## Cryptography for everybody: Perceptual Hashing to Compare Images

Some years ago, I supervised a bachelor project with the topic “image processing” at the University of Kassel (when I was working for the “Applied Information Security” group). The bachelor students had to implement image processing components in CrypTool 2.

One result of the project was the so called “ImageHash” component. One of my students implemented the “block hash” by Steinebach [1]. It is a robust (or perceptual) hash function, that allows to use it for the search for images in image databases, even when the image is different (e.g. resized or modified) from the original.

This hash algorithm works in four steps:
Step 1: Grayscale: Colors (RGB) are converted to grayscale (0x00 to 0xFF)
Step 2: Resize: Image is resized to target size e.g. 16×16 pixels
Step 3: Flip: Image is flipped horizontally and vertically until the brightest quarter of the image is in the top left corner
Step 4: Binarize: Convert grayscale pixels to black and white pixels using a threshold

In the following image, an example image is hashed using these four steps:

I still like the component and think it was a good addition to CrypTool 2. Clearly, the bachelor students got good grades for the project. Other results were the “Image processor”, the “Transcripor”, and the “Watermark Creator” in CrypTool 2 :-).

Today, I also createad a YouTube video about perceptual hashing. Here, I also show the “ImageHash” component of CrypTool 2 . You may watch it here:

Nils

[1] Steinebach, Martin. “Robust hashing for efficient forensic analysis of image sets .” International Conference on Digital Forensics and Cyber Crime. Springer, Berlin, Heidelberg, 2011

## Cryptography for everybody: The Straddling Checkerboard Cipher and Che Guevara

Today, Prof. Esslinger and I updated two templates, which are part of CrypTool 2 (CT2). Regularly, we update templates to improve their quality. A template is a pre-built graphical program (CT2 workspace) which demonstrates a cryptographic method, e.g. a cipher or a cryptographic protocol. Right now, we have about 250 templates in CrypTool 2.

We updated the “Straddling Checkerboard” cipher template as well as the “Che Guevara” cipher template, which also contains the Straddling Checkerboard cipher component for encryption.

To create his cipher, Che Guevara used a cascade of two ciphers:
(1) he encrypted his text using the Straddling Checkerboard cipher, turning his plaintext into digits and

In the template shown above, we have six different inputs: four for the straddling checkerboard: plaintext, key, alphabet, row and column assignments. Two for Vernam: an additional alphabet consisting only of digits and a one-time pad.

Actually, all inputs for the Straddling Checkerboard cipher could be kept constant or even published, since security is given as long as the one-time pad is kept secret.

If you want to know how to create a straddling checkerboard cipher in CrypTool 2, you can watch my video about it:

I hope you enjoy the Straddling Checkerboard cipher template and the Che Guevara cipher template of CrypTool 2. If you want to test these, you need the current nightly build available here: https://www.cryptool.org/de/ct2/downloads

Nils

## Cryptography for everybody: Basics of Cryptology Series (correct ordering)

In 2020 I started my “Basics of Cryptology” series on my “Cryptography for everybody” YouTube channel.

Right now, the series consists of 23 different videos. The videos cover topics from the beginnings of cryptology and basic terminology up to modern cryptology (e.g. RSA and AES).

While creating the series I jumped with the focus and created videos, as the topics came into my mind. Thus, today, I tried to order the videos to a “correct” order. Here is my current list π

Clearly, you can just watch the videos as they are present in the YouTube playlist π
Maybe, in the future, I will create a single long video, a new playlist, or maybe even re-create all videos. When I started creating my videos, my YouTube channel was new and I had not so much experience as I have now.

Nevertheless, I hope you’ll find my videos in that series interesting and helpful. If you have any questions, ideas, or wishes, feel free to write below this blogpost or directly below my videos in the comment section π

Nils

## Cryptography for everybody: Let’s Create Our Own Homophonic Substitution Cipher

In our newest video on “Cryptography for everybody”, we create a homophonic substitution cipher using CrypTool 2.

The “Substitution component” of CrypTool 2 allows to create substitution ciphers. For that, we implemented an easy-to-use syntax based on plaintext and ciphertext alphabets. An “alphabet” is just a string (some text), which consists of our “symbols”. A symbol can be one or more UTF-8 characters.

Example (simple shift cipher):
– Plaintext alphabet=”ABCDEFG…Z”
– Ciphertext alphabet=”BCDEFGH…A”

Providing these two alphabets to the substitution component would create a simple shift cipher, where each letter of the plaintext alphabet is shifted one to the left in our corresponding ciphertext alphabet. In the substitution component, letters are substituted based on their corresponding positions in the given alphabets. The first letter of the plaintext alphabet is substituted by the first letter of the ciphertext alphabet, the second by the second, etc.

But the substitution component is much more powerful. It allows also to create alphabets consisting of “words” and also allows alternative substitutions to create “homophones”.

Example (homophonic substitution cipher):
– Plaintext alphabet=”ABCDEFG…Z”
– Ciphertext alphabet=”[01|02][03|04]…[999|555]”

In this example, the letter A can be substituted by either 01 or 02. The brackets tell the substitution component that it should use everything inside the brackets as a single ciphertext symbol. The pipe symbol tells the component that we want to create alternatives. Using this syntax, we are able to create a homophonic substitution cipher, where one plaintext letter will be replaced by one of the defined homophones.

But we are not only limited to use simple two or three digit combinations. We can also create mappings like [MAXIMILIAN] in the plaintext alphabet and [1001] in the ciphertext alphabet. Doing so, we can create so-called nomenclators. How this can be done in CrypTool 2 is part of the linked YouTube video. So if you are interested in more details, you should have a look at this π