implement sha1 based totp

modules/user/src/main/TotpSecret.scala

@@ -1,7 +1,9 @@
 package lila.user
 
 import java.security.SecureRandom
-import scala.math.BigInt
+import javax.crypto.Mac
+import javax.crypto.spec.SecretKeySpec
+import scala.math.{ pow, BigInt }
 
 case class TotpSecret(val secret: Array[Byte]) {
   override def toString = "TotpSecret(****************)"
@@ -9,9 +11,31 @@ case class TotpSecret(val secret: Array[Byte]) {
   def base32: String = {
     new String(BigInt(secret).toString.toCharArray.map(_.asDigit).map(TotpSecret.base(_)))
   }
+
+  def totp(period: Long): String = {
+    // Loosely based on scala-totp-auth:
+    // https://github.com/marklister/scala-totp-auth/blob/master/src/main/scala/Authenticator.scala
+
+    val msg = BigInt(period).toByteArray.reverse.padTo(8, 0.toByte).reverse
+
+    val hmac = Mac.getInstance("HmacSha1")
+    hmac.init(new SecretKeySpec(secret, "RAW"))
+    val hash = hmac.doFinal(msg)
+
+    val offset = hash(hash.length - 1) & 0xf
+
+    val binary: Long = ((hash(offset) & 0x7f) << 24 |
+      (hash(offset + 1) & 0xff) << 16 |
+      (hash(offset + 2) & 0xff) << 8 |
+      (hash(offset + 3) & 0xff))
+
+    val otp = binary % pow(10, TotpSecret.digits).toLong
+    ("0" * TotpSecret.digits + otp.toString).takeRight(TotpSecret.digits)
+  }
 }
 
 object TotpSecret {
+  private val digits = 6
   private val base = ('A' to 'Z') ++ ('2' to '7')
 
   def apply(base32: String) = new TotpSecret(base32.map(base.indexOf(_)).foldLeft(0: BigInt)((a, b) => a * 32 + b).toByteArray)