Cyber Security
Prepared by;
Dr. Eng. Mohamed Saeid Shalaby
Data Security
Plain text - the original message.
Cipher text - the coded message.
Cipher - algorithm for transforming plain text to cipher text.
Key - info used in cipher known only to sender/receiver.
Encipher (encrypt) - converting plaintext to cipher text.
Decipher (decrypt) - recovering plain text from cipher text.
Cryptography - study of encryption principles/methods.
Cryptanalysis (code breaking) - the study of principles/ methods
of deciphering cipher text without knowing key.
Cryptology - the field of both cryptography and cryptanalysis.
Cryptography
Number of keys used
Hash functions: no key
Secret key cryptography: one key
Public key cryptography: two keys - public, private
Type of encryption operations used
Substitution / Transposition / Product
Way in which plaintext is processed
Block / Stream
Brute Force Search
• Always possible to simply try every key.
• Most basic attack, proportional to key size.
• Assume either know / recognise plaintext.
Key Size (bits) Number of Time required at 1 Time required at 106
Alternative Keys decryption/µs decryptions/µs
32 232 = 4.3 109 231 µs = 35.8 minutes 2.15 milliseconds
56 256 = 7.2 1016 255 µs = 1142 years 10.01 hours
128 2128 = 3.4 1038 2127 µs = 5.4 1024 5.4 1018 years
years
168 2168 = 3.7 1050 2167 µs = 5.9 1036 5.9 1030 years
years
26 characters 26! = 4 1026 2 1026 µs = 6.4 1012 6.4 106 years
(permutation) years
Symmetric Cipher Model
Private Cipher Model
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)
• Assume encryption algorithm is known.
• Implies a secure channel to distribute key.
Classical Substitution Ciphers
Letters of plaintext are replaced by other letters or by
numbers or symbols
Plaintext is viewed as a sequence of bits, then
substitution replaces plaintext bit patterns with cipher
text bit patterns
Caesar Ciphers
Earliest known substitution cipher.
Replaces each letter by 3rd letter on.
Example:
meet me after the toga party
PHHW PH DIWHU WKH WRJD SDUWB
Caesar Ciphers
Define transformation as:
abcdefghijklmnopqrstuvwxyz
DEFGHIJKLMNOPQRSTUVWXYZABC
Mathematically give each letter a number
abcdefghijk l m
0 1 2 3 4 5 6 7 8 9 10 11 12
n o p q r s t u v w x y Z
13 14 15 16 17 18 19 20 21 22 23 24 25
Then have Caesar cipher as:
C = E(p) = (p + k) mod (26)
p = D(C) = (C – k) mod (26)
Monoalphabetic Cipher
Rather than just shifting the alphabet.
Could shuffle (jumble) the letters arbitrarily.
Each plaintext letter maps to a different random cipher
text letter.
Key is 26 letters long;
Plain: abcdefghijklmnopqrstuvwxyz
Cipher: DKVQFIBJWPESCXHTMYAUOLRGZN
Plain text: ifwewishtoreplaceletters
Cipher text: WIRFRWAJUHYFTSDVFSFUUFYA
Disadvantages
Now have a total of 26! = 4 x 1026 keys.
Is that secure?.
Problem is language characteristics.
Human languages are redundant.
Letters are not equally commonly used.
Note that all human languages have varying letter frequencies, though the
number of letters and their frequencies varies.
One-Time Pad
If a truly random key as long as the message is used, the
cipher will be secure - One-Time pad.
E.g., a random sequence of 0’s and 1’s XORed to plaintext,
no repetition of keys.
Unbreakable since cipher text bears no statistical
relationship to the plaintext.
For any plaintext, it needs a random key of the same length
- Hard to generate large amount of keys.
Have problem of safe distribution of key.
Thank you very much
