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<?php

/**
 * Pure-PHP ASN.1 Parser
 *
 * PHP version 5
 *
 * ASN.1 provides the semantics for data encoded using various schemes.  The most commonly
 * utilized scheme is DER or the "Distinguished Encoding Rules".  PEM's are base64 encoded
 * DER blobs.
 *
 * \phpseclib3\File\ASN1 decodes and encodes DER formatted messages and places them in a semantic context.
 *
 * Uses the 1988 ASN.1 syntax.
 *
 * @author    Jim Wigginton <terrafrost@php.net>
 * @copyright 2012 Jim Wigginton
 * @license   http://www.opensource.org/licenses/mit-license.html  MIT License
 * @link      http://phpseclib.sourceforge.net
 */

namespace phpseclib3\File;

use phpseclib3\Common\Functions\Strings;
use phpseclib3\File\ASN1\Element;
use phpseclib3\Math\BigInteger;

/**
 * Pure-PHP ASN.1 Parser
 *
 * @author  Jim Wigginton <terrafrost@php.net>
 */
abstract class ASN1
{
    // Tag Classes
    // http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf#page=12
    const CLASS_UNIVERSAL        = 0;
    const CLASS_APPLICATION      = 1;
    const CLASS_CONTEXT_SPECIFIC = 2;
    const CLASS_PRIVATE          = 3;

    // Tag Classes
    // http://www.obj-sys.com/asn1tutorial/node124.html
    const TYPE_BOOLEAN           = 1;
    const TYPE_INTEGER           = 2;
    const TYPE_BIT_STRING        = 3;
    const TYPE_OCTET_STRING      = 4;
    const TYPE_NULL              = 5;
    const TYPE_OBJECT_IDENTIFIER = 6;
    //const TYPE_OBJECT_DESCRIPTOR = 7;
    //const TYPE_INSTANCE_OF       = 8; // EXTERNAL
    const TYPE_REAL              = 9;
    const TYPE_ENUMERATED        = 10;
    //const TYPE_EMBEDDED          = 11;
    const TYPE_UTF8_STRING       = 12;
    //const TYPE_RELATIVE_OID      = 13;
    const TYPE_SEQUENCE          = 16; // SEQUENCE OF
    const TYPE_SET               = 17; // SET OF

    // More Tag Classes
    // http://www.obj-sys.com/asn1tutorial/node10.html
    const TYPE_NUMERIC_STRING   = 18;
    const TYPE_PRINTABLE_STRING = 19;
    const TYPE_TELETEX_STRING   = 20; // T61String
    const TYPE_VIDEOTEX_STRING  = 21;
    const TYPE_IA5_STRING       = 22;
    const TYPE_UTC_TIME         = 23;
    const TYPE_GENERALIZED_TIME = 24;
    const TYPE_GRAPHIC_STRING   = 25;
    const TYPE_VISIBLE_STRING   = 26; // ISO646String
    const TYPE_GENERAL_STRING   = 27;
    const TYPE_UNIVERSAL_STRING = 28;
    //const TYPE_CHARACTER_STRING = 29;
    const TYPE_BMP_STRING       = 30;

    // Tag Aliases
    // These tags are kinda place holders for other tags.
    const TYPE_CHOICE = -1;
    const TYPE_ANY    = -2;

    /**
     * ASN.1 object identifiers
     *
     * @var array
     * @link http://en.wikipedia.org/wiki/Object_identifier
     */
    private static $oids = [];

    /**
     * ASN.1 object identifier reverse mapping
     *
     * @var array
     */
    private static $reverseOIDs = [];

    /**
     * Default date format
     *
     * @var string
     * @link http://php.net/class.datetime
     */
    private static $format = 'D, d M Y H:i:s O';

    /**
     * Filters
     *
     * If the mapping type is self::TYPE_ANY what do we actually encode it as?
     *
     * @var array
     * @see self::encode_der()
     */
    private static $filters;

    /**
     * Current Location of most recent ASN.1 encode process
     *
     * Useful for debug purposes
     *
     * @var array
     * @see self::encode_der()
     */
    private static $location;

    /**
     * DER Encoded String
     *
     * In case we need to create ASN1\Element object's..
     *
     * @var string
     * @see self::decodeDER()
     */
    private static $encoded;

    /**
     * Type mapping table for the ANY type.
     *
     * Structured or unknown types are mapped to a \phpseclib3\File\ASN1\Element.
     * Unambiguous types get the direct mapping (int/real/bool).
     * Others are mapped as a choice, with an extra indexing level.
     *
     * @var array
     */
    const ANY_MAP = [
        self::TYPE_BOOLEAN              => true,
        self::TYPE_INTEGER              => true,
        self::TYPE_BIT_STRING           => 'bitString',
        self::TYPE_OCTET_STRING         => 'octetString',
        self::TYPE_NULL                 => 'null',
        self::TYPE_OBJECT_IDENTIFIER    => 'objectIdentifier',
        self::TYPE_REAL                 => true,
        self::TYPE_ENUMERATED           => 'enumerated',
        self::TYPE_UTF8_STRING          => 'utf8String',
        self::TYPE_NUMERIC_STRING       => 'numericString',
        self::TYPE_PRINTABLE_STRING     => 'printableString',
        self::TYPE_TELETEX_STRING       => 'teletexString',
        self::TYPE_VIDEOTEX_STRING      => 'videotexString',
        self::TYPE_IA5_STRING           => 'ia5String',
        self::TYPE_UTC_TIME             => 'utcTime',
        self::TYPE_GENERALIZED_TIME     => 'generalTime',
        self::TYPE_GRAPHIC_STRING       => 'graphicString',
        self::TYPE_VISIBLE_STRING       => 'visibleString',
        self::TYPE_GENERAL_STRING       => 'generalString',
        self::TYPE_UNIVERSAL_STRING     => 'universalString',
        //self::TYPE_CHARACTER_STRING     => 'characterString',
        self::TYPE_BMP_STRING           => 'bmpString'
    ];

    /**
     * String type to character size mapping table.
     *
     * Non-convertable types are absent from this table.
     * size == 0 indicates variable length encoding.
     *
     * @var array
     */
    const STRING_TYPE_SIZE = [
        self::TYPE_UTF8_STRING      => 0,
        self::TYPE_BMP_STRING       => 2,
        self::TYPE_UNIVERSAL_STRING => 4,
        self::TYPE_PRINTABLE_STRING => 1,
        self::TYPE_TELETEX_STRING   => 1,
        self::TYPE_IA5_STRING       => 1,
        self::TYPE_VISIBLE_STRING   => 1,
    ];

