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From: Jerome Etienne (jmeoff.net)
Date: Wed Jan 02 2002 - 13:57:54 CST
The following text describes a security hole in the encrypted loop
device for linux. Because of it, an attacker is able to modify the
content of the encrypted device without being detected. This text
proposes to fix the hole by authenticating the device.
comments are welcome
ps: version in html, pdf and ps can be found in http://www.off.net/~jme
Vulnerability in encrypted loop device for Linux
Jerome Etienne jmeoff.net
This text describes a security hole i found in encrypted loop device for
Linux. An attacker is able to modify the content of the encrypted device
without being detected (see section 2). This text proposes to fix the hole
by authenticating the device (see section 3).
1 Threat model
Encrypting a disk device aims to protect against a off-line attacker who
would be able to access the disk between 2 legitimate mounts.
It isn't against an attacker who has access to the running computer when
the encrypted device is mounted as either (i) the attacker is root and it
can access the encrypted device anyway or (ii) he is an unprivileged user
and can be stopped with Unix's right management (i.e. user/group).
2 Attack description
The vulnerability of encrypted loop device is due to its lack of
authentication. The aim of encryption is to make the data unreadable for
anybody who doesn't know the key. It doesn't prevent an attacker from
modifying the data. People assume that an attacker won't do it because the
attacker wouldn't be able to choose the resulting clear text. But this
section shows that the attacker can choose the resulting clear text to
some extends and that modifying the cypher text data may be interesting
even if the attacker ignores the result.
This attack is only applicable to device storing data which are reused
across mounts: most file-system (e.g. ext2, reiserfs, ext3) but not swap.
In some systems, encrypted devices are stored in the same location than
the encrypted disk containing the operating system. For those systems the
attacker who can access the encrypted device, can easily modify the OS to
gain access (e.g. kernel) independtly of the encrypted device.
2.1 To insert random data
If the attacker modifies the cipher text without choosing the resulting
clear text, it will likely produce random data. The legitimate user won't
detect the modification and will use them as if they were valid. As they
likely appears random, it will result of a Denial of Service (aka DoS).
2.2 To insert chosen data
The encryption mode used by encrypted loop device is CBC[oST81,sec 5.3].
CBC allows cut/past attacks i.e. the attacker can cut encrypted data from
one part of the device and paste them in another location. As both data
sections have been encrypted by the same key, the clear text won't be
completely random data.
This lack of authentication isn't a CBC flaw. Authentication isn't
considered a aim of the encryption mode, so most modes (e.g. ECB, CFB,
OFB) doesn't authenticate the data. To use another mode would be flawed in
the same way except if they explicitly protect against forgery. Recently
some modes including authentication popped up to speed up the encryption /
authentication couple but as far as i know they are all patented.
In very short, encrypting with CBC is Cn=Enc(Cn-1 xor Pn) where Enc(x) is
encrypting x, Pn is the nth block of plain text and Cn the nth block of
cipher text. For the first block, Cn-1 is an Initial vector (aka IV) which
may be public and must be unique for a given key. The decryption is Pn =
Dec(Cn) xor Cn-1. See [oST81,sec 5.3] for a longer description of CBC.
If the attacker copies s blocks from the location m to n (aka
[Cn,...,Cn+s-1] == [Cm,...,Cm+s-1]), Pn+1 up to Pn+s-1 will the same as
Pm+1 to Pm+s-1 and Pn will likely appears random. Cn (i.e. Cm) will be
decrypted as Pn = Dec(Cm) xor Cn-1 but Cm-1 and Cn-1 are different so Pn
will likely appears random. Nevertheless Pn+1 = Dec(Cn+1) xor Cn =
Dec(Cm+1) xor Cm = Pm+1, so Pn+1=Pm+1. So if the attacker has an idea of
the content of a group of blocks in the device, he can copy them to the
Nth block, thus it can choose the content of it without being detected.
As an file-system isn't designed to appears random, its content may be
predictable to some extents (e.g. common directories and files, inode,
superblock). The attacker may use such informations to guess the contents
and do a knowledgeable cut/past. For example, an attacker knowing the
location of a password file may replace a password by another one which is
3 Proposed fixes
We propose 2 types of fixes: one which authenticate at mount time (see
section 3.1) and the other which authenticates at the cluster level (see
section 3.2). The choice between the two (see section 3.4) is a user
matter as it mostly depends on the access pattern on the encrypted device.
In the proposed fixes, the authentication is a MAC computed over the
encrypted device. The MAC is HMAC[KBC97] combined with a configured hash
function, preferably a well studied one such as SHA1[oST95] or MD5[Riv92].
The MAC secret key is derived from the pass-phrase via PKCS-5 key
derivations ([Kal00,sec 5.1]).
