Display TOTP code if entry contains OTP secret

TOTP is calculated on display and on copy to clipboard from secret
embedded in entry (either in password or in extra) and the current time.

1 files changed, 50 insertions, 0 deletions

diff --git a/app/src/main/java/com/zeapo/pwdstore/utils/Totp.java b/app/src/main/java/com/zeapo/pwdstore/utils/Totp.java
new file mode 100644
index 00000000..5e4326e2
--- /dev/null
+++ b/app/src/main/java/com/zeapo/pwdstore/utils/Totp.java
@@ -0,0 +1,50 @@
+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 Totp {
+
+    private static final String ALGORITHM = "HmacSHA1";
+    private static final int TIME_WINDOW = 30;
+    private static final int CODE_DIGITS = 6;
+
+    private static final Base32 BASE_32 = new Base32();
+
+    private Totp() {
+    }
+
+    public static String calculateCode(String secret, long epochSeconds) {
+        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;
+        }
+
+        long time = epochSeconds / TIME_WINDOW;
+        byte[] digest = mac.doFinal(ByteBuffer.allocate(8).putLong(time).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);
+    }
+}