Decryption in a smart contract

Question

I have a key pair:

The data is encrypted off-chain:

and it is saved into the following smart contract:

contract Foo {
    string data;
}

Can I decrypt stored data on-chain, assuming I provide a corresponding private key?

function decryptData(string privKey){
    ...
}

Answer 1

I don't think this is possible in the present arrangement.

SK1 would have to be transmitted to the contract, therefore revealed to all verifiers, which is everyone. In any case, that's the hurdle. In summary, if the contract can decode it, then everyone can decode it.

This is the sort of thing that might be possible with a ZKSnarks implementation, as I understand it.

This might be of assistance: Can smart contracts compute on encrypted data?

Hope it helps.

Answer 2

Yes, you can, at least in principle.

Ethereum is Turing-complete, so you can do whatever calculations you like. However, I don't know of any actual implementations that do this on-chain, and depending on the details of your encryption algorithms it may not be practical to do it within the gas limit. The upcoming Metropolis system upgrade may help, as it should include biginteger functionality that will make it easier and cheaper to do crypto stuff on-chain.

Whether this is a good idea is another question, though, depending on what you're trying to achieve. The private key and the decrypted data will be visible to anyone who can access the blockchain, and for most purposes this is the kind of thing you want to do off-chain in any case.

Answer 3

If you're trying to decrypt data on-chain, I recommend using a hashing function on your data appended to a random ethereum address for entropy. Here's how:

1. Post hashed data inside your given data variable on-chain. For all intents and purposes, this data is encrypted, as nobody but you knows the hash inputs that generated this hash output.
2. This data is a hash made by: web3.utils.soliditySha3([realData, randomAddress]) off-chain inside Web3js. You keep both these inputs a secret from the blockchain.
3. When you want to decrypt this hash, post the realData and randomAddress to a smart contract. Hash them on-chain using solidity's keccak256 function. keccak256(abi.encodePacked(uint256 realData, address randomAddress)).
4. If the on-chain hash matches the data variable, then this data has effectively been decrypted.

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Originally I wrote my answer using only signatures but that was wrong. You cannot use a signature function for this (it also costs more gas anyway). For more information: https://crypto.stackexchange.com/questions/43894/asymmetry-between-public-key-and-private-key-digital-signatures-use-the-private

If you want to make sure the hash came from an authorized user, use a mapping object to store the hashes based on a msg.sender variable. msg.sender is effectively a signature as it is.

Answer 4

No, you can't. Ethereum EVM does not provide encryption/decryption functions, they are very high level general purpose functions. But you can try to decrypt outside of the smart contract, using the not-anymore-private key found in the transaction.

See also What are effective techniques to encrypt/decrypt data stored in a smart contract?