Encryption – Recommended Way to Pass Secret in Solidity

encryption

Hi I'm an Ethereum noob just trying something out.

I have a quick question for the community here about passing secrets in Solidity. Since, we can only control who can change state variables in Solidity, the values inside variables can be seen by the public.

What's the recommended way to implement a Smart Contract that shows certain values to specific address? Can I encrypt my message with the public key (address) of the intended recipient? And let the recipient decrypt the message with their private key? How would I do that in Solidity?

Thanks

Best Answer

In the case where it is acceptable to perform the encryption and decryption off-chain, follow these four steps:

Off-chain, use an encryption library to encrypt the message using the recipient's public key associated with their Ethereum address. An example of how to this: How to encrypt a message with the public-key of an Ethereum address
Submit an Ethereum transaction invoking a method on your contract to store the encrypted message in a contract state variable.
After the transaction confirms, the recipient can make a (non-transaction) call to read the state variable contents from the contract.
Off-chain, the recipient uses their Ethereum private key to decrypt the message.

Note that only step 2 modifies the blockchain.

How would I do that in Solidity?

You probably do not want to do the decryption on-chain, since it would require including the recipient's private key as an input to a transaction, making it visible to all.

Also, I don't know if this is currently possible. The elliptic curve operations are complex and use a lot of gas. In the upcoming Metropolis hard fork, new precompiled contracts will provide gas-efficient implementations of elliptic curve operations (see EIP 213). These should make it possible for you to implement the decryption in Solidity. But, at that point the private key is revealed, so it's hard to think of a use case that makes sense.

Related Solutions

Ethereum Encryption – How to Encrypt a Message with the Public Key of an Ethereum Address

thx @Edmunx Edgar, i tried to use ECIES, but it failed to install because of a subdepencency. I now used the bitcore-lib together with bitcore-ecies. This works like expected.

EDIT: I created a npm-module which does exactly theses things and also has some performance optimisations and tutorials: github:eth-crypto.

Here is my code for anyone with the same question:

// run 'npm install eth-crypto --save'

const EthCrypto = require('eth-crypto');

// create identitiy with key-pairs and address
const alice = EthCrypto.createIdentity();

const secretMessage = 'My name is Satoshi Buterin';
const encrypted = await EthCrypto.encryptWithPublicKey(
    alice.publicKey, // encrypt with alice's publicKey
    secretMessage
);

const decrypted = await EthCrypto.decryptWithPrivateKey(
    alice.privateKey,
    encrypted
);

if(decrypted === secretMessage) console.log('success');

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Solidity – Current Methods to Send Encrypted Messages from Contracts

You can't make an Ethereum contract start with plaintext and meaningfully encrypt a message, because all contract execution has to be verifiable by all the nodes in the network, so as you say everybody would also have to be able to see your plaintext.

If you want to send a message from one user of a contract to another user of the contract, the obvious thing to do is for the contract to manage an identifier for the user who wants to receive the message - say a PGP fingerprint - and the sender to use that to look up their PGP public key on a regular key server and use that to encrypt the message for them.

You probably don't want the encrypted message to actually transit the blockchain unless it's very small as it will cost gas, but you might want to use the blockchain to let the receiver know where to find the encrypted message - for example, store the encrypted message in IPFS, and have the sender update the contract to let the receiver know the hash of the document they've put on IPFS.

Best Answer

Related Solutions

Ethereum Encryption – How to Encrypt a Message with the Public Key of an Ethereum Address

Solidity – Current Methods to Send Encrypted Messages from Contracts

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