public static final Bytes getTinkOutputPrefix(int keyId) {
    return Bytes.copyFrom(
        ByteBuffer.allocate(NON_EMPTY_PREFIX_SIZE) // BIG_ENDIAN by default
            .put(TINK_START_BYTE)
            .putInt(keyId)
            .array());
  }
All non-empty prefixes must have the same size of 5 bytes.
java_src/src/main/java/com/google/crypto/tink/internal/OutputPrefixUtil.java