4.1 Compression and Encryption
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Benefits of reducing the amount of data (compression):
• Data is sent more quickly • Less bandwidth • Less buffering on audio and video streams • Less storage is required
Types of compression
Lossy, lossless
Lossy
Non-essential data is permanently removed
Lossless
Patterns in the data are spotted and summaries in a shorter format without permanently removing any information
RLE (Run length encoding)
• A basic method of compression that summaries consecutive patterns of the same data • Works well with image and sound data where data could be repeated many times
Dictionary compression
Spots regularly occurring data and stores it separately in a dictionary
Encryption
Hashing or encrypting data so that it cannot be understood and doesn’t possess the means to decrypt it
Caesar cipher
Shifts the alphabet
Ways to break caesar cipher
Brute force, frequency analysis
Vernam cipher
The encryption key is also known as the one-time pad, is the only cipher proven to be unbreakable
Vernam cipher key
• A truly random sequence or equal in length than the plaintext and only ever used once • Shared with the recipient by hand, independently of the message and destroyed immediately after use
Algorithmic security
• Keys are determined using a computer algorithm • A key derived from an algorithm can be unpicked
Symmetric encryption (private key encryption)
Same key is used to encrypt and decrypt data
Security problems with symmetric encryption
The key must also be transferred to the recipient
Asymmetric encryption
• Only uses two separate but related keys • Public key is made public so that others wishing to send data can use this key to encrypt it • Encrypted messages can only be decrypted with the private key
Hashing functions
One way functions that provide mapping between an arbitrary length input and a usually fixed length or smaller output