/* Returns true if "candidate" is a witness for the compositeness
* of "number", false if "candidate" is a strong liar. "exponent"
* and "squareCount" are used for computation */
bool PrimeGenerator::isWitness( BigInt candidate,
const BigInt &number,
const BigInt &exponent,
unsigned long int squareCount,
const BigInt &numberMinusOne)
{
//calculate candidate = (candidate to the power of exponent) mod number
candidate.SetPowerMod(exponent, number);
//temporary variable, used to call the divide function
BigInt quotient;
for (unsigned long int i = 0; i < squareCount; i++)
{
bool maybeWitness(false);
if (candidate != BigIntOne && candidate != numberMinusOne)
maybeWitness = true;
//PrimeGenerator used to be a friend of BigInt, so the following
//statement produced the result in one call to BigInt::divide()
// BigInt::divide(candidate * candidate, number, quotient, candidate);
//That doesn't work any more, so we have to use two calls
candidate = candidate * candidate;
quotient = (candidate) / number;
candidate = (candidate) % number;
if (maybeWitness && candidate == BigIntOne)
return true; //definitely a composite number
}
if (candidate != BigIntOne)
return true; //definitely a composite number
return false; //probable prime
}
/* Returns true if "candidate" is a witness for the compositeness
* of "number", false if "candidate" is a strong liar. "exponent"
* and "squareCount" are used for computation */
bool PrimeGenerator::isWitness( BigInt candidate,
const BigInt &number,
const BigInt &exponent,
unsigned long int squareCount,
const BigInt &numberMinusOne)
{
//calculate candidate = (candidate to the power of exponent) mod number
candidate.SetPowerMod(exponent, number);
//temporary variable, used to call the divide function
BigInt quotient;
for (unsigned long int i = 0; i < squareCount; i++)
{
bool maybeWitness(false);
if (candidate != BigIntOne && candidate != numberMinusOne)
maybeWitness = true;
BigInt::divide(candidate * candidate, number, quotient, candidate);
if (maybeWitness && candidate == BigIntOne)
return true; //definitely a composite number
}
if (candidate != BigIntOne)
return true; //definitely a composite number
return false; //probable prime
}
/* Decrypts a "chunk" (a small part of a message) using "key" */
string RSA::decryptChunk(const BigInt &chunk, const Key &key)
{
BigInt a = chunk;
// The RSA decryption algorithm is a congruence equation.
a.SetPowerMod(key.GetExponent(), key.GetModulus());
// Decode the message to a readable form.
return RSA::decode(a);
}
/* Encrypts a "chunk" (a small part of a message) using "key" */
string RSA::encryptChunk(const string &chunk, const Key &key)
{
// First encode the chunk, to make sure it is represented as an integer.
BigInt a = RSA::encode(chunk);
// The RSA encryption algorithm is a congruence equation.
a.SetPowerMod(key.GetExponent(), key.GetModulus());
return a.ToString();
}
