# APPLIED CRYPTOGRAPHY PDF

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capable of applying it to new constructions that may not be covered in the book. Throughout the book we present many case studies to survey how deployed. HANDBOOK of. APPLIED. CRYPTOGRAPHY. Alfred J. Menezes. Paul C. van Oorschot bank? Fortunately, the magical mathematics of cryptography can help . Applied Cryptography, Second Edition: Protocols, Algorthms, and Source. Code in C (cloth). (Publisher: John Wiley & Sons, Inc.) Author(s): Bruce Schneier.

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Padlock icon from the Firefox Web browser , which indicates that TLS , a public-key cryptography system, is in use. Symmetric-key cryptosystems use the same key for encryption and decryption of a message, although a message or group of messages can have a different key than others. A significant disadvantage of symmetric ciphers is the key management necessary to use them securely. Each distinct pair of communicating parties must, ideally, share a different key, and perhaps for each ciphertext exchanged as well.

The number of keys required increases as the square of the number of network members, which very quickly requires complex key management schemes to keep them all consistent and secret.

Whitfield Diffie and Martin Hellman , authors of the first published paper on public-key cryptography. In a groundbreaking paper, Whitfield Diffie and Martin Hellman proposed the notion of public-key also, more generally, called asymmetric key cryptography in which two different but mathematically related keys are used—a public key and a private key.

Instead, both keys are generated secretly, as an interrelated pair. In a public-key encryption system, the public key is used for encryption, while the private or secret key is used for decryption. While Diffie and Hellman could not find such a system, they showed that public-key cryptography was indeed possible by presenting the Diffie—Hellman key exchange protocol, a solution that is now widely used in secure communications to allow two parties to secretly agree on a shared encryption key.

Other asymmetric-key algorithms include the Cramer—Shoup cryptosystem , ElGamal encryption , and various elliptic curve techniques. Ellis had conceived the principles of asymmetric key cryptography. Williamson is claimed to have developed the Diffie—Hellman key exchange. Public-key cryptography is also used for implementing digital signature schemes. A digital signature is reminiscent of an ordinary signature ; they both have the characteristic of being easy for a user to produce, but difficult for anyone else to forge.

Digital signatures can also be permanently tied to the content of the message being signed; they cannot then be 'moved' from one document to another, for any attempt will be detectable. In digital signature schemes, there are two algorithms: one for signing, in which a secret key is used to process the message or a hash of the message, or both , and one for verification, in which the matching public key is used with the message to check the validity of the signature.

Digital signatures are central to the operation of public key infrastructures and many network security schemes e.

For example, the hardness of RSA is related to the integer factorization problem, while Diffie—Hellman and DSA are related to the discrete logarithm problem. The security of elliptic curve cryptography is based on number theoretic problems involving elliptic curves.

Because of the difficulty of the underlying problems, most public-key algorithms involve operations such as modular multiplication and exponentiation, which are much more computationally expensive than the techniques used in most block ciphers, especially with typical key sizes.

As a result, public-key cryptosystems are commonly hybrid cryptosystems , in which a fast high-quality symmetric-key encryption algorithm is used for the message itself, while the relevant symmetric key is sent with the message, but encrypted using a public-key algorithm. Similarly, hybrid signature schemes are often used, in which a cryptographic hash function is computed, and only the resulting hash is digitally signed.

Breaking and reading of the Enigma cipher at Poland's Cipher Bureau , for 7 years before the war, and subsequent decryption at Bletchley Park , was important to Allied victory.

It is a common misconception that every encryption method can be broken. In connection with his WWII work at Bell Labs , Claude Shannon proved that the one-time pad cipher is unbreakable, provided the key material is truly random , never reused, kept secret from all possible attackers, and of equal or greater length than the message. In such cases, effective security could be achieved if it is proven that the effort required i.

