step2 method
Generates the decryption number
Implementation
void step2({
///Index of the chosen possible e value
required int eIndex,
}){
//Throw an error if the index is out of range
if( eIndex < 0 || _possibleE.length <= eIndex){
throw CipherError.eOutOfRange;
}
//Assign e
_e = _possibleE[eIndex];
//Find d and assign it
_d = this.e!.modInverse(phi!);
}