How's My SSL? is a cute little website that tells you how secure your TLS client is. TLS clients just like the browser you're reading this with.

How's My SSL? was originally made to help a web server developer learn what real world TLS clients were capable of. It's been expanded to give developers and the very technically-savvy a quick and easy way to learn more about the TLS tools they use.

It's also meant to impel developers to modernize and improve their TLS stacks. Many security problems come from engineers simply not knowing what worries to have. How's My SSL? is a demonstration of what those TLS client worries should be.

How's My SSL? chooses topics important to today's security environment and analyzes clients in that context. It will never be a complete audit, but it can hit the high notes. Over time, How's My SSL? will change to live in an ever more difficult security environment. It will be kept up by people who care.


Rating

How's My SSL? gives a rating to each client that connects to it. These ratings are not the final word on "how secure" a TLS client is, but captures important aspects of its security.

Clients are rated as Probably Okay if no security problems could be detected.

Clients are downgraded to Improvable if they do not support ephemeral key cipher suites, do not support session tickets, or are using TLS 1.1.

Clients are downgraded to Bad if any of the following are true:

  • It uses TLS 1.0 (instead of 1.1 or 1.2), or, worse, SSLv3 or earlier.
  • It supports known insecure cipher suites. Developers can find the list of known insecure cipher suites in the howsmyssl repository on GitHub.
  • It supports TLS compression (that is compression of the encryption information used to secure your connection) which exposes it to the CRIME attack.
  • It is susceptible to the BEAST attack.

How's My SSL? always selects the worst rating the client could receive. That is, clients rated as "Bad" may also have problems that, on their own, would have rated them as "Improvable".


What It Means

How's My SSL has a lot of jargon on it. We're going to discuss the sections on its homepage. While it won't help everyone understand all of it, those who have a glimmer of an idea of what they mean will be better off reading these.

Version

The Version section discusses the security of the highest version of the TLS protocol your client supports. (Formerly, TLS was known as "SSL".)

TLS 1.2 is latest version of TLS and it is recommended for clients to implement. It is the new normal for highly secure websites. TLS 1.2 provides access to advanced cipher suites that support elliptical curve cryptography (large efficiency wins) and AEAD block cipher modes (like the very nice GCM cipher suites). Clients using TLS 1.2 may be set to Probably Okay.

TLS 1.1 is the second most recent version of TLS. It fixes some security problems in TLS 1.0, removing the need for many of the workarounds built into clients and servers. Many deployed clients and servers of TLS 1.0 do not implement those workarounds and, so, TLS 1.1 is a good improvement. However, the modern security environment has pushed us to TLS 1.2. Clients using TLS 1.1 will be marked down to at least Improvable.

TLS 1.0 is the first version of TLS, is fairly common in the world, and requires workarounds in both the client and server to work securely for all cipher suites. TLS 1.0 is also unable to use modern cipher suites that offer greater security and efficiency. Clients using it will be marked as Bad.

SSLv3, the last version of SSL, and the first before TLS 1.0, is considered insecure. Clients using it or older versions of SSL will be marked as Bad.

Ephemeral Key Support

The Ephemeral Key Support section says if your client tells the service that it supports cipher suites that include an additional private key created for each connection to the service. This allows servers that support it to provide forward secrecy to the client.

This extra key prevents attackers from storing all of your encrypted communication with the website, then later gaining access to the website's private key, and decrypting all of your (and others') traffic to the website. This additional key, however, is expensive to transfer, especially without elliptical curve cryptography support in the client and server to make the key smaller at the same security. Clients with support for ephemeral key cipher suites will be defaulted to Probably Okay. Clients without it will be marked as Bad.

Session Ticket Support

Session tickets are a mechanism to speed up resumption of encrypted communication with a server. When a client and server first connect over TLS, they perform a complicated, time-consuming handshake. This handshake lets the client and the server communicate a shared secret (basically, a password) without exposing that secret to people tapping the line. However, it only allows the client and the specific computer that served the client communicate. Since websites can (and, often, must) serve their traffic from many physical computers, if the client disconnects and reconnects to the website, the client could connect to another machine that knew nothing about the shared secret. When that happens, the client and server have to perform the slow handshake again.

If the client supports session tickets, the physical machines that serve a website's traffic and share a special second key called a session ticket key can deterministically generate a secret shared with the client called a session ticket. If the client connects and reconnects it includes this session ticket and most of the expensive reconnection process can be skipped. This is a huge performance boost, making users happier.

However, the session ticket key living on all of the website's computers means there is a secret that could be leaked to an attacker. Worse, it undermines the security of ephemeral key cipher suites. Preventing this is often a source of important engineering work for a security engineer team. Of course, another alternative is for the website to simply turn off support for session tickets and enable support for ephemeral keys.

How's My SSL will rate clients with session ticket support as Probably Okay, but not having it will only have it defaulted to Improvable.

