1. IT443 – Network Security Administration Instructor: Bo Sheng
Cryptography Basics
IT443 – Network Security Administration
Instructor: Bo Sheng

2. Outline Basic concepts in cryptography system Secret key cryptography
Public key cryptography
Hash functions

3. Encryption/Decryption
Plaintext: a message in its original form
Ciphertext: a message in the transformed, unrecognized form
Encryption: the process that transforms a plaintext into a ciphertext
Decryption: the process that transforms a ciphertext to the corresponding plaintext
Key: the value used to control encryption/decryption.
plaintext
ciphertext
plaintext
encryption
decryption
key
key

4. Cryptanalysis “code breaking”, “attacking the cipher”
Difficulty depends on
sophistication of the cipher
amount of information available to the code breaker
Any cipher can be broken by exhaustive trials, but rarely practical

5. Caesar Cipher
Replace each letter with the one 3 letters later in the alphabet
ex.: plaintext CAT  ciphertext FDW
plaintext alphabet
ciphertext alphabet A B C D E F G H I J K … A B C D E F G H I J K …
Trivial to break

6. Mono-Alphabetic Ciphers
Generalized substitution cipher: an arbitrary (but fixed) mapping of one letter to another
26! ( 4.0*1026  288) possibilities
plaintext alphabet
ciphertext alphabet A B C D E F G H I J K … A B C D E F G H I J K …

7. Attacking Mono-Alphabetic Ciphers
Broken by statistical analysis of letter, word, and phrase frequencies of the language
Frequency of single letters in English language, taken from a large corpus of text:

8. Ciphertext Only Attacks
Ex.: attacker can intercept encrypted communications, nothing else
Breaking the cipher: analyze patterns in the ciphertext
provides clues about the encryption method/key

9. Known Plaintext Attacks
Ex.: attacker intercepts encrypted text, but also has access to some of the corresponding plaintext (definite advantage)
Makes some codes (e.g., mono-alphabetic ciphers) very easy to break

10. Chosen Plaintext Attacks
Ex.: attacker can choose any plaintext desired, and intercept the corresponding ciphertext
Allows targeted code breaking (choose exactly the messages that will reveal the most about the cipher)

11. The “Weakest Link” in Security
Cryptography is rarely the weakest link
Weaker links
Implementation of cipher
Distribution or protection of keys
… …

12. Secret Keys vs Secret Algorithms
Security by obscurity
We can achieve better security if we keep the algorithms secret
Hard to keep secret if used widely
Reverse engineering, social engineering
Publish the algorithms
Security of the algorithms depends on the secrecy of the keys
Less unknown vulnerability if all the smart (good) people in the world are examine the algorithms

13. Outline Basic concepts in cryptography system Secret key cryptography
Public key cryptography
Hash functions

14. Secret Key Cryptography
Same key is used for encryption and decryption
Also known as
Symmetric cryptography
Conventional cryptography
plaintext
ciphertext
plaintext
encryption
decryption
key
Same key
key

15. Secret Key Cryptography
Stream cipher
Block cipher
Converts one input plaintext block of fixed size k bits to an output ciphertext block of k bits
DES, IDEA, AES, …
AES
Selected from an open competition, organized by NSA
Joan Daemen and Vincent Rijmen (Belgium)
Block size=128 bits, Key Size= 128/192/256 bits

16. Key Size
Keys should be selected from a large potential set, to prevent brute force attacks
Secret key sizes
40 bits were considered adequate in 70’s
56 bits used by DES were adequate in the 80’s
128 bits are adequate for now
If computers increase in power by 40% per year, need roughly 5 more key bits per decade to stay “sufficiently” hard to break

17. Public Key Cryptography
A public/private key pair is used
Public key can be publicly known
Private key is kept secret by the owner of the key
Much slower than secret key cryptography
Also known as asymmetric cryptography
Another mode: digital signature
plaintext
ciphertext
plaintext
encryption
decryption
Public key
Private key

18. Public Key Cryptography
Digital signature
Only the party with the private key can create a digital signature.
The digital signature is verifiable by anyone who knows the public key.
The signer cannot deny that he/she has done so.
plaintext
ciphertext
plaintext
Sign
Verify
Private key
Public key

19. Public Key Cryptography
It must be computationally
easy to generate a public / private key pair
hard to determine the private key, given the public key
easy to encrypt using the public key
easy to decrypt using the private key
hard to recover the plaintext message from just the ciphertext and the public key

20. Symmetric vs Asymmetric
Symmetric algorithms are much faster
In the order of a 1000 times faster
Symmetric algorithms require a shared secret
Impractical if the communicating entities don’t have another secure channel
Both algorithms are combined to provide practical and efficient secure communication
E.g., establish a secret session key using asymmetric crypto and use symmetric crypto for encrypting the traffic

21. Outline Basic concepts in cryptography system Secret key cryptography
Public key cryptography
Hash functions

22. Hash Function Also known as
Message digest
One-way transformation
One-way function
Hash
Length of H(m) much shorter than length of m
Usually fixed lengths: 128 or 160 bits
Message of
arbitrary length
A fixed-length
short message
Hash

23. Properties of Hash Consider a hash function H
Performance: Easy to compute H(m)
One-way property: Given H(m) but not m, it’s computationally infeasible to find m
Weak collision resistance (free): Given H(m), it’s computationally infeasible to find m’ such that H(m’) = H(m).
Strong collision resistance (free): Computationally infeasible to find m1, m2 such that H(m1) = H(m2)

24. Hash Applications File / Message integrity
Check if a downloaded file is corrupted
Detect if a file has been changed by someone after it was stored
Compute a hash H(F) of file F
openssl dgst -md5 filename

25. Hash Applications Password verification
Password cannot be stored in plaintext
In a hashed format
Linux: /etc/passwd, /etc/shadow
cat /etc/shadow

26. Hash Applications User authentication Alice Bob
Alice wants to authenticate herself to Bob
Assuming they already share a secret key K
Alice
Bob
time 
“I’m Alice” R
computes Y=H(R|K) Y
verifies that Y=H(R|K)

27. Modern Hash Functions MD5 (128 bits) SHA (Secure Hash Algorithm)
Previous versions (i.e., MD2, MD4) have weaknesses.
Broken; collisions published in August 2004
Too weak to be used for serious applications
SHA (Secure Hash Algorithm)
Weaknesses were found
SHA-1 (160 bits)
Broken, but not yet cracked
Collisions in 269 hash operations, much less than the brute-force attack of 280 operations
Results were circulated in February 2005, and published in CRYPTO ’05 in August 2005
SHA-256, SHA-384, …

28. Birthday Attack What is the smallest group size k such that
The probability that at least two people in the group have the same birthday is greater than 0.5? 23
Implication for hash function H of length m
With probability at least 0.5
If we hash about 2m/2 random inputs,
Two messages will have the same hash image
m=64, 1ns per hash
Brute force (264): 1013 seconds over 300 thousand years
Birthday attack (232): 4 seconds

29. Lab 1 Sample codes eecs.mit.edu’s IP is 18.62.1.6
Assume their subnetwork use 28-bit prefix
Scan ~
dig -x short
/home/shengbo/it443/scanip.sh
/home/shengbo/it443/scanip.pl

30. Electronic Code Book (ECB)
128
M M M M4
46 + padding
Plaintext 
Key E
C C C C4
128
Ciphertext 

31. Cipher Block Chaining (CBC)
M1 M2 M3 M4
46 + padding
128
128
128
Initialization
Vector
C1 C2 C3 C4
128 E
Key