Hashes & Cryptography

The hash function takes a set of data and reduces it to a smaller fixed size. Hash is often used in programming for everything from searching data to easily detecting changes. The hash functions in Go are divided into two categories namely cryptographic and non-cryptographic. Non-cryptographic hash functions can be found under the hash package and include adler32, crc32, crc64 and fnv.

Here’s an example using crc32:

package main
import (
     "fmt"
     "hash/crc32"
)
func main() {
     h := crc32.NewIEEE()
     h.Write([]byte("test"))
     v := h.Sum32()
     fmt.Println(v)
}

Hash crc32 implements the Writer interface, so we can write bytes to it like any other Writer. Once we’ve written everything we want, we call Sum32() to return uint32. A common use for crc32 is to compare two files. If the Sum32 values for both files are the same, it is likely (though not 100% certain) that the files are the same. If the values are different then the files are definitely not the same.

The following compares two files using crc32 below.

package main

import (
	"fmt"
	"hash/crc32"
	"io/ioutil"
)

func getHash(filename string) (uint32, error) {
	bs, err := ioutil.ReadFile(filename)
	if err != nil {
		return 0, err
	}
	h := crc32.NewIEEE()
	h.Write(bs)
	return h.Sum32(), nil
}
func main() {
	h1, err := getHash("test1.txt")
	if err != nil {
		return
	}
	h2, err := getHash("test2.txt")
	if err != nil {
		return
	}
	fmt.Println(h1, h2, h1 == h2)
}

Let’s try a scenario, for example, if we fill in the files test1.txt and test2.txt as follows:

• test1.txt is filled with hello world and test2.txt is filled with hello world santekno –> data expectations from the two files are different so return (false)
• test1.txt is filled with hello world and test2.txt is filled with hello world –> the data expectations from both files are the same, so return (true)