Messages can be encrypted (it's data disguised) so that intercepted message cannot be decrypted (understood) by an interceptor.
Suppose that Alice (sender) wants to send a message to Bob (receiver). She can use encryption algorithm (word 'algorithm' can be explained as solving of problem using computer) so that encrypted message (known as ciphertext) looks unintelligible to any intruder.
In many modern cryptographic systems it does not matter that encryption technique can be known to everyone.
Historical Cryptographic systems include:
Caesar cipher (a cipher is a method for encrypting data).
For English text, the Caesar cipher would work by taking each letter in the plaintext (unencrypted text) and substituting it with the letter that is k letters later (allowing wrap-around; that is, havilng the letter 'z' followed by letter 'a') in the alphabet. For example if k = 3, then the letter 'a' in plaintext becomes 'd' in ciphertext; 'b' in plaintext becomes 'e' in ciphertext and so on.
An improvement on Caesar cipher is monoalphabetic cipher.
It also substitutes one letter of the alphabet with another letter of the alphabet. However, rather than substituting according to regular pattern (for example, substituting by shifting all letters in message by k places in alphabet, forward), any letter can be substituting by any other letter, as long as each letter has unique substitute letter and vice versa.
For example, monoalphabetic cipher (key to it) can look like this:
a b c d e f g h i j k l m n o p q r s t u v w x y z
h n m r t l o a s v p f c b g k y d i e x j z u q w
The plaintext message "bob, i love you. alice" becomes "ngn, s fgjt qgx. hfsmt".
Five hundred years ago, techniques improving on monoalphabetic encryption, known as polyalphabetic encryption were invented. The idea behind polyalphabetic encryption is to use multiple monoalphabetic ciphers, with a specific monoalphabetic cipher to encode a letter in specific position in the plaintext message.