internal void ModularPower( ChineseRemainder CRTResult,
Integer Exponent,
ChineseRemainder CRTModulus,
bool UsePresetBaseArray )
{
// The square and multiply method is in Wikipedia:
// https://en.wikipedia.org/wiki/Exponentiation_by_squaring
if( Worker.CancellationPending )
return;
if( CRTResult.IsZero())
return; // With CRTResult still zero.
if( CRTResult.IsEqual( CRTModulus ))
{
// It is congruent to zero % ModN.
CRTResult.SetToZero();
return;
}
// Result is not zero at this point.
if( Exponent.IsZero() )
{
CRTResult.SetToOne();
return;
}
Integer Result = new Integer();
CRTMath1.GetTraditionalInteger( Result, CRTResult );
Integer Modulus = new Integer();
CRTMath1.GetTraditionalInteger( Modulus, CRTModulus );
if( Modulus.ParamIsGreater( Result ))
{
// throw( new Exception( "This is not supposed to be input for RSA plain text." ));
IntMath.Divide( Result, Modulus, Quotient, Remainder );
Result.Copy( Remainder );
CRTResult.SetFromTraditionalInteger( Remainder );
}
if( Exponent.IsEqualToULong( 1 ))
{
// Result stays the same.
return;
}
if( !UsePresetBaseArray )
SetupBaseModArray( Modulus );
if( CRTBaseModArray == null )
throw( new Exception( "SetupBaseModArray() should have already been done here." ));
CRTXForModPower.Copy( CRTResult );
ExponentCopy.Copy( Exponent );
int TestIndex = 0;
CRTResult.SetToOne();
int LoopsTest = 0;
while( true )
{
LoopsTest++;
if( (ExponentCopy.GetD( 0 ) & 1) == 1 ) // If the bottom bit is 1.
{
CRTResult.Multiply( CRTXForModPower );
ModularReduction( CRTResult, CRTAccumulate );
CRTResult.Copy( CRTAccumulate );
}
ExponentCopy.ShiftRight( 1 ); // Divide by 2.
if( ExponentCopy.IsZero())
break;
// Square it.
CRTCopyForSquare.Copy( CRTXForModPower );
CRTXForModPower.Multiply( CRTCopyForSquare );
ModularReduction( CRTXForModPower, CRTAccumulate );
CRTXForModPower.Copy( CRTAccumulate );
}
ModularReduction( CRTResult, CRTAccumulate );
CRTResult.Copy( CRTAccumulate );
// Division is never used in the loop above.
// This is a very small Quotient.
// See SetupBaseMultiples() for a description of how to calculate
// the maximum size of this quotient.
CRTMath1.GetTraditionalInteger( Result, CRTResult );
IntMath.Divide( Result, Modulus, Quotient, Remainder );
// Is the Quotient bigger than a 32 bit integer?
if( Quotient.GetIndex() > 0 )
throw( new Exception( "I haven't ever seen this happen. Quotient.GetIndex() > 0. It is: " + Quotient.GetIndex().ToString() ));
QuotientForTest = Quotient.GetAsULong();
if( QuotientForTest > 2097867 )
throw( new Exception( "This can never happen unless I increase ChineseRemainder.DigitsArraySize." ));
Result.Copy( Remainder );
CRTResult.SetFromTraditionalInteger( Remainder );
}
internal void ModularPower( Integer Result, Integer Exponent, Integer GeneralBase )
{
// The square and multiply method is in Wikipedia:
// https://en.wikipedia.org/wiki/Exponentiation_by_squaring
// x^n = (x^2)^((n - 1)/2) if n is odd.
// x^n = (x^2)^(n/2) if n is even.
if( Result.IsZero())
return; // With Result still zero.
if( Result.IsEqual( GeneralBase ))
{
// It is congruent to zero % ModN.
Result.SetToZero();
return;
}
// Result is not zero at this point.
if( Exponent.IsZero() )
{
Result.SetFromULong( 1 );
return;
}
if( GeneralBase.ParamIsGreater( Result ))
{
// throw( new Exception( "This is not supposed to be input for RSA plain text." ));
IntMath.Divide( Result, GeneralBase, Quotient, Remainder );
Result.Copy( Remainder );
}
if( Exponent.IsEqualToULong( 1 ))
{
// Result stays the same.
return;
}
// This could also be called ahead of time if the base (the modulus)
// doesn't change. Like when your public key doesn't change.
SetupGeneralBaseArray( GeneralBase );
XForModPower.Copy( Result );
ExponentCopy.Copy( Exponent );
int TestIndex = 0;
Result.SetFromULong( 1 );
while( true )
{
if( (ExponentCopy.GetD( 0 ) & 1) == 1 ) // If the bottom bit is 1.
{
IntMath.Multiply( Result, XForModPower );
// Modular Reduction:
AddByGeneralBaseArrays( TempForModPower, Result );
Result.Copy( TempForModPower );
}
ExponentCopy.ShiftRight( 1 ); // Divide by 2.
if( ExponentCopy.IsZero())
break;
// Square it.
IntMath.Multiply( XForModPower, XForModPower );
// Modular Reduction:
AddByGeneralBaseArrays( TempForModPower, XForModPower );
XForModPower.Copy( TempForModPower );
}
// When AddByGeneralBaseArrays() gets called it multiplies a number
// by a uint sized digit. So that can make the result one digit bigger
// than GeneralBase. Then when they are added up you can get carry
// bits that can make it a little bigger.
// If by chance you got a carry bit on _every_ addition that was done
// in AddByGeneralBaseArrays() then this number could increase in size
// by 1 bit for each addition that was done. It would take 32 bits of
// carrying for HowBig to increase by 1.
// See HowManyToAdd in AddByGeneralBaseArrays() for why this check is done.
int HowBig = Result.GetIndex() - GeneralBase.GetIndex();
if( HowBig > 2 ) // I have not seen this happen yet.
throw( new Exception( "The difference in index size was more than 2. Diff: " + HowBig.ToString() ));
// So this Quotient has only one or two 32-bit digits in it.
// And this Divide() is only called once at the end. Not in the loop.
IntMath.Divide( Result, GeneralBase, Quotient, Remainder );
Result.Copy( Remainder );
}
