Export a method to decode Envelope payload using flexible base64 decoder

dsse/sign.go

@@ -31,6 +31,16 @@ type Envelope struct {
 	Signatures  []Signature `json:"signatures"`
 }
 
+/*
+DecodePayload returns the serialized body, decoded
+from the envelope's payload field. A flexible
+decoder is used, first trying standard base64, then
+URL-encoded base64.
+*/
+func (e *Envelope) DecodePayload() ([]byte, error) {
+	return b64Decode(e.Payload)
+}
+
 /*
 Signature represents a generic in-toto signature that contains the identifier
 of the key which was used to create the signature.

dsse/sign_test.go

@@ -331,7 +331,7 @@ func TestEcdsaSign(t *testing.T) {
 	assert.Equal(t, acceptedKeys[0].KeyID, keyID, "unexpected keyid")
 }
 
-func TestB64Decode(t *testing.T) {
+func TestDecodePayload(t *testing.T) {
 	var want = make([]byte, 256)
 	for i := range want {
 		want[i] = byte(i)
@@ -342,23 +342,35 @@ func TestB64Decode(t *testing.T) {
 	var b64StdErr = "AAECAwQFBgcICQoLDA0ODxAREhMUFRYXGBkaGxwdHh8gISIjJCUmJygpKissLS4vMDEyMzQ1Njc4OTo7PD0-P0BBQkNERUZHSElKS0xNTk9QUVJTVFVWV1hZWltcXV5fYGFiY2RlZmdoaWprbG1ub3BxcnN0dXZ3eHl6e3x9fn-AgYKDhIWGh4iJiouMjY6PkJGSk5SVlpeYmZqbnJ2en6ChoqOkpaanqKmqq6ytrq-wsbKztLW2t7i5uru8vb6_wMHCw8TFxsfIycrLzM3Oz9DR0tPU1dbX2Nna29zd3t_g4eLj5OXm5-jp6uvs7e7v8PHy8_T19vf4-fr7_P3-_w"
 
 	t.Run("Standard encoding", func(t *testing.T) {
-		got, err := b64Decode(b64Std)
+		env := &Envelope{
+			Payload: b64Std,
+		}
+		got, err := env.DecodePayload()
 		assert.Nil(t, err, "unexpected error")
 		assert.Equal(t, want, got, "wrong data")
 	})
 	t.Run("URL encoding", func(t *testing.T) {
-		got, err := b64Decode(b64Url)
+		env := &Envelope{
+			Payload: b64Url,
+		}
+		got, err := env.DecodePayload()
 		assert.Nil(t, err, "unexpected error")
 		assert.Equal(t, want, got, "wrong data")
 	})
 
 	t.Run("Standard encoding - error", func(t *testing.T) {
-		got, err := b64Decode(b64StdErr)
+		env := &Envelope{
+			Payload: b64StdErr,
+		}
+		got, err := env.DecodePayload()
 		assert.NotNil(t, err, "expected error")
 		assert.Nil(t, got, "wrong data")
 	})
 	t.Run("URL encoding - error", func(t *testing.T) {
-		got, err := b64Decode(b64UrlErr)
+		env := &Envelope{
+			Payload: b64UrlErr,
+		}
+		got, err := env.DecodePayload()
 		assert.NotNil(t, err, "expected error")
 		assert.Nil(t, got, "wrong data")
 	})

dsse/verify.go

@@ -41,7 +41,7 @@ func (ev *EnvelopeVerifier) Verify(e *Envelope) ([]AcceptedKey, error) {
 	}
 
 	// Decode payload (i.e serialized body)
-	body, err := b64Decode(e.Payload)
+	body, err := e.DecodePayload()
 	if err != nil {
 		return nil, err
 	}