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Implementing a proven-secure and cost-effective countermeasure against the compression ratio info-leak mass exploitation (CRIME) attack

Implementing a Proven-secure and Cost-effective
Countermeasure against the Compression Ratio
Info-leak Mass Exploitation (CRIME) Attack
Jayamine Alupotha, Sanduni Prasadi, Janaka
Alawatugoda and Roshan Ragel
Department of Computer Engineering
Faculty of Engineering
University of Peradeniya
Peradeniya 20400, Sri Lanka
Mohamed Fawsan
Department of Computer Science and Engineering
Faculty of Engineering
University of Moratuwa
Moratuwa 10400, Sri Lanka
Abstract—Header compression is desirable for network ap-
plications as it saves bandwidth and reduces latency. However,
when data is compressed before being encrypted, the amount of
compression leaks information about the amount of redundancy
in the plaintext. In web requests, headers contain secret web
cookies. Therefore, compression of headers before encryption
will reveal the information about the secret web cookies. This
side-channel has led to Compression Ratio Info-leak Made Easy
(CRIME) attack on web traffic protected by the SSL/TLS
protocols. In order to mitigate the CRIME attack, compression
is completely disabled at the TLS/SSL layer, which in return
increases the bandwidth consumption and latency. In a previous
work (Financial Cryptography and Data Security 2015), two
countermeasures are presented with formal security proofs,
against compression side-channel attacks, namely (1)–separating
secret cookies from user inputs and (2)–using a static compression
In this work we create a test environment to replicate the
CRIME attack and verify the attack. Moreover, we implement
a proven-secure countermeasure against the CRIME attack, in
a real world client/server setup, following the aforementioned
two countermeasures. Our implementation achieves better com-
pression ratio (closer to the original TLS/SSL compression), and
hence reduces the bandwidth usage and latency significantly
(therefore cost-effective). To the best of our knowledge, this is
the first proven-secure and cost-effective countermeasure imple-
mentation against the CRIME attack.
Index Terms– data compression, CRIME attack, SSL/TLS,
cryptography, security
Nowadays, web pages have been grown to need thousands
of requests. The headers in these requests consume significant
amount of bandwidth, increasing latency. Initially, SPDY [1]
addressed this issue by compressing header fields using DE-
FLATE algorithm [2], which compresses the redundant header
fields effectively. But that approach exposed a security risk as
demonstrated by the CRIME attack [3].
At ekoparty 2012 security conference, Duong and Rizzo
announced the CRIME attack; a compression side-channel
attack against HTTPS (Secure HyperText Transfer Protocol)
traffic [3]. The demonstration given at the ekoparty 2012
showed how to recover the secret web cookies of HTTPS
requests. The web cookies are used for web application
authentication (after log-in). Therefore, the attacker will be
able to hijack the session upon recovering the web cookie.
As the obvious fix against the CRIME attack, all the web
servers and clients disabled TLS/SSL-level (Transport Layer
Security/Secure Socket Layer) compression.
Disabling header compression effects on the performance
by increasing the bandwidth usage and latency, that obviously
increases the cost. In HTTP/2 this issue is also taken into
account and a new header field compression, HPACK [4] is
introduced. HPACK is a compression format for efficiently
representing HTTP header fields. According to HTTP/2 [5],
HPACK does not completely prevent the CRIME attack [3],
but it mitigates the risk to some extent.
A. Header Compression
Compression is a mechanism to transmit or store data by
reducing its size. Gzip [6] and DEFLATE [2] are considered
as the most common compression formats.
