[FAB-830] - Helper functions to sign KVS data

This change set introduces helper functions (and respective tests) to sign
messages at the chaincode level using the previously checked-in encshim
extension.

Change-Id: Ib41ea63c200ca16b705f92d6b61760141249decf
Signed-off-by: Alessandro Sorniotti <ale.linux@sopit.net>

core/chaincode/shim/ext/entities/message.go

@@ -0,0 +1,76 @@
+/*
+Copyright IBM Corp. All Rights Reserved.
+
+SPDX-License-Identifier: Apache-2.0
+*/
+
+package entities
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// SignedMessage is a simple struct that contains space
+// for a payload and a signature over it, and convenience
+// functions to sign, verify, marshal and unmarshal
+type SignedMessage struct {
+	// ID contains a description of the entity signing this message
+	ID []byte `json:"id"`
+
+	// Payload contains the message that is signed
+	Payload []byte `json:"payload"`
+
+	// Sig contains a signature over ID and Payload
+	Sig []byte `json:"sig"`
+}
+
+// Sign signs the SignedMessage and stores the signature in the Sig field
+func (m *SignedMessage) Sign(signer Signer) error {
+	if signer == nil {
+		return fmt.Errorf("nil signer")
+	}
+
+	m.Sig = nil
+	bytes, err := json.Marshal(m)
+	if err != nil {
+		return fmt.Errorf("sign error: json.Marshal returned %s", err)
+	}
+	sig, err := signer.Sign(bytes)
+	if err != nil {
+		return fmt.Errorf("sign error: signer.Sign returned %s", err)
+	}
+	m.Sig = sig
+
+	return nil
+}
+
+// Verify verifies the signature over Payload stored in Sig
+func (m *SignedMessage) Verify(verifier Signer) (bool, error) {
+	if verifier == nil {
+		return false, fmt.Errorf("nil verifier")
+	}
+
+	sig := m.Sig
+	m.Sig = nil
+	defer func() {
+		m.Sig = sig
+	}()
+
+	bytes, err := json.Marshal(m)
+	if err != nil {
+		return false, fmt.Errorf("sign error: json.Marshal returned %s", err)
+	}
+
+	return verifier.Verify(sig, bytes)
+}
+
+// ToBytes serializes the intance to bytes
+func (m *SignedMessage) ToBytes() ([]byte, error) {
+	return json.Marshal(m)
+}
+
+// FromBytes populates the instance from the supplied byte array
+func (m *SignedMessage) FromBytes(d []byte) error {
+	return json.Unmarshal(d, m)
+}

core/chaincode/shim/ext/entities/message_test.go

@@ -0,0 +1,49 @@
+package entities
+
+import (
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSignedMessage(t *testing.T) {
+	ent, err := GetEncrypterSignerEntityForTest("TEST")
+	assert.NoError(t, err)
+	assert.NotNil(t, ent)
+
+	m := &SignedMessage{Payload: []byte("message"), ID: []byte(ent.ID())}
+
+	err = m.Sign(ent)
+	assert.NoError(t, err)
+	v, err := m.Verify(ent)
+	assert.NoError(t, err)
+	assert.True(t, v)
+}
+
+func TestSignedMessageErr(t *testing.T) {
+	ent, err := GetEncrypterSignerEntityForTest("TEST")
+	assert.NoError(t, err)
+	assert.NotNil(t, ent)
+
+	m := &SignedMessage{Payload: []byte("message"), ID: []byte(ent.ID())}
+
+	err = m.Sign(nil)
+	assert.Error(t, err)
+	_, err = m.Verify(nil)
+	assert.Error(t, err)
+
+	m = &SignedMessage{Payload: []byte("message"), Sig: []byte("barf")}
+	_, err = m.Verify(nil)
+	assert.Error(t, err)
+}
+
+func TestSignedMessageMarshaller(t *testing.T) {
+	m1 := &SignedMessage{Payload: []byte("message"), Sig: []byte("sig"), ID: []byte("ID")}
+	m2 := &SignedMessage{}
+	b, err := m1.ToBytes()
+	assert.NoError(t, err)
+	err = m2.FromBytes(b)
+	assert.NoError(t, err)
+	assert.True(t, reflect.DeepEqual(m1, m2))
+}