RE: How do I remove padding during AES encryption/ decryption

[Bhat, Jayalakshmi Manjunath]

Hi,

Thanks for the reply. I have my sample test case like this.

#define KEYSIZE 256
#define AES_BLOCK_SIZE 32

void  test_main()
{
    char key[KEYSIZE+1];
    int I,keylen;
    char data[AES_BLOCK_SIZE] ;
    char cbuf[AES_BLOCK_SIZE];
    char pbuf[AES_BLOCK_SIZE];

 
strcpy(key,"2ea24d27bc6e40e70b0a2ab08b0831675cf1274834f98a58709edeeb56af
f547");
 
strcpy(data,"00000000000000000000000000000000000000000000000000000000000
00000");
    keylen = strlen(key);

    {
            AES_KEY ctx;
            unsigned char iv[AES_BLOCK_SIZE];
            memset(cbuf, 0,AES_BLOCK_SIZE);
            AES_set_encrypt_key(key, KEYSIZE, &ctx);
            AES_cbc_encrypt(data, cbuf, AES_BLOCK_SIZE, &ctx, iv,
AES_ENCRYPT);

            for (i =0 ; i <sizeof(data) ; i++)
                   printf("%d...input = %d \n",data[i],i);
            printf("\n");

            for (i =0 ; i <sizeof(cbuf); i++)
          printf("%d...encoded data =%d \n",cbuf[i],i);
            printf("\n");
    }

    {
            AES_KEY ctx;
            int len,pad,flag =0;
            unsigned char iv[AES_BLOCK_SIZE];
            memset(pbuf, 0,AES_BLOCK_SIZE);
            memset(iv, 0, AES_BLOCK_SIZE);
            AES_set_decrypt_key(key, KEYSIZE, &ctx);
            AES_cbc_encrypt(cbuf,pbuf, AES_BLOCK_SIZE, &ctx, iv,
AES_DECRYPT);
    }

}

Please can any tell me what could be the problem with this code?
Regards,
Jaya.

  

-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Marek Marcola
Sent: Monday, September 18, 2006 3:39 PM
To: openssl-users@openssl.org
Subject: Re: How do I remove padding during AES decryption

Hello, 
>    Please can any one tell me how do I remove the pad bytes during AES

> decyrption using AES_cbc_encryption.

Provided that block_size is size of encryption algorithm block size and
last block is in dst you may use something like that: 

    .
    .
    pad = dst[block_size - 1];

    if (pad > block_size) {
        goto err;
    }

    for (i = 1; i < pad; i++) {
        if (dst[block_size - 1 - i] != pad) {
            goto err;
        }
    }
    len = block_size - pad;
    .
    .

Proper length is returned in len.

Best regards,
--
Marek Marcola <[EMAIL PROTECTED]>

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______________________________________________________________________
OpenSSL Project                                 http://www.openssl.org
User Support Mailing List                    openssl-users@openssl.org
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