Network Working Group N. Williams
Internet-Draft Cryptonector
Intended status: Standards Track January 1, 2013
Expires: July 5, 2013
Hypertext Transport Protocol (HTTP) Session Continuation Protocol
draft-williams-websec-session-continue-proto-00
Abstract
One of the most often talked about problems in web security is
"cookies". Web cookies are a method of associating requests with
"sessions" that may have been authenticated somehow. Cookies are a
form of bearer token that leave much to be desired. This document
proposes a session "continuation" protocol for HyperText Transport
Protocol (HTTP).
Status of this Memo
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This Internet-Draft will expire on July 5, 2013.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 3
2. Session Keying . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Mixing HTTP and HTTPS . . . . . . . . . . . . . . . . . . 4
2.2. Authenticated Session Keying . . . . . . . . . . . . . . . 4
2.2.1. HTTP/Negotiate Session Keying . . . . . . . . . . . . . . 5
2.2.2. Digest . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.3. Unauthenticated Session Keying . . . . . . . . . . . . . . 5
3. Session Initiation . . . . . . . . . . . . . . . . . . . . 6
4. Session Scope: Sharing Sessions Across Servers . . . . . . 9
5. Unauthenticated to Authenticated Session Upgrade . . . . . 10
6. Session Continuation . . . . . . . . . . . . . . . . . . . 11
6.1. Session Validation and Error Handling . . . . . . . . . . 11
6.2. Session Expiration and Renewal . . . . . . . . . . . . . . 12
6.3. Alternative: Define Session Scheme for WWW-Authenticate . 12
7. Logout . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8. Inquiring Session Status . . . . . . . . . . . . . . . . . 14
9. Analysis . . . . . . . . . . . . . . . . . . . . . . . . . 15
10. IANA Considerations . . . . . . . . . . . . . . . . . . . 16
11. Security Considerations . . . . . . . . . . . . . . . . . 17
12. References . . . . . . . . . . . . . . . . . . . . . . . . 18
12.1. Normative References . . . . . . . . . . . . . . . . . . . 18
12.2. Informative References . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . 20
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1. Introduction
The motivation for this protocol is described in
[I-D.williams-websec-session-continue-prob].
We define a protocol for cryptographic "session continuation" for
HyperText Transport Protocol (HTTP) [RFC2616]. Session continuation
is the act of binding an HTTP request to a "session". A "session"
consists of all the HTTP requests by a given user (possibly an
authenticated user, or possibly an anonymous user). This protocol is
a cryptographic protocol that aims to meet all the requirements given
in [I-D.williams-websec-session-continue-prob].
The protocol consists of:
o a request header carrying a keyed Message Authentication Code
(MAC) that proves possession of a shared session key (shared
between the user and the server);
o a response header advertising a default session scope to clients;
o a session identification in the form of a URI;
o an optional facility for server-side statelessness by storing
state on the client-side, encrypted in a secret key known to the
server;
o a request header for requesting the establishment of a session;
o a response header for indicating the establishment of a session,
and including a session URI and any optional state to be repeated
by the client.
1.1. Conventions used in this document
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].
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2. Session Keying
There are two methods for keying an HTTP session:
o session keys are output by HTTP authentication;
o or session keys are asserted by the client or the server;
For the key assertion method TLS with confidentiality protection is
clearly REQUIRED for security. We've considered the possibility of
using Diffie-Hellman key agreement or RSA key transport, but as that
would duplicate functionality that is in TLS we consider that out of
scope for the time being.
In either case a single session key is produced, and that is the only
key utilized. Having a single key helps reduce session state size
and protocol complexity, but we must (and do) distinguish key usage
by prefixing a purpose indicator to the MAC input.
When keys are output by HTTP authentication there may be a key length
mismatch. In this case a Key Derivation Function (KDF) [RFC5869] is
applied to generate the session key.
[[anchor1: We could also use the TLS extractor to generate keys, but
that would be an unnecessary complication and would provide very
little additional value. Channel binding is achieved per-RFCs 5056
and 5929.]]
[[anchor2: Should we have distinct session keys for request and
response MACs? Probably, but it increases state size. Better to add
a direction indicator to the MAC plaintext.]]
2.1. Mixing HTTP and HTTPS
We expect that many sites will continue to mix HTTP and HTTPS for
various reasons. To make this possible the MAC input will include a
marker indicating HTTP or HTTPS.
2.2. Authenticated Session Keying
When an HTTP client uses HTTP authentication, and the authentication
mechanism used can establish a session key, then the client SHOULD
request session initiation using a shared session key output by the
HTTP authentication mechanism. The client MUST send the session
initiation header concurrently with the last HTTP authentication
message.
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2.2.1. HTTP/Negotiate Session Keying
[[anchor3: Write text explaining how to use the GSS PRF to exchange
keys when using HTTP/Negotiate]]
2.2.2. Digest
[[anchor4: Digest could output a session key. Do we want to bother?
(Basic certainly can't, or we'd not want it to anyways.)]]
2.3. Unauthenticated Session Keying
Sessions for unauthenticated users may appear to make little sense at
first. This is useful, for example, and just as web cookies are, for
tracking "shopping carts" when a user is window shopping, so to
speak.
For unauthenticated session initiation the client merely requests the
creation of a session with an asserted session key, for lack of a
better choice.
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3. Session Initiation
Sessions are always initiated by the client by including a Session-
Init header in the client's request carrying the client's proposal
for a session.
Servers that support sessions will respond by creating a session and
returning a session ID URI.
