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發表者：冷日發表時間： 2019/5/27 6:25:12

How to hash a string in Android?

I am working on an Android app and have a couple strings that I would like to encrypt before sending to a database. I'd like something that's secure, easy to implement, will generate the same thing every time it's passed the same data, and preferably will result in a string that stays a constant length no matter how large the string being passed to it is. Maybe I'm looking for a hash.

This snippet calculate md5 for any given string

public String md5(String s) {
    try {
        // Create MD5 Hash
        MessageDigest digest = java.security.MessageDigest.getInstance("MD5");
        digest.update(s.getBytes());
        byte messageDigest[] = digest.digest();

        // Create Hex String
        StringBuffer hexString = new StringBuffer();
        for (int i=0; i<messageDigest.length; i++)
            hexString.append(Integer.toHexString(0xFF & messageDigest[i]));
        return hexString.toString();

    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    }
    return "";
}

Source: http://www.androidsnippets.com/snippets/52/index.html

Hope this is useful for you

That function above from (http://www.androidsnippets.org/snippets/52/index.html) is flawed. If one of the digits in the messageDigest is not a two character hex value (i.e. 0x09), it doesn't work properly because it doesn't pad with a 0. If you search around you'll find that function and complaints about it not working. Here a better one found in the comment section of this page, which I slightly modified:

public static String md5(String s)
{
    MessageDigest digest;
    try
    {
        digest = MessageDigest.getInstance("MD5");
        digest.update(s.getBytes(Charset.forName("US-ASCII")),0,s.length());
        byte[] magnitude = digest.digest();
        BigInteger bi = new BigInteger(1, magnitude);
        String hash = String.format("%0" + (magnitude.length << 1) + "x", bi);
        return hash;
    }
    catch (NoSuchAlgorithmException e)
    {
        e.printStackTrace();
    }
    return "";
}

not working method:

public static String md5(String s) {
    try {
        // Create MD5 Hash
        MessageDigest digest = java.security.MessageDigest
                .getInstance("MD5");
        digest.update(s.getBytes());
        byte messageDigest[] = digest.digest();

        // Create Hex String
        StringBuffer hexString = new StringBuffer();
        for (int i = 0; i < messageDigest.length; i++)
            hexString.append(Integer.toHexString(0xFF & messageDigest[i]));
        return hexString.toString();

    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    }
    return "";
}

result - 1865e62e7129927f6e4cd9bff104f0 (length 30)
working method:

public static final String md5(final String toEncrypt) {
    try {
        final MessageDigest digest = MessageDigest.getInstance("md5");
        digest.update(toEncrypt.getBytes());
        final byte[] bytes = digest.digest();
        final StringBuilder sb = new StringBuilder();
        for (int i = 0; i < bytes.length; i++) {
            sb.append(String.format("%02X", bytes[i]));
        }
        return sb.toString().toLowerCase();
    } catch (Exception exc) {
        return ""; // Impossibru!
    }
}

result - 1865e62e7129927f6e4c0d9bff1004f0 (length 32)

private static char[] hextable = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };

public static String byteArrayToHex(byte[] array) {
    String s = "";
    for (int i = 0; i < array.length; ++i) {
        int di = (array[i] + 256) & 0xFF; // Make it unsigned
        s = s + hextable[(di >> 4) & 0xF] + hextable[di & 0xF];
    }
    return s;
}

public static String digest(String s, String algorithm) {
    MessageDigest m = null;
    try {
        m = MessageDigest.getInstance(algorithm);
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
        return s;
    }

    m.update(s.getBytes(), 0, s.length());
    return byteArrayToHex(m.digest());
}

public static String md5(String s) {
    return digest(s, "MD5");
}

The answer above is almost 100% correct. It will fail with unicode.

    MessageDigest digest;
    try {
        digest = MessageDigest.getInstance("MD5");
        byte utf8_bytes[] = tag_xml.getBytes();
        digest.update(utf8_bytes,0,utf8_bytes.length);
        hash = new BigInteger(1, digest.digest()).toString(16);
    }
    catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    }

Need the length from the byte array not the string.

With @Donut solution, with UTF-8 encoded characters (eg: é) you have to use getBytes("UTF-8"). Here is my correction of the digest method:

private static char[] hextable = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f'};

public static String byteArrayToHex(byte[] array) {
    String s = "";
    for (int i = 0; i < array.length; ++i) {
        int di = (array[i] + 256) & 0xFF; // Make it unsigned
        s = s + hextable[(di >> 4) & 0xF] + hextable[di & 0xF];
    }
    return s;
}

public static String digest(String s, String algorithm) {
    MessageDigest m = null;
    try {
        m = MessageDigest.getInstance(algorithm);
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
        return s;
    }

    try {
        m.update(s.getBytes("UTF-8"));
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
        m.update(s.getBytes());
    }
    return byteArrayToHex(m.digest());
}

public static String md5(String s) {
    return digest(s, "MD5");
}

Donut's solution in a single function:

private static char[] hextable = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e', 'f' };

private static String md5(String s)
{
    MessageDigest digest;
    try
    {
        digest = MessageDigest.getInstance("MD5");
        digest.update(s.getBytes(), 0, s.length());
        byte[] bytes = digest.digest();

        String hash = "";
        for (int i = 0; i < bytes.length; ++i)
        {
            int di = (bytes[i] + 256) & 0xFF;
            hash = hash + hextable[(di >> 4) & 0xF] + hextable[di & 0xF];
        }

        return hash;
    }
    catch (NoSuchAlgorithmException e)
    {
    }

    return "";
}

The following worked for me on Android without truncating any 0's infront:

MessageDigest md = null;
String digest = null;
    try {
        md = MessageDigest.getInstance("MD5");

        byte[] hash = md.digest(myStringToEncode.getBytes("UTF-8")); //converting byte array to Hexadecimal String
        StringBuilder sb = new StringBuilder(2*hash.length);

        for(byte b : hash){
            sb.append(String.format("%02x", b&0xff));
        }

        digest = sb.toString();

    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    }
return digest;

MessageDigest md = MessageDigest.getInstance("MD5");
md.update('yourstring');
byte[] digest = md.digest();
StringBuffer sb = new StringBuffer();
for (byte b : digest) {
    sb.append(String.format("%02x", (0xFF & b)));
}

It's late for the author, but before this, I get Integer.toHexString(0xff&b) , which strips leading 0s from the hex string. It makes me struggled for a long time. Hope useful for some guys.

if you are using guava:

public String generateMd5(String input) {
    HashFunction hf = Hashing.md5();
    Hasher hasher = hf.newHasher();
    HashCode hc = hasher.putString(input, StandardCharsets.UTF_8).hash();
    return hc.toString();
}

If you didn't have security constraints and just wanted to convert String to a unique int. I'm writing it because that what I looked for and reached here.

String my_key
int my_key.hashCode()

if you have up to 10 chars it will even be unique See also https://stackoverflow.com/a/17583653/1984636

This not missing '0'

  public static String md5(String string) {
        if (TextUtils.isEmpty(string)) {
            return "";
        }
        MessageDigest md5 = null;
        try {
            md5 = MessageDigest.getInstance("MD5");
            byte[] bytes = md5.digest(string.getBytes());
            String result = "";
            for (byte b : bytes) {
                String temp = Integer.toHexString(b & 0xff);
                if (temp.length() == 1) {
                    temp = "0" + temp;
                }
                result += temp;
            }
            return result;
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
        }
        return "";
    }

原文出處：java - How to hash a string in Android? - Stack Overflow

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