International Association
for Cryptologic Research

In 2023, Mfungo et al. presented an image encryption scheme that relies on a series of diffusion techniques and uses a chaotic map to generate three secret keys. Note that two out of three keys are dynamically generated based on the size of the original image, while the remaining key is static. The authors claim that their proposal offers $149$ bits of security. Unfortunately, we found a chosen plaintext attack that requires $2$ oracle queries and has a worse case complexity of $\mathcal O(2^{32})$. If the attacker has access to $1$ encryption oracle query and $1$ decryption oracle query, we can lower the complexity to $\mathcal O(2^{18.58})$. Encrypting an image with Mfungo et al.'s scheme has a worst case complexity of $\mathcal O(2^{33})$. Therefore, both our attacks are faster than encrypting an image. Our attacks are feasible because the dynamic keys remain unchanged for different plaintext images of the same size, and the static key remains the same for all images.