I haven't been able to find much in the way of professional reviews for Secretsync - http://getsecretsync.com/ss/
But i'm wondering how it compares to Boxcryptor and Truecrypt. How secure is Secretsync really?
It's fairly straightforward to understand SecretSync, in my opinion. It looks like they require Java, and one HOPES they are using the built-in crypto libraries to provide their 256-bit AES symmetric key crypto rather than rolling their own implementation.
HOWEVER, they are not as secure as you may require.
According to their FAQ, they do one of two things:
If they give you a key, then they know the key, but they don't have access to your files (DropBox is the file distribution medium). If you use your own passphrase, then they STILL use their own key, but it becomes a salt value for the generated key from the PBKDF2 function.
So they do not appear to use a different key per file, but rather one key for all files. This wouldn't normally be a problem with AES, but it suffers from problems associated with all of the other deficiencies in their model.
Having a symmetric key generated FOR you is an issue regardless of the asymmetric relationship between secretsync and dropbox. I'd like to know what security the organization has on their end.
According to the FAQ they appear to be susceptible to a denial of service (regardless of how they spin it): "Will I be able to decrypt my files if your service is no longer available?"
Also, because the account is web-accessible, any number of social-engineering attacks can allow the SecretSync account password to be changed by having access to the account email address. Having this account access will mean you get access to the secret key from their server. This would be more tricky to access if you generated your own key, but the complexity of the function is reduced from one of finding an AES 256-bit key to one of brute-forcing passwords (or phrases). We know humans are the weak link in security. Having access to ONE encrypted file, plus access to their server-generated key means you can decrypt any file you want (with a possible side-challenge of needing to first brute-force the user's password they may have set when signing up).
It is also not clear to me what happens if you need to change your password on the key. Do they re-encrypt everything? This is a deficiency as well.