drown: daily C challenge, 23 October 2026

Your mission

Count how many key ids appear in both sorted lists.

today's brief

March 2016. DROWN broke a modern TLS server using a completely different machine, because the two shared one RSA key and the older one still answered SSLv2 from 1995. Roughly a third of HTTPS sites were exposed the day it was published. Nobody had to touch the good server at all. Find the keys that live on both sides.

See it in action

shared_keys({1,4,7}, 3, {2,4,7,9}, 4): 2
shared_keys({1,2,3}, 3, {4,5}, 2): 0
shared_keys({5}, 1, {5}, 1): 1

Your checklist

modern holds m key ids and legacy holds l key ids.
Both lists are sorted in increasing order and neither one repeats a value.
Return how many ids appear in both lists.
Either list can be empty.

Constraints

Expected file: drown.c
Allowed functions: None. Write every helper yourself.