Use binary.BigEndian and attribute source

Signed-off-by: jolheiser <john.olheiser@gmail.com>

IMPLEMENTATION.md

@@ -1,3 +1,7 @@
+# Implementation
+
+Adapted from [source](https://spectre.app/blog/2018-01-06-algorithm/).
+
 ## User Key Derivation
 ([code](user_key.go))
 

site_key.go

@@ -8,24 +8,13 @@ import (
 func siteKey(userKey []byte, scoper Scoper, siteName string, counter int, scope Scope) []byte {
 	nameBytes := []byte(siteName)
 	scopeBytes := []byte(scoper.Scope(scope))
-
 	nameBytesLen := len(nameBytes)
-	keySalt := append(scopeBytes,
+
-		byte(nameBytesLen>>24),
+	keySalt := append(scopeBytes, bigEndian(nameBytesLen)...)
-		byte(nameBytesLen>>16),
-		byte(nameBytesLen>>8),
-		byte(nameBytesLen),
-	)
 	keySalt = append(keySalt, nameBytes...)
-	keySalt = append(keySalt,
+	keySalt = append(keySalt, bigEndian(counter)...)
-		byte(counter>>24),
-		byte(counter>>16),
-		byte(counter>>8),
-		byte(counter),
-	)
 
 	sign := hmac.New(sha256.New, userKey)
 	sign.Write(keySalt)
-
 	return sign.Sum(nil)
 }

spectre.go

@@ -1,5 +1,7 @@
 package spectre
 
+import "encoding/binary"
+
 // Spectre is a spectre client
 type Spectre struct {
 	name   string
@@ -67,3 +69,9 @@ func WithScope(s Scope) SiteOption {
 		opts.scope = s
 	}
 }
+
+func bigEndian(num int) []byte {
+	buf := make([]byte, 4)
+	binary.BigEndian.PutUint32(buf, uint32(num))
+	return buf
+}

user_key.go

@@ -6,14 +6,9 @@ func userKey(name, secret string, scoper Scoper) ([]byte, error) {
 	nameBytes := []byte(name)
 	secretBytes := []byte(secret)
 	keyScope := []byte(scoper.Scope(Authentication))
-
 	nameBytesLen := len(nameBytes)
-	keySalt := append(keyScope,
+
-		byte(nameBytesLen>>24),
+	keySalt := append(keyScope, bigEndian(nameBytesLen)...)
-		byte(nameBytesLen>>16),
-		byte(nameBytesLen>>8),
-		byte(nameBytesLen),
-	)
 	keySalt = append(keySalt, nameBytes...)
 
 	return scrypt.Key(secretBytes, keySalt, 32768, 8, 2, 64)