Automatically check if a certificate matches specific ciphers

Question

My nginx backend server supports the following ciphers:

ssl_ciphers "TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256"

I allow the client to upload their own SSL certificate and key and I want to validate that it matches one of the configured ciphers. Otherwise they lose access to the server when I restart nginx config with the newly uploaded certificate. When I use openssl x509 I get an output with something like

Signature Algorithm: ecdsa-with-SHA256

How can I use python or bash code to take this string and figure out if it's valid for my nginx config?

WRT Signature Algorithm: ecdsa-with-SHA256 - this pertains to how the leaf certificate is signed by the next certificate up in the chain. It has nothing to do with the signature by the cert over the TLS handshake. The good news is that you support several TLS 1.3 cipher suites. These can be used with certificates containing either RSA public keys or ECDH public keys. So, you should be fine with either type, as long as the client supports TLS 1.3. — mti2935
– mti2935, Commented Feb 22, 2024 at 19:45
@mti2935: 1.3 can use a cert containing RSA (classic or PSS) or ECDSA or EdDSA key, but it can never use an ECDH or XDH (or FFDH) key from a cert, they are always ephemeral. Also specifying 1.3 suites in nginx ssl_ciphers is ineffective; OpenSSL will ignore them and use its default -- which happens to be the same as this directive would have specified. — dave_thompson_085
– dave_thompson_085, Commented Mar 11, 2024 at 0:34

Esa Jokinen · Accepted Answer · 2024-03-10 07:40:55Z

Update your ciphersuites, instead

It seems you are facing a problem that you have caused by yourself by not supporting ECDSA for TLS 1.2. Instead of trying to figure out how to validate the certificated provided are RSA certificates you could update your ciphersuites to support both type of certificates by adding the corresponding ECDSA alternatives:

RSA	ECDSA
DHE-RSA-AES256-GCM-SHA384	DHE-ECDSA-AES256-GCM-SHA384
DHE-RSA-AES128-GCM-SHA256	DHE-ECDSA-AES128-GCM-SHA256
ECDHE-RSA-AES256-GCM-SHA384	ECDHE-ECDSA-AES256-GCM-SHA384
ECDHE-RSA-AES128-GCM-SHA256	ECDHE-ECDSA-AES128-GCM-SHA256

Resulting:

ssl_ciphers "TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:DHE-ECDSA-AES256-GCM-SHA384:DHE-ECDSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-GCM-SHA256"

Or switch to the recommendations from RFC9325, 4.2:

IANA name from RFC	OpenSSL name for Nginx
TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256	ECDHE-RSA-AES128-GCM-SHA256
TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384	ECDHE-RSA-AES256-GCM-SHA384
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256	ECDHE-ECDSA-AES128-GCM-SHA256
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384	ECDHE-ECDSA-AES256-GCM-SHA384

Resulting:

ssl_ciphers "TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384"

With either you would not have the problem in the first place.

If you validate, validate for all possible problems

You wrote your concern is:

Otherwise they lose access to the server when I restart Nginx config with the newly uploaded certificate.

This could happen for a number of other reasons:

The files are corrupted or not certificate and key altogether.
The client could have uploaded the key in place of the certificate and vice versa.
The key might not match the certificate.

And if HTTP Strict Transport Security (HSTS) was enabled:

The certificate does not match the hostname.
The certificate is not signed by a trusted CA.

In order to test all possibilities at once you could use an actual connection to test the certificate the same way a browser would do. You could use a staging environment with the updated configuration and try to make a connection using, e.g., curl --hsts --fail (HSTS since 7.77.0) or wget (supports HSTS).

There are no DHE-ECDSA ciphersuites for TLS1.2 (or lower), and the API used by nginx ssl_ciphers (OpenSSL's SSL[_CTX]_set_cipher_list) does not apply to TLS1.3 suites, so both of these are ignored. — dave_thompson_085
– dave_thompson_085, Commented Mar 11, 2024 at 0:28

Mario · Accepted Answer · 2024-12-07 15:45:51Z

See my comment above on why this may be a non-issue if you are using TLS 1.3.

If you still feel you need an automated way to analyze a certificate, this short Python script might help:

from cryptography import x509 from cryptography.hazmat.backends import default_backend with open('eccert.pem', 'rb') as file: certificate=file.read() certDecoded = x509.load_pem_x509_certificate(certificate, default_backend()) print(certDecoded.subject) print(certDecoded.public_key())

If you place the file eccert.pem in the same directory, you'll see that the script produces the following when run, indicating that the certificate contains an EC public key:

<cryptography.hazmat.backends.openssl.ec._EllipticCurvePublicKey object at 0x7f6539a63df0>

If you run the script against rsacert.pem instead, you'll see that it produces the following, indicating that the certificate contains an RSA public key:

<cryptography.hazmat.backends.openssl.rsa._RSAPublicKey object at 0x7f6539a63f40>

Copied below are both pem files.

eccert.pem

rsacert.pem

Thanks for the kind response. Your python code is kind of close to what I wanted but I still can't achieve my goal. What I want is basically a machine that take a certificate and a list of cipher suites in the format I show in the original question and returns True/False depending on the certificate matching one of the suites — Shahar G
– Shahar G, Commented Apr 4, 2024 at 13:58

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Automatically check if a certificate matches specific ciphers

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Update your ciphersuites, instead

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Automatically check if a certificate matches specific ciphers

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Update your ciphersuites, instead

If you validate, validate for all possible problems

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