1 files changed, 73 insertions, 0 deletions
diff --git a/app/src/main/java/dev/msfjarvis/aps/util/totp/Otp.kt b/app/src/main/java/dev/msfjarvis/aps/util/totp/Otp.kt
new file mode 100644
index 00000000..551b2e5d
--- /dev/null
+++ b/app/src/main/java/dev/msfjarvis/aps/util/totp/Otp.kt
@@ -0,0 +1,73 @@
+/*
+ * Copyright © 2014-2020 The Android Password Store Authors. All Rights Reserved.
+ * SPDX-License-Identifier: GPL-3.0-only
+ */
+
+package dev.msfjarvis.aps.util.totp
+
+import com.github.michaelbull.result.Err
+import com.github.michaelbull.result.runCatching
+import java.nio.ByteBuffer
+import java.util.Locale
+import javax.crypto.Mac
+import javax.crypto.spec.SecretKeySpec
+import kotlin.experimental.and
+import org.apache.commons.codec.binary.Base32
+
+object Otp {
+
+    private val BASE_32 = Base32()
+    private val STEAM_ALPHABET = "23456789BCDFGHJKMNPQRTVWXY".toCharArray()
+
+    init {
+        check(STEAM_ALPHABET.size == 26)
+    }
+
+    fun calculateCode(secret: String, counter: Long, algorithm: String, digits: String) = runCatching {
+        val algo = "Hmac${algorithm.toUpperCase(Locale.ROOT)}"
+        val decodedSecret = BASE_32.decode(secret)
+        val secretKey = SecretKeySpec(decodedSecret, algo)
+        val digest = Mac.getInstance(algo).run {
+            init(secretKey)
+            doFinal(ByteBuffer.allocate(8).putLong(counter).array())
+        }
+        // Least significant 4 bits are used as an offset into the digest.
+        val offset = (digest.last() and 0xf).toInt()
+        // Extract 32 bits at the offset and clear the most significant bit.
+        val code = digest.copyOfRange(offset, offset + 4)
+        code[0] = (0x7f and code[0].toInt()).toByte()
+        val codeInt = ByteBuffer.wrap(code).int
+        check(codeInt > 0)
+        if (digits == "s") {
+            // Steam
+            var remainingCodeInt = codeInt
+            buildString {
+                repeat(5) {
+                    append(STEAM_ALPHABET[remainingCodeInt % 26])
+                    remainingCodeInt /= 26
+                }
+            }
+        } else {
+            // Base 10, 6 to 10 digits
+            val numDigits = digits.toIntOrNull()
+            when {
+                numDigits == null -> {
+                    return Err(IllegalArgumentException("Digits specifier has to be either 's' or numeric"))
+                }
+                numDigits < 6 -> {
+                    return Err(IllegalArgumentException("TOTP codes have to be at least 6 digits long"))
+                }
+                numDigits > 10 -> {
+                    return Err(IllegalArgumentException("TOTP codes can be at most 10 digits long"))
+                }
+                else -> {
+                    // 2^31 = 2_147_483_648, so we can extract at most 10 digits with the first one
+                    // always being 0, 1, or 2. Pad with leading zeroes.
+                    val codeStringBase10 = codeInt.toString(10).padStart(10, '0')
+                    check(codeStringBase10.length == 10)
+                    codeStringBase10.takeLast(numDigits)
+                }
+            }
+        }
+    }
+}