The main compression method used in TLS header com-
pression is DEFLATE [2]. DEFLATE compression method is
a combination of the LZ77 algorithm and Huffman encoding,
which defines a loss-less compression. Huffman coding is used
to eliminate the redundancy of repeating symbols. It searches
for repeated strings and replaces them with back-references to
the last occurrence as (distance, length) and convert the content
into a zlib-formatted stream. Gzip is based on the DEFLATE
algorithm. Therefore, gzip provides a loss less-compression,
similar to DEFLATE compression.978-1-5386-1676-5/17/$31.00 ©2017 IEEE
ICIIS'2017 1570375904
B. Compression Side-channel Attacks
Compression side-channel attacks [7] are not new. Research
on compression side-channel attacks were started more than a
decade ago. In 2002 Kelsey described a side-channel attack [8]
provided by data compression algorithms, yielding information
about their inputs by the size of their outputs to reveal
information about plaintext. If plaintext is compressed before
the encryption, the length of the ciphertext reveals information
about the amount of compression, which in turn can reveal
information about the plaintext to the attacker. According to
Kelsey’s attack, if an attacker is allowed to choose inputs x
that are combined with a target secret sand the concatenation
xksis compressed and encrypted, observing the length of
the outputs can eventually allow the attacker to extract the
secret s. For example, to determine the first character of s,
the attacker asks to have the string x=prefix*prefix
combined with s, then compressed and encrypted, for every
possible character *; in one case, when *=s1, the amount
of redundancy is higher and the ciphertext should be shorter.
Once each character of sis found, the attack can be carried
out on the next character.
The security research community has had a lot of surprises
with SSL/TLS uncovering a few terrific attacks. First, at the
ekoparty 2011 Security Conference, Rizzo and Duong uncov-
ered a new attack on Transport Layer Security (TLS), namely
BEAST attack [9]. At the ekoparty 2012 security conference,
Rizzo and Duong released the details about the CRIME [3]
attack, which is a compression side-channel attack against
HTTPS traffic to reveal secret web cookies. In 2013 at Black
Hat USA, Prado, Harris, and Gluck announced another attack,
namely BREACH [10] attack, that targets the vulnerability
of HTTP body compression to reveal secrets like anti-CSRF
tokens and any other personally identifiable data.
C. The CRIME Attack
Many web applications use cookies in the headers of
HTTPS requests for authorization purposes. These requests
are subjected to compression and encryption respectively. The
primary target of the CRIME attack was the user’s cookie in
the HTTPS header. If the victim visited an attacker-controlled
web page, the attacker could use Javascript to cause the victim
to send HTTPS requests to URLs of the attacker’s choice on
the target server. The attacker could adaptively choose those
URLs to include a prefix to carry out Kelsey’s attack. Consider
a website with basic authentication. If a user with an active
session makes the following request:
POST /test/demo-form.php HTTP/1.1
With the above request, the attacker sends a string
JSESSIONID=6 as a URL string.
In the above case, JSESSIONID which is used to iden-
tify the authorized users, is the secret. Note that the string
JSESSIONID= is repeated in the request. DEFLATE can
take advantage of this repeated strings. In fact, if the first
character of the guess matches the first character of the actual
value of JSESSIONID (6 in this case), then DEFLATE will
compress the request even more. The request with maximum
compression ratio can be considered as the correct guess.
Because of that an attacker can exploit to recover the first
character of JSESSIONID. Then the attacker proceeds to
recover the second byte, injecting the recovered first byte and a
guessed character for the second byte. By continuing the same
process as per the first byte, attacker can recover complete
JSESSIONID byte-by-byte.
D. Our Contribution
In this work, we replicate the CRIME attack. In order to
do that, we setup a client/server environment with TLS/SSL
layer compression enabled. We establish the attack setup by
injecting a Javascript (using an attacker-controlled website) on
the client machine, to inject the URLs (of attacker’s guesses
on cookie bytes) to be sent to the target server. Moreover, we
implement the CRIME attack algorithm to recover the cookie
byte-by-byte by measuring the compression length of each
request made by the client to the target server. Our setup can
be used to test the CRIME attack and countermeasures against
Then, we implement a proven-secure countermeasure
against the CRIME attack, following the proven-secure con-
cepts of (1)–separating secrets from user inputs and (2)–using
a static compression dictionary of Alawatugoda et al. [11].
The security of this countermeasure is theoretically proven in a
well-defined security model in the work of Alawatugoda et al.
