protonmail: make SRP modulus signature failures a warning

Temporarily make signature errors non-fatal because the Go OpenPGP library is
missing some features we need:

    openpgp: unsupported feature: public key algorithm 22

This doesn't really weaken our security since we weren't checking the signature
key anyway. To completely fix this issue, we need to upstream the required
key algorithm.

Fixes #46

protonmail/srp.go

@@ -7,6 +7,7 @@ import (
 	"encoding/base64"
 	"errors"
 	"io"
+	"log"
 	"math/big"
 
 	"golang.org/x/crypto/openpgp"
@@ -22,10 +23,11 @@ func decodeModulus(msg string) ([]byte, error) {
 		return nil, errors.New("invalid modulus signed PGP block")
 	}
 
-	// TODO: check signature key
+	// TODO: check signature and signature key
 	_, err := openpgp.CheckDetachedSignature(nil, bytes.NewReader(block.Plaintext), block.ArmoredSignature.Body)
 	if err != nil && err != openpgperrors.ErrUnknownIssuer {
-		return nil, err
+		//return nil, fmt.Errorf("failed to decode modulus: %v", err)
+		log.Println("warning: failed to check SRP modulus signature:", err)
 	}
 
 	return base64.StdEncoding.DecodeString(string(block.Plaintext))