1. Computer and Information Security
Chapter 2
Crypto Basics

2. Overview Terminology How to Speak Crypto Classic Crypto Modern Crypto
Simple Substitution Cipher
Double Transposition Cipher
One Time Pad
Project VERONA
Modern Crypto

3. Part I: Crypto

4. Chapter 2: Crypto Basics
MXDXBVTZWVMXNSPBQXLIMSCCSGXSCJXBOVQXCJZMOJZCVC
TVWJCZAAXZBCSSCJXBQCJZCOJZCNSPOXBXSBTVWJC
JZDXGXXMOZQMSCSCJXBOVQXCJZMOJZCNSPJZHGXXMOSPLH
JZDXZAAXZBXHCSCJXTCSGXSCJXBOVQX
 plaintext from Lewis Carroll, Alice in Wonderland
The solution is by no means so difficult as you might
be led to imagine from the first hasty inspection of the characters.
These characters, as any one might readily guess,
form a cipher - that is to say, they convey a meaning…
- Edgar Allan Poe, The Gold Bug

5. Crypto
Cryptology  The art and science of making and breaking “secret codes”
Cryptography  making “secret codes”
Cryptanalysis  breaking “secret codes”
Crypto  all of the above (and more)

6. Basic Terminology plaintext - original message
ciphertext - 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 (codebreaking) - study of principles/ methods of deciphering ciphertext without knowing key
cryptology - field of both cryptography and cryptanalysis

7. How to Speak Crypto
A cipher or cryptosystem is used to encrypt the plaintext
The result of encryption is ciphertext
We decrypt ciphertext to recover plaintext
A key is used to configure a cryptosystem
A symmetric key cryptosystem uses the same key to encrypt as to decrypt
A public key cryptosystem uses a public key to encrypt and a private key to decrypt

8. Crypto Basic assumptions This is known as Kerckhoffs’ Principle
The system is completely known to the attacker
Only the key is secret
That is, crypto algorithms are not secret
This is known as Kerckhoffs’ Principle
Why do we make this assumption?
Experience has shown that secret algorithms are weak when exposed
Secret algorithms never remain secret
Better to find weaknesses beforehand

9. Crypto as Black Box A generic view of symmetric key crypto key key
plaintext
encrypt
decrypt
plaintext
ciphertext
A generic view of symmetric key crypto

10. Classical ciphers
Substitution- “units” of plain text are replaced with cipher text
Polyalphabetic substitution- different for each character
Transposition- “unit” of plaintext are rearranged, usually in complex order
See (

11. Caesar Cipher plain: abcdefghijklmnopqrstuvwxyz
key: defghijklmnopqrstuvwxyzabc
cipher: PHHW PH DIWHU WKH WRJD SDUWB
plain: MEET ME AFTER THE TOGA PARTY

12. Simple Substitution Plaintext: fourscoreandsevenyearsago Key:h i j k l m n o p q r s t u v w x y 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 z C
Plaintext
Ciphertext
Ciphertext:
IRXUVFRUHDQGVHYHQBHDUVDJR
Shift by 3 is a “Caesar cipher”

13. Caesar Cipher Decryption
Suppose we know a Caesar cipher is being used:
Given ciphertext:
VSRQJHEREVTXDUHSDQWV 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 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 z C
Plaintext
Ciphertext
Plaintext: spongebobsquarepants

14. “Rail-Fence” Cipher DISGRUNTLED EMPLOYEE DRLEOIGUTE MLYESNDPE

15. Simple Cipher Examples
Substitution ciphers - Caesar
Transposition ciphers – Rail Fence
Codes and Ciphers Primer

16. Not-so-Simple Substitution
Shift by n for some n  {0,1,2,…,25}
Then key is n
Example: key n = 7 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 H I J K L M N O P Q R S T U V W X Y Z A B C D E F z G
Plaintext
Ciphertext

17. Cryptanalysis I: Try Them All
A simple substitution (shift by n) is used
But the key is unknown
Given ciphertext: CSYEVIXIVQMREXIH
How to find the key?
Only 26 possible keys  try them all!
Exhaustive key search
Solution: key is n = 4

18. Least-Simple Simple Substitution
In general, simple substitution key can be any permutation of letters
Not necessarily a shift of the alphabet
For example 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 J I C A X S E Y V D K W B Q T Z R H F M P N U L G z O
Plaintext
Ciphertext
Then 26! > 288 possible keys!

