# Cryptography Cryptography is the science of securing communication and data against adversaries. It is simultaneously a branch of mathematics, a cornerstone of Computer Science, and a national security asset regulated as a munition by many governments. Every secure transaction on the internet, every encrypted military communication, every [[Blockchain]] verification depends on cryptographic primitives. The politics of cryptography — who gets strong encryption, whether governments can mandate backdoors, how quantum computing threatens current systems — runs through surveillance policy, intelligence operations, and civil liberties debates tracked across this vault. ## Symmetric Cryptography **Symmetric Encryption** uses the same key for encryption and decryption. AES (Advanced Encryption Standard), adopted by the US government in 2001, is the dominant symmetric cipher — it encrypts everything from hard drives to VPN tunnels to classified military communications. Symmetric ciphers are fast and well-suited to bulk data encryption, but they require a secure method to share the secret key between parties, which is the fundamental problem that public-key cryptography solves. ## Asymmetric Cryptography **[[Public-Key Cryptography]]**, invented independently by Diffie-Hellman (1976) and RSA (1977), uses mathematically linked key pairs: a public key for encryption and a private key for decryption. This solved the key distribution problem and enabled secure communication between parties who have never met — the foundation of internet commerce, secure email, and digital signatures. RSA's security rests on the difficulty of factoring large numbers — a problem believed to be computationally intractable for classical computers but solvable in polynomial time by [[Shor's Algorithm]] on a quantum computer. This threat drives the urgent development of [[Post-Quantum Cryptography]] by [[NSA]], NIST, and allied agencies. ## Hashing and Digital Signatures **Cryptographic hash functions** (SHA-256, SHA-3) take arbitrary input and produce a fixed-size output that is computationally infeasible to reverse or to find collisions for. Hashing verifies data integrity — if a single bit changes, the hash changes completely. [[Blockchain]] technology chains blocks together using sequential hashes, creating tamper-evident records. [[Bitcoin]] mining is literally a competition to find hash inputs meeting a difficulty target. **Digital signatures** combine hashing with public-key cryptography to provide authentication, integrity, and non-repudiation. They are the mechanism behind software update verification, secure boot chains, and the PKI (Public Key Infrastructure) that underpins HTTPS certificates for the entire web. ## Zero-Knowledge Proofs Zero-knowledge proofs allow one party to prove knowledge of a secret without revealing the secret itself. Originally a theoretical construct from the 1980s, they now power privacy-preserving cryptocurrency protocols and are being explored for identity verification systems that prove eligibility (age, citizenship, clearance level) without disclosing underlying personal data. ## Cryptography and State Power Cryptography has been regulated as a munition by the US government since the Cold War. Export controls on strong encryption were a major policy battle in the 1990s "Crypto Wars," and the tension between law enforcement's desire for access and individuals' right to privacy continues in debates over [[Encryption]] backdoors. The [[NSA]] maintains a dual role: it develops cryptographic standards to protect US government communications while simultaneously trying to break the cryptographic systems used by adversaries. The Snowden revelations showed that the NSA had deliberately weakened the Dual_EC_DRBG random number generator — a cryptographic standard it had promoted through NIST — embedding a backdoor that undermined global cryptographic security. This history, documented in [[NSA-Tech Company Relationships]], illustrates the inherent tension between security and surveillance. [[Harvest Now Decrypt Later]] — the strategy of intercepting and storing encrypted traffic today for future decryption once quantum computers mature — underscores why [[Post-Quantum Cryptography]] is a present-day national security priority, not a future concern. ## See Also - [[Encryption]] — the operational application of cryptographic principles - [[Post-Quantum Cryptography]] — preparing for the quantum threat - [[Blockchain]] — cryptographic data structures for decentralized trust - Information Theory — Shannon's theoretical foundation for cryptographic security - Signals Intelligence and Computing — intelligence agencies' relationship with cryptography