[11]. We implement this countermeasure in a real world setup,
using Apache tomcat server. We use a static table to encode
header field names, since it will increase the compression
ratio. Then, the secret cookies are separated, and after that
DEFLATE compression is taken place on the encoded header,
except on the separated secret cookies. Finally, the compressed
portion and the separated secrets are encrypted, and the
HTTPS request is created.
Our countermeasure implementation achieves better com-
pression ratio (closer to the original TLS/SSL compression),
because the header is encoded using a suitable static dictionary
and only the secrets are kept uncompressed. Therefore, our
countermeasure reduces the bandwidth usage and latency sig-
nificantly. To the best of our knowledge, this is the first proven-
secure and cost-effective (by means of bandwidth save and
reducing the latency) countermeasure implementation against
the CRIME attack.
In this section we briefly discuss about currently using
countermeasures against the CRIME attack and proposed
countermeasures in the literature.
A. Disabling Compression
Disabling compression at the HTTP level completely pre-
vents the side-channel. Therefore it defeats the CRIME attack.
Unfortunately, this solution can have a significant impact on
performance. Large number of requests are sent (including
handshake requests) to load a page of a web application. These
requests take a considerable amount of the network bandwidth
and has an impact on network latency. This is the widely used
fix against the CRIME attack.
B. TLS Extension to Hide The Length
The risk of CRIME attack can be somewhat mitigated by
hiding the length of requests. Because of that attacker has
to identify the length first and it will take longer time and
the attacker has to send large number of requests. In 2013,
Pironti and Mavrogiannopoulos purposed a TLS extension [12]
to allow arbitrary amount of padding in any TLS ciphersuite.
But this approach does not eliminate the risk completely. The
attacker can recover the exact length of the request by sending
multiple requests for single guess and averaging the sizes of
the requests. This countermeasure delays the attack.
C. Static Dictionary Compression
In the static dictionary compression, the dictionary used for
compression does not adapt to the plaintext being compressed,
but instead is preselected in advance based on the expected
distribution of plaintext messages, for example including com-
mon words and phrases to be used in the context (in our case
the context of header field names). Therefore, a secret cookie
(which is normally a set of random characters), has a negligible
chance to be appeared in the static dictionary and being
compressed. It is statistically proven that this countermeasure
can make the attackers probability of recovering the secret
cookie by launching the CRIME attack negligible [11].
D. Separating Secrets From User Inputs
Separating secrets from user inputs [11] has been proven
as an effective solution which can completely eliminate the
CRIME attack. The sole idea if this countermeasure is to
separate the secret cookie from rest of the information that are
being compressed. Since the secret cookie is not compressed
the origin of the compression leakage is shut. Since the other
information is compressed it is possible to achieve significant
compression ratio.
Although disabling header compression completely miti-
gates the CRIME attack, it has a drastic impact on network
latency and bandwidth usage. As an example, average band-
width consumption by headers in Facebook is 11.8% of the
total bandwidth consumption of requests-responses (Figure
1), which is 14.7% in Gmail (Figure 2). We measure this
for 2000 exchanges (requests-responses) in each website with
95% confidence level and ±0.7confidence interval. This result
shows that headers acquire considerable amount of the total
bandwidth consumption of a website. Hence, in reduction of
network latency and bandwidth consumption, header compres-
sion is very useful.
Fig. 1. Request-Response size vs header percentage for Facebook
Fig. 2. Request-Response size vs header percentage for Gmail
A. Prototype
In order to test the proposed countermeasure, the CRIME
attack is remodeled and a test environment is created. For a
web application to be vulnerable to this side-channel attack,
it must,
be served from a server that uses HTTP-level header
sent requests through a browser that supports header
compression and
reflect a secret (such as a cookie) in HTTP request header.
Additionally, while not strictly a requirement, the attack is
helped greatly if no noise occurs in the side-channel. This is
because the difference in size of the request headers measured
by the attacker can be quite small.
The attack is carried out with the assumption that the
attacker has the ability to view the victims encrypted traffic.
