Data Protection and Security

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Secret Key Cryptography

I.I

Symmetric Encryption

Stream Ciphers

Stream ciphers differ from block ciphers in several aspects:

• They always contain a secret “state” which evolves with time during encryption.
• They usually produce streams of bits rather than blocks.

Thus two main parts of a stream cipher are:

• State-transition function
• Filter (producing the output of the stream cipher)

Stream ciphers in general can be implemented more efficiently than block ciphers. Therefore, stream ciphers were ruling the world of encryption for a long time. Appearance of fast block ciphers and the convenience of use of block ciphers in a stream-like behaviour via CM, OFB or CBC modes of operation have caused a shift of interest. Moreover, generally designing a secure stream cipher is more challenging than designing a secure block cipher.