Somewhat OT, encryption question

[Robert Moskowitz]

On 11/26/2014 10:18 PM, Bruno Wolff III wrote:
> On Wed, Nov 26, 2014 at 20:33:51 -0500,
>  Robert Moskowitz <rgm at htt-consult.com> wrote:
>>
>> On 11/26/2014 07:10 PM, Bruno Wolff III wrote:
>>> On Wed, Nov 26, 2014 at 20:47:25 +0000,
>>> Bill Oliver <vendor at billoblog.com> wrote:
>>>> On Wed, 26 Nov 2014, Bill Oliver wrote:
>>>>
>>>> Actually, let me be more specific.  Let's say I have data on a flash
>>>> drive that is encrypted using gpg.  We can even say the flash drive
>>>> itself is encrypted.
>>>>
>>>> Now let's say that flash drive is stolen, lost, etc. *and* the
>>>> passphrase is compromised.  I want the data on the flash drive to be
>>>> available *only on one computer* even if the passphrase is known.
>>>
>>> If you don't need to decrypt data in the field, you can use public 
>>> key encryption. You won't be able to decrypt the data without the 
>>> private key. (Which you wouldn't have with you or the flash drive.)
>>
>> NOBODY encrypts lots of data with asymmetric cryptography. Rather, 
>> using RSA say, you create a random AES key, encrypt the data with 
>> that, THEN encrypt the little key data with the public key.
>
> Yes of course. The idea was that you could do the encryption with say 
> pgp, erase the originally (carefully) and then you wouldn't be able to 
> decrypt the data without having the private key (which would be on 
> some other device than the computer the file was on). Using asymmetric 
> cryptography is what allows you to do that. Just using a symmetric key 
> wouldn't. That the bulk of the encryption is really done symmetricly 
> with just the symmetric key encrypted with the public key is just an 
> immplementation detail.
>
>> If your private key is on a USB dongle with your software supporting 
>> it, it all works together.
>
> But that doesn't seem to be what he wants. He wants to make sure that 
> that having the encrypted data, and his passphrase is not good enough 
> to recover the plaintext. If the private key is on the same machine he 
> is using his passphrase on, then they may both may be compromised 
> together (the scenario didn't say how the passphrase was compromised 
> so it isn't clear of this is a likely or unlikely case).

The private key is on the dongle.  Some dongles actually do the key 
operation on the dongle so the private key never leaves it.  But it has 
been a decade since I worked with these devices.

A shell script that is on a USB device (but I still prefer a microSD 
card and a USB card reader) that reads something on the computer plus 
the 'passcode' to construct a pgp 'passphrase' might do it.