It certainly would be more convenient to store my KeePass database on either S3, Dropbox, or better yet SpiderOak. My fear is having my cloud storage account compromised then having the credentials recovered by either brute force or some other attack vector. How safe is this? What risks do I need to know about?
It is hard to quantify exactly, but if you have the DB on a mobile device then I wouldn't say this is particularly any less secure. KeePass encrypts the DB because the file remaining secure isn't expected to be a guarantee. It's certainly preferable that the DB file not get in the wild, but if your security depends on the encrypted file remaining confidential, then you have bigger problems than whether to use cloud storage or not.
A sufficiently strong master password should prevent brute forcing at least long enough for a breach to be detected and for you to change the passwords within it. In this way, it may even be slightly preferable to having a local copy on a mobile device as someone may compromise the file if you take your eyes off your device even momentarily and it would be much harder to identify that breach occurred.
If you want to secure it even further, you can add another layer of security by encrypting the file you store in cloud storage online. The master password provides pretty good security as long as you choose a difficult to brute force password (long and truly random), but it still can't compete with an actual long encryption key. If you encrypt the file that you store online and then keep that key with you protected by a similar master password, now the online component alone is much, much harder to decrypt (likely impossible if done correctly) and if your key file gets compromised, you simply re-encrypt your online DB immediately with a new key. You're still in trouble if someone can compromise your cloud account first and get the file, but it requires two points of compromise instead of one.
Personally, I'd probably end up using my OwnCloud (which is self hosted), but I have the advantage of having my own personal web server and I realize that's not an option everyone can take advantage of. (The only reason I haven't is that I don't have a particular need to coordinate a key database in that manner.) A public cloud based solution should work as a fine second option though.
You can increase the resiliency of your KeePass database to brute force by increasing the number of PBKDF2 iterations when deriving the database encryption key from your password. You can do this in KeePass under File > Database settings > Security. Personally, I use around 5,000,000 rounds (1 s delay). Remember that mobile devices are slower.
I use the KeePass-Dropbox combination. The password database is encrypted using a key derived from a strong master password. Even if somebody acquires your encrypted password database through your cloud account, a strong enough master password renders brute-force attacks infeasible.
Simply put: Use a strong master password and stop worrying about this.
The cloud is inherently untrustworthy, and files kept on it should be considered vulnerable, so you need strong encryption to protect you. KeePass offers that. However, you then need to be able to trust every client you enter your password into. If you read them on an iPhone, do you trust the platform? Do you shield your password from the cameras on the subway when you enter it, every time? How about your laptop?
You also need to consider the value of what you're protecting. Is this safeguarding your retirement funds? Your fantasy bowling league scores? Political dissention that is illegal in your country? For some cases it's simply not worth the risk of making a mistake.
So yes, as long as you trust KeePass, and trust your devices, and trust your ability to keep your master key secured, don't worry about keeping the database in Dropbox.
Even if your KP database were to be compromised from Dropbox, using both a strong password, and additionally a keyfile not stored in Dropbox should give you security beyond any known means of electronic attack (as long as your devices aren't already compromised).
The keyfile should be stored in a separate secure location, such as a USB drive which you can secure physically. This provides 3 layers of protection:
- Dropbox account (assume low security)
- Your strong password (strong)
- Your keyfile (as secure as you make it)
While a strong enough cipher should be able to resist brute force attack, consider that by storing your password database in the cloud you give the potential attacker much more information than he could get from e.g. a lost phone with the same database.
Every time you modify a password, the attacker gets a new database, which he can use together with older versions in differential analysis attacks. You will need a stronger cipher to resist that, compared to simple COA. If he also happens to have one of your passwords (e.g. by cracking a poorly protected password database from one of the sites you visit), he may be able to identify that password in your database. This would open doors for KPA which may be close to trivial in case you reuse passwords.
You're also giving the attacker information about your password-related activity. He will know how often you change your passwords, and may be able to deduce which passwords you change.
So, if you decide to store your password database in a public cloud,
Pick a strong encryption algorithm, resistant to differential cryptoanalysis. Make sure every password in the database is individually salted.
Change the master password regularly, preferably as often as you do with your most valued password.
Don't reuse passwords.
I would suggest a solution that involves the following:
- KeePass Database file stored on local computer with Full Disk encryption (e.g. Veracrypt)
- KeePass Key File stored on an external USB disk
- Cloud Storage 1 (e.g. Dropbox) to hold the Database file
- Cloud Storage 2 (e.g. Google Drive) to hold a backup of the Key File
- Enable 2FA on both Cloud Storage accounts (Mobile authenticator app preferable over Text Message codes)
As everyone mentioned, having a strong master password for you database file is very important to thwart brute force attacks. But if you combine that with a key file, then it becomes practically impossible to brute force.
If Cloud Storage 1 is compromised, then the database file itself without the key file will be impossible to brute force, unless there's a serious security vulnerability in Keepass's encryption implementation.
If Cloud Storage 2 is compromised, then the key file itself if useless without the database file. It's just an extension to you master password, it doesn't contain any data.
I would say that it would be highly unlikely for both Cloud Storages to be compromised at the same time by the same attacker. You'd have to be a high profile individual for that (e.g. Politician, Billionaire, Secret Agent etc.) :)
If you're not one of the above and are willing to sacrifice a bit of security in exchange for ease of access (e.g. you keep forgetting where you put the damn USB disk), then store both Database File and Key file on the local computer. However, in this case the local computer becomes the weak spot, if compromised, the attacker would have access to the key file, so would just need to brute force the master password on the database, so best to set a strong one. This would be the same as not using a key file at all.
I am a strong believer in defense in depth on this.
No matter how strong your password is, it is also one that cannot be changed.
So to protect my Keepass DB, I am using three layers:
- The database has a strong pass phrase (not just a password). I am not using a key file because, for me, the risk of losing the key file outweighs the security benefits.
- The database is stored on an encrypted file system (I'm using encfs. By itself, it has of course been shown to be insecure, but it still adds a layer of protection).
- The encrypted version is uploaded to an owncloud server, running on my own hardware, and of course over HTTPS.
On why the password cannot be changed: of course you can change the password, and it will actually re-encrypt the Keepass database, but you cannot retroactively change it on old copies.
Especially (but not only) if you store the database in the cloud, you have to assume that an adversary got a hold of the database at some earlier point in time, with an old password. That may give her the information she is after.
I say go with the "encrypted at client" cloud solution of your choice, encrypt with the most PBKDF2 iterations you are comfortable with, largest keyfile you can live with, and most importantly, store the information you keep close to you (locally) on a SELF ENCRYPTING drive. Put an encrypted filesystem on top if you must, but encryption of data that is prone to loss or theft, like a USB drive, needs to be automatic.
If a group is able to compromise a cloud service enough to discover that you have a kdb to be cracked, they may have more motivation to plant a key logger/root kit on your personal devices. Of course, if such a group can see you downloading the software from sourceforge or an app store, they obtain similar information. In fact, this post is a much easier tip-off, so concern for this attack vector may be for individuals more paranoid than ourselves.