A Method for Obtaining Digital Signatures and Public-Key Cryptosystems
An encryption method is presented with the novel
property that publicly revealing an encryption 
key does not thereby reveal the corresponding decryption
key.  This has two important consequences: (1) 
Couriers or other secure means are not needed to transmit
keys, since a message can be enciphered using 
an encryption key publicly revealed by the intended recipient.
 Only he can decipher the message, since 
only he knows the corresponding decryption key.  (2)
A message can be "signed" using a privately held 
decryption key.  Anyone can verify this signature using
the corresponding publicly revealed encryption 
key.  Signatures cannot be forged, and a signer cannot
later deny the validity of his signature.  This 
has obvious applications in "electronic mail" and "electronic
funds transfer" systems.  A message is 
encrypted by representing it as a number M, raising
M to a publicly specified power e, and then taking 
the remainder when the result is divided by the publicly
specified product, n, of two large secret prime 
numbers p and q.  Decryption is similar;only a different,
secret, power d is used, where e * d = 1 (mod(p-1) 
* (q-1)).  the security of the system rests in part on
the difficulty of factoring the published divisor, 
n.
CACM February, 1978
Rivest, R.L.
Shamir, A.
Adleman, L.
