rfc:session-create-id

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rfc:session-create-id [2016/08/10 09:03]
yohgaki Update description because I added collision detection in the function.
rfc:session-create-id [2020/03/26 12:47] (current)
cmb this RFC has been implemented long ago
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   * Version: 0.1   * Version: 0.1
   * Created Date: 2016-04-07   * Created Date: 2016-04-07
-  * Modified Date: 2016-04-07+  * Modified Date: 2016-08-10
   * Author: Yasuo Ohgaki <yohgaki@ohgaki.net>   * Author: Yasuo Ohgaki <yohgaki@ohgaki.net>
-  * Status: Under Discussion+  * Status: Implemented (PHP 7.1)
   * First Published at: http://wiki.php.net/rfc/session-create-id   * First Published at: http://wiki.php.net/rfc/session-create-id
  
 ===== Introduction ===== ===== Introduction =====
-Session ID is created by session internal bin_to_reabable() function. bin_to_readable() creates readable string from binary data depending. New session_create_id() uses bin_to_readable() to create user defined session ID string. Session ID may use 'a'-'z', 'A'-'Z', ',', '-'. Without session_create_id(), user has to implement their own bin_to_readable() in user land.+Session ID is created by session internal bin_to_readable() function. bin_to_readable() creates readable string from binary data depending. New session_create_id() uses bin_to_readable() to create user defined session ID string. Session ID may use 'a'-'z', 'A'-'Z', ',', '-'. Without session_create_id(), user has to implement their own bin_to_readable() in user land.
  
 ===== Proposal ===== ===== Proposal =====
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 Note: User defined session ID creation function(handler) is not documented, but it's there. Note: User defined session ID creation function(handler) is not documented, but it's there.
 http://php.net/manual/en/book.session.php http://php.net/manual/en/book.session.php
-It's not documented because Object based save handler has non standard method name. i.e. create_sid() rather than createSid(). Rename will be proposed by different RFC. If you would like to see how it is used, please refer to phpt files. +<del>It's not documented because Object based save handler has non standard method name. i.e. create_sid() rather than createSid(). Rename will be proposed by different RFC.</del>(Documented after this RFC is created) If you would like to see how it is used, please refer to phpt files. 
  
