[Cryptography Team] Re: KryptOn MakoEnvelope signedAndSealedFrom:to:object:

[Chris Muller]

> ECB, CTR ("Counter"), EAX and GGM are all modes of
> operation for block
> ciphers. This wikipedia page
>
http://en.wikipedia.org/wiki/Block_cipher_modes_of_operation
> provides a
> good overview of the different modes, and why ECB is
> a bad choice

Ok, yes, KryptOn 1.0 is using ECB mode.  This is very
easily changed, though, the method is
BlockCipher>>#makoEncrypt:.

> (I was particularly struck by the spectacular
> failure of ECB mode to
> encrypt the sample image!)

That surprised me too, but then thinking about it
makes sense because all the background pixels are the
same color, therefore they are encrypting to the same
bits.  So the background color changes but the
"disturbed" pixels (other than the background color)
are causing the original shape of the original image
to show through.

> With specific reference to a Mako signed-sealed
> envelope, probably the
> best thing to do is to perform the public-key
> signing operation on the
> original data, and then encrypt-and-MAC the signed
> data as a separate
> step. The thing to do is to change the way envelopes
> are sealed (the
> signing process can be left alone) to be an
> encrypt-and-MAC operation
> rather than a simple encrypt-only operation with no
> integrity
> protection. For instance, Rijndael in EAX or GGM
> mode would do nicely
> for the enciphering step.

Ok, if this is the right thing to do then I will try
to make these changes.

> Another thing to watch out for is the key-exchange
> protocol, which can
> be really sensitive.

Ok, if you have any suggestions please let me know
here on the cryptography list.

Regards,
  Chris