PHP RFC: Session ID without hashing

• Version: 1.0
• Create Date: 2016-04-06
• Modified Date: 2016-07-02
• Author: Yasuo Ohgaki yohgaki@ohgaki.net
• Status: Voting
• First Published at: http://wiki.php.net/rfc/session-id-without-hashing

The objective of this RFC is session module code cleanup, remove unneeded code complexity and redundancy.

• Hash function is not required to generate session ID with CSPRNG.
• Hash function removal results in less number of INI config.

NOTE: It is meaningless applying hash to CS safe random bytes.

Since PHP 7, there is php_random_bytes() function. Session ID generation does not need hashing for secure session ID generation. Session module may simply convert random bytes to readable characters.

As a bonus, simple session generation performance increased 2X or more.

• w/ Patch: Requests per second: 2278.59 [#/sec] (mean)
• w/o Patch: Requests per second: 899.36 [#/sec] (mean)

Use php_random_bytes() generated binary random value directly and convert it to session ID by using session internal bin_to_readable() function.

Remove hash and RNG related codes and settings from session module.

• session.hash_function
• session.hash_bits_per_character
• session.entropy_file
• session.entropy_length

Add new config for session ID generation.

• session.sid_length (Session ID string length. Default: 48)
• session.sid_bits_per_character (Outstanding bits per char. Default: 5)

NOTE: New session ID is far less likely to have collisions. Even if system has broken CSPRNG, session module has session ID collision detection already.

Min/max length of session ID: 22 - 256 (22 is the same length as MD5 hash with_hash_bits_per_chars)

Changed default

• session.use_strict_mode (0 to 1) - Changed as insurance of broken PRNG implementation.

If PRNG has vulnerability that generates predictable value, exposing raw PRNG state helps attackers. Although, this is unlikely with modern PRNGs, but risk is there.

To mitigate risk, additional bytes (+60 bytes) are read from PRNG. Additional bytes should be good enough for predictable session ID with broken PRNG implementation.

None. session.sid_length, session.sid_bits_per_character may set to compatible value. Default is incompatible to older versions, if you need session ID string compatibility set INI to

• session.sid_length=32 (Session ID string length. Default: 48)
• session.sid_bits_per_character=4 (Outstanding bits per char. Default: 5)

PHP 7.1

None

Session module

None

None

hardcoded default and php.ini-* default values are the same.

• session.sid_length (Session ID string length. Default: 48)
• session.sid_bits_per_character (Outstanding bits per char. Default: 5)

None

3rd party and user save handlers are unaffected.

This project requires 2/3 majority.

Session ID without hashing
Real name	Yes	No
bishop (bishop)
danack (danack)
derick (derick)
dmitry (dmitry)
galvao (galvao)
guilhermeblanco (guilhermeblanco)
jgmdev (jgmdev)
kguest (kguest)
laruence (laruence)
lcobucci (lcobucci)
leigh (leigh)
mariano (mariano)
rasmus (rasmus)
stas (stas)
tyrael (tyrael)
yohgaki (yohgaki)
zeev (zeev)
zimt (zimt)
Final result:	13	5
This poll has been closed.

Session ID without hashing Re-vote
Real name	Yes	No
davey (davey)
dm (dm)
guilhermeblanco (guilhermeblanco)
leigh (leigh)
ocramius (ocramius)
pollita (pollita)
sammyk (sammyk)
stas (stas)
yohgaki (yohgaki)
Count:	9	0

• 2nd Vote starts 2016-07-24 and ends 2015-07-31 23:59:59
• 1st Vote starts 2016-07-02 and ends 2016-07-11

• https://github.com/php/php-src/pull/1850

After the project is implemented, this section should contain

1. the version(s) it was merged to
2. a link to the git commit(s)
3. a link to the PHP manual entry: http://svn.php.net/viewvc?view=revision&revision=339678

• RFC Enable session.use_strict_mode by default.