如何解密密文分区-AES，Java

Question

I , wrote java method, which encrypts - decrypts data.

    String message = "Hello world 123456";

    // Creating Key. Key of size = 128
    byte [] raw ={-49, -44, 51, -114, 58, 79, 83, -38, 107, 64, 67, -108, -52, 109, 85, 77};
    SecretKeySpec skeySpec = new SecretKeySpec(raw, "AES");
    Cipher cipher = Cipher.getInstance("AES");
    cipher.init(Cipher.ENCRYPT_MODE, skeySpec);

    // Encript
    byte[] encrypted = cipher.doFinal(message.getBytes());
    System.out.println("encripted: " + Arrays.toString(encrypted));
    System.out.println("encripted: "+ asHex(encrypted));

    cipher.init(Cipher.DECRYPT_MODE, skeySpec);
    byte[] decript = cipher.doFinal(encrypted, 0, 32);

Everything works well! encrypted bytes size is 32. That is correct. But can I dencrypt part to part (block to block - for instance, decrypt first block, then second) ? I mean to decript firstly 16 byte and then second 16 byte and etc..

might be something like this.

byte[] decript = cipher.doFinal(encrypted, 0, 16);

I have such error:

javax.crypto.BadPaddingException: Given final block not properly padded

In the other word, can I decrypt AES cipher text, block to block?

Update: I have encrypted video file. It is for about 1 gb. I want to write my video player, which will open my file. If I will decript full video there will be memory problem. So I will give bytes of stream step to step to my player (block to block). Can I use AES for that?

Answer 1

You are currently encrypting using ECB mode encryption. The algorithm string for Cipher , "AES" , uses the provider default for mode of encryption and padding. For the SunJCE that translates to "AES/ECB/PKCS5Padding" (which actually performs PKCS#7 padding).

ECB encrypts decrypts blocks of 16 bytes. This means that the message is conceptually converted to a concatenation of 16 byte blocks, and each block is then encrypted separately. As each block is encrypted separately, you can also decrypt the blocks separately. That means starting at a block boundary, and then decrypting x times the block size. As you don't want to perform the PKCS#7 unpadding until you reach the last block, you should use an update method instead of a doFinal method (or you can use "AES/ECB/NoPadding" ).

If you want to encrypt files you should definitely take a look at java.nio , and Cipher methods that use ByteBuffer as input / output.

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Note that this is only a direct answer; you should not be using ECB mode encryption. Use CTR with a unique IV (nonce) per key/file combination instead to provide confidentiality of the stream.