md5.Sum() and md5.New().Sum() not match

[Terry Wu]

See : http://play.golang.org/p/SnTt2BxHQe

package main


import (
 "crypto/md5"
 "fmt"
)


func main() {
 h := md5.New()
 fmt.Printf("%x\n%x\n", h.Sum([]byte("abc")), md5.Sum([]byte("abc")))
}

OUTPUT:

616263d41d8cd98f00b204e9800998ecf8427e
900150983cd24fb0d6963f7d28e17f72

is it suppose to be like this? why ?

[Ian Lance Taylor]

When you call md5.New you get a value that implements the Hash
interface (https://golang.org/pkg/hash/#Hash). See the description of
the Sum method of the Hash interface. It's quite different from what
md5.Sum does. In particular note that the output of h.sum starts with
616263, which is the ASCII hex code for "abc".

Ian

[Manlio Perillo]

This is what you are doing:

fmt.Printf("%x\n%x%x\n", h.Sum([]byte("abc")), []byte("abc"), md5.Sum([]byte("")))

You are appending the md5 hash of the empty string, to the string "abc".

Read the documentation.

Regards  Manlio 

[dja...@gmail.com]

http://play.golang.org/p/rDjD8VuyAL

[Peter Waller]

I'm thankful that I don't think I happen to have made this mistake in production code - out of habit I only ever wrote .Sum(nil).

I'm very surprised to hear this is the case, especially since the Hash documentation says that .Size() returns the number of bytes that .Sum() will return, implying that the number of bytes returned by .Sum() is a constant.

Can anyone explain why I might want this surprising behaviour where Sum(x) prepends bytes x to the hash?

(Obviously, I'm aware this isn't going to change. I'm asking so that I can hang more baubles from my tree of knowledge; not make this mistake in the future and correct other people who might otherwise make the mistake).

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[James Bardin]

On Monday, October 26, 2015 at 9:05:43 AM UTC-4, Peter Waller wrote:

I'm thankful that I don't think I happen to have made this mistake in production code - out of habit I only ever wrote .Sum(nil).

I'm very surprised to hear this is the case, especially since the Hash documentation says that .Size() returns the number of bytes that .Sum() will return, implying that the number of bytes returned by .Sum() is a constant.

Can anyone explain why I might want this surprising behaviour where Sum(x) prepends bytes x to the hash?

The most useful feature isn't that Sum prepends the bytes from x, it's that Sum "appends" the output to x. Normally you still pass in a zero length slice. This lets you use (and reuse) a preallocated slice for the output, so that Sum doesn't need do any other allocations.

[Peter Waller]

On 26 October 2015 at 15:25, James Bardin <j.ba...@gmail.com> wrote:

The most useful feature isn't that Sum prepends the bytes from x, it's that Sum "appends" the output to x. Normally you still pass in a zero length slice. This lets you use (and reuse) a preallocated slice for the output, so that Sum doesn't need do any other allocations.

Aha. That makes sense. I wasn't thinking with my zero-allocations hat on.