wolfSSH is following the industry common practice of removing SHA-1 as a default configuration option. SHA-1 has been considered broken for a while now and shouldn’t be used for security purposes. [RFC 8332](https://datatracker.ietf.org/doc/html/rfc8332) recognizes this for the SSH protocol and offers new RSA-based algorithms for signing authentication messages.
In the wolfSSH v1.4.15 release, we were heavy-handed when it came to disabling SHA-1 and removed it from the compile using a preprocessor flag. There was an option to add it back in, but its use wasn’t clear. This was a mistake.
For wolfSSH v1.4.17, we restored SHA-1 to the library, but it is “soft-disabled.” This means it is not offered in the default list of algorithms available during key exchange. One may add the algorithm “ssh-rsa” back as an available algorithm, along with DHE using SHA-1, at runtime. To support this, there is now a set of functions to set the algorithm lists used during key exchange and to poll the library on which algorithms are enabled in the build. Please see the wolfSSH manual section 13 for more information on the [Key Exchange Algorithm Configuration].
If you have questions about any of the above, please contact us at facts@wolfSSL.com or call us at +1 425 245 8247.
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