## What is Shannon theorem in cryptography?

Shannon ‘s 1949 definition: A cipher provides perfect secrecy against a ciphertext-only attack if the plaintext and the ciphertext, considered as random variables, are independent. What does “unbreakable” mean? the best guessing rule that he/she would use without having seen the ciphertext.

## What is Shannon secrecy?

Theorem(Shannon perfect secrecy) Suppose a cryptosystem with |K|=|C|=|P| . The cryptosystem has perfect secrecy if and only if. each key is used with equal probability 1/|K| , for every plaintext x and ciphertext y there is a unique key k such that e_k(x)=y .

**What is cipher perfect secrecy?**

A ciphertext maintains perfect secrecy if the attacker’s knowledge of the contents of the message is the same both before and after the adversary inspects the ciphertext, attacking it with unlimited resources. That is, the message gives the adversary precisely no information about the message contents.

### How do you prove a cipher is perfectly secure?

Put formally: Definition 2 If a cryptographic scheme meets the following condition: ∀(m1,m2) ∈ M,∀c ∈ C : P[k ← G(·) : Enc(m1,k) = c] = P[k ← G(·) : Enc(m2,k) = c] Then it is perfectly secure. Note that perfect security is only over a domain of messages.

### What are the two main goals of cryptography?

The Main Goals of cryptography

- Data Privacy(confidentiality)
- Data Authenticity(it came from from where it claims)
- Data integrity(it has not been modified on the way) in the digital world.

**What are the two main goals of cryptography according to Claude Shannon?**

In cryptography, confusion and diffusion are two properties of the operation of a secure cipher identified by Claude Shannon in his 1945 classified report A Mathematical Theory of Cryptography. These properties, when present, work to thwart the application of statistics and other methods of cryptanalysis.

#### What is difference between encryption and decryption?

Encryption is the process of converting normal message (plaintext) into meaningless message (Ciphertext). Whereas Decryption is the process of converting meaningless message (Ciphertext) into its original form (Plaintext). While decryption is the process of converting meaningless message into its original form.

#### How does Vernam cipher work?

The Vernam cipher is, in theory, a perfect cipher. Instead of a single key, each plaintext character is encrypted using its own key. This key — or key stream — is randomly generated or is taken from a one-time pad, e.g. a page of a book. The key must be equal in length to the plaintext message.

**What is Monoalphabetic Cipher example?**

Examples of monoalphabetic ciphers would include the Caesar-shift cipher, where each letter is shifted based on a numeric key, and the atbash cipher, where each letter is mapped to the letter symmetric to it about the center of the alphabet.

## How do I get perfect secrecy?

[Enc(K, m) = c] = pc. In other words, what perfect secrecy requires is that, given a ciphertext, every message in the message space is exactly as likely to be the underlying plaintext. To put it yet another way, the plaintext is independent of the ciphertext.

## Which ciphers are secure?

Currently, the most secure and most recommended combination of these four is: Elliptic Curve Diffie–Hellman (ECDH), Elliptic Curve Digital Signature Algorithm (ECDSA), AES 256 in Galois Counter Mode (AES256-GCM), and SHA384. See the full list of ciphers supported by OpenSSL.