TLS Compression

TLS compression was a specification of a way to make the data in the encrypted connection smaller in order to speed up the transfer of data across those connections. Unfortunately, applying this technique makes it easy for attackers to learn information about the encrypted data if the attackers can make the user's computer make connections to the website. In practice, this turns out to be easy. This is now referred to as the CRIME attack. While some clients have mitigations in place, it's been determined that the best way to avoid the attack is to disable TLS compression entirely. Clients that do so will be defaulted to Probably Okay while clients that leave it enabled will be marked as Bad.

BEAST Vulnerability

The BEAST vulnerability is an attack on TLS 1.0 connections using a cipher suite in a cipher block chaining (CBC) mode. An attacker is able to decrypt the data on a TLS connection matching that criteria. Upgrading to TLS 1.1 or greater or not using CBC cipher suites prevents the attack (though, RC4 has its own issues). Another mitigation strategy is called 1/n-1 record splitting. 1/n-1 record splitting allows TLS 1.0 clients using CBC cipher modes to mitigate the attack by sending only a single byte in the first encrypted application data record of the connection, which fixes up the initialization vector for the rest of the connection.

A caveat here: Certain TLS libraries, like Java's javax.net.ssl, use a slightly different form of 1/n-1 record splitting that is not detectable by How's My SSL? as most requests to How's My SSL? are too small to trigger it. However, these libraries have usually implemented TLS 1.2 and clients should use it to remove this error.

If the client supports CBC mode cipher suites on TLS 1.0 and does not implement the 1/n-1 record splitting mitigation detectable by How's My SSL?, it will be marked as Bad.

Clients using versions of TLS greater than 1.0 or not using CBC cipher suites or are using both but also implement 1/n-1 record splitting, will be marked as Probably Okay

Insecure Cipher Suites

This section displayed any cipher suites the client supports that are known to be insecure. Insecure cipher suites make it easy for attackers to decrypt data on tapped lines, or make it easy for the attacker to pretend to be the website, stealing secrets directly from the client. Cipher suites are found here for one of many reasons. Insecure cipher suites are only a problem if the server also supports it. Of course, some of these suites don't require the server to prove that they can serve the traffic for the site they claim to, making them easy to man-in-the-middle.

The first among these reasons is if the size of the keys created by the cipher suite are too small. Small keys are easier to guess (or "brute force"), and there are many cipher suites that have been defined with far, far too small keys.

A cipher suite supported by the client will be put here if it does not ensure that the server you connected to was actually for the website you wanted. Cipher suites are to do so by inspecting the certificate the server sends and verifying it trusts the other entities (called "certificate authorities") that have "signed" the certificate. This is called the "certificate authority trust model", and if this sounds crazy to you on many levels, you are not alone. It's currently the best security we have, however. TACK is one good attempt to, in a backwards compatible way, replace it.

A cipher suite may also be added if it's a "null cipher suite" which is a funny way of saying the cipher does not encrypt the data at all. In a truly bizarre set of decisions these were put in the SSL/TLS specifications even though offer no privacy or security for the user or the web service. These are giant, glowing "rob me" signs on the web.

Finally, a cipher suite can be added here if it is known to be obsolete in some way. Currently, some NSS clients (like Firefox) will allow for the use of under-specified or broken cipher suites that were meant to be killed off after the death of SSLv3 and were never really specified well in the first place.

Any client supporting an insecure cipher suite will be marked as Bad.

Unknown Cipher Suites Supported

This section notes any cipher suites that How's My SSL did not recognize. How's My SSL has a very complete knowledge of cipher suites both specified and in use. If a cipher suite is unknown, the client is being, well, "interesting", and "interesting" usually means broken. Without a specification, a cipher suite has not been publicly vetted by the community of cryptographers and the vast, vast, vast majority of unvetted security systems are entirely insecure. Clients supporting unknown cipher suites will be marked as Bad.

Given Cipher Suites

This section is one of the most often needed by developers and was the original reason this webapp was made. It simply prints out the standard names of the cipher suites that the client says it supports in the same order the client gave them to the server. This ordering is sometimes used to determine which cipher suite a client prefers, but TLS servers tend to choose their own preference of those cipher suits. No rating will be given in this section.


TLS vs SSL

Okay, last thing. The jargon around is a little funny, so let's be a little more explicit. The 'S' in "HTTPS" is the TLS protocol. When folks refer to the "TLS" they are referring to the most common of modern protocols of encrypting data across the internet. "SSL", when used by experts, refers to the older versions of these protocols. In general, people use "SSL" and "TLS" interchangeably, but that's changing towards everyone saying "TLS". "TLS" is what everyone will call it in the future, while "SSL" is the phrase everyone knows right now.

(Heck, at some point, this site will redirect to howsmytls.com instead of it redirecting here.)


Contributing

Have feedback? Leave it on the howsmyssl-upkeep mailing list. Notice a bug? Create an issue on the Github repository.

Feedback is very welcome.


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