資料加密解密

加密：透過演算法將明文變為密文

單向加密：密文無法還原成明文，如 MD5、SHA
雙向加密：密文可以還原成明文

對稱加密（私鑰）：加密解密是同一把金鑰，如 DES、AES
非對稱加密（公鑰）：公鑰加密私鑰解密、私鑰加密公鑰解密，如 RSA、DSA

單向加密

以雜湊計算出16進位的「摘要」，相當於指紋
即同一資料的摘要會相同
常用於比對資料傳輸的一致性

使用 java.security.MessageDigest 的範例

// "MD2", "MD5", "SHA1", "SHA-256", "SHA-384", "SHA-512"
public static String sumString(String algorithm, String string) throws NoSuchAlgorithmException {
    return md(algorithm, string.getBytes());
}

public static String sumFile(String algorithm, String path) throws NoSuchAlgorithmException, IOException {
    byte[] input = Files.readAllBytes(Paths.get(path));
    return md(algorithm, input);
}

private static String md(String algorithm, byte[] input) throws NoSuchAlgorithmException {
    MessageDigest md = MessageDigest.getInstance(algorithm);
    md.update(input);
    byte[] hash = md.digest();
    return DatatypeConverter.printHexBinary(hash);
}

測試

String[] algorithms = new String[] { "MD2", "MD5", "SHA1", "SHA-256", "SHA-384", "SHA-512" };
String[] strings = new String[] { "AAAAAAAAAAA", "BBBBBBBBBBB", "CCCCCCCCCCC" };
String[] files = new String[] { "C:/aaa.txt", "C:/bbb.jpg", "C:/ccc.pdf" };

for (String string : strings) {
    System.out.println();
    System.out.println("字串: " + string);
    for (String algorithm : algorithms) {
        System.out.println(algorithm + ": " + sumString(algorithm, string));
    }
}

for (String file : files) {
    System.out.println();
    System.out.println("檔案: " + file);
    for (String algorithm : algorithms) {
        System.out.println(algorithm + ": " + sumFile(algorithm, file));
    }
}

AES 加密

encryption algorithm/operation modes/padding scheme

AES
AES/ECB/NoPadding
AES/ECB/PKCS5Padding
Blowfish/ECB/PKCS5Padding
DES/ECB/PKCS5Padding
AES/CBC/PKCS5Padding
Blowfish/CBC/PKCS5Padding
DES/CBC/PKCS5Padding
AES/CBC/PKCS7Padding
Blowfish/CBC/PKCS7Padding
DES/CBC/PKCS7Padding
RSA/NONE/NoPadding
AES/GCM/NoPadding
RSA/None/OAEPWithSHA-1AndMGF1Padding
RSA/None/OAEPWITHSHA-256ANDMGF1PADDING
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