Android之RSA解密

加密解密接觸的最多的莫過於MD5的加密解密，今天我們介紹一下RSA的解密。RSA一般有兩個鑰，一個公鑰用於加密，一個私鑰用於解密。

下面簡單的說一下，這個過程：

1）我們命名一個PRIVATE_KEY類型爲String的祕鑰，將祕鑰值賦給PRIVATE_KEY

2）將PRIVATE_KEY轉化爲PrivateKey對象，方法如下：

/**
 * 從字符串中加載私鑰
 * @param privateKeyStr
 * @throws Exception
 */
public static PrivateKey loadPrivateKey(String privateKeyStr) throws Exception {
    try {
        byte[] buffer = Base64Utils.decode(privateKeyStr);
        PKCS8EncodedKeySpec keySpec = new PKCS8EncodedKeySpec(buffer);
        KeyFactory keyFactory = KeyFactory.getInstance("RSA");
        return (RSAPrivateKey) keyFactory.generatePrivate(keySpec);
    } catch (NoSuchAlgorithmException e) {
        throw new Exception("無此算法");
    } catch (InvalidKeySpecException e) {
        throw new Exception("私鑰非法");
    } catch (NullPointerException e) {
        throw new Exception("私鑰數據爲空");
    }
}

3）將需解密的字符串轉化成byte[]

byte[] needJiemiData = Base64Utils.decode(moniData);

4)RSA解密數據

/**
 * RSA解密
 *
 * @param privateKey 私鑰
 * @param cipherData 密文數據
 * @return 明文
 * @throws Exception 解密過程中的異常信息
 */
public static byte[] decrypt(PrivateKey privateKey, byte[] cipherData) throws Exception {
    if (privateKey == null) {
        throw new Exception("解密私鑰爲空, 請設置");
    }
    Cipher cipher = null;
    try {
        cipher = Cipher.getInstance("RSA/ECB/PKCS1Padding");
        cipher.init(Cipher.DECRYPT_MODE, privateKey);
        return cipher.doFinal(cipherData);
    } catch (NoSuchAlgorithmException e) {
        throw new Exception("無此解密算法");
    } catch (NoSuchPaddingException e) {
        e.printStackTrace();
        return null;
    } catch (InvalidKeyException e) {
        throw new Exception("解密私鑰非法,請檢查");
    } catch (IllegalBlockSizeException e) {
        throw new Exception("密文長度非法");
    } catch (BadPaddingException e) {
        throw new Exception("密文數據已損壞");
    }
}

最後這邊貼一下Base64Utils代碼

public class Base64Utils {
    private static char[] base64EncodeChars = new char[]
            {'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T',
                    'U', 'V', 'W', 'X', 'Y', 'Z', 'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
                    'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z', '0', '1', '2', '3', '4', '5',
                    '6', '7', '8', '9', '+', '/'};
    private static byte[] base64DecodeChars = new byte[]
            {-1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63, 52, 53,
                    54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -1, -1, -1, -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
                    12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1, -1, 26, 27, 28, 29,
                    30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1,
                    -1, -1, -1};

    public static String encode(byte[] data) {
        StringBuffer sb = new StringBuffer();
        int len = data.length;
        int i = 0;
        int b1, b2, b3;
        while (i < len) {
            b1 = data[i++] & 0xff;
            if (i == len) {
                sb.append(base64EncodeChars[b1 >>> 2]);
                sb.append(base64EncodeChars[(b1 & 0x3) << 4]);
                sb.append("==");
                break;
            }
            b2 = data[i++] & 0xff;
            if (i == len) {
                sb.append(base64EncodeChars[b1 >>> 2]);
                sb.append(base64EncodeChars[((b1 & 0x03) << 4) | ((b2 & 0xf0) >>> 4)]);
                sb.append(base64EncodeChars[(b2 & 0x0f) << 2]);
                sb.append("=");
                break;
            }
            b3 = data[i++] & 0xff;
            sb.append(base64EncodeChars[b1 >>> 2]);
            sb.append(base64EncodeChars[((b1 & 0x03) << 4) | ((b2 & 0xf0) >>> 4)]);
            sb.append(base64EncodeChars[((b2 & 0x0f) << 2) | ((b3 & 0xc0) >>> 6)]);
            sb.append(base64EncodeChars[b3 & 0x3f]);
        }
        return sb.toString();
    }

    public static byte[] decode(String str) {
        try {
            return decodePrivate(str);
        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        }
        return new byte[]
                {};
    }

    private static byte[] decodePrivate(String str) throws UnsupportedEncodingException {
        StringBuffer sb = new StringBuffer();
        byte[] data = null;
        data = str.getBytes("US-ASCII");
        int len = data.length;
        int i = 0;
        int b1, b2, b3, b4;
        while (i < len) {

            do {
                b1 = base64DecodeChars[data[i++]];
            } while (i < len && b1 == -1);
            if (b1 == -1) {
                break;
            }

            do {
                b2 = base64DecodeChars[data[i++]];
            } while (i < len && b2 == -1);
            if (b2 == -1) {
                break;
            }
            sb.append((char) ((b1 << 2) | ((b2 & 0x30) >>> 4)));

            do {
                b3 = data[i++];
                if (b3 == 61) {
                    return sb.toString().getBytes("iso8859-1");
                }
                b3 = base64DecodeChars[b3];
            } while (i < len && b3 == -1);
            if (b3 == -1) {
                break;
            }
            sb.append((char) (((b2 & 0x0f) << 4) | ((b3 & 0x3c) >>> 2)));

            do {
                b4 = data[i++];
                if (b4 == 61) {
                    return sb.toString().getBytes("iso8859-1");
                }
                b4 = base64DecodeChars[b4];
            } while (i < len && b4 == -1);
            if (b4 == -1) {
                break;
            }
            sb.append((char) (((b3 & 0x03) << 6) | b4));
        }
        return sb.toString().getBytes("iso8859-1");
    }

}