转载于：https://bkssl.com/document/nodejs-csharp-ecdh.html

nodejs的ECDH算法在进行computeSecret的时候不会自动进行HASH运算，但C#的ECDH算法必须指定HASH算法。

两边算法必须使用相同的椭圆曲线和Hash算法，例如下面用例都是用的SHA256。
双方交换公钥，计算得到共享秘钥：SharedSecret。

用例使用readline去交换公钥，实际一般运行在其他协议中，像SSH2等。
单纯秘钥交换并不安全，需要增加其他手段保证不被中间人监听、修改，例如ssh2使用serverhostkey对秘钥交换的数据进行签名。

nodejs测试代码

import readline from 'readline';

import crypto  from 'crypto';

function computeHash(data, alg){
    const hashAlg = crypto.createHash(alg || 'SHA256');
    hashAlg.update(data);
    return hashAlg.digest();
}

const reader = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});

const ecdh = crypto.createECDH('prime256v1');

const e = ecdh.generateKeys();

console.log("Local Public Key:" + e.toString('base64'));

reader.question('Please Input Remote PublicKey:', key => {
    console.log("Remote Public Key:" + key);
    let sharedSecret = ecdh.computeSecret(Buffer.from(key, 'base64'));

    //计算共享秘钥，需要先HASH运算，以兼容C#
    sharedSecret = computeHash(sharedSecret);
    console.log("Shared Secret: " + sharedSecret.toString('base64'));
})


// 控制台输出
// Local Public Key:BGLb5Sddo8N5sd1/+a7vhwb66QFMqAJtNFvEISecSzrBP3DjEr3R6vHeefI100yVDCnPQLNmgv56ampL+tNBc2c=
// Please Input Remote PublicKey:BASkBjbrTNysfXrcqyCOWuVgWVRF8a3VTufPLl99PFdY4+oj2d2kImu1OoZLMMoRKwul02gP27dWRoMCGicHjuQ=
// Remote Public Key:BASkBjbrTNysfXrcqyCOWuVgWVRF8a3VTufPLl99PFdY4+oj2d2kImu1OoZLMMoRKwul02gP27dWRoMCGicHjuQ=
// Shared Secret: EvhnVuNRa3dBqIFDyfSQfYmRF4/fPgJHDRZG5xB6HnA=

C#测试代码
 

var ecdh = ECDiffieHellmanHelper.New();
byte[] localPublicKey = ECDiffieHellmanHelper.CreateKeyExchange(ecdh);

Console.WriteLine("Local Public Key: {0}", localPublicKey.ToBase64String());


Console.WriteLine("Please Input Remote Public Key:");
string remotePublicKey = Console.ReadLine();

Console.WriteLine("Remote Public Key: {0}", remotePublicKey);

byte[] sharedSecret = ECDiffieHellmanHelper.DecryptKeyExchange(ecdh, remotePublicKey.AsBase64String());

Console.WriteLine("Shared Secret: {0}", sharedSecret.ToBase64String());

// 控制台输出
// Local Public Key: BASkBjbrTNysfXrcqyCOWuVgWVRF8a3VTufPLl99PFdY4+oj2d2kImu1OoZLMMoRKwul02gP27dWRoMCGicHjuQ=
// Please Input Remote Public Key:
// BGLb5Sddo8N5sd1/+a7vhwb66QFMqAJtNFvEISecSzrBP3DjEr3R6vHeefI100yVDCnPQLNmgv56ampL+tNBc2c=
// Remote Public Key: BGLb5Sddo8N5sd1/+a7vhwb66QFMqAJtNFvEISecSzrBP3DjEr3R6vHeefI100yVDCnPQLNmgv56ampL+tNBc2c=
// Shared Secret: EvhnVuNRa3dBqIFDyfSQfYmRF4/fPgJHDRZG5xB6HnA=

C#的ECDiffieHellmanHelper类实现

internal class ECPublicKey : ECDiffieHellmanPublicKey
{
    private readonly ECParameters parameters;

    public ECPublicKey(ECParameters parameters)
    {
        this.parameters = parameters;
    }

    public override ECParameters ExportParameters() => parameters;

    public override ECParameters ExportExplicitParameters() => parameters;

}

public class ECDiffieHellmanHelper
{
    //可以选择其他的曲线
    private static readonly ECCurve curve = ECCurve.NamedCurves.nistP256;


    public static ECDiffieHellman New(ECCurve curve) => ECDiffieHellman.Create(curve);
    public static ECDiffieHellman New() => ECDiffieHellman.Create(curve);
    public static byte[] CreateKeyExchange(ECDiffieHellman ecdh)
    {
        var args = ecdh.PublicKey.ExportParameters();

        int blockSize = args.Q.X.Length;
        byte[] buffer = new byte[blockSize * 2 + 1];
        buffer[0] = 0x4;
        args.Q.X.CopyTo(buffer, 1);
        args.Q.Y.CopyTo(buffer, blockSize + 1);

        return buffer;
    }

    public static byte[] DecryptKeyExchange(ECDiffieHellman ecdh, byte[] serverPublicKey)
    {
        ReadOnlySpan<byte> bytes = new(serverPublicKey);

        int length = (serverPublicKey.Length - 1) / 2;

        ECPublicKey thirdPublicKey = new (new ECParameters
        {
            Curve = curve,
            D = null,
            Q = new ECPoint
            {
                X = bytes.Slice(1, length).ToArray(),
                Y = bytes.Slice(1 + length, length).ToArray(),
            }
        });

        return ecdh.DeriveKeyFromHash(thirdPublicKey, HashAlgorithmName.SHA256);
    }
}