Schedule changes

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Hey folks. We’ve switched up the schedule after the midterm, we’re going to jump to chapter 7 starting on March 2. Please read chapter 7 over the weekend. See the schedule page for more info. Thanks and good luck on the test! Mike

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One Response to “Schedule changes”

  1. Matthew Flaschen Says:

    After class, a few people were talking about the proper order of signing or MACing (http://en.wikipedia.org/wiki/Message_authentication_code) and encrypting. When you are using public key encryption, you actually can’t simply sign-then-encrypt, or encrypt-then-sign. This article (http://world.std.com/~dtd/sign_encrypt/sign_encrypt7.html) explains why. The summary version is that either version is vulnerable to surreptitious forwarding.

    Imagine Alice writes a message, “I love you”, signs it with her private key, then encrypts the signed data with Bob’s public key. She sends it to Bob, who then decrypts it, reencrypts it with Charlie’s key, and secretly passes it to Charlie. Charlie then decrypts it, and sees that it is signed by Alice’s private key. This can obviously cause embarrassment for both Alice and Charlie.

    On the other hand, say Alice proves a complex theorem, encrypts the result with Bob’s public key, signs the encrypted data with her private key, then tries to send it to Bob. But her network admin Eve is evil. So Eve intercepts the message (and drops the original packets), removes Alice’s signature, then re-signs it with Eve’s own private key. She then sends that message to Bob. Bob then thinks that Eve proved the theorem.

    So, used naïvely, neither approach works. The article notes that, “Either S/E/S or E/S/E suffices to reveal any alteration of the sender’s crypto layers, as long as the receiving client knows how to detect the alterations.” The article explains three other possible solutions too.

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