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[CODEC-341] Fix Base16 custom alphabet decode table
Derive Base16 decode tables from custom encode tables so a configured codec can decode its own output. Reject encode tables that do not contain exactly 16 unique byte values. Reviewed-by: OpenAI Codex Reviewed-by: Anthropic Claude Code
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Lines changed: 78 additions & 8 deletions

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‎src/changes/changes.xml‎

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@@ -45,6 +45,7 @@ The <action> type attribute can be add,update,fix,remove.
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<body>
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<release version="1.22.1" date="YYYY-MM-DD" description="This is a feature and maintenance release. Java 8 or later is required.">
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<!-- FIX -->
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<action type="fix" issue="CODEC-341" dev="ggregory" due-to="Ruiqi Dong, Gary Gregory">Base16.Builder.setEncodeTable(byte...) can create a codec that cannot decode its own output.</action>
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<action type="add" issue="CODEC-337" dev="pkarwasz" due-to="Ruiqi Dong, Gary Gregory">Digest ALL reuses System.in, so only the first algorithm sees the real input (#431).</action>
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<!-- ADD -->
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<!-- UPDATE -->

‎src/main/java/org/apache/commons/codec/binary/Base16.java‎

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@@ -33,7 +33,7 @@
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* </p>
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* <p>
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* The only additional feature above those specified in RFC 4648 is support for working with a lower-case alphabet in addition to the default upper-case
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* alphabet.
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* alphabet, and configuring a custom 16-byte alphabet with {@link Builder#setEncodeTable(byte...)}.
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* </p>
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*
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* @see Base16InputStream
@@ -78,10 +78,18 @@ public Base16 get() {
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return new Base16(this);
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}
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/**
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* Sets the Base16 encode table and derives the matching decode table.
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*
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* @param encodeTable 16 unique bytes, null resets to the default upper-case table.
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* @return {@code this} instance.
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* @throws IllegalArgumentException if {@code encodeTable} does not contain 16 unique bytes.
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*/
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@Override
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public Builder setEncodeTable(final byte... encodeTable) {
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super.setDecodeTableRaw(Arrays.equals(encodeTable, LOWER_CASE_ENCODE_TABLE) ? LOWER_CASE_DECODE_TABLE : UPPER_CASE_DECODE_TABLE);
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return super.setEncodeTable(encodeTable);
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final byte[] table = encodeTable != null ? encodeTable : UPPER_CASE_ENCODE_TABLE;
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super.setDecodeTableRaw(toDecodeTable(table));
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return super.setEncodeTable(table);
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}
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/**
@@ -91,8 +99,7 @@ public Builder setEncodeTable(final byte... encodeTable) {
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* @return {@code this} instance.
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*/
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public Builder setLowerCase(final boolean lowerCase) {
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setEncodeTableRaw(lowerCase ? LOWER_CASE_ENCODE_TABLE : UPPER_CASE_ENCODE_TABLE);
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return asThis();
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return setEncodeTable(lowerCase ? LOWER_CASE_ENCODE_TABLE : UPPER_CASE_ENCODE_TABLE);
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}
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}
@@ -152,6 +159,32 @@ public Builder setLowerCase(final boolean lowerCase) {
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/** Mask used to extract 4 bits, used when decoding character. */
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private static final int MASK_4_BITS = 0x0f;
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private static byte[] toDecodeTable(final byte[] encodeTable) {
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if (Arrays.equals(encodeTable, UPPER_CASE_ENCODE_TABLE)) {
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return UPPER_CASE_DECODE_TABLE;
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}
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if (Arrays.equals(encodeTable, LOWER_CASE_ENCODE_TABLE)) {
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return LOWER_CASE_DECODE_TABLE;
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}
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if (encodeTable.length != 1 << BITS_PER_ENCODED_BYTE) {
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throw new IllegalArgumentException("Base16 encode table must contain 16 entries.");
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}
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int max = -1;
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for (final byte b : encodeTable) {
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max = Math.max(max, b & 0xff);
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}
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final byte[] decodeTable = new byte[max + 1];
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Arrays.fill(decodeTable, (byte) -1);
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for (int i = 0; i < encodeTable.length; i++) {
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final int b = encodeTable[i] & 0xff;
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if (decodeTable[b] != -1) {
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throw new IllegalArgumentException("Duplicate value in Base16 encode table: " + b);
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}
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decodeTable[b] = (byte) i;
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}
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return decodeTable;
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}
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/**
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* Constructs a new builder.
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*
@@ -241,8 +274,9 @@ void decode(final byte[] data, int offset, final int length, final Context conte
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private int decodeOctet(final byte octet) {
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int decoded = -1;
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if ((octet & 0xff) < decodeTable.length) {
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decoded = decodeTable[octet];
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final int b = octet & 0xff;
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if (b < decodeTable.length) {
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decoded = decodeTable[b];
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}
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if (decoded == -1) {
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throw new IllegalArgumentException("Invalid octet in encoded value: " + (int) octet);
@@ -282,7 +316,8 @@ void encode(final byte[] data, final int offset, final int length, final Context
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*/
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@Override
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public boolean isInAlphabet(final byte octet) {
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return isInAlphabet((byte) (octet & 0xff), decodeTable);
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final int b = octet & 0xff;
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return b < decodeTable.length && decodeTable[b] != -1;
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}
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/**

‎src/test/java/org/apache/commons/codec/binary/Base16Test.java‎

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@@ -149,6 +149,40 @@ void testConstructors() {
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new Base16(false, CodecPolicy.STRICT);
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}
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@Test
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void testCustomEncodeTableAffectsDecodeTable() {
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final byte[] encodeTable = "0123456789ABCDEF".getBytes(StandardCharsets.US_ASCII);
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final byte tmp = encodeTable[0];
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encodeTable[0] = encodeTable[1];
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encodeTable[1] = tmp;
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final Base16 base16 = Base16.builder().setEncodeTable(encodeTable).get();
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final byte[] encoded = base16.encode(new byte[] { 1 });
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assertEquals("10", new String(encoded, StandardCharsets.US_ASCII), "Custom Base16 alphabet encoding test");
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assertArrayEquals(new byte[] { 1 }, base16.decode(encoded), "Custom Base16 alphabet decoding test");
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}
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@Test
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void testCustomEncodeTableRejectsDuplicates() {
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final byte[] encodeTable = "00123456789ABCDE".getBytes(StandardCharsets.US_ASCII);
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assertThrows(IllegalArgumentException.class, () -> Base16.builder().setEncodeTable(encodeTable));
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}
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@Test
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void testCustomEncodeTableRejectsInvalidLength() {
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assertThrows(IllegalArgumentException.class,
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() -> Base16.builder().setEncodeTable("0123456789ABCDE".getBytes(StandardCharsets.US_ASCII)));
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}
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@Test
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void testBuilderSetLowerCaseDecodesOwnOutput() {
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final Base16 base16 = Base16.builder().setLowerCase(true).get();
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final byte[] data = { (byte) 0xab };
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final byte[] encoded = base16.encode(data);
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assertEquals("ab", new String(encoded, StandardCharsets.US_ASCII), "Base16 builder lower-case encoding test");
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assertArrayEquals(data, base16.decode(encoded), "Base16 builder lower-case decoding test");
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}
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@Test
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void testDecodeSingleBytes() {
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final String encoded = "556E74696C206E6578742074696D6521";

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