Element IntegerGroup::EncodeBytes(const QByteArray &in) const
{
// We can store p bytes minus 2 bytes for padding and one more to be safe
const int can_read = BytesPerElement();
if(can_read < 1) qFatal("Illegal parameters");
if(in.count() > can_read) qFatal("Cannot encode: string is too long");
// Add initial 0xff byte and trailing 0x00 byte
QByteArray padded;
padded.append(0xff);
padded.append(in.left(can_read));
padded.append((char)0x00);
padded.append(0xff);
// Change byte of padded string until the
// integer represented by the byte arry is a quadratic
// residue. We need to be sure that every plaintext
// message is a quadratic residue modulo p
const int last = padded.count()-2;
for(unsigned char pad=0x00; pad < 0xff; pad++) {
padded[last] = pad;
Element element(new IntegerElementData(Integer(padded)));
if(IsElement(element)) {
return element;
}
}
qFatal("Could not encode message as quadratic residue");
return Element(new IntegerElementData(Integer(1)));
}
Element CppECGroup::EncodeBytes(const QByteArray &in) const
{
/*
* See the article
* "Encoding And Decoding of a Message in the
* Implementation of Elliptic Curve Cryptography
* using Koblitz’s Method" for details on how this works.
*
* k == MessageSerializationParameter defines the percentage
* chance that we won't be able to encode a given message
* in a given elliptic curve point. The failure probability
* is 2^(-k).
*
* We can store b = log_2(p/k) bytes in every
* elliptic curve point, where p is the security
* parameter (prime size) of the elliptic curve.
*
* For p = 2^256, k = 256, b = 224 (minus 2 padding bytes)
*/
if(in.count() > BytesPerElement()) {
qFatal("Failed to serialize over-sized string");
}
// Holds the data to be encoded plus a leading and a trailing
// 0xFF byte
QByteArray data;
data.append(0xff);
data += in;
data.append(0xff);
// r is an encoding of the string in a big integer
CryptoPP::Integer r(reinterpret_cast<const byte*>(data.constData()), data.count());
//qDebug() << "r" << FromCppInteger(r)).GetByteArray().toHex();
Q_ASSERT(r < _curve.FieldSize());
Element point;
CryptoPP::Integer x, y;
for(int i=0; i<_k; i++) {
// x = rk + i mod p
x = ((r*_k)+i);
Q_ASSERT(x < _curve.FieldSize());
if(SolveForY(x, point)) {
return point;
}
}
qFatal("Failed to find point");
return Element(new CppECElementData(CryptoPP::ECPPoint()));
}
Element ByteGroup::EncodeBytes(const QByteArray &in) const
{
const int can_read = BytesPerElement();
if(can_read < 1) qFatal("Illegal parameters");
if(in.count() > can_read) qFatal("Cannot encode: string is too long");
QByteArray out(_n_bytes, 0);
Utils::Serialization::WriteInt(in.count(), out, 0);
for(int i=0; i<can_read && i<in.count(); i++) {
out[i+4] = in[i];
}
return Element(new ByteElementData(out));
}
Element CompositeIntegerGroup::EncodeBytes(const QByteArray &in) const
{
// We can store p bytes minus 2 bytes for padding and one more to be safe
const int can_read = BytesPerElement();
if(can_read < 1) qFatal("Illegal parameters");
if(in.count() > can_read) qFatal("Cannot encode: string is too long");
// Add initial 0xff byte and trailing 0x00 byte
QByteArray padded;
padded.append(0xff);
padded.append(in.left(can_read));
padded.append(0xff);
return Element(new IntegerElementData(Integer(padded)));
}
