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

Fixes #1041 (support ed25519-sha256 for DKIM)

irc/email/dkim.go

@@ -4,9 +4,18 @@
 package email
 
 import (
+	"bytes"
+	"crypto"
+	"crypto/ed25519"
+	"crypto/rsa"
+	"crypto/x509"
+	"encoding/pem"
 	"errors"
-	dkim "github.com/toorop/go-dkim"
+	"fmt"
+
 	"os"
+
+	dkim "github.com/emersion/go-msgauth/dkim"
 )
 
 var (
@@ -17,38 +26,77 @@ type DKIMConfig struct {
 	Domain   string
 	Selector string
 	KeyFile  string `yaml:"key-file"`
-	keyBytes []byte
+	privKey  crypto.Signer
+}
+
+func (dkim *DKIMConfig) Enabled() bool {
+	return dkim.Domain != ""
 }
 
 func (dkim *DKIMConfig) Postprocess() (err error) {
-	if dkim.Domain != "" {
-		if dkim.Selector == "" || dkim.KeyFile == "" {
-			return ErrMissingFields
-		}
-		dkim.keyBytes, err = os.ReadFile(dkim.KeyFile)
-		if err != nil {
-			return err
-		}
+	if !dkim.Enabled() {
+		return nil
 	}
+
+	if dkim.Selector == "" || dkim.KeyFile == "" {
+		return ErrMissingFields
+	}
+
+	keyBytes, err := os.ReadFile(dkim.KeyFile)
+	if err != nil {
+		return fmt.Errorf("Could not read DKIM key file: %w", err)
+	}
+	dkim.privKey, err = parseDKIMPrivKey(keyBytes)
+	if err != nil {
+		return fmt.Errorf("Could not parse DKIM key file: %w", err)
+	}
+
 	return nil
 }
 
-var defaultOptions = dkim.SigOptions{
-	Version:               1,
-	Canonicalization:      "relaxed/relaxed",
-	Algo:                  "rsa-sha256",
-	Headers:               []string{"from", "to", "subject", "message-id", "date"},
-	BodyLength:            0,
-	QueryMethods:          []string{"dns/txt"},
-	AddSignatureTimestamp: true,
-	SignatureExpireIn:     0,
+func parseDKIMPrivKey(input []byte) (crypto.Signer, error) {
+	if len(input) == 0 {
+		return nil, errors.New("DKIM private key is empty")
+	}
+
+	// raw ed25519 private key format
+	if len(input) == ed25519.PrivateKeySize {
+		return ed25519.PrivateKey(input), nil
+	}
+
+	d, _ := pem.Decode(input)
+	if d == nil {
+		return nil, errors.New("Invalid PEM data for DKIM private key")
+	}
+
+	if rsaKey, err := x509.ParsePKCS1PrivateKey(d.Bytes); err == nil {
+		return rsaKey, nil
+	}
+
+	if k, err := x509.ParsePKCS8PrivateKey(d.Bytes); err == nil {
+		switch key := k.(type) {
+		case *rsa.PrivateKey:
+			return key, nil
+		case ed25519.PrivateKey:
+			return key, nil
+		default:
+			return nil, fmt.Errorf("Unacceptable type for DKIM private key: %T", k)
+		}
+	}
+
+	return nil, errors.New("No acceptable format for DKIM private key")
 }
 
 func DKIMSign(message []byte, dkimConfig DKIMConfig) (result []byte, err error) {
-	options := defaultOptions
-	options.PrivateKey = dkimConfig.keyBytes
-	options.Domain = dkimConfig.Domain
-	options.Selector = dkimConfig.Selector
-	err = dkim.Sign(&message, options)
-	return message, err
+	options := dkim.SignOptions{
+		Domain:                 dkimConfig.Domain,
+		Selector:               dkimConfig.Selector,
+		Signer:                 dkimConfig.privKey,
+		HeaderCanonicalization: dkim.CanonicalizationRelaxed,
+		BodyCanonicalization:   dkim.CanonicalizationRelaxed,
+	}
+	input := bytes.NewBuffer(message)
+	output := bytes.NewBuffer(make([]byte, 0, len(message)+1024))
+	err = dkim.Sign(output, input, &options)
+	return output.Bytes(), err
 }

irc/email/email.go

@@ -233,7 +233,7 @@ func SendMail(config MailtoConfig, recipient string, msg []byte) (err error) {
 		}
 	}
 
-	if config.DKIM.Domain != "" {
+	if config.DKIM.Enabled() {
 		msg, err = DKIMSign(msg, config.DKIM)
 		if err != nil {
 			return