What is RSA?
RSA is one of the first usable public-key cryptosystems and is widely used for secure data transmission. RSA stands for Ron Rivest, Adi Shamir and Leonard Adleman, who were the first to publicly describe its algorithm in 1977. In such a cryptosystem, the encryption key is public and differs from the decryption key which is kept secret.
When using RSA, you create and publish the product of two large prime numbers along with an auxiliary value, as their public key. The prime factors must be kept secret. Anyone can use the public key to encrypt a message, but with currently published methods, if the public key is large enough, only someone with knowledge of the prime factors can feasibly decode the message. Breaking RSA encryption is known as the RSA problem.