An attacker might accomplish this with a network protocol
analyzer like Wireshark or a proxy server. Therefore, the
CRIME attacker can be an ISP (Internet Service Provider),
network administrator, the government or any party who can
eavesdrop encrypted traffic (Man-In-The-Middle). It is also
assumed that the attacker has the ability to cause the victim
to send HTTP requests to the secure web server. This can
be accomplished by coercing the victim to visit an attacker-
controlled site (which will contain a JavaScript code that sends
requests to the SSL/TLS-protected server with injected values
in the request header).
This is an active, online attack. The attack proceeds byte-
by-byte. The attacker will coerce the victim to send a small
number of requests to guess the first byte of the target secret
cookie. The attacker then measures the size of the request
header. With that information, the CRIME attack algorithm
determines the correct value for the first character of the
secret cookie. Since the attack relies on LZ77 loss-less data
compression algorithm, the first byte of the target secret must
be correctly guessed before the second byte is attempted.
Figure 3 shows the CRIME attack setup.
Fig. 3. CRIME attack setup
B. CRIME Attack Algorithm
Huffman coding is a loss-less data compression algorithm
[13]. In this algorithm, input characters are assigned with
variable-length codes. Frequency of the corresponding char-
acter in the content defines the length of the assigned code.
The most frequent character gets the smallest character set and
the least frequent character gets the largest character set.
When the attacker guesses a character correctly and injects
the guess to the request, the resulting string will have a longer
repeated character set than in the previous string in the request.
This results more compression. Differently, if an attacker
guesses a character incorrectly, the length of the compressed
request will be equal or less than the length of compressed
request of the correct guess. Having this length equal or less
than the length of the correct guess seems problematic. This
could be solved using Two-Tries Method used by Duong and
Rizzo. Instead of sending one request for each guess, two
requests are sent as follows:
Let the assumed secret for the first byte is X,
First send a request with <url>/secret=X (let the
compressed and encrypted request length is L1 in this
Then send a request with <url>/Xsecret= (let the
compressed and encrypted request length is L2 in this
Get the minimum for L1 by considering all the possible
characters in place of X. If that L1 is less than corresponding
L2, then the guessed character is taken as a possible value.
Recovering the secret will act in unexpected manner when
there are repeated characters in the secret itself. For example,
suppose the target secret is ABCABXYZ. Further, suppose
the first five characters of the secret: ABCAB are recovered.
When trying to guess the next character it will likely return
Cand Xas correct values. This is because ABCABC alone is
compressible while ABCABX matches the actual secret. This
is mitigated by checking how well our guess compresses by
itself. If a particular guess compresses by itself beyond a
certain threshold, it is discarded.
Block ciphers pose and additional challenge to the CRIME
attack algorithm. A tipping point is the point at which any
additional incompressible character causes the injected string
to overflow into an additional block. When the injected string
is aligned to a tipping point, it is assumed that only correct
guesses are fit within the current block, allowing to use the
CRIME attack algorithm the same way as for stream ciphers.
In order to get our injected values aligned to a tipping point, a
filler string (that is not repeated in the injected string) is added
to the request.
Currently, our CRIME attack algorithm has an accuracy of
93% to recover 4 bytes of any length cookie of hexadecimal
C. Pitfalls
Despite the straightforward nature of the CRIME attack,
actually exploiting it to create a real testing environment is
challenging. This is mainly because the header compression is
completely removed from all servers and browsers. Therefore,
an older version of a browser that allows header compression
is used. A server is configured to enable HTTP-level header
compression using OpenSSL library.
We implement our countermeasure using an Apache tomcat
server and tested it with a Java socket client.
When implementing the countermeasure, in order to have
a better header compression, static dictionary is adopted for
encoding the header field names. In general, static dictionary
compression schemes work by advancing through the string
xand looking to see if the current substring appears in the
dictionary D: if it does, then an encoding of the index of the
substring is recorded, otherwise an encoding of the current
substring is recorded. Our countermeasure adopts a static table
as the static dictionary, which consists of a predefined static
list of header field names. We use that static dictionary for
encoding the header fields. Figure 4 shows an example for a
static dictionary.