3.1 Authenticating at mount time
As we need to authenticate the device across mounts and not while it is
mounted (see section 1), it is sufficient to authenticate the whole device
during mount operations. It slows down mount operations but they are
rather infrequent so we consider the trade-off delay/security acceptable.
The MAC is verified during mount operations and generated during unmount
operations. It isn't supposed to be valid while the device is mounted.
The MAC generation is done when unmounting the device. The MAC is computed
over all the sectors of the device and the result is appended in the
device file after all the sectors.
The MAC verification is done when mounting the device. The MAC is computed
over all the sectors of the device. If the result is equal to the MAC
appended to the block device, the verification succeed, else it failed.
The verification may fail (i) if an attacker attempted to modify the
device during 2 legitimates mounts or (ii) if the device hasn't been
cleanly unmounted (e.g. computer crash). It is impossible to automatically
distinguish both cases with certainty. So if the verification fails, the
user is notified and the mount operation may be stopped depending on
3.2 Authenticating at cluster level
To authenticate the whole device at mount time, may be considered
prohibitive by some users, so this section describe an alternative which
authenticate the device at the cluster level. A cluster is a group of one
or more sectors, the exact number depends on configuration. In this case,
the MAC is verified each time a cluster is read from the disk and
generated at each write.
If the device isn't cleanly unmounted, the authentication of one or more
cluster may fail (e.g. the super block). This case will be detected at
mount time. But if an attacker forges data in the device, it will be
detected only when the user read the modified data. The kernel will read
the forged cluster and the authentication will fail. It may report it with
a printk with a rate limitor, it isn't clean but i don't see any better
3.3 MAC location
Currently the encrypted loop file-system is stored in a regular file of a
hosting file-system. Its size is a multiple of a sector size (i.e. 512
byte). The MAC could be stored in a separate file or included in the
regular file. To store the MAC in a separate file, generates problems
while managing the loop device file (e.g. copy, backup). The administrator
must not forget to copy the MAC file when he copies the device file, else
the copied device won't be usable anymore. To store the MAC in the same
file as the clusters doesn't has this disadvantage.
To authenticate at the cluster level will increase the access time of each
cluster but won't affect mount operation. The exact increase depends on
the MAC and encryption algorithms. As a rule of thumb, MAC algorithms are
typically 3 times faster than encryption ones so the time dedicated to
cryptography for each block will increase by around 30%. To authenticate
at mount time will largely slow down the mount operations but won't affect
every access once mounted.
The authentication at mount time will detect forgery at mount time,
whereas the alternative detects it only when the forged cluster is read,
possibly a long time after the modification. Users may consider that it is
easier to diagnose who forged it if they have a better idea of when the
To authenticate the whole device at mount time requires a single MAC per
device, so the space overhead (typically 16 byte) is negligible compared
to the device's size. To authenticate at the cluster level requires a MAC
per cluster, it is significantly more but some people may consider it
still negligible, especially with cheap disks.
The choice between the two mostly depends on the access pattern on the
encrypted device. If the device is used for interactive purpose, the
increased access latency may be unsuitable. If the access latency is
important or if every block is frequently modified, to authenticate only
once at mount time may be more interesting. If the user can't stand long
mount operations, to authenticate at cluster level will be more suitable.
As only the final user knows the type access made on his encrypted device,
he should be the one able to choose between the two.
Thanks to Andy Kleen and Phil Swan for their useful comments.
This text described an vulnerability in encrypted loop device which allows
an attacker to modify the encrypted device without being detected (see
section 2). We propose a fix which authenticate the whole device during
mount operation (see section 3.1). This fix slows down mount operations
but we consider the trade-off longer delay vs additional security very
reasonable as mount operations are rather infrequent. We propose another
fix which authenticate at cluster level for people who can't stand long
mount operation. The choice between the two is a final user matter.
The authentication may be optionally disabled thus if an user considers
the trade-off delay/security not in favor of security, he may choose to be
vulnerable to this attack and disable it. Nevertheless the author thinks
encrypted loop device must be secure by default.
B. Kaliski. Pkcs 5: Password-based cryptography specification
version 2.0. Request For Comment (Informational) RFC2898,
H. Krawczyk, M. Bellare, and R. Canetti. Hmac: Keyed-hashing for
message authentication. Request For Comment (Informational)
RFC2104, February 1997.
National Institute of Standards and Technology. implementing and
using the nbs data encryption standard. Federal information
processing standards fips74, April 1981.
National Institute of Standards and Technology. Secure hash
standard. Federal information processing standards fips180-1,
R. Rivest. The md5 message-digest algorithm. Request For Comment
(Informational) RFC1321, April 1992.