    /**
     * Parse BER-encoding
     *
     * Serves a similar purpose to openssl's asn1parse
     *
     * @param Element|string $encoded
     * @return ?array
     */
    public static function decodeBER($encoded)
    {
        if ($encoded instanceof Element) {
            $encoded = $encoded->element;
        }

        self::$encoded = $encoded;

        $decoded = self::decode_ber($encoded);
        if ($decoded === false) {
            return null;
        }

        return [$decoded];
    }

    /**
     * Parse BER-encoding (Helper function)
     *
     * Sometimes we want to get the BER encoding of a particular tag.  $start lets us do that without having to reencode.
     * $encoded is passed by reference for the recursive calls done for self::TYPE_BIT_STRING and
     * self::TYPE_OCTET_STRING. In those cases, the indefinite length is used.
     *
     * @param string $encoded
     * @param int $start
     * @param int $encoded_pos
     * @return array|bool
     */
    private static function decode_ber($encoded, $start = 0, $encoded_pos = 0)
    {
        $current = ['start' => $start];

        if (!isset($encoded[$encoded_pos])) {
            return false;
        }
        $type = ord($encoded[$encoded_pos++]);
        $startOffset = 1;

        $constructed = ($type >> 5) & 1;

        $tag = $type & 0x1F;
        if ($tag == 0x1F) {
            $tag = 0;
            // process septets (since the eighth bit is ignored, it's not an octet)
            do {
                if (!isset($encoded[$encoded_pos])) {
                    return false;
                }
                $temp = ord($encoded[$encoded_pos++]);
                $startOffset++;
                $loop = $temp >> 7;
                $tag <<= 7;
                $temp &= 0x7F;
                // "bits 7 to 1 of the first subsequent octet shall not all be zero"
                if ($startOffset == 2 && $temp == 0) {
                    return false;
                }
                $tag |= $temp;
            } while ($loop);
        }

        $start += $startOffset;

        // Length, as discussed in paragraph 8.1.3 of X.690-0207.pdf#page=13
        if (!isset($encoded[$encoded_pos])) {
            return false;
        }
        $length = ord($encoded[$encoded_pos++]);
        $start++;
        if ($length == 0x80) { // indefinite length
            // "[A sender shall] use the indefinite form (see 8.1.3.6) if the encoding is constructed and is not all
            //  immediately available." -- paragraph 8.1.3.2.c
            $length = strlen($encoded) - $encoded_pos;
        } elseif ($length & 0x80) { // definite length, long form
            // technically, the long form of the length can be represented by up to 126 octets (bytes), but we'll only
            // support it up to four.
            $length &= 0x7F;
            $temp = substr($encoded, $encoded_pos, $length);
            $encoded_pos += $length;
            // tags of indefinte length don't really have a header length; this length includes the tag
            $current += ['headerlength' => $length + 2];
            $start += $length;
            extract(unpack('Nlength', substr(str_pad($temp, 4, chr(0), STR_PAD_LEFT), -4)));
            /** @var integer $length */
        } else {
            $current += ['headerlength' => 2];
        }

        if ($length > (strlen($encoded) - $encoded_pos)) {
            return false;
        }

        $content = substr($encoded, $encoded_pos, $length);
        $content_pos = 0;

        // at this point $length can be overwritten. it's only accurate for definite length things as is

        /* Class is UNIVERSAL, APPLICATION, PRIVATE, or CONTEXT-SPECIFIC. The UNIVERSAL class is restricted to the ASN.1
           built-in types. It defines an application-independent data type that must be distinguishable from all other
           data types. The other three classes are user defined. The APPLICATION class distinguishes data types that
           have a wide, scattered use within a particular presentation context. PRIVATE distinguishes data types within
           a particular organization or country. CONTEXT-SPECIFIC distinguishes members of a sequence or set, the
           alternatives of a CHOICE, or universally tagged set members. Only the class number appears in braces for this
           data type; the term CONTEXT-SPECIFIC does not appear.

             -- http://www.obj-sys.com/asn1tutorial/node12.html */
        $class = ($type >> 6) & 3;
        switch ($class) {
            case self::CLASS_APPLICATION:
            case self::CLASS_PRIVATE:
            case self::CLASS_CONTEXT_SPECIFIC:
                if (!$constructed) {
                    return [
                        'type'     => $class,
                        'constant' => $tag,
                        'content'  => $content,
                        'length'   => $length + $start - $current['start']
                    ] + $current;
                }

                $newcontent = [];
                $remainingLength = $length;
                while ($remainingLength > 0) {
                    $temp = self::decode_ber($content, $start, $content_pos);
                    if ($temp === false) {
                        break;
                    }
                    $length = $temp['length'];
                    // end-of-content octets - see paragraph 8.1.5
                    if (substr($content, $content_pos + $length, 2) == "\0\0") {
                        $length += 2;
                        $start += $length;
                        $newcontent[] = $temp;
                        break;
                    }
                    $start += $length;
                    $remainingLength -= $length;
                    $newcontent[] = $temp;
                    $content_pos += $length;
                }

                return [
                    'type'     => $class,
                    'constant' => $tag,
                    // the array encapsulation is for BC with the old format
                    'content'  => $newcontent,
                    // the only time when $content['headerlength'] isn't defined is when the length is indefinite.
                    // the absence of $content['headerlength'] is how we know if something is indefinite or not.
                    // technically, it could be defined to be 2 and then another indicator could be used but whatever.
                    'length'   => $start - $current['start']
                ] + $current;
        }

        $current += ['type' => $tag];

