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1.3.1
| Question | Answer |
|---|---|
| Compression | Reduces file size, download times, storage requirements + makes best use of bandwidth when transmitting the file. The file must be uncompressed to be used. |
| Colour depth | amount of bits per pixel in an image |
| Lossy (compression) | - Data is permanently removed - Big size reductions - Used on image, video or sound files (JPEG , MP3 , MP4) - Removes frequencies or shades of colour that the human eye can't detect, so the two files look the same, but with difference at bit level |
| Lossless (compression) | - Data temporarily removed; uncompressed file can be retained perfectly - Less storage size difference then, lossy - Documents (text) are best (e.g. ZIP, PNG), but can be used on images + audio - Techniques: - RLE - Dictionary coding |
| RLE (Lossless) | Run-length encoding: - used on images with repeating colours, some text files - consecutive repetitions (runs) of the same data are replaced by the number of times the data is repeated and the data itself (frequency/data pairs) AABBBC= 2A 3B 1C |
| Dictionary coding (Lossless) | - used in text, sometimes images - Index built; each data item is recorded alongside an index reference - Stores the dictionary and the sequence of occurrences 1 = ‘hello’, 2 = ‘world’, 3 = ‘,’, 4 = [space] 142341 (hello world, hello) |
| Encryption | makes data unreadable to third parties To decrypt, the correct cipher and key must be known. |
| Cipher | algorithm for encryption/decryption |
| Key | value required to encrypt/decrypt data |
| Symmetric encryption | - Same key is used to encrypt/decrypt data - Key may be intercepted or key production process duplicated quite easily by hackers |
| Asymmetric encryption | - Uses a key pair - Sender will encrypt data with receiver’s public key and sender's private key (known as a combined encryption key), and receiver will decrypt with receivers private key and senders public key |
| key pair | (a public and private key) - The public key is used to encrypt data and can be freely shared - The private key is used to decrypt data and is kept secret |
| Hash function | transforms a string of characters into a fixed-length hash value that represents the original input string. |
| Hashing properties | - One-way process - Same input always gives same output |
| Hashing uses | - passwords storage - digital signatures - proof of state - hash tables |
| Passwords storage | the hashed version of the password is stored. When a password is entered, it is hashed and compared |
| Digital signatures | verify that a received message is the same as the sent message. A hashed version of the text is sent alongside the message that the receiver can encrypt the text to compare the two |
| proof of state | proves data hasn’t changed over a period of time, by hashing all the data and later hashing it all again and comparing |
| Hash tables | a data structure designed for fast data access. Each piece of data is stored using a key that is hashed to provide a memory location. We can then perform operations at a constant speed: - Searching - Insertion - Deletion |
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