This means it must be shown that no efficient method as opposed to the time-consuming brute force method can be found to break the cipher. Since no such proof has been found to date, the one-time-pad remains the only theoretically unbreakable cipher. There are a wide variety of cryptanalytic attacks, and they can be classified in any of several ways.

A common distinction turns on what Eve an attacker knows and what capabilities are available.

In a ciphertext-only attack , Eve has access only to the ciphertext good modern cryptosystems are usually effectively immune to ciphertext-only attacks. In a known-plaintext attack , Eve has access to a ciphertext and its corresponding plaintext or to many such pairs.

In a chosen-plaintext attack , Eve may choose a plaintext and learn its corresponding ciphertext perhaps many times ; an example is gardening , used by the British during WWII. In a chosen-ciphertext attack , Eve may be able to choose ciphertexts and learn their corresponding plaintexts. For example, a simple brute force attack against DES requires one known plaintext and decryptions, trying approximately half of the possible keys, to reach a point at which chances are better than even that the key sought will have been found.

But this may not be enough assurance; a linear cryptanalysis attack against DES requires known plaintexts with their corresponding ciphertexts and approximately DES operations. Public-key algorithms are based on the computational difficulty of various problems.

The most famous of these are the difficulty of integer factorization of semiprimes and the difficulty of calculating discrete logarithms , both of which are not yet proven to be solvable in polynomial time using only a classical Turing-complete computer. Much public-key cryptanalysis concerns designing algorithms in P that can solve these problems, or using other technologies, such as quantum computers. For instance, the best known algorithms for solving the elliptic curve-based version of discrete logarithm are much more time-consuming than the best known algorithms for factoring, at least for problems of more or less equivalent size.

Thus, other things being equal, to achieve an equivalent strength of attack resistance, factoring-based encryption techniques must use larger keys than elliptic curve techniques. For this reason, public-key cryptosystems based on elliptic curves have become popular since their invention in the mids.

While pure cryptanalysis uses weaknesses in the algorithms themselves, other attacks on cryptosystems are based on actual use of the algorithms in real devices, and are called side-channel attacks. If a cryptanalyst has access to, for example, the amount of time the device took to encrypt a number of plaintexts or report an error in a password or PIN character, he may be able to use a timing attack to break a cipher that is otherwise resistant to analysis.

An attacker might also study the pattern and length of messages to derive valuable information; this is known as traffic analysis [52] and can be quite useful to an alert adversary. Poor administration of a cryptosystem, such as permitting too short keys, will make any system vulnerable, regardless of other virtues.

Social engineering and other attacks against humans e. Cryptographic primitives Much of the theoretical work in cryptography concerns cryptographic primitives —algorithms with basic cryptographic properties—and their relationship to other cryptographic problems. More complicated cryptographic tools are then built from these basic primitives.

These primitives provide fundamental properties, which are used to develop more complex tools called cryptosystems or cryptographic protocols, which guarantee one or more high-level security properties.

Note however, that the distinction between cryptographic primitives and cryptosystems, is quite arbitrary; for example, the RSA algorithm is sometimes considered a cryptosystem, and sometimes a primitive. Typical examples of cryptographic primitives include pseudorandom functions , one-way functions , etc. Cryptosystems One or more cryptographic primitives are often used to develop a more complex algorithm, called a cryptographic system, or cryptosystem.

Cryptosystems e. Cryptosystems use the properties of the underlying cryptographic primitives to support the system's security properties.

As the distinction between primitives and cryptosystems is somewhat arbitrary, a sophisticated cryptosystem can be derived from a combination of several more primitive cryptosystems. In many cases, the cryptosystem's structure involves back and forth communication among two or more parties in space e.

Such cryptosystems are sometimes called cryptographic protocols. More complex cryptosystems include electronic cash [53] systems, signcryption systems, etc. Some more 'theoretical'[ clarification needed ] cryptosystems include interactive proof systems , [54] like zero-knowledge proofs , [55] systems for secret sharing , [56] [57] etc.

