On modular decomposition of integers

At Crypto 2001, Gallant et al. showed how to exploit fast endomorphisms on some specific classes of elliptic curves to obtain fast scalar multiplication. The GLV method works by decomposing scalars into two small portions using multiplications, divisions, and rounding operations in the rationals. We present a new simple method based on the extended Euclidean algorithm that uses notably different operations than that of traditional decomposition. We obtain strict bounds on each component. Additionally, we examine the use of random decompositions, useful for key generation or cryptosystems requiring ephemeral keys. Specifically, we provide a complete description of the probability distribution of random decompositions and give bounds for each component in such a way that ensures a concrete level of entropy. This is the first analysis on distribution of random decompositions in GLV allowing the derivation of the entropy and thus an answer to the question first posed by Gallant in 1999.