Please, I insist, can you come up with an example using small baby numbers?
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ECKeyPairGenerator gen = new ECKeyPairGenerator();
var secureRandom = new SecureRandom();
var ps = Org.BouncyCastle.Asn1.Sec.SecNamedCurves.GetByName("secp256k1");
var ecParams = new ECDomainParameters(ps.Curve, ps.G, ps.N, ps.H);
var keyGenParam = new ECKeyGenerationParameters(ecParams, secureRandom);
gen.Init(keyGenParam);
// Generate private key #1.
AsymmetricCipherKeyPair kp1 = gen.GenerateKeyPair();
ECPrivateKeyParameters priv1 = (ECPrivateKeyParameters)kp1.Private;
byte[] hexpriv1 = priv1.D.ToByteArrayUnsigned();
Debug.WriteLine("Private key 1 is " + Bitcoin.ByteArrayToString(hexpriv1));
// Get the public key for #1 (multiplying it by constant G defined by secp256k1).
ECPoint pub1 = ps.G.Multiply(priv1.D);
// Generate private key #2.
AsymmetricCipherKeyPair kp2 = gen.GenerateKeyPair();
ECPrivateKeyParameters priv2 = (ECPrivateKeyParameters)kp2.Private;
byte[] hexpriv2 = priv2.D.ToByteArrayUnsigned();
Debug.WriteLine("Private key 2 is " + Bitcoin.ByteArrayToString(hexpriv2));
// Get the public key for #2.
ECPoint pub2 = ps.G.Multiply(priv2.D);
// Add the two public keys together.
ECPoint pubsum = pub1.Add(pub2);
// Turn that into a Bitcoin address.
byte[] pubbytes = Bitcoin.PubKeyToByteArray(pubsum);
string pubhashsum = Bitcoin.PubHexToPubHash(pubbytes);
string pubaddr = Bitcoin.PubHashToAddress(pubhashsum, "Bitcoin");
Debug.WriteLine("Bitcoin address of summing public keys: " + pubaddr);
// Combine the two private keys together.
Org.BouncyCastle.Math.BigInteger privsum = priv1.D.Add(priv2.D).Mod(ps.N);
// Turn the combined privkey into a pubkey...
ECPoint pub12 = ps.G.Multiply(privsum);
// ... then a Bitcoin address.
pubbytes = Bitcoin.PubKeyToByteArray(pub12);
pubhashsum = Bitcoin.PubHexToPubHash(pubbytes);
pubaddr = Bitcoin.PubHashToAddress(pubhashsum, "Bitcoin");
Debug.WriteLine("Bitcoin address of multiplying private keys: " + pubaddr);
Debug.WriteLine("Combined private key: " + Bitcoin.ByteArrayToString(privsum.ToByteArrayUnsigned()));
Private key 1 is B2 55 38 35 39 4D 34 8C 85 54 BB EB 14 02 29 91 D4 89 E4 EF 4A F7 20 51 D3 59 5F 8C 23 E1 BC B9
Private key 2 is E2 DC 50 E3 79 E0 EC F8 2D C0 5E 6D A5 08 89 CC 8A 4C CA C2 3F 40 76 76 CD 15 3C 49 C9 AB AB 41
Bitcoin address of summing public keys: 12mMm5FypacB3rcehrBpqaNqwYvF9PSjjE
Bitcoin address of multiplying private keys: 12mMm5FypacB3rcehrBpqaNqwYvF9PSjjE
Combined private key: 95 31 89 18 B3 2E 21 84 B3 15 1A 58 B9 0A B3 5F A4 27 D2 CA DA EE F6 8C E0 9C 3D 49 1D 57 26 B9