Internet Engineering Task Force S. Barbato
Internet-Draft S. Dorigotti
Intended status: Informational T. Fossati, Ed.
Expires: April 11, 2013 KoanLogic
October 8, 2012
SCS: Secure Cookie Sessions for HTTP
draft-secure-cookie-session-protocol-07
Abstract
This document provides an overview of SCS, a small cryptographic
protocol layered on top of the HTTP cookie facility, that allows its
users to produce and consume authenticated and encrypted cookies, as
opposed to usual cookies, which are un-authenticated and sent in
clear text.
An interesting property, rising naturally from the given
confidentiality and authentication properties, is that by using SCS
cookies, it is possible to avoid storing the session state material
on the server side altogether. In fact, an SCS cookie presented by
the user agent to the origin server can always be validated (i.e.
possibly recognized as self-produced, fresh, untampered material)
and, as such, be used to safely restore application state.
Hence, typical use cases may include devices with little or no
storage offering some functionality via an HTTP interface, as well as
web applications with high availability or load balancing
requirements which would prefer to handle application state without
the need to synchronize the pool through shared storage or peering.
Another noteworthy application scenario is represented by the
distribution of authorized web content (e.g. by CDNs), where an SCS
token can be used, either in a cookie or embedded in the URI, to
provide evidence of the entitlement to access the associated resource
by the requesting user agent.
Nevertheless, its security properties allow SCS to be used whenever
the privacy and integrity of cookies is a concern, by paying an
affordable price in terms of increased cookie size, additional CPU
clock cycles needed by the symmetric key encryption and HMAC
algorithms, and related key management, which can be made a nearly
transparent task.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on April 11, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. SCS Protocol . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. SCS Cookie Description . . . . . . . . . . . . . . . . . . 5
3.1.1. ATIME . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.2. DATA . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1.3. TID . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.4. IV . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.5. AUTHTAG . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Crypto Transform . . . . . . . . . . . . . . . . . . . . . 8
3.2.1. Cipher Set . . . . . . . . . . . . . . . . . . . . . . 8
3.2.2. Compression . . . . . . . . . . . . . . . . . . . . . 9
3.2.3. Cookie Encoding . . . . . . . . . . . . . . . . . . . 9
3.2.4. Outbound Transform . . . . . . . . . . . . . . . . . . 9
3.2.5. Inbound Transform . . . . . . . . . . . . . . . . . . 10
3.3. PDU Exchange . . . . . . . . . . . . . . . . . . . . . . . 11
3.3.1. Cookie Attributes . . . . . . . . . . . . . . . . . . 12
3.3.1.1. Expires . . . . . . . . . . . . . . . . . . . . . 12
3.3.1.2. Max-Age . . . . . . . . . . . . . . . . . . . . . 12
3.3.1.3. Domain . . . . . . . . . . . . . . . . . . . . . . 12
3.3.1.4. Secure . . . . . . . . . . . . . . . . . . . . . . 12
4. Key Management and Session State . . . . . . . . . . . . . . . 12
5. Cookie Size Considerations . . . . . . . . . . . . . . . . . . 14
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
8.1. Security of the Cryptographic Protocol . . . . . . . . . . 15
8.2. Impact of the SCS Cookie Model . . . . . . . . . . . . . . 15
8.2.1. Old cookie replay . . . . . . . . . . . . . . . . . . 15
8.2.2. Cookie Deletion . . . . . . . . . . . . . . . . . . . 17
8.2.3. Cookie Sharing or Theft . . . . . . . . . . . . . . . 17
8.2.4. Session Fixation . . . . . . . . . . . . . . . . . . . 17
8.3. Advantages of SCS over Server-side Sessions . . . . . . . 18
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1. Normative References . . . . . . . . . . . . . . . . . . . 18
9.2. Informative References . . . . . . . . . . . . . . . . . . 19
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 19
A.1. No Compression . . . . . . . . . . . . . . . . . . . . . . 19
A.2. Use Compression . . . . . . . . . . . . . . . . . . . . . 20
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
SCS is a small cryptographic protocol layered on top of the HTTP
cookie facility [RFC6265], that allows its users to produce and
consume authenticated and encrypted cookies, as opposed to usual
cookies, which are un-authenticated and sent in clear text.