        // decode UNIVERSAL tags
        switch ($tag) {
            case self::TYPE_BOOLEAN:
                // "The contents octets shall consist of a single octet." -- paragraph 8.2.1
                if ($constructed || strlen($content) != 1) {
                    return false;
                }
                $current['content'] = (bool) ord($content[$content_pos]);
                break;
            case self::TYPE_INTEGER:
            case self::TYPE_ENUMERATED:
                if ($constructed) {
                    return false;
                }
                $current['content'] = new BigInteger(substr($content, $content_pos), -256);
                break;
            case self::TYPE_REAL: // not currently supported
                return false;
            case self::TYPE_BIT_STRING:
                // The initial octet shall encode, as an unsigned binary integer with bit 1 as the least significant bit,
                // the number of unused bits in the final subsequent octet. The number shall be in the range zero to
                // seven.
                if (!$constructed) {
                    $current['content'] = substr($content, $content_pos);
                } else {
                    $temp = self::decode_ber($content, $start, $content_pos);
                    if ($temp === false) {
                        return false;
                    }
                    $length -= (strlen($content) - $content_pos);
                    $last = count($temp) - 1;
                    for ($i = 0; $i < $last; $i++) {
                        // all subtags should be bit strings
                        if ($temp[$i]['type'] != self::TYPE_BIT_STRING) {
                            return false;
                        }
                        $current['content'] .= substr($temp[$i]['content'], 1);
                    }
                    // all subtags should be bit strings
                    if ($temp[$last]['type'] != self::TYPE_BIT_STRING) {
                        return false;
                    }
                    $current['content'] = $temp[$last]['content'][0] . $current['content'] . substr($temp[$i]['content'], 1);
                }
                break;
            case self::TYPE_OCTET_STRING:
                if (!$constructed) {
                    $current['content'] = substr($content, $content_pos);
                } else {
                    $current['content'] = '';
                    $length = 0;
                    while (substr($content, $content_pos, 2) != "\0\0") {
                        $temp = self::decode_ber($content, $length + $start, $content_pos);
                        if ($temp === false) {
                            return false;
                        }
                        $content_pos += $temp['length'];
                        // all subtags should be octet strings
                        if ($temp['type'] != self::TYPE_OCTET_STRING) {
                            return false;
                        }
                        $current['content'] .= $temp['content'];
                        $length += $temp['length'];
                    }
                    if (substr($content, $content_pos, 2) == "\0\0") {
                        $length += 2; // +2 for the EOC
                    }
                }
                break;
            case self::TYPE_NULL:
                // "The contents octets shall not contain any octets." -- paragraph 8.8.2
                if ($constructed || strlen($content)) {
                    return false;
                }
                break;
            case self::TYPE_SEQUENCE:
            case self::TYPE_SET:
                if (!$constructed) {
                    return false;
                }
                $offset = 0;
                $current['content'] = [];
                $content_len = strlen($content);
                while ($content_pos < $content_len) {
                    // if indefinite length construction was used and we have an end-of-content string next
                    // see paragraphs 8.1.1.3, 8.1.3.2, 8.1.3.6, 8.1.5, and (for an example) 8.6.4.2
                    if (!isset($current['headerlength']) && substr($content, $content_pos, 2) == "\0\0") {
                        $length = $offset + 2; // +2 for the EOC
                        break 2;
                    }
                    $temp = self::decode_ber($content, $start + $offset, $content_pos);
                    if ($temp === false) {
                        return false;
                    }
                    $content_pos += $temp['length'];
                    $current['content'][] = $temp;
                    $offset += $temp['length'];
                }
                break;
            case self::TYPE_OBJECT_IDENTIFIER:
                if ($constructed) {
                    return false;
                }
                $current['content'] = self::decodeOID(substr($content, $content_pos));
                if ($current['content'] === false) {
                    return false;
                }
                break;
            /* Each character string type shall be encoded as if it had been declared:
               [UNIVERSAL x] IMPLICIT OCTET STRING

                 -- X.690-0207.pdf#page=23 (paragraph 8.21.3)

               Per that, we're not going to do any validation.  If there are any illegal characters in the string,
               we don't really care */
            case self::TYPE_NUMERIC_STRING:
                // 0,1,2,3,4,5,6,7,8,9, and space
            case self::TYPE_PRINTABLE_STRING:
                // Upper and lower case letters, digits, space, apostrophe, left/right parenthesis, plus sign, comma,
                // hyphen, full stop, solidus, colon, equal sign, question mark
            case self::TYPE_TELETEX_STRING:
                // The Teletex character set in CCITT's T61, space, and delete
                // see http://en.wikipedia.org/wiki/Teletex#Character_sets
            case self::TYPE_VIDEOTEX_STRING:
                // The Videotex character set in CCITT's T.100 and T.101, space, and delete
            case self::TYPE_VISIBLE_STRING:
                // Printing character sets of international ASCII, and space
            case self::TYPE_IA5_STRING:
                // International Alphabet 5 (International ASCII)
            case self::TYPE_GRAPHIC_STRING:
                // All registered G sets, and space
            case self::TYPE_GENERAL_STRING:
                // All registered C and G sets, space and delete
            case self::TYPE_UTF8_STRING:
                // ????
            case self::TYPE_BMP_STRING:
                if ($constructed) {
                    return false;
                }
                $current['content'] = substr($content, $content_pos);
                break;
            case self::TYPE_UTC_TIME:
            case self::TYPE_GENERALIZED_TIME:
                if ($constructed) {
                    return false;
                }
                $current['content'] = self::decodeTime(substr($content, $content_pos), $tag);
                break;
            default:
                return false;
        }

        $start += $length;

        // ie. length is the length of the full TLV encoding - it's not just the length of the value
        return $current + ['length' => $start - $current['start']];
    }

    /**
     * ASN.1 Map
     *
     * Provides an ASN.1 semantic mapping ($mapping) from a parsed BER-encoding to a human readable format.
     *
     * "Special" mappings may be applied on a per tag-name basis via $special.
     *
     * @param array $decoded
     * @param array $mapping
     * @param array $special
     * @return array|bool|Element|string|null
     */
    public static function asn1map(array $decoded, $mapping, $special = [])
    {
        if (isset($mapping['explicit']) && is_array($decoded['content'])) {
            $decoded = $decoded['content'][0];
        }

        switch (true) {
            case $mapping['type'] == self::TYPE_ANY:
                $intype = $decoded['type'];
                // !isset(self::ANY_MAP[$intype]) produces a fatal error on PHP 5.6
                if (isset($decoded['constant']) || !array_key_exists($intype, self::ANY_MAP) || (ord(self::$encoded[$decoded['start']]) & 0x20)) {
                    return new Element(substr(self::$encoded, $decoded['start'], $decoded['length']));
                }
                $inmap = self::ANY_MAP[$intype];
                if (is_string($inmap)) {
                    return [$inmap => self::asn1map($decoded, ['type' => $intype] + $mapping, $special)];
                }
                break;
            case $mapping['type'] == self::TYPE_CHOICE:
                foreach ($mapping['children'] as $key => $option) {
                    switch (true) {
                        case isset($option['constant']) && $option['constant'] == $decoded['constant']:
                        case !isset($option['constant']) && $option['type'] == $decoded['type']:
                            $value = self::asn1map($decoded, $option, $special);
                            break;
                        case !isset($option['constant']) && $option['type'] == self::TYPE_CHOICE:
                            $v = self::asn1map($decoded, $option, $special);
                            if (isset($v)) {
                                $value = $v;
                            }
                    }
                    if (isset($value)) {
                        if (isset($special[$key])) {
                            $value = $special[$key]($value);
                        }
                        return [$key => $value];
                    }
                }
                return null;
            case isset($mapping['implicit']):
            case isset($mapping['explicit']):
            case $decoded['type'] == $mapping['type']:
                break;
            default:
                // if $decoded['type'] and $mapping['type'] are both strings, but different types of strings,
                // let it through
                switch (true) {
                    case $decoded['type'] < 18: // self::TYPE_NUMERIC_STRING == 18
                    case $decoded['type'] > 30: // self::TYPE_BMP_STRING == 30
                    case $mapping['type'] < 18:
                    case $mapping['type'] > 30:
                        return null;
                }
        }

