What is RSA Encryption ?

The ultimate in modern cryptography is public/private key cryptosystems. One of the first and most successful is the RSA algorithm. It is fairly simple yet very powerful, and it is at the heart of some of the best encryption software available.

In cryptography, RSA (which stands for Rivest, Shamir and Adleman who first publicly described it) is an algorithm for public-key cryptography. It is the first algorithm known to be suitable for signing as well as encryption, and one of the first great advances in public key cryptography. RSA is widely used in electronic commerce protocols, and is believed to be secure given sufficiently long keys and the use of up-to-date implementations.

The RSA algorithm involves three steps: key generation, encryption and decryption.

RSA involves a public key and a private key. The public key can be known to everyone and is used for encrypting messages. Messages encrypted with the public key can only be decrypted using the private key.

The algorithm is based on modular exponentiation. Numbers e, d and N are chosen with the property that if A is a number less than N, then (Ae mod N)d mod N = A.

This means that you can encrypt A with e and decrypt using d. Conversely you can encrypt using d and decrypt using e (though doing it this way round is usually referred to as signing and verification).

• The pair of numbers (e,N) is known as the public key and can be published.
• The pair of numbers (d,N) is known as the private key and must be kept secret.

The number e is known as the public exponent, the number d is known as the private exponent, and N is known as the modulus. When talking of key lengths in connection with RSA, what is meant is the modulus length.

An algorithm that uses different keys for encryption and decryption is said to be asymmetric.

Anybody knowing the public key can use it to create encrypted messages, but only the owner of the secret key can decrypt them.

The Original RSA Patent as filed with the U.S. Patent Office by Rivest; Ronald L. (Belmont, MA), Shamir; Adi (Cambridge, MA), Adleman; Leonard M. (Arlington, MA), December 14, 1977, U.S. Patent 4,405,829.