Exchanging Data

Actual data is removed from the file in order to reduce its size. An algorithm is used to strip out the least important data. The original file cannot be restored as data is physically removed.

Reduces the file size but allows the original data be perfectly reconstructed from the compressed data. Actual data is still removed however the data is encoded in such a way that the original can be recreated. Less effect at reducing file size than lossy.

Run length encoding is used when there is repeated redundant data. It stores the item once and then an index with how often the item is repeated.

Dictionary coding is when an index is built where every data item in the file is recorded along with an indexed reference. The final compressed file will just consist of the dictionary index along with the sequence of occurrences needed to recreate the original file.

• Reduce download times • Reduce requirements on file storage • Make best use of bandwidth

multimedia files e.g. MP3, JPEG, MPEG

text files and computer programs

Encryption is the process of encoding a message so that it can be read only by the sender and the intended recipient.

Symmetric Encryption is when the same key is used to encrypt and decrypt the message. Both party’s must know the key and also keep it secret.

There is a security risk as the key may be intercepted or the process of creating the key may be duplicated meaning the data can be decrypted by a third party.

Asymmetric Encryption (Public Key Cryptography) is when a public key and a private key is used. The public key is used to encrypt the data and the private key is used to decrypt the data. If you use person X’s public key to encrypt the data only person X’s private key will be able to decrypt it

Hashing is the process used to transform a data item into something different. A hashing function provides a mapping between an arbitrary length input and a fixed length output. It is a one-way transformation meaning you cannot get back to the original form.

• generating disk address - storing and checking passwords

hash total is encrypted using the users private key. If the receiver can decryptit using the senders public key they know that the message is authentic

used for transfering data e.g. online shopping

Used when the same person is accessing and saving data e.g. for backing up

• encrypted very quickly - simple and easy

• no movement of keys, more secure - can be used for digital signatures

• not as fast

Databases are structured, persistent collections of data.

single table database. It is inefficient as it is difficult to query and leads to data redundancy which can cause errors.

A relational database has more than one related table. It is more efficient that a flat file database as no data is duplicated meaning querying is easier.

when the relationships between tables are shown in abstracted view.

one-to-one one-to-many many-to-many