Class Slides

The slides for Lesson 04 is available here.

Class Video

The video for all lessons are available in the video page

Notes

• review of last class

• the following converts block cipher to stream cipher

  • Cipher feedback (CFB)

  • Output Feed Back(OFB)

  • Counter (CTR)

• Cipher feed back (CFB)

  • couldn't understand from lecture so watched this: https://youtu.be/Xm37066R38E

• Output Feed Back(OFB)

  • not clearly understooed

• Counter (CTR)

  • use use counter that starts for a number and increases by one

  • each counter is enctypted with the key

  • the encrypted counter and plain text is XOR-ed to produce ciphertext

  • it goes on until finished

• advantage of (CTR)

  • can do parallal process

• GCM block mode

  • advantage is that we can check the intrigity of the message

• how GCM works

  • take an initial value (IV), do XOR (speculating as not clear from lecture) with counter 0 value

  • enctypt the XOR-ed value

  • increase counter

  • use the value of first step as next steps IV do XOR (speculating as not clear from lecture) with counter 1 and then encrypt it.

  • XOR the encrypted value and plain text to get the ciphertext

  • now again XOR the ciphertext with Auth data which will be used as the Auth data for next step

  • in the final round we XOR Auth data, length of the key and enctypted the XOR-ed value of second step to produce an Auth Tag

• Use of GCM

  • used in networking with AES so called AES-GCM Authinticated Encryption

  • Galios hash function [see more at slide 28]

  • for a network packet with header, sequence and data, AES-GCM usages the header as Additional Auth data, sequence as IV and data as plain-text and produces a new packet with same header, sequence and encrypted ciphertext and a new auth tag as ICV

• IDEA

• Blowfish

• RC5

  • was used by WAP

• Cast-128

• Demo in Java

• Key Escrow Standerd

  • devide the key in two part and put one to escrow agency and another to govt

  • never got implemented due to public opposition

• advantage and disadvantage [see slide]

Source