TooTallNate/Java-WebSocket · error · IllegalArgumentException

Source array with length

Error message

Source array with length %d cannot have offset of %d and still process four bytes.

What it means

decode4to3 must be able to read exactly 4 bytes starting at srcOffset. If srcOffset is negative or srcOffset + 3 >= source.length, fewer than 4 bytes are available and the method throws IllegalArgumentException reporting the source length and offset. The bounds are intentionally strict because the decoder always consumes a full 4-byte quantum.

Solutions

1. Only call the offset-based decode when srcOffset + 4 <= source.length; handle trailing 2-3 bytes via the standard API.
2. Use the high-level Base64.decode(String/byte[]) which chunks correctly, instead of manual offset arithmetic.
3. Validate that the Base64 payload length is a multiple of 4 (after padding) before decoding.

Example fix

// before
for (int i = 0; i < src.length; i += 4) {
    decode4to3(src, i, dest, i / 4 * 3);
}

// after
for (int i = 0; i + 3 < src.length; i += 4) {
    decode4to3(src, i, dest, i / 4 * 3);
}

if (src != null && srcOffset >= 0 && srcOffset + 4 <= src.length) {
    decode4to3(src, srcOffset, dest, destOffset);
}

static boolean canReadFourBytes(byte[] a, int off) {
    return a != null && off >= 0 && off + 4 <= a.length;
}

try {
    Base64.decode(src, off, len, dest, destOff);
} catch (IllegalArgumentException e) {
    // source slice too small — fall back to full decode
    decoded = Base64.decode(src);
}

When it happens

Trigger: Calling decode overloads with (source, offset, length/destOffset) where the source slice has fewer than 4 bytes remaining, e.g. decoding a 2-3 byte tail incorrectly or offsetting into a truncated array.

Common situations: Manual chunking loops that don't stop at len < 4; decoding truncated Base64 payloads; wrong offset arithmetic when skipping headers/magic bytes.

Related errors

• Cannot have offset of
• Destination array with length
• Cannot have length offset:
• Source array was null.
• Destination array was null.

AI-assisted analysis of TooTallNate/Java-WebSocket@afeacbf8c0 (2026-09-09). Data as JSON: /api/errors/3fdcfb6e553bb4e2. Report an issue: GitHub.

Appendix: source

Thrown at src/main/java/org/java_websocket/util/Base64.java:814

   * @return the number of decoded bytes converted
   * @throws IllegalArgumentException if source or destination arrays are null, if srcOffset or
   *                                  destOffset are invalid or there is not enough room in the
   *                                  array.
   * @since 1.3
   */
  private static int decode4to3(
      byte[] source, int srcOffset,
      byte[] destination, int destOffset, int options) {

    // Lots of error checking and exception throwing
    if (source == null) {
      throw new IllegalArgumentException("Source array was null.");
    }   // end if
    if (destination == null) {
      throw new IllegalArgumentException("Destination array was null.");
    }   // end if
    if (srcOffset < 0 || srcOffset + 3 >= source.length) {
      throw new IllegalArgumentException(String.format(
          "Source array with length %d cannot have offset of %d and still process four bytes.",
          source.length, srcOffset));
    }   // end if
    if (destOffset < 0 || destOffset + 2 >= destination.length) {
      throw new IllegalArgumentException(String.format(
          "Destination array with length %d cannot have offset of %d and still store three bytes.",
          destination.length, destOffset));
    }   // end if

    final byte[] DECODABET = getDecodabet(options);

    // Example: Dk==
    if (source[srcOffset + 2] == EQUALS_SIGN) {
      // Two ways to do the same thing. Don't know which way I like best.
      //int outBuff =   ( ( DECODABET[ source[ srcOffset    ] ] << 24 ) >>>  6 )
      //              | ( ( DECODABET[ source[ srcOffset + 1] ] << 24 ) >>> 12 );
      int outBuff = ((DECODABET[source[srcOffset]] & 0xFF) << 18)
          | ((DECODABET[source[srcOffset + 1]] & 0xFF) << 12);

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