Minecraft player auth SHA-1 digest.

sha1.go

// You can test this online at: https://play.golang.org/p/hhayRT1VWgj
package main

import (
	"crypto/sha1"
	"encoding/hex"
	"fmt"
	"io"
	"strings"
)

var examples = map[string]string{
	"Notch": "4ed1f46bbe04bc756bcb17c0c7ce3e4632f06a48",
	"jeb_":  "-7c9d5b0044c130109a5d7b5fb5c317c02b4e28c1",
	"simon": "88e16a1019277b15d58faf0541e11910eb756f6",
}

func main() {
	for username, hash := range examples {
		digest := AuthDigest(username)
		fmt.Printf("%q = %s\n", username, digest)
		fmt.Println("> Matches wiki example?", yesno(hash == digest))
	}
}

func yesno(b bool) string {
	if b {
		return "Yes"
	}
	return "No"
}

// AuthDigest computes a special SHA-1 digest required for Minecraft web
// authentication on Premium servers (online-mode=true).
// Source: http://wiki.vg/Protocol_Encryption#Server
//
// Also many, many thanks to SirCmpwn and his wonderful gist (C#):
// https://gist.github.com/SirCmpwn/404223052379e82f91e6
func AuthDigest(s string) string {
	h := sha1.New()
	io.WriteString(h, s)
	hash := h.Sum(nil)

	// Check for negative hashes
	negative := (hash[0] & 0x80) == 0x80
	if negative {
		hash = twosComplement(hash)
	}

	// Trim away zeroes
	res := strings.TrimLeft(hex.EncodeToString(hash), "0")
	if negative {
		res = "-" + res
	}

	return res
}

// little endian
func twosComplement(p []byte) []byte {
	carry := true
	for i := len(p) - 1; i >= 0; i-- {
		p[i] = byte(^p[i])
		if carry {
			carry = p[i] == 0xff
			p[i]++
		}
	}
	return p
}

Excuse me, I think your comment over the function twosComplement(:[]byte):[]byte is wrong / misleading. I am pretty sure the hash is big endian, with the MSB being located at p[0] and the LSB being located at p[len(p)-1].

To encode a []byte base 16, you would use encoding/hex instead of `fmt.Sprintf("%x") for better performance:

fmt.Sprintf("%x", hash) == hex.EncodeToString(hash)

Thanks a lot