Like all the other "... is hard" problems in past PlaidCTF this one is about CVE-2015-3193.<span>OpenSSL 1.0.2 versions before 1.0.2e on x86_64 contain a bug in the function BN_mod_exp() and sometimes it produces wrong results. So, the solution is here:https://github.com/hannob/bignum-fuzz/blob/master/CVE-2015-3193-openssl-vs-gcrypt-modexp.c</span>