Implementing ECDSA from Scratch Without Libraries

Introduction In the previous article, we used the Web Crypto API's crypto.subtle to sign and verify with ECDSA. The API made it easy, but the internals remained a black box. In this article, we implement ECDSA signing and verification from scratch using only basic arithmetic and mod — no crypto libraries. We output intermediate values at every step to see exactly what's happening. The code and explanations in this article were developed through conversation with AI (Claude). The idea of using a