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How to encrypt or decrypt a file in Kotlin

How to encrypt or decrypt a file in Kotlin.

Here's a step-by-step tutorial on how to encrypt or decrypt a file in Kotlin.

Step 1: Setting Up the Project​

  1. Create a new Kotlin project in your preferred IDE.
  2. Add the following dependency to your project's build.gradle file:
implementation 'org.bouncycastle:bcprov-jdk15on:1.68'

This will enable the use of the Bouncy Castle library for encryption and decryption.

Step 2: Encrypting a File​

  1. Create a function named encryptFile to perform the encryption:
import org.bouncycastle.jce.provider.BouncyCastleProvider
import java.io.File
import java.io.FileOutputStream
import java.security.Security
import javax.crypto.Cipher
import javax.crypto.CipherOutputStream
import javax.crypto.SecretKey
import javax.crypto.SecretKeyFactory
import javax.crypto.spec.PBEKeySpec
import javax.crypto.spec.PBEParameterSpec

fun encryptFile(inputFile: File, outputFile: File, password: String) {
    Security.addProvider(BouncyCastleProvider())

    val salt = byteArrayOf(0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08)
    val iterationCount = 1000

    val keySpec = PBEKeySpec(password.toCharArray())
    val secretKeyFactory = SecretKeyFactory.getInstance("PBEWithSHA256And256BitAES-CBC-BC")
    val secretKey: SecretKey = secretKeyFactory.generateSecret(keySpec)

    val cipher = Cipher.getInstance("PBEWithSHA256And256BitAES-CBC-BC")
    val pbeParamSpec = PBEParameterSpec(salt, iterationCount)
    cipher.init(Cipher.ENCRYPT_MODE, secretKey, pbeParamSpec)

    val inputStream = inputFile.inputStream()
    val outputStream = CipherOutputStream(FileOutputStream(outputFile), cipher)

    inputStream.copyTo(outputStream)

    outputStream.flush()
    outputStream.close()
    inputStream.close()
}
  1. In the main function, call the encryptFile function with the appropriate file paths and password:
fun main() {
    val inputFile = File("path/to/input/file.txt")
    val outputFile = File("path/to/output/encrypted_file.txt")
    val password = "mysecretpassword"

    encryptFile(inputFile, outputFile, password)
}

Step 3: Decrypting a File​

  1. Create a function named decryptFile to perform the decryption:
fun decryptFile(inputFile: File, outputFile: File, password: String) {
    Security.addProvider(BouncyCastleProvider())

    val salt = byteArrayOf(0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08)
    val iterationCount = 1000

    val keySpec = PBEKeySpec(password.toCharArray())
    val secretKeyFactory = SecretKeyFactory.getInstance("PBEWithSHA256And256BitAES-CBC-BC")
    val secretKey: SecretKey = secretKeyFactory.generateSecret(keySpec)

    val cipher = Cipher.getInstance("PBEWithSHA256And256BitAES-CBC-BC")
    val pbeParamSpec = PBEParameterSpec(salt, iterationCount)
    cipher.init(Cipher.DECRYPT_MODE, secretKey, pbeParamSpec)

    val inputStream = CipherInputStream(inputFile.inputStream(), cipher)
    val outputStream = FileOutputStream(outputFile)

    inputStream.copyTo(outputStream)

    outputStream.flush()
    outputStream.close()
    inputStream.close()
}
  1. In the main function, call the decryptFile function with the appropriate file paths and password:
fun main() {
    val inputFile = File("path/to/input/encrypted_file.txt")
    val outputFile = File("path/to/output/decrypted_file.txt")
    val password = "mysecretpassword"

    decryptFile(inputFile, outputFile, password)
}

That's it! You now have the necessary code to encrypt and decrypt a file in Kotlin using the Bouncy Castle library. Remember to replace the file paths with the actual paths to your input and output files, and choose a strong and secure password for encryption.

This tutorial used the PBEWithSHA256And256BitAES-CBC-BC algorithm for encryption and decryption. Feel free to explore other algorithms supported by Bouncy Castle based on your specific requirements.