initial commit

src/commonMain/kotlin/com/infendro/otp/HOTP.kt

@@ -0,0 +1,58 @@
+package com.infendro.otp
+
+import com.infendro.hash.HashFunction
+import com.infendro.hash.SHA1
+import com.infendro.mac.HMAC
+import com.infendro.otp.util.toByteArray
+import com.infendro.otp.util.toInt
+import kotlin.experimental.and
+import kotlin.math.pow
+
+class HOTP(
+    val function: HashFunction = SHA1,
+    val length: Int = 6,
+) {
+    fun verify(
+        secret: ByteArray,
+        counter: Long,
+        otp: String,
+    ): Boolean {
+        return generate(secret, counter) == otp
+    }
+
+    fun generate(
+        secret: ByteArray,
+        counter: Long,
+    ): String {
+        if (counter < 0)
+            throw Exception()
+
+        val hash = generateHash(
+            secret,
+            counter.toByteArray()
+        )
+        return otp(hash)
+    }
+
+    private fun generateHash(
+        secret: ByteArray,
+        value: ByteArray,
+    ): ByteArray {
+        return HMAC(function).hash(secret, value)
+    }
+
+    private fun otp(
+        hash: ByteArray,
+    ): String {
+        val offset = (hash.last() and 0xF).toInt()
+
+        var truncatedHash = hash
+            .drop(offset)
+            .take(4)
+            .toInt() and 0x7FFFFFFF
+        truncatedHash %= 10.0.pow(length).toInt()
+
+        return truncatedHash.toString()
+            .padStart(length, '0')
+    }
+}