xdr, keypair: Add helpers to create CAP-40 decorated signatures

keypair/from_address.go

@@ -75,6 +75,10 @@ func (kp *FromAddress) SignDecorated(input []byte) (xdr.DecoratedSignature, erro
 	return xdr.DecoratedSignature{}, ErrCannotSign
 }
 
+func (kp *FromAddress) SignPayloadDecorated(input []byte) (xdr.DecoratedSignature, error) {
+	return xdr.DecoratedSignature{}, ErrCannotSign
+}
+
 func (kp *FromAddress) Equal(a *FromAddress) bool {
 	if kp == nil && a == nil {
 		return true

keypair/full.go

@@ -109,10 +109,18 @@ func (kp *Full) SignDecorated(input []byte) (xdr.DecoratedSignature, error) {
 		return xdr.DecoratedSignature{}, err
 	}
 
-	return xdr.DecoratedSignature{
-		Hint:      xdr.SignatureHint(kp.Hint()),
-		Signature: xdr.Signature(sig),
-	}, nil
+	return xdr.NewDecoratedSignature(sig, kp.Hint()), nil
+}
+
+// SignPayloadDecorated returns a decorated signature using this key's hint and
+// the last four bytes of both the input and the signature on that input.
+func (kp *Full) SignPayloadDecorated(input []byte) (xdr.DecoratedSignature, error) {
+	sig, err := kp.Sign(input)
+	if err != nil {
+		return xdr.DecoratedSignature{}, err
+	}
+
+	return xdr.NewDecoratedSignatureForPayload(sig, kp.Hint(), input), nil
 }
 
 func (kp *Full) Equal(f *Full) bool {

keypair/full_test.go

@@ -106,4 +106,12 @@ var _ = Describe("keypair.Full", func() {
 		})
 	})
 
+	Describe("SignPayloadDecorated()", func() {
+		It("returns the correct xdr struct", func() {
+			sig, err := subject.SignPayloadDecorated(message)
+			Expect(err).To(BeNil())
+			Expect(sig.Hint).To(BeEquivalentTo(payloadHint))
+			Expect(sig.Signature).To(BeEquivalentTo(signature))
+		})
+	})
 })

keypair/main.go

@@ -40,6 +40,7 @@ type KP interface {
 	Sign(input []byte) ([]byte, error)
 	SignBase64(input []byte) (string, error)
 	SignDecorated(input []byte) (xdr.DecoratedSignature, error)
+	SignPayloadDecorated(input []byte) (xdr.DecoratedSignature, error)
 }
 
 // Random creates a random full keypair

keypair/main_test.go

@@ -19,12 +19,13 @@ func TestBuild(t *testing.T) {
 }
 
 var (
-	address   = "GBRPYHIL2CI3FNQ4BXLFMNDLFJUNPU2HY3ZMFSHONUCEOASW7QC7OX2H"
-	seed      = "SDHOAMBNLGCE2MV5ZKIVZAQD3VCLGP53P3OBSBI6UN5L5XZI5TKHFQL4"
-	hint      = [4]byte{0x56, 0xfc, 0x05, 0xf7}
-	message   = []byte("hello")
-	signature = []byte{
-		0x2E, 0x75, 0xcc, 0x20, 0xd5, 0x19, 0x11, 0x1c, 0xaa, 0xaa, 0xdd, 0xdf,
+	address     = "GBRPYHIL2CI3FNQ4BXLFMNDLFJUNPU2HY3ZMFSHONUCEOASW7QC7OX2H"
+	seed        = "SDHOAMBNLGCE2MV5ZKIVZAQD3VCLGP53P3OBSBI6UN5L5XZI5TKHFQL4"
+	hint        = [4]byte{0x56, 0xfc, 0x05, 0xf7}
+	payloadHint = [4]byte{0x33, 0x90, 0x69, 0x98}
+	message     = []byte("hello")
+	signature   = []byte{
+		0x2e, 0x75, 0xcc, 0x20, 0xd5, 0x19, 0x11, 0x1c, 0xaa, 0xaa, 0xdd, 0xdf,
 		0x46, 0x4b, 0xb6, 0x50, 0xd2, 0xea, 0xf0, 0xa5, 0xd1, 0x8d, 0x74, 0x56,
 		0x93, 0xa1, 0x61, 0x00, 0xf2, 0xa4, 0x93, 0x7b, 0xc1, 0xdf, 0xfa, 0x8b,
 		0x0b, 0x1f, 0x61, 0xa2, 0x76, 0x99, 0x6d, 0x7e, 0xe8, 0xde, 0xb2, 0xd0,

