feat: add gpg integration

This commit adds gpg system integration. This is done through two new
packages: gpgbin, which handles the system calls and parsing; and gpg
which is mostly a copy of emersion/go-pgpmail with modifications to
interface with package gpgbin. gpg includes tests for many cases, and
by it's nature also tests package gpgbin. I separated these in case an
external dependency is ever used for the gpg sys-calls/parsing (IE we
mirror how go-pgpmail+openpgp currently are dependencies)

Two new config options are introduced:
* pgp-provider. If it is not explicitly set to "gpg", aerc will default to
it's internal pgp provider
* pgp-key-id: (Optionally) specify a key by short or long keyId

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>
Acked-by: Koni Marti <koni.marti@gmail.com>
Acked-by: Robin Jarry <robin@jarry.cc>

1 files changed, 165 insertions, 0 deletions

diff --git a/lib/crypto/gpg/reader.go b/lib/crypto/gpg/reader.go
new file mode 100644
index 00000000..bf977ed4
--- /dev/null
+++ b/lib/crypto/gpg/reader.go
@@ -0,0 +1,165 @@
+// reader.go largerly mimics github.com/emersion/go-gpgmail, with changes made
+// to interface with the gpg package in aerc
+
+package gpg
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"io"
+	"mime"
+	"strings"
+
+	"git.sr.ht/~rjarry/aerc/lib/crypto/gpg/gpgbin"
+	"git.sr.ht/~rjarry/aerc/models"
+	"github.com/emersion/go-message/textproto"
+)
+
+type Reader struct {
+	Header         textproto.Header
+	MessageDetails *models.MessageDetails
+}
+
+func NewReader(h textproto.Header, body io.Reader) (*Reader, error) {
+	t, params, err := mime.ParseMediaType(h.Get("Content-Type"))
+	if err != nil {
+		return nil, err
+	}
+
+	if strings.EqualFold(t, "multipart/encrypted") && strings.EqualFold(params["protocol"], "application/pgp-encrypted") {
+		mr := textproto.NewMultipartReader(body, params["boundary"])
+		return newEncryptedReader(h, mr)
+	}
+	if strings.EqualFold(t, "multipart/signed") && strings.EqualFold(params["protocol"], "application/pgp-signature") {
+		micalg := params["micalg"]
+		mr := textproto.NewMultipartReader(body, params["boundary"])
+		return newSignedReader(h, mr, micalg)
+	}
+
+	var headerBuf bytes.Buffer
+	textproto.WriteHeader(&headerBuf, h)
+
+	return &Reader{
+		Header: h,
+		MessageDetails: &models.MessageDetails{
+			Body: io.MultiReader(&headerBuf, body),
+		},
+	}, nil
+}
+
+func Read(r io.Reader) (*Reader, error) {
+	br := bufio.NewReader(r)
+
+	h, err := textproto.ReadHeader(br)
+	if err != nil {
+		return nil, err
+	}
+	return NewReader(h, br)
+}
+
+func newEncryptedReader(h textproto.Header, mr *textproto.MultipartReader) (*Reader, error) {
+	p, err := mr.NextPart()
+	if err != nil {
+		return nil, fmt.Errorf("gpgmail: failed to read first part in multipart/encrypted message: %v", err)
+	}
+
+	t, _, err := mime.ParseMediaType(p.Header.Get("Content-Type"))
+	if err != nil {
+		return nil, fmt.Errorf("gpgmail: failed to parse Content-Type of first part in multipart/encrypted message: %v", err)
+	}
+	if !strings.EqualFold(t, "application/pgp-encrypted") {
+		return nil, fmt.Errorf("gpgmail: first part in multipart/encrypted message has type %q, not application/pgp-encrypted", t)
