forked from 3wordchant/capsul-flask
352 lines
18 KiB
HTML
352 lines
18 KiB
HTML
{% extends 'base.html' %}
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{% block title %}About SSH{% endblock %}
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{% block content %}
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<div class="row full-margin"><h1>Understanding the Secure Shell Protocol (SSH)</h1></div>
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{% endblock %}
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{% block subcontent %}
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<div class="long-form">
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<p>
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In order to use our service, you will have to use the Secure Shell protocol (SSH) to connect to your capsul.
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</p>
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<p>
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<a href="https://en.wikipedia.org/wiki/SSH_(Secure_Shell)">SSH</a> is a very old tool, created back when the internet was a different place, with different use cases and concerns.
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In many ways, the protocol has failed to evolve to meet the needs of our 21st century global internet.
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Instead, the users of SSH (tech heads, sysadmins, etc) have had to evolve our processes to work around SSH's limitations.
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</p>
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<p>
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These days, we use SSH + public-key cryptography to establish secure connections to our servers.
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If you are not familiar with the concept of public key cryptography, cryptographic signatures,
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or diffie-hellman key exchange, you may wish to see
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<a href="https://en.wikipedia.org/wiki/Public-key_cryptography">the wikipedia article</a> for a refresher.
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</p>
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<div class="row half-margin"><h1>Public Key Crypto and Key Exchange: The TL;DR</h1></div>
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<p>
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Computers can generate <b>"key pairs"</b> which consist of a public key and a private key. Given a <b>public key pair A</b>:
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</p>
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<ol>
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<li>
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A computer which has access to <b>public key A</b> can encrypt data,
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and then <b>ONLY</b> a computer which has access <b>private key A</b> can decrypt & read it
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</li>
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<li>
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Likewise, a computer which has access to <b>private key A</b> can encrypt data,
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and any a computer which has access <b>public key A</b> can decrypt it,
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thus <b>PROVING</b> the message must have come from someone who posesses <b>private key A</b>
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</li>
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</ol>
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<p>
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Key exchange is a process in which two computers, Computer A and Computer B (often referred to as Alice and Bob)
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both create key pairs, so you have <b>key pair A</b> and <b>key pair B</b>, for a total of 4 keys:
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</p>
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<ol>
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<li><b>public key A</b></li>
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<li><b>private key A</b></li>
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<li><b>public key B</b></li>
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<li><b>private key B</b></li>
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</ol>
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<p>
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In simplified terms, during a key exchange,
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</p>
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<ol>
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<li><b>computer A</b> sends <b>computer B</b> its public key</li>
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<li><b>computer B</b> sends <b>computer A</b> its public key</li>
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<li><b>computer A</b> sends <b>computer B</b>
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a message which is encrypted with <b>computer B</b>'s public key</li>
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<li><b>computer B</b> sends <b>computer A</b>
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a message which is encrypted with <b>computer A</b>'s public key</li>
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</ol>
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<p>
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The way this process is carried out allows A and B to communicate with each-other securely, which is great, <br/><br/>
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<b><u>HOWEVER, there is a catch!!</u></b>
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</p>
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<p>
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When computers A and B are trying to establish a secure connection for the first time,
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we assume that the way they communicate right now is NOT secure. That means that someone on the network
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between A and B can read & modify
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all messages they send to each-other! You might be able to see where this is heading...
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</p>
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<p>
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When <b>computer A</b> sends its public key to <b>computer B</b>,
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someone in the middle (lets call it <b>computer E, or Eve</b>) could record that message, save it,
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and then replace it with a forged message to <b>computer B</b> containing <b>public key E</b>
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(from a key pair that <b>computer E</b> generated).
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If this happens, when <b>computer B</b> sends an encrypted message to <b>computer A</b>,
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B thinks that A's public key is actually <b>public key E</b>, so it will use <b>public key E</b> to encrypt.
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And again, <b>computer E</b> in the middle can intercept the message, and they can decrypt it as well
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because they have <b>private key E</b>.
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Finally, they can relay the same message to <b>computer A</b>, this time encrypted with <b>computer A</b>'s public key.
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This is called a <a href="https://en.wikipedia.org/wiki/Man-in-the-middle_attack">Man In The Middle (MITM)</a> attack.
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</p>
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<p>
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Without some additional verification method,
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<b><u>Computer A AND Computer B can both be duped and the connection is NOT really secure</u></b>.