By having a non-tamperable proof of authorship attached, each SCS
cookie can always be validated by the originator, making it possible
for a server to handle clients' session state without the need to
store it locally. In fact, an SCS enabled server could completely
delegate the application state storage to the client (e.g. a web
browser) and use it, in all respects, as a remote storage device.
The result of the cryptographic transformations applied to state data
can be used to ensure that its information authenticity and
confidentiality attributes are the same as if they were stored
privately on server-side.
The no-storage requirement, which is the key design constraint of
SCS, makes it an ideal candidate in the following settings:
a. devices with little or no storage -- typically embedded devices
which provide functionality such as software updates,
configuration, device monitoring, etc. via an HTTP interface;
b. web applications with high availability or load balancing
requirements, which may delegate handling of the application
state to clients instead of using shared storage or forced
peering, to enhance overall parallelism.
It is worth noting that a peculiar difference between SCS, when used
in strict no-storage mode, and usual "server-side" cookie sessions
arises as soon as we carefully consider the roles of the playing
entities. In the "server-side" model, the server acts a triple role
as the "generator", the "owner", and the "verifier" of cookie
credentials. Instead, a server implementing SCS in no-storage mode,
acts the "generator" and "verifier" roles only -- the "owner" being
inapplicable for obvious reasons.
In all respects, the Server grants the custody of the generated
cookie to the Client, whose trust model needs to be taken into
consideration when designing applications that use SCS this way. The
consequences of such discrepancy (e.g. deliberate deletion of a
cookie, explicit privilege revocation, etc.) will be analyzed in
Section 8.2.
An SCS server can be implemented within a web application by means of
a user library that exposes the core SCS functionality and leaves
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explicit control over SCS cookies to the programmer, or
transparently, by hiding, for example, a "diskless session" facility
behind a generic session API abstraction. SCS implementers are free
to choose the model that best suites their needs.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. SCS Protocol
The SCS protocol defines:
o the SCS cookie structure and encoding (Section 3.1);
o the cryptographic transformations involved in SCS cookie creation
and verification (Section 3.2);
o the HTTP-based PDU exchange (Section 3.3).
o the underlying key management model (Section 4).
Note that the PDU is transmitted to the client as an opaque data
block, hence no interpretation nor validation is necessary. The
single requirement for client-side support of SCS is cookie
activation on the user agent. The origin server is sole actor
involved in the PDU manipulation process, which greatly simplifies
the crypto operations -- especially key management, which is usually
a pesky task.
In the following sections we assume S to be one or more
interchangeable HTTP server entities (e.g. a server pool in a load-
balanced or high-availability environment) and C to be the client
with a cookie-enabled browser, or any user agent with equivalent
capabilities.
3.1. SCS Cookie Description
S and C exchange a cookie (Section 3.3), whose cookie-value consists
of a sequence of adjacent non-empty values, each of which is the 'URL
and Filename safe' Base-64 encoding [RFC4648] of a specific SCS
field.
(Hereafter the encoded and raw versions of each SCS field are
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distinguished based on the presence, or lack thereof, of the 'e'
prefix in their name, e.g. eATIME and ATIME.)
Each SCS field is separated by its left and/or right sibling by means
of the %x7c ASCII character (i.e. '|'), as follows:
scs-cookie = scs-cookie-name "=" scs-cookie-value
scs-cookie-name = token
scs-cookie-value = eDATA "|" eATIME "|" eTID "|" eIV "|" eAUTHTAG
eDATA = 1*base64url-character
eATIME = 1*base64url-character
eTID = 1*base64url-character
eIV = 1*base64url-character
eAUTHTAG = 1*base64url-character
Figure 1
Confidentiality is limited to the application state information (i.e.
the DATA field), while integrity and authentication apply to the
entire cookie-value.
The following subsections describe the syntax and semantics of each
SCS cookie field.
3.1.1. ATIME
Absolute timestamp relating to the last read or write operation
performed on session DATA, encoded as a HEX string holding the number
of seconds since UNIX epoch (i.e. since 00:00:00, Jan 1 1970.)
This value is updated with each client contact and is used to
identify expired sessions. If the delta between the received ATIME
value and the current time on S, is larger than a predefined
"session_max_age" (which is chosen by S as an application-level
parameter), a session is considered to be no longer valid, and is
therefore rejected.
Such an expiration error may be used to force user logout from an SCS
cookie based session, or hooked in the web application logics to the
display of a HTML form asking re-validation of user credentials.
