=============
Cryptography
=============

Signer key
----------

The 64 bytes private signer key is a concatination whose last 32 bytes
is the public key. A convenience function ``crypto_sign_ed25519_sk_to_pk()``
can also be used to extract this public key.

.. code-block:: c

   #included <sodium.h>

   unsigned char skpk[crypto_sign_SECRETKEYBYTES]; // 64
   unsigned char pk[crypto_sign_PUBLICKEYBYTES];   // 32
   crypto_sign_keypair(pk, skpk);

   unsigned char pubkey[crypto_sign_PUBLICKEYBYTES];
   crypto_sign_ed25519_sk_to_pk(pubkey, skpk);

The value of the two arrays *pubkey* and *pk* should match.

----

Hash
----

We decide the hash ``h`` to be of size 13 bytes, and then compute
the hash of the file.

.. code-block:: c

   #included <sodium.h>

   std::ifstream f("c:/stuff/buf-64.dat", std::ios::binary);
   std::vector<unsigned char> buf(std::istreambuf_iterator<char> {f}, {});

   unsigned char h[13];
   crypto_generichash(h, sizeof h, buf.data(), buf.size(), nullptr, 0);