1. Cryptographic Algorithms and Protocols
Classical Encryption Techniques
Slides 02
5-Feb-19
Cryptographic Algorithms and Protocols

2. Cryptographic Algorithms and Protocols
Symmetric Encryption
Terminologies
Substitution Ciphers
Shift Cipher
Substitution Cipher
Caesar Cipher
Monoalphabetic Cipher
Playfair Cipher
Vigener Cipher
One Time Pad
Transposition Ciphers
Rail Fence cipher
Row Transposition Cipher
Product Ciphers
Rotor Machines Cipher
Steganography
5-Feb-19
Cryptographic Algorithms and Protocols

3. Cryptographic Algorithms and Protocols
Symmetric Encryption
Hacker
Insecure Channel
Encryption
Algorithm
Decryption
Algorithm M M M M M M
Key
Generator
Secure Channel
Key
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Cryptographic Algorithms and Protocols

4. Asymmetric Encryption
Cryptanalyst
Cipher Text
Hacker
Plain Text
Encipher
Decipher M M M M M M
Key
Generator
Plain Text
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Cryptographic Algorithms and Protocols

5. Cryptographic Algorithms and Protocols
Terminologies
Symmetric Encryption
Conventional encryption
Single Key encryption
Private key encryption
Asymmetric Encryption
Public Key encryption
One key is public and the other is secret
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Cryptographic Algorithms and Protocols

6. Cryptographic Algorithms and Protocols
Terminologies
Plaintext: the original message
Ciphertext: the coded message
Cipher: algorithm for transforming plaintext to ciphertext
Key: info used in cipher known only to sender/receiver
Encipher (encrypt): converting plaintext to ciphertext
Decipher (decrypt): recovering ciphertext from plaintext
Cryptography: study of encryption principles/methods
Cryptanalysis (code-breaking): the study of principles/ methods of deciphering ciphertext without knowing key
Cryptology: the field of both cryptography and cryptanalysis
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Cryptographic Algorithms and Protocols

7. Cryptographic Algorithms and Protocols
Terminologies
The Encryption Process:
EK(M) = C
where
M: Plain Text
E: Cipher Function
K: Secret Key
C: Cipher Text
The Decryption Process:
M = DK(C)
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Cryptographic Algorithms and Protocols

8. Cryptographic Algorithms and Protocols
Cryptography
can characterize by:
type of encryption operations used
substitution / transposition / product
number of keys used
single-key or private / two-key or public
way in which plaintext is processed
block / stream
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Cryptographic Algorithms and Protocols

9. Cryptographic Algorithms and Protocols
Brute Force Search
I will try every possible key!
The time to find the key depends on the key size
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Cryptographic Algorithms and Protocols

10. Cryptographic Algorithms and Protocols
Caesar/Shift Cipher
Example:
K = 3
A  D, B  E , C  F, … , X  A , Y  B Z  C
HELLO  KHOOR
Click here to try the Java Applet
5-Feb-19
Cryptographic Algorithms and Protocols

11. Caesar Cipher – Cryptanalysis
Assume a Hacker caught this Cipher message:
GB OR BE ABG GB OR: GUNG VF GUR DHRFGVBA:
JURGURE 'GVF ABOYRE VA GUR ZVAQ GB FHSSRE
GUR FYVATF NAQ NEEBJF BS BHGENTRBHF SBEGHAR,
BE GB GNXR NEZF NTNVAFG N FRN BS GEBHOYRF,
NAQ OL BCCBFVAT RAQ GURZ? GB QVR: GB FYRRC;
AB ZBER; NAQ OL N FYRRC GB FNL JR RAQ
GUR URNEG-NPUR NAQ GUR GUBHFNAQ ANGHENY FUBPXF
GUNG SYRFU VF URVE GB, 'GVF N PBAFHZZNGVBA
QRIBHGYL GB OR JVFU'Q. GB QVR: GB FYRRC;
GB FYRRC: CREPUNAPR GB QERNZ: NL, GURER'F GUR EHO;
SBE VA GUNG FYRRC BS QRNGU JUNG QERNZF ZNL PBZR
JURA JR UNIR FUHSSYRQ BSS GUVF ZBEGNY PBVY,
ZHFG TVIR HF CNHFR: GURER'F GUR ERFCRPG
GUNG ZNXRF PNYNZVGL BS FB YBAT YVSR;
What is the key?
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Cryptographic Algorithms and Protocols

