Offchain encryption in Ethereum dapp

Question

Im wrting a dapp which holds / manages sensitive data which might need to be encrypted and decrypted and push again into chain.

I know it is not safe to encrypt data in smart contract,which is why Im thinking of off-chain encryption mechanism . I thought to encrypt data using public key and then decrypt using private key . However I found that popular wallet s/w like metamask doesnt allow this.

1.What are the possible ways to acheive this or any other better ways? (I thought managing users eth keys is not so safe)

2.If using custom keys for encryption, Is it ok using a keymanagement tool like Google KMS or Amazon Kms to manage keys and encrypt data?

Answer 1

You can encrypt data with the public key and decrypt it with the private key offchain with standard encryption, in particular using Elliptic Curve Integrated Encryption Scheme (ECIES)

An example in python using ethereum keys:
https://github.com/tritone11/eth-ecies-py

In Rust:
https://github.com/tritone11/rust-eth-ecies

In javascript:
https://github.com/libertylocked/eth-ecies

In python, pyelliptic is sometimes a pain in the ass, you can obtain the same result following this:
https://github.com/ethereum/trinity/blob/master/p2p/ecies.py

Answer 2

MetaMask does not support encryption/decryption but is support off-chain signing that you can verify on-chain.

The signature will use the user private key. This way it is 100% authentic.

For the data, you can save off-chain or on-chain. But instead of encryption and decryption, you have to use ask the user to sign and you can later verify the signature on and off-chain.

Actually, there is already the EIP712 standard for this. You can check how it works with MetaMask here: https://medium.com/metamask/eip712-is-coming-what-to-expect-and-how-to-use-it-bb92fd1a7a26

Bellow is how the user is requested to sign a specific data: