Is Lock safe?
The short answer is yes.
But in reality, this is a bit longer; in many parts of the application it talks about blocking due to many users not being familiar with term such as encrypting, but what really happens is that your information is encrypted using AES; the same state-of-the-art encryption algorithm used as the national standard in the United States. Lock uses 256-bit keys for encryption, which means that it would take millions of years for a criminal to decrypt your data using a brute force attack.
Is my information protected?
The difference from other applications is that it has only one permission. It can only access external storage (SD Card) to encrypt your files. It has no access to the internet, SMS or other networks. Therefore Lock has no way to send your data out of your phone.
On the other hand, Lock is hosted on github and anyone who wishes to may see it.
Can I share encrypted files?
Yes, to share an encrypted file, you have to open it and find the share icon in the action bar of the files viewer that incorporates Lock.