Encrypting Communication

To encrypt communications between endpoints, several protocols exist. The most populars being TLS, SSH and IPSEC. Each usually being used for different purposes. Yet, many developers will often feel the need to re-invent the wheel and create their own "proprietary" protocol. You've heard the saying "don't roll your own crypto", so you know you should use TLS. If you're not using TLS because you have an excellent reason not to use TLS, you are allowed to consider the Noise protocol framework, but keep in mind that even Noise requires you to understand what you're doing.


While TLS is usually used between browsers and webservers, there are no limitations as to how one can use TLS and to what kind of endpoints are able to use TLS to protect their communications. Because of this initial setting though, TLS is commonly encountered as a protocol that authenticates the server only and does not care about the client (perhaps the client authenticates later via a password in the application layer). Yet, this does not mean that TLS is limited to this configuration and client authentication (via certificates) as part of the protocol is totally possible.

Note that TLS is sometimes seen deeply integrated with another kind of protocol. For example the QUIC protocol (sometimes refered to as TCP 2.0) has encryption by default thanks to TLS.

What Versions of SSL/TLS Are Secure?

The design of SSL/TLS has had many broken versions in the past. For this reason, practically nobody uses SSL anymore and more secure versions of TLS (1.2 and 1.3) are pushed for adoption.

Version Secure Vulnerabilities
TLS 1.1 yes /
TLS 1.2 yes /
TLS 1.3 yes /

Ideally, only TLS 1.1, 1.2 and 1.3 should be supported. Hyper ideally, only the last version (TLS 1.3) should be supported.

Can TLS 1.0 And 1.1 Still Be Supported?

Unfortunately, many clients continue to use older versions and it is sometimes tricky to continue to support them. This leads to the question, can we support older version securely?

Note that if you need to follow strong regulations like the PCI DSS, Anything under TLS 1.1 has been deprecated which should facilitate this answer.

Non-web browser <-> web server setup. Both POODLE (SSL 3.0) and BEAST (TLS 1.0) are attacks that usually imply a browser-based client. This is because the attack has several requirements:

  • the attacker needs a way to execute malicious code on the client-side in order to emite specific SSL/TLS packets (this is usually done by serving malicious javascript to the client)
  • each client's encrypted packet content is in part known (usually true for browsers since we know what HTTP requests look like)
  • what we want to steal is repeatedly and automatically sent in every requests emitted by the client (we are usually targeting a cookie)

For this reason, most of the non-browser-webserver scenarios might not be vulnerable to these attacks. Nonetheless attacks only get better and one should not rely on a specific scenario to mitigate a powerful attack.

Web browser <-> web server setup. While SSL 3.0 is pretty much broken and can even endanger secure versions of TLS (DROWN), we know ways to mitigate the BEAST attack on TLS 1.0. These mitigations are implemented in all modern browsers which let us conclude that BEAST is no longer a threat. With that in mind, supporting TLS 1.0 is possible, but not recommended.

Can SSL 3.0 Still Be Supported?

While some companies still support SSL 3.0, it is a difficult endeavor as the protocol is completely broken by the POODLE attack and the numerous attacks on RC4. You should not support it unless you have extremely good reasons and you know what you're doing.

Is It Possible To Support Several Versions of TLS?

The first problem in supporting several versions of TLS is in the certificates presented by the server (and possibly the client). If they are the same accross different versions of TLS, we are doing what is effectively called "key re-use". The next question "Can I re-use a server's private key?" approaches the problems with that.

In addition, clients will often "fallback" to older versions if they cannot connect to more recent versions. This is all done in good faith but can in some cases be induced by malicious man-in-the-middle attackers. This is what we call downgrade attacks. Fortunately, we have mitigations against these which need to be implemented on both side of the protocol.

On the client-side of things, a fake tls_fallback_scsv cipher suite can be sent after doing a fallback to signal to the server what happened. The server can then check if both endpoints indeed support a more recent version of TLS than is being negotiated.

On the server-side of things, TLS 1.3-enabled servers who support lower TLS versions must include a hint that they support TLS 1.3 in their random value.

Note that these mitigations are "best-effort", if the other side does not implement them they will be ignored.

Can I Re-use A Server's Private Key?

Has research has shown [Key Reuse: Theory and Practice, DROWN]. Key re-use accross applications and protocols can lead to severe vulnerabilities. For this reason, it is advised to use a server's private key only for the purpose it was designed for.

How To Read A Ciphersuite?

A Cipher suite is just a series of algorithms used in by the TLS connection. As different algorithms can be supported by a server, a client will often advertise a list which the server can choose from. There exist four important algorithms used by TLS that are negotiable:

  • a key exchange algorithm
  • a cipher for authenticated encryption
  • a signature algorithm
  • a hash function

Dedepending on the version of TLS in used, each of these might or might not be included in the cipher suite (if they are not, they will be included in some other field). For example this is a TLS 1.3 cipher suite:


While this is a TLS 1.2 cipher suite:


While this is a TLS 1.1 cipher suite:


As you can see, these can be pretty cryptic. Note that OpenSSL has renamed all ciphersuites which makes the task of understanding them even more difficult. Fortunately the OWASP has a translation table.

