Stage en Cryptologie H/F CEA

Transciphering is a cryptographic technique that re-encrypts data from one scheme to another without an intermediate decryption step. This process drastically reduces the overhead induced by the size of homomorphic ciphertexts during their transmission and storage.
The core idea of transciphering is to convert data encrypted with a classical symmetric encryption scheme into a Homomorphic Encryption (HE) scheme.

How Transciphering Works ?
Consider a scenario with a plaintext message m, a symmetric scheme SYM with key k, and a Homomorphic Encryption scheme HE with a public key pk.
The message is initially encrypted as SYM.Enc_k(m).
Transciphering uses the evaluation capability of the HE scheme to perform the symmetric decryption process entirely within the homomorphic domain. The process requires the symmetric key k to be encrypted under HE public key, HE.Enc_pk(k).
By using the homomorphic evaluation key evk, the result is a message encrypted directly under HE public key pk:
HE.Eval_evk(SYM.Dec(HE.Enc_pk(k), SYM.Enc_k(m))) = HE.Enc_pk(m)
The final result, HE.Enc_pk(m), can now be used for homomorphic computation.

Application and Goal
The primary application of transciphering is to allow clients to outsource the cumbersome process of homomorphic encryption to a more resource-abundant party, such as a Cloud or Edge server (often modeled as an honest-but-curious third party).
This approach lets clients use a fast symmetric scheme for initial encryption and data transmission, relying on the server to perform the resource-intensive transciphering step before homomorphic computation begins.

Internship Focus
During this internship, we will revisit and analyze transciphering techniques, focusing on recent advances
and optimizations in Homomorphic Encryption schemes to improve efficiency and performance. For example, we will study AES transciphering.