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Petr Gladkikh
Asked: 2020-09-11 04:38:18 +0800 CST

How to diagnose "CA certificate too weak" error, how to use the CA cert anyway?

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I have to use a service that uses self-signed certificate (from Ubuntu). I have added the company's CA to the trusted list (Ubuntu). After that "self signed certificate in chain" error is gone but now I get "CA certificate too weak" error. E.g.

curl -vvv https://someservicehost.net
 *   Trying 93.184.216.34:443...
 * TCP_NODELAY set
 * Connected to someservicehost.net (93.184.216.34) port 443 (#0)
 * ALPN, offering h2
 * ALPN, offering http/1.1
 * successfully set certificate verify locations:
 *   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
 * TLSv1.3 (OUT), TLS handshake, Client hello (1):
 * TLSv1.3 (IN), TLS handshake, Server hello (2):
 * TLSv1.2 (IN), TLS handshake, Certificate (11):
 * TLSv1.2 (OUT), TLS alert, bad certificate (554):
 * SSL certificate problem: CA certificate key too weak
 * Closing connection 0
curl: (60) SSL certificate problem: CA certificate key too weak

I tried to search it but there is surprisingly little information on this. So I have 2 questions

  • How do I find exactly which properties of this certificate make it "weak"? Is it hash, key length, something else? I deliberately omitted the certificate details as I want to know how do I diagnose the problem not the problem with this particular certificate.
  • How do I force client software to trust it? I think I do not have a choice of upgrading the CA cert.
ubuntu openssl curl ssl-certificate-errors

1 Answers

  1. At the time of writing this, example.net used a certificate signed by the DigiCert SHA2 Secure Server CA intermediate CA, which in turn is signed by the DigiCert Global Root CA root CA. Both CA certificates use a 2048-bit RSA key. However, if you are behind a corporate TLS proxy, the actual CA might only use a 1024-bit key (you didn't provide any details on it), so adapt the rest of the answer according to the key size of that CA certificate. If you can't avoid the proxy, a better approach, than what's described below, would be to inquire whether a CA certificate with a stronger key could be employed on it.

    When OpenSSL verifies the certificate chain, it checks the key size against the configured security level. According to the documentation, level 1 corresponds to a minimum of 80 bits of security, level 2 to 112 bits, level 3 to 128 bits, level 4 to 192 bits and level 5 to 256 bits.

    RSA 2048-bit keys are currently deemed to have 112 bits of security, so if the above level is 3 or higher, the key is rejected as too weak. So to accept such a certificate, you'd have to reduce the security level to 2 or lower.

    The level can be configured at compile time or via configuration (it can also be changed by the program that links libssl/libcrypto, but it looks like curl doesn't do so). The default level is 1 according to the documentation linked above.

    So first check the compile options (openssl version -f) and then the default openssl.cnf file on your system (the directory where it's located can be determined via openssl version -d). There might be a setting called CipherSuites that sets SECLEVEL (e.g. CipherString = DEFAULT@SECLEVEL=3 would set it to level 3).

    To override the level, either change the default config file, or use a custom file via OPENSSL_CONF environment variable. For instance, the following file sets the default security level to 2 (see the documentation for details):

    openssl_conf = openssl_conf_sect

[openssl_conf_sect]
ssl_conf = ssl_sect

[ssl_sect]
system_default = system_default_sect

[system_default_sect]
CipherString = DEFAULT@SECLEVEL=2

    It may be used e.g. with OPENSSL_CONF=/path/to/this.cnf curl -vvv https://example.net.

    As pointed out in the comments by @dave_thompson_085, curl allows configuring the cipher suites on the command line. So instead of fiddling with the configuration, you may simply change the security level via --ciphers option:

    curl -vvv --ciphers DEFAULT@SECLEVEL=2 https://example.net
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