使用C / C ++与Java进行DES加密

Question

Just wondering why same plain text encrypted in Java and C doesn't generate same cipher text. For DES algo I noticed

input text = "text", key = "test"

C cipher text len = 24 Java generates cipher text len = 8

For 256Bit AES I noticed similar difference
C cipher text len = 32 Java generates cipher text len = 16

here is my "C" code

char* enc(const char* text, const char* keyStr)
{
    EVP_CIPHER_CTX ctx;
    unsigned char key[32] = {0};
    unsigned char iv[16] = {0};
    unsigned char in[16] = {0};
    unsigned char out[32]; /* at least one block longer than in[] */
    memset(out, 0, 32);
    int outlen1, outlen2;

    memcpy(in, text, strlen(text));
    memcpy(key, keyStr, strlen(keyStr));

    EVP_EncryptInit(&ctx, EVP_aes_256_cbc(), key, iv);    
    EVP_EncryptUpdate(&ctx, out, &outlen1, in, sizeof(in));
    EVP_EncryptFinal(&ctx, out + outlen1, &outlen2);

    char* ret  = (char*)malloc(outlen1 + outlen2+1);
    memset(ret, 0, outlen1 + outlen2+1);
    memcpy(ret, out, outlen1 + outlen2);

    EVP_CIPHER_CTX_cleanup(&ctx);

    return ret;
}

Here is the "Java" code

public static byte[] enc(byte[] input, byte[] keyStr){
        byte[] output = null;

        try {           
            byte[] newKey = getByteArrays(keyStr, 0, 32);
            SecretKey secretKey = new SecretKeySpec(newKey, "AES");
            Cipher cipher = Cipher.getInstance("AES/CBC/PKCS5Padding");  
            //Cipher cipher = Cipher.getInstance("DES/CBC/PKCS5Padding");
            String ivStr = "";
            byte[] ivKey = getByteArrays(ivStr.getBytes("UTF-8"), 0, 16);
            IvParameterSpec ips = new IvParameterSpec(ivKey);
            cipher.init(Cipher.ENCRYPT_MODE, secretKey, ips, null);         
            output = cipher.doFinal(input);

        } catch (Exception e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }       
        return output;
    }

I have list of entries encrypted using C , I want to decrypt those entries in Java.

following update helped to get same length , both in 'C' and 'JAVA'

EVP_EncryptUpdate(&ctx, out, &outlen1, in, strlen(text));

No Just wondering why ciphertext returned by "C" and "JAVA" are different for same text and key, I'm expecting them tobe same

Answer 1

The reason the C code outputs 32 bytes of ciphertext is because you are padding your input string with zeros before passing it to the encryption function.

In this line:

EVP_EncryptUpdate(&ctx, out, &outlen1, in, sizeof(in));

the function has no idea that you padded your data with zeros. Since sizeof(in) is 16, it takes 16 as the length of your plaintext. And with PKCS5 padding (and AES), 16 bytes will pad to 32 bytes.

In your Java code you didn't manually pad your input, you just passed it straight to the encryption function. So the encryption function sees your plaintext size as 4 bytes, which pads to 16 with PKCS5 padding.

So your solution is simple: don't manually pad your input in the C code. Just pass the plaintext directly to EVP_EncryptUpdate . It's already designed to handle unpadded plaintext.