Итак, я нашел эквивалентный код в Kotlin:
private fun decrypt(cryptedData: ByteArray): String {
val cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding") //"PKCS1Padding" is the default padding
cipher.init(Cipher.DECRYPT_MODE, key)
val res = cipher.doFinal(cryptedData).toHexString()
val decrypted = hexToAscii(res)
return decrypted
}