Are low security curves for ECDSA not supported? I am working in a very bandwidth restricted environment, and we are looking to implement ECDSA digital signatures. Although they are no longer recommended, we might be looking to use curves that are not listed as supported by most libs, including OpenSSL: https://en.wikipedia.org/wiki/Comparison_of_TLS_implementations#Supported_elliptic_curves

We are looking at curves such as secp112r2, secp128r1, sect131r1 and so on. This is not to discuss security of these, but their support.

In Cygwin, a huge load of curves is available, including the mentioned low-secutity curves. But in the URL these curves are not listed. Our team has observed that in some other OpenSSL sources (that are not through Cygwin), these curves are not supported. Are they no longer supported by OpenSSL, and if so - why are they available to me through Cygwin?

And by the way - I assume curve support is rather complicated, so that it is not possible to "import" a curve that is not supported in a given library.
I'm not sure, but I guess that two different openssl version or just builds could be done with or without support of Elliptic Curves with unsecure security levels.

Furthermore your curves have very low security level, and you shouldn't use them if security is a concern (and if it's not, you probably don't need ECDSA at all).

However, your assumption that "curve support is rather complicated" is wrong.

With libcrypto, the OpenSSL library, not the command line tool, you can use EC_GROUP_get_curve_GF to create your own elliptic curve. If you put in the parameters of those curves, you will be able to use them, with the library, even if the parameters are not encoded in OpenSSL.