xdr/decorated_signature.go

@@ -0,0 +1,38 @@
+package xdr
+
+// NewDecoratedSignature constructs a decorated signature structure directly
+// from the given signature and key hint. Note that the key hint should
+// correspond to the key that created the signature, but this helper cannot
+// ensure that.
+func NewDecoratedSignature(sig []byte, keyHint [4]byte) DecoratedSignature {
+	return DecoratedSignature{
+		Hint:      SignatureHint(keyHint),
+		Signature: Signature(sig),
+	}
+}
+
+// NewDecoratedSignatureForPayload creates a decorated signature with a hint
+// that uses the key hint, the last four bytes of signature, and the last four
+// bytes of the input that got signed. Note that the signature should be the
+// signature of the payload via the key being hinted, but this construction
+// method cannot ensure that.
+func NewDecoratedSignatureForPayload(
+	sig []byte, keyHint [4]byte, payload []byte,
+) DecoratedSignature {
+	hint := [4]byte{}
+	// copy the last four bytes of the payload into the hint
+	if len(payload) >= len(hint) {
+		copy(hint[:], payload[len(payload)-4:])
+	} else {
+		copy(hint[:], payload[:])
+	}
+
+	for i := 0; i < len(keyHint); i++ {
+		hint[i] ^= keyHint[i]
+	}
+
+	return DecoratedSignature{
+		Hint:      SignatureHint(hint),
+		Signature: Signature(sig),
+	}
+}

xdr/decorated_signature_test.go

@@ -0,0 +1,50 @@
+package xdr_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stellar/go/xdr"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDecoratedSignatures(t *testing.T) {
+	signature := []byte{0, 1, 2, 3, 4, 5, 6, 7, 8}
+	keyHint := [4]byte{9, 10, 11, 12}
+
+	testCases := []struct {
+		payload             []byte
+		expectedHint        [4]byte
+		expectedPayloadHint [4]byte
+	}{
+		{
+			[]byte{13, 14, 15, 16, 17, 18, 19, 20, 21},
+			keyHint,
+			[4]byte{27, 25, 31, 25},
+		},
+		{
+			[]byte{18, 19, 20},
+			keyHint,
+			[4]byte{27, 25, 31, 12},
+		},
+	}
+
+	for _, testCase := range testCases {
+		t.Run(
+			fmt.Sprintf("%d-byte decorated sig", len(testCase.payload)),
+			func(t *testing.T) {
+				decoSig := xdr.NewDecoratedSignature(signature, keyHint)
+				assert.EqualValues(t, testCase.expectedHint, decoSig.Hint)
+				assert.EqualValues(t, signature, decoSig.Signature)
+			})
+
+		t.Run(
+			fmt.Sprintf("%d-byte decorated sig for payload", len(testCase.payload)),
+			func(t *testing.T) {
+				decoSig := xdr.NewDecoratedSignatureForPayload(
+					signature, keyHint, testCase.payload)
+				assert.EqualValues(t, testCase.expectedPayloadHint, decoSig.Hint)
+				assert.EqualValues(t, signature, decoSig.Signature)
+			})
+	}
+}

xdr/signer_key_test.go

@@ -55,7 +55,6 @@ func TestSignerKey_SetAddress(t *testing.T) {
 		Name    string
 		Address string
 	}{
-
 		{
 			Name:    "AccountID",
 			Address: "GA3D5KRYM6CB7OWQ6TWYRR3Z4T7GNZLKERYNZGGA5SOAOPIFY6YQHES5",
@@ -69,8 +68,8 @@ func TestSignerKey_SetAddress(t *testing.T) {
 			Address: "XBU2RRGLXH3E5CQHTD3ODLDF2BWDCYUSSBLLZ5GNW7JXHDIYKXZWGTOG",
 		},
 		{
-			"SignedPayload",
-			"PA7QYNF7SOWQ3GLR2BGMZEHXAVIRZA4KVWLTJJFC7MGXUA74P7UJUAAAAAOQCAQDAQCQMBYIBEFAWDANBYHRAEISCMKBKFQXDAMRUGY4DUAAAAFGBU",
+			Name:    "SignedPayload",
+			Address: "PA7QYNF7SOWQ3GLR2BGMZEHXAVIRZA4KVWLTJJFC7MGXUA74P7UJUAAAAAOQCAQDAQCQMBYIBEFAWDANBYHRAEISCMKBKFQXDAMRUGY4DUAAAAFGBU",
 		},
 	}