Only the owner of the private key knows it, so text enciphered cannot be read by anyone except the owner of the private key
Only the owner of the private key knows it, so text enciphered with private key must have been generated by the owner
Enciphered letters cannot be changed undetectably without knowing private key
Message enciphered with private key came from someone who knew it
Quintuple (E, D, M, K, C):
Example: Cæsar cipher
The sender and the receiver share a common (secret) key:
Two basic algorithm types:
Rearrange letters in plaintext to produce ciphertext
Example (Rail-Fence Cipher)
HELLO WORLD
HLOOL
ELWRD
Ciphertext is H
LOOL ELWRD
Change characters in plaintext to produce ciphertext
Example (Cæsar cipher)
HELLO WORLD
- Key is 3, usually written as letter ‘D’
Ciphertext is KHOOR ZRUOG
f(c) frequency of character c in ciphertext;
p(x) is frequency of character x in English;
Φ(i) correlation of frequency of letters in ciphertext with corresponding letters in English, assuming key is i;
Φ(i) = Σ0 ≤ c ≤ 25 f(c)p(c – i) so here;
Φ(i) = 0.1p(6 – i) + 0.1p(7 – i) + 0.1p(10 – i) + 0.3p(14 – i) + 0.2p(17 – i) + 0.1p(20 – i) + 0.1p(25 – i).
Key is too short
So make it longer
Like Cæsar cipher, but use a phrase
Example
THE BOY HAS THE BALL
VIG
key
VIGVIGVIGVIGVIGV
plain
THEBOYHASTHEBALL
cipher
OPKWWECIYOPKWIRG
Tableau shown has relevant rows, columns only
Example encipherments:
key V, letter T: follow V column down to T row (giving “O”).
Key I, letter H: follow I column down to H row (giving “P”).
period: length of key
In earlier example, period is 3
tableau: table used to encipher and decipher
Vigènere cipher has key letters on top, plaintext letters on the left
polyalphabetic: the key has several different letters
Cæsar cipher is monoalphabetic
A Vigenère cipher with a random key at least as long as the message
DXQR
. Equally likely to correspond to plaintext DOIT
(key AJIY
) and to plaintext DONT
(key AJDY
) and any other 4 letters;A block cipher:
A product cipher:
Cipher consists of 16 rounds (iterations) each with a round key generated from the user-supplied key.
Considered too weak
Diffie, Hellman said in a few years technology would allow DES to be broken in days.
Design decisions not public
Key space DES = 256 ≈ 7,2056 ·1016
A PC with 500 Mhz that is able to test a key in a clock cycle, needs:
144.115.188 seconds ≈ 834 days ≈ 2 years and 3 months to test 255 ≈ 3,6 ·1016 keys.
4 weak keys
12 semi-weak keys
Complementation property
DESk(m) = c ⇒ DESk‘ (m‘) = c‘
S-boxes exhibit irregular properties;
Distribution of odd, even numbers non-random;
Outputs of fourth box depends on input to third box.
How to cipher text longer than 64 bit?
plaintext x = x1x2…xn (in n blocks of 64 bit).
Ciphered text y = y1y2…yn.
Plaintext x=x1x2…xn (in n blocks of 64 bit).
Ciphered text y = y1y2…yn.
Inizialization vector IV is public.
Plaintext x=x1x2…xn (diviso in n blocchi di 64 bit).
Ciphered text y = y1y2…yn.
1. Course Introduction: Security basic concepts
2. Access Control models: Authentication and authorization mechanisms
6. Role Based Access Control standard (v3)
7. XACML: extensible Access Control Markup Language
8. Authentication Protocols in distributed system
10. Java Authentication and Authorization Service (JAAS)
11. Network security
12. Network security, security protocols: PGP, SSL