        if (isset($mapping['implicit'])) {
            $decoded['type'] = $mapping['type'];
        }

        switch ($decoded['type']) {
            case self::TYPE_SEQUENCE:
                $map = [];

                // ignore the min and max
                if (isset($mapping['min']) && isset($mapping['max'])) {
                    $child = $mapping['children'];
                    foreach ($decoded['content'] as $content) {
                        if (($map[] = self::asn1map($content, $child, $special)) === null) {
                            return null;
                        }
                    }

                    return $map;
                }

                $n = count($decoded['content']);
                $i = 0;

                foreach ($mapping['children'] as $key => $child) {
                    $maymatch = $i < $n; // Match only existing input.
                    if ($maymatch) {
                        $temp = $decoded['content'][$i];

                        if ($child['type'] != self::TYPE_CHOICE) {
                            // Get the mapping and input class & constant.
                            $childClass = $tempClass = self::CLASS_UNIVERSAL;
                            $constant = null;
                            if (isset($temp['constant'])) {
                                $tempClass = $temp['type'];
                            }
                            if (isset($child['class'])) {
                                $childClass = $child['class'];
                                $constant = $child['cast'];
                            } elseif (isset($child['constant'])) {
                                $childClass = self::CLASS_CONTEXT_SPECIFIC;
                                $constant = $child['constant'];
                            }

                            if (isset($constant) && isset($temp['constant'])) {
                                // Can only match if constants and class match.
                                $maymatch = $constant == $temp['constant'] && $childClass == $tempClass;
                            } else {
                                // Can only match if no constant expected and type matches or is generic.
                                $maymatch = !isset($child['constant']) && array_search($child['type'], [$temp['type'], self::TYPE_ANY, self::TYPE_CHOICE]) !== false;
                            }
                        }
                    }

                    if ($maymatch) {
                        // Attempt submapping.
                        $candidate = self::asn1map($temp, $child, $special);
                        $maymatch = $candidate !== null;
                    }

                    if ($maymatch) {
                        // Got the match: use it.
                        if (isset($special[$key])) {
                            $candidate = $special[$key]($candidate);
                        }
                        $map[$key] = $candidate;
                        $i++;
                    } elseif (isset($child['default'])) {
                        $map[$key] = $child['default'];
                    } elseif (!isset($child['optional'])) {
                        return null; // Syntax error.
                    }
                }

                // Fail mapping if all input items have not been consumed.
                return $i < $n ? null : $map;

            // the main diff between sets and sequences is the encapsulation of the foreach in another for loop
            case self::TYPE_SET:
                $map = [];

                // ignore the min and max
                if (isset($mapping['min']) && isset($mapping['max'])) {
                    $child = $mapping['children'];
                    foreach ($decoded['content'] as $content) {
                        if (($map[] = self::asn1map($content, $child, $special)) === null) {
                            return null;
                        }
                    }

                    return $map;
                }

                for ($i = 0; $i < count($decoded['content']); $i++) {
                    $temp = $decoded['content'][$i];
                    $tempClass = self::CLASS_UNIVERSAL;
                    if (isset($temp['constant'])) {
                        $tempClass = $temp['type'];
                    }

                    foreach ($mapping['children'] as $key => $child) {
                        if (isset($map[$key])) {
                            continue;
                        }
                        $maymatch = true;
                        if ($child['type'] != self::TYPE_CHOICE) {
                            $childClass = self::CLASS_UNIVERSAL;
                            $constant = null;
                            if (isset($child['class'])) {
                                $childClass = $child['class'];
                                $constant = $child['cast'];
                            } elseif (isset($child['constant'])) {
                                $childClass = self::CLASS_CONTEXT_SPECIFIC;
                                $constant = $child['constant'];
                            }

                            if (isset($constant) && isset($temp['constant'])) {
                                // Can only match if constants and class match.
                                $maymatch = $constant == $temp['constant'] && $childClass == $tempClass;
                            } else {
                                // Can only match if no constant expected and type matches or is generic.
                                $maymatch = !isset($child['constant']) && array_search($child['type'], [$temp['type'], self::TYPE_ANY, self::TYPE_CHOICE]) !== false;
                            }
                        }

                        if ($maymatch) {
                            // Attempt submapping.
                            $candidate = self::asn1map($temp, $child, $special);
                            $maymatch = $candidate !== null;
                        }

                        if (!$maymatch) {
                            break;
                        }

                        // Got the match: use it.
                        if (isset($special[$key])) {
                            $candidate = $special[$key]($candidate);
                        }
                        $map[$key] = $candidate;
                        break;
                    }
                }

                foreach ($mapping['children'] as $key => $child) {
                    if (!isset($map[$key])) {
                        if (isset($child['default'])) {
                            $map[$key] = $child['default'];
                        } elseif (!isset($child['optional'])) {
                            return null;
                        }
                    }
                }
                return $map;
            case self::TYPE_OBJECT_IDENTIFIER:
                return isset(self::$oids[$decoded['content']]) ? self::$oids[$decoded['content']] : $decoded['content'];
            case self::TYPE_UTC_TIME:
            case self::TYPE_GENERALIZED_TIME:
                // for explicitly tagged optional stuff
                if (is_array($decoded['content'])) {
                    $decoded['content'] = $decoded['content'][0]['content'];
                }
                // for implicitly tagged optional stuff
                // in theory, doing isset($mapping['implicit']) would work but malformed certs do exist
                // in the wild that OpenSSL decodes without issue so we'll support them as well
                if (!is_object($decoded['content'])) {
                    $decoded['content'] = self::decodeTime($decoded['content'], $decoded['type']);
                }
                return $decoded['content'] ? $decoded['content']->format(self::$format) : false;
            case self::TYPE_BIT_STRING:
                if (isset($mapping['mapping'])) {
                    $offset = ord($decoded['content'][0]);
                    $size = (strlen($decoded['content']) - 1) * 8 - $offset;
                    /*
                       From X.680-0207.pdf#page=46 (21.7):

