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Debugging a cryptography implementation

Claude Code debugged a fresh Go implementation of the ML-DSA signature algorithm, finding why Verify rejected valid signatures and two separate Sign bugs.

Resultrc_F2B5GN7A

It located all three bugs in one shot each, the Verify bug within minutes, saving the author hours of debugging.

ShownBlog · Nov 1 · Last checked Oct 13, 2026

Overview

How it was set up

Claude Code v2.0.28 with Opus 4.1, no system prompt. Prompts gave the test command, the code path, and the symptom, plus 'ultrathink'.

Worth knowing

The proposed fixes were mediocre or incomplete. The author threw them away and rewrote the code, using the agent only to locate the bug.

A prompt to start from

I am implementing [algorithm] in [language], and [symptom, e.g. tests fail against a known good test vector]. Run the tests with [test command]. The code is in [path]. Figure out what is going on. ultrathink

Written by Agentic Feats from the source. Not the author's exact prompt.

Sources 1

Originalwords.filippo.io/claude-debugging

History

Source published: 2025-11-01
Added to Agentic Feats: 2026-10-13
Pick: No
Markdown copy: /c/debugging-a-cryptography-implementation-claude-code.md

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