3.1.2. DATA
Block of encrypted and optionally compressed data, possibly
containing the current session state. Note that no restriction is
imposed on clear text structure: the protocol is completely agnostic
as to inner data layout.
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Generally speaking, the plain text is the "normal" cookie that would
have been exchanged by S and C if SCS wasn't used.
3.1.3. TID
This identifier is equivalent to a SPI in a Data Security SA
[RFC3740]) and consists of an ASCII string that uniquely identifies
the transform set (keys and algorithms) used to generate this SCS
cookie.
SCS assumes that a key-agreement/distribution mechanism exists for
environments in which S consists of multiple servers, which provides
a unique external identifier for each transform set shared amongst
pool members.
Please note that the said mechanism may safely downgrade to a
periodic key-refresh in case there is one only server in the pool and
key is generated in place -- i.e. it is not handled from an external
source.
3.1.4. IV
Initialization Vector used for the encryption algorithm
(Section 3.2).
In order to avoid providing correlation information to a possible
attacker with access to a sample of SCS cookies created using the
same TID, the IV MUST be created randomly for each SCS cookie.
3.1.5. AUTHTAG
Authentication tag based on the plain string concatenation of the
base64url encoded DATA, ATIME, TID and IV fields, framed by the "|"
separator (see also the definition of the Concat() function in
Section 3.2):
AUTHTAG = HMAC(base64url(DATA) "|"
base64url(ATIME) "|"
base64url(TID) "|"
base64url(IV))
Note that, from a cryptographic point of view, the "|" character
provides explicit authentication of the length of each supplied
field, which results in a robust countermeasure against splicing
attacks.
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3.2. Crypto Transform
SCS could potentially use any combination of primitives capable of
performing authenticated encryption. In practice an encrypt-then-mac
approach [Kohno] with CBC-mode encryption and HMAC [RFC2104]
authentication was chosen.
The two algorithms MUST be associated with two independent keys.
The following conventions will be used in the algorithm description
(Section 3.2.4 and Section 3.2.5):
o Enc/Dec(): block encryption/decryption functions (Section 3.2.1);
o HMAC(): authentication function (Section 3.2.1);
o Comp/Uncomp(): compression/decompression functions
(Section 3.2.2);
o e/d(): cookie value encoding/decoding functions (Section 3.2.3);
o RAND(): random number generator [RFC4086];
o Concat(): string concatenation function. It takes an arbitrary
number of base64url encoded strings and returns the string
obtained by juxtaposing each of the inputs in the exact order in
which they are listed, separated by the "|" char. Example:
Concat("akxI", "MTM", "Hadvo") = "akxI|MTM|Hadvo".
Note that using "|" as the framing byte in the Concat() function
is arbitrary: any symbol with empty intersection with the
base64url alphabet is safe to be used (as long as it is allowed by
the cookie-value ABNF in [RFC6265]).
3.2.1. Cipher Set
Implementors MUST support at least the following algorithms:
o AES-CBC-128 for encryption;
o HMAC-SHA1 with a 128 bit key for authenticity and integrity,
which appear to be sufficiently secure in a wide range of use cases
[Bellare], are widely available, and can be implemented in a few
kilobytes of memory, providing an extremely valuable feature in
constrained devices.
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One should consider using larger cryptographic key lengths (192 or
256 bit) according to the actual security and overall system
performance requirements.
3.2.2. Compression
Compression, which may be useful or even necessary when handling
large quantities of data, is not compulsory (in such case Comp/Uncomp
are replaced by an identity matrix). If this function is enabled,
DEFLATE [RFC1951] format MUST be supported.
Some advice to SCS users: compression should not be enabled when
handling relatively short and entropic state such as pseudo random
session identifiers. Instead, large and quite regular state blobs
could get a significant boost when compressed.
3.2.3. Cookie Encoding
SCS cookie values MUST be encoded using the URL and filename safe
alphabet (i.e. base64url) defined in section 5 of Base-64 [RFC4648].
This encoding is very wide-spread, falls smoothly into the encoding
rules defined in Section 4.1.1 of [RFC6265], and can be safely used
to supply SCS based authorization tokens within an URI (e.g. in a
query string or straight into a path segment).
