ECDSA Nonce Recovery
As part of signing something using DSA (digital signature algorithm) one must select a secret, cryptographically secure random number $k$ to be used as a nonce. $k$ must never be reused. Why you ask? Well you could ask Sony, or I could just tell you that you can recover $k$ given two signature / message pairs that used the same $k$ and signing key, which can lead to the signing key being compromised. I've signed two messages $(z_1, z_2)$ with the same $k$ (using the NIST curve P-192), resulting in the signatures $(s_1, r_1)$ and $(s_2, r_2)$. Your job is to recover $k$ (submit your answer in hex). Some reading to get started (of most relevance is section 2.3).
z1 = 78963682628359021178354263774457319969002651313568557216154777320971976772376 s1 = 5416854926380100427833180746305766840425542218870878667299 r1 = 5568285309948811794296918647045908208072077338037998537885
z2 = 62159883521253885305257821420764054581335542629545274203255594975380151338879 s2 = 1063435989394679868923901244364688588218477569545628548100 r2 = 5568285309948811794296918647045908208072077338037998537885
n = 6277101735386680763835789423176059013767194773182842284081