Basic Cryptography

Piyush P Kurur

March 9, 2015

Cryptography

What is Cryptography?

  • Sending secret message

  • Authenticating messages

Secret key Cryptography

  • Both parties need to share a secret

  • Usually fast

  • Unfortunately not very easy to setup.

Public key methods

  • A public key known to every one.

  • A secret key known only to the you.

  • Encryption can be done using the public key

  • Decryption needs the private key.

Known Public key algorithms

  • RSA

  • Elliptic curve based ones.

Digital signature.

  • Easy to verify the signature

  • Impossible to generate S without secret key

Integrity checking

Problem

  • A large database D, or a collection of files

  • How can I check if the data is not modified

Cryptographic hashes

  • Given a string X, h(X) is a small string

  • Given h(X) impossible to find Y such that h(Y) = h(X)

  • Impossible to find X≠Y such that h(X) = h(Y).

For integrity checking

  • You can compute the hash

  • Digitally sign it and send it.

Gnu Privacy Guard

Concepts

  • Master keys

  • Sub keys

  • Fingerprints

  • Revocation

Exporting and importing

  • To send your key you can export it

  • You can import the public key of others