[OTR-dev] working with the libgcrypt mpi type and getting the actual integer value

[Bjorn Kuiper]

On 2/28/2013 7:19 AM, Kjell Braden wrote:
> On 2013-02-27 00:04, Bjorn Kuiper wrote:
>> Hi,
>>
>> I'm trying to develop a C# implementation of the Off-the-record
>> library, but I'm having some problems with the libgcrypt library.
>>
>> Can somebody explain to me how you can print out the actual integer
>> value of an hex'd MPI ?
>>
>> Basically i want to know the integer value of this 'private' key
>>
>> 48BFDA215C31A9F0B226B3DB11F862450A0F30DA
>
> That *is* the integer value. It's in base 16 though.
>
>>
>> I think it is
>>
>> 415325779662433871844955547383752003988573073626
>
> In base 10, yes.
>
>
>
>
>> but I'm unable to find a way to confirm this using libgcrypt
>>
>> I tried the following code:
>>
>> -- code snippit gcry_mpi_t cript_prime; char buffer[50] = {0}; char
>> number[50] = {0};
>>
>> cript_prime = gcry_mpi_new(50);
>>
>> strcpy(number,"415325779662433871844955547383752003988573073626");
>> gcry_mpi_scan(&cript_prime,GCRYMPI_FMT_USG,number,sizeof(number),NULL);
>>
>>
>> gcry_mpi_print(GCRYMPI_FMT_USG,buffer,sizeof(buffer),NULL,cript_prime);
>>
>>  printf("The number tested is: %s\n",buffer);
>>
>> printf("trying to convert to HEX\n");
>>
>> /* get actual value */ gcry_mpi_print (GCRYMPI_FMT_HEX, buffer,
>> sizeof(buffer), NULL, cript_prime); /* print actual value */
>> printf("result: %s\n", buffer); -- end of code snippit
>>
>> which results in:
>>
>> -- output The number tested is:
>> 415325779662433871844955547383752003988573073626 trying to convert to
>> HEX result: 415325779662433871844955547383752003988573073626 -- end
>> of output
>>
>
>
> That's because your buffer is too small and the second gcry_mpi_print
> fails (which you never realize because you don't check the return code).
> The buffer remains unmodified.
>
> If you initialized your buffer with:
>> char buffer[101] = {0};
>
> you get this output:
>> The number tested is: 415325779662433871844955547383752003988573073626
>> trying to convert to HEX
>> result:
>> 3431353332353737393636323433333837313834343935353534373338333735323030333938383537333037333632360000
>>
>
> 0x34 => ascii 4
> 0x31 => ascii 1
> 0x35 => ascii 5
> ... you get the idea
>
>
> Conclusion:
> GRYMPI_FMT_USG does not print the integer in a string representation but
> instead uses the bytes to store a variable length unsigned integer.
> GCRYMPI_FMT_HEX does indeed print the integer in a string
> representation, but in hexadecimal instead of decimal.
>
> libgcrypt does not seem to offer decimal string output.
>

Thanks Kjell,

Indeed, I was looking for the base 10 value. that particular comment of 
you helped me out a lot. I could even check the value here: 
http://www.unitconversion.org/numbers/base-10-to-base-16-conversion.html

I guess I was over-thinking some of these things. Anyway, I'm glad that 
my thoughts were correct and i got it confirmed.

Thank you again.