"To understand how public key cryptography works, let's consider
secure communications in general. One way to send a confidential
message to someone is to agree on an obfuscation system in
advance--like substituting each letter in the message with the next
one in the alphabet.
"A more sophisticated method would be to use encryption software
which uses an encryption algorithm, known as a cipher. The message
(known as plaintext ) is entered and passed to the algorithm along
with a key--a string of characters that you supply--comes out in
encrypted form (known as ciphertext.) This unintelligible jumble of
characters can only be converted back to the original plaintext by
passing the message through the same cipher and supplying the same
key. This is known as a symmetric encryption system."