B FOR 204 Lecture Notes - Lecture 9: Public Key Infrastructure, Digital Signature, Public-Key Cryptography
2 Jun 2018
School
Department
Course
Professor
BFOR 204
Fundamentals Information and Cybersecurity
Cryptography - II
One-way Functions
• Easy to compute, hard to reverse
• Example: f (A) = YA (mod p) f -1(YA) is called “discrete log”
• Hard to compute
• Could always do exhaustive search to find p and Y
• Here, there are p -1 choices
Symmetric Encryption/Decryption Model
Requirements:
• There are two requirements for secure use of symmetric encryption:
o A strong encryption algorithm
o Sender and receiver must have obtained copies of the secret key in a secure
fashion and must keep the key secure
• The security of symmetric encryption depends on the secrecy of the key, not the
secrecy of the algorithm
o This makes it feasible for widespread use
o Manufacturers can and have developed low-cost chip implementations of
data encryption algorithms
o These chips are widely available and incorporated into a number of products
Symmetric Key Distribution using symmetric encryption
• For symmetric encryption to work, the two parties to an exchange must share the
same key, and that key must be protected from access by others
• Frequent key changes are usually desirable to limit the amount of data compromised
if an attacker learns the key
• Key distribution technique
o The means of delivering a key to two parties that wish to exchange data,
without allowing others to see the key
Problems with Diffie Hellman Key Exchange and Symmetric Encryption
• If you have 1000 friends with whom you wish to exchange secret messages, then
you will need to first exchange keys with each of the 1000 friends and then
exchange messages.
• You will need to remember 1000 secret keys!
• Not very feasible.
• Symmetric Encryption and Diffie Hellman Key Exchange work the best when the
number of parties involved are fewer.
o Fewer keys to exchange
o More efficient
Asymmetric algorithms
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