Fix method so it allows necessary padding

Author: julia-zack

src/main/java/co/rsk/bitcoinj/script/ScriptChunk.java

@@ -183,18 +183,22 @@ private int decodePositiveNConsideringEncoding() {
             throw new IllegalArgumentException("Chunk has null data.");
         }
         int dataLength = data.length;
-        byte moreSignificantByte = data[dataLength - 1];
+        byte mostSignificantByte = data[dataLength - 1];
 
-        if (moreSignificantByte == 0x00) {
-            throw new IllegalArgumentException("Number from chunk does not have minimal encoding.");
-        }
-
-        int signByte = moreSignificantByte & 0x80;
+        int signByte = mostSignificantByte & 0x80;
         boolean isPositive = signByte == 0;
         if (!isPositive) {
             throw new IllegalArgumentException("Number from chunk is not positive.");
         }
 
+        boolean hasPadding = mostSignificantByte == 0;
+        if (hasPadding) {
+            boolean shouldAllowPadding = (data[dataLength - 2] & 0x80) != 0;
+            if (!shouldAllowPadding) {
+                throw new IllegalArgumentException("Number from chunk does not have minimal encoding.");
+            }
+        }
+
         if (dataLength > 4) {
             throw new IllegalArgumentException("Number from chunk has more than 4 bytes.");
         }

src/test/java/co/rsk/bitcoinj/script/ScriptChunkTest.java

@@ -180,8 +180,10 @@ public void decodePositiveN_forZero_throwsIAE() {
     @Test
     public void decodePositiveN_forNumberWithoutMinimalEncoding_throwsIAE() {
         byte zero = 0x00;
-        for (int n=0; n<20; n++) {
-            int i = (1 << n); // i = (2^n)
+
+        // from 1 to 15, numbers are small numbers and therefore pushed as opcodes
+        // with null chunk data
+        for (int i=1; i<16; i++) {
             byte[] numWithoutMinimalEncoding = new byte[] {(byte) i, zero};
 
             ScriptBuilder builder = new ScriptBuilder();
@@ -192,6 +194,23 @@ public void decodePositiveN_forNumberWithoutMinimalEncoding_throwsIAE() {
             assertFalse(chunk.isPositiveN());
             assertThrows(IllegalArgumentException.class, chunk::decodePositiveN);
         }
+
+        for (int n=4; n<20; n++) {
+            int i = (1 << n); // i = (2^n)
+            ScriptBuilder builder = new ScriptBuilder();
+            ScriptChunk chunk = builder.number(i).build().getChunks().get(0);
+            byte[] chunkData = chunk.data;
+
+            // add zero padding so the number does not have minimal encoding
+            byte [] numWithoutMinimalEncoding = Arrays.copyOf(chunkData, chunkData.length + 1);
+            numWithoutMinimalEncoding[chunkData.length] = zero;
+
+            // build the chunk for the number without minimal encoding
+            builder = new ScriptBuilder();
+            ScriptChunk newChunk = builder.data(numWithoutMinimalEncoding).build().chunks.get(0);
+            assertFalse(newChunk.isPositiveN());
+            assertThrows(IllegalArgumentException.class, newChunk::decodePositiveN);
+        }
     }
 
     @Test