What Algorithms And Cipher Suites Are Not Secure?

There are so many combination of algorithms and ciphersuites available that it is impossible to write up a blacklist. Instead, we will happily do with a whitelist. A perfect combination of algorithms would be a bouquet from this list:

Key Exchange. X448, X25519, ECDHE (with secp256r1, secp384r1 or secp521r1)

Signature. EdDSA (with Curve448 or Curve25519), ECDSA (with secp256r1, secp384r1 or secp521r1), RSA-PSS with SHA-256, SHA-384 or SHA-512) and RSA-PKCS1 (with SHA-256, SHA-385 or SHA-512).

Authenticated Encryption. AES-128-GCM, AES-256-GCM, Chacha20-Poly1305

Hash. SHA-256, SHA-384 or SHA-521

Nonetheless, this doesn't mean that other algorithms are not secure. Here is a list of other acceptable algorithms:

RSA key exchange. This key exchange has had a lot of issues with the Bleichenbacher attack on RSA encryption with PKCS#1 v1.5. Implementations are expected not to leak any information about the correctness of the decryption, but a lot of them still fail this test (the bleichenbacher attack has been re-discovered again and again over the years). Hence this mode should be avoided solely because attacks against it are extremely practical and hard to mitigate against. In addition, this key exchange does not provide forward secrecy and is one of the reason why it has been removed from TLS 1.3.

Diffie-Hellman (FFDH). Not to mix with ECDH, the Diffie-Hellman key exchange has had its fair share of issues. The LOGJAM research found out that the real-world usage of the key exchange had a poor security stance while it was also found that the protocol was easy to backdoor and that some standards might actually have been. While the protocol can be used correctly by using known good groups (default in TLS 1.3) or by testing the groups supported by a TLS server, protocols and browsers are moving away from it in favor of ECDH and its smaller keys.

DSA. As for FFDH and ECDH, DSA has seen its key sizes greatly reduced by the invention of ECDSA. Which is why pretty much no-one still use DSA. In addition, multi-key attacks are weaker against ECDH as well. This makes DSA not a bad algorithm but a poorly supported one.

AES-CBC. The infamous cipher has unfortunately been wrongly implemented in SSL/TLS from the start, and has led to several practical attacks: POODLE on SSL 3.0, BEAST on TLS 1.0 and Lucky13 on all versions of the protocol. That is to say, there exist client-side mitigations for BEAST (client-side) and the Lucky13 attack should not be possible if the implementation does not leak information about the correctness of the decryption. That being said, implementations have been found to repeatidly make mistakes there (Lucky13 has been re-discovered many times) and thus AES-CBC should be quickly deprecated in favor of authenticated ciphers like AES-GCM or Chacha20-Poly1305.

AES-CCM. Rarely used but sometimes needed, it is plagued by inconvenience rather than insecurity. Although there exist an AES-CCM-8 version which should be avoided.

3DES also called DES-CBC3 or DES-EDE (for Encryption-Decryption-Encryption) has been targeted by the Sweet32 research which has helped with its deprecation. Although the attack remains highly impractical, which makes 3DES fine (and still actively used in the banking world) we have better cipher nowadays.

TLS can support many more ciphers through various extensions. But it would be too long for us to list why each of them should not be included. Wikipedia has a useful table that shows what ciphers are deemed secure or not.

Is Renegotiation A Problem In TLS?

Yes and no. But this is enough to make us think that renegotiation is bringing more issues than it solves and should thus be disabled.

Is Compression A Problem In TLS?

TLS Compression and compression at the application both should be disabled because of the numerous compression attacks.

What is DER, .PEM, ASN.1, x.509, BASE64?

Read this.

Is 0-RTT (early Data) Secure In TLS 1.3?

TLS 1.3 introduces a new concept called "0-RTT" or "early data" which allows clients to send encrypted requests to servers during their very first flight of messages. Because of the design of this feature, these messages are not forward-secure and are replayable. This is a problem if such requests can mutate the state of the server (what we sometimes call "non-idempotent" requests).

This feature has been pushed by big players who want to get faster sessions, but their downsides have been criticized heavily which leads us to NOT recommend activating this feature in TLS 1.3.

If this feature MUST be used, know that TLS 1.3 clients should maintain a whitelist of requests that are safe to send as early data and TLS 1.3 servers should also maintain such a whitelist.

How To Assess x.509 Certificates?

While nothing constrain TLS endpoints to use certificates, implementations are generally biased towards its use. Both servers and clients can use them to allow the other side of the connection to authenticate them. These certificates contain public keys that will get used for the handshake's key exchange. These public keys should be supporting secure key exchange algorithms (see What Algorithms And Cipher Suites Are Not Secure?) and be of the correct size. The same should be verified on the entire chain of certificates used by both endpoints if a public key infrastructure is in use.

