1. Encryption involves encoding messages so their meaning is obscured, while decryption is the reverse process of decoding encrypted messages. 2. Classical encryption techniques include symmetric key encryption where senders and receivers share a common key, and asymmetric key encryption which uses public and private key pairs. 3. Symmetric ciphers like the Caesar cipher and Vigenere cipher use substitution or transposition of letters to encrypt messages, while transposition ciphers rearrange letters in the message. Modern algorithms like Triple DES and Blowfish are also summarized.

1. Basic Encryption and Decryption

2. • Encryption: a process of encoding a message
so that its meaning is not obvious
• Decryption: the reverse process

3. Encode(encipher) vs.
Decode(decipher)
• Encoding: the process of translating entire
words or phrases to other words or phrases
• Enciphering: translating letters or symbols
individually
• Encryption: the group term that covers both
encoding and enciphering

4. Basic operations
• plaintext to ciphertext: encryption
C = E(P)
• ciphertext to plaintext: decryption:
P = D(C)
requirement: P = D(E(P))

5. Classical Encryption
Techniques
• Symmetric key encryption
• Asymmetric key encryption

6. Symmetric Key Encryption
• Sender and recipient share a common key
• Was the only type of cryptography, prior to
invention of public-key in 1970’s
• All traditional schemes are symmetric / single
key / private-key encryption algorithms, with
a single key, used for both encryption and
decryption, since both sender and receiver are
equivalent, either can encrypt or decrypt
messages using that common key.

7. Symmetric Cipher Model

8. Requirements
• Two requirements for secure use of symmetric
encryption:
– a strong encryption algorithm
– a secret key known only to sender / receiver
Y = EK(X)
X = DK(Y)
Here, plaintext X, ciphertext Y, key K, encryption
algorithm Ek, decryption algorithm Dk.
• Assume encryption algorithm is known
• Implies a secure channel to distribute key

9. Types of Ciphers
• Substitution ciphers
• Permutation (or transposition) ciphers

10. Classical Substitution Ciphers
• A substitution cipher replaces one
symbol with another.
• Substitution ciphers can be categorized
as either monoalphabetic ciphers or
polyalphabetic ciphers.

11. Monoalphabetic Ciphers
• In monoalphabetic substitution, the
relationship between a symbol in the
plaintext to a symbol in the ciphertext is
always one-to-one.

12. • The simplest monoalphabetic cipher is
the additive cipher. This cipher is
sometimes called a shift cipher and
sometimes a Caesar cipher

13. Caesar Cipher
• Earliest known substitution cipher
• First attested use in military affairs
• Replaces each letter by 3rd letter on
• example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB

14. • Note: when letters are involved, the following
conventions are used in this course: Plaintext
is always in lowercase; ciphertext is in
uppercase

15. • can define transformation as:
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
D E F G H I J K L M N O P Q R S T U V W X Y Z A B C
• mathematically give each letter a number
• then have Caesar cipher as:
C = E(p) = (p + k) mod (26)
p = D(C) = (C – k) mod (26)

16. When the cipher is additive, the plaintext, ciphertext, and key are integers
in Z26

17. • This mathematical description uses modulo
arithmetic (ie clock arithmetic). Here, when
you reach Z you go back to A and start again.
Mod 26 implies that when you reach 26, you
use 0 instead (ie the letter after Z, or 25 + 1
goes to A or 0).
• Example: howdy (7,14,22,3,24) encrypted
using key f (5) is MTBID

18. Example
• Use the additive cipher with key = 15 to
encrypt the message “hello”.

19. • Use the additive cipher with key = 15 to
decrypt the message “WTAAD”.

20. Cryptanalysis of Caesar Cipher
• only have 26 possible keys, of which only 25
are of any use, since mapping A to A etc
doesn't really obscure the message.
• Advantage : easy to use
• Disadvantage: simple structure and easy to
break

21. Polyalphabetic Ciphers
• another approach to improving security is to
use multiple cipher alphabets
• called polyalphabetic substitution ciphers
• makes cryptanalysis harder with more
alphabets to guess and flatter frequency
distribution
• use a key to select which alphabet is used for
each letter of the message

22. Vigenere Cipher
• Vigenere key stream does not depend
on the plaintext characters; it depends
only on the position of the character in
the plaintext

24. Example
• encrypt the message She is listening using the
6-character keyword “PASCAL”.
• The initial key stream is (15, 0, 18, 2, 0, 11).
The key stream is the repetition of this initial
key stream (as many times as needed

25. TRANSPOSITION CIPHERS

26. TRANSPOSITION CIPHERS
• A transposition cipher does not
substitute one symbol for another,
instead it changes the location of the
symbols.

27. Keyless Transposition Ciphers
• Simple transposition ciphers, which were
used in the past, are keyless.
• Text is written into a table column by
column and then is transmitted row by
row.
• Text is written into a table and row by row,
then is transmitted column by column.

28. Example: Rail fence cipher.
• The ciphertext is created reading the
pattern row by row.
• For example, to send the message “Meet
me at the park” to Bob, Alice writes
She then creates the ciphertext
“MEMATEAKETETHPR

29. Example
• Alice and Bob can agree on the number
of columns and use the second method.
Alice writes the same plaintext, row by
row, in a table of four columns.
She then creates the ciphertext
“MMTAEEHREAEKTTP”.

30. Keyed Transposition Ciphers
• Divide the plaintext into groups of
predetermined size, called blocks, and
then use a key to permute the characters
in each block separately.

31. Steganography
• an alternative to encryption
• hides existence of message
– using only a subset of letters/words in a longer
message marked in some way
– using invisible ink
– hiding in LSB in graphic image or sound file
• has drawbacks
– high overhead to hide relatively few info bits

32. CONVENTIONAL ENCRYPTION
ALGORITHMS

33. CONVENTIONAL ENCRYPTION ALGORITHMS
• Triple DES
• International Data Encryption Algorithm (IDEA)
• Blowfish
• RC5
• CAST-128
• RC2
• Characteristics of Advanced Symmetric Block
Ciphers