Security Flashcards
Symmetric key cryptography
sender n receiver keys identical
public key cryptography
encryption key is public
decryption key is private
block ciphers / transposition ciphers
the plaintext is divided into fixed-size blocks and encrypted one block at a time
DES (Data Encryption Standard)
-symmetric key
-64-bit block size
-56-bit key
-19 stages
(3 stages of transposition)
(16 stages of:
Exclusive ORing with a key (derived from input key)
substitution function
transposition function,
permutation
AES (Advanced Encryption Standard)
- symmetric key
- safer than DES
- 128-bit block size
- more bits in key
- brute force would take trillions of years
IDEA (International Data Encryption Algorithm)
- symmetric key
- 64-bit block size
- 128-bit keys
-8-iteration encryption iterations are: addition multiplications exclusive ORing permutation using different key derived from input key
RSA steps (6)
Public key
SLOW
1) Choose 2 "large" primes: 'p' and 'q' 2) Let n = p*q Let z = (p-1) * (q-1) 3) Let 'e' < n, relatively prime to z 4) Find a number d s.t: e*d = 1 mod z (e*d / z = ? + 1) find d.
5) public key = (e,n)
6) private key = (d, n)
A is relatively prime to Z
A and Z share no common positive factors except for 1.
public key: (e,n)
private key: (d,n)
How to encrypt plaintext block M < n to a Ciphertext block C?
C = M^e mod n
public key: (e,n)
private key: (d,n)
How to decrypt a ciphertext block C < n to a plaintext block M?
M = C^d mod n
How to provide confidentiality AND authentication?
Alice has private key and wants to send to Bob
1) Alice encodes message with private key (Anyone w/ public key can decode this)
2) Alice encodes THIS with public key (Only someone w/ private key can decode this)
3) Bob decodes message with private key
4) Bob decodes THIS with public key. If he gets the message, then it was Alice who sent it (because only she could have encoded something w/ her private key)
Hash function H
-Accepts a message M of any size and produces a fixed size block H(M) [digest of M]
Hash function must be: (3)
- Computationally efficient
- For any digest d, it must be computational infeasible to find an M’ s.t. H(M’) = d
- For any message M, it must be computationally infeasible to find M’ s.t. H(M’) = H(M)
Message Integrity using a Hash Function and Public Key
Encrypt digest with private key.
Decrypt digest with public key
So only small piece of data is using RSA, better performance
Key Distribution Center (KDC)
shares different secret key with each registered user.
Alice and Bob know their own symmetric keys for communicating with KDC
-KDC sends Alice a session key (R1) AND R1 encrypted with Bob’s key
Bob decrypts R1 and knows to use that to communicate with Alice