                       "When a "NamedBitList" is used in defining a bitstring type ASN.1 encoding rules are free to add (or remove)
                        arbitrarily any trailing 0 bits to (or from) values that are being encoded or decoded. Application designers should
                        therefore ensure that different semantics are not associated with such values which differ only in the number of trailing
                        0 bits."
                    */
                    $bits = count($mapping['mapping']) == $size ? [] : array_fill(0, count($mapping['mapping']) - $size, false);
                    for ($i = strlen($decoded['content']) - 1; $i > 0; $i--) {
                        $current = ord($decoded['content'][$i]);
                        for ($j = $offset; $j < 8; $j++) {
                            $bits[] = (bool) ($current & (1 << $j));
                        }
                        $offset = 0;
                    }
                    $values = [];
                    $map = array_reverse($mapping['mapping']);
                    foreach ($map as $i => $value) {
                        if ($bits[$i]) {
                            $values[] = $value;
                        }
                    }
                    return $values;
                }
                // fall-through
            case self::TYPE_OCTET_STRING:
                return $decoded['content'];
            case self::TYPE_NULL:
                return '';
            case self::TYPE_BOOLEAN:
            case self::TYPE_NUMERIC_STRING:
            case self::TYPE_PRINTABLE_STRING:
            case self::TYPE_TELETEX_STRING:
            case self::TYPE_VIDEOTEX_STRING:
            case self::TYPE_IA5_STRING:
            case self::TYPE_GRAPHIC_STRING:
            case self::TYPE_VISIBLE_STRING:
            case self::TYPE_GENERAL_STRING:
            case self::TYPE_UNIVERSAL_STRING:
            case self::TYPE_UTF8_STRING:
            case self::TYPE_BMP_STRING:
                return $decoded['content'];
            case self::TYPE_INTEGER:
            case self::TYPE_ENUMERATED:
                $temp = $decoded['content'];
                if (isset($mapping['implicit'])) {
                    $temp = new BigInteger($decoded['content'], -256);
                }
                if (isset($mapping['mapping'])) {
                    $temp = (int) $temp->toString();
                    return isset($mapping['mapping'][$temp]) ?
                        $mapping['mapping'][$temp] :
                        false;
                }
                return $temp;
        }
    }

    /**
     * DER-decode the length
     *
     * DER supports lengths up to (2**8)**127, however, we'll only support lengths up to (2**8)**4.  See
     * {@link http://itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf#p=13 X.690 paragraph 8.1.3} for more information.
     *
     * @param string $string
     * @return int
     */
    public static function decodeLength(&$string)
    {
        $length = ord(Strings::shift($string));
        if ($length & 0x80) { // definite length, long form
            $length &= 0x7F;
            $temp = Strings::shift($string, $length);
            list(, $length) = unpack('N', substr(str_pad($temp, 4, chr(0), STR_PAD_LEFT), -4));
        }
        return $length;
    }

    /**
     * ASN.1 Encode
     *
     * DER-encodes an ASN.1 semantic mapping ($mapping).  Some libraries would probably call this function
     * an ASN.1 compiler.
     *
     * "Special" mappings can be applied via $special.
     *
     * @param Element|string|array $source
     * @param array $mapping
     * @param array $special
     * @return string
     */
    public static function encodeDER($source, $mapping, $special = [])
    {
        self::$location = [];
        return self::encode_der($source, $mapping, null, $special);
    }

    /**
     * ASN.1 Encode (Helper function)
     *
     * @param Element|string|array|null $source
     * @param array $mapping
     * @param int $idx
     * @param array $special
     * @return string
     */
    private static function encode_der($source, array $mapping, $idx = null, array $special = [])
    {
        if ($source instanceof Element) {
            return $source->element;
        }

        // do not encode (implicitly optional) fields with value set to default
        if (isset($mapping['default']) && $source === $mapping['default']) {
            return '';
        }

        if (isset($idx)) {
            if (isset($special[$idx])) {
                $source = $special[$idx]($source);
            }
            self::$location[] = $idx;
        }

        $tag = $mapping['type'];

        switch ($tag) {
            case self::TYPE_SET:    // Children order is not important, thus process in sequence.
            case self::TYPE_SEQUENCE:
                $tag |= 0x20; // set the constructed bit

                // ignore the min and max
                if (isset($mapping['min']) && isset($mapping['max'])) {
                    $value = [];
                    $child = $mapping['children'];

                    foreach ($source as $content) {
                        $temp = self::encode_der($content, $child, null, $special);
                        if ($temp === false) {
                            return false;
                        }
                        $value[] = $temp;
                    }
                    /* "The encodings of the component values of a set-of value shall appear in ascending order, the encodings being compared
                        as octet strings with the shorter components being padded at their trailing end with 0-octets.
                        NOTE - The padding octets are for comparison purposes only and do not appear in the encodings."

                       -- sec 11.6 of http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf  */
                    if ($mapping['type'] == self::TYPE_SET) {
                        sort($value);
                    }
                    $value = implode('', $value);
                    break;
                }

                $value = '';
                foreach ($mapping['children'] as $key => $child) {
                    if (!array_key_exists($key, $source)) {
                        if (!isset($child['optional'])) {
                            return false;
                        }
                        continue;
                    }

                    $temp = self::encode_der($source[$key], $child, $key, $special);
                    if ($temp === false) {
                        return false;
                    }

                    // An empty child encoding means it has been optimized out.
                    // Else we should have at least one tag byte.
                    if ($temp === '') {
                        continue;
                    }

                    // if isset($child['constant']) is true then isset($child['optional']) should be true as well
                    if (isset($child['constant'])) {
                        /*
                           From X.680-0207.pdf#page=58 (30.6):

