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The Enterprise Security API (ESAPI) project is an OWASP project to create simple strong security controls for every web platform. Security controls are not simple to build. You can read about the hundreds of pitfalls for unwary developers on the OWASP website. By providing developers with a set of strong controls, we aim to eliminate some of the complexity of creating secure web applications. This can result in significant cost savings across the SDLC.

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/*
 * OWASP Enterprise Security API (ESAPI)
 * 
 * This file is part of the Open Web Application Security Project (OWASP)
 * Enterprise Security API (ESAPI) project. For details, please see
 * http://www.owasp.org/index.php/ESAPI.
 *
 * Copyright (c) 2009 - The OWASP Foundation
 */
package org.owasp.esapi.codecs;

/** Encode and decode to/from hexadecimal strings to byte arrays. */
public class Hex {

    /** Output byte representation as hexadecimal representation.
     * 
     * @param b				Bytes to encode to hexadecimal representation.
     * @param leading0x		If true, return with leading "0x".
     * @return				Hexadecimal representation of specified bytes.
     */
    public static String toHex(byte[] b, boolean leading0x) {
        StringBuffer hexString = new StringBuffer();
        if ( leading0x ) {
            hexString.append("0x");
        }
        for ( int i = 0; i < b.length; i++ ) {
            int j = b[i] & 0xff;        // Convert byte to int.
            String hex = Integer.toHexString(j);
            if (hex.length() == 1) {
                hexString.append('0');  // Add leading zero.
            }
            hexString.append(hex);      // Hex digit(s).
        }
        return hexString.toString();
    }

    /**
     * Output byte representation as hexadecimal representation.
     * Alias for toHex() method.
     * 
     * @param b				Bytes to encode to hexadecimal representation.
     * @param leading0x		If true, return with leading "0x".
     * @return				Hexadecimal representation of specified bytes.
     */
    public static String encode(byte[] b, boolean leading0x) {
        return toHex(b, leading0x);
    }

    /**
     * Decode hexadecimal-encoded string and return raw byte array.
     * Important note: This method preserves leading 0 filled bytes on the
     * conversion process, which is important for cryptographic operations
     * in dealing with things like keys, initialization vectors, etc. For
     * example, the string "0x0000face" is going to return a byte array
     * whose length is 4, not 2.
     * 
     * @param hexStr	Hexadecimal-encoded string, with or without leading "0x".
     * @return			The equivalent byte array.
     */
    public static byte[] fromHex(String hexStr) {
        String hexRep = hexStr;
        if ( hexStr.startsWith("0x") ) {
            // Then skip over the leading 0x.
            hexRep = hexStr.substring(2);
        }
        int len = hexRep.length() / 2;
        byte[] rawBytes = new byte[len];
        for (int i = 0; i < len; i++) {
            String substr = hexRep.substring(i * 2, (i * 2) + 2);
            rawBytes[i] = (byte)(Integer.parseInt(substr, 16));
        }
        return rawBytes;
    }

    /** Decode hexadecimal-encoded string and return raw byte array.
     * Alias for fromHex() method.
     * 
     * @param hexStr	Hexadecimal-encoded string, with or without leading "0x".
     * @return			The equivalent byte array. 
     */
    public static byte[] decode(String hexStr) {
        return fromHex(hexStr);
    }
}