What is the X in X-mas?
(The initials of the authors)
1. RSA Data Security, Inc.
2. Their cryptography systems, especially RSA encryption.
The RSA algorithm was first described in the paper:
[R. Rivest, A. Shamir, L. Adleman, "A Method for Obtaining Digital Signatures and Public-key Cryptosystems". CACM 21,2; 1978]