| Internet Engineering Task Force | S. Barbato |
| Internet-Draft | S. Dorigotti |
| Intended status: Informational | T. Fossati, Ed. |
| Expires: June 05, 2013 | KoanLogic |
| December 02, 2012 |
SCS: Secure Cookie Sessions for HTTP
draft-secure-cookie-session-protocol-09
This memo defines a generic URI and HTTP header friendly envelope for carrying symmetrically encrypted, authenticated, and origin-timestamped tokens. It also describes one possible usage of such tokens via a simple protocol based on HTTP cookies.
SCS use cases cover a wide spectrum of applications, ranging from distribution of authorized content via HTTP (e.g. with out-of-band signed URIs), to securing browser sessions with diskless embedded devices (e.g. SOHO routers), or web servers with high availability or load balancing requirements that may want to delegate the handling of the application state to clients instead of using shared storage or forced peering.
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Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
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This memo defines a generic URI and HTTP header friendly envelope for carrying symmetrically encrypted, authenticated, and origin-timestamped tokens.
It is generic in that it does not force any specific format upon the authenticated information - which makes SCS tokens flexible, easy, and secure to use in many different scenarios.
It is URI and HTTP header friendly, as it has been explicitly designed to be compatible with both the ABNF "token" syntax [RFC2616] (the one used for e.g. Set-Cookie and Cookie headers), and the path or query syntax of HTTP URIs.
This memo also describes one possible usage of such tokens via a simple protocol based on HTTP cookies that allows to establish "client mode" sessions. This is not their sole possible usage, and while no other operational patterns are outlined here, it is expected that SCS tokens may be easily employed as a building block for other kind of HTTP based applications that need to carry in-band secured information.
When SCS tokens are used to implement client mode cookie sessions, the SCS implementer must fully understand the security implications entailed by the act of delegating the whole application state to the client (browser). In this regard, some hopefully useful security considerations have been collected in Section 8.2. Please note that they may not cover all possible scenarios though, and must therefore be weighed carefully against the specific application threat model.
An SCS server may be implemented within a web application by means of a user library that exposes the core SCS functionality and leaves explicit control over SCS tokens to the programmer, or transparently, for example by hiding a "diskless session" facility behind a generic session API abstraction. SCS implementers are free to choose the model that best suits their needs.
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].
The SCS protocol defines:
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 the 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.
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 distinguished based on the presence, or lack thereof, of the 'e' prefix in their name, e.g. eATIME and ATIME.)
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
Each SCS field is separated by its left and/or right sibling by means of the %x7c ASCII character (i.e. '|'), as follows:
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.
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 the 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 expiration error may be used to force user logout from an SCS cookie based session, or hooked in the web application logic to display an HTML form requiring re-validation of user credentials.
Block of encrypted and optionally compressed data, possibly containing the current session state. Note that no restriction is imposed on the clear text structure: the protocol is completely agnostic as to inner data layout.
Generally speaking, the plain text is the "normal" cookie that would have been exchanged by S and C if SCS had not been used.
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.
Such mechanism may safely downgrade to a periodic key-refresh if there is only one server in the pool and the key is generated in place - i.e. it is not handled by an external source.
However, when many servers act concurrently upon the same pool, a more sophisticated protocol, whose specification is out of the scope of the present document, must be devised (ideally one that is able to handle key agreement for dynamic peer groups in a secure and efficient way, e.g. [CLIQUES], [Steiner]).
Initialization Vector used for the encryption algorithm (see 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.
AUTHTAG = HMAC(base64url(DATA) "|"
base64url(ATIME) "|"
base64url(TID) "|"
base64url(IV))
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 Box() function in Section 3.2):
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 3.2.5 and Section 3.2.6):
Note that that the adoption of "|" as the framing symbol in the Box() function is arbitrary: any char allowed by the cookie-value ABNF in [RFC6265] is safe to be used as long it has empty intersection with the base64url alphabet.
It is also worth noting that the role of the framing symbol, which provides an implicit length indicator for each of the atoms, is key to the correctness and security of SCS.
This is especially relevant when the authentication tag is computed (see Section 3.1.5). More specifically, the explicit inclusion of the framing symbol within the HMAC input seals the integrity of the blob as a whole together with each of its composing atoms in their exact position.
This feature makes the protocol robust against attacks aimed at disrupting the security of SCS PDUs by freely moving boundaries between adjacent atoms.
Implementors MUST support at least the following algorithms: [Bellare], [RFC6194], are widely available, and can be implemented in a few kilobytes of memory, providing an extremely valuable feature for constrained devices.
which appear to be sufficiently secure in a broad range of use cases
One should consider using larger cryptographic key lengths (192 or 256 bit) according to the actual security and overall system performance requirements.
