(buf[:])
+	if err != nil {
+		return
+	}
+	return w.w.Close()
+}
+
+// noOpCloser is like an ioutil.NopCloser, but for an io.Writer.
+type noOpCloser struct {
+	w io.Writer
+}
+
+func (c noOpCloser) Write(data []byte) (n int, err os.Error) {
+	return c.w.Write(data)
+}
+
+func (c noOpCloser) Close() os.Error {
+	return nil
+}
+
+// SerializeSymmetricallyEncrypted serializes a symmetrically encrypted packet
+// to w and returns a WriteCloser to which the to-be-encrypted packets can be
+// written.
+func SerializeSymmetricallyEncrypted(w io.Writer, c CipherFunction, key []byte) (contents io.WriteCloser, err os.Error) {
+	if c.keySize() != len(key) {
+		return nil, error.InvalidArgumentError("SymmetricallyEncrypted.Serialize: bad key length")
+	}
+	writeCloser := noOpCloser{w}
+	ciphertext, err := serializeStreamHeader(writeCloser, packetTypeSymmetricallyEncryptedMDC)
+	if err != nil {
+		return
+	}
+
+	_, err = ciphertext.Write([]byte{symmetricallyEncryptedVersion})
+	if err != nil {
+		return
+	}
+
+	block := c.new(key)
+	blockSize := block.BlockSize()
+	iv := make([]byte, blockSize)
+	_, err = rand.Reader.Read(iv)
+	if err != nil {
+		return
+	}
+	s, prefix := cipher.NewOCFBEncrypter(block, iv, cipher.OCFBNoResync)
+	_, err = ciphertext.Write(prefix)
+	if err != nil {
+		return
+	}
+	plaintext := cipher.StreamWriter{S: s, W: ciphertext}
+
+	h := sha1.New()
+	h.Write(iv)
+	h.Write(iv[blockSize-2:])
+	contents = &seMDCWriter{w: plaintext, h: h}
+	return
+}
diff --git a/src/pkg/crypto/openpgp/packet/symmetrically_encrypted_test.go b/src/pkg/crypto/openpgp/packet/symmetrically_encrypted_test.go
index 5543b20297..ba5606e6ce 100644
--- a/src/pkg/crypto/openpgp/packet/symmetrically_encrypted_test.go
+++ b/src/pkg/crypto/openpgp/packet/symmetrically_encrypted_test.go
@@ -9,6 +9,7 @@ import (
 	"crypto/openpgp/error"
 	"crypto/sha1"
 	"encoding/hex"
+	"io"
 	"io/ioutil"
 	"os"
 	"testing"
@@ -76,3 +77,48 @@ func testMDCReader(t *testing.T) {
 }
 
 const mdcPlaintextHex = "a302789c3b2d93c4e0eb9aba22283539b3203335af44a134afb800c849cb4c4de10200aff40b45d31432c80cb384299a0655966d6939dfdeed1dddf980"
+
+func TestSerialize(t *testing.T) {
+	buf := bytes.NewBuffer(nil)
+	c := CipherAES128
+	key := make([]byte, c.keySize())
+
+	w, err := SerializeSymmetricallyEncrypted(buf, c, key)
+	if err != nil {
+		t.Errorf("error from SerializeSymmetricallyEncrypted: %s", err)
+		return
+	}
+
+	contents := []byte("hello world\n")
+
+	w.Write(contents)
+	w.Close()
+
+	p, err := Read(buf)
+	if err != nil {
+		t.Errorf("error from Read: %s", err)
+		return
+	}
+
+	se, ok := p.(*SymmetricallyEncrypted)
+	if !ok {
+		t.Errorf("didn't read a *SymmetricallyEncrypted")
+		return
+	}
+
+	r, err := se.Decrypt(c, key)
+	if err != nil {
+		t.Errorf("error from Decrypt: %s", err)
+		return
+	}
+
+	contentsCopy := bytes.NewBuffer(nil)
+	_, err = io.Copy(contentsCopy, r)
+	if err != nil {
+		t.Errorf("error from io.Copy: %s", err)
+		return
+	}
+	if !bytes.Equal(contentsCopy.Bytes(), contents) {
+		t.Errorf("contents not equal got: %x want: %x", contentsCopy.Bytes(), contents)
+	}
+}
diff --git a/src/pkg/crypto/openpgp/s2k/s2k.go b/src/pkg/crypto/openpgp/s2k/s2k.go
index 80b81bd3a5..da926a76ed 100644
--- a/src/pkg/crypto/openpgp/s2k/s2k.go
+++ b/src/pkg/crypto/openpgp/s2k/s2k.go
@@ -123,6 +123,26 @@ func Parse(r io.Reader) (f func(out, in []byte), err os.Error) {
 	return nil, error.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.
