ECDiffieHellman - mbedTLS 与 C#

Question

我需要在 ARM cortex M3 和 PC 之间使用椭圆曲线 Diffie Hellman。在 ARM 上，我使用 mbed TLS. On the PC side I want to us C# and the ECDiffieHellman(Cng) class.

我可以在 ARM 和 ARM 上做 ECDH，但是当我尝试用 PC 替换一侧时我确实遇到了麻烦。

问题出在密钥交换上。我发现，密钥由 mbed TLS 导出为 <LEN><0x04><X><Y>.

因此，在 C# 中，我使用

导入 public 密钥

私有静态EC

DiffieHellmanPublicKey ToPublicKey(byte[] publicKey)
{
    var keyLength = 32;

    if (publicKey[0] != (2 + 2 * keyLength) - 1)
        throw new ArgumentException("Invalid key length", nameof(publicKey));
    if (publicKey[1] != 0x04)
        throw new ArgumentException("Invalid key format", nameof(publicKey));

    var parameters = new ECParameters()
    {
        Curve = ECCurve.NamedCurves.brainpoolP256r1,
        Q = new ECPoint()
        {
            X = publicKey.Skip(2).Take(keyLength).ToArray(),
            Y = publicKey.Skip(2 + keyLength).Take(keyLength).ToArray()
        }
    };

    using (var tmp = ECDiffieHellman.Create(parameters))
    {
        return tmp.PublicKey;
    }
}

生成我试过的密钥

var ecdh = new ECDiffieHellmanCng(ECCurve.NamedCurves.brainpoolP256r1);
ecdh.KeyDerivationFunction = ECDiffieHellmanKeyDerivationFunction.Tls;
ecdh.Seed = new byte[32];
ecdh.Label = Encoding.ASCII.GetBytes("ECDiffieHellman");

new RNGCryptoServiceProvider().GetBytes(ecdh.Seed);

// ...

var sharedSecret = ecdh.DeriveKeyMaterial(peersPublicKey);

我的问题是生成的共享密钥与 mbed TLS 生成的共享密钥长度不同，不匹配。

有人已经解决了这个问题吗？

谢谢！

编辑 1：

我忘了说，我在 ARM 上使用裸 ECDH。所以我认为没有任何密钥派生函数在执行。对结果进行散列 (SHA256) 以匹配 C# 端是否足够（在此处将 SHA256 配置为 KDF 时）？

Answer 1

我解决了这个问题。简而言之：只需散列 ECDH 结果。

C/C++边:

EC 迪菲赫尔曼

mbedtls_ecdh_init(...);
mbedtls_ecdh_setup(...);
mbedtls_ecdh_make_public(...); //make own public key and send it to peer
mbedtls_ecdh_read_public(...); //reed peers public key
mbedtls_ecdh_calc_secret(...); //note: i pass in my own RND func because of no OS
mbedtls_ecdh_free(...);

SHA256

mbedtls_sha256_init(...);
mbedtls_sha256_starts_ret(...);
mbedtls_sha256_update_ret(...);
mbedtls_sha256_finish_ret(...);
mbedtls_sha256_free(...);

C# 端:

private void EllipticCurveDiffieHellman()
{
    var ecdh = new ECDiffieHellmanCng(ECCurve.NamedCurves.brainpoolP256r1);

    ecdh.KeyDerivationFunction = ECDiffieHellmanKeyDerivationFunction.Hash;
    ecdh.HashAlgorithm = CngAlgorithm.Sha256;

    //get relevant point from own public key
    var ownPublicKey = ecdh.PublicKey.ExportExplicitParameters().Q;

    var peersPublicKey = SendPublicKey(ownPublicKey); //key exchange
    var sharedSecret = ecdh.DeriveKeyMaterial(peersPublicKey);

    Console.WriteLine("Key: " + HexValue.Parse(sharedSecret.ToArray()));
}

DiffieHellmanPublicKey ToPublicKey(byte[] publicKey)
{
    var keyLength = 32;

    if (publicKey[0] != (2 + 2 * keyLength) - 1)
        throw new ArgumentException("Invalid key length", nameof(publicKey));
    if (publicKey[1] != 0x04)
        throw new ArgumentException("Invalid key format", nameof(publicKey));

    var parameters = new ECParameters()
    {
        Curve = ECCurve.NamedCurves.brainpoolP256r1,
        Q = new ECPoint()
        {
            X = publicKey.Skip(2).Take(keyLength).ToArray(),
            Y = publicKey.Skip(2 + keyLength).Take(keyLength).ToArray()
        }
    };

    using (var tmp = ECDiffieHellman.Create(parameters))
    {
        return tmp.PublicKey;
    }
}

ECDiffieHellman - mbedTLS 与 C#

ECDiffieHellman - mbedTLS vs C#

c#

c

elliptic-curve

diffie-hellman

mbedtls