3.2.4. Outbound Transform
The output data transformation as seen by the server (the only actor
which explicitly manipulates SCS cookies) is illustrated by the
pseudo-code in Figure 2.
1. IV := RAND()
2. ATIME := NOW
3. DATA := Enc(Comp(plain-text-cookie-value), IV)
4. AUTHTAG := HMAC(Concat(e(DATA), e(ATIME), e(TID), e(IV)))
Figure 2
A new Initialization Vector is randomly picked (step 1.). As
previously mentioned (Section 3.1.4) this step is necessary to avoid
providing correlation information to an attacker.
A new ATIME value is taken as the current timestamp according to the
server clock (step 2.).
Since the only user of the ATIME field is the server, it is
unnecessary for it to be synchronized with the client -- though it
needs to use a fairly stable clock. However, if multiple servers are
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active in a load-balancing configuration, clocks SHOULD be
synchronized to avoid errors in the calculation of session expiry.
The plain text cookie value is then compressed (if needed) and
encrypted by using the key-set identified by TID (step 3.).
If the length of (compressed) state is not a multiple of the block
size, its value MUST be filled with as many padding bytes of equal
value as the pad length -- as defined by the scheme given in Section
6.3 of [RFC5652].
Then the authentication tag, which encompasses each SCS field (along
with lengths, and relative positions) is computed by HMAC'ing the
"|"-separated concatenation of their base64url representations using
the key-set identified by TID (step 4.).
Finally the SCS cookie-value is created as follows:
scs-cookie-value = Concat(e(DATA), e(ATIME), e(TID), e(IV),
e(tag))
3.2.5. Inbound Transform
The inbound transformation is described in Figure 3. In it, each of
the 'e'-prefixed names has to be interpreted as the base64url encoded
value of the corresponding SCS field.
0. If (split_fields(scs-cookie-value) == ok)
1. tid' := d(eTID)
2. If (tid' is available)
3. tag' := d(eAUTHTAG)
4. tag := HMAC(Concat(eDATA, eATIME, eTID, eIV))
5. If (tag = tag')
6. atime' := d(eATIME)
7. If (NOW - atime' <= session_max_age)
8. iv' := d(eIV)
data' := d(eDATA)
9. state := Uncomp(Dec(data', iv'))
10. Else discard PDU
11. Else discard PDU
12. Else discard PDU
13. Else discard PDU
Figure 3
First of all, the inbound scs-cookie-value is broken into its
component fields which MUST be exactly 5, and each at least of the
minimum length specified in Figure 1 (step 0.). In case any of these
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preliminary checks fails, the PDU is discarded (step 13.); else TID
is decoded to allow key-set lookup (step 1.).
If the cryptographic credentials (encryption and authentication
algorithms and keys identified by TID) are unavailable (step 12.),
the inbound SCS cookie is discarded since its value has no chance to
be interpreted correctly. This may happen for several reasons: e.g.,
if a device without storage has been reset and loses the credentials
stored in RAM, if a server pool node desynchronizes, or in case of a
key compromise that forces the invalidation of all current TID's,
etc.
When a valid key-set is found (step 2.), the AUTHTAG field is decoded
(step 3.) and the (still) encoded DATA, ATIME, TID and IV fields are
supplied to the primitive that computes the authentication tag (step
4.).
If the tag computed using the local key-set matches the one carried
by the supplied SCS cookie, we can be confident that the cookie
carries authentic material; otherwise the SCS cookie is discarded
(step 11.).
Then the age of the SCS cookie (as deduced by ATIME field value and
current time provided by the server clock) is decoded and compared to
the maximum time-to-live defined by the session_max_age parameter.
In case the "age" check is passed, the DATA and IV fields are finally
decoded (step 8.), so that the original plain text data can be
extracted from the encrypted and optionally compressed blob (step
9.).
Note that steps 5. and 7. allow any altered packets or expired
sessions to be discarded, hence avoiding unnecessary state decryption
and decompression.
3.3. PDU Exchange
SCS can be modeled in the same manner as a typical store-and-forward
protocol, in which the endpoints are S, consisting of one or more
HTTP servers, and the client C, an intermediate node used to
"temporarily" store the data to be successively forwarded to S.
In brief, S and C exchange an immutable cookie data block
(Section 3.1): the state is stored on the client at the first hop and
then restored on the server at the second, as in Figure 4.
