OpenSSL Testing a Signature Algorithm

In this post we’ll look at how to test whether a server supports a certain signature algorithm when using TLS.

1. Testing RSA PKCS1 Signature Algorithm
2. Testing RSA PSS RSAE Signature Algorithm
3. Testing DSA Signature Algorithm
4. Testing ECDSA Signature Algorithm
5. Verifying Results

Below we have the SSLScan results of github.com. Let’s see how to verify if a certain signature algorithm is supported by the server.

For all of the below commands SHA256 may be replaced with the desired hash function:

• SHA1
• SHA224
• SHA256
• SHA384
• SHA512

Testing RSA PKCS1

An example algorithm in this category is rsa_pkcs1_sha256:

openssl s_client -connect HOST:PORT -sigalgs "RSA+SHA256"

Testing RSA PSS RSAE

An example algorithm in this category is rsa_pss_rsae_sha256:

openssl s_client -connect HOST:PORT -sigalgs "RSA-PSS+SHA256"

Testing DSA

An example algorithm in this category is dsa_sha256:

openssl s_client -connect HOST:PORT -sigalgs "DSA+SHA256"

Testing ECDSA

An example algorithm in this category is ecdsa_secp256r1_sha256:

openssl s_client -connect HOST:PORT -sigalgs "ECDSA+SHA256"

Verifying Results

When running one of the above commands you should see the certificate printed out, and below that you should see two fields Peer signing digest: and Peer signature type:. These will be the values that you provided to the -sigalgs parameter.

If the certificate is NOT printed, then the signature algorithm you provided is not supported by the server.

The below image is the output of the following command:

openssl s_client -connect github.com:443 -sigalgs "RSA-PSS+SHA256"

The highest version of TLS supported by the server will be used by default. If you want to test a signature algorithm for TLSv1.2 or TLSv1.3 you can use the flags -tls1_2 and -tls1_3 respectively:

openssl s_client -connect github.com:443 -tls1_2 -sigalgs "RSA-PSS+SHA256"

Notes

• Make sure you specify a port number at the end of the host name!
• SSLScan results can be incorrect. Some of the algorithms shown may not work. You can use the aforementioned commands to verify the results.