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# One way functions MD5 algorithm

MD5

MD5 is an improved version of MD4. Although more complex than MD4, it is similar in design and also produces a 128-bit hash.

Description of MD5

After some initial processing, MD5 processes the input text in 512-bit blocks, divided into 16 32-bit sub-blocks. The output of the algorithm is a set of four 32-bit blocks, which concatenate to form a single 128-bit hash value.

First, the message is padded so that its length is just 64 bits short of being a multiple of 512. Thispadding is a single 1-bit added to the end of the message, followed by as many zeros as are required. Then, a 64-bit representation of the message’s length (before padding bits were added) is appended to the result. These two steps serve to make the message length an exact multiple of 512 bits in length (required for the rest of the algorithm), while ensuring that different messages will not lookь the same after padding.

The main loop of the algorithm begins.MD5 has four rounds of 16 operations each.This loop continues for as many 512-bit blocks as are inthe message. The four variables are copied into different variables: a gets A, b gets B, c gets C, and d gets D. The main loop has four rounds (MD4 had only three rounds), all very similar. Each round uses a different operation 16 times. Each operation performs a nonlinear function on three of a, b, c, and d. Then it adds that result to the fourth variable, a sub-block of the text and a constant. Then it rotates that result to the right a variable number of bits and adds the result to one of a, b, c, or d. Finally the result replaces one of a, b, c, or d.

Security of MD5

The improvements of MD5 over MD4:

1. A fourth round has been added.

2. Each step now has a uniqe additive constanty

3. The function G in round 2 was changed from((X┴Y)*( X┴Z)*( Y┴Z)) TO((X┴Z)* (Y┴Z)) to make G less symmetric

4. Each step now adds in the result of the previous step. This promotes a faster avalanche effect

5. The order in which message sub-blocks are accessed in rounds 2 and 3 is changed, to make these patterns less alike

6. The left circular shift amounts in each round have been approximately optimized, to yield a faster avalanche effect.

One way functions SHA algorithm. SHA-256, SHA-384, and SHA-512.

Secure Hash Algorithm (SHA)

The Secure Hash Algorithm (SHA) for use with the Digital Signature Standard.According to the Federal Register. This proposed standard specified a Secure Hash Algorithm (SHA) for use with the proposed Digital Signature Standard. Additionally, for applications not requiring a digital signature, the SHA is to be used whenever a secure hash algorithm is required for Federal applications.

Description of SHA

First, the message is padded to make it a multiple of 512 bits long. Padding is exactly the same as in MD5: First append a one, then as many zeros as necessary to make it 64 bits short of a multiple of 512, and finally a 64-bit representation of the length of the message before padding.   SHA-256 SHA-512 SHA-384

SHA-384 is defined in the exact same manner as SHA-512 with the following two

exceptions:

(1) The initial hash value H(0) is based on the fractional parts of the square roots of the ninth through sixteenth primes.

(2) The final 384-bit hash is obtained by truncating the SHA-512-based hash output to its left-most 384 bits.

It is possible to use a symmetric block cipher algorithm as a one-way hash function. The idea is that if the block algorithm is secure, then the one-way hash function will also be secure.

The general scheme is as follows   1. Security methods. Kerckhoff’s Principle

3. Attack. . Type of attacks. Threats to computer systems. Threat Models. Threat Trees

4. Basic methods of encryption. Classification of encryption methods. Examples of simple methods.

5. Caesar’s cipher generalized. Other monoalphabetic ciphers. The problems.

7. Polyalphabetic ciphers. Vigenere Tableau.

8.

9. New standard. AES. Mathematical basic. Algorithm. Key Schedule

10. Symmetric cryptosystem. DES

11. Symmetric cryptosystem. 3DES

12. Symmetric cryptosystem. AES

13. Block Cipher Modes

14. Stream Ciphers. PRG

15. Stream Ciphers. RC4

16. Public key crypto. RSA

17. Public key crypto. Diffie-Hellman

18. Linear Cryptoanalysis. Example

19. Differential Cryptoanalysis. Example

20. Protocols. Definitions. Rules of communication. Types of protocols. Problems.

21. Secure Protocols. Three types of Protocol

22. Secure Elections. Simplistic Protocol #1,2

23. Secure Elections. Voting with Blind Signatures

24. Secure Elections. Election with two organization

25. Digital Cash Protocol

26. Key management.. Certification problem. Certificate

27. Authority. X.509. Certificate Hierarchy

28. PGP. Key Management in PGP. PGP’s Web of Trust. IDEA. Key Schedule. Standard ANSI X9.17. Working with PGP

29. Protocols. Digital Cash. Examples. Key management

30. One way functions. Properties. Collision-resistance. Example.

31. One way functions MD5 algorithm.

32. One way functions SHA algorithm. SHA-256, SHA-384, and SHA-512.

33. One way functions. Using symmetric algorithms

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