https://cs.lozic.me/areas/cryptography/Recent content in Cryptography on CS TheoremsHugoen-usFri, 08 Oct 2027 12:00:00 +0100https://cs.lozic.me/posts/t036-ip-equals-pspace/Fri, 08 Oct 2027 12:00:00 +0100https://cs.lozic.me/posts/t036-ip-equals-pspace/<h2 id="symptom">Symptom</h2> <p>You have a claim you cannot check. A cloud provider ran a computation on a petabyte you do not have and reports an answer. A model was trained on data you cannot access. A database returned an aggregate over rows you are not permitted to read.</p>https://cs.lozic.me/posts/t110-the-no-cloning-theorem/Fri, 02 Jul 2027 12:00:00 +0100https://cs.lozic.me/posts/t110-the-no-cloning-theorem/<h2 id="symptom">Symptom</h2> <p>You have spent this whole part learning that information wants to be copied. Entropy bounds compression (T018), Huffman hits the bound (T020), redundancy buys error correction (T021). Every result so far treats &ldquo;make another copy&rdquo; as the free operation — the thing you do without thinking, the reason <code>cp</code> has no interesting failure modes.</p>https://cs.lozic.me/posts/t074-the-birthday-bound/Fri, 06 Nov 2026 12:00:00 +0100https://cs.lozic.me/posts/t074-the-birthday-bound/<h2 id="symptom">Symptom</h2> <p>You need a short ID for uploads. Eight hex characters feels generous, so you take the first 32 bits of a hash and move on.</p> <p>At about 80,000 uploads, two files collide, and one of them silently overwrites the other, and the bug report says the customer&rsquo;s invoice contains someone else&rsquo;s line items.</p>