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</p>
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<div class="row half-margin"><h1>Authenticating Public Keys: A Tale of Two Protocols</h1></div>
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<p>
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Now that we have seen how key exhange works,
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and we understand that in order to prevent MITM attacks, all participants have to have a way of knowing
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whether a given public key is authentic or not, I can explain what I meant when I said
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</p>
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<p>
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> [SSH] has failed to evolve to meet the needs of our 21st century global internet
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</p>
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<p>
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In order to explain this, let's first look at how a different, more modern protocol,
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<a href="https://en.wikipedia.org/wiki/Transport_Layer_Security">Transport Layer Security (or TLS)</a> solved this problem.
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TLS, (still sometimes called by its olde name "Secure Sockets Layer", or SSL) was created to enable HTTPS, to allow
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internet users to log into web sites securely and purchase things online by entering their credit card number.
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Of course, this required security that actually works; if someone could MITM attack the connection, they could easily
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steal tons of credit card numbers and passwords.
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</p>
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<p>
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In order to enable this, a new standard called <a href="https://en.wikipedia.org/wiki/X.509">X.509</a> was created.
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X.509 dictates the data format of certificates and keys (public keys and private keys), and it also defines
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a simple and easy way to determine whether a given certificate (public key) is authentic.
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X.509 introduced the concept of a Certificate Authority, or CA.
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These CAs were supposed to be bank-like public institutions of power which everyone could trust.
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The CA would create a key pair on an extremely secure computer, and then a CA Certificate (the public side of that key pair)
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would be distributed along with every copy of Windows, Mac OS, and Linux. Then folks who wanted to run a secure web server
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could generate thier OWN key pair for thier web server,
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and pay the CA to sign thier web server's X.509 certificate (public key) with the highly protected CA private key.
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Critically, issue date, expiration date, and the domain name of the web server, like foo.example.com, would have to be included
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in the x.509 certiciate along with the public key.
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This way, when the user types https://foo.example.com into thier web browser:
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</p>
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<ol>
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<li>The web browser sends a TLS ClientHello request to the server</li>
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<li>
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The server responds with a ServerHello & ServerCertificate message
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<ul>
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<li>The ServerCertificate message contains the X.509 certificate for the web server at foo.example.com</li>
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</ul>
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</li>
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<li>The web browser inspects the X.509 certificate
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<ul>
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<li>
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Is the current date in between the issued date and expiry date of the certificate?
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If not, display an <a href="https://expired.badssl.com/">EXPIRED_CERTIFICATE error</a>.
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</li>
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<li>
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Does the domain name the user typed in, foo.example.com, match the domain name in the certificate?
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If not, display a <a href="https://wrong.host.badssl.com/">BAD_CERT_DOMAIN error</a>.
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</li>
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<li>
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Does the certificate contain a valid CA signature?
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(can the signature on the certificate be decrypted by one of the CA Certificates included with the operating system?)
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If not, display an <a href="https://untrusted-root.badssl.com/">UNKNOWN_ISSUER error</a>.
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</li>
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</ul>
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</li>
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<li>Assuming all the checks pass, the web browser trusts the certificate and connects</li>
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</ol>
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<p>
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This system enabled the internet to grow and flourish:
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purchasing from a CA was the only way to get a valid X.509 certificate for a website,
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and guaranteeing authenticity was in the CA's business interest.
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The CAs kept their private keys behind razor wire and armed guards, and followed strict rules to ensure that only the right
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people got thier certificates signed.
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Only the CAs themselves or anyone who had enough power to force them to create a fraudulent certificate
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would be able to execute MITM attacks.
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</p>
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<p>
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The TLS+X.509 Certificate Authority works well for HTTP and other application protocols, because
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</p>
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<ul>
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<li>Most internet users don't have the patience to manually verify the authenticity of digital certificates.</li>
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<li>Most internet users don't understand or care how it works; they just want to connect right now.</li>
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<li>Businesses and organizations that run websites are generally willing to jump through hoops and
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subjugate themselves to authorities in order to offer a more secure application experience to thier users.</li>
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<li>The centralization & problematic power dynamic which CAs represent
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is easily swept under the rug, if it doesn't directly or noticably impact the average person, who cares?</li>
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</ul>
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<p>
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However, this would never fly with SSH. You have to understand, SSH does not come from Microsoft, it does not come from Apple,
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in fact, it does not even come from Linux or GNU. <a href="https://www.openssh.com/">SSH comes from BSD</a>.
