The Vulnerability

As there is no fixed-point type in Solidity, developers are required to implement their own using the standard integer data types. There are a number of pitfalls developers can run into during this process. We will try to highlight some of these in this section.

This simple token buying/selling contract has some obvious problems. Although the mathematical calculations for buying and selling tokens are correct, the lack of floating-point numbers will give erroneous results. For example, when buying tokens on line 8, if the value is less than the initial division will result in 0, leaving the result of the final multiplication as 0 (e.g., 200 wei divided by 1e18 equals 0). Similarly, when selling tokens, any number of tokens less than 10 will also result in 0 ether. In fact, rounding here is always down, so selling 29 tokens will result in .

The issue with this contract is that the precision is only to the nearest ether (i.e., 1e18 wei). This can get tricky when dealing with decimals in ERC20 tokens when you need higher precision.

Preventative Techniques

You should ensure that any ratios or rates you are using allow for large numerators in fractions. For example, we used the rate tokensPerEth in our example. It would have been better to use weiPerTokens, which would be a large number. To calculate the corresponding number of tokens we could do msg.value/weiPerTokens. This would give a more precise result.

Another tactic to keep in mind is to be mindful of order of operations. In our example, the calculation to purchase tokens was msg.value/weiPerEth*tokenPerEth. Notice that the division occurs before the multiplication. (Solidity, unlike some languages, guarantees to perform operations in the order in which they are written.) This example would have achieved a greater precision if the calculation performed the multiplication first and then the division; i.e., .

Real-World Example: Ethstick

The does not use extended precision; however, it deals with wei. So, this contract will have issues of rounding, but only at the wei level of precision. It has some more serious flaws, but these relate back to the difficulty in getting entropy on the blockchain (see Entropy Illusion). For a further discussion of the Ethstick contract, we’ll refer you to another post by Peter Vessenes, .