19. Cryptanalysis II: Be Clever
We know that a simple substitution is used
But not necessarily a shift by n
Find the key given the ciphertext:
PBFPVYFBQXZTYFPBFEQJHDXXQVAPTPQJKTOYQWIPBVWLXTOXBTFXQWAXBVCXQWAXFQJVWLEQNTOZQGGQLFXQWAKVWLXQWAEBIPBFXFQVXGTVJVWLBTPQWAEBFPBFHCVLXBQUFEVWLXGDPEQVPQGVPPBFTIXPFHXZHVFAGFOTHFEFBQUFTDHZBQPOTHXTYFTODXQHFTDPTOGHFQPBQWAQJJTODXQHFOQPWTBDHHIXQVAPBFZQHCFWPFHPBFIPBQWKFABVYYDZBOTHPBQPQJTQOTOGHFQAPBFEQJHDXXQVAVXEBQPEFZBVFOJIWFFACFCCFHQWAUVWFLQHGFXVAFXQHFUFHILTTAVWAFFAWTEVOITDHFHFQAITIXPFHXAFQHEFZQWGFLVWPTOFFA

20. Cryptanalysis II Ciphertext:
PBFPVYFBQXZTYFPBFEQJHDXXQVAPTPQJKTOYQWIPBVWLXTOXBTFXQWAXBVCXQWAXFQJVWLEQNTOZQGGQLFXQWAKVWLXQWAEBIPBFXFQVXGTVJVWLBTPQWAEBFPBFHCVLXBQUFEVWLXGDPEQVPQGVPPBFTIXPFHXZHVFAGFOTHFEFBQUFTDHZBQPOTHXTYFTODXQHFTDPTOGHFQPBQWAQJJTODXQHFOQPWTBDHHIXQVAPBFZQHCFWPFHPBFIPBQWKFABVYYDZBOTHPBQPQJTQOTOGHFQAPBFEQJHDXXQVAVXEBQPEFZBVFOJIWFFACFCCFHQWAUVWFLQHGFXVAFXQHFUFHILTTAVWAFFAWTEVOITDHFHFQAITIXPFHXAFQHEFZQWGFLVWPTOFFA
Analyze this message using statistics below
Ciphertext frequency counts: 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 21 26 6 10 12 51 25 9 3 1 15 28 42 27 4 24 22 Z 8

21. Cryptanalysis: Terminology
Cryptosystem is secure if best known attack is to try all keys
Exhaustive key search, that is
Cryptosystem is insecure if any shortcut attack is known
But then an insecure cipher might be harder to break than a secure cipher!
This is counterintuitive…

22.  Double Transposition Plaintext: attackxatxdawn
Permute rows
and columns 
Ciphertext: xtawxnattxadakc
Key is matrix size and permutations: rows (3,5,1,4,2) and columns (1,3,2)

23. One-Time Pad: Encryption
e=000 h=001 i=010 k=011 l=100 r=101 s=110 t=111
Encryption: Plaintext  Key = Ciphertext h e i l t r
001
000
010
100
111
101
Plaintext:
Key:
111
101
110
100
000
001 s r l h t
Ciphertext:

24. One-Time Pad: Decryption
e=000 h=001 i=010 k=011 l=100 r=101 s=110 t=111
Decryption: Ciphertext  Key = Plaintext s r l h t
110
101
100
001
111
Ciphertext:
Key:
111
101
110
100
000
001
010 h e i l t r
Plaintext:

25. One-Time Pad Double agent claims sender used following “key” s r l h t
110
101
100
001
111
Ciphertext:
“key”:
101
111
000
100
110
011
010
001 k i l h t e r
“Plaintext”:
e=000 h=001 i=010 k=011 l=100 r=101 s=110 t=111

26. One-Time Pad Or sender is captured and claims the key is… s r l h t h
110
101
100
001
111
Ciphertext:
“key”:
111
101
000
011
110
001
100
010 h e l i k s
“Plaintext”:
e=000 h=001 i=010 k=011 l=100 r=101 s=110 t=111

27. One-Time Pad Summary Provably secure…
Ciphertext provides no info about plaintext
All plaintexts are equally likely
…but, only when be used correctly
Pad must be random, used only once
Pad is known only to sender and receiver
Note: pad (key) is same size as message
So, why not distribute msg instead of pad?

