RSA PKCS1 V1.5 Signature Validation Problem

[Mark Satterfield]

I am using LibTomCrypt and LibTomMath on my ARM embedded platform. I am creating a signature with a data hash  on a windows platform using

python pyOpenSSL package and placing that signature on the embedded platform. I am using libTomCrypt to validate the signature. The hash value

pulled from the signature is extracted correctly but when I generate a hash 256 value over the data (same data as signature hash was created from)

I get a mismatch. The calculated hash does not match. Any ideas? I tried defining MP_16BIT but that did not work. I am running ENDIAN_LITTLE.

[Mark Satterfield]

This is duplicate post of my previous post, Sorry for the extra data.

[Steffen Jaeckel]

Hi Mark,

how do you create the signature?
What's your code for the verification?

MP_16BIT has not much to do with that, only if you built libtommath with
MP_16BIT (which is definitely not recommended).

Cheers,
Steffen
--
Steffen Jaeckel - s_ja...@gmx.de
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[Mark Satterfield]

On Windows:

    from OpenSSL import crypto

    private_key = crypto.PKey()

    

    # read private file for from file

    with open(expand_filename(_('output.private_key_file')), 'rb') as f:

        keystring = f.read()

      

    private_key = crypto.load_privatekey(crypto.FILETYPE_ASN1, keystring)

    # generate hash over data and sign it, sw_bin is data from a binary ELF file

    sign = crypto.sign(private_key, sw_bin, "sha256")

On ARM processor platform to calculate hash256:

     static int hash_index;

     static const char* hash_method = "sha256";

/**

 * Register the hash method and set hash index for method

 * @return void

 */

void

set_hash_index()

{

if( register_hash(&sha256_desc) == -1 )

{

print_string("Error: registering sha256 hash\n");

}

hash_index = find_hash(hash_method);

}

/*

 * Get the hash index

 * @return int - hash index

 */

int

get_hash_index()

{

return(hash_index);

}

/**

 * calculate a hash256 over data

 * 

 * @param data - array of data

 * @param datasize - length of array

 * @param unsigned char* - address where calculated hash is stored

 * @param unsigned long* - address where length of hash is stored

 * @return bool - true if hash calculated otherwise false 

 */

bool

get_data_hash(unsigned char* data,

 int datasize,

 unsigned char* hash,

 unsigned long* hash_len)

{

/* required to engage math library */

ltc_mp = ltm_desc;

set_hash_index();

if( (hash_memory(get_hash_index(), data, datasize, hash, hash_len)) != CRYPT_OK )

{

print_string("Failed to calculate hash on data\n");

return(false);

}

return(true);

}

The get_data_hash function calculate the hash, data points to bin file data in flash, and datasize is length of data.

On Friday, April 28, 2017 at 2:05:59 AM UTC-4, Mark Satterfield wrote:

[Mark Satterfield]

Replaced the sha256 function with one I found on the internet and now the hash calculation matches the hash calculation from the rsa signature code.

On Friday, April 28, 2017 at 2:05:59 AM UTC-4, Mark Satterfield wrote: