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Docs Solidity by example

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Leonardo Alt
2019-12-13 16:19:49 +01:00
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docs/examples/blind-auction.rst
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Blind Auction
*************
In this section, we will show how easy it is to create a
completely blind auction contract on Ethereum.
We will start with an open auction where everyone
can see the bids that are made and then extend this
contract into a blind auction where it is not
possible to see the actual bid until the bidding
In this section, we will show how easy it is to create a completely blind
auction contract on Ethereum. We will start with an open auction where
everyone can see the bids that are made and then extend this contract into a
blind auction where it is not possible to see the actual bid until the bidding
period ends.
.. _simple_auction:
@@ -17,16 +15,12 @@ period ends.
Simple Open Auction
===================
The general idea of the following simple auction contract
is that everyone can send their bids during
a bidding period. The bids already include sending
money / ether in order to bind the bidders to their
bid. If the highest bid is raised, the previously
highest bidder gets her money back.
After the end of the bidding period, the
contract has to be called manually for the
beneficiary to receive their money - contracts cannot
activate themselves.
The general idea of the following simple auction contract is that everyone can
send their bids during a bidding period. The bids already include sending money
/ Ether in order to bind the bidders to their bid. If the highest bid is
raised, the previously highest bidder gets their money back. After the end of
the bidding period, the contract has to be called manually for the beneficiary
to receive their money - contracts cannot activate themselves.
::
@@ -161,38 +155,31 @@ activate themselves.
Blind Auction
=============
The previous open auction is extended to a blind auction
in the following. The advantage of a blind auction is
that there is no time pressure towards the end of
the bidding period. Creating a blind auction on a
transparent computing platform might sound like a
contradiction, but cryptography comes to the rescue.
The previous open auction is extended to a blind auction in the following. The
advantage of a blind auction is that there is no time pressure towards the end
of the bidding period. Creating a blind auction on a transparent computing
platform might sound like a contradiction, but cryptography comes to the
rescue.
During the **bidding period**, a bidder does not
actually send her bid, but only a hashed version of it.
Since it is currently considered practically impossible
to find two (sufficiently long) values whose hash
values are equal, the bidder commits to the bid by that.
After the end of the bidding period, the bidders have
to reveal their bids: They send their values
unencrypted and the contract checks that the hash value
is the same as the one provided during the bidding period.
During the **bidding period**, a bidder does not actually send their bid, but
only a hashed version of it. Since it is currently considered practically
impossible to find two (sufficiently long) values whose hash values are equal,
the bidder commits to the bid by that. After the end of the bidding period,
the bidders have to reveal their bids: They send their values unencrypted and
the contract checks that the hash value is the same as the one provided during
the bidding period.
Another challenge is how to make the auction
**binding and blind** at the same time: The only way to
prevent the bidder from just not sending the money
after they won the auction is to make her send it
together with the bid. Since value transfers cannot
be blinded in Ethereum, anyone can see the value.
Another challenge is how to make the auction **binding and blind** at the same
time: The only way to prevent the bidder from just not sending the money after
they won the auction is to make them send it together with the bid. Since value
transfers cannot be blinded in Ethereum, anyone can see the value.
The following contract solves this problem by
accepting any value that is larger than the highest
bid. Since this can of course only be checked during
the reveal phase, some bids might be **invalid**, and
this is on purpose (it even provides an explicit
flag to place invalid bids with high value transfers):
Bidders can confuse competition by placing several
high or low invalid bids.
The following contract solves this problem by accepting any value that is
larger than the highest bid. Since this can of course only be checked during
the reveal phase, some bids might be **invalid**, and this is on purpose (it
even provides an explicit flag to place invalid bids with high value
transfers): Bidders can confuse competition by placing several high or low
invalid bids.
::
@@ -296,24 +283,6 @@ high or low invalid bids.
msg.sender.transfer(refund);
}
// This is an "internal" function which means that it
// can only be called from the contract itself (or from
// derived contracts).
function placeBid(address bidder, uint value) internal
returns (bool success)
{
if (value <= highestBid) {
return false;
}
if (highestBidder != address(0)) {
// Refund the previously highest bidder.
pendingReturns[highestBidder] += highestBid;
}
highestBid = value;
highestBidder = bidder;
return true;
}
/// Withdraw a bid that was overbid.
function withdraw() public {
uint amount = pendingReturns[msg.sender];
@@ -339,4 +308,22 @@ high or low invalid bids.
ended = true;
beneficiary.transfer(highestBid);
}
// This is an "internal" function which means that it
// can only be called from the contract itself (or from
// derived contracts).
function placeBid(address bidder, uint value) internal
returns (bool success)
{
if (value <= highestBid) {
return false;
}
if (highestBidder != address(0)) {
// Refund the previously highest bidder.
pendingReturns[highestBidder] += highestBid;
}
highestBid = value;
highestBidder = bidder;
return true;
}
}