[PATCH v4] KEYS: encrypted: fix key instantiation with user-provided data

[Mimi Zohar]

On Fri, 2022-10-14 at 08:40 +0200, Nikolaus Voss wrote:
> On Thu, 13 Oct 2022, Mimi Zohar wrote:
> > On Thu, 2022-10-13 at 08:39 +0200, Nikolaus Voss wrote:
> >> Commit cd3bc044af48 ("KEYS: encrypted: Instantiate key with user-provided
> >> decrypted data") added key instantiation with user provided decrypted data.
> >> The user data is hex-ascii-encoded but was just memcpy'ed to the binary buffer.
> >> Fix this to use hex2bin instead.
> >>
> >> Old keys created from user provided decrypted data saved with "keyctl pipe"
> >> are still valid, however if the key is recreated from decrypted data the
> >> old key must be converted to the correct format. This can be done with a
> >> small shell script, e.g.:
> >>
> >> BROKENKEY=abcdefABCDEF1234567890aaaaaaaaaa
> >> NEWKEY=$(echo -ne $BROKENKEY | xxd -p -c32)
> >> keyctl add user masterkey "$(cat masterkey.bin)" @u
> >> keyctl add encrypted testkey "new user:masterkey 32 $NEWKEY" @u
> >>
> >> It is encouraged to switch to a new key because the effective key size
> >> of the old keys is only half of the specified size.
> >
> > Both the old and new decrypted data size is 32 bytes.  Is the above
> > statement necessary, especially since the Documentation example does
> > the equivalent?
> 
> The old key has the same byte size but all bytes must be within the 
> hex-ascíi range of characters, otherwise it is refused by the kernel.
> So if you wanted a 32 bytes key you get 16 effective bytes for the key. 
> In the above example the string size of the $BROKENKEY is 32, while 
> the string size of the $NEWKEY is 64.
> 
> If you do
> 
> $ echo $NEWKEY
> 6162636465664142434445463132333435363738393061616161616161616161
> 
> for the example, the range problem is obvious, so $NEWKEY is still broken. 
> That's why it should only be used to recover data which should be 
> reencypted with a new key. If you count exactly, the effective key size is 
> _slightly_ longer than half of the specified size, but it is still a 
> severe security problem.

So the issue with NEWKEY isn't the "effective key size of the old keys
is only half of the specified size", but that the old key, itself, is
limited to the hex-ascii range of characters.

Mimi