International Association
for Cryptologic Research

We use theta groups to study $2$-isogenies between Kummer lines, with a particular focus on the Montgomery model. This allows us to recover known formula, along with more efficient forms for translated isogenies, which require only $2S+2m_0$ for evaluation. We leverage these translated isogenies to build a hybrid ladder for scalar multiplication on Montgomery curves with rational $2$-torsion which cost $3M+6S+2m_0$ by bits, compared to $5M+4S+1m_0$ for the standard Montgomery ladder.