Fig. 4. Example static dictionary
In the client-side, first the header fields of the request
are replaced by the relevant encoding taken from the static
dictionary. Then the secrets in the request header are separated.
The encoded request without secrets is compressed and then
encrypted together with the separated secrets. In the server-
side (tomcat server), the content is decrypted and then the
compressed content is decompressed. Then, the secrets are
appended to the uncompressed request, to regenerate the
request in its original form. Then the request is decoded using
the static dictionary.
Similarly the response header is encoded using the static
dictionary at the server-side. If secrets are available, then they
are separated out. After that, the response header excluding the
secrets is compressed. Finally both the compressed content and
the separated secrets are encrypted before sending to the client.
At the client-side, the response is decrypted, then the response
header without secrets is decompressed. The separated secrets
are appended to the response header and finally the response
header is decoded to regenerate the original header.
Figure 5 shows the format of a request/response header,
that is used in our countermeasure. In order to identify
requests/responses with compressed headers, an indicator of
value 0is added to the beginning of the request/response.
Next 2 bytes are allocated for the compressed-content’s length.
After that the compressed request/response header (without the
secret cookies) is added. Separated secret cookies are added
after ’:’ character. End of the header is denoted by \r\n.
Fig. 5. Format of a request/response header in our countermeasure
The actual request header is rearranged as shown below.
Consider the following request:
POST /test/demo-form.php HTTP/1.1
This request header is encoded using the static dictionary
illustrated in Figure 4:
POST /test/demo-form.php HTTP/1.1
Then the request header is rearranged by separating the secrets:
POST /test/demo-form.php HTTP/1.1
Then, it is compressed using DEFLATE compression algo-
rithm, and used as the ”compressed-content”. The secret cook-
ies JSESSIONID=672CA12B are used as the ”Cookies”.
Then the new request header is encrypted. Since the secrets
do not compress with the request header (together with the
attacker-injected values), the CRIME attack can be completely
eliminated. Because we wipe out the origin of the attack.
Figure 6 and 7 illustrates the compression ratios (equation
(1)) of the headers for Facebook and Gmail respectively,
when using our countermeasure vs the original SSL/TLS-layer
compression (vulnerable to the CRIME attack). Although the
cookies are separated and kept uncompressed, the headers are
significantly compressed with the use of our countermeasure
implementation (closer to the compression ratio of original
TLS/SSL-layer compression). This is useful in reduction of
the bandwidth usage and decreasing the latency.
Compression Ratio =Compressed Header Size
Header Size ×100 (1)
In this work we develop a test environment to replicate
the CRIME attack and verified the attack. Further, we im-
plement a practical countermeasure, namely static dictio-
nary header encoding and separating secret cookies in re-
quest/response headers, against the CRIME attack in a real
world client/server setup. To the best of our knowledge, this
is the first proven-secure and cost-effective countermeasure
implementation against the CRIME attack. We also implement
this countermeasure in the server-side, in a way that it works
compatibly with the Mozilla Firefox browser.
As a future work we will look at how to improve the
compressibility of headers. Particularly, using an adaptive
encoding dictionary this can be achieved, given that the secret
cookie is excluded when constructing the dictionary. This
direction should be further studied. Moreover, we will look
at the BREACH attack [10] and work on implementations of
Fig. 6. Compression ratios of headers of Facebook for our countermeasure
vs original SSL/TLS-layer compression
Fig. 7. Compression ratios of headers of Gmail for our countermeasure vs
original SSL/TLS-layer compression
proven-secure countermeasures against it. Particularly, it will
be possible to adopt the two proven-secure countermeasures
of Alawatugoda et al. [11]. Differently, we will have to deal
with the HTTP response body.
Janaka Alawatugoda acknowledges the postdoctoral fellow-
ship grant NRC 16-020 of the National Research Council, Sri
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RFC 7541 HPACK: Header Compression for HTTP/2
  • R Peon
  • H Ruellan
Length hiding padding for the transport layer security protocol
  • A Pironti
  • N Mavrogiannopoulos
Breach: reviving the crime attack
  • Y Gluck
  • N Harris
  • A Prado