                           "The tagging construction specifies explicit tagging if any of the following holds:
                            ...
                            c) the "Tag Type" alternative is used and the value of "TagDefault" for the module is IMPLICIT TAGS or
                            AUTOMATIC TAGS, but the type defined by "Type" is an untagged choice type, an untagged open type, or
                            an untagged "DummyReference" (see ITU-T Rec. X.683 | ISO/IEC 8824-4, 8.3)."
                         */
                        if (isset($child['explicit']) || $child['type'] == self::TYPE_CHOICE) {
                            $subtag = chr((self::CLASS_CONTEXT_SPECIFIC << 6) | 0x20 | $child['constant']);
                            $temp = $subtag . self::encodeLength(strlen($temp)) . $temp;
                        } else {
                            $subtag = chr((self::CLASS_CONTEXT_SPECIFIC << 6) | (ord($temp[0]) & 0x20) | $child['constant']);
                            $temp = $subtag . substr($temp, 1);
                        }
                    }
                    $value .= $temp;
                }
                break;
            case self::TYPE_CHOICE:
                $temp = false;

                foreach ($mapping['children'] as $key => $child) {
                    if (!isset($source[$key])) {
                        continue;
                    }

                    $temp = self::encode_der($source[$key], $child, $key, $special);
                    if ($temp === false) {
                        return false;
                    }

                    // An empty child encoding means it has been optimized out.
                    // Else we should have at least one tag byte.
                    if ($temp === '') {
                        continue;
                    }

                    $tag = ord($temp[0]);

                    // if isset($child['constant']) is true then isset($child['optional']) should be true as well
                    if (isset($child['constant'])) {
                        if (isset($child['explicit']) || $child['type'] == self::TYPE_CHOICE) {
                            $subtag = chr((self::CLASS_CONTEXT_SPECIFIC << 6) | 0x20 | $child['constant']);
                            $temp = $subtag . self::encodeLength(strlen($temp)) . $temp;
                        } else {
                            $subtag = chr((self::CLASS_CONTEXT_SPECIFIC << 6) | (ord($temp[0]) & 0x20) | $child['constant']);
                            $temp = $subtag . substr($temp, 1);
                        }
                    }
                }

                if (isset($idx)) {
                    array_pop(self::$location);
                }

                if ($temp && isset($mapping['cast'])) {
                    $temp[0] = chr(($mapping['class'] << 6) | ($tag & 0x20) | $mapping['cast']);
                }

                return $temp;
            case self::TYPE_INTEGER:
            case self::TYPE_ENUMERATED:
                if (!isset($mapping['mapping'])) {
                    if (is_numeric($source)) {
                        $source = new BigInteger($source);
                    }
                    $value = $source->toBytes(true);
                } else {
                    $value = array_search($source, $mapping['mapping']);
                    if ($value === false) {
                        return false;
                    }
                    $value = new BigInteger($value);
                    $value = $value->toBytes(true);
                }
                if (!strlen($value)) {
                    $value = chr(0);
                }
                break;
            case self::TYPE_UTC_TIME:
            case self::TYPE_GENERALIZED_TIME:
                $format = $mapping['type'] == self::TYPE_UTC_TIME ? 'y' : 'Y';
                $format .= 'mdHis';
                // if $source does _not_ include timezone information within it then assume that the timezone is GMT
                $date = new \DateTime($source, new \DateTimeZone('GMT'));
                // if $source _does_ include timezone information within it then convert the time to GMT
                $date->setTimezone(new \DateTimeZone('GMT'));
                $value = $date->format($format) . 'Z';
                break;
            case self::TYPE_BIT_STRING:
                if (isset($mapping['mapping'])) {
                    $bits = array_fill(0, count($mapping['mapping']), 0);
                    $size = 0;
                    for ($i = 0; $i < count($mapping['mapping']); $i++) {
                        if (in_array($mapping['mapping'][$i], $source)) {
                            $bits[$i] = 1;
                            $size = $i;
                        }
                    }

                    if (isset($mapping['min']) && $mapping['min'] >= 1 && $size < $mapping['min']) {
                        $size = $mapping['min'] - 1;
                    }

                    $offset = 8 - (($size + 1) & 7);
                    $offset = $offset !== 8 ? $offset : 0;

                    $value = chr($offset);

                    for ($i = $size + 1; $i < count($mapping['mapping']); $i++) {
                        unset($bits[$i]);
                    }

                    $bits = implode('', array_pad($bits, $size + $offset + 1, 0));
                    $bytes = explode(' ', rtrim(chunk_split($bits, 8, ' ')));
                    foreach ($bytes as $byte) {
                        $value .= chr(bindec($byte));
                    }

                    break;
                }
                // fall-through
            case self::TYPE_OCTET_STRING:
                /* The initial octet shall encode, as an unsigned binary integer with bit 1 as the least significant bit,
                   the number of unused bits in the final subsequent octet. The number shall be in the range zero to seven.

                   -- http://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf#page=16 */
                $value = $source;
                break;
            case self::TYPE_OBJECT_IDENTIFIER:
                $value = self::encodeOID($source);
                break;
            case self::TYPE_ANY:
                $loc = self::$location;
                if (isset($idx)) {
                    array_pop(self::$location);
                }

                switch (true) {
                    case !isset($source):
                        return self::encode_der(null, ['type' => self::TYPE_NULL] + $mapping, null, $special);
                    case is_int($source):
                    case $source instanceof BigInteger:
                        return self::encode_der($source, ['type' => self::TYPE_INTEGER] + $mapping, null, $special);
                    case is_float($source):
                        return self::encode_der($source, ['type' => self::TYPE_REAL] + $mapping, null, $special);
                    case is_bool($source):
                        return self::encode_der($source, ['type' => self::TYPE_BOOLEAN] + $mapping, null, $special);
                    case is_array($source) && count($source) == 1:
                        $typename = implode('', array_keys($source));
                        $outtype = array_search($typename, self::ANY_MAP, true);
                        if ($outtype !== false) {
                            return self::encode_der($source[$typename], ['type' => $outtype] + $mapping, null, $special);
                        }
                }

