Introduce (warrant) Canary ERC

EIPS/eip-canary-standard.md

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+## Preamble
+
+    EIP: 801
+    Title: ERC-801 Canary Standard
+    Author: ligi<ligi@ligi.de>
+    Type: Standard
+    Category: ERC
+    Status: Draft
+    Created: 2017-12-16
+
+
+## Simple Summary
+
+A standard interface for canary contracts.
+
+## Abstract
+
+The following standard allows the implementation of canaries within contracts.
+This standard provides basic functionality to check if a canary is alive, keeping the canary alive and optionally manage feeders.
+
+## Motivation
+
+The canary can e.g. be used as a [warrant canary](https://en.wikipedia.org/wiki/Warrant_canary).
+A standard interface allows other applications to easily interface with canaries on Ethereum - e.g. for visualizing the state, automated alarms, applications to feed the canary or contracts (e.g. insurance) that use the state.
+
+## Specification
+
+### Methods
+
+#### isAlive()
+
+Returns if the canary was feed properly to signal e.g. that no warrant was received.
+
+``` js
+function isAlive() constant returns (bool alive)
+```
+
+#### timeToLive()
+
+The time in seconds that can elapse between feed() calls without starving the canary dead.
+
+``` js
+function timeToLive() constant returns (uint256 ttl)
+```
+
+#### feed()
+
+Extend the life of the canary by timeToLive() seconds.
+
+**NOTE**: this should trigger the event listed below
+
+``` js
+function feed()
+```
+
+#### poison()
+
+Kills the canary instantly. E.g. in a urgent case when a warrant was received.
+
+``` js
+function poison()
+```
+
+#### (optional) addFeeder(address feeder)
+
+Add address that is allowed to feed (and therefore also poison) the canary.
+
+``` js
+function addFeeder(address feeder)
+```
+
+#### (optional) removeFeeder(address feeder)
+
+Remove address that is allowed to feed the canary.
+
+``` js
+function removeFeeder(address feeder)
+```
+
+#### (optional) getFeederCount()
+
+Returns the count of addresses that are allowed to feed the canary.
+
+``` js
+function getFeederCount() constant returns (int count)
+```
+
+#### (optional) getFeederByIndex(unit8 index)
+
+Returns the address of the feeder at the given index.
+
+``` js
+function getFeederByIndex(unit8 index) constant returns (address feederAddress)
+```
+
+#### (optional) needsAllFeeders()
+
+When true: all feeders need to call feed() in the intervals defined by timeToLive() - analog to a multisig.
+When false: all the feeders can extend the lifespan of the canary.
+
+This does not affect poisoning - one feeder is always enough to kill the canary.
+
+``` js
+function needsAllFeeders() constant returns (bool needsAllFeeders)
+```
+
+### Events
+
+#### Feed
+
+MUST trigger when the canary got food.
+
+``` js
+event Feed(address feeder)
+```
+
+## Implementation
+
+TODO
+
+## Copyright
+Copyright and related rights waived via [CC0](https://creativecommons.org/publicdomain/zero/1.0/).