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Filename : Encoding.php
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<?php

namespace PleskGrafana\Defuse\Crypto;

use PleskGrafana\Defuse\Crypto\Exception as Ex;
final class Encoding
{
    const CHECKSUM_BYTE_SIZE = 32;
    const CHECKSUM_HASH_ALGO = 'sha256';
    const SERIALIZE_HEADER_BYTES = 4;
    /**
     * Converts a byte string to a hexadecimal string without leaking
     * information through side channels.
     *
     * @param string $byte_string
     *
     * @throws Ex\EnvironmentIsBrokenException
     *
     * @return string
     */
    public static function binToHex($byte_string)
    {
        $hex = '';
        $len = Core::ourStrlen($byte_string);
        for ($i = 0; $i < $len; ++$i) {
            $c = \ord($byte_string[$i]) & 0xf;
            $b = \ord($byte_string[$i]) >> 4;
            $hex .= \pack('CC', 87 + $b + ($b - 10 >> 8 & ~38), 87 + $c + ($c - 10 >> 8 & ~38));
        }
        return $hex;
    }
    /**
     * Converts a hexadecimal string into a byte string without leaking
     * information through side channels.
     *
     * @param string $hex_string
     *
     * @throws Ex\BadFormatException
     * @throws Ex\EnvironmentIsBrokenException
     *
     * @return string
     * @psalm-suppress TypeDoesNotContainType
     */
    public static function hexToBin($hex_string)
    {
        $hex_pos = 0;
        $bin = '';
        $hex_len = Core::ourStrlen($hex_string);
        $state = 0;
        $c_acc = 0;
        while ($hex_pos < $hex_len) {
            $c = \ord($hex_string[$hex_pos]);
            $c_num = $c ^ 48;
            $c_num0 = $c_num - 10 >> 8;
            $c_alpha = ($c & ~32) - 55;
            $c_alpha0 = ($c_alpha - 10 ^ $c_alpha - 16) >> 8;
            if (($c_num0 | $c_alpha0) === 0) {
                throw new Ex\BadFormatException('Encoding::hexToBin() input is not a hex string.');
            }
            $c_val = $c_num0 & $c_num | $c_alpha & $c_alpha0;
            if ($state === 0) {
                $c_acc = $c_val * 16;
            } else {
                $bin .= \pack('C', $c_acc | $c_val);
            }
            $state ^= 1;
            ++$hex_pos;
        }
        return $bin;
    }
    /**
     * Remove trialing whitespace without table look-ups or branches.
     *
     * Calling this function may leak the length of the string as well as the
     * number of trailing whitespace characters through side-channels.
     *
     * @param string $string
     * @return string
     */
    public static function trimTrailingWhitespace($string = '')
    {
        $length = Core::ourStrlen($string);
        if ($length < 1) {
            return '';
        }
        do {
            $prevLength = $length;
            $last = $length - 1;
            $chr = \ord($string[$last]);
            /* Null Byte (0x00), a.k.a. \0 */
            // if ($chr === 0x00) $length -= 1;
            $sub = $chr - 1 >> 8 & 1;
            $length -= $sub;
            $last -= $sub;
            /* Horizontal Tab (0x09) a.k.a. \t */
            $chr = \ord($string[$last]);
            // if ($chr === 0x09) $length -= 1;
            $sub = (0x8 - $chr & $chr - 0xa) >> 8 & 1;
            $length -= $sub;
            $last -= $sub;
            /* New Line (0x0a), a.k.a. \n */
            $chr = \ord($string[$last]);
            // if ($chr === 0x0a) $length -= 1;
            $sub = (0x9 - $chr & $chr - 0xb) >> 8 & 1;
            $length -= $sub;
            $last -= $sub;
            /* Carriage Return (0x0D), a.k.a. \r */
            $chr = \ord($string[$last]);
            // if ($chr === 0x0d) $length -= 1;
            $sub = (0xc - $chr & $chr - 0xe) >> 8 & 1;
            $length -= $sub;
            $last -= $sub;
            /* Space */
            $chr = \ord($string[$last]);
            // if ($chr === 0x20) $length -= 1;
            $sub = (0x1f - $chr & $chr - 0x21) >> 8 & 1;
            $length -= $sub;
        } while ($prevLength !== $length && $length > 0);
        return (string) Core::ourSubstr($string, 0, $length);
    }
    /*
     * SECURITY NOTE ON APPLYING CHECKSUMS TO SECRETS:
     *
     *      The checksum introduces a potential security weakness. For example,
     *      suppose we apply a checksum to a key, and that an adversary has an
     *      exploit against the process containing the key, such that they can
     *      overwrite an arbitrary byte of memory and then cause the checksum to
     *      be verified and learn the result.
