# **Case**

Very easy case, you have 100 DAI/USDC/USDT. You wanna transfer some to your friend but you have no ETH in your wallet :(

# **Okay, let's find the solution starting with DAI**

## Simple Way

Of course, you can “just transfer some dust ETH to the account”

\
BUT:

 ![](https://cdn.hackernoon.com/images/XJZlYDr4hZaCqfTQZ1W9SDhlNu93-2022-07-28T19:22:29.296Z-cl65fadkg002p0as6hsu2awqk)

So let’s find another way //

## Another way

Let's dive into the code https://etherscan.io/toke /0x6b175474e89094c44da98b954eedeac495271d0f#code  \n 

Scrolling it down… aand see that picture:

```clike
// --- Approve by signature ---
function permit(address holder, address spender, uint256 nonce, uint256 expiry,
               bool allowed, uint8 v, bytes32 r, bytes32 s) external
{
   bytes32 digest =
       keccak256(abi.encodePacked(
           "\x19\x01",
           DOMAIN_SEPARATOR,
           keccak256(abi.encode(PERMIT_TYPEHASH,
                                 holder,
                                 spender,
                                 nonce,
                                 expiry,
                                 allowed))
   ));

   require(holder != address(0), "Dai/invalid-address-0");
   require(holder == ecrecover(digest, v, r, s), "Dai/invalid-permit");
   require(expiry == 0 || now <= expiry, "Dai/permit-expired");
   require(nonce == nonces[holder]++, "Dai/invalid-nonce");
   uint wad = allowed ? uint(-1) : 0;
   allowance[holder][spender] = wad;
   emit Approval(holder, spender, wad);
}
```

\
There is a permit function(see reference __<https://eips.ethereum.org/EIPS/eip-2612>__).

\
Function designed to be callable from any account, so if you want to give allowance for transfer, you don’t need to make a transaction anymore. Rather, you can generate a permit signature off-chain and send it to your friend by any messanger, even WhatsApp :)

### Deeper explanation

Arguments:

* holder - address who gives allowance
* spender - address who can spend allowance
* nonce - sequence number of permit (used to avoid __[replay attack](https://en.wikipedia.org/wiki/Replay_attack)__)
* expiry - deadline timestamp, if you didn’t use permit till that timestamp it will be invalidated
* allowed - boolean flag, if true you make infinite allowance, if false - revoke allowance
* v, r, s - ethereum signature, more __[details](https://medium.com/mycrypto/the-magic-of-digital-signatures-on-ethereum-98fe184dc9c7)__

  \

How that works:

The key idea is elementary, the user makes the signature of “allowance” using own private key, then the contract can verify that using built-in ecrecover function. “Allowance” is just the set of parameters of permit (like holder, spender, etc.) and service constants (see EIP for details).

\
Algorithm:

* User chooses parameters
* User calculates keccak256 hash of paraments + service info
* User signs hash from the previous step
* User sends signature and parameters to friend off-chain
* Friend sent a transaction with the permit and transfer tokens from the users’ account

\
## What about USDC/USDT ?

### USDC

Here is the __[contract](https://etherscan.io/address/0xa2327a938febf5fec13bacfb16ae10ecbc4cbdcf#code#L2126)__ source code, basically, there is similar mechanics with the permit. But for one-time transfers is better to use __[transferWithAuthorization](https://etherscan.io/address/0xa2327a938febf5fec13bacfb16ae10ecbc4cbdcf#code#L2035)__ function.

### USDT

Here is the __[contract](https://etherscan.io/token/0xdac17f958d2ee523a2206206994597c13d831ec7#code)__ souce code

 ![](https://cdn.hackernoon.com/images/XJZlYDr4hZaCqfTQZ1W9SDhlNu93-yia2g01.jpeg)

USDT doesn’t have permit mechanics and it’s cannot be implemented since the contract is non-upgradable.

 \n 

Tether

USD Coin

Chain

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