Add :digits option (#39)

Author: LoicBersier

lib/nimble_totp.ex

@@ -55,12 +55,20 @@ defmodule NimbleTOTP do
   [eqrcode](https://github.com/SiliconJungles/eqrcode) to generate the QR
   code as **SVG**.
 
+  If you use more than 6 digits for the totp token you will need to specify
+  it in the otpauth uri with the `digits` option.
+
   Example:
 
       uri = NimbleTOTP.otpauth_uri("Acme", "alice", secret)
       #=> "otpauth://totp/Acme:alice?secret=MFRGGZA&issuer=Acme"
       uri |> EQRCode.encode() |> EQRCode.svg()
       #=> "<?xml version=\\"1.0\\" standalone=\\"yes\\"?>\\n<svg version=\\"1.1\\" ...
+       
+      uri = NimbleTOTP.otpauth_uri("Acme", "alice", secret, digits: 8)
+      #=> "otpauth://totp/Acme:alice?secret=MFRGGZA&issuer=Acme"
+      uri |> EQRCode.encode() |> EQRCode.svg()
+      #=> "<?xml version=\\"1.0\\" standalone=\\"yes\\"?>\\n<svg version=\\"1.1\\" ...
 
   ### Generating a Time-Based One-Time Password
 
@@ -128,7 +136,7 @@ defmodule NimbleTOTP do
   """
 
   import Bitwise
-  @totp_size 6
+  @default_digits 6
   @default_totp_period 30
 
   @typedoc "Unix time in seconds, `t:DateTime.t/0` or `t:NaiveDateTime.t/0`."
@@ -214,6 +222,8 @@ defmodule NimbleTOTP do
       *in seconds*) to be used. Default is `System.os_time(:second)`.
     * `:period` - The period (in seconds) in which the code is valid. Default is `30`.
       If this option is given to `verification_code/2`, it must also be given to `valid?/3`.
+    * `:digits` - The desired length of the totp. Default is 6.
+      If this option is given to `verification_code/2`, it must also be given to `valid?/3` and `otpauth_uri/3`/`otpauth_uri/4`.
 
   ## Examples
 
@@ -226,18 +236,21 @@ defmodule NimbleTOTP do
   def verification_code(secret, opts \\ []) when is_binary(secret) and is_list(opts) do
     time = opts |> Keyword.get_lazy(:time, fn -> System.os_time(:second) end) |> to_unix()
     period = Keyword.get(opts, :period, @default_totp_period)
+    digits = Keyword.get(opts, :digits, @default_digits)
+
+    digits not in 6..10 && raise ArgumentError, "digits must be between 6 and 10"
 
-    verification_code(secret, time, period)
+    verification_code(secret, time, period, digits)
   end
 
-  @spec verification_code(binary(), integer(), pos_integer()) :: binary()
-  defp verification_code(secret, time, period) do
+  @spec verification_code(binary(), integer(), pos_integer(), integer()) :: binary()
+  defp verification_code(secret, time, period, digits) do
     secret
     |> hmac(time, period)
     |> hmac_truncate()
-    |> rem(1_000_000)
+    |> rem(Integer.pow(10, digits))
     |> to_string()
-    |> String.pad_leading(@totp_size, "0")
+    |> String.pad_leading(digits, "0")
   end
 
   defp hmac(secret, time, period) do
@@ -273,6 +286,9 @@ defmodule NimbleTOTP do
     * `:period` - The period (in seconds) in which the code is valid. Default is `30`.
       If this option is given to `verification_code/2`, it must also be given to `valid?/3`.
 
+    * `:digits` - The desired length of the totp. Default is 6.
+      If this option is given to `verification_code/2`, it must also be given to `valid?/3` and `otpauth_uri/3`/`otpauth_uri/4`.
+
   ## Preventing TOTP code reuse
 
   The `:since` option can be used to prevent TOTP codes from being reused. When set
@@ -302,18 +318,30 @@ defmodule NimbleTOTP do
   @spec valid?(binary(), String.t(), [option() | validate_option()]) :: boolean()
   def valid?(secret, otp, opts \\ [])
 
-  def valid?(secret, <<a1, a2, a3, a4, a5, a6>>, opts) do
+  def valid?(secret, otp, opts) when is_binary(otp) do
     time = opts |> Keyword.get(:time, System.os_time(:second)) |> to_unix()
     period = Keyword.get(opts, :period, @default_totp_period)
+    digits = Keyword.get(opts, :digits, @default_digits)
 
