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))
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.
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
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
23.
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
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