1. MSIT 543 Cryptography Concepts and Techniques Symmetric Key Encryption
Dr Jeffrey A Robinson

2. How Encryption Works Substitution Cyphers Codes Keys
See classic cyphers at
Codes
ASCII, Grey binary code, Morse code, EBCDIC ( )
Keys
Keywords and phrases (books etc.)

3. Symmetric Key Encryption
Extracts from
Part 7
Prevent eavesdropping
Or unauthorized access to information

4. Symmetric Key Encryption
Consider Alice and Bob and people who want to securely exchange messages, but Eve is a potential eavesdropper.

5. Symmetric Key Encryption
One way discrete functions

6. Symmetric Key Encryption
First, in open text, Bob and Alice agree on a prime (3) and a modulus to use (Eve can eavesdrop)

7. Symmetric Key Encryption
Next Alice picks a PRIVATE KEY (random number 54) and uses the prime and modulus to generate a result (15) that she sends to Bob (Eve can still eavesdrop)

8. Symmetric Key Encryption
Next Bob picks a random number (24) and generates a result and send the result back to Alice (16). (Eve can still eavesdrop and know both results but neither random number)

9. Symmetric Key Encryption
Bob and Alice now know their own number and the result of the other person.
(Eve only knows the results)

10. Symmetric Key Encryption
And here is the heart of the trick
Alice takes Bob’s result and raises it to the power of her private number, which is the same as the original prime raised to both private numbers

11. Symmetric Key Encryption
This generates a new number which will become the encryption key used by Alice

12. Symmetric Key Encryption
Similarly, Bob takes Alice’s result and raises it to his private number which is the same as the original prime raised to both private numbers

13. Symmetric Key Encryption
Bob and Alice now share a common (symmetric) encryption key that they can use to encrypt messages
Eve cannot know the result (the shared key) without knowing both private numbers

14. Symmetric Key Encryption
Thus Bob and Alice now share a common, symmetric, but completely private key.
Now…consider this with numbers that are 128 or 256 digits long.

15. Symmetric Key Encryption - DES
The Data Encryption Standard (DES, was a predominant symmetric-key algorithm for the encryption of electronic data through the 1970’s.
Developed in the early 1970s at IBM and based on an earlier design by Horst Feistel, the algorithm was submitted to the National Bureau of Standards (NBS) following the agency's invitation to propose a candidate for the protection of sensitive, unclassified electronic government data.

16. Symmetric Key Encryption - DES
However, controversies arose out of classified design elements, a relatively short key length of the symmetric-key block cipher design, and the involvement of the NSA, nourishing suspicions about a backdoor.
DES is now considered to be insecure for many applications. This is chiefly due to the 56-bit key size being too small
In January, 1999, distributed.net and the Electronic Frontier Foundation collaborated to publicly break a DES key in 22 hours and 15 minutes

17. Symmetric Encryption Keys - TDES
In cryptography, Triple DES (3DES) is the common name for the Triple Data Encryption Algorithm (TDEA or Triple DEA) symmetric-key block cipher, which applies the Data Encryption Standard (DES) cipher algorithm three times to each data block.
The original DES cipher's key size of 56 bits was generally sufficient when that algorithm was designed, but the availability of increasing computational power made brute-force attacks feasible. Triple DES provides a relatively simple method of increasing the key size of DES to protect against such attacks, without the need to design a completely new block cipher algorithm.

18. Newer Algorithms
Newer symmetric key encryption algorithms have been developed (using longer keys, thus making them more difficult to crack). These include:
AES
Blowfish