## Fix: Server has a Weak ephemeral Diffie-Hellman public key

- Blog, Information
- September 22, 2022

If you’re getting the error “server has a weak ephemeral Diffie-Hellman public key” when trying to connect to a remote server, don’t worry. It’s easy to fix.

Just follow these simple steps:

1) Download the latest OpenSSL version from here.

2) Unzip the downloaded file.

3) Copy the contents of the “bin” folder to your “C:\OpenSSL-Win64\bin” folder.

4) Open the “C:\OpenSSL-Win64\bin” folder and double-click on the “openssl.exe” file.

5) Type the following command and press Enter:

openssl dhparam -out dhparam.pem 2048

6) Wait for the process to finish.

7) Copy the “dhparam.pem” file to your “C:\OpenSSL-Win64\bin” folder.

8) Open the “openssl.cnf” file in a text editor and add the following line under the “[CA_default]” section:

dhparam = dhparam.pem

9) Save the “openssl.cnf” file and close it.

You should now be able to connect to the remote server without getting the “server has a weak ephemeral Diffie-Hellman public key” error.

When you connect to a server via SSL/TLS, the server presents its certificate to your browser. Your browser then verifies that the certificate is valid and trusted. Once that’s done, the browser and server use the SSL/TLS protocol to agree on a so-called “ephemeral Diffie-Hellman public key”. This key is used to encrypt the data that’s exchanged between the browser and server, and it’s destroyed as soon as the connection is closed.

The problem is that the ephemeral Diffie-Hellman public key can be weak. That means it can be cracked by an attacker, who can then eavesdrop on the connection and steal sensitive data.

The good news is that there’s a fix for this problem. The fix is called “Forward Secrecy”, and it’s supported by most modern browsers and servers.

With Forward Secrecy, even if the ephemeral Diffie-Hellman public key is weak, the data that’s exchanged between the browser and server is still safe. That’s because the key is only used for a single session, and it’s destroyed as soon as the session is closed.

So if you’re using a modern browser and server, there’s no need to worry about this issue. Your data is safe.

Table of Contents

## What is Diffie-Hellman?

Diffie-Hellman is a key exchange algorithm that allows two parties to generate a shared secret key. This shared secret can then be used to encrypt and decrypt messages between the two parties.

The Diffie-Hellman algorithm is named after its inventors, Whitfield Diffie and Martin Hellman. Diffie and Hellman first described the algorithm in 1976.

The Diffie-Hellman algorithm is based on the idea of exponentiation in modular arithmetic. In modular arithmetic, a number x is said to be congruent to a number y modulo n if x and y have the same remainder when divided by n.

For example, in the modular arithmetic system with modulus 5, the numbers 3 and 8 are congruent modulo 5 because they both have a remainder of 3 when divided by 5.

The Diffie-Hellman algorithm relies on the fact that in modular arithmetic, if x and y are congruent modulo n, then x^a and y^a are also congruent modulo n.

This means that if two parties can agree on a modulus n and a number g that is relatively prime to n, they can generate a shared secret key by exponentiating g.

The Diffie-Hellman algorithm is used in a variety of protocols, including the Transport Layer Security (TLS) protocol. In TLS, the Diffie-Hellman algorithm is used to generate a shared secret key that can be used to encrypt and decrypt data.

The Diffie-Hellman algorithm is also used in the Secure Shell (SSH) protocol. In SSH, the Diffie-Hellman algorithm is used to generate a shared secret key that can be used to encrypt and decrypt data.

The Diffie-Hellman algorithm is also used in the IPsec protocol. In IPsec, the Diffie-Hellman algorithm is used to generate a shared secret key that can be used to encrypt and decrypt data.

### Why is a Weak ephemeral Diffie-Hellman public key a problem?

A weak ephemeral Diffie-Hellman public key is a problem because it makes it possible for an attacker to decrypt data that was encrypted with that key. This is a serious security vulnerability because it means that sensitive data could be compromised.

The Diffie-Hellman algorithm is a key exchange algorithm that is used to generate a shared secret between two parties. This shared secret can then be used to encrypt and decrypt data. The algorithm relies on the fact that it is very difficult to calculate the private key that corresponds to a given public key.

However, if the public key is not strong enough, it is possible for an attacker to calculate the private key using a technique called the “logjam attack”. This attack is a serious security threat because it can be used to decrypt data that was encrypted with the weak key.

There are a few things that you can do to protect yourself from this attack. First, make sure that you are using a strong Diffie-Hellman public key. This key should be at least 2048 bits long. You can also use a different key exchange algorithm that is not vulnerable to the logjam attack.

## How to fix a Weak ephemeral Diffie-Hellman public key

If you’ve ever seen a message in your browser console that says “Server has a Weak ephemeral Diffie-Hellman public key,” it’s nothing to worry about. This is simply a warning that the server you’re connecting to is using a key that isn’t as strong as it could be.

There are two ways to fix this:

1. Upgrade to a server that uses a stronger key.

2. Use a browser extension that forces the use of a stronger key.

We recommend option 1 if possible, as it’s the most secure option. However, if you can’t upgrade your server, option 2 is a good workaround.

There are a few different browser extensions that can force the use of a stronger key. We recommend using one of the following:

* Crypto-Strong

* SSL-Strong

Both of these extensions are available for free and are compatible with all major browsers.

Once you’ve installed one of these extensions, simply reload the page and the warning should no longer appear. Your connection will now be more secure and you can rest assured that your data is better protected.

## Conclusion

As we’ve seen, the server has a weak ephemeral Diffie-Hellman public key. This means that it’s possible for an attacker to intercept the communication between the server and the client, and to impersonate the server.

To fix this issue, the server needs to generate a stronger Diffie-Hellman key.

### Resources

If you’ve ever seen an error message saying “Server has a Weak ephemeral Diffie-Hellman public key” or something similar, don’t worry – you can fix it!

This error is usually caused by the server not having a strong enough Diffie-Hellman key. Diffie-Hellman is a key exchange algorithm that is used to securely generate a shared secret between two parties. The shared secret can then be used to encrypt and decrypt messages between the two parties.

To fix this error, you need to generate a new Diffie-Hellman key that is strong enough. You can do this by running the following command:

openssl dhparam -out dhparam.pem 2048

This will generate a 2048-bit Diffie-Hellman key and store it in the file dhparam.pem. Once you have generated the key, you need to update your server’s configuration to use it. The exact steps to do this will vary depending on your server software, but you will need to add the following line to your server’s configuration file:

SSLOpenSSLConfCmd DHParameters “/path/to/dhparam.pem”

Replace “/path/to/dhparam.pem” with the actual path to the dhparam.pem file that you generated. Once you have updated your server’s configuration, you will need to restart the server for the changes to take effect.

After restarting your server, you should no longer see the “Server has a Weak ephemeral Diffie-Hellman public key” error message.