2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/90819We show in some detail how to implement Shor's efficient quantum algorithm for discrete logarithms for the particular case of elliptic curve groups. It turns out that for this problem a smaller quantum computer can solve problems further beyond current computing than for integer factorisation. A 160 bit elliptic curve cryptographic key could be broken on a quantum computer using around 1000 qubits while factoring the security-wise equivalent 1024 bit RSA modulus would require about 2000 qubits. In this paper we only consider elliptic curves over GF($p$) and not yet the equally important ones over GF($2^n$) or other finite fields. The main technical difficulty is to implement Euclid's gcd algorithm to compute multiplicative inverses modulo $p$. As the runtime of Euclid's algorithm depends on the input, one difficulty encountered is the ``quantum halting problem''.34 pages Latex, essentially published version, but not using Journal style fileQuantum Physicstext