Fix: use verifyTime in the config time instead of Now()

When decrypting message, we have to use verifyTime in the config
otherwise signatures not valid at verifyTime but valid at Now()
will be seen as valid.

crypto/keyring_message.go

@@ -5,6 +5,7 @@ import (
 	"crypto"
 	"io"
 	"io/ioutil"
+	"time"
 
 	"github.com/ProtonMail/go-crypto/openpgp"
 	"github.com/ProtonMail/go-crypto/openpgp/packet"
@@ -190,6 +191,7 @@ func asymmetricDecrypt(
 		encryptedIO,
 		privateKey,
 		verifyKey,
+		verifyTime,
 	)
 	if err != nil {
 		return nil, err
@@ -215,7 +217,10 @@ func asymmetricDecrypt(
 
 // Core for decryption+verification (all) functions.
 func asymmetricDecryptStream(
-	encryptedIO io.Reader, privateKey *KeyRing, verifyKey *KeyRing,
+	encryptedIO io.Reader,
+	privateKey *KeyRing,
+	verifyKey *KeyRing,
+	verifyTime int64,
 ) (messageDetails *openpgp.MessageDetails, err error) {
 	privKeyEntries := privateKey.entities
 	var additionalEntries openpgp.EntityList
@@ -228,7 +233,19 @@ func asymmetricDecryptStream(
 		privKeyEntries = append(privKeyEntries, additionalEntries...)
 	}
 
-	config := &packet.Config{Time: getTimeGenerator()}
+	config := &packet.Config{
+		Time: func() time.Time {
+			if verifyTime == 0 {
+				/*
+					We default to current time while decrypting and verifying
+					but the caller will remove signature expiration errors later on.
+					See processSignatureExpiration().
+				*/
+				return getNow()
+			}
+			return time.Unix(verifyTime, 0)
+		},
+	}
 
 	messageDetails, err = openpgp.ReadMessage(encryptedIO, privKeyEntries, nil, config)
 	if err != nil {