review feedback: follow the spec and pad shares when generating the s…

…hare commitment instead of not allowing chains

x/lazyledgerapp/types/payformessage.go

@@ -205,7 +205,10 @@ func (msg *SignedTransactionDataPayForMessage) GetSigners() []sdk.AccAddress {
 // https://github.com/lazyledger/lazyledger-specs/blob/master/rationale/message_block_layout.md#non-interactive-default-rules
 func CreateCommit(k uint64, namespace, message []byte) ([]byte, error) {
 	// break message into shares
-	shares := ChunkMessage(message)
+	shares := chunkMessage(message)
+
+	// add padding if necessary
+	shares = addSharePadding(shares)
 
 	// organize shares for merkle mountain range
 	heights := PowerOf2MountainRange(uint64(len(shares)), k)
@@ -233,8 +236,8 @@ func CreateCommit(k uint64, namespace, message []byte) ([]byte, error) {
 	return merkle.HashFromByteSlices(subTreeRoots), nil
 }
 
-// ChunkMessage breaks the message into 256 byte pieces
-func ChunkMessage(message []byte) [][]byte {
+// chunkMessage breaks the message into 256 byte pieces
+func chunkMessage(message []byte) [][]byte {
 	var shares [][]byte
 	for i := 0; i < len(message); i += ShareSize {
 		end := i + ShareSize
@@ -246,6 +249,22 @@ func ChunkMessage(message []byte) [][]byte {
 	return shares
 }
 
+// addSharePadding will add padding to the last share if necessary
+func addSharePadding(shares [][]byte) [][]byte {
+	if len(shares) == 0 {
+		return shares
+	}
+
+	// add padding to the last share if necessary
+	if len(shares[len(shares)-1]) != ShareSize {
+		padded := make([]byte, ShareSize)
+		copy(padded, shares[len(shares)-1])
+		shares[len(shares)-1] = padded
+	}
+
+	return shares
+}
+
 // PowerOf2MountainRange returns the heights of the subtrees for binary merkle
 // mountian range
 func PowerOf2MountainRange(l, k uint64) []uint64 {

x/lazyledgerapp/types/payformessage_test.go

@@ -120,3 +120,30 @@ func TestGetCommitmentSignBytes(t *testing.T) {
 		assert.Equal(t, tt.expected, res)
 	}
 }
+
+func TestShareChunkingAndPadding(t *testing.T) {
+	type test struct {
+		input  []byte
+		expect [][]byte
+	}
+	tests := []test{
+		{
+			input:  []byte{1},
+			expect: [][]byte{append([]byte{1}, bytes.Repeat([]byte{0}, ShareSize-1)...)},
+		},
+		{
+			input:  bytes.Repeat([]byte{1}, ShareSize),
+			expect: [][]byte{bytes.Repeat([]byte{1}, ShareSize)},
+		},
+	}
+	for _, tt := range tests {
+		shares := chunkMessage(tt.input)
+		shares = addSharePadding(shares)
+		for _, share := range shares {
+			if len(share) != ShareSize {
+				t.Errorf("invalid share length: got %d wanted core.ShareSize (%d)", len(share), ShareSize)
+			}
+		}
+		assert.Equal(t, tt.expect, shares)
+	}
+}