[Info-vax] Current state of file/disk encryption on VMS

[Stephen Hoffman]

On 2022-09-02 21:26:26 +0000, Alexander Schreiber said:

> Stephen Hoffman <seaohveh at hoffmanlabs.invalid> wrote:
>> On 2022-09-01 20:45:30 +0000, Alexander Schreiber said:
>> 
>>> Stephen Hoffman <seaohveh at hoffmanlabs.invalid> wrote:
>>>> 
>>>> If BACKUP is encrypting data before performing data compression, that's 
>>>> a design bug in BACKUP.
>>> 
>>> Well, that is actually the right thing do to from a crypto security 
>>> point of view. Compressed files tend to have specified headers and 
>>> structures, which means that "compress, then encrypt" potentially 
>>> enables a nice automatic known plaintext attack. And I suspect that is 
>>> the reason it was done this way.
>> 
>> If your chosen encryption reveals your plaintext, your encryption needs 
>> help. Whether ghostly penguins, or otherwise.
> 
> *sigh*
> 
> That is _not_ what "know plaintext attack" means. It means you have the 
> encrypted message and somehow, through other means, got (part) of the 
> plaintext. E.g. in WW2 ciphers where broken by this because the used 
> (known) standard headers and known standard text in known
> places (e.g. standard greeting of "Heil Hitler" - now run the brute 
> forcer until it finds a key that makes the ciphertext decrypt to that).
> 
> With modern fileformats, standard headers serve this role - e.g. if you 
> know that you have an encrypted JPEG image, you know that the plaintext 
> has a certain header structure (in this case, including the string 
> "JFIF") you have some help ;-)
> 
> One way to guard against this (as has been pointed out elsewhere in 
> this thread) is to use proper encryption algorithms and protocols (e.g. 
> AES with random IV in CBC mode), correctly implemented.

The "ghostly penguins" was a reference to a famous example of the 
foibles of using AES-ECB using an image of the Linux penguin.

The plaintext references in my reply were to recovering the data, and 
not to a known-plaintext attack.

Attempting data compression after data encryption is somewhere in the 
range of futile, unnecessary, and resource-wasteful.

And if data compression performed after data encryption does provide a 
substantial storage savings, your chosen encryption algorithm is 
suspect.

More generally, the OpenVMS APIs here are either not helpful or wholly 
missing, whether with how BACKUP mis-sequences here, or more generally 
around API designs for data protection and network connection security 
that are inherently prone to errors.


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