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What is Scapy

Scapy is a phyton utility to send, sniff, dissect (aufgliedern) and forge (fälschen) ip packets. It’s used to:

  • to create tools that can probe, scan or attack networks
  • ist very powerfool for interactive packet manipulation
  • used to create attack signatures for IDS/IPS systems
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Whats the idea of Cypertext

A cryptographic algorithm is used to encrypt a plaintext message

           

Message, Plaintext --> [Encryption] -- Ciphertext --> [Decryption] --> Message, Plaintext

                                            ^----------------key-------------------^

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What is Kerckhoffs's Princple

A cryptographic system must be secure even if everything about the system, except the key, is public knowledge.

  • Algorithms are known public, anyone can text them
  • «The enemy knows the system»
  • Public exposure may help to find weaknesses more quickly
  • Most believe in this principle, but some also think it’s saver to keep the key and the algorithm secret
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What is a SP-Network

Uses repeated substitution (Replacing bytes with others) and permutation (Swapping bytes around) operations. Repeated for multiple rounds.

wikipedia: https://en.wikipedia.org/wiki/Substitution%E2%80%93permutation_network

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How does the Caesar cipher work

To encrypt a message each letter of the alphabet is shift three letters to the right. It is a substitution cipher that is mono-alphabetic.

 

Example 

Here’s an example of the Caesar cipher in action.

Khuh’v dq hadpsoh ri wkh Fdhvdu flskhu lq dfwlrq.

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How does XOR work and why is it interessting for cryptographics

A function which takes 2 inputs returns true if one of the inputs is true and the other is false.

e.g.: 

A B O

0  0  0

0  1  1

1  0  1

1  1  0

 

doing the process again with A and O gives back B again. This is very usefull, because this process can be thought of as A encrypting and decrypting B.

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What is the one time pad

The one time pad uses XOR to encrypt and decrypt a message. 

+ once the key is gone there is no way using statistics to retreive the original message

- the size of the key is the same as the file e.g. 1 GB file = 1GB key

- if a key is used more than once, the cipher is broken, because it can be reversed then

 

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What is a stream cipher and which advanteages and disadvantages does it have

The idea is to create a one time pad by generating an infinite pseudo random-keystream

  • Encryption of long continuous streams possible
  • Extremely fast with low memory footprint à ideal for low-power devices
  • If designed well it can seek (suchen, ermitteln) to any location in the stream
  • The keystream must appear statistically random
  • A key + nonce must never be reused
  • Stream ciphers do not protect the ciphertext (no guaranteed integrity)