To read a certificate in PEM or DER format you can use the OpenSSL command-line interface:

openssl x509 -in yourcert.pem -noout -text

But other tools like certstrap exist.

To figure out if a certificate is correctly written, check the common x.509 certificate validation/creation pitfalls and use tools like zlint and webPKI.

To manually modify a certificate you need to understand the DER encoding.

To install certificates on your computer easily, there is mkcert.

To follow the life of certificates, or to check if a certificate was mis-issued, crt.sh is useful.

Certificate Validation

  • lowercase issues
  • long un-ordered chains
  • lots of extensions

How To Test A TLS Server As A Blackbox?

There is no easy way to test a TLS server as a blackbox, and it should be avoided if it can. Because of the complexity of TLS implementations, you should always prefer a whitebox assessment.

That being said, many tools exist to scan and detect easy problems (misconfiguration usually):

Unfortunately outputs obtained from these tools are not always clear and can also be false positives. Online scanners also exist:

There are stronger tools like TLS Attacker and ROBOT detect which will look for kwown cryptographic attacks that are wrongly mitigated by the TLS implementation.

The OpenSSL CLI can be used as a quick-and-dirty client:

$ openssl s_client -connect google.com:443

Socat can also be handful:

$ socat stdio openssl-connect:google.com:443,cert=$HOME/etc/client.pem,cafile=$HOME/etc/server.crt

TKTK fragmentation

How To Test A TLS Client As A Blackbox?

Tools like TLS Attacker can also be used to test clients against known cryptographic attacks.

To test TLS clients that are meant to accept any valid certificates, one handy website is badssl.com which is a collection of webservers serving "bad" certificates. Your client should always refuse these.

Some TLS clients are meant to only accept a fixed set of certificates, for example a mobile application connecting to https://api.mywebsite.com should not accept a server broadcasting valid certificate for https://www.evil.com. To test these you can try to man-the-middle the connection via tools like mitmproxy.

Furthermore, wireshark can be used to analyze if clients correctly split AES-CBC payloads to mitigate against the BEAST attack.

Downgrade prevention can be tested as discussed in How To Support Several Versions Of TLS?.

Unfortunately the list is infinite and it is hard to figure out what to test. As with TLS servers, there are no secrets: TLS clients tests should be code-assisted.

How to configure a TLS server correctly?

To configure Apache / Nginx / Lighttpd / HAProxy / AWS ELB, Mozilla's SSL config generator is the go-to resource. Mozilla also publishes a server-side TLS set of security consideration.

For language-specific configurations, refer to:

How to Analyze a TLS handshake?

Wireshark is the best tool to analyze a TLS handshake. There exist good explanations out there to understand what is going on.

code review of a TLS implementation

Implementing TLS is very hard, and very few people are capable of doing this securely. See the numerous articles on BearSSL. It would take an entire book to go through the many pitfalls of implementing such a protocol. It almost always makes more sense to use an already existing library like BearSSL or BoringSSL. But if you really have to do it, here are some pointers:

x.509 parsing. Parsing of certificates has been dubbed the most dangerous code in the world, and this is not for nothing. It is incredibly hard to parse x.509 certificates correctly. There has been many types of vulnerabilities that have been discovered: denial of services due to unordered certificate chains, null terminators in the middle of the common name field that allow for domain name spoofing, incorrect signatures that still get validated, etc. For this reason, it always makes sense to fuzz them. For this, frankencerts are useful, and pretty much any serious TLS library has corpus available for fuzzing them. In addition, note that certificates should be encoded using DER, but not any other encoding (like BER).

Incorrect state machine. Transitions between different types of state can be tricky, for example you shouldn't accept a ChangeCipherState message after a Finished message. Their consequences can also be devastating. Fuzzing can sometimes yield good results.

Padding errors.

Cryptographic Algorithms. whycheproof

What libraries should I use?

  • Unfortunately SSL/TLS has suffered from many implementation flaws
  • the most famous one being heartbleed of course
  • BearSSL
  • BoringSSL (libreSSL?)
  • Everest https://project-everest.github.io/

What about DTLS?

  • A final mention about DTLS, which is just TLS over UDP (instead of TCP)
  • It's pretty much the same as TLS, except that some concerns like amplification attacks and IP spoofing have been taken care of by DTLS
  • The idea is that DTLS will perform some sort of TCP handshake before doing any crypto/handshake

What about SSH?

  • There's not much to say about SSH, besides the fact that you're probably using this in one
  • that is, when you authenticate to a server
  • no real indication to give, besides how the keys should be generated and stored.
  • Depending on your algorithm, you can refer to the different section of this website
  • otherwise, check www.keylength.com

What about IPSEC?

  • nope, use wireguard if you can

What about VPNs?

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