我正在使用Firebase实时数据库制作聊天应用程序。我知道Firebase十分安全（只要您的规则正确），但是我自己可以使用我的应用程序阅读人们的所有聊天记录。
我想停止此操作，为此，我需要一种解密和加密方法。我尝试使用Caesar的解密，但是在此过程中失败了。
String encrypt(String talk, int key){
for(int i=0;i<talk.length;i++)
//can't think of how to procceed from here
我想知道是否有一种方法可以在这里实现凯撒的加密，否则，我应该使用哪种其他加密？
答案 0 :(得分：0)
您可以在客户端上生成一些加密密钥（例如，基于用户的凭据），然后将其安全地存储在客户端（例如，在KeyStore中或根据其他最低SDK版本使用其他方法）。然后使用加密密钥和AES（或任何其他标准）在发送/接收时对消息进行加密/解密。
答案 1 :(得分：0)
在使用此词时，凯撒密码实际上不是“密码”或加密方法。它实际上是25种不同编码的集合。根据定义，编码不是加密的，也不安全。如果您正在寻找用于生产的解决方案，那么Caesar Cipher绝对不是。破坏微不足道，完全不提供安全性。
您应该应该做的是清楚地定义自己要保护的威胁模型和攻击媒介。从这里开始，您应该咨询具有实际加密经验的人，以设计出解决所发现问题的方法。
尽管您可能不会这样做，但没人会这样做，因为他们认为他们更了解。如果您决定不这样做，则至少要花一些时间来学习一些基本的密码学概念。
这是我的own repository here中的一些Java代码，展示了一种使用给定密码加密和解密字符串的安全方法：
public class SecureCompatibleEncryptionExamples {
private final static String ALGORITHM_NAME = "AES/GCM/NoPadding";
private final static int ALGORITHM_NONCE_SIZE = 12;
private final static int ALGORITHM_TAG_SIZE = 128;
private final static int ALGORITHM_KEY_SIZE = 128;
private final static String PBKDF2_NAME = "PBKDF2WithHmacSHA256";
private final static int PBKDF2_SALT_SIZE = 16;
private final static int PBKDF2_ITERATIONS = 32767;
public static String encryptString(String plaintext, String password) throws NoSuchAlgorithmException, InvalidKeySpecException, InvalidKeyException, InvalidAlgorithmParameterException, NoSuchPaddingException, IllegalBlockSizeException, BadPaddingException {
// Generate a 128-bit salt using a CSPRNG.
SecureRandom rand = new SecureRandom();
byte[] salt = new byte[PBKDF2_SALT_SIZE];
rand.nextBytes(salt);
// Create an instance of PBKDF2 and derive a key.
PBEKeySpec pwSpec = new PBEKeySpec(password.toCharArray(), salt, PBKDF2_ITERATIONS, ALGORITHM_KEY_SIZE);
SecretKeyFactory keyFactory = SecretKeyFactory.getInstance(PBKDF2_NAME);
byte[] key = keyFactory.generateSecret(pwSpec).getEncoded();
// Encrypt and prepend salt.
byte[] ciphertextAndNonce = encrypt(plaintext.getBytes(StandardCharsets.UTF_8), key);
byte[] ciphertextAndNonceAndSalt = new byte[salt.length + ciphertextAndNonce.length];
System.arraycopy(salt, 0, ciphertextAndNonceAndSalt, 0, salt.length);
System.arraycopy(ciphertextAndNonce, 0, ciphertextAndNonceAndSalt, salt.length, ciphertextAndNonce.length);
// Return as base64 string.
return Base64.getEncoder().encodeToString(ciphertextAndNonceAndSalt);
}
public static String decryptString(String base64CiphertextAndNonceAndSalt, String password) throws NoSuchAlgorithmException, InvalidKeySpecException, InvalidKeyException, InvalidAlgorithmParameterException, IllegalBlockSizeException, BadPaddingException, NoSuchPaddingException {
// Decode the base64.
byte[] ciphertextAndNonceAndSalt = Base64.getDecoder().decode(base64CiphertextAndNonceAndSalt);
// Retrieve the salt and ciphertextAndNonce.
byte[] salt = new byte[PBKDF2_SALT_SIZE];
byte[] ciphertextAndNonce = new byte[ciphertextAndNonceAndSalt.length - PBKDF2_SALT_SIZE];
System.arraycopy(ciphertextAndNonceAndSalt, 0, salt, 0, salt.length);
System.arraycopy(ciphertextAndNonceAndSalt, salt.length, ciphertextAndNonce, 0, ciphertextAndNonce.length);
// Create an instance of PBKDF2 and derive the key.
