Re: Hash functions and streaming

From: Walter Roberson (
Date: 10/24/05

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    Date: Mon, 24 Oct 2005 20:47:09 +0000 (UTC)

    In article <>,
    frank <> wrote:
    :I have just heard (although apparently it's old news) that the SHA-1
    :has been fundamentally broken. It doesn't take 2**80 hashes for a
    :collision to occur, but only 2**69 hashes.

    :I don't really understand what problem is caused by someone finding a

    :So, my questions are:

    :1. Does this mean that they have reversed the hash back to plaintext?


    :2. Or have they found some plaintext that hashes to the same value as
    :some other plaintext? And if so, why is this considered dangerous?


    This is dangerous because the new plaintext might say "This is an
    billed media stream; automatically pay $25,000 into the following
    bank account..."

    :4. If the SHA-1 message digest was not encrypted, what is the worst
    :that someone could do if they could create a collision?

    See above. Or worse.

    :5. If the stream is very long and the compromised block is just 60
    :seconds or less of that stream, could a hash collision of that one
    :block provide a vulnerability for the rest of the stream? Even though
    :each block will have a completely different hash?

    Yes. Typically in streaming media, blocks identify themselves as
    to their type. The injected block doesn't have to identify itself
    as pure media data: it could identify itself as some other
    block type that the player happens to have a buffer overflow
    vulnerability for.

    I am spammed, therefore I am.

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