/**
     * Combine user credentials into an appropriately formatted string.
     *
     * @param credentials An array of credential strings. Credential
     *                    strings may contain any character but ';'
     *                    (semicolon), '\\' (backslash), and control
     *                    characters (with ASCII codes 0-31 and 127).
     *
     * @return <CODE>null</CODE> only if any credential strings are invalid.
     */
    public String getCredentialsString(String[] credentials) {

        // Create a buffer with which to generate the credentials string.
        StringBuffer buffer = new StringBuffer();

        // Verify and add each credential to the buffer.
        if (credentials != null) {
            for (int i = 0; i < credentials.length; ++i) {
                if (i > 0) buffer.append(';');
                for (int j = 0, n = credentials[i].length(); j < n; ++j) {
                    char c = credentials[i].charAt(j);
                    if (c != ';' && c != '\\' && c >= ' ' && c != 127) {
                        buffer.append(c);
                    } else {
                        return null;
                    }
                }
            }
        }

        // Return the credentials string.
        return buffer.toString();

    }