How to shuffle ciphertexts?

[lzjluzijie]

Hi, I am new to homomorphic encryption. I didn't find how to shuffle ciphertexts in this library. Is there any way to do it? Thank you.

[justalittlenoob]

Hi @lzjluzijie
This library does not have shuffle functionality. If you want to improve ciphertext security, how about using this ?

pailliercryptolib/ipcl/include/ipcl/pub_key.hpp

Line 91 in fdc2135

void applyObfuscator(std::vector<BigNumber>& ciphertext) const;

[lzjluzijie]

What exactly is this function doing? My current use case is that Alice holds the private key and an array of ciphertext, sent the ciphertext of the array to Bob. Bob adds the an array of data to the ciphertext, shuffles the result ciphertext, and send back to Alice. So Alice and decrypt the results, but don't know what Bob's array is, since the order is changed.

[justalittlenoob]

In your usage scenario, the result returned by Bob cannot be decrypted. As you said, the order is changed.
This library can adapt to this scenario：

• Alice: has pk & sk, an array A. CT_A = pk.encrypt(A)
• Bob: get pk & CT_A from Alice, has an array B. CT_B = pk.encrypt(B)
• Bob: CT_RES = CT_A + CT_B
• Bob: send CT_RES to Alice.
• Alice: RES = sk.decrypt(CT_RES).

[lzjluzijie]

Sorry I didn't explain my needs clearly. The steps you mentioned is exactly want I am doing now. However, I don't want Alice to decrypt the RES like this directly, since Alice would learn the values of B by RES-A. If Bob can shuffle CT_RES to CT_RES', Alice can still decrypt the same results RES', a permutation of RES, but in a different order, so Alice cannot compute B directly since RES'-A is meaningless.

[justalittlenoob]

Okay, I think I got your point. This library doesn't provide any related functions (shuffle), so you can only implement it yourself.

[lzjluzijie]

I see. Can you give me any hint about how to get started?