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Public Key Cryptography Invented

November 1976 to February 1978.Cybersecurity.Invention.Date precision, month.Evidence grade, primary.2 primary sources

Drivers:

Technological capabilityResearch breakthroughSecurity incident

The growth of computer networking made secure communication essential. Traditional key distribution could not scale. Academic research sought mathematical solutions to enable secure communication without prior shared secrets.

Before public key cryptography, two people who wanted to communicate secretly needed to meet first to agree on a secret code. Public key cryptography is like having a special padlock: anyone can lock a box with your public lock (encrypt a message), but only you have the key to open it (your private key). This means strangers can send you secrets without ever meeting you.

Public Key Cryptography Invented event plate

Structured atlas record showing date, domain, evidence grade, source count, and predecessor and successor links.

Event plate: Public Key Cryptography Invented Convergence-divergence layout. The central hero card carries the event year, type, title, evidence grade, domain and era band. 0 predecessor cards on the left feed in with red arrows labelled "absorbs". 0 successor cards on the right derive with red arrows labelled "spawns". Key terms below the hero pin the vocabulary the event introduced. EVENT PLATE Source: https://ieeexplore.ieee.org/document/1055638 1976 - INVENTION Public Key CryptographyInvented primary evidence Domain: AI and machine learning Era band: E6 AI-scale systems KEY TERMS - VOCABULARY THE EVENT INTRODUCED public key private key RSA Diffie-Hellman Convergence-divergence: predecessors absorbed, successors spawned Hero card carries year, evidence and domain. 0 predecessors flow in from the left; 0 successors flow out to the right. Key termsbelow pin the vocabulary the event introduced.

Forecasts and counterfactuals stay labelled as opinion in the event data. Source: Computer History Museum.

Before

All practical encryption required both parties to share a secret key in advance. This 'key distribution problem' made secure communication difficult at scale. Meeting in person or using trusted couriers was impractical for electronic communication. Symmetric encryption could not enable secure communication between strangers.

What changed

Diffie and Hellman introduced the concept of public key cryptography in 1976, enabling secure key exchange over insecure channels. In 1977-78, Rivest, Shamir, and Adleman (RSA) created a practical public key system that also enabled digital signatures. For the first time, secure communication was possible without pre-shared secrets.

How it happened

Whitfield Diffie and Martin Hellman published 'New Directions in Cryptography' in November 1976, describing key exchange and the concept of trapdoor functions. Ron Rivest, Adi Shamir, and Leonard Adleman at MIT discovered a concrete implementation (RSA) in 1977, publishing in February 1978. The RSA algorithm's security relies on the difficulty of factoring large prime numbers.

Outcomes

  • Solved the key distribution problem for the digital age
  • Enabled secure e-commerce and online banking
  • Made digital signatures possible, enabling authentication
  • Created foundation for SSL/TLS, PGP, and modern security

Limitations

  • Public key operations are computationally expensive
  • Key management remains complex in practice
  • Vulnerable to quantum computing attacks (future threat)
  • Implementation errors can undermine security

Lessons learnt

  • Mathematical breakthroughs can transform security
  • Open publication accelerates adoption and scrutiny
  • Theoretical concepts require practical implementations
  • Security assumptions can be challenged by new computing paradigms

Stakeholders and artefacts

Organisations

  • Stanford UniversityacademiaDiffie-Hellman research
  • MITacademiaRSA development

Individuals

  • Whitfield DiffieCo-inventor, Stanford UniversityCo-invented public key cryptography and key exchange
  • Martin HellmanCo-inventor, Stanford UniversityCo-invented public key cryptography and key exchange
  • Ronald RivestCo-inventor, MITCo-invented RSA algorithm
  • Adi ShamirCo-inventor, MITCo-invented RSA algorithm
  • Leonard AdlemanCo-inventor, MITCo-invented RSA algorithm

Artefacts

  • Diffie-Hellman Key ExchangeprotocolProtocol for secure key exchange over insecure channels
  • RSA AlgorithmprotocolPublic key cryptosystem for encryption and digital signatures
  • Public KeyspecificationOpenly shared key for encryption, paired with private key

Key terms

public keyprivate keyRSADiffie-Hellmanasymmetric encryptiondigital signature

Causality

Made possible: Data Encryption Standard (DES) Published; SSL/TLS Protocol Evolution.

On this course

Read in the path Cybersecurity: Threats and Defences.

Sources

1Whitfield Diffie, Martin E. Hellman. "New Directions in Cryptography". Stanford University, 1976-11.peer reviewedieeexplore.ieee.org/document/1055638
2Ronald L. Rivest, Adi Shamir, Leonard Adleman. "A Method for Obtaining Digital Signatures and Public-Key Cryptosystems". MIT, 1978-02.peer revieweddl.acm.org/doi/10.1145/359340.359342