ed.  One last note: if you store a Truecrypt volume in the "cloud", it's already compromised: you can compare differences between versions and derive the keys that way, since the data is being intercepted bit-by-bit at the ISP itself.  If that were the case, Truecrypt would already be worthless, because someone could image your drive, let you work on it, make another image, and compare the two.  Luckily, thats not how encryption works, and the attack you describe cant be generalized without knowing a lot more about how things are encrypted. You appear to be describing a "Known-Plaintext attack", which Wikipedia notes modern encryption schemes (such as AES) are not vulnerable to.http://en.wikipedia.org/wiki/Known-plaintext_attack  Truecrypt is only "vulnerable" if you use crappy keys, or a crappy encryption algorithm (DES or something), or if you get keylogged.  Two comparisons aren't enough - tens, hundreds, thousands? Probably enough. I'm talking about a Truecrypt volume stored on iCloud or Google Drive - each time you update it, if that traffic is peeped, you can conceivably build a picture of the bit-by-bit differences in the volume itself and thereby compromise it. Am I capable of doing that? No: however, the method is recognized.  I