Cryptography Public Key.pdf - 2 INTRODUCTION TO CRYPTOGRAPHY 2.1 WHAT IS CRYPTOGRAPHY The word cryptography comes from the Greek words

Cryptography Public Key.pdf - 2 INTRODUCTION TO...

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39 2 I NTRODUCTION TO C RYPTOGRAPHY 2.1 W HAT I S C RYPTOGRAPHY ? The word cryptography comes from the Greek words κρυπτο ( hidden or secret ) and γραφη ( writing ). Oddly enough, cryptography is the art of secret writing. More generally, people think of cryptography as the art of mangling information into apparent unintelligibility in a manner allow- ing a secret method of unmangling. The basic service provided by cryptography is the ability to send information between participants in a way that prevents others from reading it. In this book we will concentrate on the kind of cryptography that is based on representing information as numbers and mathematically manipulating those numbers. This kind of cryptography can provide other ser- vices, such as integrity checking—reassuring the recipient of a message that the message has not been altered since it was generated by a legitimate source authentication—verifying someone’s (or something’s) identity But back to the traditional use of cryptography. A message in its original form is known as plaintext or cleartext . The mangled information is known as ciphertext . The process for produc- ing ciphertext from plaintext is known as encryption . The reverse of encryption is called decryp- tion . While cryptographers invent clever secret codes, cryptanalysts attempt to break these codes. These two disciplines constantly try to keep ahead of each other. Ultimately, the success of the cryptographers rests on the ciphertext plaintext plaintext encryption decryption
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40 I NTRODUCTION TO C RYPTOGRAPHY 2.1.1 Fundamental Tenet of Cryptography If lots of smart people have failed to solve a problem, then it probably won’t be solved (soon). Cryptographic systems tend to involve both an algorithm and a secret value. The secret value is known as the key . The reason for having a key in addition to an algorithm is that it is difficult to keep devising new algorithms that will allow reversible scrambling of information, and it is diffi- cult to quickly explain a newly devised algorithm to the person with whom you’d like to start com- municating securely. With a good cryptographic scheme it is perfectly OK to have everyone, including the bad guys (and the cryptanalysts) know the algorithm because knowledge of the algo- rithm without the key does not help unmangle the information. The concept of a key is analogous to the combination for a combination lock. Although the concept of a combination lock is well known (you dial in the secret numbers in the correct sequence and the lock opens), you can’t open a combination lock easily without knowing the combination. 2.1.1 Computational Difficulty It is important for cryptographic algorithms to be reasonably efficient for the good guys to compute. The good guys are the ones with knowledge of the keys.
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  • Spring '16
  • Magu
  • Cryptography, Alice, public key cryptography

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