You can freely build on How RSA Public-Key Encryption Keeps Digital Messages Secret

What this patent covers

This patent outlines a cryptographic system for secure communication. A sender transforms a message, represented as a number M, into a secret ciphertext C. This is done by calculating M raised to a specific power 'e' and then finding the remainder when that result is divided by a large composite number 'n' (C ≡ M^e (mod n)), as described in Claim 1. The numbers 'e' and 'n' form the public key. The intended receiver then takes this ciphertext C and transforms it back into the original message M' using their secret private key, which involves raising C to a different power 'd' and again finding the remainder when divided by 'n' (M' ≡ C^d (mod n)). For example, if Alice wants to send a secret message to Bob, she uses Bob's public key (e, n) to encrypt her message. Only Bob, who knows the secret 'd' (his private key), can decrypt it.

What is now free to use

All 1 claims of US 4405829 are in the public domain. Specifically:

The 1 dependent claim add narrowing limitations and are also free.

What is NOT covered

Patent expiry frees this specific invention. Separately-patented improvements made after expiry may still be protected.

• Does not cover symmetric encryption systems where the same key is used for both encryption and decryption.

• Does not cover cryptographic methods that do not rely on modular exponentiation (M^e mod n) for encryption and decryption.

• Does not cover systems where the modulus 'n' is not the product of two prime numbers, 'p' and 'q', as specified in Claim 1.

• Does not cover encryption schemes that do not use a public exponent 'e' that is relatively prime to lcm(p-1, q-1), as defined in Claim 1.

• Does not cover methods for key exchange that don't rely on the specific mathematical properties of RSA, such as Diffie-Hellman.

Who is building on this today

Many companies and organizations continue to build on the principles of RSA. Major technology companies like Google, Apple, Microsoft, and Amazon rely on RSA for securing their online services and protecting user data. Cybersecurity firms like Palo Alto Networks and Fortinet integrate RSA into their security products. Open-source projects like OpenSSL also widely implement RSA for various cryptographic tasks.

Products built on expired version of this technology

How to cite this patent in your documentation

Massachusetts Institute of Technology. US Patent 4405829. Cryptographic communications system and method. Granted 1983, expired 2000. Now in the public domain.

Note: This is a convenience citation. Consult a patent attorney for formal freedom-to-operate analysis.

PatentBrief is an educational resource and does not provide legal advice. Patent expiration information is derived from USPTO records and may not reflect continuation patents, divisional filings, or separately-patented improvements. For commercial use or production decisions, obtain a formal freedom-to-operate (FTO) opinion from a registered patent attorney.