diff options
| -rw-r--r-- | docs/frequently-asked-questions.rst | 18 | ||||
| -rw-r--r-- | docs/units-and-global-variables.rst | 12 |
2 files changed, 12 insertions, 18 deletions
diff --git a/docs/frequently-asked-questions.rst b/docs/frequently-asked-questions.rst index 8fd0c041..e4eee32b 100644 --- a/docs/frequently-asked-questions.rst +++ b/docs/frequently-asked-questions.rst @@ -125,24 +125,6 @@ Example:: } } -Are timestamps (``now,`` ``block.timestamp``) reliable? -======================================================= - -This depends on what you mean by "reliable". -In general, they are supplied by miners and are therefore vulnerable. - -Unless someone really messes up the blockchain or the clock on -your computer, you can make the following assumptions: - -You publish a transaction at a time X, this transaction contains same -code that calls ``now`` and is included in a block whose timestamp is Y -and this block is included into the canonical chain (published) at a time Z. - -The value of ``now`` will be identical to Y and X <= Y <= Z. - -Never use ``now`` or ``block.hash`` as a source of randomness, unless you know -what you are doing! - Can a contract function return a ``struct``? ============================================ diff --git a/docs/units-and-global-variables.rst b/docs/units-and-global-variables.rst index 64795306..887535da 100644 --- a/docs/units-and-global-variables.rst +++ b/docs/units-and-global-variables.rst @@ -73,6 +73,18 @@ Block and Transaction Properties This includes calls to library functions. .. note:: + Do not rely on ``block.timestamp``, ``now`` and ``block.blockhash`` as a source of randomness, + unless you know what you are doing. + + Both the timestamp and the block hash can be influenced by miners to some degree. + Bad actors in the mining community can for example run a casino payout function on a chosen hash + and just retry a different hash if they did not receive any money. + + The current block timestamp must be strictly larger than the timestamp of the last block, + but the only guarantee is that it will be somewhere between the timestamps of two + consecutive blocks in the canonical chain. + +.. note:: If you want to implement access restrictions in library functions using ``msg.sender``, you have to manually supply the value of ``msg.sender`` as an argument. |