How do I use OpenSSL to make a public and private key using a .crt and .key file?

I know this is probably a bad question but I am so confused. I have a tls certificate and a tls key file, tls.crt and tls.key. I think I'm supposed to convert them into a public and private key, relatively, in order to use openssl.

side note: Am I approaching this wrong? Is there some TLS application to encrypt/decrypt?

To convert the public key, I use

$ openssl x509 -pubkey tls.crt -noout > pubkey

This created a public key that I was able to encrypt a message file with by doing

$ opensssl rsautl -encrypt -inkey pubkey -pubin -in <message file> -out <encrypted output>

I have successfully encrypted the file but now I don't know how to decrypt it. I don't know how to convert my private key (tls.key) into an ssl private key. This is what stumps me. Shouldn't I be doing some tls encryption/decryption? If not, and I am supposed to be using openssl, how should I convert tls.key into a private key usable by openssl rsautl -decrypt?

Solution

tl;dr: The key file is the key file you want. No conversion should be required.

The trick here is in the following question: "What is a certificate?" The answer is that it is a signed public key that goes along with a secure private key. It (usually) gets signed by a CA ("Certificatation Authority").

The basic process is:

Entity that wants a certificate creates a private/public key pair.
Entity send the public key and some information about the Entity to the CA.
The CA performs "some level" of validation on the Entity, verifying their identity. Usually this is just some level of proof that the requestor owns the web site the certificate is to be used for.
The CA takes the provided public key and the other info provided, uses their private key to generate a signature for that data, which then can be verified by decrypting the signature using the CA's public key. That becomes your certificate.

So, you took your own public key from the certificate. The private key (tls.key) should be the key file you created in the first step - just use it on your encrypted data.