12. Caesar Cipher – Cryptanalysis
GB OR BE ABG GB OR: GUNG VF GUR DHRFGVBA:
JURGURE 'GVF ABOYRE VA GUR ZVAQ GB FHSSRE
GUR FYVATF NAQ NEEBJF BS BHGENTRBHF SBEGHAR,
BE GB GNXR NEZF NTNVAFG N FRN BS GEBHOYRF,
NAQ OL BCCBFVAT RAQ GURZ? GB QVR: GB FYRRC;
AB ZBER; NAQ OL N FYRRC GB FNL JR RAQ
GUR URNEG-NPUR NAQ GUR GUBHFNAQ ANGHENY FUBPXF
GUNG SYRFU VF URVE GB, 'GVF N PBAFHZZNGVBA
QRIBHGYL GB OR JVFU'Q. GB QVR: GB FYRRC;
GB FYRRC: CREPUNAPR GB QERNZ: NL, GURER'F GUR EHO;
SBE VA GUNG FYRRC BS QRNGU JUNG QERNZF ZNL PBZR
JURA JR UNIR FUHSSYRQ BSS GUVF ZBEGNY PBVY,
ZHFG TVIR HF CNHFR: GURER'F GUR ERFCRPG
GUNG ZNXRF PNYNZVGL BS FB YBAT YVSR;
N could be I or A  key is 5 or 13
Comparing Letter frequency between Cipher and English texts
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Cryptographic Algorithms and Protocols

13. Caesar Cipher – Cryptanalysis
Try the shift:
1, 6, 13, or 17
The common shift is 13
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Cryptographic Algorithms and Protocols

14. Monoalphabetic Cipher
Plain: abcdefghijklmnopqrstuvwxyz
Cipher:DKVQFIBJWPESCXHTMYAUOLRGZN
EXAMPLE
Plaintext: ifwewishtoreplaceletters
Ciphertext:WIRFRWAJUHYFTSDVFSFUUFYA
Click here to try the Java Applet
5-Feb-19
Cryptographic Algorithms and Protocols

15. Cryptographic Algorithms and Protocols
Playfair Cipher
a 5X5 matrix of letters based on a keyword
fill in letters of keyword (sans duplicates)
fill rest of matrix with other letters
eg. using the keyword CHARLES
Click here to try the Java Applet L R A H C F D B S E N M K I G U T Q P O Z Y X W V
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Cryptographic Algorithms and Protocols

16. Cryptographic Algorithms and Protocols
Vigenère Cipher
An improvement over the Playfair
It is a polyalphabetic substitution cipher
Effectively multiple Caesar ciphers
Key is multiple letters long K = k1 k2 ... kd
ith letter specifies ith alphabet to use
Use each alphabet in turn
Repeat from start after d letters in message
Decryption simply works in reverse
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Cryptographic Algorithms and Protocols

17. Cryptographic Algorithms and Protocols
Vigenère Cipher
KEY
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Cryptographic Algorithms and Protocols

18. Cryptographic Algorithms and Protocols
Vigenère Cipher
write the plaintext out
write the keyword repeated above it
use each key letter as a caesar cipher key
encrypt the corresponding plaintext letter
eg using keyword deceptive
key: deceptivedeceptivedeceptive
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGRZGVTWAVZHCQYGLMGJ
Click here to try the Java Applet
5-Feb-19
Cryptographic Algorithms and Protocols

