1 files changed, 49 insertions, 0 deletions
diff --git a/app/src/main/java/com/zeapo/pwdstore/utils/Otp.java b/app/src/main/java/com/zeapo/pwdstore/utils/Otp.java
new file mode 100644
index 00000000..44e2aa64
--- /dev/null
+++ b/app/src/main/java/com/zeapo/pwdstore/utils/Otp.java
@@ -0,0 +1,49 @@
+package com.zeapo.pwdstore.utils;
+
+import android.util.Log;
+
+import org.apache.commons.codec.binary.Base32;
+
+import java.math.BigInteger;
+import java.nio.ByteBuffer;
+import java.security.InvalidKeyException;
+import java.security.NoSuchAlgorithmException;
+import java.util.Arrays;
+
+import javax.crypto.Mac;
+import javax.crypto.spec.SecretKeySpec;
+
+public class Otp {
+    public static final int TIME_WINDOW = 30;
+
+    private static final String ALGORITHM = "HmacSHA1";
+    private static final int CODE_DIGITS = 6;
+
+    private static final Base32 BASE_32 = new Base32();
+
+    private Otp() {
+    }
+
+    public static String calculateCode(String secret, long counter) {
+        SecretKeySpec signingKey = new SecretKeySpec(BASE_32.decode(secret), ALGORITHM);
+
+        Mac mac = null;
+        try {
+            mac = Mac.getInstance(ALGORITHM);
+            mac.init(signingKey);
+        } catch (NoSuchAlgorithmException e) {
+            Log.e("TOTP", ALGORITHM + " unavailable - should never happen", e);
+            return null;
+        } catch (InvalidKeyException e) {
+            Log.e("TOTP", "Key is malformed", e);
+            return null;
+        }
+
+        byte[] digest = mac.doFinal(ByteBuffer.allocate(8).putLong(counter).array());
+        int offset = digest[digest.length - 1] & 0xf;
+        byte[] code = Arrays.copyOfRange(digest, offset, offset + 4);
+        code[0] = (byte) (0x7f & code[0]);
+        String strCode = new BigInteger(code).toString();
+        return strCode.substring(strCode.length() - CODE_DIGITS);
+    }
+}