Enter 26-letter key: AABCDEFGHIJ K LMNOPQRSTU VWXY

That key is illegal

Enter 26-letter key: BCDEFGHIJ K LMNOPQRSTU VWXYZ A

Plaintext: HAL9 000

Ciphertext: IBM9 000

The first key entered by the user is only eight characters and is short to

be a valid key. The second key attempted is of the right length but

misses the letter Z . The user gets it right at the third try.

2. Secret key cryptography scheme

Suppose that A wants to send a message to B. Both of them share the

same secret key. In order to encrypt the message, A XORs his message

with the shared secret key. B also XORs the message with his (the

same) secret key to decrypt the message.

For example, if the key is:

K ey = 0011

A’s message = 0101

A’s message XORed with the key: 0011 XOR 0101 = 0110

B receives = 0110

Applying the secret key to get the original message, B gets: 0110 XOR

key = 0101

TheXOR works for three reasons: It is associative, any binary value

XORed with itself is 0, Any binary value XORed with 0 is itself.

The advantages of secret key cryptography include performing XOR

is very fast and well tested.

The disadvantages are that the key must remain secret, exchanging

keys with someone must be done in secret, each of the communicating

pair of people needs to share a key.

Public key cryptography scheme

Here, there are two parts to the key: a secret part and a public part.

A needs to obtain his public key before sending a message to B. The B

has published his public on his homepage for anyone to download as

he likes to be contacted only via encrypted messages. A has obtained