Richard Heathfield <rjh@cpax.org.uk> wrote:
> On 05/01/2025 06:17, Rich wrote:
>> Stefan Claas <pollux@tilde.club> wrote:
>>> Rich wrote:
>>>
>>>> If instead you mean some kind of "special, PNG aware, encryptor that
>>>> only encrypted the bitmap data of a PNG", but left the file as
>>>> otherwise a proper PNG image structure, then that is slightly tricky
>>>> (and an algorithm that is only useful for PNG's alone).
>>>
>>> Yes, this is what I mean.
>>
>> Which brings up the question of: why?
>>
>> Why go to the trouble to create an encryptor that is specalized for
>> just encrypting the internal bitmap data within a PNG, leaving the rest
>> as a PNG file, when a generic "byte stream" encryptor will encrypt the
>> entire PNG with no extra effort?
>
> Sorry to come late to the party.
>
> I can think of two places where image formats can be useful in
> cryptography.
>
> Firstly, lossless image formats can be used to steganographically
> hide small ciphertexts in the low bits without seriously
> degrading the image.
>
> Secondly, if you want an eyeball on how tangled your bits are,
> rip off the image's metadata, encrypt the bitmap data, bolt the
> metadata back on, and visually inspect the resulting image and
> look for patterns (or, if you used Tux - of course you did - any
> remnants of penguin).

Yes, all useful, and all also trivial to do with the netpbm image
formats. Pad to a square/rectangular size if need be, slap on two
lines of text for metadata at the front, then view the image with an
image viewer.

Stefan appeared to be making a program that, given an actual working
PNG image file, would root around in the PNG file format itself, and
encrypt just the internal bitmap data.

On 10/02/2025 02:19, Rich wrote:

<snip>

> Stefan appeared to be making a program that, given an actual working
> PNG image file, would root around in the PNG file format itself, and
> encrypt just the internal bitmap data.

Been there, done that, lost the t-shirt, and just found it again
on a /very/ old drive.

--
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within

On a sunny day (Mon, 10 Feb 2025 00:59:04 +0000) it happened Richard
Heathfield <rjh@cpax.org.uk> wrote in <vobj0o$t1cd$1@dont-email.me>:

>On 05/01/2025 06:17, Rich wrote:
>> Stefan Claas <pollux@tilde.club> wrote:
>>> Rich wrote:
>>>
>>>> If instead you mean some kind of "special, PNG aware, encryptor that
>>>> only encrypted the bitmap data of a PNG", but left the file as
>>>> otherwise a proper PNG image structure, then that is slightly tricky
>>>> (and an algorithm that is only useful for PNG's alone).
>>>
>>> Yes, this is what I mean.
>>
>> Which brings up the question of: why?
>>
>> Why go to the trouble to create an encryptor that is specalized for
>> just encrypting the internal bitmap data within a PNG, leaving the rest
>> as a PNG file, when a generic "byte stream" encryptor will encrypt the
>> entire PNG with no extra effort?
>
>Sorry to come late to the party.
>
>I can think of two places where image formats can be useful in
>cryptography.
>
>Firstly, lossless image formats can be used to steganographically
>hide small ciphertexts in the low bits without seriously
>degrading the image.
>
>Secondly, if you want an eyeball on how tangled your bits are,
>rip off the image's metadata, encrypt the bitmap data, bolt the
>metadata back on, and visually inspect the resulting image and
>look for patterns (or, if you used Tux - of course you did - any
>remnants of penguin).

Reading this I got a weird idea:
what if of you use the 'Alien problem' (discussed here long ago)
and use picture hight and picture width as the ratio for a simple ASCII message?
Leave the picture unaltered, use resize...
Have not tried it though.