PBEKeySpec pwSpec = new PBEKeySpec(password.toCharArray(), salt, PBKDF2_ITERATIONS, ALGORITHM_KEY_SIZE);
SecretKeyFactory keyFactory = SecretKeyFactory.getInstance(PBKDF2_NAME);
byte[] key = keyFactory.generateSecret(pwSpec).getEncoded();
// Decrypt and return result.
return new String(decrypt(ciphertextAndNonce, key), StandardCharsets.UTF_8);
}
public static byte[] encrypt(byte[] plaintext, byte[] key) throws InvalidKeyException, InvalidAlgorithmParameterException, NoSuchAlgorithmException, NoSuchPaddingException, IllegalBlockSizeException, BadPaddingException {
// Generate a 96-bit nonce using a CSPRNG.
SecureRandom rand = new SecureRandom();
byte[] nonce = new byte[ALGORITHM_NONCE_SIZE];
rand.nextBytes(nonce);
// Create the cipher instance and initialize.
Cipher cipher = Cipher.getInstance(ALGORITHM_NAME);
cipher.init(Cipher.ENCRYPT_MODE, new SecretKeySpec(key, "AES"), new GCMParameterSpec(ALGORITHM_TAG_SIZE, nonce));
// Encrypt and prepend nonce.
byte[] ciphertext = cipher.doFinal(plaintext);
byte[] ciphertextAndNonce = new byte[nonce.length + ciphertext.length];
System.arraycopy(nonce, 0, ciphertextAndNonce, 0, nonce.length);
System.arraycopy(ciphertext, 0, ciphertextAndNonce, nonce.length, ciphertext.length);
return ciphertextAndNonce;
}
public static byte[] decrypt(byte[] ciphertextAndNonce, byte[] key) throws InvalidKeyException, InvalidAlgorithmParameterException, IllegalBlockSizeException, BadPaddingException, NoSuchAlgorithmException, NoSuchPaddingException {
// Retrieve the nonce and ciphertext.
byte[] nonce = new byte[ALGORITHM_NONCE_SIZE];
byte[] ciphertext = new byte[ciphertextAndNonce.length - ALGORITHM_NONCE_SIZE];
System.arraycopy(ciphertextAndNonce, 0, nonce, 0, nonce.length);
System.arraycopy(ciphertextAndNonce, nonce.length, ciphertext, 0, ciphertext.length);
// Create the cipher instance and initialize.
Cipher cipher = Cipher.getInstance(ALGORITHM_NAME);
cipher.init(Cipher.DECRYPT_MODE, new SecretKeySpec(key, "AES"), new GCMParameterSpec(ALGORITHM_TAG_SIZE, nonce));
// Decrypt and return result.
return cipher.doFinal(ciphertext);
}
}
答案 2 :(得分：-1)
您可以使用Caesar的加密，但是要在接收方解密消息，还必须将key存储在Firebase中。
但是您可以使用rand()来获取key，并在存储它之前对它进行任何数学运算，这对您来说是不容易理解的，这也是随机的。
这看起来确实很难，但事实并非如此。凯撒的加密和解密代码如下所示：
private String encryptMessage(String talk, int k){
// make the string encrypted before sending to the database
k = k % 26 + 26;
StringBuilder encoded = new StringBuilder();
for (char i : talk.toCharArray()) {
if (Character.isLetter(i)) {
if (Character.isUpperCase(i)) {
encoded.append((char) ('A' + (i - 'A' + k) % 26 ));
}
else {
encoded.append((char) ('a' + (i - 'a' + k) % 26 ));
}
}
else {
encoded.append(i);
}
}
return encoded.toString();
}
private String decryptMessage(String m, int key){
// make string readable on the receiver's device
return encryptMessage(m,26-key);
}