Basics of Cryptography

The Basics of Cryptography

When Julius Caesar sent messages to his generals, he didn't trust his

messengers. So he replaced every A in his messages with a D, every B with an

E, and so on through the alphabet. Only someone who knew the “shift by 3”

rule could decipher his messages.

And so we begin.

Encryption and decryption

Data that can be read and understood without any special measures is called

plaintext or cleartext. The method of disguising plaintext in such a way as to

hide its substance is called encryption. Encrypting plaintext results in

unreadable gibberish called ciphertext. You use encryption to ensure that

information is hidden from anyone for whom it is not intended, even those

who can see the encrypted data. The process of reverting ciphertext to its

original plaintext is called decryption.

What is cryptography?

Cryptography is the science of using mathematics to encrypt and decrypt data.

Cryptography enables you to store sensitive information or transmit it across

insecure networks (like the Internet) so that it cannot be read by anyone except

the intended recipient.

While cryptography is the science of securing data, cryptanalysis is the science

of analyzing and breaking secure communication. Classical cryptanalysis

involves an interesting combination of analytical reasoning, application of

mathematical tools, pattern finding, patience, determination, and luck.

Cryptanalysts are also called attackers.

Cryptology embraces both cryptography and cryptanalysis

Strong cryptography

“There are two kinds of cryptography in this world: cryptography that will stop your
kid sister from reading your files, and cryptography that will stop major governments
from reading your files. This book is about the latter.”
--Bruce Schneier, Applied Cryptography: Protocols, Algorithms, and Source
Code in C.
PGP is also about the latter sort of cryptography.
Cryptography can be strong or weak, as explained above. Cryptographic
strength is measured in the time and resources it would require to recover the
plaintext. The result of strong cryptography is ciphertext that is very difficult to
decipher without possession of the appropriate decoding tool. How difficult?
Given all of today’s computing power and available time—even a billion
computers doing a billion checks a second—it is not possible to decipher the
result of strong cryptography before the end of the universe.
One would think, then, that strong cryptography would hold up rather well
against even an extremely determined cryptanalyst.Who’s really to say? No
one has proven that the strongest encryption obtainable today will hold up
under tomorrow’s computing power. However, the strong cryptography
employed by PGP is the best available today. Vigilance and conservatism will
protect you better, however, than claims of impenetrability.
How does cryptography work?
A cryptographic algorithm, or cipher, is a mathematical function used in the
encryption and decryption process. A cryptographic algorithm works in
combinationwith a key—a word, number, or phrase—to encrypt the plaintext.
The same plaintext encrypts to different ciphertext with different keys. The
security of encrypted data is entirely dependent on two things: the strength of
the cryptographic algorithm and the secrecy of the key.
A cryptographic algorithm, plus all possible keys and all the protocols that
make it work comprise a cryptosystem. PGP is a cryptosystem.