rlp: fix bug in decodeLength (#2081)

Fixes bug in RLP `decodeLength()` by ensuring that calculations of length should use units of bytes instead of bits.

category: bug 
ticket: #2076

eth2util/rlp/rlp.go

@@ -101,9 +101,9 @@ func decodeLength(item []byte) (offset int, length int, err error) {
 	}
 
 	if prefix < 0xc0 {
-		length = int(prefix - 0xb7)
-		if length > 63 || length < 0 {
-			return 0, 0, errors.New("invalid length prefix")
+		length = int(prefix - 0xb7)    // length of the string in bytes in binary form
+		if length > 8 || length <= 0 { // This is impossible based on outer if else checks
+			panic("length not in expected range [1,8]")
 		}
 
 		offset := 1 + length
@@ -125,8 +125,8 @@ func decodeLength(item []byte) (offset int, length int, err error) {
 	}
 
 	length = int(prefix - 0xf7)
-	if length > 63 || length < 0 {
-		return 0, 0, errors.New("invalid length prefix")
+	if length > 8 || length <= 0 { // This is impossible based on outer if else checks
+		panic("length not in expected range [1,8]")
 	}
 
 	offset = 1 + length

eth2util/rlp/rlp_internal_test.go

@@ -0,0 +1,30 @@
+// Copyright © 2022-2023 Obol Labs Inc. Licensed under the terms of a Business Source License 1.1
+
+package rlp
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDecodeLength(t *testing.T) {
+	t.Run("ok", func(t *testing.T) {
+		items := []byte{0xb8, 0x01, 0x0a}
+		offset, length, err := decodeLength(items)
+		require.NoError(t, err)
+		require.Equal(t, 2, offset)
+		require.Equal(t, 1, length)
+	})
+
+	t.Run("prefix absent", func(t *testing.T) {
+		_, _, err := decodeLength(nil)
+		require.ErrorContains(t, err, "input too short")
+	})
+
+	t.Run("prefix provided but length absent", func(t *testing.T) {
+		items := []byte{0xbf}
+		_, _, err := decodeLength(items)
+		require.ErrorContains(t, err, "input too short")
+	})
+}