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<a href="https://en.wikipedia.org/wiki/BSD">Berkeley Software Distribution</a>. Most people don't even know
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what BSD is. It's <i>Deep Nerdcore</i> material. The people who maintain SSH are not playing around, they would never
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allow themselves to be subjugated by so-called "Certificate Authorities".
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So, what are they doing instead? Where is SSH at? Well, back when it was created, computer security was easy —
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a very minimal defense was enough to deter attackers.
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In order to help prevent these MITM attacks, instead of something like X.509, SSH employs a policy called
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<a href="https://en.wikipedia.org/wiki/Trust_on_first_use">Trust On First Use (TOFU)</a>.
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</p>
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<p>
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The SSH client application keeps a record of every server it has ever connected to
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in a file <span class="code">~/.ssh/known_hosts</span>.
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</p>
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<p>
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(the tilde <span class="code">~</span> here represents the user's home directory,
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<span class="code">/home/username</span> on linux,
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<span class="code">C:\Users\username</span> on Windows, and
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<span class="code">/Users/username</span> on MacOS).
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</p>
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<p>
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If the user asks the SSH client to connect to a server it has never seen before,
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it will print a prompt like this to the terminal:
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</p>
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<pre class="code">The authenticity of host 'fooserver.com (69.4.20.69)' can't be established.
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ECDSA key fingerprint is SHA256:EXAMPLE1xY4JUVhYirOVlfuDFtgTbaiw3x29xYizEeU.
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Are you sure you want to continue connecting (yes/no/[fingerprint])?</pre>
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<p>
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Here, the SSH client is displaying the fingerprint (<a href="https://en.wikipedia.org/wiki/SHA-2">SHA256 hash</a>)
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of the public key provided by the server at <span class="code">fooserver.com</span>.
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Back in the day, when SSH was created, servers lived for months to years, not minutes, and they were installed by hand.
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So it would have been perfectly reasonable to call the person installing the server on thier
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<a href="https://nokiamuseum.info/nokia-909/">Nokia 909</a>
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and ask them to log into it & read off the host key fingerprint over the phone.
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After verifing that the fingerprints match in the phone call, the user would type <span class="code">yes</span>
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to continue.
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</p>
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<p>
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After the SSH client connects to a server for the first time, it will record the server's IP address and public key in the
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<span class="code">~/.ssh/known_hosts</span> file. All subsequent connections will simply check the public key
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the server presents against the public key it has recorded in the <span class="code">~/.ssh/known_hosts</span> file.
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If the two public keys match, the connection will continue without prompting the user, however, if they don't match,
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the SSH client will display a scary warning message:
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</p>
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<pre class="code">
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@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
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@ WARNING: POSSIBLE DNS SPOOFING DETECTED! @
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@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
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The ECDSA host key for fooserver.com has changed,
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and the key for the corresponding IP address 69.4.20.42
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is unknown. This could either mean that
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DNS SPOOFING is happening or the IP address for the host
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and its host key have changed at the same time.
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@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
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@ WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED! @
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@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
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IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
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Someone could be eavesdropping on you right now (man-in-the-middle attack)!
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It is also possible that a host key has just been changed.
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The fingerprint for the ECDSA key sent by the remote host is
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SHA256:EXAMPLEpDDefcNcIROtFpuTiHC1j3iNU74aaKFO03+0.
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Please contact your system administrator.
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Add correct host key in /root/.ssh/known_hosts to get rid of this message.
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Offending ECDSA key in /root/.ssh/known_hosts:1
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remove with:
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ssh-keygen -f "/root/.ssh/known_hosts" -R "fooserver.com"
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ECDSA host key for fooserver.com has changed and you have requested strict checking.
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Host key verification failed.
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</pre>
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<p>
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This is why it's called <b>Trust On First Use</b>:
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SSH protocol assumes that when you type <span class="code">yes</span> in response to the prompt during your first connection,
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you <b>really did</b> verify that the server's public key fingerprint matches.
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If you type <span class="code">yes</span> here without checking the server's host key somehow, you could add an attackers public key to the trusted
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list in your <span class="code">~/.ssh/known_hosts</span> file; if you type <span class="code">yes</span> blindly, you are
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<b>completely disabling all security of the SSH connection</b>.
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It can be fully man-in-the-middle attacked & you are
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vulnerable to surveillance, command injection, even emulation/falsification of the entire stream.
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</p>
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<p>
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So what are technologists to do? Most cloud providers don't "provide" an easy way to get the SSH host public keys
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for instances that users create on thier platform. For example, see this
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<a href="https://serverfault.com/questions/941915/verify-authenticity-of-ssh-host-on-digital-ocean-droplet-freebsd">
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question posted by a frustrated user trying to secure thier connection to a digitalocean droplet</a>.
