https://yn-labs.com/en/tags/crypto/Recent content in Crypto on 万屋猫LabsHugoenMon, 13 Jul 2026 11:00:00 +0900https://yn-labs.com/en/writeups/ctf/kalmarctf-2026-rbg-plus-polynomial-gcd/Mon, 13 Jul 2026 11:00:00 +0900https://yn-labs.com/en/writeups/ctf/kalmarctf-2026-rbg-plus-polynomial-gcd/<h2 id="overview">Overview</h2> <p>KalmarCTF 2026 Crypto challenge &ldquo;RBG+&rdquo; (106pts). It combines RSA with an LCG (linear congruential generator): the flag <code>m</code> is encrypted with RSA, and the exponent <code>e</code> is updated by the LCG. The tricky part is that <strong>the output is given as &ldquo;the sum of two ciphertexts&rdquo;</strong>. Because the two terms are added together, they cannot be separated into individual <code>m^e mod N</code> values, so the RSA structure cannot be used as-is (each value is the integer sum of the two terms taken <code>mod N</code>, and the sum can exceed <code>N</code>. This integrality makes the later argument easier, but the essence of the attack is that the value is &ldquo;hidden inside a sum&rdquo;).</p>