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BRIEF INTRODUCTION TO CRYPTOGRAPHY
By PAGVAC
February 8, 2004

What will I learn from this file?
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What cryptography is
How encryption and decryption works
Cryptography terms
Symmetric cryptography
Asymmetric cryptography
PGP

Brief Introduction to Cryptography
Cryptography is the art and science of keeping messages secure
...
Basically, the point of cryptography is to allow any user to keep his
data secure and not readable from not desired individuals
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In order to
fully understand some cryptography-related terms, we are going to use the following
example
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Suppose that the information you are sending him is quite sensitive and it is extremely
important to you that only your boss gets to read the message
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This text file at this point is readable and
therefore unsecured
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So now you need to
make the plaintext file unreadable
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The cyphertext file cannot be read and it looks as a
sequence of non-sense characters
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After you email the cyphertext file to you boss then he would decrypt it, which means
that he would convert it back to its plaintext form so he could read it
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Plaintext

Figure 1

Encryption

Ciphertext

Decryption

Plaintext

Now you must be wondering why would only your boss decrypt your ciphertext file
and not someone else
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In order for only a specific person to be able to decrypt a file a key
is used
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Only the people who own the key can
open the door, and at the same time you keep you key hidden from other people so that no
intruders get into your house
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This key is just a
sequence of characters such as “firebird” or “this is my key”
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Key
Plaintext

Encryption

Key
Ciphertext

Decryption

Plaintext

Figure 2
The key you choose is really up to you, although you should know that the longer your
key is, the harder it will be for a cryptanalyst (a ciphertext breaker) to decrypt the file
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There are all sorts of encryption algorithms out there, although this subject is
really outside the scope of this paper
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In a symmetric algorithm the key used to encrypt and decrypt a certain
message is the same in both cases
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The example discussed at the beginning of this paper uses a
symmetric algorithm to encrypt and decrypt the text file sent by email
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If
anyone had access to the key, he could decrypt and encrypt the message with no problem
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As you
can guess from the term “asymmetric” now each of the two parties have a different key
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The first one
is used to encrypt the message, whereas the private key is used to decrypt it
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However, the private key should only be known by whoever is supposed to decrypt the
message
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We will
use your boss again
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So you talk to him about asymmetric cryptography and he likes the concept
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He uses this key to decrypt
messages that his employees send to him
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Any of his employees will be
able to encrypt a message in order to send it to their boss, but only their boss will be able
to decrypt it and therefore be able to read the message, since only he knows the private
key
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Which tool to choose is really up to you
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You can find many different versions of PGP at
http://www
...
org/
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PGP Desktop has mainly three parts: PGPkeys, PGPmail, and
PGPdisk
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With PGPmail you can encrypt email messages
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There are
also other secondary tasks you can perform with PGP Desktop such as securing your ICQ
communications, completely deleting files (Windows by itself does not truly delete files),
and create self-decrypting archives

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