use emersion/go-msgauth for DKIM (#2242)

Fixes #1041 (support ed25519-sha256 for DKIM)

vendor/github.com/emersion/go-msgauth/dkim/sign.go

@@ -0,0 +1,346 @@
+package dkim
+
+import (
+	"bufio"
+	"bytes"
+	"crypto"
+	"crypto/rand"
+	"crypto/rsa"
+	"encoding/base64"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+	"time"
+
+	"golang.org/x/crypto/ed25519"
+)
+
+var randReader io.Reader = rand.Reader
+
+// SignOptions is used to configure Sign. Domain, Selector and Signer are
+// mandatory.
+type SignOptions struct {
+	// The SDID claiming responsibility for an introduction of a message into the
+	// mail stream. Hence, the SDID value is used to form the query for the public
+	// key. The SDID MUST correspond to a valid DNS name under which the DKIM key
+	// record is published.
+	//
+	// This can't be empty.
+	Domain string
+	// The selector subdividing the namespace for the domain.
+	//
+	// This can't be empty.
+	Selector string
+	// The Agent or User Identifier (AUID) on behalf of which the SDID is taking
+	// responsibility.
+	//
+	// This is optional.
+	Identifier string
+
+	// The key used to sign the message.
+	//
+	// Supported Signer.Public() values are *rsa.PublicKey and
+	// ed25519.PublicKey.
+	Signer crypto.Signer
+	// The hash algorithm used to sign the message. If zero, a default hash will
+	// be chosen.
+	//
+	// The only supported hash algorithm is crypto.SHA256.
+	Hash crypto.Hash
+
+	// Header and body canonicalization algorithms.
+	//
+	// If empty, CanonicalizationSimple is used.
+	HeaderCanonicalization Canonicalization
+	BodyCanonicalization   Canonicalization
+
+	// A list of header fields to include in the signature. If nil, all headers
+	// will be included. If not nil, "From" MUST be in the list.
+	//
+	// See RFC 6376 section 5.4.1 for recommended header fields.
+	HeaderKeys []string
+
+	// The expiration time. A zero value means no expiration.
+	Expiration time.Time
+
+	// A list of query methods used to retrieve the public key.
+	//
+	// If nil, it is implicitly defined as QueryMethodDNSTXT.
+	QueryMethods []QueryMethod
+}
+
+// Signer generates a DKIM signature.
+//
+// The whole message header and body must be written to the Signer. Close should
+// always be called (either after the whole message has been written, or after
+// an error occurred and the signer won't be used anymore). Close may return an
+// error in case signing fails.
+//
+// After a successful Close, Signature can be called to retrieve the
+// DKIM-Signature header field that the caller should prepend to the message.
+type Signer struct {
+	pw        *io.PipeWriter
+	done      <-chan error
+	sigParams map[string]string // only valid after done received nil
+}
+
+// NewSigner creates a new signer. It returns an error if SignOptions is
+// invalid.
+func NewSigner(options *SignOptions) (*Signer, error) {
+	if options == nil {
+		return nil, fmt.Errorf("dkim: no options specified")
+	}
+	if options.Domain == "" {
+		return nil, fmt.Errorf("dkim: no domain specified")
+	}
+	if options.Selector == "" {
+		return nil, fmt.Errorf("dkim: no selector specified")
+	}
+	if options.Signer == nil {
+		return nil, fmt.Errorf("dkim: no signer specified")
+	}
+
+	headerCan := options.HeaderCanonicalization
+	if headerCan == "" {
+		headerCan = CanonicalizationSimple
+	}
+	if _, ok := canonicalizers[headerCan]; !ok {
+		return nil, fmt.Errorf("dkim: unknown header canonicalization %q", headerCan)
+	}
+
+	bodyCan := options.BodyCanonicalization
+	if bodyCan == "" {
+		bodyCan = CanonicalizationSimple
+	}
+	if _, ok := canonicalizers[bodyCan]; !ok {
+		return nil, fmt.Errorf("dkim: unknown body canonicalization %q", bodyCan)
+	}
+
+	var keyAlgo string
+	switch options.Signer.Public().(type) {
+	case *rsa.PublicKey:
+		keyAlgo = "rsa"
+	case ed25519.PublicKey:
+		keyAlgo = "ed25519"
+	default:
+		return nil, fmt.Errorf("dkim: unsupported key algorithm %T", options.Signer.Public())
+	}
+
+	hash := options.Hash
+	var hashAlgo string
+	switch options.Hash {
+	case 0: // sha256 is the default
+		hash = crypto.SHA256
+		fallthrough
+	case crypto.SHA256:
+		hashAlgo = "sha256"
+	case crypto.SHA1:
+		return nil, fmt.Errorf("dkim: hash algorithm too weak: sha1")
+	default:
+		return nil, fmt.Errorf("dkim: unsupported hash algorithm")
+	}
+
+	if options.HeaderKeys != nil {
+		ok := false
+		for _, k := range options.HeaderKeys {
+			if strings.EqualFold(k, "From") {
+				ok = true
+				break
+			}
+		}
+		if !ok {
+			return nil, fmt.Errorf("dkim: the From header field must be signed")
+		}
+	}
+
+	done := make(chan error, 1)
+	pr, pw := io.Pipe()
+
+	s := &Signer{
