Score:2

Crypto

It possible to encrypt .bmp bitmap files using Stream Cipher Chacha20?

akez

10/11/22, 6:15 PM

Maybe this sounds like a stupid question. I'm sorry for that.

I just wanted to know is it possible to encrypt a bitmap image file like *.BMP, and generate the ciphertext which is an obscure bitmap image where each pixel is random (The image maybe would looks like an analog TV without an antenna)?

562

1 + 2

encryption

stream-cipher

chacha

decryption

Paul Uszak

10/11/22, 9:04 PM

You mean to only encrypt the raster, so that the entire file still remains a valid .BMP?

akez

10/12/22, 12:50 AM

@Paul Uszak, yes exactly.

Score:1

Crypto

kelalaka

10/11/22, 9:06 PM

Yes, it is possible to encrypt a BMP image with any stream cipher.

First of all, you need to cut the first 54-byte of the BMP image into another file. At this point, the data part begins. Then the rest can be encrypted with any stream cipher. After that prepend the header to the encrypted file.

Here the example with OpenSSL. I will use AES with CTR mode where CTR mode turns any block cipher into a stream cipher.

#/bin/bash
#original file is tux.bmp
head -c 54 tux.bmp > tux_head.part
tail -c +55 tux.bmp > tux_data.part
openssl enc -aes-128-ctr -e -in tux_data.part -out aes-ctr-enc-data.part -K "01234567890123450123456789012345" -iv "00000000000000010000000000000001"
cp tux_head.part encrypted_tux.bmp
cat aes-ctr-enc-data.part >> encrypted_tux.bmp

I've used AES-CTR since OpenSSL has ChaCha20-Poly1305 as a cipher suite that is an authenticated encryption. It will add tag, so it is not useful here.

The input image

The output image

0 + 10

Paul Uszak

10/11/22, 9:36 PM

Needs `in` to be `-in`.

Paul Uszak

10/11/22, 9:39 PM

And there's an oddness. Have you tried this? Mine appears with a green tinge. Is that a 'feature' of the BMP format?

kelalaka

10/11/22, 10:19 PM

@PaulUszak I've corrected, tested, and added the result into the answer, Could you check with your file?

Paul Uszak

10/11/22, 11:18 PM

Nope. Just did it on Tux and it comes out with the same green tinge, not the style you have :-( I suspect that it's compatibility options with GIMP as I used that to create my BMPs. Encryption works perfectly though :-) I'll leave it here. Good answer.

kelalaka

10/11/22, 11:21 PM

Interesting. I've used a viewer, now tested with GIMP, too. It is working.

Paul Uszak

10/11/22, 11:32 PM

https://pasteboard.co/K6biu3Y.bmp done as `openssl enc -aes-128-ctr -e -in pic_data.part -out aes-ctr-enc-data.part -K "aaaaaa" -iv "bbbbbb"`

kelalaka

10/11/22, 11:41 PM

Did you combine the header part and the encrypted raster?

Paul Uszak

10/11/22, 11:46 PM

Yes. Anyway, viewers wouldn't display a raw data raster without header information. You'd have to import raw, set dimensions and assign a format and palette. It's something I've done by GIMPing. I'm special that way... The chat bot is shouting at me.

akez

10/12/22, 2:20 AM

@Paul Uszak Chacha20 is using 128-bit constant, a 256-bit key, a 32-bit counter, and a 96-bit nonce. Which means, it will increase the file size of the file, but that's fine with me. The problem is is it still possible to display the ciphertext as a raster bitmap?

kelalaka

10/12/22, 7:48 AM

It will increase the file size to only 96-bit, nothing more. My example already displays the raster as an image, did your check? One can easily use programming to achieve this.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: It possible to encrypt .bmp bitmap files using Stream Cipher Chacha20?

TH: เป็นไปได้ไหมที่จะเข้ารหัสไฟล์บิตแมป .bmp โดยใช้ Stream Cipher Chacha20?

RO: Este posibil să criptați fișierele bitmap .bmp folosind Stream Cipher Chacha20?

RU: Можно ли зашифровать растровые файлы .bmp с помощью Stream Cipher Chacha20?

VI: Có thể mã hóa các tệp bitmap .bmp bằng Stream Cipher Chacha20 không?

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