Both sentences work perfectly fine in polite company. The second is a tiny bit more precise due to doors generally requiring a particular key -- "the" key. No one would misunderstand the first sentence -- it is spoken all the time, completely normal wording. Everyone understands how doors and keys work. We need a key to get in and we all understand we need the key that fits this door.


Back to your original question. If someone is insisting that one of the sentences is correct and the other one is wrong, then the correct sentence is number 2. (But remember, with something as simple and ordinary as doors with keys, both are used in ordinary conversation and no one makes an issue of it.)


Edit to mention this one last bit about doors and keys. Our front door requires a key for entry, as is true for most homes. There is only one particular key "format" that will work in the lock, as is also commonly true. However, we have 3 identical keys for that door -- one on my keyring, one on my Mr's keyring, and a spare hidden on the property. So technically, it is correct to say we need "a" key to get in, the requirement is still "a particular key", of which there are several.