openssl 1.0.2 fips: unexpected failure with 512 bit RSA

[jmayclin]

Problem:

In #4612 there was an unexpected failure when attempting to use 512 bit RSA certs with OpenSSL 1.0.2 FIPS. We did not see this failure in any of the other libcryptos that we use.

While no one should be using 512 bit RSA certs, it would still be valuable to understand and document why OpenSSL 1.0.2 FIPs behaves this particular way.

Requirements/Acceptance Criteria

We should be able to explain why this failed for OpenSSL 1.0.2 FIPs only, and then document that knowledge.