Bug #75178 [NEW]: bcpowmod() misbehaves for non-integer base or modulus
From: cmb
Operating system: *
PHP version: 7.0.23
Package: BC math related
Bug Type: Bug
Bug description:bcpowmod() misbehaves for non-integer base or modulus
Description:
------------
bcpowmod() accepts non-integral operands, writing a warning to
stderr (see bug #75169) and simply going on for non-integer base
and modulus, while a non-integer exponent is truncated. The
results appear to be nonsensical.
Since bcpowmod() is a simple wrapper for bc_raisemod() which
implements the so called right-to-left binary method for modular
exponentiation[1], the function is unsuitable for non-integral
operands, so the warnings are correct, but base and modulus would
have to be truncated as well.[2]
[1]
<https://en.wikipedia.org/wiki/Modular_exponentiation#Right-to-left_binary_method>
[2]
<https://github.com/php/php-src/blob/php-7.0.23/ext/bcmath/libbcmath/src/raisemod.c#L69>
Test script:
---------------
<?php
var_dump(bcpowmod('4.1', '4', '3', 3));
var_dump(bcpowmod('4', '4', '3.1', 3));
Expected result:
----------------
bc math warning: non-zero scale in base
string(5) "1.000"
bc math warning: non-zero scale in modulus
string(5) "1.000"
Actual result:
--------------
bc math warning: non-zero scale in base
string(5) "0.276"
bc math warning: non-zero scale in modulus
string(5) "0.250"
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Edit bug report at https://bugs.php.net/bug.php?id=75178&edit=1
--
Try a snapshot (PHP 5.4): https://bugs.php.net/fix.php?id=75178&r=trysnapshot54
Try a snapshot (PHP 5.5): https://bugs.php.net/fix.php?id=75178&r=trysnapshot55
Try a snapshot (trunk): https://bugs.php.net/fix.php?id=75178&r=trysnapshottrunk
Fixed in SVN: https://bugs.php.net/fix.php?id=75178&r=fixed
Fixed in release: https://bugs.php.net/fix.php?id=75178&r=alreadyfixed
Need backtrace: https://bugs.php.net/fix.php?id=75178&r=needtrace
Need Reproduce Script: https://bugs.php.net/fix.php?id=75178&r=needscript
Try newer version: https://bugs.php.net/fix.php?id=75178&r=oldversion
Not developer issue: https://bugs.php.net/fix.php?id=75178&r=support
Expected behavior: https://bugs.php.net/fix.php?id=75178&r=notwrong
Not enough info: https://bugs.php.net/fix.php?id=75178&r=notenoughinfo
Submitted twice: https://bugs.php.net/fix.php?id=75178&r=submittedtwice
register_globals: https://bugs.php.net/fix.php?id=75178&r=globals
PHP 4 support discontinued: https://bugs.php.net/fix.php?id=75178&r=php4
Daylight Savings: https://bugs.php.net/fix.php?id=75178&r=dst
IIS Stability: https://bugs.php.net/fix.php?id=75178&r=isapi
Install GNU Sed: https://bugs.php.net/fix.php?id=75178&r=gnused
Floating point limitations: https://bugs.php.net/fix.php?id=75178&r=float
No Zend Extensions: https://bugs.php.net/fix.php?id=75178&r=nozend
MySQL Configuration Error: https://bugs.php.net/fix.php?id=75178&r=mysqlcfg
Thread (2 messages)
- cmb@php.net