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dh key too small

Today I encoutered the dh key too small issue when running curl and wget commands.

And most of the reasons is that server is passing a weak DH key to client. This is the most pages say in the Internet. However today I found the issue only happens on Kali Linux. This should be the hint that there is some wrong in client side instead of server side.

I checked /etc/ssl/openssl.cnf. No surprise there is an extra configuration for SSL security level:

[system_default_sect]
MinProtocol = TLSv1.2
CipherString = DEFAULT@SECLEVEL=2

The default security level is 1 for OpenSSL. So setting it up to 2, some of the websites may not meet level 2 security requirement and caused the "dh key to small" issue.

Recorded in this page, https://www.openssl.org/docs/man1.1.0/man3/SSL_CTX_get_security_level.html:

DSA and DH keys shorter than 2048 bits and ECC keys shorter than 224 bits are prohibited.

Workaround

  1. You can turn down the security level in /etc/ssl/openssl.cnf
  2. You can add --cipher 'DEFAULT:!DH' as command line parameter
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