extracting mpi from a S-exp crypted with RSA
mo at g10code.com
Wed Apr 20 16:22:41 CEST 2005
On Tue, Apr 19, 2005 at 07:00:14PM +0000, flebool wrote:
> the crypted S-exp CRYPTED is represented as:
> (a A-MPI)))
> Calling the function:
> int legth=gcrypt_sexp_length(CRYPTED);
> I get length=2.
Correct, you have two objects in that S-Expression:
2. (rsa (a A-MPI))
> i want to get the mpi (a A-MPI),(that i've understood to be the crypted text
> in MPI format) but the function
> gcry_mpi_t internal_mpi = gcry_sexp_nth_mpi(CRYPTED,1,GCRYMPI_FMT_USG);
You need to distringuish between the S-Expression containing the MPI
and it's label `(a A-MPI)' and the actual MPI value, which is `A-MPI'.
So, before you can use gcry_sexp_nth_mpi(), you need to extract the
a = (enc-val (rsa (a A-MPI)));
b = sexp_nth (a, 1); // extract `(rsa (a A-MPI))'
mpi = sexp_mpi (b, 1; // extract `A-MPI'.
Alternatively, you can also use gcry_sexp_find_token().
> for completeness,i post the output of a
> writt= gcry_sexp_sprint ( CRYPTED , GCRYSEXP_FMT_DEFAULT , buf_test2 ,(size_t)
By the way: GCRYSEXP_FMT_ADVANCED is much more appropriate for output
intended to be human-readable.
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