Elliptic Curve Cryptography

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Elliptic Curve Cryptography (ECC) is based on the algebraic structure of elliptic curves over finite fields. The use of elliptic curves in cryptography was independently suggested by Neal Koblitz and Victor Miller in 1985.

From a high level, Crypto++ offers a numbers of schemes and alogrithms which operate over elliptic curves. Fields include both Fp and F2m, and schemes include:

Ján Jančár showed Crypto++ 8.2 and below leaked timing information in elliptic curve gear. You should upgrade to Crypto++ 8.3 and above. Also see Issue 869, Elliptic Curve timing leaks.

Construction/Initialization of Keys

If you want to perform two-step construction and initialization of a private key, then perform the following. Note that Initialize takes a RandomNumberGenerator, which causes private key generation. An Initialize that lacks the PRNG does not generate a private key.

While the example below is written for ECIES, the technique applies to all Diffie-Hellman schemes over elliptic curves due to Crypto++'s use of interface programming. That means it works equally well for ECMQV, ECDSA, ECMQV and ECNR. 

// From Wei Dai in a private email
ECIES<ECP>::Decryptor d;
d.AccessKey().GenerateRandom(GlobalRNG(), MakeParameters(Name::GroupOID(), ASN1::secp256r1()));

Alternative, but non-preferred methods include the following. 

ECIES<ECP>::Decryptor decryptor;
decryptor.AccessKey().AccessGroupParameters().Initialize(prng, ASN1::secp256r1());

As yet another alternative, you can set the private exponent (multiplicand) directly: 

ECIES<ECP>::Decryptor decryptor;
decryptor.AccessKey().AccessGroupParameters().Initialize(ASN1::secp256r1());

Integer x(prng, Integer::One(), decryptor.AccessKey().GetGroupParameters().GetSubgroupOrder()-1);
decryptor.AccessKey().SetPrivateExponent(x);

Minimizing Key Size for Persistence

Keys can be serialized in a number of different formats. Some formats are better for interoperability, while others are better for minimizing size. Keys and their formats are covered in detail at Keys and Formats.

Keys and Formats does not discuss minimizing a serialized key's size. Taking from Wei Dai on the Crypto++ mailing list:

To minimize the size of public and private keys, what you need to do is

encode only the private exponent of the private key, and the public point of

the public key.

// save private exponent
privateKey.GetPrivateExponent().DEREncode(privFile);

// load private exponent
Integer x;
x.BERDecode(privFile);
privateKey.AccessGroupParameters().Initialize(CryptoPP::ASN1::secp160k1());
privateKey.SetPrivateExponent(x);

// save public element
publicKey.GetGroupParameters().GetCurve().EncodePoint(pubFile, publicKey.GetPublicElement(), true);

// load public element
ECP::Point p;
publicKey.AccessGroupParameters().Initialize(CryptoPP::ASN1::secp160k1());
publicKey.GetGroupParameters().GetCurve().DecodePoint(p, pubFile, publicKey.GetGroupParameters().GetCurve().EncodedPointSize(true));
publicKey.SetPublicElement(p);

Curve Operations

Sometimes you may want to perform curve operations directly, and not in the context of, say, a higher level encryptor or signer. The code below shows you how to exponentiate, multiple and add using the lower level primitives over secp256r1. 

#include "integer.h"
#include "eccrypto.h"
#include "osrng.h"
#include "oids.h"

#include <iostream>
#include <iomanip>

int main(int argc, char* argv[])
{
    using namespace CryptoPP;
    typedef DL_GroupParameters_EC<ECP> GroupParameters;
    typedef DL_GroupParameters_EC<ECP>::Element Element;

    AutoSeededRandomPool prng;    
    GroupParameters group;
    group.Initialize(ASN1::secp256r1());

    // private key
    Integer x(prng, Integer::One(), group.GetMaxExponent());
    
    std::cout << "Private exponent:" << std::endl;
    std::cout << "  " << std::hex << x << std::endl;
    
    // public key
    Element y = group.ExponentiateBase(x);

    std::cout << "Public element:" << std::endl;
    std::cout << "  " << std::hex << y.x << std::endl;
    std::cout << "  " << std::hex << y.y << std::endl;
    
    // element addition
    Element u = group.GetCurve().Add(y, ECP::Point(2,3));

    std::cout << "Add:" << std::endl;
    std::cout << "  " << std::hex << u.x << std::endl;
    std::cout << "  " << std::hex << u.y << std::endl;

    // scalar multiplication
    Element v = group.GetCurve().ScalarMultiply(u, Integer::Two());

    std::cout << "Mult:" << std::endl;
    std::cout << "  " << std::hex << v.x << std::endl;
    std::cout << "  " << std::hex << v.y << std::endl;

    return 0;
}

The program produces output similar to the following. 

$ ./test.exe
Private exponent:
  b48e35e8d60918f815857503b034681bc59db689dee0ffc35a140e365bb056dch
Public element:
  bb9c8daaace9712f368bc98cf004a4594a14f9c330e2db141906ec67f05ab8d8h
  e37e5e161aae15f54f20d67b665311717305932a1479427fe063d84c5be82a1dh
Sum:
  f5055cd23f23f5721d8a5f6f87bd61206e972a97c19478200cb0b1f24af398ach
  107a532732098c4d051efc7f54d9bda78020a6e68f95e01a33700bab56a91f9ah
Mult:
  46628d3e4f43da4fd001c652682d33f608c34ce3cf6c13f45b9bd014cbb83ed4h
  3b58f98bd0d70196036b77f6fcca6fe206bdf3beda4b2b604d5cb8ae0327a57ch

Non-standard Curves

Crypto++ supplies a set of standard curves approved by ANSI, Brainpool, and NIST. Crypto++ does not provide curve generation functionality. If you need a custom curve, see Elliptic Curve Builder for Windows or Ján Jančár ecgen on Linux.

Downloads

dpval-2.zip - Elliptic Curve Domain Parameter Validation. The program dumps the public and private keys, and validates the curve per Certicom's SEC 2 Whitepaper. In addition, the program demonstrates mathematics with the point of infinity and scalar multiplications using Crypto++.

ecctest.zip - Exercises encryption and decryption using ANSI, Brainpool, and NIST curves by way of #define

ECDSA-Test.zip - Crypto++ ECDSA sample program

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