Koblitz, N. A Course in Number Theory and Cryptography. 2nd ed. Springer, New York, 1994.
Pomerance, C. “Cryptology and Computational Number Theory—An Introduction,” in Cryptology and Computational Number Theory, Pomerance, C., ed. Proceedings of Symposia in Applied Mathematics, vol. 42, American Mathematical Society, Providence, RI, 1990. Thisbook gives an excellent account of how the method of repeated squares is used in cryptography.