Pseudo-Random Generator

This sixth blog in the Cryptography Series explores two fundamental ideas regarding Pseudorandom Generators (PRGs). First, we will learn about the Blum-Micali method, an intelligent sequential construction that allows us to build a PRG capable of extending its output to an arbitrary length from a base PRG that only stretches its input slightly. Next, the post introduces a core security standard: next-bit unpredictability. The highlight of this post is the proof of a classic and surprising result: a PRG is considered secure (indistinguishable from a truly random sequence) if and only if no adversary can efficiently predict its next bit. This provides a solid bridge between statistical theory and practical predictability. [Read More]

This post explores the stream cipher, a practical solution to the long-key problem of the One-Time Pad. We delve into how a “pseudo-random generator” (PRG) can “stretch” a short key (seed) into a long keystream for encryption. The blog’s core focuses on the formal security definition of a PRG—the concept of “computational indistinguishability”—and presents a detailed proof demonstrating that a secure PRG leads to a semantically secure stream cipher. [Read More]