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| − | <center><font size= | + | <center> <font size= 5> |
| − | + | ==[[2015_Summer_Cryptography_Paar|The Paar Lectures on Introductory Cryptography]]== | |
| + | [http://slectures.projectrhea.org Slectures] by [[User:Agarwa65|Divya Agarwal]] and [[User:Khmarsh|Katie Marsh]] | ||
</font size> | </font size> | ||
| + | </center> | ||
| − | + | <center><font size= 4>4. Stream Ciphers </font size> | |
| − | + | ||
| − | + | ||
</center> | </center> | ||
---- | ---- | ||
---- | ---- | ||
| + | |||
| + | Introductory lecture to Stream Ciphers - Prof. Paar | ||
=[http://www.youtube.be/watch?v=rheateam@gmail.com Link to video on youtube]= | =[http://www.youtube.be/watch?v=rheateam@gmail.com Link to video on youtube]= | ||
| − | <youtube> | + | <youtube>https://www.youtube.com/watch?v=ChORQIRUin4</youtube> |
---- | ---- | ||
| + | ''' | ||
| + | == Accompanying Lecture Notes == | ||
| + | ''' | ||
| + | ---- | ||
| + | |||
| + | '''Stream Ciphers vs. Block Ciphers''' | ||
| + | |||
| + | * Stream Ciphers | ||
| + | ** Encrypt bits individually by adding a bit from a ''key stream'' to a plaintext bit. | ||
| + | ** Usually small and fast, common in embedded devices (e.g., A5/1 for GSM phones) | ||
| + | |||
| + | * Block Ciphers | ||
| + | ** Encrypt an entire block of plaintext bits at a time with the same key. | ||
| + | ** Are common for Internet applications | ||
| + | |||
| + | In this lecture we study Stream Ciphers. | ||
| + | |||
| + | '''Encryption and Decryption of Stream Ciphers''' | ||
| + | |||
| + | Each plaintext bit <math>x_i</math> is encrypted by adding a secret key stream bit <math>s_i</math> modulo 2. Refer figure below. | ||
| + | |||
| + | [[File:EDStreamCiphers.PNG|500px|thumb|left|Fig 1:Encryption and Decryption with Stream Ciphers]] | ||
| + | |||
| + | * Encryption : <math> y_i = e_{si} = x_i + s_i mod2 </math> where <math>x_i,s_i,y_i \in \{0,1\} </math> | ||
| + | * Decryption : <math> x_i = e_{si} = y_i + s_i mod2 </math> | ||
| + | |||
| + | Encryption and decryption are the same functions. | ||
| + | |||
| + | '''Why is Modulo 2 Addition a Good Encryption Function?''' | ||
| + | |||
| + | * Modulo 2 addition is equivalent to XOR operation. | ||
| + | * For perfectly random key stream <math>s_i</math>, each ciphertext output bit has a 50% chance to be 0 or 1, which is a Good statistic property. | ||
| + | |||
| + | '''How do we get a good Key Stream?''' | ||
| + | |||
| + | Security of stream cipher depends entirely on the key stream <math>s_i</math>: | ||
| + | * It should be random. | ||
| + | * Must be reproducible at both sender and receiver ends. | ||
| + | |||
| + | There are several ways to produce random key streams, they are listed below: | ||
| + | * Random number genrators(RNGs) | ||
| + | ** True Random number genrators(TRNGs) | ||
| + | ** Pseudo Random number genrators(PRNG) | ||
| + | ** Cryptographically Secure Pseudorandom number genrators(CSRNG) | ||
| + | |||
| + | * One-Time Pad(OTP) | ||
| + | |||
| + | * Linear feedback shift registers(LFSRs) | ||
| + | |||
| + | |||
| + | The complete lecture on Stream Ciphers is available [https://www.youtube.com/watch?v=AELVJL0axRs here]. | ||
| + | |||
| + | ---- | ||
| + | == References== | ||
| + | * C. Paar. Understanding Cryptography. Lecture Notes. Dept. of Electr. Eng. and Information Sciences, Ruhr University. | ||
| + | * C. Paar and J. Pelzl. Understanding Cryptography. A textbook for Student and Practitioners. Springer 2010. | ||
| + | |||
---- | ---- | ||
==[[2015_Summer_Cryptography_Paar_Stream Ciphers_Divya Agarwal and Kathryn Marsh_comments | Questions and comments]]== | ==[[2015_Summer_Cryptography_Paar_Stream Ciphers_Divya Agarwal and Kathryn Marsh_comments | Questions and comments]]== | ||
Latest revision as of 05:18, 24 June 2015
Contents
The Paar Lectures on Introductory Cryptography
Slectures by Divya Agarwal and Katie Marsh
Introductory lecture to Stream Ciphers - Prof. Paar
Link to video on youtube
Accompanying Lecture Notes
Stream Ciphers vs. Block Ciphers
- Stream Ciphers
- Encrypt bits individually by adding a bit from a key stream to a plaintext bit.
- Usually small and fast, common in embedded devices (e.g., A5/1 for GSM phones)
- Block Ciphers
- Encrypt an entire block of plaintext bits at a time with the same key.
- Are common for Internet applications
In this lecture we study Stream Ciphers.
Encryption and Decryption of Stream Ciphers
Each plaintext bit $ x_i $ is encrypted by adding a secret key stream bit $ s_i $ modulo 2. Refer figure below.
- Encryption : $ y_i = e_{si} = x_i + s_i mod2 $ where $ x_i,s_i,y_i \in \{0,1\} $
- Decryption : $ x_i = e_{si} = y_i + s_i mod2 $
Encryption and decryption are the same functions.
Why is Modulo 2 Addition a Good Encryption Function?
- Modulo 2 addition is equivalent to XOR operation.
- For perfectly random key stream $ s_i $, each ciphertext output bit has a 50% chance to be 0 or 1, which is a Good statistic property.
How do we get a good Key Stream?
Security of stream cipher depends entirely on the key stream $ s_i $:
- It should be random.
- Must be reproducible at both sender and receiver ends.
There are several ways to produce random key streams, they are listed below:
- Random number genrators(RNGs)
- True Random number genrators(TRNGs)
- Pseudo Random number genrators(PRNG)
- Cryptographically Secure Pseudorandom number genrators(CSRNG)
- One-Time Pad(OTP)
- Linear feedback shift registers(LFSRs)
The complete lecture on Stream Ciphers is available here.
References
- C. Paar. Understanding Cryptography. Lecture Notes. Dept. of Electr. Eng. and Information Sciences, Ruhr University.
- C. Paar and J. Pelzl. Understanding Cryptography. A textbook for Student and Practitioners. Springer 2010.
Questions and comments
If you have any questions, comments, etc. please post them here.
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