initial commit

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

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+package com.infendro.otp
+
+import com.infendro.hash.HashFunction
+import com.infendro.hash.SHA1
+import kotlin.time.Duration
+import kotlin.time.Duration.Companion.seconds
+import kotlin.time.ExperimentalTime
+import kotlin.time.Instant
+
+@ExperimentalTime
+class TOTP(
+    function: HashFunction = SHA1,
+    length: Int = 6,
+    val period: Duration = 30.seconds,
+) {
+    val hotp = HOTP(function, length)
+
+    fun verify(
+        secret: ByteArray,
+        instant: Instant,
+        otp: String,
+    ): Boolean {
+        return generate(secret, instant) == otp
+    }
+
+    fun generate(
+        secret: ByteArray,
+        instant: Instant,
+    ): String {
+        return hotp.generate(
+            secret,
+            counter(instant)
+        )
+    }
+
+    private fun counter(
+        instant: Instant,
+    ): Long {
+        val ms = instant.toEpochMilliseconds()
+        val d = period.inWholeMilliseconds
+        return ms / d
+    }
+}