lnencrypt: Moves the crypto functions in the chanbackup package into its own package called lnencrypt

The functions inside of the crypto.go file in chanbackup (like EncryptPayloadToWriter and DecryptPayloadFromReader) can be used by a lot of things outside of just the chanbackup package. We can't just reference them directly from the chanbackup package because it's likely that it would generate circular dependencies. Therefore we need to move these functions into their own package to be referenced by chanbackup and whatever new functionality that needs them

chanbackup/multi.go

@@ -6,6 +6,7 @@ import (
 	"io"
 
 	"github.com/lightningnetwork/lnd/keychain"
+	"github.com/lightningnetwork/lnd/lnencrypt"
 	"github.com/lightningnetwork/lnd/lnwire"
 )
 
@@ -89,7 +90,12 @@ func (m Multi) PackToWriter(w io.Writer, keyRing keychain.KeyRing) error {
 
 	// With the plaintext multi backup assembled, we'll now encrypt it
 	// directly to the passed writer.
-	return encryptPayloadToWriter(multiBackupBuffer, w, keyRing)
+	e, err := lnencrypt.KeyRingEncrypter(keyRing)
+	if err != nil {
+		return fmt.Errorf("unable to generate encrypt key %v", err)
+	}
+
+	return e.EncryptPayloadToWriter(multiBackupBuffer.Bytes(), w)
 }
 
 // UnpackFromReader attempts to unpack (decrypt+deserialize) a packed
@@ -99,7 +105,11 @@ func (m *Multi) UnpackFromReader(r io.Reader, keyRing keychain.KeyRing) error {
 	// We'll attempt to read the entire packed backup, and also decrypt it
 	// using the passed key ring which is expected to be able to derive the
 	// encryption keys.
-	plaintextBackup, err := decryptPayloadFromReader(r, keyRing)
+	e, err := lnencrypt.KeyRingEncrypter(keyRing)
+	if err != nil {
+		return fmt.Errorf("unable to generate encrypt key %v", err)
+	}
+	plaintextBackup, err := e.DecryptPayloadFromReader(r)
 	if err != nil {
 		return err
 	}