-    <<e1, e2, e3, e4, e5, e6>> = verification_code(secret, time, period)
+    digits not in 6..10 && raise ArgumentError, "digits must be between 6 and 10"
 
-    (bxor(e1, a1) ||| bxor(e2, a2) ||| bxor(e3, a3) ||| bxor(e4, a4) ||| bxor(e5, a5) |||
-       bxor(e6, a6)) === 0 and not reused?(time, period, opts)
+    code = verification_code(secret, time, period, digits)
+
+    byte_size(code) == byte_size(otp) and validate_digits(code, otp) == 0 and
+      not reused?(time, period, opts)
   end
 
   def valid?(_secret, _otp, _opts), do: false
 
+  @spec validate_digits(integer(), integer()) :: :error | integer()
+  defp validate_digits(<<e, e_rest::binary>>, <<a, a_rest::binary>>) do
+    bxor(e, a) ||| validate_digits(e_rest, a_rest)
+  end
+
+  defp validate_digits(<<>>, <<>>) do
+    0
+  end
+
   @spec reused?(integer(), pos_integer(), [option() | validate_option()]) :: boolean()
   defp reused?(time, period, opts) do
     if since = Keyword.get(opts, :since) do

test/nimble_totp_test.exs

@@ -128,9 +128,31 @@ defmodule NimbleTOTPTest do
         assert NimbleTOTP.valid?(secret, code, time: date_time)
         assert NimbleTOTP.valid?(secret, code, time: naive_date_time)
 
-        refute NimbleTOTP.valid?(secret, "abcdef", time: time)
-        refute NimbleTOTP.valid?(secret, "abcdef", time: date_time)
-        refute NimbleTOTP.valid?(secret, "abcdef", time: naive_date_time)
+        refute NimbleTOTP.valid?(secret, "abcdefgh", time: time)
+        refute NimbleTOTP.valid?(secret, "abcdefgh", time: date_time)
+        refute NimbleTOTP.valid?(secret, "abcdefgh", time: naive_date_time)
+      end
+    end
+
+    test "returns true if it matches the verification code with a length of 8" do
+      time = System.os_time(:second)
+      date_time = DateTime.from_unix!(time, :second)
+      naive_date_time = DateTime.to_naive(date_time)
+
+      for _ <- 1..1000 do
+        secret = NimbleTOTP.secret()
+
+        code = NimbleTOTP.verification_code(secret, time: time, digits: 8)
+        assert code == NimbleTOTP.verification_code(secret, time: date_time, digits: 8)
+        assert code == NimbleTOTP.verification_code(secret, time: naive_date_time, digits: 8)
+
+        assert NimbleTOTP.valid?(secret, code, time: time, digits: 8)
+        assert NimbleTOTP.valid?(secret, code, time: date_time, digits: 8)
+        assert NimbleTOTP.valid?(secret, code, time: naive_date_time, digits: 8)
+
+        refute NimbleTOTP.valid?(secret, "abcdefgh", time: time, digits: 8)
+        refute NimbleTOTP.valid?(secret, "abcdefgh", time: date_time, digits: 8)
+        refute NimbleTOTP.valid?(secret, "abcdefgh", time: naive_date_time, digits: 8)
       end
     end
 
@@ -161,14 +183,21 @@ defmodule NimbleTOTPTest do
       end
     end
 
-    test "returns false if the code does not have 6 digits" do
+    test "returns false if the code does not have 6 digits for default totp_length" do
       time = System.os_time(:second)
       secret = NimbleTOTP.secret()
       code = NimbleTOTP.verification_code(secret, time: time)
       refute NimbleTOTP.valid?(secret, "", time: time)
       refute NimbleTOTP.valid?(secret, binary_part(code, 0, 5), time: time)
       refute NimbleTOTP.valid?(secret, <<?0, code::binary>>, time: time)
     end
+
+    test "returns false if the totp_length is under 6 or above 10" do
+      time = System.os_time(:second)
+      secret = NimbleTOTP.secret()
+      assert_raise ArgumentError, "length must be between 6 and 10", fn -> NimbleTOTP.verification_code(secret, time: time, digits: 5) end
+      assert_raise ArgumentError, "length must be between 6 and 10", fn -> NimbleTOTP.verification_code(secret, time: time, digits: 11) end
+    end
   end
 
   defp to_unix(naive_datetime),