libssl-dev : Depends: libssl1.0.0 (= 1.0.1k-3) but 1.0.1k-3+deb8u4 is to be installed

Can someone help me figure out how to fix this. I am trying to install secp256k1

Could you please specify your distribution and version.
libbssl1 was removed for security reasons though still installable from old-backports, old-versions and other dated/insecure sources (only install it if you know what your doing).
https://packages.debian.org/jessie/libssl1.0.0 (jessie/security - you need the security repo for this)
https://packages.debian.org/jessie-b...ts/libssl1.0.0 (jessie-backports - you need backports for this)
https://packages.debian.org/sid/libssl1.0.0 (debports, sid - you need debports or sid for this)

*(last two options should work for testing as well)
*Link to the correct name/location if needed.
deb8u4 is just a security fix, link to whatever name your binary is asking for and it should just work.

Gnome 3.14.1
Kali GNU/Linux 2.0 (sana) 32-bit

I need to install Secp256k1 but I am having issues, which source shall I use?

Thanks for your help

usually if the version number is greater then what it says it still works, unless a function that the software needs within the depends has been removed.

I'm not a Kali user here, and I'm not sure if they use debian repo?

If the version is >= and the package manager is griping about some extraneous label, one of my favorite methods is to download the package and use:which turns all dependency problems into warnings instead of breaking the install.

After you use that, you should run: to make sure all other dependencies are met.

This only fixes dpkg's warnings/dependencies and not those from external sources.
In the OP's case his version should just work when properly linked (using the expected name for his binary) or by changing it in the install/make/whatever script (something upstream should have done already).