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.

Fig 1:Encryption and Decryption with Stream Ciphers

• 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

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