Creating and destroying an EVP_CTX_MD structure with every hash
operation is very expensive. Speed things up by caching one within
the krb5_crypto structure. krb5_crypto can already only be safely
used by one thread at a time - adding a message digest context here
shouldn't introduce any further threading risks.
Users of the stashed context must be careful to ensure that they
call no other hash functions whilst they are in the middle of using
the context.
lib/krb5/crypto.c was a large, monolithic block of code which made
it very difficult to selectively enable and disable particular
alogrithms.
Reorganise crypto.c into individual files for each encryption and
salt time, and place the structures which tie everything together
into their own file (crypto-algs.c)
Add a non-installed library (librfc3961) and test program
(test_rfc3961) which builds a minimal rfc3961 crypto library, and
checks that it is usable.