     *
     *      In this scenario, the adversary can extract the key one byte at
     *      a time by overwriting it with their guess of its value and then
     *      asking if the checksum matches. If it does, their guess was right.
     *      This kind of attack may be more easy to implement and more reliable
     *      than a remote code execution attack.
     *
     *      This attack also applies to authenticated encryption as a whole, in
     *      the situation where the adversary can overwrite a byte of the key
     *      and then cause a valid ciphertext to be decrypted, and then
     *      determine whether the MAC check passed or failed.
     *
     *      By using the full SHA256 hash instead of truncating it, I'm ensuring
     *      that both ways of going about the attack are equivalently difficult.
     *      A shorter checksum of say 32 bits might be more useful to the
     *      adversary as an oracle in case their writes are coarser grained.
     *
     *      Because the scenario assumes a serious vulnerability, we don't try
     *      to prevent attacks of this style.
     */
    /**
     * INTERNAL USE ONLY: Applies a version header, applies a checksum, and
     * then encodes a byte string into a range of printable ASCII characters.
     *
     * @param string $header
     * @param string $bytes
     *
     * @throws Ex\EnvironmentIsBrokenException
     *
     * @return string
     */
    public static function saveBytesToChecksummedAsciiSafeString($header, #[\SensitiveParameter] $bytes)
    {
        // Headers must be a constant length to prevent one type's header from
        // being a prefix of another type's header, leading to ambiguity.
        Core::ensureTrue(Core::ourStrlen($header) === self::SERIALIZE_HEADER_BYTES, 'Header must be ' . self::SERIALIZE_HEADER_BYTES . ' bytes.');
        return Encoding::binToHex($header . $bytes . \hash(self::CHECKSUM_HASH_ALGO, $header . $bytes, \true));
    }
    /**
     * INTERNAL USE ONLY: Decodes, verifies the header and checksum, and returns
     * the encoded byte string.
     *
     * @param string $expected_header
     * @param string $string
     *
     * @throws Ex\EnvironmentIsBrokenException
     * @throws Ex\BadFormatException
     *
     * @return string
     */
    public static function loadBytesFromChecksummedAsciiSafeString($expected_header, #[\SensitiveParameter] $string)
    {
        // Headers must be a constant length to prevent one type's header from
        // being a prefix of another type's header, leading to ambiguity.
        Core::ensureTrue(Core::ourStrlen($expected_header) === self::SERIALIZE_HEADER_BYTES, 'Header must be 4 bytes.');
        /* If you get an exception here when attempting to load from a file, first pass your
           key to Encoding::trimTrailingWhitespace() to remove newline characters, etc.      */
        $bytes = Encoding::hexToBin($string);
        /* Make sure we have enough bytes to get the version header and checksum. */
        if (Core::ourStrlen($bytes) < self::SERIALIZE_HEADER_BYTES + self::CHECKSUM_BYTE_SIZE) {
            throw new Ex\BadFormatException('Encoded data is shorter than expected.');
        }
        /* Grab the version header. */
        $actual_header = (string) Core::ourSubstr($bytes, 0, self::SERIALIZE_HEADER_BYTES);
        if ($actual_header !== $expected_header) {
            throw new Ex\BadFormatException('Invalid header.');
        }
        /* Grab the bytes that are part of the checksum. */
        $checked_bytes = (string) Core::ourSubstr($bytes, 0, Core::ourStrlen($bytes) - self::CHECKSUM_BYTE_SIZE);
        /* Grab the included checksum. */
        $checksum_a = (string) Core::ourSubstr($bytes, Core::ourStrlen($bytes) - self::CHECKSUM_BYTE_SIZE, self::CHECKSUM_BYTE_SIZE);
        /* Re-compute the checksum. */
        $checksum_b = \hash(self::CHECKSUM_HASH_ALGO, $checked_bytes, \true);
        /* Check if the checksum matches. */
        if (!Core::hashEquals($checksum_a, $checksum_b)) {
            throw new Ex\BadFormatException("Data is corrupted, the checksum doesn't match");
        }
        return (string) Core::ourSubstr($bytes, self::SERIALIZE_HEADER_BYTES, Core::ourStrlen($bytes) - self::SERIALIZE_HEADER_BYTES - self::CHECKSUM_BYTE_SIZE);
    }
}