【Kotlinプログラム入門】。今回は入門レベル1 暗号化・復号化の使い方・書き方を勉強してみましょう！

package com.sample.sample

import android.os.Bundle
import androidx.activity.ComponentActivity
import androidx.activity.compose.setContent
import androidx.compose.foundation.layout.Column
import androidx.compose.foundation.layout.fillMaxSize
import androidx.compose.material3.MaterialTheme
import androidx.compose.material3.Surface
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Modifier
import androidx.compose.ui.tooling.preview.Preview
import com.sample.sample.ui.theme.SampleTheme
//暗号化・復号化で使用
import javax.crypto.Cipher
import javax.crypto.spec.IvParameterSpec
import javax.crypto.spec.SecretKeySpec
import java.nio.charset.StandardCharsets
import java.util.Base64

class MainActivity : ComponentActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContent {
            SampleTheme {
                // A surface container using the 'background' color from the theme
                Surface(
                    modifier = Modifier.fillMaxSize(),
                    color = MaterialTheme.colorScheme.background
                ) {
                    //暗号化・復号化のサンプル
                    Sample("ポンコツ２人組の気ままにプログラム")
                }
            }
        }
    }
}

/*
 * 暗号化・復号化のサンプル
 */
@Composable
fun Sample(plainText: String, modifier: Modifier = Modifier) {
    //暗号化・復号化に用いるキー：16バイト
    val key = "0123456789123456"
    //暗号化
    val encryptedText = encrypt(plainText, key)
    //復号化
    val decryptedText = decrypt(encryptedText, key)
    //結果を表示
    Column {
        Text(text = "暗号化されたテキスト: $encryptedText", modifier = modifier)
        Text(text = "復号化されたテキスト: $decryptedText", modifier = modifier)
    }
}

/*
 * 暗号化
 */
fun encrypt(plainText: String, key: String): String {
    val cipher = Cipher.getInstance("AES/CBC/PKCS5Padding")
    val secretKeySpec = SecretKeySpec(key.toByteArray(StandardCharsets.UTF_8), "AES")
    val iv = ByteArray(cipher.blockSize)
    val ivParameterSpec = IvParameterSpec(iv)
    cipher.init(Cipher.ENCRYPT_MODE, secretKeySpec, ivParameterSpec)
    val cipherText = cipher.doFinal(plainText.toByteArray(StandardCharsets.UTF_8))
    return Base64.getEncoder().encodeToString(cipherText)
}

/*
 * 復号化
 */
fun decrypt(cipherText: String, key: String): String {
    val cipher = Cipher.getInstance("AES/CBC/PKCS5Padding")
    val secretKeySpec = SecretKeySpec(key.toByteArray(StandardCharsets.UTF_8), "AES")
    val iv = ByteArray(cipher.blockSize)
    val ivParameterSpec = IvParameterSpec(iv)
    cipher.init(Cipher.DECRYPT_MODE, secretKeySpec, ivParameterSpec)
    val decryptedText = cipher.doFinal(Base64.getDecoder().decode(cipherText))
    return String(decryptedText, StandardCharsets.UTF_8)
}

@Preview(showBackground = true)
@Composable
fun GreetingPreview() {
    SampleTheme {
        Sample("ポンコツ２人組の気ままにプログラム")
    }
}