Pre-sale tokens locking mechanism

Question

I was looking into the ERC20 Token Standards and Crowdsale. There are various sample contracts including the one from OpenZeppelin which exhibits locking the token transfers until the crowdsale is over.

Instead of a simple locking and unlocking mechanism, can we lock the tokens sold in pre-sale for a longer period than the tokens sold in the subsequent sales? How can we do that without changing the ERC20 standards?

Answer 1

Locking tokens once the crowdsale is over is a functionality that would have to go on the token contract. If you are using Open Zeppelin there's a Pausable contract you can use and make your token inherit from that in order to prevent tokens to be moved around.

But yes, that would require adding extra logic into your ERC20 token.

Answer 2

Presale specific lock ups can be done without need to modify EIP-20 tokens.

1. Presale buyers deposit in the presale contract
2. Presale contract buys from the main token sale, tokens are transferred on the presale contract
3. Presale buyers need to claim their tokens by calling claim() function on the presale contract
4. claim() is not available until the lock up period is over

Here is an example:

https://github.com/TokenMarketNet/ico/blob/master/contracts/PreICOProxyBuyer.sol#L231