                $filters = self::$filters;
                foreach ($loc as $part) {
                    if (!isset($filters[$part])) {
                        $filters = false;
                        break;
                    }
                    $filters = $filters[$part];
                }
                if ($filters === false) {
                    throw new \RuntimeException('No filters defined for ' . implode('/', $loc));
                }
                return self::encode_der($source, $filters + $mapping, null, $special);
            case self::TYPE_NULL:
                $value = '';
                break;
            case self::TYPE_NUMERIC_STRING:
            case self::TYPE_TELETEX_STRING:
            case self::TYPE_PRINTABLE_STRING:
            case self::TYPE_UNIVERSAL_STRING:
            case self::TYPE_UTF8_STRING:
            case self::TYPE_BMP_STRING:
            case self::TYPE_IA5_STRING:
            case self::TYPE_VISIBLE_STRING:
            case self::TYPE_VIDEOTEX_STRING:
            case self::TYPE_GRAPHIC_STRING:
            case self::TYPE_GENERAL_STRING:
                $value = $source;
                break;
            case self::TYPE_BOOLEAN:
                $value = $source ? "\xFF" : "\x00";
                break;
            default:
                throw new \RuntimeException('Mapping provides no type definition for ' . implode('/', self::$location));
        }

        if (isset($idx)) {
            array_pop(self::$location);
        }

        if (isset($mapping['cast'])) {
            if (isset($mapping['explicit']) || $mapping['type'] == self::TYPE_CHOICE) {
                $value = chr($tag) . self::encodeLength(strlen($value)) . $value;
                $tag = ($mapping['class'] << 6) | 0x20 | $mapping['cast'];
            } else {
                $tag = ($mapping['class'] << 6) | (ord($temp[0]) & 0x20) | $mapping['cast'];
            }
        }

        return chr($tag) . self::encodeLength(strlen($value)) . $value;
    }

    /**
     * BER-decode the OID
     *
     * Called by _decode_ber()
     *
     * @param string $content
     * @return string
     */
    public static function decodeOID($content)
    {
        static $eighty;
        if (!$eighty) {
            $eighty = new BigInteger(80);
        }

        $oid = [];
        $pos = 0;
        $len = strlen($content);
        // see https://github.com/openjdk/jdk/blob/2deb318c9f047ec5a4b160d66a4b52f93688ec42/src/java.base/share/classes/sun/security/util/ObjectIdentifier.java#L55
        if ($len > 4096) {
            //throw new \RuntimeException("Object identifier size is limited to 4096 bytes ($len bytes present)");
            return false;
        }

        if (ord($content[$len - 1]) & 0x80) {
            return false;
        }

        $n = new BigInteger();
        while ($pos < $len) {
            $temp = ord($content[$pos++]);
            $n = $n->bitwise_leftShift(7);
            $n = $n->bitwise_or(new BigInteger($temp & 0x7F));
            if (~$temp & 0x80) {
                $oid[] = $n;
                $n = new BigInteger();
            }
        }
        $part1 = array_shift($oid);
        $first = floor(ord($content[0]) / 40);
        /*
          "This packing of the first two object identifier components recognizes that only three values are allocated from the root
           node, and at most 39 subsequent values from nodes reached by X = 0 and X = 1."

          -- https://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf#page=22
        */
        if ($first <= 2) { // ie. 0 <= ord($content[0]) < 120 (0x78)
            array_unshift($oid, ord($content[0]) % 40);
            array_unshift($oid, $first);
        } else {
            array_unshift($oid, $part1->subtract($eighty));
            array_unshift($oid, 2);
        }

        return implode('.', $oid);
    }

    /**
     * DER-encode the OID
     *
     * Called by _encode_der()
     *
     * @param string $source
     * @return string
     */
    public static function encodeOID($source)
    {
        static $mask, $zero, $forty;
        if (!$mask) {
            $mask = new BigInteger(0x7F);
            $zero = new BigInteger();
            $forty = new BigInteger(40);
        }

        if (!preg_match('#(?:\d+\.)+#', $source)) {
            $oid = isset(self::$reverseOIDs[$source]) ? self::$reverseOIDs[$source] : false;
        } else {
            $oid = $source;
        }
        if ($oid === false) {
            throw new \RuntimeException('Invalid OID');
        }

        $parts = explode('.', $oid);
        $part1 = array_shift($parts);
        $part2 = array_shift($parts);

        $first = new BigInteger($part1);
        $first = $first->multiply($forty);
        $first = $first->add(new BigInteger($part2));

        array_unshift($parts, $first->toString());

        $value = '';
        foreach ($parts as $part) {
            if (!$part) {
                $temp = "\0";
            } else {
                $temp = '';
                $part = new BigInteger($part);
                while (!$part->equals($zero)) {
                    $submask = $part->bitwise_and($mask);
                    $submask->setPrecision(8);
                    $temp = (chr(0x80) | $submask->toBytes()) . $temp;
                    $part = $part->bitwise_rightShift(7);
                }
                $temp[strlen($temp) - 1] = $temp[strlen($temp) - 1] & chr(0x7F);
            }
            $value .= $temp;
        }

        return $value;
    }

    /**
     * BER-decode the time
     *
     * Called by _decode_ber() and in the case of implicit tags asn1map().
     *
     * @param string $content
     * @param int $tag
     * @return \DateTime|false
     */
    private static function decodeTime($content, $tag)
    {
        /* UTCTime:
           http://tools.ietf.org/html/rfc5280#section-4.1.2.5.1
           http://www.obj-sys.com/asn1tutorial/node15.html

           GeneralizedTime:
           http://tools.ietf.org/html/rfc5280#section-4.1.2.5.2
           http://www.obj-sys.com/asn1tutorial/node14.html */

        $format = 'YmdHis';

        if ($tag == self::TYPE_UTC_TIME) {
            // https://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf#page=28 says "the seconds
            // element shall always be present" but none-the-less I've seen X509 certs where it isn't and if the
            // browsers parse it phpseclib ought to too
            if (preg_match('#^(\d{10})(Z|[+-]\d{4})$#', $content, $matches)) {
                $content = $matches[1] . '00' . $matches[2];
            }
            $prefix = substr($content, 0, 2) >= 50 ? '19' : '20';
            $content = $prefix . $content;
        } elseif (strpos($content, '.') !== false) {
            $format .= '.u';
        }

        if ($content[strlen($content) - 1] == 'Z') {
            $content = substr($content, 0, -1) . '+0000';
        }

        if (strpos($content, '-') !== false || strpos($content, '+') !== false) {
            $format .= 'O';
        }

        // error supression isn't necessary as of PHP 7.0:
        // http://php.net/manual/en/migration70.other-changes.php
        return @\DateTime::createFromFormat($format, $content);
    }

    /**
     * Set the time format
     *
     * Sets the time / date format for asn1map().
     *
     * @param string $format
     */
    public static function setTimeFormat($format)
    {
        self::$format = $format;
    }

    /**
     * Load OIDs
     *
     * Load the relevant OIDs for a particular ASN.1 semantic mapping.
     * Previously loaded OIDs are retained.
     *
     * @param array $oids
     */
    public static function loadOIDs(array $oids)
    {
        self::$reverseOIDs += $oids;
        self::$oids = array_flip(self::$reverseOIDs);
    }

