ImageFE
Functional encryption for images.
Introduction
In the traditional cryptography framework, a decryptor either recovers the entire plaintext from the ciphertext or recovers nothing. FE allows fine-grained control over the amount of information each specific decryptor can reveal from the ciphertext.
ImageFE adopts a simplified FE scheme. There are two participants, the trusted encryptor and the decryptor. The encryptor i) generates a universal ciphertext from a given plaintext; ii) generates a set of different function keys. The type and amount of information a decryptor can reveal is determined by the function key it receives.
Methodology
Efficient FE only exists for inner products and quadratic polynomials. ImageFE selects several convolution kernels to obfuscate an image. Note that convolution can be considered as a sequence of inner products. Given an original image, ImageFE derives a function key for every convolution kernel. Decrypting with a function key has the same effect as applying the corresponding convolution kernel to the original image. The function key that corresponds to the identity kernel can recover the original image.
Currently, ImageFE uses Decisional Diffie-Hellman (DDH) as the underlying FE algorithm.
Application
Consider the use case of blockchain. The ciphertext can be released to all users. A function key can be protected by public-key encryption schemes widely used by blockchains. The encryption and transmission of function keys to specific users can be triggered automatically by smart contracts. In this way, ImageFE can be naturally integrated with DEFI and NFT.
Use ImageFE
Dependency
Building from source
cd imgfe
cd cmd
go build -o imgfe.exe
Usages
The enc command takes an original image and a specification of convolution kernels. It produces the ciphertext and the function keys.
The dec command takes the ciphertext and a function key. It produces an image that is equivalent to the result of applying the corresponding convolution kernel to the original image. Multiple function keys in one mode share the same ciphertext.
Help messages
./imgfe.exe enc -h
./image.exe dec -h
Down resolution mode
downsize_partial yields an image that is scaled down (kernel_offset+1) folds.
downsize_full yields the original image.
./imgfe.exe enc -kernel_type="downsize_partial,downsize_full"
./imgfe.exe dec -image_file="../workspace/cart_partial.png" -key_file="../workspace/func_key_downsize_partial.json"
./imgfe.exe dec -image_file="../workspace/cart_full.png" -key_file="../workspace/func_key_downsize_full.json"
Blur mode
gaussian_blur and box_blur yield blurred images. Kernel size equals (kernel_offset*2+1).
identity yields the original image.
./imgfe.exe enc -kernel_type="identity,gaussian_blur,box_blur"
./imgfe.exe dec -image_file="../workspace/cart_gaussian_blur.png" -key_file="../workspace/func_key_gaussian_blur.json"
./imgfe.exe dec -image_file="../workspace/cart_identity.png" -key_file="../workspace/func_key_identity.json"
Miscellaneous
Contact
The Dappanomics Lab is founded by Northwestern faculties, alumni and students. We are currently developing DApps surrounding NFT and DEFI. If you are interested in our project, please contact us at you.li at u.northwestern.edu .
Acknowledgements
Dr. Tilen Marc for his kind support.
The FENTEC project.