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1. dump-state:
S --> C
Set-Cookie: ANY_COOKIE_NAME=KrdPagFes_5ma-ZUluMsww|MTM0...
Expires=...; Path=...; Domain=...;
2. restore-state:
C --> S
Cookie: ANY_COOKIE_NAME=KrdPagFes_5ma-ZUluMsww|MTM0...
Figure 4
3.3.1. Cookie Attributes
In the following sub paragraphs a series of recommendations is
provided in order to maximize SCS PDU fitness in the generic cookie
ecosystem.
3.3.1.1. Expires
SCS cookies MUST include an Expires attribute which shall be set to a
value consistent with session_max_age.
For maximum compatibility with existing user agents the timestamp
value MUST be encoded in rfc1123-date format which requires a 4-digit
year.
3.3.1.2. Max-Age
Since not all UAs support this attribute, it MUST NOT be present in
any SCS cookie.
3.3.1.3. Domain
SCS cookies MUST include a Domain attribute compatible with
application usage.
A trailing '.' MUST NOT be present in order to minimize the
possibility of a user agent ignoring the attribute value.
3.3.1.4. Secure
This attribute MUST always be asserted when SCS sessions are carried
over a TLS channel.
4. Key Management and Session State
This specification provides some common recommendations and practices
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relevant to cryptographic key management.
In the following, the term 'key' references both encryption and HMAC
keys.
o The key SHOULD be generated securely following the randomness
recommendations in [RFC4086];
o the key SHOULD only be used to generate and verify SCS PDUs;
o the key SHOULD be replaced regularly as well as any time the
format of SCS PDUs or cryptographic algorithms changes.
Furthermore, to preserve the validity of active HTTP sessions upon
renewal of cryptographic credentials (whenever the value of TID
changes), an SCS server MUST be capable of managing at least two
transforms contemporarily: the currently instantiated one, and its
predecessor.
Each transform set SHOULD be associated with an attribute pair:
"refresh" and "expiry", which is used to identify the exposure limits
(in terms of time or quantity of encrypted and/or authenticated
bytes, etc) of related cryptographic material.
In particular, the "refresh" attribute specifies the time limit for
substitution of transform set T with new material T'. From that
moment onwards, and for an amount of time determined by "expiry", all
new sessions will be created using T', while the active T-protected
ones go through a translation phase in which:
o the inbound transformation authenticates and decrypts/decompresses
using T (identified by TID);
o the outbound transformation encrypts/compresses and authenticates
using T'.
T' {not valid yet} |---------------------|----------------
| translation stage |
T ----------------|---------------------| {no longer valid}
refresh refresh + expiry
Figure 5
As shown in Figure 5, the duration of the HTTP session MUST fit
within the lifetime of a given transform set (i.e. from creation time
until "refresh" + "expiry").
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In practice, this should not be an obstacle because the longevity of
the two entities (HTTP session and SCS transform set) should differ
by one or two orders of magnitude.
An SCS server may take this into account by determining the duration
of a session adaptively according to the expected deletion time of
the active T, or by setting the "expiry" value to at least the
maximum lifetime allowed by an HTTP session.
Since there is only one refresh attribute also in situations with
more than one key (e.g. one for encryption and one for
authentication) within the same T, the smallest value is chosen.
5. Cookie Size Considerations
In general, SCS cookies are bigger than their plain text
counterparts. This is due to a couple of different factors:
o inflation of the Base-64 encoding of the state data (approx. 1.4
times the original size, including the encryption padding), and
o the fixed size increment (approx. 80/90 bytes) due to SCS fields
and framing overhead.
While the former is a price the user must always pay proportionally
to the original data size, the latter is a fixed quantum, which can
be huge on small amounts of data, but is quickly absorbed as soon as
data becomes big enough.
The following table compares byte lengths of SCS cookies (with a four
bytes' TID) and corresponding plain text cookies in a worst case
scenario, i.e. when no compression is in use (or applicable).
plain | SCS
-------+-------
11 | 128
102 | 256
285 | 512
651 | 1024
1382 | 2048
2842 | 4096
The largest uncompressed cookie value that can be safely supplied to
SCS is about 2.8KB.