   * https://github.com/php/php-src/blob/master/ext/session/tests/save_handler.inc   * https://github.com/php/php-src/blob/master/ext/session/tests/save_handler.inc
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 If session data is stored in database, administrators are able to list active session per uid easily and efficiently. If session data is stored in database, administrators are able to list active session per uid easily and efficiently.
 +
 +===== Discussions =====
 +
 +== Should have str_bin2readable() instead of session_create_id() ==
 +
 +Internal php_bin_to_readable() function that converts binary to chars has issues.
 + - It's not designed to reversible. i.e. str_readable2bin() is not possible. It cannot be implemented usable way at least.
 + - session_create_id() is mandatory function for session ID validation enabled user save handlers. There should be default session ID creation function. Otherwise, users have to implement by themselves and users could easily implement improper or broken session_create_id() function.
 +
 +== User land session_create_id() is easy ==
 +
 +It would be easy with quick, dirty and insecure way. However, proper, precise and compatible session_create_id() is complex to do the exactly what proposed session_create_id() does.
 +
 +<code php>
 +function session_create_id(string $prefix)
 +{
 +    $encoded = base64_encode(random_bytes(ini_get('session.sid_length')*2));
 +    // Use same charset as PHP
 +    $sid = substr(rtrim(strtr($encoded, '+/', ',-'), '='), 0,
 +                          ini_get('session.sid_length');
 +    // This code does not consider session.hash_bits_per_character.
 +    // It must be handled also, but omitted here.
 +    // In addition, above code reduces session ID security by 
 +    // eliminating certain patterns of bits. i.e. Removing 
 +    // certain chars in BASE64 encoding string removes certain
 +    // patterns of bits. As a result, randomness of session ID
 +    // is spoiled.
 +
 +    $sid .= $prefix;
 +
 +    // Now validate SID so that it does not have collisions
 +    when session is active, connect to database and validate SID
 +      try to fetch sid
 +        if sid is there
 +          try again to generate SID few times
 +      if SID validation failed
 +         fatal error
 +      return safe SID
 +   when session is inactive
 +      return unvalidated SID
 +}
 +</code>
 +
 +Something like above code is **required** to implement recommended user session save handlers currently. 
 +
 +== The default 128 bits Session ID is large enough to ignore collisions ==
 +
 +Brute force session ID hijack risk is described here.
 +https://www.owasp.org/index.php/Insufficient_Session-ID_Length
 +
 +  The expected number of seconds required to guess a valid session
 +  identifier is given by the equation:
 +  
 +  (2^B+1)/(2*A*S)
 +  
 +  Where:
 +  
 +  B is the number of bits of entropy in the session identifier
 +  A is the number of guesses an attacker can try each second
 +  S is the number of valid session identifiers that are valid and
 +  available to be guessed at any given time
 +
 +It says
 +
 +''Now assume a 128 bit session identifier that provides 64 bits of entropy. With a very large web site, an attacker might try 10,000 guesses per second with 100,000 valid session identifiers available to be guessed. Given these assumptions, the expected time for an attacker to successfully guess a valid session identifier is greater than 292 years.''
 +
 +292 years may sound long enough. However, even though the document explicitly does not states "Session manager must validate session ID for possible collisions", but it is clear it assumes session manager that validates session ID.
 +
 +Let me paraphrase OWASP's document to show why.
 +
 +''Now assume a 128 bit session identifier that provides 64 bits of entropy. With a very large web site, legitimate users might create 10,000 new session ID per second (New and regenerated session) with 10,000,000 valid session identifiers available to be collided. Given these assumptions, the expected time of web system to successfully has collided identifier is greater than 2 years on average.''
 +
 +NOTE: It's about probability. Expectation is "on average" and could be much shorter.
 +
 +Assumption for security should be pessimistic. OWASP makes pessimistic assumption for entropy in session ID, probably because proving "CSPRNG generates good quality of random bytes" is difficult.
 +
 +10M active sessions are possible even with relatively small sites because there are users who use very long session ID life time for "auto login". (This is not a recommended auto login implementation, though) 10K new session ID per second is possible for relatively small sites also because OWASP recommends session ID regeneration for every 15 minutes or less for certain usage.
 +
 +In addition to above, current session management implementation does not support timestamp based session data management. i.e. https://wiki.php.net/rfc/precise_session_management This makes situation even worse.
 +
 +Somebody wins lottery. Even 1 in millions/hundreds years could happen. IMHO, it is not reasonable to argue "Session ID collision very rare and cannot happen" or "Session ID is safe without collision detection, can ignore collisions", and tell poor user "We do know it may happen, but you just had rare bad luck. Even though protection could be implemented, whatever consequences are your responsibility. It's the PHP way".
 +
 +If there are users who really do not want collision detection at all, they should do it by their own responsibility and risk. e.g.
 +
 +<code php>
 +if (session_status() == PHP_SESSION_ACTIVE) {
 +  session_commit();
 +}
 +ini_set('session.use_strict_mode', 0);
 +// NIST requires SHA2 or better hash for collision sensitive usage.
 +$new_sid = hash('sha512', random_bytes(128));
 +session_id($new_sid);
 +session_start()
 +</code>
  
  
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 ===== Proposed PHP Version(s) ===== ===== Proposed PHP Version(s) =====
  
-PHP 7.1+PHP 7.2
  
 ===== Voting Choices ===== ===== Voting Choices =====
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 This project requires a 2/3 majority This project requires a 2/3 majority
  
-<doodle title="Add session_create_id() frunction" auth="Yasuo Ohgaki" voteType="single" closed="false">+<doodle title="Add session_create_id() frunction" auth="Yasuo Ohgaki" voteType="single" closed="true">
    * Yes    * Yes
    * No    * No
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 After the project is implemented, this section should contain  After the project is implemented, this section should contain 
   - the version(s) it was merged to   - the version(s) it was merged to
 +    - PHP 7.1 and master (Merged to 7.1 also by RM permission)
   - a link to the git commit(s)   - a link to the git commit(s)
 +    - http://git.php.net/?p=php-src.git;a=commitdiff;h=7ee9f81c54ca888beae1c6aaeaccee1260012076
   - a link to the PHP manual entry for the feature   - a link to the PHP manual entry for the feature
 +    -  http://svn.php.net/viewvc?view=revision&revision=339956
  
 ===== References ===== ===== References =====
rfc/session-create-id.1470819788.txt.gz · Last modified: 2017/09/22 13:28 (external edit)