What is the difference between an unconditionally secure cipher and a computationally secure cipher?

• Ciphertext only. One possible attack under these circumstances is the brute-force approach of trying all possible keys. If the key space is very large, this becomes impractical. Thus, the opponent must rely on an analysis of the ciphertext itself, generally applying various statistical tests to it.  
• Known plaintext. The analyst may be able to capture one or more plaintext messages as well as their encryptions. With this knowledge, the analyst may be able to deduce the key on the basis of the way in which the known plaintext is transformed.  
• Chosen plaintext. If the analyst is able to choose the messages to encrypt, the analyst may deliberately pick patterns that can be expected to reveal the structure of the key.

1. the cost of breaking the cipher exceeds the value of the encrypted information, and
2. the time required to break the cipher exceeds the useful lifetime of the information.

1. There is the practical problem of making large quantities of random keys. Any heavily used system might require millions of random characters on a regular basis. Supplying truly random characters in this volume is a significant task. 
2. Even more daunting is the problem of key distribution and protection. For every message to be sent, a key of equal length is needed by both sender and receiver. Thus, a mammoth key distribution problem exists.

A cipher can be said to be computationally secure if it cannot be cracked in 'reasonable time'. This is a concept that relates to how long it will take a computer to carry out a task.

It is said that a cryptosystem is computationally secure when the computational requirements become so large that it becomes practically infeasible to break their cipher in a reasonable time period. Second, the cryptosystem relies on mathematical problems that are assumed to be difficult to solve.

The definition of an unconditionally secure cryptosystem states that the cryptosystem cannot be broken even with infinitely computational ressources and time. However, since most books define the keyspace K to be finite, then with infinite time any computational device can perform an exhaustive keysearch.

AES is the current encryption standard (Daemen and Rijmen, 2002), but it is also reported to be vulnerable to algebraic attacks (Courtois and Pieprzyk, 2002). Therefore the unconditional security (theoretical security, perfect secrecy and perfect security), of cryptosystem will become more and more important.

Brute-force attack:

The attacker tries every possible key on a piece of ciphertext until an intelligible translation into plaintext is obtained. On average, half of all possible keys must be tried to achieve success.

Philip Zimmermann is the creator of Pretty Good Privacy (PGP), an email encryption program that was made available to the public via FTP download. Originally designed as a human rights tool, PGP became the most widely used email encryption software in the world.

A differential cryptanalysis attack is a type of chosen plaintext attack on block ciphers that analyzes pairs of plaintexts rather than single plaintexts, so the analyst can determine how the targeted algorithm works when it encounters different types of data.

An encryption scheme is said to be computationally secure if: the cost of breaking the cipher exceeds the value of the encrypted information, and. the time required to break the cipher exceeds the useful lifetime of the information.Jan 17, 2017

Cryptanalysis is the process of studying cryptographic systems to look for weaknesses or leaks of information.

Also next observation can be presented here that a public key cryptosystem can never provide unconditional security. This is because an opponent, on observing a ciphertext y, can encrypt each possible plaintext in turn using the public key until he finds the unique x such that y = eK(x).

Block ciphers have the advantage of high diffusion and strong tamper resistance without detection. They have the disadvantage of slower encryption speed since the entire block must be captured for encryption/decryption. Block ciphers also breed errors since a mistake in just one symbol could alter the whole block.

OTPs are provably unconditionally secure, this means that you can not break them with any amount of compute time. It is mathematically impossible to break a OTP. ... This is due to convenience, in order to use a OTP you have to trade pads with each person you wish to communicate with.

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.

The basic idea of the information theoretic approach to securely transmit conferential messages (without using an encryption key) to a legitimate receiver is to use the inherent randomness of the physical medium (including noises and channel fluctuations due to fading) and exploit the difference between the channel to ...

A block cipher is an encryption method that applies a deterministic algorithm along with a symmetric key to encrypt a block of text, rather than encrypting one bit at a time as in stream ciphers. For example, a common block cipher, AES, encrypts 128 bit blocks with a key of predetermined length: 128, 192, or 256 bits.Dec 19, 2014

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