Simplify sample code for verifying host identify

[0xced]

• Include using directives to make it clear that System.Linq and Renci.SshNet are required
• Remove unnecessary new byte[] in expectedFingerPrint declaration
• Use discard (i.e. _) instead of unused sender argument (C# 7.0)
• Use Enumerable.SequenceEqual instead of manually comparing byte arrays
• using var client without braces (C# 8.0 feature)

[A9G-Data-Droid]

I would like a note about how to get the key into byte[] format.

Related reading: https://winscp.net/eng/docs/faq_hostkey

That only works if you have the ability to get the public key from a known good source.

I like to store it in a Base64 string. So I'm pulling it from a config file like this:

byte[] expectedFingerPrint = Convert.FromBase64String(ServerFingerPrint);

If you want to pull the fingerprint from your code so you can accept the server fingerprint you need something like this:

client.HostKeyReceived += (_, e) =>
{
    ServerFingerPrint = Convert.ToBase64String(e.FingerPrint);
    e.CanTrust = expectedFingerPrint.SequenceEqual(e.FingerPrint);
};

Then when .CanTrust is false you can give the user the option of saving this fingerprint, which is a normal workflow in most SSH\SFTP clients.

This at least serves to notify you when the fingerprint has changed, which is better than nothing.

[WojciechNagorski]

Old PR, closing for now.

[0xced]

Sure it's old but I think it's still relevant, isn't it?

If there are conflicts with the current state of the develop branch I'd be happy to resolve them.

I'd understand if you don't want to merge this because you'd prefer to keep the sample code compatible with an older C# version or maybe for another reason that I haven't thought about. But closing simply because the pull request old is really disappointing and will likely discourage me to further contribute to this project. 😞

[WojciechNagorski]

@0xced regarding this comment #897 (comment)

the check should look like below, right?

client.HostKeyReceived += (_, e) =>
{
    ServerFingerPrint = Convert.ToBase64String(e.FingerPrint);
    e.CanTrust = expectedFingerPrint.SequenceEqual(e.FingerPrint);
};

If you prepare a good PR, I will definitely merge it.

[0xced]

Well, Adam said

I like to store it in a Base64 string.

Personally, I store them in this format: 97:70:33:82:fd:29:3a:73:39:af:6a:07:ad:f8:80:49 because it's an MD5 hash (and that's how Microsoft displays it in their Azure DevOps SSH - Public Keys page):

using System.Globalization;
using System.Linq;
using Renci.SshNet;

const string expectedFingerPrint = "97:70:33:82:fd:29:3a:73:39:af:6a:07:ad:f8:80:49";
using var client = new SshClient(host: "ssh.dev.azure.com", port: 22, username: "git", password: "");
client.HostKeyReceived += (_, e) =>
{
    var md5FingerPrint = string.Join(':', e.FingerPrint.Select(x => x.ToString("x2", NumberFormatInfo.InvariantInfo)));
    e.CanTrust = md5FingerPrint == expectedFingerPrint;
};
client.Connect();

It would even be misleading to use base64 since it's an MD5 hash and usually the fingerprint displayed as base64 are SHA256 hashes, see also GitHub's SSH key fingerprints.

I still think my pull request is good as is. 🤷

[A9G-Data-Droid]

@0xced That's a great point! In newer versions of OpenSSH, Base64 encoded SHA-256 is the default instead of hexadecimal MD5. This topic is related to #613.

I agree that the way you have shown is industry standard, as OpenSSH also reports this colon separated hex format.

The example you give in this PR is good, better than it was. I like your new example better. I would use BitConverter instead of rolling my own.

using System.Globalization;
using System.Linq;
using Renci.SshNet;

const string expectedFingerPrint = "97:70:33:82:fd:29:3a:73:39:af:6a:07:ad:f8:80:49";
using var client = new SshClient("sftp.foo.com", "guest", "pwd")
client.HostKeyReceived += (_, e) =>
{
    var md5FingerPrint = BitConverter.ToString(e.FingerPrint).Replace("-", ":");
    e.CanTrust = md5FingerPrint == expectedFingerPrint;
};

client.Connect();

[0xced]

Related: I just submitted #1163 which adds two new (well documented) properties: MD5FingerPrint and SHA256FingerPrint. Inspired by this discussion of course. 😉

And if #1163 gets merged the sample code could be improved even further.

About BitConverter vs string.Join() we should benchmark it (both speed and allocations) before choosing the best implementation.

[WojciechNagorski]

closed by #1186. Readme has been updated.

[WojciechNagorski]

Version 2023.0.0 has been published https://www.nuget.org/packages/SSH.NET/2023.0.0