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Is any 256-bit combination a bitcoin private key?

I’ve recently been implementing a simple bitcoin wallet and some libraries seem to disagree with address derivation from the same WIF private key, and thus wouldn’t sign transactions.After that I used different libraries to implement the same thing and they successfully signed transactions, but then the network refused to accept them. When I did tis with a private key that was derived from a mnemonic (in a specific way), it worked.

So, I’m really wondering whether there is a specific requierement imposed on bitcoin’s private keys.