Cryptography 101

What cryptography can do and what it can't!



Dennis "@the_metalgamer" Fink

18 March 2015

at "cryptoParty: Let's hack towards a better future!" by C3L

Cryptography

  • Combines other scientific disciplines (maths, computer engineering etc...)
  • Multiple subcategories
  • Encryption is one part of it
  • On the one side: Message/Data and Key
  • Key: Think of it as a password
  • Magic: The function that crunches the two inputs together
  • Produces data

Note: There is no 100% security!

  • Everything can be compromised

If you go to Google over an encrypted channel, they still can figure out what you do and who you are

  • Encryption is not equal to anonymity (but you can learn it here how to be anonymous)
  • You often need both

Only as strong as the weakest link in the chain!

  • Example: If you send an encrypted email to your friend and then he prints it out unencrypted, the whole encryption in useless, although people who eavesdropped could not read the message

It is always a trade-off between strength and usability/speed!

There is one encryption algorithm which is 100% secure. It's called a one-time pad. The key should be totally random and really used only once, but for this you had to always generate a new key for transfering a new message and you had to somehow exchange the key with your communication partner.

Types of encryption

  • Symmetric Encryption
    • Problem of how to securely exchange the key
    • Usually faster than asymmetric encryption
  • Asymmetric Encryption
    • One key is called the public key which everyone in the world can have
    • The other one is called the secret key which only you should have
    • Usually slower than symmetric encryption
  • Often we use a combination of both

Encryption paradigms

  • Link encryption
    • Example: Imagine you would like to send a post-card to your mother and you wanted to do it encrypted. With link encryption you would encrypt the message and the address of the reciever with the key provided by the postal service. Then you would put in the letter box. If someone would break the box open could not read your message, but the postal service would need to decrypt it to read the address and then they could also read the message.
  • End-to-end encryption
    • Same example as with link encrytion. But instead of encrypting the message with the key of the postal service, you would encrypt the message with the key you shared with your mother. The address cannot not be encrypted, because the postal service needs access to it. An third party could not read the message, but could read the address
  • You need both
    • First we encrypt the message using end-to-end encryption and then encrypt the complete post-card with Link encryption

Want to know more?

Take the course at KhanAcademy:

Thank you for your attention!



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