+func Serialize(w io.Writer, key []byte, rand io.Reader, passphrase []byte) os.Error {
+	var buf [11]byte
+	buf[0] = 3 /* iterated and salted */
+	buf[1], _ = HashToHashId(crypto.SHA1)
+	salt := buf[2:10]
+	if _, err := io.ReadFull(rand, salt); err != nil {
+		return err
+	}
+	const count = 65536 // this is the default in gpg
+	buf[10] = 96        // 65536 iterations
+	if _, err := w.Write(buf[:]); err != nil {
+		return err
+	}
+
+	Iterated(key, crypto.SHA1.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 {
diff --git a/src/pkg/crypto/openpgp/s2k/s2k_test.go b/src/pkg/crypto/openpgp/s2k/s2k_test.go
index 75bc47ec10..27d2e9ae0b 100644
--- a/src/pkg/crypto/openpgp/s2k/s2k_test.go
+++ b/src/pkg/crypto/openpgp/s2k/s2k_test.go
@@ -7,6 +7,7 @@ package s2k
 import (
 	"bytes"
 	"crypto/sha1"
+	"crypto/rand"
 	"encoding/hex"
 	"testing"
 )
@@ -95,3 +96,26 @@ func TestParse(t *testing.T) {
 		}
 	}
 }
+
+
+func TestSerialize(t *testing.T) {
+	buf := bytes.NewBuffer(nil)
+	key := make([]byte, 16)
+	passphrase := []byte("testing")
+	err := Serialize(buf, key, rand.Reader, passphrase)
+	if err != nil {
+		t.Errorf("failed to serialize: %s", err)
+		return
+	}
+
+	f, err := Parse(buf)
+	if err != nil {
+		t.Errorf("failed to reparse: %s", err)
+		return
+	}
+	key2 := make([]byte, len(key))
+	f(key2, passphrase)
+	if !bytes.Equal(key2, key) {
+		t.Errorf("keys don't match: %x (serialied) vs %x (parsed)", key, key2)
+	}
+}
diff --git a/src/pkg/crypto/openpgp/write.go b/src/pkg/crypto/openpgp/write.go
index a1ede564e2..48c86f604e 100644
--- a/src/pkg/crypto/openpgp/write.go
+++ b/src/pkg/crypto/openpgp/write.go
@@ -9,6 +9,7 @@ import (
 	"crypto/openpgp/armor"
 	"crypto/openpgp/error"
 	"crypto/openpgp/packet"
+	"crypto/rand"
 	_ "crypto/sha256"
 	"io"
 	"os"
@@ -81,3 +82,36 @@ func detachSign(w io.Writer, signer *Entity, message io.Reader, sigType packet.S
 
 	return sig.Serialize(w)
 }
+
+// FileHints contains metadata about encrypted files. This metadata is, itself,
+// encrypted.
+type FileHints struct {
+	// IsBinary can be set to hint that the contents are binary data.
+	IsBinary bool
+	// FileName hints at the name of the file that should be written. It's
+	// truncated to 255 bytes if longer. It may be empty to suggest that the
+	// file should not be written to disk. It may be equal to "_CONSOLE" to
+	// suggest the data should not be written to disk.
+	FileName string
+	// EpochSeconds contains the modification time of the file, or 0 if not applicable.
+	EpochSeconds uint32
+}
+
+// SymmetricallyEncrypt acts like gpg -c: it encrypts a file with a passphrase.
+// The resulting WriteCloser MUST be closed after the contents of the file have
+// been written.
+func SymmetricallyEncrypt(ciphertext io.Writer, passphrase []byte, hints *FileHints) (plaintext io.WriteCloser, err os.Error) {
+	if hints == nil {
+		hints = &FileHints{}
+	}
+
+	key, err := packet.SerializeSymmetricKeyEncrypted(ciphertext, rand.Reader, passphrase, packet.CipherAES128)
+	if err != nil {
+		return
+	}
+	w, err := packet.SerializeSymmetricallyEncrypted(ciphertext, packet.CipherAES128, key)
+	if err != nil {
+		return
+	}
+	return packet.SerializeLiteral(w, hints.IsBinary, hints.FileName, hints.EpochSeconds)
+}
diff --git a/src/pkg/crypto/openpgp/write_test.go b/src/pkg/crypto/openpgp/write_test.go
index a74a84b2b7..8551aeb638 100644
--- a/src/pkg/crypto/openpgp/write_test.go
+++ b/src/pkg/crypto/openpgp/write_test.go
@@ -7,6 +7,8 @@ package openpgp
 import (
 	"bytes"
 	"crypto/rand"
+	"os"
+	"io"
 	"testing"
 	"time"
 )
@@ -85,3 +87,36 @@ func TestNewEntity(t *testing.T) {
 		t.Errorf("results differed")
 	}
 }
+
+func TestSymmetricEncryption(t *testing.T) {
+	buf := new(bytes.Buffer)
+	plaintext, err := SymmetricallyEncrypt(buf, []byte("testing"), nil)
+	if err != nil {
+		t.Errorf("error writing headers: %s", err)
+		return
+	}
+	message := []byte("hello world\n")
+	_, err = plaintext.Write(message)
+	if err != nil {
+		t.Errorf("error writing to plaintext writer: %s", err)
+	}
+	err = plaintext.Close()
+	if err != nil {
+		t.Errorf("error closing plaintext writer: %s", err)
+	}
+
+	md, err := ReadMessage(buf, nil, func(keys []Key, symmetric bool) ([]byte, os.Error) {
+		return []byte("testing"), nil
+	})
+	if err != nil {
+		t.Errorf("error rereading message: %s", err)
+	}
+	messageBuf := bytes.NewBuffer(nil)
+	_, err = io.Copy(messageBuf, md.UnverifiedBody)
+	if err != nil {
+		t.Errorf("error rereading message: %s", err)
+	}
+	if !bytes.Equal(message, messageBuf.Bytes()) {
+		t.Errorf("recovered message incorrect got '%s', want '%s'", messageBuf.Bytes(), message)
+	}
+}