[to state the obvious, few people on this cryptography list are using
   pgp, and I appear to have been influenced not to as well; I'm just
   trying to help myself process both situations.  do you use pgp?  do you
   believe there is any reason to?  this is a cryptography list.]
   The mpi_powm implementations are in mpi/mpi-pow.c
   : [1]https://dev.gnupg.org/source/libgcrypt/browse/master/mpi/mpi-mpow.
   c
   What's exciting, is there are actually two different implementations in
   that one file.  The old one called "simple exponentiation" starts on
   line 49 and goes through line 353.  The second one occupies the second
   half of the file and starts on line 405.
   They're both quite big, but a lot of it looks like boilerplate.
   To learn modular algorithms from this file most efficiently for
   cracking PGP as a software developer, we want to review the parts of
   these implementations that directly relate to what is needed to
   implement modular arithmetic.

References

   1. https://dev.gnupg.org/source/libgcrypt/browse/master/mpi/mpi-mpow.c