    /**
     * Set filters
     *
     * See \phpseclib3\File\X509, etc, for an example.
     * Previously loaded filters are not retained.
     *
     * @param array $filters
     */
    public static function setFilters(array $filters)
    {
        self::$filters = $filters;
    }

    /**
     * String type conversion
     *
     * This is a lazy conversion, dealing only with character size.
     * No real conversion table is used.
     *
     * @param string $in
     * @param int $from
     * @param int $to
     * @return string
     */
    public static function convert($in, $from = self::TYPE_UTF8_STRING, $to = self::TYPE_UTF8_STRING)
    {
        // isset(self::STRING_TYPE_SIZE[$from] returns a fatal error on PHP 5.6
        if (!array_key_exists($from, self::STRING_TYPE_SIZE) || !array_key_exists($to, self::STRING_TYPE_SIZE)) {
            return false;
        }
        $insize = self::STRING_TYPE_SIZE[$from];
        $outsize = self::STRING_TYPE_SIZE[$to];
        $inlength = strlen($in);
        $out = '';

        for ($i = 0; $i < $inlength;) {
            if ($inlength - $i < $insize) {
                return false;
            }

            // Get an input character as a 32-bit value.
            $c = ord($in[$i++]);
            switch (true) {
                case $insize == 4:
                    $c = ($c << 8) | ord($in[$i++]);
                    $c = ($c << 8) | ord($in[$i++]);
                    // fall-through
                case $insize == 2:
                    $c = ($c << 8) | ord($in[$i++]);
                    // fall-through
                case $insize == 1:
                    break;
                case ($c & 0x80) == 0x00:
                    break;
                case ($c & 0x40) == 0x00:
                    return false;
                default:
                    $bit = 6;
                    do {
                        if ($bit > 25 || $i >= $inlength || (ord($in[$i]) & 0xC0) != 0x80) {
                            return false;
                        }
                        $c = ($c << 6) | (ord($in[$i++]) & 0x3F);
                        $bit += 5;
                        $mask = 1 << $bit;
                    } while ($c & $bit);
                    $c &= $mask - 1;
                    break;
            }

            // Convert and append the character to output string.
            $v = '';
            switch (true) {
                case $outsize == 4:
                    $v .= chr($c & 0xFF);
                    $c >>= 8;
                    $v .= chr($c & 0xFF);
                    $c >>= 8;
                    // fall-through
                case $outsize == 2:
                    $v .= chr($c & 0xFF);
                    $c >>= 8;
                    // fall-through
                case $outsize == 1:
                    $v .= chr($c & 0xFF);
                    $c >>= 8;
                    if ($c) {
                        return false;
                    }
                    break;
                case ($c & (PHP_INT_SIZE == 8 ? 0x80000000 : (1 << 31))) != 0:
                    return false;
                case $c >= 0x04000000:
                    $v .= chr(0x80 | ($c & 0x3F));
                    $c = ($c >> 6) | 0x04000000;
                    // fall-through
                case $c >= 0x00200000:
                    $v .= chr(0x80 | ($c & 0x3F));
                    $c = ($c >> 6) | 0x00200000;
                    // fall-through
                case $c >= 0x00010000:
                    $v .= chr(0x80 | ($c & 0x3F));
                    $c = ($c >> 6) | 0x00010000;
                    // fall-through
                case $c >= 0x00000800:
                    $v .= chr(0x80 | ($c & 0x3F));
                    $c = ($c >> 6) | 0x00000800;
                    // fall-through
                case $c >= 0x00000080:
                    $v .= chr(0x80 | ($c & 0x3F));
                    $c = ($c >> 6) | 0x000000C0;
                    // fall-through
                default:
                    $v .= chr($c);
                    break;
            }
            $out .= strrev($v);
        }
        return $out;
    }

    /**
     * Extract raw BER from Base64 encoding
     *
     * @param string $str
     * @return string
     */
    public static function extractBER($str)
    {
        /* X.509 certs are assumed to be base64 encoded but sometimes they'll have additional things in them
         * above and beyond the ceritificate.
         * ie. some may have the following preceding the -----BEGIN CERTIFICATE----- line:
         *
         * Bag Attributes
         *     localKeyID: 01 00 00 00
         * subject=/O=organization/OU=org unit/CN=common name
         * issuer=/O=organization/CN=common name
         */
        if (strlen($str) > ini_get('pcre.backtrack_limit')) {
            $temp = $str;
        } else {
            $temp = preg_replace('#.*?^-+[^-]+-+[\r\n ]*$#ms', '', $str, 1);
            $temp = preg_replace('#-+END.*[\r\n ]*.*#ms', '', $temp, 1);
        }
        // remove new lines
        $temp = str_replace(["\r", "\n", ' '], '', $temp);
        // remove the -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- stuff
        $temp = preg_replace('#^-+[^-]+-+|-+[^-]+-+$#', '', $temp);
        $temp = preg_match('#^[a-zA-Z\d/+]*={0,2}$#', $temp) ? Strings::base64_decode($temp) : false;
        return $temp != false ? $temp : $str;
    }

    /**
     * DER-encode the length
     *
     * DER supports lengths up to (2**8)**127, however, we'll only support lengths up to (2**8)**4.  See
     * {@link http://itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf#p=13 X.690 paragraph 8.1.3} for more information.
     *
     * @param int $length
     * @return string
     */
    public static function encodeLength($length)
    {
        if ($length <= 0x7F) {
            return chr($length);
        }

        $temp = ltrim(pack('N', $length), chr(0));
        return pack('Ca*', 0x80 | strlen($temp), $temp);
    }

    /**
     * Returns the OID corresponding to a name
     *
     * What's returned in the associative array returned by loadX509() (or load*()) is either a name or an OID if
     * no OID to name mapping is available. The problem with this is that what may be an unmapped OID in one version
     * of phpseclib may not be unmapped in the next version, so apps that are looking at this OID may not be able
     * to work from version to version.
     *
     * This method will return the OID if a name is passed to it and if no mapping is avialable it'll assume that
     * what's being passed to it already is an OID and return that instead. A few examples.
     *
     * getOID('2.16.840.1.101.3.4.2.1') == '2.16.840.1.101.3.4.2.1'
     * getOID('id-sha256') == '2.16.840.1.101.3.4.2.1'
     * getOID('zzz') == 'zzz'
     *
     * @param string $name
     * @return string
     */
    public static function getOID($name)
    {
        return isset(self::$reverseOIDs[$name]) ? self::$reverseOIDs[$name] : $name;
    }
}