Ajax Javascript HTML CSS PHP C#
Bookmark this on Delicious Share on Facebook Slashdot It! Digg

# bcpowmod

## Code Examples / Notes » bcpowmod

ewilde aht bsmdevelopment dawt com

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);
}

william

The author's statement:
"A natural number is any positive non-zero integer."
should be, imo, something like:
"In this context only positive non-zero integers are considered to be natural numbers."
http://mathworld.wolfram.com/NaturalNumber.html
http://planetmath.org/encyclopedia/NaturalNumber.html
http://en.wikipedia.org/wiki/Natural_number

laysoft

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);
}
}

rrasss

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."