I, as many others, used TrueCrypt. It's not a good solution anymore because all encrypted files are stored in a single volume file.
I also used BoxCryptor, its advantage is that it encrypts each file separately, this avoids the need of creating a big volume file and allows its use with WebDAV sharing services.
But BoxCryptor is gone. It was changed for a "server-side encrypting" solution, as many services out there. I can't accept a service like that is secure, all of them claim to be able to share "encrypted" files without requiring my private key. So, if there is a way to decrypt files without my key, then the least thing these services are is secure.
I tried now Cryptomator, it's a new FOSS solution that works similar to how BoxCryptor used to work, adding the feature of encrypting folders and files names. It seemed awesome, the best of TrueCrypt and BoxCryptor together! But it's VERY slow, to the point of hanging any process that tries even to list the contents of a folder inside its vault. Cryptomator will be great in some years, but it's too unstable for now.
Can somebody suggest me any solution, that's really secure and doesn't just claim to be, that works offline and doesn't require sending files to a remote server, that doesn't share anything that only I should be able to decrypt, and doesn't require big volume files?