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6. Acknowledgements
We would like to thank Jim Schaad, David Wagner and Lorenzo Cavallaro
for their valuable feedback on this document.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
8.1. Security of the Cryptographic Protocol
From a cryptographic architecture perspective, the described
mechanism can be easily traced to an "encode then encrypt then MAC"
scheme (Encode-then-EtM) as described in [Kohno].
Given a "provably-secure" encryption scheme and MAC (as for the
algorithms mandated in Section 3.2.1), Kohno et al. [Kohno]
demonstrate that their composition results in a secure authenticated
encryption scheme.
8.2. Impact of the SCS Cookie Model
The fact that the server does not own the cookie it produces, gives
rise to a series of consequences that must be clearly understood when
one envisages the use of SCS as a cookie provider and validator for
his/her application.
In the following paragraphs, a set of different attack scenarios
(together with corresponding countermeasures where applicable) are
identified and analyzed.
8.2.1. Old cookie replay
SCS doesn't address replay of old cookie values.
In fact, there is nothing that guarantees an SCS application about
the client having returned the most recent version of the cookie.
As with "server-side" sessions, if an attacker gains possession of a
given user's cookies - via simple passive interception or another
technique - he/she will always be able to restore the state of an
intercepted session by representing the captured data to the server.
The ATIME value along with the session_max_age configuration
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parameter allow SCS to mitigate the chances of an attack (by forcing
a time window outside of which a given cookie is no longer valid),
but cannot exclude it completely.
A countermeasure against the "passive interception and replay"
scenario can be applied at transport/network level using the anti-
replay services provided by e.g., SSL/TLS [RFC5246] or IPsec
[RFC4301].
Anyway, a generic solution is still out of scope: an SCS application
wishing to be replay-resistant must put in place some ad hoc
mechanism to prevent clients (both rogue and legitimate) from (a)
being able to replay old cookies as valid credentials and/or (b)
getting any advantage by replaying them.
In the following, some typical use cases are illustrated:
o Session inactivity timeout scenario (implicit invalidation): use
the session_max_age parameter if a global setting is viable, else
place an explicit TTL in the cookie (e.g.
validity_period="start_time, duration") that can be verified by
the application each time the Client presents the SCS cookie.
o Session voidance scenario (explicit invalidation): put a randomly
chosen string into each SCS cookie (cid="$(random())") and keep a
list of valid session cid's against which the SCS cookie presented
by the client can be checked. When a cookie needs to be
invalidated, delete the corresponding cid from the list. The
described method has the drawback that, in case a non-permanent
storage is used to archive valid cid's, a reboot/restart would
invalidate all sessions (It can't be used when |S| > 1).
o One-shot transaction scenario (ephemeral): this is a variation on
the previous theme when sessions are consumed within a single
request/response. Put a nonce="$(random())" within the state
information and keep a list of not-yet-consumed nonces in RAM.
Once the client presents its cookie credential, the embodied nonce
is deleted from the list and will be therefore discarded whenever
replayed.
It may be noteworthy that despite the chances of preventing replay in
some well established circumstances by using aforementioned
mechanisms, if the attacker is able to use the cookie before the
legitimate client gets a chance to, then the impersonation attack
will always succeed.
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8.2.2. Cookie Deletion
A direct, and important, consequence of the missing owner role in SCS
is that a client could intentionally delete its cookie and return
nothing.
The application protocol has to be designed so there is no incentive
to do so, for instance:
o it is safe for the cookie to represent some kind of positive
capability - the possession of which increases the client's
powers;
o It is not safe to use the cookie to represent negative
capabilities - where possession reduces the client's powers-, or
for revocation.
Note that this behavior is not equivalent to cookie removal in the
"server-side" cookie model, because in case of missing cookie backup
by other parties (e.g. the application using SCS), the Client could
simply make it disappear once and for all.
8.2.3. Cookie Sharing or Theft
Just like with plain cookies, SCS doesn't prevent sharing (both
voluntary and illegitimate) of cookies between multiple clients.
In the context of voluntary cookie sharing, using HTTPS is useless:
Client certificates are just as shareable as cookies, hence
equivalently to the "server-side" cookie model, there seems to be no
way to prevent this threat.
The theft could be mitigated by securing the wire (e.g. via HTTPS,
IPsec, VPN, ...), thus reducing the opportunity of cookie stealing to
a successful attack on the protocol endpoints.
8.2.4. Session Fixation
Session fixation vulnerabilities [Kolsec] are not addressed by SCS.