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.
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 a URI (e.g. in a query string or straight into a path segment).
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(Box(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 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:
The inbound transformation is described in Figure 3. Each of the 'e'-prefixed names shown 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(Box(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 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.
If the "age" check passes, 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.
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.
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
In the following sub paragraphs a series of recommendations is provided in order to maximize SCS PDU fitness in the generic cookie ecosystem.
If an SCS cookie includes an Expires attribute, then the attribute MUST 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.
Since not all UAs support this attribute, it MUST NOT be present in any SCS cookie.
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.
This attribute MUST always be asserted when SCS sessions are carried over a TLS channel.
This attribute SHOULD always be asserted.
This specification provides some common recommendations and practices relevant to cryptographic key management.
In the following, the term 'key' references both encryption and HMAC keys.
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:
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").
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.
It is critical for the correctness of the protocol that, in case multiple equivalent SCS servers are used in a pool, all of them share the same view of time (see also Section 3.2.5) and keying material.
As far as the latter is concerned, SCS does not mandate the use of any specific key sharing mechanism, and will keep working correctly as long as the said mechanism is able to provide a single, coherent, view of the keys shared by pool members - while conforming to the recommendations given in this section.
In general, SCS cookies are bigger than their plain text counterparts. This is due to the following reasons:
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.
plain | SCS
-------+-------
11 | 128
102 | 256
285 | 512
651 | 1024
1382 | 2048
2842 | 4096
The following table compares byte lengths of SCS cookies (with a four byte TID) and corresponding plain text cookies in a worst case scenario, i.e. when no compression is in use (or applicable).
We would like to thank Jim Schaad, David Wagner, Lorenzo Cavallaro, Willy Tarreau, Tobias Gondrom, John Michener, Sean Turner, Barry Leiba, Robert Sparks, Stephen Farrell, Stewart Bryant, and Nevil Brownlee for their valuable feedback on this document.
This memo includes no request to IANA.
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 3.2.2), Kohno et al. [Kohno] demonstrate that their composition results in a secure authenticated encryption scheme.
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.
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 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].
A native solution is not in scope with the security properties inherent to an SCS cookie. Hence, 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:
It is worth noting that in all but the latter scenario, if an attacker is able to use the cookie before the legitimate Client gets a chance to, then the impersonation attack will always succeed.
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:
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.
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 only as a separate secure transport provider is useless: in fact, Client certificates are just as shareable as cookies. Instead, using some form of secure channel binding (as illustrated in Section 8.2.1) may cancel this risk.
The risk of 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.
In order to reduce the attack window on stolen cookies, an application may choose to generate cookies whose lifetime is upper bounded by the browsing session lifetime (i.e. by not attaching an Expires attribute to them.)
Session fixation vulnerabilities [Kolsec] are not addressed by SCS.
A more sophisticated protocol involving active participation of the UA in the SCS cookie manipulation process would be needed: e.g. some form of challange-response exchange initiated by the Server in the HTTP response and replied to by the UA in the next chained HTTP request.
Unfortunately the present specification which is based on [RFC6265] sees the UA as a completely passive actor, whose role is to blindly paste the cookie value set by the Server.
Nevertheless, the SCS cookies wrapping mechanism may be used in the future as a building block for a more robust HTTP state management protocol.
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 protect the cookie-value which is normally plain text and tamperable.
Furthermore, neither of the common vulnerabilities of server-side sessions (SID prediction and 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 in general, no slicing nor altering operations can be done over an SCS PDU without controlling the cryptographic key-set.
| [RFC5246] | Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008. |
| [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. |
| [RFC5056] | Williams, N., "On the Use of Channel Bindings to Secure Channels", RFC 5056, November 2007. |
| [Bellare] | Bellare, M.B., "New Proofs for NMAC and HMAC: Security Without Collision-Resistance", 2006. |
| [Kohno] | Kohno, T.K., Palacio, A.P. and J.B. Black, "Building Secure Cryptographic Transforms, or How to Encrypt and MAC", 2003. |
| [Kolsec] | Kolsec, M.K., "Session Fixation Vulnerability in Web-based Applications", 2002. |
| [Steiner] | Steiner, M., Tsudik, G. and M. Waidner, "Diffie-Hellman Key Distribution Extended to Group Communication", 1996. |
| [CLIQUES] | Steiner, M., Tsudik, G. and M. Waidner, "Cliques: A New Approach to Group Key Agreement", 1996. |
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.
The following parameters:
produce the following tokens:
The same parameters as above, except ATIME and IV:
produce the following tokens:
In both cases, the resulting SCS cookie is obtained via ordered concatenation of the produced tokens, as described in Section 3.1.