BoxCryptor Classic is described as 'Boxcryptor Classic is the first product of the Boxcryptor family to secure data in the cloud - officially released in 2011. It grew from a solution originally specific for Dropbox with basic features, to one that supports all main cloud providers, is available on all platforms' and is an app in the security & privacy category. There are more than 25 alternatives to BoxCryptor Classic for a variety of platforms, including Windows, Mac, Linux, iPhone and Android apps. The best BoxCryptor Classic alternative is Cryptomator, which is both free and Open Source. Other great apps like BoxCryptor Classic are VeraCrypt, TrueCrypt, Cryptsync and Encrypto.
Cryptomator provides transparent, client-side encryption for your cloud. Protect your documents from unauthorized access. Cryptomator is free and open source software, so you can rest assured there are no backdoors.
Ensure your data's security by encrypting entire partitions, storage devices, or system drives with VeraCrypt. Benefit from transparent and automatic real-time encryption, hardware acceleration on modern processors, and robust security algorithms that are resistant to brute-force attacks.
[TrueCrypt is not exactly "Open Source", but the source is available. See the reviews to get more information.]
CryptSync is a small utility that synchronizes two folders while encrypting the contents in one folder. That means one of the two folders has all files unencrypted (the files you work with) and the other folder has all the files encrypted.
Boxcryptor encrypt files transparently
Encrypto lets you encrypt files before sending them to friends or coworkers. Just drop a file into Encrypto, set a password, and then send it however you normally would — but this time with added security. It’s free for both Mac and Windows.
Boxcryptor is an easy-to-use encryption software optimized for the cloud. It allows the secure use of cloud storage services without sacrificing comfort.
LibreCrypt (formerly DoxBox) is an open-source disk encryption for Windows and the successor of FreeOTFE
It isnt an option for cloud sync
S.S.E. File Encryptor is a cross-platform encrypter/decrypter of files and folders.
Perfecto Encryptor will encrypt your data fast and securely! This unique program will help you to keep any of your files and folders in encrypted form. And only you will can decrypt your files.
Encrypts and protects your files on your machine, before they hit your disk and the cloud: Collaborate with others, end to end encrypted.
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Veracrypt creates disk containers that are then used to store files. Boxcryptor Classic uses EncFS to individually encrypt files.
This makes BC favorable for clouds because then you're uploading the files themselves (encrypted), versus one huge file that could take forever to decrypt.