# Public-Key Cryptography An encryption system in which each party holds two mathematically linked keys: a **public key** (shared freely) used to encrypt messages, and a **private key** (kept secret) used to decrypt them. Anyone can encrypt a message to you using your public key, but only you can read it. The same math works in reverse for digital signatures: you sign with your private key, and anyone can verify the signature with your public key. Invented independently by Whitfield Diffie and Martin Hellman (1976) and by Ron Rivest, Adi Shamir, and Leonard Adleman (the RSA algorithm, 1977) — though British intelligence agency GCHQ had classified an equivalent discovery by James Ellis, Clifford Cocks, and Malcolm Williamson several years earlier and kept it secret until 1997. ## Why it matters here Public-key cryptography is the original case study in the pattern the KB tracks with [[Dual-Use AI]] and [[Chip Export Controls]]: a technology invented for civilian purposes that the U.S. government classified as a weapon, restricted as a munition, and eventually lost control of. The "Crypto Wars" of the 1990s established the legal and political precedents that the current AI restriction debates are replaying almost move for move. **The munitions classification.** Under the Arms Export Control Act (1976) and ITAR, strong cryptography was classified as a Category XIII munition — the same legal category as tanks, missiles, and flamethrowers. Exporting encryption software with key lengths above 40 bits required a State Department license, effectively banning American companies from shipping secure products internationally. Phil Zimmermann's release of PGP (Pretty Good Privacy) in 1991 triggered a three-year federal criminal investigation on the theory that publishing encryption software on the internet constituted illegal arms export. **The Clipper Chip.** In 1993, the Clinton administration proposed the Clipper Chip — a government-designed encryption chip for consumer telephones that would give law enforcement backdoor access through a "key escrow" system. The proposal unified an unlikely coalition of civil libertarians, cryptographers, and tech companies against it. In 1994, AT&T researcher Matt Blaze published a paper demonstrating a fundamental flaw in the escrow mechanism. The Clipper Chip was effectively dead by 1996. **The First Amendment argument.** The decisive legal battles came in the mid-1990s. In *Bernstein v. United States* (1995–1999), the Ninth Circuit ruled that software source code is speech protected by the First Amendment, and that export restrictions on encryption were an unconstitutional prior restraint. In *Junger v. Daley* (2000), the Sixth Circuit reached the same conclusion. These rulings — that code is speech — are now being cited in challenges to the [[Anthropic Model Export Directive]], arguing that restricting access to AI models raises the same constitutional issues. **The resolution.** In September 1999, the Clinton administration effectively surrendered, removing virtually all restrictions on the export of retail encryption products regardless of key length. The government lost because the technology was too useful, too widely distributed, and too fundamental to internet commerce to suppress. Within a decade, the same encryption the government had classified as a weapon became the backbone of e-commerce, banking, and secure communications worldwide. **The AI parallel.** The Crypto Wars established that the U.S. government will classify transformative technologies as weapons, attempt to restrict their distribution, face legal and market resistance, and eventually adapt. The current cycle — [[Chip Export Controls]], the [[Anthropic Model Export Directive]], the Trump administration's oscillation between restriction and deregulation — is the same pattern with higher stakes and faster timelines. The key difference: AI capabilities are harder to distribute as openly as encryption source code was, which may give the government more leverage than it had in the 1990s. ## Technical foundations The security of public-key cryptography rests on mathematical problems that are easy in one direction but computationally infeasible in reverse — "trapdoor functions." RSA relies on the difficulty of factoring large prime numbers; Diffie-Hellman and elliptic curve cryptography (ECC) rely on the discrete logarithm problem. These problems are vulnerable to quantum computers running [[Shor's Algorithm]], which is why [[Post-Quantum Cryptography]] and the [[Harvest Now Decrypt Later]] threat are active concerns. ## See Also - [[Harvest Now Decrypt Later]] — the quantum threat to current encryption - [[Post-Quantum Cryptography]] — the next generation of encryption - [[Dual-Use AI]] — the current iteration of the technology-as-weapon classification pattern - [[Chip Export Controls]] — the semiconductor version of the same restriction impulse - [[Anthropic Model Export Directive]] — the AI model version of the Crypto Wars - [[Splinternet]] — the broader fragmentation of technology access along national lines - [[Cybersecurity Lexicon]] — the full cybersecurity vocabulary - [[US Government Technology Restrictions Timeline]] — the chronological arc from cryptography to AI