From 572bbdc339b154cb274dc44252ca2d6c11663712 Mon Sep 17 00:00:00 2001
From: Andrea Franz <andrea@gravityblast.com>
Date: Fri, 15 Mar 2019 10:26:38 +0100
Subject: [PATCH] parse sign response and calculate v

---
 command_set.go     |  2 +-
 types/signature.go | 82 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 83 insertions(+), 1 deletion(-)
 create mode 100644 types/signature.go

diff --git a/command_set.go b/command_set.go
index 0096173..1ba9aa6 100644
--- a/command_set.go
+++ b/command_set.go
@@ -199,7 +199,7 @@ func (cs *CommandSet) Sign(data []byte) (*types.Signature, error) {
 		return nil, err
 	}
 
-	return types.ParseSignature(resp.Data)
+	return types.ParseSignature(data, resp.Data)
 }
 
 func (cs *CommandSet) mutualAuthenticate() error {
diff --git a/types/signature.go b/types/signature.go
new file mode 100644
index 0000000..1a28053
--- /dev/null
+++ b/types/signature.go
@@ -0,0 +1,82 @@
+package types
+
+import (
+	"bytes"
+
+	"github.com/ethereum/go-ethereum/crypto"
+	"github.com/status-im/keycard-go/apdu"
+)
+
+var (
+	TagSignatureTemplate = uint8(0xA0)
+)
+
+type Signature struct {
+	pubKey []byte
+	r      []byte
+	s      []byte
+	v      byte
+}
+
+func ParseSignature(message, resp []byte) (*Signature, error) {
+	pubKey, err := apdu.FindTag(resp, TagSignatureTemplate, uint8(0x80))
+	if err != nil {
+		return nil, err
+	}
+
+	r, err := apdu.FindTagN(resp, 0, TagSignatureTemplate, uint8(0x30), uint8(0x02))
+	if err != nil {
+		return nil, err
+	}
+
+	if len(r) > 32 {
+		r = r[len(r)-32:]
+	}
+
+	s, err := apdu.FindTagN(resp, 1, TagSignatureTemplate, uint8(0x30), uint8(0x02))
+	if err != nil {
+		return nil, err
+	}
+
+	v, err := calculateV(message, pubKey, r, s)
+	if err != nil {
+		return nil, err
+	}
+
+	return &Signature{
+		pubKey: pubKey,
+		r:      r,
+		s:      s,
+		v:      v,
+	}, nil
+}
+
+func (s *Signature) R() []byte {
+	return s.r
+}
+
+func (s *Signature) S() []byte {
+	return s.s
+}
+
+func (s *Signature) V() byte {
+	return s.v
+}
+
+func calculateV(message, pubKey, r, s []byte) (v byte, err error) {
+	rs := append(r, s...)
+	for i := 0; i < 2; i++ {
+		v = byte(i)
+		sig := append(rs, v)
+		rec, err := crypto.Ecrecover(message, sig)
+		if err != nil {
+			return v, err
+		}
+
+		if bytes.Equal(pubKey, rec) {
+			return v, nil
+		}
+	}
+
+	return v, err
+}