The iotuil package has been deprecated since go1.16; the various
functions it provided now exist in the os and io packages. Replace all
instances of ioutil with either io or os, as appropriate.
Initial implementation constructed the hash input incorrectly. New test uses a key modulus that is actually on the openssl weak key list instead of a random placeholder.
Adds a basic truncated modulus hash check for RSA keys that can be used to check keys against the Debian `{openssl,openssh,openvpn}-blacklist` lists of weak keys generated during the [Debian weak key incident](https://wiki.debian.org/SSLkeys).
Testing is gated on adding a new configuration key to the WFE, RA, and CA configs which contains the path to a directory which should contain the weak key lists.
Fixes#157.
Aaron Gable