+		pw:   pw,
+		done: done,
+	}
+
+	closeReadWithError := func(err error) {
+		pr.CloseWithError(err)
+		done <- err
+	}
+
+	go func() {
+		defer close(done)
+
+		// Read header
+		br := bufio.NewReader(pr)
+		h, err := readHeader(br)
+		if err != nil {
+			closeReadWithError(err)
+			return
+		}
+
+		// Hash body
+		hasher := hash.New()
+		can := canonicalizers[bodyCan].CanonicalizeBody(hasher)
+		if _, err := io.Copy(can, br); err != nil {
+			closeReadWithError(err)
+			return
+		}
+		if err := can.Close(); err != nil {
+			closeReadWithError(err)
+			return
+		}
+		bodyHashed := hasher.Sum(nil)
+
+		params := map[string]string{
+			"v":  "1",
+			"a":  keyAlgo + "-" + hashAlgo,
+			"bh": base64.StdEncoding.EncodeToString(bodyHashed),
+			"c":  string(headerCan) + "/" + string(bodyCan),
+			"d":  options.Domain,
+			//"l": "", // TODO
+			"s": options.Selector,
+			"t": formatTime(now()),
+			//"z": "", // TODO
+		}
+
+		var headerKeys []string
+		if options.HeaderKeys != nil {
+			headerKeys = options.HeaderKeys
+		} else {
+			for _, kv := range h {
+				k, _ := parseHeaderField(kv)
+				headerKeys = append(headerKeys, k)
+			}
+		}
+		params["h"] = formatTagList(headerKeys)
+
+		if options.Identifier != "" {
+			params["i"] = options.Identifier
+		}
+
+		if options.QueryMethods != nil {
+			methods := make([]string, len(options.QueryMethods))
+			for i, method := range options.QueryMethods {
+				methods[i] = string(method)
+			}
+			params["q"] = formatTagList(methods)
+		}
+
+		if !options.Expiration.IsZero() {
+			params["x"] = formatTime(options.Expiration)
+		}
+
+		// Hash and sign headers
+		hasher.Reset()
+		picker := newHeaderPicker(h)
+		for _, k := range headerKeys {
+			kv := picker.Pick(k)
+			if kv == "" {
+				// The Signer MAY include more instances of a header field name
+				// in "h=" than there are actual corresponding header fields so
+				// that the signature will not verify if additional header
+				// fields of that name are added.
+				continue
+			}
+
+			kv = canonicalizers[headerCan].CanonicalizeHeader(kv)
+			if _, err := io.WriteString(hasher, kv); err != nil {
+				closeReadWithError(err)
+				return
+			}
+		}
+
+		params["b"] = ""
+		sigField := formatSignature(params)
+		sigField = canonicalizers[headerCan].CanonicalizeHeader(sigField)
+		sigField = strings.TrimRight(sigField, crlf)
+		if _, err := io.WriteString(hasher, sigField); err != nil {
+			closeReadWithError(err)
+			return
+		}
+		hashed := hasher.Sum(nil)
+
+		// Don't pass Hash to Sign for ed25519 as it doesn't support it
+		// and will return an error ("ed25519: cannot sign hashed message").
+		if keyAlgo == "ed25519" {
+			hash = crypto.Hash(0)
+		}
+
+		sig, err := options.Signer.Sign(randReader, hashed, hash)
+		if err != nil {
+			closeReadWithError(err)
+			return
+		}
+		params["b"] = base64.StdEncoding.EncodeToString(sig)
+
+		s.sigParams = params
+		closeReadWithError(nil)
+	}()
+
+	return s, nil
+}
+
+// Write implements io.WriteCloser.
+func (s *Signer) Write(b []byte) (n int, err error) {
+	return s.pw.Write(b)
+}
+
+// Close implements io.WriteCloser. The error return by Close must be checked.
+func (s *Signer) Close() error {
+	if err := s.pw.Close(); err != nil {
+		return err
+	}
+	return <-s.done
+}
+
+// Signature returns the whole DKIM-Signature header field. It can only be
+// called after a successful Signer.Close call.
+//
+// The returned value contains both the header field name, its value and the
+// final CRLF.
+func (s *Signer) Signature() string {
+	if s.sigParams == nil {
+		panic("dkim: Signer.Signature must only be called after a succesful Signer.Close")
+	}
+	return formatSignature(s.sigParams)
+}
+
+// Sign signs a message. It reads it from r and writes the signed version to w.
+func Sign(w io.Writer, r io.Reader, options *SignOptions) error {
+	s, err := NewSigner(options)
+	if err != nil {
+		return err
+	}
+	defer s.Close()
+
+	// We need to keep the message in a buffer so we can write the new DKIM
+	// header field before the rest of the message
+	var b bytes.Buffer
+	mw := io.MultiWriter(&b, s)
+
+	if _, err := io.Copy(mw, r); err != nil {
+		return err
+	}
+	if err := s.Close(); err != nil {
+		return err
+	}
+
+	if _, err := io.WriteString(w, s.Signature()); err != nil {
+		return err
+	}
+	_, err = io.Copy(w, &b)
+	return err
+}
+
+func formatSignature(params map[string]string) string {
+	sig := formatHeaderParams(headerFieldName, params)
+	return sig
+}
+
+func formatTagList(l []string) string {
+	return strings.Join(l, ":")
+}
+
+func formatTime(t time.Time) string {
+	return strconv.FormatInt(t.Unix(), 10)
+}