Message Digests (HASH)

Message digests or hashes are commonly 128 bits to 160 bits in length and provide a digital identifier for each digital file or document. Message digest functions also called hash functions, are used to produce digital summaries of information called message digests. Message digest functions are mathematical functions that process information to "produce a different message digest for each unique document". Identical documents have the same message digest; but if even one of the bits for the document changes, the message digest changes.

Figure. Example of the Message Digest Process

Because message digests are much shorter than the data from which the digests are generated and the digests have a finite length, duplicate message digests called collisions can exist for different data sets. However, good message digest functions use one-way functions to ensure that it is mathematically and computationally infeasible to reverse the message digest process and discover the original data.

Message digests are commonly used in conjunction with public key technology to create digital signatures or "digital thumbprints" that are used for authentication, integrity, and nonrepudiation. Message digests also are commonly used with digital signing technology to provide data integrity for electronic files and documents.

Secret Keys - A description

We all know how the lock on a door works. There are a series of small round bars called tumblers that, when lined up correctly, drop out of the way, removing the obstruction, so you can turn the key and open the door. But, just because you know how a door lock works does not mean that you can find the right key. It takes a lot of time and trouble to find the correct key.

Similarly,

Talented people can reverse-engineer software that uses an algorithm and know its internal working. Well, if that is true, how do you keep the data safe if everyone knows how it’s done? The solution is in the key. The key is a complex sequence of alpha-numeric characters, used as an input to the encryption algorithm. If you can keep the key unknown and unknowable, that goes a very long way in keeping the data safe from prying eyes.

LONGER KEY IS BETTER

For keys to be secure, they must be lengthy i.e. a 128-bit key is more secure than a 56-bit key. Longer keys are generally much harder to crack. Look at it this way, of the house keys shown below, which do you think would be safer to use?

Symmetric Algorithms

Symmetric algorithms use one key to encrypt data and the same key to decrypt it.

Your front door key is symmetric — you use the same key to lock as well as unlock your door. The secret to the security of your front door is that you have the key with you and you don’t give a copy to anyone else. If you do trust someone else with the key, it will always be an exact copy of the one you have.

DES

Triple DES

IDEA

AES

CRYPTOGRAPHY

Cryptography is the practice and study of hiding information.

Related Terms :
Plaintext: Decrypted or unencrypted data (it doesn’t have to be text only)

Encrypt: Scrambling data using an algorithm to make it unrecognizable
Decrypt: Unscrambling data to its original format
Cipher: Another word for algorithm
Secret key: The secret key is a complex sequence of alpha-numeric characters, used as an input to the encryption algorithm. The algorithm will produce a different output depending on the specific key being used at the time. They allow you to scramble and unscramble data. As the same key is used to open or close a door lock, similarly, a same secret key must be used both at sender's and receiver's end to encrypt and decrypt the data. MORE ON SECRET KEYS

Ciphertext: This is the scrambled unrecognisable message produced as output. It depends on the plaintext and the secret key. For a given message, two different keys will produce two different ciphertexts.

Do read this WIKIPEDIA entry on Cryptography.