使用 PKS5 填充 javax.crypto 的 3DESede 时密文长度错误

Question

我在我的 android 应用程序中使用以下代码来加密 Triple DES using the Encrypted Code Book (ECB) mode with three independent keys (aka 3DESede), which are provided as a 24 byte sized key array. Therefore I use the Java Crypto API. This works pretty well, but if I encrypt an eight character string I get a 16 byte cipher text, which should not happen as 3DES operates on chunks of 64 bit (resp. 8 byte). Same holds for the PKCS5 padding 中的字符串，因为这也在 64 位块上运行。所以我的问题是导致这个问题的原因是什么？

private static byte[] encryptText(String plaintext, byte[] keyBytes) throws Exception {
    // Get plaintext as ASCII byte array
    final byte[] plainBytes;
    if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.KITKAT) {
        plainBytes = plaintext.getBytes(StandardCharsets.US_ASCII);
    } else {
        plainBytes = plaintext.getBytes("US-ASCII");
    }

    // Generate triple DES key from byte array
    final DESedeKeySpec keySpec = new DESedeKeySpec(keyBytes);
    SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("DESede");
    SecretKey key = keyFactory.generateSecret(keySpec);

    // Setup the cipher
    final Cipher c3des = Cipher.getInstance("DESede/ECB/PKCS5Padding");
    c3des.init(Cipher.ENCRYPT_MODE, key);

    // Return ciphertext
    return c3des.doFinal(plainBytes);
}

Answer 1

PKCS5Padding 在与 DES 一起使用时添加 1-8 个字节的填充。如果您加密 8 个字节，您将获得 8 个额外的填充字节以获得偶数个块。

如果您使用Cipher.getInstance("DES/ECB/NoPadding")并加密8个字节，您将得到8个字节的密文。

Answer 2

当使用 PKCS#5 填充时，它必须始终添加填充，否则在解密时将无法确定是否添加了填充。因此，即使输入数据是块大小的精确倍数，也必须添加填充，这将是 8 个字节。

见PKCS padding:

If the original data is an integer multiple of N bytes, then an extra block of bytes with value N is added. This is necessary so the deciphering algorithm can determine with certainty whether the last byte of the last block is a pad byte indicating the number of padding bytes added or part of the plaintext message. Consider a plaintext message that is an integer multiple of N bytes with the last byte of plaintext being 01. With no additional information, the deciphering algorithm will not be able to determine whether the last byte is a plaintext byte or a pad byte. However, by adding N bytes each of value N after the 01 plaintext byte, the deciphering algorithm can always treat the last byte as a pad byte and strip the appropriate number of pad bytes off the end of the ciphertext; said number of bytes to be stripped based on the value of the last byte.

PKCS#5 padding is identical to PKCS#7 padding, except that it has only been defined for block ciphers that use a 64-bit (8 byte) block size. In practice the two can be used interchangeably.

使用 PKS5 填充 javax.crypto 的 3DESede 时密文长度错误

Wrong cipher text length when using 3DESede with PKS5 padding of javax.crypto

java

encryption

android

3des