ConFoo Montreal 2017 Calling for Papers

# bcpowmod

(PHP 5)

bcpowmodRaise an arbitrary precision number to another, reduced by a specified modulus

### Description

string bcpowmod ( string \$left_operand , string \$right_operand , string \$modulus [, int \$scale ] )

Use the fast-exponentiation method to raise left_operand to the power right_operand with respect to the modulus modulus.

### Parameters

left_operand

The left operand, as a string.

right_operand

The right operand, as a string.

modulus

The modulus, as a string.

scale

This optional parameter is used to set the number of digits after the decimal place in the result. You can also set the global default scale for all functions by using bcscale().

### Return Values

Returns the result as a string, or NULL if modulus is 0.

### Notes

Note:

Because this method uses the modulus operation, numbers which are not positive integers may give unexpected results.

### Examples

The following two statements are functionally identical. The bcpowmod() version however, executes in less time and can accept larger parameters.

``` <?php\$a = bcpowmod(\$x, \$y, \$mod);\$b = bcmod(bcpow(\$x, \$y), \$mod);// \$a and \$b are equal to each other.?> ```

• bcpow() - عددی با دقت دلخواه به توان عدد دیگر
• bcmod() - دریافت قدر مطلق عددی با دقت دلخواه

``` However, if you read his full note, you see this paragraph:"The function bcpowmod(v, e, m) is supposedly equivalent to bcmod(bcpow(v, e), m).  However, for the large numbers used as keys in the RSA algorithm, the bcpow function generates a number so big as to overflow it.  For any exponent greater than a few tens of thousands, bcpow overflows and returns 1."So you still can, and should (over bcmod(bcpow(v, e), m) ), use his function if you are using larger exponents, "any exponent greater than a few tens of thousand." ```
``` Versions of PHP prior to 5 do not have bcpowmod in their repertoire.  This routine simulates this function using bcdiv, bcmod and bcmul.  It is useful to have bcpowmod available because it is commonly used to implement the RSA algorithm. The function bcpowmod(v, e, m) is supposedly equivalent to bcmod(bcpow(v, e), m).  However, for the large numbers used as keys in the RSA algorithm, the bcpow function generates a number so big as to overflow it.  For any exponent greater than a few tens of thousands, bcpow overflows and returns 1.This routine will iterate through a loop squaring the result, modulo the modulus, for every one-bit in the exponent.  The exponent is shifted right by one bit for each iteration.  When it has been reduced to zero, the calculation ends.This method may be slower than bcpowmod but at least it works.function PowModSim(\$Value, \$Exponent, \$Modulus)  {  // Check if simulation is even necessary.  if (function_exists("bcpowmod"))    return (bcpowmod(\$Value, \$Exponent, \$Modulus));  // Loop until the exponent is reduced to zero.  \$Result = "1";  while (TRUE)    {    if (bcmod(\$Exponent, 2) == "1")      \$Result = bcmod(bcmul(\$Result, \$Value), \$Modulus);    if ((\$Exponent = bcdiv(\$Exponent, 2)) == "0") break;    \$Value = bcmod(bcmul(\$Value, \$Value), \$Modulus);    }  return (\$Result);  } ```
``` I found a better way to emulate bcpowmod on PHP 4, which works with very big numbers too:function powmod(\$m,\$e,\$n) {    if (intval(PHP_VERSION)>4) {        return(bcpowmod(\$m,\$e,\$n));    } else {        \$r="";        while (\$e!="0") {            \$t=bcmod(\$e,"4096");            \$r=substr("000000000000".decbin(intval(\$t)),-12).\$r;            \$e=bcdiv(\$e,"4096");        }        \$r=preg_replace("!^0+!","",\$r);        if (\$r=="") \$r="0";        \$m=bcmod(\$m,\$n);        \$erb=strrev(\$r);        \$q="1";        \$a[0]=\$m;        for (\$i=1;\$i<strlen(\$erb);\$i++) {            \$a[\$i]=bcmod(bcmul(\$a[\$i-1],\$a[\$i-1]),\$n);        }        for (\$i=0;\$i<strlen(\$erb);\$i++) {            if (\$erb[\$i]=="1") {                \$q=bcmod(bcmul(\$q,\$a[\$i]),\$n);            }        }        return(\$q);    }} ```