1. Algorithms, Digital Signatures, and Pragmatics
Cryptography
Algorithms, Digital Signatures, and Pragmatics
Frequently, presenters must deliver material of a technical nature to an audience unfamiliar with the topic or vocabulary. The material may be complex or heavy with detail. To present technical material effectively, use the following guidelines from Dale Carnegie Training®.
Consider the amount of time available and prepare to organize your material. Narrow your topic. Divide your presentation into clear segments. Follow a logical progression. Maintain your focus throughout. Close the presentation with a summary, repetition of the key steps, or a logical conclusion.
Keep your audience in mind at all times. For example, be sure data is clear and information is relevant. Keep the level of detail and vocabulary appropriate for the audience. Use visuals to support key points or steps. Keep alert to the needs of your listeners, and you will have a more receptive audience.
Arun A Tharuvai
CSC8530
November 25, 2003

2. Introduction Cryptographic Algorithms Digital Signatures
Cryptography Pragmatics
In your opening, establish the relevancy of the topic to the audience. Give a brief preview of the presentation and establish value for the listeners. Take into account your audience’s interest and expertise in the topic when choosing your vocabulary, examples, and illustrations. Focus on the importance of the topic to your audience, and you will have more attentive listeners.

3. Cryptographic Algorithms
Convert plaintext message M to ciphertext {M}K using an encryption algorithm E, and a key K
E(K,M) = {M} K
Secrecy should lie in the key, not the algorithm
If you have several points, steps, or key ideas use multiple slides. Determine if your audience is to understand a new idea, learn a process, or receive greater depth to a familiar concept. Back up each point with adequate explanation. As appropriate, supplement your presentation with technical support data in hard copy or on disc, , or the Internet. Develop each point adequately to communicate with your audience.

4. Cryptographic Algorithms
Symmetric Algorithms (Secret key)
Same key is used for encryption and decryption, and shared by both parties
Asymmetric Algorithms (Public key)
A public key, used by anyone for encryption
A corresponding private key is used for decryption.

5. Cryptographic Algorithms
Block Ciphers
Operate on fixed-size blocks of data. 64 bits is a common size. Useful for non-realtime data, including and data.
Stream Ciphers
Operate on single bits of data. A stream of bits is used by a function known as a keystream generator. In practice very similar to block ciphers.
CBC mode
Each plain text block is combined with the preceding ciphertext block using XOR before it is encrypted
Prevents similar blocks from encrypting to the same result.

6. Design Techniques Confusion Diffusion
Use of reversible mathematical operations like XOR and bitshifting to combine each block of plaintext with a key
Diffusion
Use of techniques like transposing portions of each plaintext to reduce regular patterns in plaintext

7. Secret-key Algorithms
TEA
DES
IDEA
AES (Rijndael)

8. DES Data Encryption Standard
Designed to be fast in hardware and slow in software. 56-bit key is used to encrypt 64 bit blocks
Bit permutation, combined with 16 rounds of performing the XOR operation with different 48-bit subsets of the key.
Chosen in In 1998, the EFF showed that it was possible to build a machine to crack DES keys in < 3 days for under $250,000, including design costs.
Triple DES (3DES) – applies DES three times using 2 keys, as follows: E3DES(K1,K2,M) = EDES (K1,DDES (K2,EDES (K1,M))) and is effectively as strong as a 112-bit private key. However, it’s very slow.

9. Public-key Algorithms
RSA
Diffie-Helman
El-Gamal
Elliptic curve algorithms

10. RSA Most widely used public key encryption standard.
Based upon the difficulty of factoring the product of two very large numbers.
To generate a key-pair
Choose two large prime numbers, P and Q.
N = P * Q
Z = (P-1)* (Q-1)
d is any number relatively prime to Z.
e is a number, such that e*d = 1 mod Z
The encryption key is e,N and the decryption key is the pair d,N
To encrypt plaintext M, E(e,N,M) = Me mod N
To decrypt ciphertext c, D(d,N,C) = cd mod N

11. Hybrid Protocols SSL/TLS SSH PGP/GPG
Negotiable encryption and authentication algorithms.
SSH
PGP/GPG
Uses RSA to encrypt a secret key which is then used for encrypting a document via IDEA, or 3DES

12. Properties of Digital Signatures
Authentic
Unforgeable
Non-repudiable

13. Digital Signatures
Digital Signing
Digest Functions

14. Public Key Signatures
A computes a digest of M, H(M), and encrypts it with his private key.
A then sends it along with M to B
B then uses A’s K_pub to decrypt

15. Secret Key signatures
Useful when two participants have already agreed upon shared key via a different channel, or public key cryptography.
A concatenates M with K, and computes the digest, H(M+K) = h, sending M,h
B concatenates M with K, H(M+K) = h’ comparing with h. If they’re equal the message was sent by someone with K.

16. Secure digest Functions – properties
Ease of computing hash
Difficulty of generating message from hash
Difficulty of finding another message that maps to the same hash value

17. Secure Digest Functions
MD5
SHA
Symmetric algorithm using CBC

18. Secure Digest Functions
Birthday attack
I am writing {this memo | } to { demand | request | unform you} that {Fred | Mr. Fred Jones} {must | } be { fired | terminated} {at once | immediately}. As the {July 11| 11 July} {memo| memorandum} {from | issued by} {personnel| human resources} states, to meet {our | the corporate} {quarterly | third quarter} budget {targets | goals}, {we must eliminate all discretionary spending | all discretionary spending must be eliminated.}
{Despite | Ignoring } that {memo | memorandum | order }, Fred { ordered | purchased } {PostIts | nonessential supplies} in a flagrant disregard for the company’s {budgetary crisis | current financial difficulties}.

19. Certificate standards, Authorities
X.509
SPKI

20. Cryptography Pragmatics
Performance
Legal Issues
Key sizes and speed

21. Cryptographic Performance
Key/hash size
Speed (kb/s)
Secret
TEA
128
700
DES 56
350
3DES
112
120
IDEA
Public
RSA
512 7
2048 1
Message Digest
MD5
1740
SHA
160
750

22. Legal and Political Issues
Until recently, there were severe restrictions on cryptography, especially export controls, including teaching of such knowledge to foreign nationals in the US.
The FBI and NSA wanted restrictions for ease of decrypting both foreign and domestic communications.
Algorithms approved for export were limited to 40-bit encryption. Easily breakable with modern technology.
Proposals in the mid-90s called for mandatory key-recovery for products exported, and even those used internally.
Currently, export controls now only exist on the T-7 countries.
As of 2000, Cryptographic source code can be exported as long as copies are sent to the Bureau of Industry and Security of Commerce department.

23. Conclusion
The end
Determine the best close for your audience and your presentation. Close with a summary; offer options; recommend a strategy; suggest a plan; set a goal. Keep your focus throughout your presentation, and you will more likely achieve your purpose.

24. References
Charlie Kaufman, Radia Perlman, Mike Speciner, Network Security: Private Communication in a Public World, Prentice Hall, Inc, Upper Saddle River, New Jersey, pp.
Schneier, B., Applied Cryptography, 2nd ed., John Wiley & Sons, New York, 1996, 675 pp.
Electric Frontier Foundation, Cracking DES: Secrets of Encryption Research, Wiretap Politics, and Chip Design, O’Reilly & Associates, Sevastopol, California, 1998, online at
George Coulouris, Jean Dollimore, Tim Kindberg, Distributed Systems: Concepts and Design 3rd ed, Addison-Wesley, New York, 2001, pp