1. Data Compression and Encryption zUnderstand reversible and non-reversible data compression. zIdentify Huffman code and FAX compression algorithm zUnderstand the encryption and decryption

2. Why Data Compression zData compression is developed to reduce the number of characters required in a transmission to increase transmission efficiency. zMessages are composed of fixed length of bits coded to represent characters.

3. Data Compression System

4. Data compression techniques zReversible: the data by the receiver can be recovered after data compression. zNonreversible: the data cannot be recovered by the receiver.

5. Huffman code - Reversible zAssigns fewer bits to the most commonly used characters and allocates fewest bits to the less commonly used characters. zcan achieve 50% of reduction of original codes.

6. Example of Coding Tree (Huffman)

7. Information zInformation, I is related to yI = log 2 p zwhere p is the probability of occurrence. The number of bits, n, to carry out the information is related to 1/log 2 P.

8. Adaptive scanning - Reversible zUses a dictionary to store frequently occurring strings of characters. zThe common character string will be replaced by a shorter code during transmission and will be reversed after finished.

9. FAX compression - Reversible zFAX treats each facsimile line as a series of white and black runs. zThe runs are coded based on their length, and the code is transmitted instead of the full bit map zFor 40 black spots, instead of sending 40 "dot"s, it will send "40b” where “b” is for black dots.

10. Data Compression - Nonreversible zOften called data compression, which permanently eliminates the irrelevant portions of the data. zThe sender is sending a data of having five decimal places 23.45612, the sender can chop the decimal places to four, 23.4561 before sending it out.

11. Voice Compression zVoice is sampled at 8K signalling rate and is assigned 8 bits leading to a data rate of 64K bps (8x8K = 64K bps). zData compression can be applied to reduce to 8K bps for a single voice. zThe mobile phone you are using can carry data at the rate of 9600 bps.

12. Video Compression zVideo after compression is a variable bit rate (VBR), which means that the bandwidth required to transmit the data varies with time. zCommercial package can achieve up to 26-to-1 or more.

13. Video Compression format

14. Data Encryption zThe aim is to ensure the data transmission in the Communication network is secure. zThat is to say only the user who knows the method to revert (decrypt) to data. zThe process of converting the original data into another message is called encryption zThe reverse process to convert back into the original message is called decryption.

15. Plaintext and Ciphertext zThe original message is called plaintext. zThe encrypted message is called ciphertext. zplaintext: I am a student zciphertext: 7864%^*hj&*%#ii

16. Web Security zThe word cryptography comes from the Greek, which means secret writing. It consists of four basic elements zPlaintext: The is the original message to be sent and is usually human-readable. zCiphertext: This is the plaintext after encryption. zCryptographic algorithm: This is the mathematical operations used to encrypt/decrypt the plaintext. zKey: This is a secret key used to encrypt/decrypt the message.

17. Symmetric Cryptography zSymmetric Cryptography means the same key is used to encrypt the plaintext and decrypt the ciphertext.

18. Asymmetric Cryptography zThis algorithm can be achieved through a set of keys. This key, usually termed as private key, must be well informed to the remote before zit can be converted into the plaintext. zThe sender will use its own key to encrypt the message while the remote will use the private key (not the same key of sender) to decrypt the received message.

19. Transposition zTransposition means to transpose the original order of pattern into different order. zFor instance, a byte of 1100 1000 after transposition becomes 10100100.

20. Example of Transposition

21. Substitution zIt means to substitute the complete set of bits by a different set of bits. zFor instance, the original plaintext is 1011 0011 and the key is 11000011. zAfter substitution the ciphertext becomes 01110000

22. Decryption