Illustration of "Cryptology"

Cryptology embraces two branche: Cryptanalysis & Cryptography

6 mains concepts of cryptography:

  • Confidentiality/Secrecy: Only the destination can read the message
  • Authentication: The destination can be sure the message is sent by the sender
  • Integrity: The message cannot be changed during the transmission
  • Source nonrepudiation: The source cannot deny sending the message
  • Destination nonrepudiation: The destination cannot deny receiving the message
  • Signatures: Third parties can be sure that the message is sent by the sender

Cryptology embraces two branche: Cryptanalysis & Cryptography

6 mains concepts of cryptography:

  • Confidentiality/Secrecy: Only the destination can read the message
  • Authentication: The destination can be sure the message is sent by the sender
  • Integrity: The message cannot be changed during the transmission
  • Source nonrepudiation: The source cannot deny sending the message
  • Destination nonrepudiation: The destination cannot deny receiving the message
  • Signatures: Third parties can be sure that the message is sent by the sender

Old Ciphers

Caesar

Each character of the message is shifted with a key (size 1) (if key = 1, then A-> B).

General Substitution Cipher

Each character is replaced by an other (substitute a character to another).

Vigenère Cipher

Each character is shifted using a key (size N). (if key = 123, then ABCD -> BDFE)

One-time pad

The Vigenère Cipher can be improved to be unbreakable if the key is random, used one time, and as long as the message.

Security

Unconditional security

The ciphertext provides no information to the adversaries (no stats, ...). Whatever the configuration. Not a realist goal nor interesting.

Computational security

  • The cost of cracking the ciphertext must exceed the value of the plaintext.
  • or The time required to break the ciphertext must exceed the lifetime of the plaintext.
  • or The chance of breaking the cipher can be made small by increasing the key size