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Can anyone describe how we can do what is said in the below sentence which is in Mark Stamp crypto slides (PPTX) on slide number 101 :

Can do a little better - about 1.5 “encryptions”

The whole slide is as follow :

Confidentiality and Integrity

  • Encrypt with one key, MAC with another key
  • Why not use the same key?
    • Send last encrypted block (MAC) twice?
    • This cannot add any security!
  • Using different keys to encrypt and compute MAC works, even if keys are related
    • But, twice as much work as encryption alone
    • Can do a little better - about 1.5 "encryptions"
  • Confidentiality and integrity with same work as one encryption is a research topic
otus
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user32182
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    Do you have a link to the specific slides? If so, please edit it into the question. – Maarten Bodewes Mar 04 '16 at 13:53
  • The 1.5 "encryptions" is definitely referring AES-GCM. TL;DR: You only encrypt with AES (in CTR mode) and then do one binary finite field multiplication which roughly takes the time of 0.5 AES encryptions. Concerning the last point: OCB can do this and it's subject to the on-going CAESAR competition. – SEJPM Mar 04 '16 at 16:02

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