# Cryptography

Mind Map by Cher Bachar, updated more than 1 year ago
 Created by Cher Bachar over 6 years ago
177
2

### Description

Computer science (Cryptography) Mind Map on Cryptography, created by Cher Bachar on 06/22/2013.

## Resource summary

Cryptography
1 History
1.1 Book- David Kahn- The code breakers (1996)
1.2 Ciphers
1.2.1 Symmetric Ciphers

Annotations:

• Because both D and E use the same secret key; K
1.2.1.1 E; Encryption algorithm
1.2.1.2 D; Dicription algorithm
1.2.2 1. Substitution ciphers

Annotations:

• substituting one letter with another
1.2.2.1 Solving- cipher text only attack
1.2.2.1.1 frequency of english letters
1.2.2.1.1.1 E: 12.7%
1.2.2.1.1.2 T: 9.1%
1.2.2.1.1.3 A: 8.1%
1.2.2.1.1.4 The rest appear similarly
1.2.2.1.2 frequency of pairs of letters
1.2.2.1.2.1 'he', 'ar', 'in', 'th'
1.2.2.2 size of key space
1.2.2.2.1 26!
1.2.3 Caesar cipher
1.2.3.1 Not a cipher- doesn't have a key
1.2.3.2 A fixed substitution
1.2.3.3 Easy to solve because it's not random
1.2.4 2. Vigener cipher (16th century, Rome)
1.2.4.1 k= cipher text (e.g. crypto) repeated
1.2.4.2 m= message
1.2.4.3 c= k+m
1.2.4.4 Solving
1.2.4.4.1 length of the key, e.g. 6
1.2.4.4.2 break c into groups of 6
1.2.4.4.3 work out the most common letter in the set and substract by E
1.2.5 3. Rotor machines (1870-1943)

Annotations:

• ciphers used to decode an electric mechine
1.2.5.1 Disk rotates by one
1.2.5.2 Enigma machine
1.2.6 4. Data encryption standards; DES (1974)

Annotations:

• Federal law- a standard for encryptions DES: key = 2^56, block size = 64 bits&gt;&gt; currently insecure
1.2.6.1 Today
1.2.6.1.1 AES (2001)
1.2.6.1.2 Salsa20 (2008)
2 What is it?
2.1 Uses
2.1.1 Encryption of files
2.1.2 Protection
2.1.3 User authentication
2.1.4 Limitations
2.1.4.1 Not a solution for all security problems
2.1.4.2 Useless unless implemented correctly
2.2 1. Secret Keys
2.2.1 Single use

Annotations:

• used to encrypt one message
2.2.2 Multiple use

Annotations:

• can be used to encrypt multiple messages- needs more machinery and security
2.3 2. Digital signatures- how to

Annotations:

• how to create a digital signature that will be hard to forge
2.4 3. Anonymous communication
2.4.1 Mix net

Annotations:

• when sending messages they get encrypted in the process, so a person would only be able to tell the output and not the identity of the sender
2.4.2 bidirectional
2.4.3 anonymous digital cash

Annotations:

• Need to prevent double spending-  If someone spends more than once the identity becomes revealed
2.4.4 Secure Multi-party computation
2.4.4.1 'Trusted party'

Annotations:

• So there is a middle party which gets all the data before revealing the output
2.4.4.2 No trusted party

Annotations:

• There is always a way to compute the output without a middle party
2.5 4. Secure communication
2.6 Three steps:
2.6.1 Precisely specific threat model

Annotations:

• What an attacker can do to an encryption &gt;&gt; make it unforgable
2.6.2 Provide a construction
2.6.3 Proving that breaking a construction under threat model will solve an underlying hard problem

Annotations:

• If an attacker can break the construction they can use that to make it harder to break
2.7 Crypto-magic
2.7.1 Privately outsourcing computation

Annotations:

• allows to compute encrypted data- e.g. running a search on google on encryption data
2.7.2 Zero knowledge

Annotations:

• Proving you solved something without providing the solution &gt;&gt; e.g. proving that you know what the prime numbers are of a certain value N-pxq

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