28. Real-World One-Time Pad
Project VENONA
Encrypted spy messages from U.S. to Moscow in 30’s, 40’s, and 50’s
Nuclear espionage, etc.
Thousands of messages
Spy carried one-time pad into U.S.
Spy used pad to encrypt secret messages
Repeats within the “one-time” pads made cryptanalysis possible

29. VENONA Decrypt (1944) “Ruth” == Ruth Greenglass
[C% Ruth] learned that her husband [v] was called up by the army but he was not sent to the front. He is a mechanical engineer and is now working at the ENORMOUS [ENORMOZ] [vi] plant in SANTA FE, New Mexico. [45 groups unrecoverable]
detain VOLOK [vii] who is working in a plant on ENORMOUS. He is a FELLOWCOUNTRYMAN [ZEMLYaK] [viii]. Yesterday he learned that they had dismissed him from his work. His active work in progressive organizations in the past was cause of his dismissal. In the FELLOWCOUNTRYMAN line LIBERAL is in touch with CHESTER [ix]. They meet once a month for the payment of dues. CHESTER is interested in whether we are satisfied with the collaboration and whether there are not any misunderstandings. He does not inquire about specific items of work [KONKRETNAYa RABOTA]. In as much as CHESTER knows about the role of LIBERAL's group we beg consent to ask C. through LIBERAL about leads from among people who are working on ENOURMOUS and in other technical fields.
“Ruth” == Ruth Greenglass
“Liberal” == Julius Rosenberg
“Enormous” == the atomic bomb

30. Codebook Cipher Literally, a book filled with “codewords”
Zimmerman Telegram encrypted via codebook
Februar
fest
finanzielle
folgender
Frieden
Friedenschluss
: :
Modern block ciphers are codebooks!
More about this later…

31. Codebook Cipher: Additive
Codebooks also (usually) use additive
Additive - book of “random” numbers
Encrypt message with codebook
Then choose position in additive book
Add additives to get ciphertext
Send ciphertext and additive position (MI)
Recipient subtracts additives before decrypting
Why use an additive sequence?

32. Zimmerman Telegram Perhaps most famous codebook ciphertext ever
A major factor in U.S. entry into World War I

33. Zimmerman Telegram Decrypted
British had recovered partial codebook
Then able to fill in missing parts

34. Random Historical Items
Crypto timeline
Spartan Scytale - transposition cipher
Caesar’s cipher
Poe’s short story: The Gold Bug
Election of 1876

35. Election of 1876 “Rutherfraud” Hayes vs “Swindling” Tilden
Popular vote was virtual tie
Electoral college delegations for 4 states (including Florida) in dispute
Commission gave all 4 states to Hayes
Vote on straight party lines
Tilden accused Hayes of bribery
Was it true?

36. Election of 1876 Encrypted messages by Tilden supporters later emerged
Cipher: Partial codebook, plus transposition
Codebook substitution for important words
ciphertext plaintext
Copenhagen Greenbacks
Greece Hayes
Rochester votes
Russia Tilden
Warsaw telegram
: :

37. Election of 1876 Apply codebook to original message
Pad message to multiple of 5 words (total length, 10,15,20,25 or 30 words)
For each length, a fixed permutation applied to resulting message
Permutations found by comparing several messages of same length
Note that the same key is applied to all messages of a given length

38. Election of 1876
Ciphertext: Warsaw they read all unchanged last are idiots can’t situation
Codebook: Warsaw  telegram
Transposition: 9,3,6,1,10,5,2,7,4,8
Plaintext: Can’t read last telegram. Situation unchanged. They are all idiots.
A weak cipher made worse by reuse of key
Lesson? Don’t overuse keys!

