I've created a new linux instance on Amazon EC2, and as part of that downloaded the .pem file to allow me to SSH in.

When I tried to ssh with:

ssh -i myfile.pem <public dns>

I got:

@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@         WARNING: UNPROTECTED PRIVATE KEY FILE!          @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
Permissions 0644 for 'amazonec2.pem' are too open.
It is recommended that your private key files are NOT accessible by others.
This private key will be ignored.
bad permissions: ignore key: amazonec2.pem
Permission denied (publickey).

Following this post I tried to chmod +600 the .pem file, but now when I ssh I just get

Permission denied (publickey).

What school-boy error am I making here? The .pem file is in my home folder (in macOS). Its permissions look like this:

-rw-------@   1 mattroberts  staff    1696 19 Nov 11:20 amazonec2.pem

The problem is wrong set of permissions on the file.

Easily solved by executing -

chmod 400 mykey.pem

Taken from AWS instructions -

Your key file must not be publicly viewable for SSH to work. Use this command if needed: chmod 400 mykey.pem

400 protects it by making it read only and only for the owner.