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/*
* 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)
}
}
}
}
}
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