Am Fundamentals
Am Fundamentals
Am Fundamentals
Nalaka Dissanayake
Cryptography components
Encryption
Categories of cryptography
Symmetric-key cryptography
31.6
SYMMETRIC-KEY CRYPTOGRAPHY
Symmetric-key cryptography started thousands of years
ago when people needed to exchange secrets (for
example, in a war). We still mainly use symmetric-key
cryptography in our network security.
Traditional ciphers
A substitution cipher
Solution
The cipher is probably monoalphabetic because both
occurrences of L’s are encrypted as O’s.
Example
Solution
The cipher is not monoalphabetic because each
occurrence of L is encrypted by a different character.
The first L is encrypted as N; the second as Z.
Shift cipher
Solution
We decrypt one character at a time. Each character is
shifted 15 characters up. Letter W is decrypted to H.
Letter T is decrypted to E. The first A is decrypted to L.
The second A is decrypted to L. And, finally, D is
decrypted to O. The plaintext is HELLO.
Transposition cipher
Solution
We first remove the spaces in the message. We then divide
the text into blocks of four characters. We add a bogus
character Z at the end of the third block. The result is
HELL OMYD EARZ. We create a three-block ciphertext
ELHLMDOYAZER.
Example
Solution
The result is HELL OMYD EARZ. After removing the
bogus character and combining the characters, we get the
original message “HELLO MY DEAR.”
Rotation cipher
ASYMMETRIC-KEY CRYPTOGRAPHY