39. Early 20th Century WWI - Zimmerman Telegram
“Gentlemen do not read each other’s mail”
Henry L. Stimson, Secretary of State, 1929
WWII - golden age of cryptanalysis
Midway/Coral Sea
Japanese Purple (codename MAGIC)
German Enigma (codename ULTRA)

40. Post-WWII History
Claude Shannon - father of the science of information theory
Computer revolution - lots of data to protect
Data Encryption Standard (DES), 70’s
Public Key cryptography, 70’s
CRYPTO conferences, 80’s
Advanced Encryption Standard (AES), 90’s
The crypto genie is out of the bottle…

41. Claude Shannon The founder of Information Theory
1949 paper: Comm. Thy. of Secrecy Systems
Fundamental concepts
Confusion  obscure relationship between plaintext and ciphertext
Diffusion  spread plaintext statistics through the ciphertext
Proved one-time pad is secure
One-time pad is confusion-only, while double transposition is diffusion-only

42. Cryptography Classified according to three independent dimensions:
The type of operations used for transforming plaintext to ciphertext
Substitution
Transposition
Product
The number of keys used
Symmetric (single key or secret- key or private-key)
Asymmetric (two-keys, or public-key encryption)
The way in which the plaintext is processed
Block- a block at a time
Stream- one element at a time

43. Taxonomy of Cryptography
Symmetric Key
Same key for encryption and decryption
Two types: Stream ciphers, Block ciphers
Public Key (or asymmetric crypto)
Two keys, one for encryption (public), and one for decryption (private)
And digital signatures  nothing comparable in symmetric key crypto
Hash algorithms
Can be viewed as “one way” crypto

44. Cryptanalysis Process of attempting to discover the plaintext or key
An encryption scheme is computationally secure if the ciphertext meets one of these criteria:
cost of breaking the cipher exceeds the value of the information
time requires to break the cipher exceeds the useful lifetime of the information

45. Taxonomy of Cryptanalysis
From perspective of info available to Trudy
Ciphertext only
Known plaintext
Chosen plaintext
“Lunchtime attack”
Protocols might encrypt chosen data
Adaptively chosen plaintext
Related key
Forward search (public key crypto)
And others…

46. Cryptanalysis
The process of attempting to discover the plaintext or key
Alan Turing broke the
Enigma Code in WWII

47. Enigma
The Enigma was a wooden box with a keyboard and a bank of lettered lights corresponding to the keys. To encrypt a message, a plaintext character was typed in, and after scrambling, the appropriate light was turned on to give the ciphertext character. See

48. Cryptanalysis Objective to recover key not just message
General approaches:
cryptanalytic attack
brute-force attack
If either succeeds all key use compromised

49. Techniques When only ciphertext is known:
Most difficult problem
Brute force – using all possible keys
Easiest to defend against, since opponent hast least amount of information
When some plain-text is known:
Opponent may identify word patterns, type of file, some context, enabling decoding

50. Cryptanalysis
A brute force approach involves trying every possible key until the translation is obtained.
Some new low cost chips have made this approach more reasonable.
Greatest security problem is maintaining the security of the key
See types of attacks in Stallings
(p.31) summarized on next slides.

51. Cryptanalytic Attacks
ciphertext only
only know algorithm & ciphertext, is statistical, know or can identify plaintext
known plaintext
know/suspect plaintext & ciphertext
chosen plaintext
select plaintext and obtain ciphertext
chosen ciphertext
select ciphertext and obtain plaintext
chosen text
select plaintext or ciphertext to en/decrypt

52. Types of Attacks

53. Average time required for exhaustive key search
Key Size (bits)
Number of Alternative Keys
Time required at 1 decryption/µs
Time required at decryptions/µs 32
232 = 4.3  109
231 µs = 35.8 min.
2.15 milliseconds 56
256 = 7.2  1016
255 µs = 1142 years
10.01 hours
128
2128 = 3.4  1038
2127 µs =  years
5.4  1018 years
168
2168 = 3.7  1050
2167 µs =  years
5.9  1030 years
26 characters (permutation)
26! = 4  1026
2  1026 µs= 6.4  years
6.4  106 years

54. Computationally Secure
An encryption scheme is said to be computationally secure if:
The cost of breaking the cipher exceeds the value of the encrypted information or
The time required to break the cipher exceeds the useful lifetime of the information.

55. Recommended Reading
Stallings, W. Cryptography and Network Security: Principles and Practice, 5th edition. Prentice Hall, 2011
Scneier, B. Applied Cryptography, New York: Wiley, 1996
Mel, H.X. Baker, D. Cryptography Decrypted. Addison Wesley, 2001