A more sophisticated protocol involving an active participation by
the UA in the SCS cookie manipulation would be needed: e.g. some form
of challange-response exchange initiated by the Server on the HTTP
response and replied by the UA on the next chained HTTP request.
Unfortunately the present specification which bases on [RFC6265] sees
the UA as a completely passive character, whose role is to blindly
paste the cookie value set by the Server.
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Nevertheless, the SCS cookies wrapping mechanism may be used in the
future as a building block for a more robust HTTP state management
protocol.
8.3. Advantages of SCS over Server-side Sessions
Note that all the above-mentioned vulnerabilities also apply to plain
cookies, making SCS at least as secure, but there are a few good
reasons to consider its security level enhanced.
First of all, the confidentiality and authentication features
provided by SCS protects the cookie-value which is normally plain
text and tamperable.
Furthermore, none of the common vulnerabilities of server-side
sessions (SID prediction, SID brute forcing) can be exploited when
using SCS, unless the attacker possesses encryption and HMAC keys
(both current ones and those relating to the previous set of
credentials).
More generally no slicing nor altering operations can be done over an
SCS PDU without controlling the cryptographic key-set.
9. References
9.1. Normative References
[RFC1951] Deutsch, P., "DEFLATE Compressed Data Format Specification
version 1.3", RFC 1951, May 1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, October 2006.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.
[RFC6265] Barth, A., "HTTP State Management Mechanism", RFC 6265,
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April 2011.
9.2. Informative References
[Bellare] Bellare, M., "New Proofs for NMAC and HMAC: Security
Without Collision-Resistance", 2006.
[Kohno] Kohno, T., Palacio, A., and J. Black, "Building Secure
Cryptographic Transforms, or How to Encrypt and MAC",
2003.
[Kolsec] Kolsec, M., "Session Fixation Vulnerability in Web-based
Applications", 2002.
[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security
Architecture", RFC 3740, March 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
Appendix A. Examples
The examples in this section have been created using the 'scs' test
tool bundled with LibSCS, a free and opensource reference
implementation of the SCS protocol that can be found at
<http://github.com/koanlogic/libscs>.
A.1. No Compression
The following parameters:
o Plain text cookie: "a state string"
o AES-CBC-128 key: "123456789abcdef"
o HMAC-SHA1 key: "12345678901234567890"
o TID: "tid"
o ATIME: 1347265955
o IV:
\xb4\xbd\xe5\x24\xf7\xf6\x9d\x44\x85\x30\xde\x9d\xb5\x55\xc9\x4f
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produce the following tokens:
o DATA: DqfW4SFqcjBXqSTvF2qnRA
o ATIME: MTM0NzI2NTk1NQ
o TID: OHU7M1cqdDQt
o IV: tL3lJPf2nUSFMN6dtVXJTw
o AUTHTAG: AznYHKga9mLL8ioi3If_1iy2KSA
A.2. Use Compression
The same parameters as above, except ATIME and IV:
o Plain text cookie: "a state string"
o AES-CBC-128 key: "123456789abcdef"
o HMAC-SHA1 key: "12345678901234567890"
o TID: "tid"
o ATIME: 1347281709
o IV:
\x1d\xa7\x6f\xa0\xff\x11\xd7\x95\xe3\x4b\xfb\xa9\xff\x65\xf9\xc7
produce the following tokens:
o DATA: PbE-ypmQ43M8LzKZ6fMwFg-COrLP2l-Bvgs
o ATIME: MTM0NzI4MTcwOQ
o TID: akxIKmhbMTE8
o IV: HadvoP8R15XjS_up_2X5xw
o AUTHTAG: A6qevPr-ugHQChlr_EiKYWPvpB0
In both cases, the resulting SCS cookie is obtained via ordered
concatenation of the produced tokens, as described in Section 3.1.
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Authors' Addresses
Stefano Barbato
KoanLogic
Via Marmolada, 4
Vitorchiano (VT), 01030
Italy
Email: tat@koanlogic.com
Steven Dorigotti
KoanLogic
Via Maso della Pieve 25/C
Bolzano, 39100
Italy
Email: stewy@koanlogic.com
Thomas Fossati (editor)
KoanLogic
Via di Sabbiuno 11/5
Bologna, 40136
Italy
Phone: +39 051 644 82 68
Email: tho@koanlogic.com
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