android信息-摘要校验信息

在android中，常用检验信息.防止信息被篡改。

最简单方式md5:

public static String md5( String plainText) {
		String result = null;
		try {
			MessageDigest md = MessageDigest.getInstance("MD5");
			md.update(plainText.getBytes());
			byte b[] = md.digest();
			int i;
			StringBuffer buf = new StringBuffer("");
			for (int offset = 0; offset < b.length; offset++) {
				i = b[offset];
				if (i < 0)
					i += 256;
				if (i < 16)
					buf.append("0");
				buf.append(Integer.toHexString(i));
			}
			// result = buf.toString(); //md5 32bit
			// result = buf.toString().substring(8, 24))); //md5 16bit
			result = buf.toString().substring(8, 24);
			System.out.println("mdt 16bit: " + buf.toString().substring(8, 24));
			System.out.println("md5 32bit: " + buf.toString());
		} catch (NoSuchAlgorithmException e) {
			e.printStackTrace();
		}
		return result;
	}

但是这样也不能完全保证传递的信息正确。用户可能同时改变数据和md5值。这样也无法保证数据完全正确。

下面用加密信息摘要校验，更安全。

import java.io.UnsupportedEncodingException;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Arrays;

public class DigestUtil {

	private static String encodingCharset = "UTF-8";
	
	/**
	 * @param aValue
	 * @param aKey
	 * @return
	 */
	public static String hmacSign(String aValue, String aKey) {
		byte k_ipad[] = new byte[64];
		byte k_opad[] = new byte[64];
		byte keyb[];
		byte value[];
		try {
			keyb = aKey.getBytes(encodingCharset);
			value = aValue.getBytes(encodingCharset);
		} catch (UnsupportedEncodingException e) {
			keyb = aKey.getBytes();
			value = aValue.getBytes();
		}

		Arrays.fill(k_ipad, keyb.length, 64, (byte) 54);
		Arrays.fill(k_opad, keyb.length, 64, (byte) 92);
		for (int i = 0; i < keyb.length; i++) {
			k_ipad[i] = (byte) (keyb[i] ^ 0x36);
			k_opad[i] = (byte) (keyb[i] ^ 0x5c);
		}

		MessageDigest md = null;
		try {
			md = MessageDigest.getInstance("MD5");
		} catch (NoSuchAlgorithmException e) {

			return null;
		}
		md.update(k_ipad);
		md.update(value);
		byte dg[] = md.digest();
		md.reset();
		md.update(k_opad);
		md.update(dg, 0, 16);
		dg = md.digest();
		return toHex(dg);
	}

	public static String toHex(byte input[]) {
		if (input == null)
			return null;
		StringBuffer output = new StringBuffer(input.length * 2);
		for (int i = 0; i < input.length; i++) {
			int current = input[i] & 0xff;
			if (current < 16)
				output.append("0");
			output.append(Integer.toString(current, 16));
		}

		return output.toString();
	}

	/**
	 * 
	 * @param args
	 * @param key
	 * @return
	 */
	public static String getHmac(String[] args, String key) {
		if (args == null || args.length == 0) {
			return (null);
		}
		StringBuffer str = new StringBuffer();
		for (int i = 0; i < args.length; i++) {
			str.append(args[i]);
		}
		return (hmacSign(str.toString(), key));
	}

	/**
	 * @param aValue
	 * @return
	 */
	public static String digest(String aValue) {
		aValue = aValue.trim();
		byte value[];
		try {
			value = aValue.getBytes(encodingCharset);
		} catch (UnsupportedEncodingException e) {
			value = aValue.getBytes();
		}
		MessageDigest md = null;
		try {
			md = MessageDigest.getInstance("SHA");
		} catch (NoSuchAlgorithmException e) {
			e.printStackTrace();
			return null;
		}
		return toHex(md.digest(value));

	}
	
	}
}

调用：String den = DigestUtil.hmacSign(string, key);只要客户端和服务器端同一个key加密检验信息，就能保证信息完整。