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Besides using the provider's HTTPS-based console to log into the machine & directly read the public key,
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providers also recommend using a "userdata script".
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This script would run on boot & upload the machine's SSH public keys to a
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trusted location like <a href="https://www.backblaze.com/b2/cloud-storage.html">Backblaze B2</a> or
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<del>Amazon S3</del><sup><a href="#ref_1">[1]</a></sup>, for an application to retrieve later.
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As an example, I wrote a
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<a href="https://git.sequentialread.com/forest/rootsystem/src/1cdbe53974d20da97d9f522d4bd62c34487817c0/terraform-modules/gateway-instance-digitalocean/upload_known_hosts.tpl#L5">
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userdata script which does this</a>
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for my own cloud compute management tool called
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<a href="https://git.sequentialread.com/forest/rootsystem">rootsystem</a>.
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Later in the process, rootsystem will
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<a href="https://git.sequentialread.com/forest/rootsystem/src/1cdbe53974d20da97d9f522d4bd62c34487817c0/host-key-poller/main.go#L33">
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download the public keys from the Object Storage provider
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and add them to the ~/.ssh/known_hosts file</a>
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before finally
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<a href="https://git.sequentialread.com/forest/rootsystem/src/1cdbe53974d20da97d9f522d4bd62c34487817c0/terraform-modules/ansible-threshold-server/main.tf#L32">
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invoking the ssh client against the cloud host</a>.
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</p>
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<p>
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Personally, I think it's disgusting and irresponsible to require users to go through that much work
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just to be able to connect to their instance securely. However, this practice appears to be an industry standard.
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It's gross, but it's where we're at right now.
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</p>
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<p>
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So for <a href="https://capsul.org">capsul</a>, we obviously wanted to do better.
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We wanted to make this kind of thing as easy as possible for the user,
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so I'm proud to announce as of today, capsul SSH host key fingerprints will be displayed on the capsul detail page,
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as well as the host's SSH public keys themselves in <span class="code">~/.ssh/known_hosts</span> format.
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Users can simply copy and paste these keys into thier <span class="code">~/.ssh/known_hosts</span> file and connect
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with confidence that they are not being MITM attacked.
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</p>
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<div class="row half-margin"><h1>It's 2021. Can't we do better than this? What's next?</h1></div>
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<p>
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Glad you asked 😜.
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</p>
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<p>
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TLS is great, except it has one problem: the X.509 CA system centralizes power and structurally invites abuse.
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Power corrupts, and absolute power corrupts absolutely. But there is hope for the future: with the invention of Bitcoin
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in 2009, we now have a new tool to use for authority-free secure consensus. Some bright folks have forked Bitcoin to produce
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<a href="https://www.namecoin.org/">Namecoin</a>, a DNS-like public blockchain which is
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<a href="https://en.bitcoin.it/wiki/Merged_mining_specification">merge-mined</a> with Bitcoin, and which allows users to
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<a href="https://sequentialread.com/how-to-register-a-namecoin-bit-domain-with-electrum-nmc/">
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register and trade names, including domain names</a>.
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In fact, Namecoin features a
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<a href="https://github.com/namecoin/proposals/blob/master/ifa-0003.md">
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specification for associating public keys with domain names
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</a>
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and easy-to-use client software packages capable of resolving these
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<a href="https://www.namecoin.org/download/betas/#ncdns">names</a>
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&
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<a href="https://www.namecoin.org/download/betas/#ncp11">
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public</a>
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<a href="https://www.namecoin.org/resources/presentations/Grayhat_2020/Namecoin_TLS_Part_2_Grayhat_2020_Monero_Village.pdf">
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keys</a>,
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capable of replacing both the DNS system and X.509 Certificate Authority system.
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</p>
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<p>
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For more information on how to get started with Namecoin, see my
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<a href="https://sequentialread.com/how-to-register-a-namecoin-bit-domain-with-electrum-nmc/">
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Namecoin guide for webmasters</a>.
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</p>
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<p>
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Cheers and best wishes,<br/>
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Forest
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</p>
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<hr/>
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<p>
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<sup id="ref_1">[1]</sup> <a href="https://www.doitwithoutdues.com/">fuck amazon</a>
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</p>
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</div>
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{% endblock %}
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{% block pagesource %}/templates/about-ssh.html{% endblock %}
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