+	}
+
+	metadata, err := textproto.ReadHeader(bufio.NewReader(p))
+	if err != nil {
+		return nil, fmt.Errorf("gpgmail: failed to parse application/pgp-encrypted part: %v", err)
+	}
+	if s := metadata.Get("Version"); s != "1" {
+		return nil, fmt.Errorf("gpgmail: unsupported PGP/MIME version: %q", s)
+	}
+
+	p, err = mr.NextPart()
+	if err != nil {
+		return nil, fmt.Errorf("gpgmail: failed to read second part in multipart/encrypted message: %v", err)
+	}
+	t, _, err = mime.ParseMediaType(p.Header.Get("Content-Type"))
+	if err != nil {
+		return nil, fmt.Errorf("gpgmail: failed to parse Content-Type of second part in multipart/encrypted message: %v", err)
+	}
+	if !strings.EqualFold(t, "application/octet-stream") {
+		return nil, fmt.Errorf("gpgmail: second part in multipart/encrypted message has type %q, not application/octet-stream", t)
+	}
+
+	md, err := gpgbin.Decrypt(p)
+	if err != nil {
+		return nil, fmt.Errorf("gpgmail: failed to read PGP message: %v", err)
+	}
+
+	cleartext := bufio.NewReader(md.Body)
+	cleartextHeader, err := textproto.ReadHeader(cleartext)
+	if err != nil {
+		return nil, fmt.Errorf("gpgmail: failed to read encrypted header: %v", err)
+	}
+
+	t, params, err := mime.ParseMediaType(cleartextHeader.Get("Content-Type"))
+	if err != nil {
+		return nil, err
+	}
+
+	if md.IsEncrypted && !md.IsSigned && strings.EqualFold(t, "multipart/signed") && strings.EqualFold(params["protocol"], "application/pgp-signature") {
+		// RFC 1847 encapsulation, see RFC 3156 section 6.1
+		micalg := params["micalg"]
+		mr := textproto.NewMultipartReader(cleartext, params["boundary"])
+		mds, err := newSignedReader(cleartextHeader, mr, micalg)
+		if err != nil {
+			return nil, fmt.Errorf("gpgmail: failed to read encapsulated multipart/signed message: %v", err)
+		}
+		mds.MessageDetails.IsEncrypted = md.IsEncrypted
+		mds.MessageDetails.DecryptedWith = md.DecryptedWith
+		mds.MessageDetails.DecryptedWithKeyId = md.DecryptedWithKeyId
+		return mds, nil
+	}
+
+	var headerBuf bytes.Buffer
+	textproto.WriteHeader(&headerBuf, cleartextHeader)
+	md.Body = io.MultiReader(&headerBuf, cleartext)
+
+	return &Reader{
+		Header:         h,
+		MessageDetails: md,
+	}, nil
+}
+
+func newSignedReader(h textproto.Header, mr *textproto.MultipartReader, micalg string) (*Reader, error) {
+	micalg = strings.ToLower(micalg)
+	p, err := mr.NextPart()
+	if err != nil {
+		return nil, fmt.Errorf("gpgmail: failed to read signed part in multipart/signed message: %v", err)
+	}
+	var headerBuf bytes.Buffer
+	textproto.WriteHeader(&headerBuf, p.Header)
+	var msg bytes.Buffer
+	headerRdr := bytes.NewReader(headerBuf.Bytes())
+	fullMsg := io.MultiReader(headerRdr, p)
+	io.Copy(&msg, fullMsg)
+
+	sig, err := mr.NextPart()
+	if err != nil {
+		return nil, fmt.Errorf("gpgmail: failed to read pgp part in multipart/signed message: %v", err)
+	}
+
+	md, err := gpgbin.Verify(&msg, sig)
+	if err != nil {
+		return nil, fmt.Errorf("gpgmail: failed to read PGP message: %v", err)
+	}
+	if md.Micalg != micalg && md.SignatureError == "" {
+		md.SignatureError = "gpg: header hash does not match actual sig hash"
+	}
+
+	return &Reader{
+		Header:         h,
+		MessageDetails: md,
+	}, nil
+}