Shannon Security(Perfect Secrecy)
Ciphertext should reveal no "infomation" about Plaintext:
The resulting ciphertext will be impossible to deccrypt or break if the following four conditions are met:
- The key must at least as long as the plaintext.
- The key must be random.
- The key must never be reused in whole or in part.
- The key must be kept completely secret by the communicating parties.
In cryptography, the one-time pad (OTP) is an encryption technique that cannot be cracked, but requires the use of a single-use pre-shared key that is not smaller than the message being sent.
Pseudo Random Number Generator(PRNG)
- an algorithm for generating a sequence of numbers whose properties approximate the properties of sequences of random numbers.
- The same seed always gives the same output stream
Requirements for Cryptographic Hash Functions
- preimage resistant
if given it is computationally infeasible to find a value s.t.
- 2-nd preimage resistant
if given it is computationally infeasible to find a value , s.t. and
- collision resistant
if it is computationally infeasible to find two distinct values , s.t.
Usages of Cryptographics Hash Functions
- Software integrity
- Message authentication
- One-time passwords
- Digital signature
The level of security of a hash function that output n bits, is about n/2 bits.
Message Authentication Code(MAC)
- A MAC scheme is a hash family, used for message authentication
- The sender and the receiver share secret
- The sender sends
- The receiver receives and verifies that , if so, then accepts the message as from the sender