How to calculate sha256 file checksum in Go - file

Question

I need utility for Windows that calculates sha256 file checksum so that when I download
fedora I can verify checksum from here: https://fedoraproject.org/static/checksums/Fedora-18-i386-CHECKSUM
Microsoft utility from http://support.microsoft.com/kb/889768 does only md5 and sha1.
I don't want to use other downloadable tools that are not signed and not available from https or from sources that I don't know about, because it does not make any sense to download unsigned code over unencrypted connection or from untrusted source to verify signature of another code to trust it.
Luckily google provides possibility to use https for all downloads so I can download Go over secure connection and start from there.
Here is simple code that does that for a small file, but it's not very good for big files because it's not streaming.
package main
import (
"io/ioutil"
"crypto/sha256"
"os"
"log"
"encoding/hex"
)
func main() {
hasher := sha256.New()
s, err := ioutil.ReadFile(os.Args[1])
hasher.Write(s)
if err != nil {
log.Fatal(err)
}
os.Stdout.WriteString(hex.EncodeToString(hasher.Sum(nil)))
}
How to make it to use streams so that it works on any file size.

Answer 1

The crypto/sha256 godoc actually has a snippet that shows how to do that (it's basically the same code as James):
package main
import (
"crypto/sha256"
"fmt"
"io"
"log"
"os"
)
func main() {
f, err := os.Open("file.txt")
if err != nil {
log.Fatal(err)
}
defer f.Close()
h := sha256.New()
if _, err := io.Copy(h, f); err != nil {
log.Fatal(err)
}
fmt.Printf("%x", h.Sum(nil))
}

Answer 2

The SHA256 hasher implements the io.Writer interface, so one option would be to use the io.Copy() function to copy the data from an appropriate io.Reader in blocks. Something like this should do:
f, err := os.Open(os.Args[1])
if err != nil {
log.Fatal(err)
}
defer f.Close()
if _, err := io.Copy(hasher, f); err != nil {
log.Fatal(err)
}

Answer 3

Full example of md5sum:
func md5sum(filePath string) (string, error) {
file, err := os.Open(filePath)
if err != nil {
return "", err
}
defer file.Close()
hash := md5.New()
if _, err := io.Copy(hash, file); err != nil {
return "", err
}
return hex.EncodeToString(hash.Sum(nil)), nil
}
EncodeToString does not omits leading 0 bytes, so fmt.Println(hex.EncodeToString([]byte{0x00, 0x00, 0xA, 0xB, 0xC})) gives
00000a0b0c