Add key generation offset (#104)

* Add key generation offset

* Bump version to 2.1.2

crypto/gopenpgp.go

@@ -8,6 +8,7 @@ import "time"
 type GopenPGP struct {
 	latestServerTime int64
 	latestClientTime time.Time
+	generationOffset int64
 }
 
 var pgp = GopenPGP{}

crypto/key.go

@@ -435,7 +435,7 @@ func generateKey(
 	cfg := &packet.Config{
 		Algorithm:     packet.PubKeyAlgoRSA,
 		RSABits:       bits,
-		Time:          getTimeGenerator(),
+		Time:          getKeyGenerationTimeGenerator(),
 		DefaultHash:   crypto.SHA256,
 		DefaultCipher: packet.CipherAES256,
 	}

crypto/time.go

@@ -13,6 +13,11 @@ func UpdateTime(newTime int64) {
 	}
 }
 
+// SetKeyGenerationOffset updates the offset when generating keys.
+func SetKeyGenerationOffset(offset int64) {
+	pgp.generationOffset = offset
+}
+
 // GetUnixTime gets latest cached time.
 func GetUnixTime() int64 {
 	return getNow().Unix()
@@ -49,3 +54,19 @@ func getDiff() (int64, error) {
 func getTimeGenerator() func() time.Time {
 	return getNow
 }
+
+// getNowKeyGenerationOffset returns the current time with the key generation offset.
+func getNowKeyGenerationOffset() time.Time {
+	extrapolate, err := getDiff()
+
+	if err != nil {
+		return time.Unix(time.Now().Unix()+pgp.generationOffset, 0)
+	}
+
+	return time.Unix(pgp.latestServerTime+extrapolate+pgp.generationOffset, 0)
+}
+
+// getKeyGenerationTimeGenerator Returns a time generator function with the key generation offset.
+func getKeyGenerationTimeGenerator() func() time.Time {
+	return getNowKeyGenerationOffset
+}