Cryptography attack
In cryptography, a
padding oracle attack
is an attack which uses the
padding
validation of a cryptographic message to decrypt the ciphertext. In cryptography, variable-length plaintext messages often have to be padded (expanded) to be compatible with the underlying
cryptographic primitive
. The attack relies on having a "padding
oracle
" who freely responds to queries about whether a message is correctly padded or not. The information could be directly given, or leaked through a
side-channel
.
The earliest well-known attack that uses a padding oracle is
Bleichenbacher's attack
of 1998, which attacks
RSA
with
PKCS #1 v1.5
padding.
[1]
The term "padding oracle" appeared in literature in 2002,
[2]
after
Serge Vaudenay
's attack on the
CBC mode decryption
used within symmetric
block ciphers
.
[3]
Variants of both attacks continue to find success more than one decade after their original publication.
[1]
[4]
[5]
Asymmetric cryptography
[
edit
]
In 1998,
Daniel Bleichenbacher
published a seminal paper on what became known as
Bleichenbacher's attack
(also known as "million message attack"). The attack uses a padding oracle against
RSA
with
PKCS #1 v1.5
padding, but it does not include the term. Later authors have classified his attack as a padding oracle attack.
[1]
Manger (2001) reports an attack on the replacement for PKCS #1 v1.5 padding, PKCS #1 v2.0 "OAEP".
[6]
Symmetric cryptography
[
edit
]
In symmetric cryptography, the padding
oracle attack
can be applied to the
CBC mode of operation
. Leaked data on padding validity can allow attackers to decrypt (and sometimes encrypt) messages through the oracle using the oracle's key, without knowing the encryption key.
Compared to Bleichenbacher's attack on RSA with PKCS #1 v1.5, Vaudenay's attack on CBC is much more efficient.
[1]
Both attacks target cryptosystems commonly-used for the time: CBC is the original mode used in
Secure Sockets Layer
(SSL) and had continued to be supported in TLS.
[4]
A number of mitigations have been performed to prevent the decryption software from acting as an oracle, but newer
attacks based on timing
have repeatedly revived this oracle. TLS 1.2 introduces a number of
authenticated encryption with additional data
modes that do not rely on CBC.
[4]
Padding oracle attack on CBC encryption
[
edit
]
The standard implementation of CBC decryption in block ciphers is to decrypt all ciphertext blocks, validate the padding, remove the
PKCS7 padding
, and return the message's plaintext.
If the server returns an "invalid padding" error instead of a generic "decryption failed" error, the attacker can use the server as a padding oracle to decrypt (and sometimes encrypt) messages.
The mathematical formula for CBC decryption is
As depicted above, CBC decryption XORs each plaintext block with the previous block.
As a result, a single-byte modification in block
will make a corresponding change to a single byte in
.
Suppose the attacker has two ciphertext blocks
and wants to decrypt the second block to get plaintext
.
The attacker changes the last byte of
(creating
) and sends
to the server.
The server then returns whether or not the padding of the last decrypted block (
) is correct (a valid PKCS#7 padding).
If the padding is correct, the attacker now knows that the last byte of
is
, the last two bytes are 0x02, the last three bytes are 0x03, …, or the last eight bytes are 0x08. The attacker can modify the second-last byte (flip any bit) to ensure that the last byte is 0x01. (Alternatively, the attacker can flip earlier bytes and
binary search
for the position to identify the padding. For example, if modifying the third-last byte is correct, but modifying the second-last byte is incorrect, then the last two bytes are known to be 0x02, allowing both of them to be decrypted.) Therefore, the last byte of
equals
.
If the padding is incorrect, the attacker can change the last byte of
to the next possible value.
At most, the attacker will need to make 256 attempts to find the last byte of
, 255 attempts for every possible byte (256 possible, minus one by
pigeonhole principle
), plus one additional attempt to eliminate an ambiguous padding.
[7]
After determining the last byte of
, the attacker can use the same technique to obtain the second-to-last byte of
.
The attacker sets the last byte of
to
by setting the last byte of
to
.
The attacker then uses the same approach described above, this time modifying the second-to-last byte until the padding is correct (0x02, 0x02).
If a block consists of 128 bits (
AES
, for example), which is 16 bytes, the attacker will obtain plaintext
in no more than 256?16 = 4096 attempts. This is significantly faster than the
attempts required to bruteforce a 128-bit key.
Encrypting messages with Padding oracle attack (CBC-R)
[
edit
]
CBC-R
[8]
turns a decryption oracle into an encryption oracle, and is primarily demonstrated against padding oracles.
Using padding oracle attack CBC-R can craft an initialization vector and ciphertext block for any plaintext:
- decrypt any ciphertext
P
i
= PODecrypt( C
i
) XOR C
i?1
,
- select previous cipherblock
C
x?1
freely,
- produce valid ciphertext/plaintext pair
C
x-1
= P
x
XOR PODecrypt( C
i
)
.
To generate a ciphertext that is
N
blocks long, attacker must perform
N
numbers of padding oracle attacks. These attacks are chained together so that proper plaintext is constructed in reverse order, from end of message (
C
N
) to beginning message (
C
0
, IV). In each step, padding oracle attack is used to construct the IV to the previous chosen ciphertext.
The CBC-R attack will not work against an encryption scheme that authenticates ciphertext (using a
message authentication code
or similar) before decrypting.
Attacks using padding oracles
[
edit
]
The original attack against CBC was published in 2002 by
Serge Vaudenay
.
[3]
Concrete instantiations of the attack were later realised against SSL
[9]
and IPSec.
[10]
[11]
It was also applied to several
web frameworks
, including
JavaServer Faces
,
Ruby on Rails
[12]
and
ASP.NET
[13]
[14]
[15]
as well as other software, such as the
Steam
gaming client.
