MD5 hashing function (like mkpasswd --hash=md5)

salted_md5.go

func md5Crypt(password string, salt string, prefix string) string {

	// start with a hash of password and salt
	initBin := md5.Sum([]byte(password + salt + password))

	// begin an initial string with hash and salt
	initText := bytes.NewBufferString(password + prefix + salt)

	// add crap to the string willy-nilly
	for i := len(password); i > 0; i -= 16 {
		lim := i
		if lim > 16 {
			lim = 16
		}
		initText.Write(initBin[0:lim])
	}

	// add more crap to the string willy-nilly
	for i := len(password); i > 0; i >>= 1 {
		if (i & 1) == 1 {
			initText.WriteByte(byte(0))
		} else {
			initText.WriteByte(password[0])
		}
	}

	// Begin our hashing in earnest using our initial string
	bin := md5.Sum(initText.Bytes())

	n := bytes.NewBuffer([]byte{})

	for i := 0; i < 1000; i++ {
		// prepare to make a new muddle
		n.Reset()

		// alternate password+crap+bin with bin+crap+password
		if (i & 1) == 1 {
			n.WriteString(password)
		} else {
			n.Write(bin[:])
		}

		// usually add the salt, but not always
		if i%3 != 0 {
			n.WriteString(salt)
		}

		// usually add the password but not always
		if i%7 != 0 {
			n.WriteString(password)
		}

		// the back half of that alternation
		if (i & 1) == 1 {
			n.Write(bin[:])
		} else {
			n.WriteString(password)
		}

		// replace bin with the md5 of this muddle
		bin = md5.Sum(n.Bytes())
	}

	// At this point we stop transliterating the PHP code and flip back to
	// reading the Apache source. The PHP uses their base64 library, but that
	// uses the wrong character set so needs to be repaired afterwards and reversed
	// and it is just really weird to read.

	result := bytes.NewBuffer([]byte{})

	// This is our own little similar-to-base64-but-not-quite filler
	fill := func(a byte, b byte, c byte) {
		v := (uint(a) << 16) + (uint(b) << 8) + uint(c) // take our 24 input bits

		for i := 0; i < 4; i++ { // and pump out a character for each 6 bits
			result.WriteByte("./0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz"[v&0x3f])
			v >>= 6
		}
	}

	// The order of these indices is strange, be careful
	fill(bin[0], bin[6], bin[12])
	fill(bin[1], bin[7], bin[13])
	fill(bin[2], bin[8], bin[14])
	fill(bin[3], bin[9], bin[15])
	fill(bin[4], bin[10], bin[5]) // 5?  Yes.
	fill(0, 0, bin[11])

	resultString := string(result.Bytes()[0:22]) // we wrote two extras since we only need 22.

	return resultString
}