19. Vigenère Cipher – Autokey Cryptanalysis
Ideally, we need a key as long as the message
Vigenère proposed the autokey cipher
Example:
key: deceptivewearediscoveredsav
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLA
knowing keyword can recover the first few letters
Click here to try the Java Applet
5-Feb-19
Cryptographic Algorithms and Protocols

20. Cryptographic Algorithms and Protocols
One-Time Pad
To build unbreakable Cipher using Vigenère, the key must be random.
Each key is used once
Example:
key: sdvbncfhjkledtghyrtustryuil
plaintext: wearediscoveredsaveyourself
ciphertext:ZICVTWQNGKZEIIGASXSTSLVVWLA
Difficult to secure the key
Click here to try the Java Applet
5-Feb-19
Cryptographic Algorithms and Protocols

21. Cryptographic Algorithms and Protocols
One-Time Pad
Different decryptions using two different keys:
Example:
KEY: pxlmvmsydoftyrvzwctnlebnecvgdupathfzzlmnyih
Plain: mr mustard with the candlestick in the hall
Cipher:ANKYODKYUREOFJBYOJXMNEOQPGMZLWZ_AUERFPLUYTS
KEY: mvbpnvmykbigeejqviptrdycxlqytiyshydbnqbuoq
Plain: miss scarlet with the knife in the library
Click here to try the Java Applet
5-Feb-19
Cryptographic Algorithms and Protocols

22. Transposition Ciphers
Hiding the message by rearranging the letter order without altering the actual letters used
Still can be recognised since ciphertext have the same frequency distribution as the original text
Transposition Ciphers form the second basic building block of ciphers. The core idea is to rearrange the order of basic units (letters/bytes/bits) without altering their actual values.
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Cryptographic Algorithms and Protocols

23. Cryptographic Algorithms and Protocols
Rail Fence cipher
write message letters out diagonally over a number of rows
then read off cipher row by row
eg. write message out as:
m e m a t r h t g p r y
e t e f e t e o a a t
giving ciphertext
MEMATRHTGPRYETEFETEOAAT
Example message is: "meet me after the toga party" with a rail fence of depth 2.
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Cryptographic Algorithms and Protocols

24. Row Transposition Ciphers
a more complex scheme
write letters of message out in rows over a specified number of columns
then reorder the columns according to some key before reading off the rows
M = attackpostponeduntiltwoam
Key:
Plaintext: a t t a c k p
o s t p o n e
d u n t i l t
w o a m x y z
C: TTNAAPTMTSUOAODWCOIXKNLYPETZ
The key is wrong in the book.
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Cryptographic Algorithms and Protocols

25. Cryptographic Algorithms and Protocols
Product Ciphers
ciphers using substitutions or transpositions are not secure because of language characteristics
hence consider using several ciphers in succession to make harder, but:
two substitutions make a more complex substitution
two transpositions make more complex transposition
but a substitution followed by a transposition makes a new much harder cipher
this is bridge from classical to modern ciphers
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Cryptographic Algorithms and Protocols

26. Cryptographic Algorithms and Protocols
Rotor Machines
before modern ciphers, rotor machines were most common product cipher
were widely used in WW2
German Enigma, Allied Hagelin, Japanese Purple
implemented a very complex, varying substitution cipher
used a series of cylinders, each giving one substitution, which rotated and changed after each letter was encrypted
with 3 cylinders have 263=17576 alphabets
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Cryptographic Algorithms and Protocols

27. Cryptographic Algorithms and Protocols
Rotor Machines
A  B, B  I, C  E <one cycle> A  E, B  Q, C  T
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Cryptographic Algorithms and Protocols

28. Cryptographic Algorithms and Protocols
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
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Cryptographic Algorithms and Protocols

29. Cryptographic Algorithms and Protocols
Summary
have considered:
classical cipher techniques and terminology
monoalphabetic substitution ciphers
cryptanalysis using letter frequencies
Playfair ciphers
polyalphabetic ciphers
transposition ciphers
product ciphers and rotor machines
stenography
5-Feb-19
Cryptographic Algorithms and Protocols