Re: NSA and crypto

David A. Scott wrote:
I see you are aruging with little Tommy much has not changed he has
no concept of entropy or Unicity Distance. It is strange he seems
to think if the file is compressed and has same amount of entropy
as a long uncompressed file that it may be easier to break the shorter
file that has a higher entropy density. However he does have a point he
just does not see it. The gzip file with AES may be easier to break
than AES alone since the gzip carries with it a signature that is its
not bijective even if you carefully remove all the headers.

Let me explain the problem so you can understand.

Suppose I have some ciphertext C and I guess a key K. I can decrypt it
to a plaintext P' which through your bijective codec will decompress to
some string P''. All strings P' are valid to the decompressor. And
all of them produce strings that have symbols from a given alphabet.

So far so good.

However, you haven't said how you make sure that all decodings follow a
proper grammar. For instance, I decrypt a JPEG and decompress to some
string of octets. The string won't be a valid JPEG and therefore I can
reject the guessed key as invalid.

Does that make sense now?

Have you got that through your thick f'ing skull yet?

Tom

.

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