/// <summary>
/// Dencrypt text on elliptic curve and return the message
/// </summary>
/// <param name="LstEncr"></param>
/// <param name="SecretKey"></param>
/// <returns></returns>
public static string EcDecrypt(List <EcModPoint> LstEncr, BigInteger SecretKey, EllipticCurveType EcType)
{
BigInteger p;
BigInteger n;
BigInteger a;
BigInteger b;
// Set params
switch (EcType)
{
case EllipticCurveType.SEC256K1:
p = SEC256K1_P;
n = SEC256K1_N;
a = SEC256K1_A;
b = SEC256K1_B;
break;
case EllipticCurveType.M383:
p = M383_P;
n = M383_N;
a = M383_A;
b = M383_B;
break;
case EllipticCurveType.E521:
p = E521_P;
n = E521_N;
a = E521_A;
b = E521_B;
break;
default:
break;
}
// 1) get kG
var kG = LstEncr.First();
// 2) compute kPb = k(SkG) = Sk(kG)
var kPb = ECKeyPairGenerator(p, a, b, kG, n, SecretKey);
// set -kPb
var negkPb = new EcModPoint {
x = kPb.x, y = -kPb.y
};
// remove kG from the list
LstEncr.RemoveAt(0);
// 3) decrypt - compute Pc - kPb = Pm (plain message)
var strMsg = string.Empty;
foreach (var pnt in LstEncr)
{
var decPnt = pointAdd(pnt, negkPb, p);
// data from x coord
var arrUShort = Base65536Helper.ToArray(decPnt.x);
var bytes = arrUShort.ToByteArray();
var str = Encoding.Unicode.GetString(bytes);
strMsg = string.Concat(strMsg, str);
// data from y coord
arrUShort = Base65536Helper.ToArray(decPnt.y);
bytes = arrUShort.ToByteArray();
str = Encoding.Unicode.GetString(bytes);
strMsg = string.Concat(strMsg, str);
}
return(strMsg);
}
/// <summary>
/// Encrypt text on elliptic curve and return a list of points on the elliptic curve
/// </summary>
/// <param name="Text"></param>
/// <param name="PubKey"></param>
/// <returns></returns>
public static List <EcModPoint> EcEncrypt(string Text, EcModPoint PubKey, EllipticCurveType EcType)
{
BigInteger p;
BigInteger n;
BigInteger a;
BigInteger b;
// Set params
switch (EcType)
{
case EllipticCurveType.SEC256K1:
p = SEC256K1_P;
n = SEC256K1_N;
a = SEC256K1_A;
b = SEC256K1_B;
break;
case EllipticCurveType.M383:
p = M383_P;
n = M383_N;
a = M383_A;
b = M383_B;
break;
case EllipticCurveType.E521:
p = E521_P;
n = E521_N;
a = E521_A;
b = E521_B;
break;
default:
break;
}
// return the digits of P base 65536
ushort[] digits = Base65536Helper.ToArray(p);
var partitionLen = digits.Length - 1;
// return a string array with string spit in groups of digits
var partitionStrings = Text.SplitInGroup(partitionLen);
// 1) create a list of big integers out of partitioned message
List <BigInteger> lstBiMsg = new List <BigInteger>();
foreach (var partitionStr in partitionStrings)
{
// encode the partioned str in Unicode
var encUtf8 = Encoding.Unicode.GetBytes(partitionStr);
// convert to ushort array
var arrShort = encUtf8.ToUShortArray();
// convert to Base 65536 big integer
var bi65536 = Base65536Helper.FromArray(arrShort);
// add to the list
lstBiMsg.Add(bi65536);
}
// 2) pair with ascii 32 (space) if list count is odd
var lstCnt = lstBiMsg.Count;
if (0 != lstCnt % 2)
{
lstBiMsg.Add(32);
lstCnt++;
}
// 3) build the pairs
List <EcModPoint> lstPm = new List <EcModPoint>();
for (int i = 0; i < lstCnt; i += 2)
{
var pnt = new EcModPoint {
x = lstBiMsg[i], y = lstBiMsg[i + 1]
};
lstPm.Add(pnt);
}
// get a random big integer k
var k = BigIntegerExtensions.NextBigInteger(185, DateTime.Now.Millisecond);
// 4) compute k*G for the chosen curve
EcModPoint kG = null;
switch (EcType)
{
case EllipticCurveType.SEC256K1:
kG = SecP256k1KeyPairGenerator(k);
break;
case EllipticCurveType.M383:
kG = M383KeyPairGenerator(k);
break;
case EllipticCurveType.E521:
kG = E521KeyPairGenerator(k);
break;
default:
break;
}
// 5) compute kPb = k(SkG)
var kPb = ECKeyPairGenerator(p, a, b, PubKey, n, k);
// 6) compute Pm + kPb = Pc (encrypted data)
List <EcModPoint> lstEncr = new List <EcModPoint>();
// first row is kG
lstEncr.Add(kG);
foreach (var pnt in lstPm)
{
var pntAdded = pointAdd(pnt, kPb, p);
lstEncr.Add(pntAdded);
}
return(lstEncr);
}
