Implement the Precompile

Now, we'll implement the logic of the precompile in Go. We'll hash our string using the MD5 algorithm.

SHA-256 Precompile Implementation

Before defining the logic of our MD5 precompile, let's look at the logic of the function hashWithSHA256 (located in sha256/contract.go), which computes the SHA-256 hash of a string:

sha256/contract.go

import (
    "crypto/sha256"
    //...
)
 
// ...
 
func hashWithSHA256(accessibleState contract.AccessibleState, caller common.Address, addr common.Address, input []byte, suppliedGas uint64, readOnly bool) (ret []byte, remainingGas uint64, err error) {
    if remainingGas, err = contract.DeductGas(suppliedGas, HashWithSHA256GasCost); err != nil {
        return nil, 0, err
    }
    // attempts to unpack [input] into the arguments to the HashWithSHA256Input.
    // Assumes that [input] does not include selector
    // You can use unpacked [inputStruct] variable in your code
    inputStruct, err := UnpackHashWithSHA256Input(input)
    if err != nil {
        return nil, remainingGas, err
    }
 
    // CUSTOM CODE STARTS HERE
    _ = inputStruct // CUSTOM CODE OPERATES ON INPUT
 
    var output [32]byte // CUSTOM CODE FOR AN OUTPUT
 
    output = sha256.Sum256([]byte(inputStruct))
    
    packedOutput, err := PackHashWithSHA256Output(output)
    if err != nil {
        return nil, remainingGas, err
    }
 
    // Return the packed output and the remaining gas
    return packedOutput, remainingGas, nil
}

As you can see, we're performing the following steps:

Line 1: Importing the sha256 function from the crypto library (at the top of the Go file)
Line 15: Unpacking the input to the variable inputStruct. It doesn't make sense that the variable has Struct in its name, but you will see why it is done like this when we have multiple inputs in a later example
Line 24: Calling the sha256 function and assign its result to the output variable
Line 26: Packing the output into a byte array
Line 32: Returning the packed output, the remaining gas and nil, since no error has occurred

Implementing the MD5 Precompile in `contract.go`

Go ahead and implement the md5/contract.go for the MD5 precompile. You should only have to write a few lines of code. If you're unsure which function to use, the following documentation might help: Go Documentation Crypto/md5

SHA-256 Precompile Implementation

Implementing the MD5 Precompile in contract.go

Solution

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Implementing the MD5 Precompile in `contract.go`