Re: [PATCH v4] KEYS: encrypted: fix key instantiation with user-provided data
From: Nikolaus Voss
Date: Fri Oct 14 2022 - 07:39:50 EST
On Fri, 14 Oct 2022, Mimi Zohar wrote:
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.
The latter resulting in the former. If for BROKENKEY 32 bytes were
specified, a brute force attacker knowing the key properties would only
need to try at most 2^(16*8) keys, as if the key was only 16 bytes long.
This is what I mean with "effective size" in contrast to the key's byte
size which is 32 in my example.
The security issue is a result of the combination of limiting the input
range to hex-ascii and using memcpy() instead of hex2bin(). It could have
been fixed either by allowing binary input or using hex2bin() (and
doubling the ascii input key length). I chose the latter.
Niko