Different hashes during CRAM-MD5 authentication

As an exercise, I'm trying to implement a mock SMTP server with CRAM-MD5 authentication in Go (without following RFC 2195, since it looks like it doesn't matter to the client what format the pre-hashed challenge is in; I also assume there is only one user "bob" with password "pass"). But I can't seem to get it right as the hash in response is always different from what I have on the server. I send the email using Go as such (running it as a separate package):

{...}
smtp.SendMail("localhost:25", smtp.CRAMMD5Auth("bob", "pass"),
   "bob@localhost", []string{"alice@localhost"}, []byte("Hey Alice!\n"))
{...}

Here's what I do when I get the authentication acknowledgement from the client:

{...}
case strings.Contains(ms, "AUTH CRAM-MD5"):
	rndbts = make([]byte, 16) // Declared at package level
	b64b := make([]byte, base64.StdEncoding.EncodedLen(16))
	rand.Read(rndbts)
	base64.StdEncoding.Encode(b64b, rndbts)
	_, err = conn.Write([]byte(fmt.Sprintf("334 %x\n", b64b)))
{...}

And this is what I do with the client's response:

{...}
 {
	ms = strings.TrimRight(ms, "\r\n") // The response to the challenge
	ds, _ := base64.StdEncoding.DecodeString(ms)
	s := strings.Split(string(ds), " ")
	login := s[0] // I can get the login from the response.
	h := hmac.New(md5.New, []byte("pass"))
	h.Write(rndbts)
	c := make([]byte, 0, ourHash.Size()) // From smtp/auth.go, not sure why we need this.
	validPass := hmac.Equal(h.Sum(c), []byte(s[1]))
	{...}	
}
{...}

And the validPass is never true. I omitted error handling from the excerpts for brevity, but they're there in the actual code (though they're always nil). Why are the hashes different? I have looked at the source code for net/smtp, and it seems to me that I'm going in the right direction, but not quite.

I hope this helps! Runnable version is at https://play.golang.org/p/-8shx_IcLV. Also note that you'll need to fix your %x (should be %s) so you're sending the right challenge down to the client. Right now I think you're trying to hex-encode your base64 string.

Once you've fixed that, I believe this code should help you to construct the right response string on the server and compare it to what the client sent.

// Example values taken from http://susam.in/blog/auth-cram-md5/

challenge := []byte("<17893.1320679123@tesseract.susam.in>")
username := []byte("alice")
password := []byte("wonderland")
clientResponse := []byte("YWxpY2UgNjRiMmE0M2MxZjZlZDY4MDZhOTgwOTE0ZTIzZTc1ZjA=")

// hash the challenge with the user's password
h := hmac.New(md5.New, password)
h.Write(challenge)
hash := h.Sum(nil)

// encode the result in lowercase hexadecimal
hexEncoded := hex.EncodeToString(hash)

// prepend the username and a space
toEncode := []byte(string(username) + " " + hexEncoded)

// base64-encode the whole thing
b64Result := make([]byte, base64.StdEncoding.EncodedLen(len(toEncode)))
base64.StdEncoding.Encode(b64Result, toEncode)

// check that this is equal to what the client sent
if hmac.Equal(b64Result, clientResponse) {
	fmt.Println("Matches!")
}