The Free On-line Dictionary of Computing (30 December 2018):

RSA encryption

    A public-key cryptosystem for both
   encryption and authentication, invented in 1977 by Ron
   Rivest, Adi Shamir, and Leonard Adleman.  Its name comes from
   their initials.

   The RSA algorithm works as follows.  Take two large prime
   numbers, p and q, and find their product n = pq; n is called
   the modulus.  Choose a number, e, less than n and relatively
   prime to (p-1)(q-1), and find its reciprocal mod (p-1)(q-1),
   and call this d.  Thus ed = 1 mod (p-1)(q-1); e and d are
   called the public and private exponents, respectively.  The
   public key is the pair (n, e); the private key is d.  The
   factors p and q must be kept secret, or destroyed.  It is
   difficult (presumably) to obtain the private key d from the
   public key (n, e).  If one could factor n into p and q,
   however, then one could obtain the private key d.  Thus the
   entire security of RSA depends on the difficulty of factoring;
   an easy method for factoring products of large prime numbers
   would break RSA.

   RSA FAQ (http://rsa.com/rsalabs/faq/faq_home.html).

   (2004-07-14)