Arakne / ArakneUtils

public final class XorCipher {

    private final @MinLen(1) String key;

    public XorCipher(@MinLen(1) String key) {

        this.key = key;

    }

    /**

     * Get the key used by the cipher

     *

     * @return The key

     */

    @Pure

    public @MinLen(1) String key() {

        return key;

    }

    /**

     * Encrypt the value using the current key

     * The encrypted value is an upper case hexadecimal string

     *

     * https://github.com/Emudofus/Dofus/blob/1.29/dofus/aks/Aks.as#L297

     *

     * @param value Value to encrypt

     * @param keyOffset Offset to use for the key

     *

     * @return Encrypted value

     */

    @SideEffectFree

    @SuppressWarnings("cast.unsafe") // XOR will return an int, so cast to char cannot be checked

    public String encrypt(String value, @NonNegative int keyOffset) {

        final String plain = escape(value);

        final StringBuilder encrypted = new StringBuilder(plain.length() * 2);

        for (int i = 0; i < plain.length(); ++i) {

            final char c = plain.charAt(i);

            final char k = key.charAt((i + keyOffset) % key.length());

            final char e = (char) (c ^ k);

            if (e < 16) {

                encrypted.append('0');

            }

            encrypted.append(Integer.toHexString(e));

        }

        return encrypted.toString().toUpperCase();

    }

    /**

     * Decrypt the value using the current key

     *

     * https://github.com/Emudofus/Dofus/blob/1.29/dofus/aks/Aks.as#L314

     *

     * @param value Value to decrypt. Must be a valid hexadecimal string

     * @param keyOffset Offset to use on the key. Must be the same used for encryption.

     *

     * @return The decrypted value

     *

     * @throws IllegalArgumentException When an invalid string is given

     * @throws NumberFormatException When an invalid hexadecimal string is given

     */

    @SideEffectFree

    @SuppressWarnings({

        "cast.unsafe",  // XOR will return an int, so cast to char cannot be checked

        "array.access.unsafe.high.range" // i / 2 is not correctly resolved

    })

    public String decrypt(String value, @NonNegative int keyOffset) {

        if (value.length() % 2 != 0) {

            throw new IllegalArgumentException("Invalid encrypted value");

        }

        final char[] decrypted = new char[value.length() / 2];

        for (int i = 0; i < value.length() - 1; i += 2) {

            final char k = key.charAt((i / 2 + keyOffset) % key.length());

            final char c = (char) Integer.parseInt(value.substring(i, i + 2), 16);

            decrypted[i / 2] = (char) (c ^ k);

        }

        try {

            return URLDecoder.decode(new String(decrypted), StandardCharsets.UTF_8.toString());

        } catch (UnsupportedEncodingException e) {

            throw new IllegalArgumentException("Invalid UTF-8 character", e);

        }

    }

    /**

     * Escape value using URL encode

     *

     * https://github.com/Emudofus/Dofus/blob/1.29/dofus/aks/Aks.as#L275

     */

    @SideEffectFree

    private static String escape(String value) {

        final StringBuilder escaped = new StringBuilder(value.length());

        try {

            for (int i = 0; i < value.length(); ++i) {

                final char c = value.charAt(i);

                if (c < 32 || c > 127 || c == '%' || c == '+') {

                    escaped.append(URLEncoder.encode(String.valueOf(c), StandardCharsets.UTF_8.toString()));

                } else {

                    escaped.append(c);

                }

            }

        } catch (UnsupportedEncodingException e) {

            throw new IllegalArgumentException("Invalid UTF-8 character", e);

        }

        return escaped.toString();