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Transcript

Data Compression and Security Chapter 20, Exploring the Digital Domain Digital Compression Concepts Compression techniques are used to replace a file with another that is smaller Decompression techniques expands the compressed file to recover original data -- exactly or in facsimile A pair of compression/decompression techniques that work together is called a codec for short Types of Codecs Codecs that upon decompression always reproduce the original file exactly are called lossless codecs Codecs that reproduce only an approximation of the original file upon decompression are called lossy codecs Codecs that take approximately the same amount of time to compress and decompress a file are referred to as symmetric codecs By contrast, codecs that feature simple fast decompression but significantly slower compression are called asymmetric codecs Compression Encoding Compression is an encoding process that filters the original file in several successive stages Codec Methods Syntactic encoding (also called entropy encoding) methods attempt to reduce the redundancy of symbolic patterns in a file without any regard to the type of information represented Semantic methods consider special properties of the type of information represented to reduce nonessential information in a file Hybrid methods combine both syntactic and semantic methods Compressing Text and Numerical Data Lossless Syntactic Methods Run-Length Encoding (RLE) looks for repeated sequences of symbols widely used for fax (facsimile) transmissions Huffman Codes exploits frequency distribution of symbols in a source adaptive Huffman coding builds its own frequency tables rather than use predefined statistics Lempel-Ziv-Welch (LZW) compression based on recognizing patterns of strings in original file fast and yields good results (50% typical reduction) Compressing Images GIF (Graphic Interchange Format) Codec employs LZW method for lossless compression TIFF (Tagged Image File Format) Codec lossless syntactic method JPEG (Joint Photographics Experts Group) Codec umbrella term covering several lossy and lossless methods baseline method is most commonly used one -lossy method based on a hybrid method Compressing Video Video compression employs both spatial and temporal compression spatial techniques compress individual frames temporal methods compress data in frames over time QuickTime and AVI (Audio Video Interleaved) are two popular (and incompatible with each other) formats Compressing Video Some Additional Methods DVI (Digital Video Interactive) Motion-JPEG MPEG (Motion Pictures Experts Group) The px64 Standard Temporal Compression in Video Lossy strategies for eliminating redundancy of information between frames employ temporal compression -- referred to as interframe compression Sequence of frames are considered together key frames difference frames Used in QuickTime and DVI Temporal Video Compression (cont’d) MPEG and related codecs employ a more complex frame-referencing method intrapictures (I pictures) predicted pictures (P pictures) bidirectional pictures (B pictures) Compressing Audio A widely used method is ADPCM (Adaptive Differential Pulse Code Modulation) ADPCM lossy method employs a differencing technique related to those used in video compression used in DVI MP3 employs psychoacoustic methods filter out parts of the signal most people do not hear uses methods to measure the amount of quantization that will just mask noise Encryption and Data Security Cryptography is the art and science of keeping message secret Encryption techniques convert data into a secret code for transmission The process of retrieving the original message at the receiver is called decryption Encryption Keys Keys are essential information -- usually a numerical parameter(s) -- needed for encryption and/or decryption algorithms Encryption keys are used to encode plaintext as encoded ciphertext Decryption keys are used to decode ciphertext and recover the original plaintext Decryption keys are sometimes discovered by brute force methods employing computers to search large potential key spaces Symmetric or Secret Key Ciphers Secret key ciphers use a single secret key (or set of keys) for both encryption and decryption The secret key must be transferred securely in order for secret key methods to be secure Data Encryption Standard (DES) is a US government sponsored secret key cipher. DES uses a 56-bit key. International Data Encryption Algorithm (IDEA) has been proposed to replace DES. It uses a 128-bit key. Longer keys make it more difficult for brute force discovery of the secret key Asymmetric or Public Key Ciphers The first practical public key algortihm was published by Rivest, Shamir, and Adleman in 1976 and is know as RSA (for their last names) Public key ciphers employ an algortihm with two keys -- a public key and a private key A sender looks up the recipient's public key and uses it to encode a message The recipient then decodes the message with his or her private key (this private key is necessary to decode the message) Asymmetric or Public Key Ciphers Illustrated More on Public Key Methods No attempt is made to keep secret the actual encryption and decryption algorithms for public key methods -- security depends on only the recipient knowing his or her private key Public key ciphers are more secure than secret key ciphers, but are not as efficient since they require longer keys and more computing in the encryption and decryption processes For sake of efficiency, sometimes secret key encryption is used and the secret key is communicated employing public key methods -the combination of a secret key encoded message and public key encoded value of the secret key is called a digital envelope Authentication The process used to verify the identity of a respondent is called authentication Authentication is very important for electronic commerce and other network transactions Authentication exploits the symmetry of public and private keys To authenticate that a person is who they say they are: send that person a nonsense message and ask them to encode it with their private key and return it to you when the message is returned, if the person is who they claim to be, you should be able to recover your nonsense message using their public key (which presumably you know) Encryption and National Security An escrowed secret key cipher is a secret key cipher in which a trusted third party controls the secret key. DES is an example of such a cipher, and the US government holds the escrowed 56-bit secret key The International Trafficking in Arms Regulation (ITAR) prohibits the export of secret key cipher systems with secret keys longer than 40 bits Encryption and National Security A major governments can break ciphers with 40-bit or shorter keys by brute force Limiting longer secret key ciphers is an attempt to retain the ability to break codes when this is deemed necessary for national security The ITAR law has been debated for a number of years Public key ciphers have complicated the debate further -and it continues The basic issue is privacy versus the national security Summary Compressing data means reducing the effective size of a data file for storage or transmission Particular paired compression/decompression methods are called codecs Codecs that cannot reproduce the original file exactly are called lossy methods; those that reproduce the original exactly are called lossless methods Text and numbers usually require lossless methods Images, video, and sound codecs are usually lossy Summary (cont’d) Syntactic methods attempt to reduce the redundancy of symbolic patterns in a file without any regard to the type of information represented Semantic methods exploit characteristics inherent in the type of information being represented The use of codecs is not an exact science -- the effectiveness and suitability of any method will depend on the exact nature of the original file and the intended use for the compressed file Summary (cont’d) With the increasing access to and ease of transmitting sensitive and confidential information come significant security risks Encryption techniques are used to encode messages for secure transmission The two primary encryption/decryption methods are: secret key (symmetric key) ciphers public key (asymmetric key) ciphers Summary (cont’d) Public key ciphers are more secure, but secret key ciphers are more efficient Public key encryption is used for authentication over computer networks An active national (and international) debate continues over government control and regulation of encryption/decryption methods