Legal issues See also: Cryptography laws in different nations Prohibitions Cryptography has long been of interest to intelligence gathering and law enforcement agencies.

Because of its facilitation of privacy , and the diminution of privacy attendant on its prohibition, cryptography is also of considerable interest to civil rights supporters. Accordingly, there has been a history of controversial legal issues surrounding cryptography, especially since the advent of inexpensive computers has made widespread access to high quality cryptography possible.

## Applied Cryptography ( Bruce Schneier)

In some countries, even the domestic use of cryptography is, or has been, restricted. Until , France significantly restricted the use of cryptography domestically, though it has since relaxed many of these rules. In China and Iran , a license is still required to use cryptography. Probably because of the importance of cryptanalysis in World War II and an expectation that cryptography would continue to be important for national security, many Western governments have, at some point, strictly regulated export of cryptography.

After World War II, it was illegal in the US to sell or distribute encryption technology overseas; in fact, encryption was designated as auxiliary military equipment and put on the United States Munitions List. However, as the Internet grew and computers became more widely available, high-quality encryption techniques became well known around the globe. Export controls Main article: Export of cryptography In the 's, there were several challenges to US export regulation of cryptography.

Bernstein , then a graduate student at UC Berkeley , brought a lawsuit against the US government challenging some aspects of the restrictions based on free speech grounds.

## Applied cryptography: protocols, algorithms, and source code in C

The case Bernstein v. United States ultimately resulted in a decision that printed source code for cryptographic algorithms and systems was protected as free speech by the United States Constitution. The treaty stipulated that the use of cryptography with short key-lengths bit for symmetric encryption, bit for RSA would no longer be export-controlled.

Since this relaxation in US export restrictions, and because most personal computers connected to the Internet include US-sourced web browsers such as Firefox or Internet Explorer , almost every Internet user worldwide has potential access to quality cryptography via their browsers e.

Many Internet users don't realize that their basic application software contains such extensive cryptosystems. These browsers and email programs are so ubiquitous that even governments whose intent is to regulate civilian use of cryptography generally don't find it practical to do much to control distribution or use of cryptography of this quality, so even when such laws are in force, actual enforcement is often effectively impossible. The technique became publicly known only when Biham and Shamir re-discovered and announced it some years later.

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The entire affair illustrates the difficulty of determining what resources and knowledge an attacker might actually have.

Another instance of the NSA's involvement was the Clipper chip affair, an encryption microchip intended to be part of the Capstone cryptography-control initiative.

Clipper was widely criticized by cryptographers for two reasons. The cipher algorithm called Skipjack was then classified declassified in , long after the Clipper initiative lapsed. The classified cipher caused concerns that the NSA had deliberately made the cipher weak in order to assist its intelligence efforts. Covering the latest developments in practical cryptographic techniques, this new edition shows programmers who design computer applications, networks, and storage systems how they can build security into their software and systems.

What's new in the Second Edition? Reviews "the definitive publicly available text on the theory and practice of cryptography" Computer Shopper, January He is a contributing editor to Dr. He is the author of E-Mail Security Wiley and is a frequent lecturer on cryptography, computer security, and privacy. Free Access.

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Returning user.Forgot password? The written assignment will be to discuss a research paper in one of the topics covered in the course. In such cases, effective security could be achieved if it is proven that the effort required i.

In other words, the letters in the alphabet are shifted three in one direction to encrypt and three in the other direction to decrypt.

Public-key cryptography Main article: Public-key cryptography Public-key cryptography, where different keys are used for encryption and decryption. Just as the development of digital computers and electronics helped in cryptanalysis, it made possible much more complex ciphers.

Protocols for secure computing. Use of asymmetric systems enhances the security of communication.

Some more 'theoretical'[ clarification needed ] cryptosystems include interactive proof systems , [54] like zero-knowledge proofs , [55] systems for secret sharing , [56] [57] etc. Anyway, it was also written by our host Bruce, and contains lots of advice about how to use crypto responsibly.