{
  "patent_number": "US 4200770",
  "country": "US",
  "title": "How to Create a Secret Code Key Without Meeting First",
  "original_title": "Cryptographic apparatus and method",
  "summary": "This 1980 patent describes a way for two people to create a secret code key over a public channel, like the internet, without ever meeting or sharing the key directly.",
  "what_it_does": "This patent explains a method and apparatus for generating a secret cryptographic key between two parties, let's call them Alice and Bob, who can only communicate over an insecure channel. Alice starts with a secret number (signal A) and Bob starts with his own secret number (signal B). They each transform their secret number using a special mathematical process that's easy to do but incredibly hard to undo (infeasible to invert). Alice sends her transformed number (transformed A) to Bob, and Bob sends his transformed number (transformed B) to Alice. Then, Alice takes Bob's transformed number (transformed B) and combines it with her original secret number (signal A) to create a shared secret key. Bob does the same, taking Alice's transformed number (transformed A) and combining it with his original secret number (signal B). The magic is that both Alice and Bob end up with the exact same secret key, but an eavesdropper who only sees the transformed numbers (transformed A and transformed B) cannot figure out the key or the original secret numbers.",
  "what_it_does_not_cover": [
    "Generating a secret key by having both parties meet in person beforehand.",
    "Using a pre-agreed secret key that is shared via a secure channel before communication begins.",
    "Methods where the transformation process is easily reversible or invertible by an eavesdropper.",
    "Systems where the final shared secret key can be deduced solely from the publicly exchanged transformed signals.",
    "Creating a secure key that is not identical for both parties involved in the communication."
  ],
  "filed": "1977-09-06",
  "granted": "1980-04-29",
  "expires": "1997-09-06",
  "status": "expired",
  "holder": "Leland Stanford Junior University",
  "holder_url": "https://patentbrief.org/company/leland-stanford-junior-university",
  "inventors": [
    {
      "name": "Martin E. Hellman",
      "url": "https://patentbrief.org/inventor/martin-e-hellman"
    },
    {
      "name": "Ralph C. Merkle",
      "url": "https://patentbrief.org/inventor/ralph-c-merkle"
    },
    {
      "name": "Bailey W. Diffie",
      "url": "https://patentbrief.org/inventor/bailey-w-diffie"
    }
  ],
  "times_cited": 708,
  "tags": [
    "telecommunications",
    "software",
    "consumer_electronics",
    "semiconductors"
  ],
  "abstract": "A cryptographic system transmits a computationally secure cryptogram over an insecure communication channel without prearrangement of a cipher key. A secure cipher key is generated by the conversers from transformations of exchanged transformed signals. The conversers each possess a secret signal and exchange an initial transformation of the secret signal with the other converser. The received transformation of the other converser's secret signal is again transformed with the receiving converser's secret signal to generate a secure cipher key. The transformations use non-secret operations that are easily performed but extremely difficult to invert. It is infeasible for an eavesdropper to invert the initial transformation to obtain either conversers' secret signal, or duplicate the latter transformation to obtain the secure cipher key.",
  "url": "https://patentbrief.org/patent/us/4200770/diffie-hellman-public-key-exchange",
  "markdown_url": "https://patentbrief.org/patent/us/4200770/diffie-hellman-public-key-exchange/md",
  "google_patents_url": "https://patents.google.com/patent/US4200770",
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}