[16]
In 2012 it was shown to be effective against
PKCS 11
cryptographic tokens.
[1]
While these earlier attacks were fixed by most
TLS
implementors following its public announcement, a new variant, the
Lucky Thirteen attack
, published in 2013, used a timing side-channel to re-open the vulnerability even in implementations that had previously been fixed. As of early 2014, the attack is no longer considered a threat in real-life operation, though it is still workable in theory (see
signal-to-noise ratio
) against a certain class of machines. As of 2015
[update]
, the most active area of development for attacks upon cryptographic protocols used to secure Internet traffic are
downgrade attack
, such as Logjam
[17]
and Export RSA/FREAK
[18]
attacks, which trick clients into using less-secure cryptographic operations provided for compatibility with legacy clients when more secure ones are available. An attack called
POODLE
[19]
(late 2014) combines both a downgrade attack (to SSL 3.0) with a padding oracle attack on the older, insecure protocol to enable compromise of the transmitted data. In May 2016 it has been revealed in
CVE
-
2016-2107
that the fix against Lucky Thirteen in OpenSSL introduced another timing-based padding oracle.
[20]
[21]
References
[
edit
]
- ^
a
b
c
d
e
Romain Bardou; Riccardo Focardi; Yusuke Kawamoto; Lorenzo Simionato; Graham Steel; Joe-Kai Tsay (2012).
Efficient Padding Oracle Attacks on Cryptographic Hardware
.
Rr-7944
(report).
INRIA
. p. 19.
- ^
Black, John; Urtubia, Hector (2002).
Side-Channel Attacks on Symmetric Encryption Schemes: The Case for Authenticated Encryption
. USENET Security '02.
- ^
a
b
Serge Vaudenay (2002).
Security Flaws Induced by CBC Padding Applications to SSL, IPSEC, WTLS...
(PDF)
. EUROCRYPT 2002.
Similar attack model was used by Bleichenbacher against PKCS#1 v1.5 [5] and by Manger against PKCS#1 v2.0 [13]. This paper shows that similar attacks are feasible in the symmetric key world.
- ^
a
b
c
Sullivan, Nick (12 February 2016).
"Padding oracles and the decline of CBC-mode cipher suites"
.
The Cloudflare Blog
.
- ^
Hanno Bock; Juraj Somorovsky; Craig Young.
"ROBOT attack: Return Of Bleichenbacher's Oracle Threat"
. Retrieved
27 February
2018
.
- ^
Manger, James (2001).
"A Chosen Ciphertext Attack on RSA Optimal Asymmetric Encryption Padding (OAEP) as Standardized in PKCS #1 v2.0"
(PDF)
. Telstra Research Laboratories.
- ^
Is the padding oracle attack deterministic
- ^
Juliano Rizzo; Thai Duong (25 May 2010).
Practical Padding Oracle Attacks
(PDF)
. USENIX WOOT 2010.
- ^
Brice Canvel; Alain Hiltgen; Serge Vaudenay; Martin Vuagnoux (2003),
Password Interception in a SSL/TLS Channel
(PDF)
.
- ^
Jean Paul Degabriele; Kenneth G. Paterson (2007),
Attacking the IPsec Standards in Encryption-only Configurations
(PDF)
, archived from
the original
on 19 December 2018
, retrieved
25 September
2018
.
- ^
Jean Paul Degabriele; Kenneth G. Paterson (2010),
On the (In)Security of IPsec in MAC-then-Encrypt Configurations
,
CiteSeerX
10.1.1.185.1534
.
- ^
Juliano Rizzo; Thai Duong (25 May 2010).
Practical Padding Oracle Attacks
(PDF)
. USENIX WOOT 2010.
- ^
Thai Duong; Juliano Rizzo (2011).
Cryptography in the Web: The Case of Cryptographic Design Flaws in ASP.NET
(PDF)
. IEEE Symposium on Security and Privacy 2011.
- ^
Dennis Fisher (13 September 2010).
"
'Padding Oracle' Crypto Attack Affects Millions of ASP.NET Apps"
.
Threat Post
. Archived from
the original
on 13 October 2010.
- ^
Vlad Azarkhin (19 September 2010).
"
"Padding Oracle" ASP.NET Vulnerability Explanation"
. Archived from
the original
on 23 October 2010
. Retrieved
11 October
2010
.
- ^
"Breaking Steam Client Cryptography"
.
Steam Database
. Retrieved
1 May
2016
.
- ^
Matthew Green;
Nadia Heninger
; Paul Zimmerman; et al. (2015),
Imperfect Forward Secrecy: How Diffie?Hellman Fails in Practice
(PDF)
. For further information see
https://www.weakdh.org
Archived
22 December 2019 at the
Wayback Machine
.
- ^
Matthew Green (3 March 2015).
"Attack of the week: FREAK (or 'factoring the NSA for fun and profit')"
.
; see
https://www.freakattack.com
Archived
5 March 2015 at the
Wayback Machine
for more information.
- ^
Matthew Green (14 October 2014).
"Attack of the week: POODLE"
.
; for further information, see
https://www.poodle.io
- ^
OpenSSL Security Advisory [3rd May 2016]
, 3 May 2016
- ^
Yet Another Padding Oracle in OpenSSL CBC Ciphersuites
, Cloudflare, 4 May 2016
|
---|
Protocols and technologies
| |
---|
Public-key infrastructure
| |
---|
See also
| |
---|
History
| |
---|
Implementations
| |
---|
Notaries
| |
---|
Vulnerabilities
| Theory
| |
---|
Cipher
| |
---|
Protocol
| |
---|
Implementation
| |
---|
|
---|