Code360 powered by Coding Ninjas X Code360 powered by Coding Ninjas X
Table of contents
Visual Threshold Scheme
Frequently Asked Questions
What is cryptography?
Where does cryptography get its use?
What makes visual cryptography crucial?
What is a visual threshold scheme?
What is the application of the visual threshold scheme?
Last Updated: Mar 27, 2024

Visual Threshold Schemes

Author Mayank Goyal
0 upvote
Master Python: Predicting weather forecasts
Ashwin Goyal
Product Manager @


Visual cryptography uses the human brain's visual system to execute computations. According to Naor and Shamir, the secret might be a rectangular image, I, made up of only black and white pixels. They took into account a threshold scheme scenario in which the shares for this "secret" are similar images made up of black and white pixels, and reconstructing the secret entails superimposing a subset of shares.

visual threshold scheme

The reconstruction is carried out "visually" by simply stacking the shares if the shares are printed on transparencies. Therefore, a scheme of this kind is referred to as a visual threshold scheme.

Visual Threshold Scheme

Naor and Shamir introduced visual cryptography. With the help of a novel cryptographic paradigm, a hidden image can be divided into n shares, each printed on a transparency. Only some of the n participants are eligible to recover the original image. Therefore the shares are divided among them. The secret image is recreated by stacking k (2 ≤ k ≤ n) of these transparencies from the group of eligible participants. The original image cannot be decoded when fewer than k transparencies are placed.

(k, n)-visual threshold scheme is the name of the resulting cryptographic scheme. Contrary to conventional cryptography algorithms, where a significant amount of processing is required to reconstruct the plain text, human visual systems perform the reconstruction. Therefore no computations are required during decoding.

Black and white graphics were used in the Naor and Shamir systems. The concept was further developed to include both color and grayscale images. Black and white photos can be encrypted in several effective methods, while grayscale and color images are more difficult to secure. Color photographs are particularly challenging to encrypt for two reasons.

  • First, it doesn't appear that a color image can be encrypted using a tiny pixel expansion (a method has a pixel expansion of m if each original image pixel is stored as m subpixels on each transparency).
  • The brightness of the colors in the reconstructed image is the second issue with color image encryption. The brightness of the reconstructed image is subpar in every scheme for color visual cryptography that has been put forth so far.

This is because, after stacking the required number of transparencies, the proportion of subpixels that retain the color of the original encoded pixel is relatively minimal.

Since the ratio is:

The number of subpixels that possess the true color / the total number of subpixels.

It requires a different definition because it affects the brightness of the reconstructed image.

Get the tech career you deserve, faster!
Connect with our expert counsellors to understand how to hack your way to success
User rating 4.7/5
1:1 doubt support
95% placement record
Akash Pal
Senior Software Engineer
326% Hike After Job Bootcamp
Himanshu Gusain
Programmer Analyst
32 LPA After Job Bootcamp
After Job


Assume we are working in a (k, n)VTS for color images and that the (i, j)th pixel of the secret image has color c. When k transparencies from qualifying participants are layered together, the color ratio of the (i, j)th pixel in the reconstructed image is the ratio above.

The encryption method may use different color ratios for the various pixels in the reconstructed image. In this situation, the color ratio of a scheme might be defined as the minimum value of the ratio defined in; the minimum is taken over all potential distinct colored pixels. On the other hand, the encryption method can be so regular that every pixel, regardless of color, has the same color ratio. In this instance, we don't need to independently specify the color ratio for every pixel of the rebuilt image. We shall claim that the encoding technique achieves a color ratio of R in this scenario if the color ratio of each pixel equals R.

The scheme has the following advantages:

  • The color ratio has a lower bound, thanks to the scheme. For instance, our color scheme's color ratio is lower constrained by 1/4 if the secret image comprises cyan, yellow, and green colors. This lower bound is not affected by the sharing count and only depends on the number of colors in the hidden image. Other color picture schemes are unable to offer such a lower bound.
  • In contrast to other schemes, our scheme has a very high color ratio. Our photographs are brighter as a result. The color ratio of our scheme will be correct if the secret image includes the colors C = c1, c2,..., ck such that no two colors, ci, and cj, may be joined to form a third color, cl.

Must Read Lower Bound in C++

Frequently Asked Questions

What is cryptography?

Confidentiality, integrity, non-repudiation, and authentication are all cryptography techniques. Hash values, symmetric cryptography, and asymmetric cryptography are the three categories.

Where does cryptography get its use?

Numerous uses of cryptography exist, including e-commerce, computer passwords, and banking transactions. Three different types of cryptographic methods are generally utilized.

What makes visual cryptography crucial?

In all cases of abnegation, betrayal, and combination occurrences, visual cryptography improves security (confidentiality). Contrary to traditional encryption, visual cryptography requires minimal work and does not require a complicated decryption procedure.

What is a visual threshold scheme?

With the help of a novel cryptographic paradigm, it is a scheme in which a hidden image can be divided into n shares, each printed on a transparency. Only some of the n participants are eligible to recover the original image. Therefore the shares are divided among them.

What is the application of the visual threshold scheme?

Various applications, including biometric privacy, secret picture sharing, access control, information hiding, print and scan, watermarking, financial document sharing, and visual authentication, benefit greatly from using VSS techniques.


In this article, we learned about the visual cryptography scheme. Later, we looked into its definition and some of its primary applications of it. Refer to our Guided Path on Coding Ninjas Studio to upskill yourself in Data Structures and AlgorithmsCompetitive ProgrammingJavaScriptSystem DesignOperating SystemCompiler DesignAutomata Theory, and many more! If you want to test your competency in coding, check out the mock test series and participate in the contests hosted on Coding Ninjas Studio!

Check out this article - Converting NFA to DFA

Nevertheless, consider our paid courses to give your career an edge over others.

Previous article
A Simplified (t, t)-threshold Scheme
Next article
What is Transport Layer Security (TLS)?
Live masterclass