MelissaA
Brenton
Shilpa Karhik
A bluffers guide to cryptography
July 13th 2006 03:58
Today's post is going to be a bluffer's guide to cryptography. It will be a little shorter then my last few posts, so if you want to know more then please feel free to ask me to write another post on it.
What is cryptography?
Cryptography is an interdisciplinary subject that deals with issues of information security by drawing on mathematics, computer science, linguistics and other disciplines.
A major part of cryptography is encryption, or the conversion of plaintext to ciphertext, which is done by the use of an algorithm called a cipher, which is controlled by a key.
The history of cryptography
Classical cryptography used two main techniques to encrypt messages. These were transposition cyphers (which reordered the letters in a word), and substitution cyphers, which replaced letters (or groups of letters), with other letters (or groups of letters).
Polyalphabetic cyphers, which use a key, where the translation of each letter from plaintext to ciphertext and back is different depending on the key used, made this technique stronger. However, it eventually fell out of favour due to its weak protection against frequency analysis (a technique which used the fact that certain letters of the alphabet are more common than others to figure out the encryption techique.
Computers changed the face of cryptography and encouraged coding based on bits, rather than letters. Computers have generally made for much more complicated ciphers. Cipher security these days often relies on areas where computers are weak, as such as the factorisation of large numbers.
Techniques of cryptography
There are two main techniques of cryptography today: symmetrical and asymmetrical key encryption. Symmetrical key encryption involves the use of the same (or similar) key to encrypt and decrypt a message, whereas asymmetrical key encryption involves the use of one key (the public key) to encrypt a message, and another (the private key) to descrypt it.
The most famous symmetrical encryption cyphers are DES and AES, both of which are block ciphers. This means that a block of plaintext is converted, using a key, to a block of cyphertext which is the same length.
RSA is probably the most famous asymmetric cypher, and it involves encryption with a public key, and decryption with a private one.
Codebreaking
Codebreaking, or cryptanalysis, is currently lagging behind security measures. However, it still manages to break many systems in surprising ways. The techniques commonly used in cryptanalysis include known-plaintext attacks where the codebreaker knows a message in both plaintext and ciphertext, as well as ciphertext-only attacks, where the codebreaker knows only the ciphertext.
As public keys are based on computational problems attacks on these types of cyphers are generally based around trying to solve those computational problems.
There will be more on this in my next post.
Well, that's it for today.
Adam
What is cryptography?
Cryptography is an interdisciplinary subject that deals with issues of information security by drawing on mathematics, computer science, linguistics and other disciplines.
A major part of cryptography is encryption, or the conversion of plaintext to ciphertext, which is done by the use of an algorithm called a cipher, which is controlled by a key.
The history of cryptography
Classical cryptography used two main techniques to encrypt messages. These were transposition cyphers (which reordered the letters in a word), and substitution cyphers, which replaced letters (or groups of letters), with other letters (or groups of letters).
Polyalphabetic cyphers, which use a key, where the translation of each letter from plaintext to ciphertext and back is different depending on the key used, made this technique stronger. However, it eventually fell out of favour due to its weak protection against frequency analysis (a technique which used the fact that certain letters of the alphabet are more common than others to figure out the encryption techique.
Computers changed the face of cryptography and encouraged coding based on bits, rather than letters. Computers have generally made for much more complicated ciphers. Cipher security these days often relies on areas where computers are weak, as such as the factorisation of large numbers.
Techniques of cryptography
There are two main techniques of cryptography today: symmetrical and asymmetrical key encryption. Symmetrical key encryption involves the use of the same (or similar) key to encrypt and decrypt a message, whereas asymmetrical key encryption involves the use of one key (the public key) to encrypt a message, and another (the private key) to descrypt it.
The most famous symmetrical encryption cyphers are DES and AES, both of which are block ciphers. This means that a block of plaintext is converted, using a key, to a block of cyphertext which is the same length.
RSA is probably the most famous asymmetric cypher, and it involves encryption with a public key, and decryption with a private one.
Codebreaking
Codebreaking, or cryptanalysis, is currently lagging behind security measures. However, it still manages to break many systems in surprising ways. The techniques commonly used in cryptanalysis include known-plaintext attacks where the codebreaker knows a message in both plaintext and ciphertext, as well as ciphertext-only attacks, where the codebreaker knows only the ciphertext.
As public keys are based on computational problems attacks on these types of cyphers are generally based around trying to solve those computational problems.
There will be more on this in my next post.
Well, that's it for today.
Adam
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