[bgbgb]

I wrote a simple piece of code that encrypts (and decrypts upon a key given) most of the files on a windows computer. I was wondering what the most secure way I could facilitate a trade of bitcoin to decryption key for a ransomware situation. I don't think I'll ever use it but was curious how real ransomware hackers do that final step.

Also, the key is just sitting there in the code (but kind of hidden because I obfuscate when its turned into an executable) so is there anyway to encrypt files on someone's computer without that computer being able to access the encryption key?

[Lynx]

If you want to give the key upon payment, condider doing so through a hidden service on tor.
Encrypting files without the key? I guess you could delete the key after encryption...?

[Knautschtabak]

Also, the key is just sitting there in the code (but kind of hidden because I obfuscate when its turned into an executable) so is there anyway to encrypt files on someone's computer without that computer being able to access the encryption key?

You need to use an asymmetric encryption algorithm like RSA instead of symmetric ones.
Asymmetric means that the key for decryption is a different one than the key for encryption. That way the encryption key is of no use for anyone.