4. Introduction
• Symmetric ciphers use symmetric algorithms
to encrypt and decrypt data.
• A symmetric key algorithm uses the same key to
encrypt data as it does to decrypt data.
• It have many important advantages like speed. But they
lack in other areas like security and management.
7. How the process works
If P is the plaintext, C is the ciphertext, and K isthe key,
We assume that Bob creates P1; we prove that P1 =P:
8. Traditional symmetric-key ciphers can be classified into two
categories:
1. Substitution Ciphers
Monoalphabetic Ciphers
Polyalphabetic Ciphers
2. Transposition Ciphers
Keyless Transposition Ciphers
Keyed Transposition Ciphers
Combining two approaches
Categories of Traditional Symmetric- Key
9. Substitution ciphers
It is a classical encryption technique where the characters present
in the original message are replaced by the other characters.
A substitution cipher replaces one symbol
with another.
11. Mono alphabetic cipher
In mono alphabetic substitution, the relationship between a
symbol in the plaintext to a symbol in the cipher text is always
one-one.
13. Use the additivecipherwith key = 15 to encryptthe
message “hello”.
Solution
We apply the encryption algorithm to the plaintext, character by
character:
Additive cipher
14. Use the additive cipher with key = 15 to decrypt the
message “WTAAD”.
Solution
We apply the decryption algorithm to the plaintext character by
character:
Additive cipher
15. Shift Cipher and Caesar Cipher
• Historically, additive ciphers are called shift
ciphers. Julius Caesar used an additive cipher to
communicate with his officers. For this reason,
additive ciphers are sometimes referred to as the
Caesar cipher. Caesar used a key of 3 for his
communications.
16. Example
• Plain text : Meet me Tomorrow
• Cipher Text: phhw ph wrpruurz
Note:
we have replaced “m” with “p” which occur
third place from m.
Shift Cipher and Caesar Cipher
17. Cryptanalysis
Eve has intercepted the ciphertext “UVACLYFZLJBYL”.
how she can use a brute-force attack to break the cipher.
Solution
Eve tries keys from 1 to 7. With a key of 7, the plaintext is “not very
secure”, which makes sense.
19. Eve has intercepted the following ciphertext. Using a
statistical attack, find the plaintext.
Solution
When Eve tabulates the frequency of letters in this ciphertext,
she gets: I =14, V =13, S =12, and so on. The most common
character is I with 14 occurrences. This means key = 4.
Cryptanalysis
20. Multiplicative cipher
In a multiplicative cipher, the plaintext and ciphertext are
integers in Z26; the key is an integer in Z26*.
C
P
21. Multiplicative cipher
We use a multiplicative cipher to encrypt the message “hello”
with a key of 7. The ciphertext is “XCZZU”.
22. Affine cipher
• The affine cipher is a combination of additive cipher and multiplicative
cipher.
• The key space is 26 * 12 (key space of additive * key space of
multiplicative) i.e. 312.
• It is relatively secure than the above two as the key space is larger.
23. Affine cipher
The affine cipher uses a pair of keys in which the first key is from
Z26* and the second is from Z26. The size of the key domain is
26 × 12 = 312.
Use an affine cipher to encrypt the message “hello” with the key
pair (7, 2).
24. Mono-alphabetic substitution
cipher
• A mono-alphabetic cipher (aka simple substitution cipher)
is a substitution cipher where each letter of the plain text is
replaced with another letter of the alphabet. It uses a fixed
key which consist of the 26 letters of a “shuffled alphabet”.