The Session-Init proposal header's value consists of a comma-
separated list of proposal parameters:
session-param = token "=" ( token | quoted-string )
Session-Init = 1#session-param
Figure 1: Session-Init request header
The following session parameters are defined:
Key-Method The type of keying: "auth" (key will be output by HTTP
authentication), "c-assert" (key is asserted in this Session-Init)
or "s-assert" (the server is expected to assert a key). In the
"auth" case the Session-Init MUST also carry a nonce and a MAC.
This session-param MUST be present.
Key The key that the client asserts, if the client asserts a key
(Key-Method=c-assert). This may also be included when Key-Method
is "auth" in case the server's implementation of HTTP
authentication does not output a key, but only when using HTTPS.
Key-Length The length of the master session key, as a count of key
bits, in base-10. The value MUST NOT be less than 96 or larger
than 256. If absent the key length SHALL be 128 bits.
MAC-Algs The MAC algorithms supported by the client. This document
defines only "HMAC-SHA-1" (HMAC with SHA-1), "HMAC-SHA-1-96" (HMAC
with SHA-1 and truncation to 96 bits), "HMAC-SHA256" (HMAC with
SHA256), and "HMAC-SHA256-128" (HMAC with SHA256 and truncation to
128 bits). All of these use HMAC [RFC2104]. Clients and servers
MUST support HMAC-SHA-1-96 and HMAC-SHA256-128. If absent the
default value is "HMAC-SHA256-128".
KDF-Algs A list of KDF algorithms. This is needed only when Key-
Method is "auth". The following are specified here: "HKDF-SHA-1"
(HKDF [RFC5869] with SHA-1) and "HKDF-SHA256" (HKDF with SHA256).
Clients and servers MUST support HKDF-SHA256. If absent and Key-
Method is "auth" then the default value is HKDF-SHA256.
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Channel-Binding-Types A comma-separated list of channel binding
[RFC5056] types. Clients and servers MUST support 'tls-server-
end-point' [RFC5929] when using HTTPS. (Note the need to use
quoted-string when the list has more than one item.) If absent
the default is 'tls-server-end-point'.
Nonce A 128-bit nonce, base64-encoded. This session-param MUST be
present.
Previous-Session-URI The URI of a previous session. See Section 5.
Previous-Session-State The session state for the previous session,
if any. See Section 5.
Unprotected-Allowed If present the value MUST be "true", and
indicates that HTTP and HTTPS are both allowed for this session.
Otherwise only HTTPS is allowed for this session.
The server responds with a Session-Assign header:
session-params = 1#session-param
Session-Init-Value = <the value of the Session-Init header>
MAC-input = Session-Init-Value "," session-params
MAC = <base64-encoding of MAC taken over the MAC-input>
Session-Assign = session-params ["," MAC]
Figure 2: Session-Assign response header
The MAC is OPTIONAL when using HTTPS, REQUIRED otherwise.
The session-params for Session-Assign are:
Key-Method If the client requested "auth" as the key method but the
server's implementation of HTTP authentication could not output a
key then this session-param MUST be present with a value of
"c-assert" (if the client included a Key in its Session-Init) or
"s-assert" (otherwise).
Key The server-asserted key, if the client requested a server-
asserted key.
MAC-Alg The name of the MAC algorithm selected by the server from
the client's proposal (REQUIRED).
KDF-Alg The selected KDF algorithm (when the client's selected Key-
Method is "auth").
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URI The URI of the session (REQUIRED).
State Server-side state to be stored on the client (OPTIONAL). Note
that servers MAY choose to store server-side state in cookies
instead.
Previous-Session Indicates whether the previous session was
recognized and accepted ("accepted"), rejected ("rejected"), or
unknown ("unknown"). This session-param MUST be present when the
client's Session-Init had a Previous-Session-URI session-param.
Host-Scope A DNS domainname (in A-label form) that the session can
be used with. Multiple Scope parameters are allowed. If the
domainname starts with a '.' then the session may be used with all
server hosts whose domainnames are sub-domains of the given Host-
Scope domainname. The server's fully-qualified hostname is always
part of the session's host scope.
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4. Session Scope: Sharing Sessions Across Servers
A service might be composed of multiple related servers, each with a
different hostname. As a result the service may require a client to
use the same session across the service's component servers. We
provide a mechanism by which the server may indicate a set of such
servers to the client: the Host-Scope session-param in the server's
Session-Assign response header.
[XXX We need a way to constrain this for privacy protection reasons.
It's not yet clear how the client can judge which Host-Scope
paramters to accept or ignore, only that must be allowed to do so.]
To facilitate interoperable session sharing across heterogeneous
server implementations we define a session resource -named by its
session URI- that can be obtained with a properly-authenticated GET
by authorized entities. The session resource's representation is a
application/json document type, containing a JSON-serialized
associative array with the following REQUIRED keys:
Master-Key The session's master key, base64-encoded.
MAC-Alg The MAC algorithm for this session.
[[anchor5: We probably want each server to see a different master
key, in which case we probably want to use a KDF with the server's
hostname as part of the salt.]]
[[anchor6: We probably want to define some OPTIONAL keys for this
object, such as "User", "User-URI", "HTTP-Auth-Scheme-Used", "HTTP-
Auth-Scheme-<param>", and so on, as well as an application-specific
namespace of keys (e.g., "App-<appname>" or "<URN>").]]
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5. Unauthenticated to Authenticated Session Upgrade
A client might first establish an unauthenticated session then
authenticate the user later. When authentication is done the client
might wish to preserve any state associated with the preceding
unauthenticated session. The client does this by sending a Session-
Init at authentication time with a 'Previous-Session-URI' session-
param and, if there was server-assigned session state, a 'Previous-
Session-State' session-param.
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6. Session Continuation
Once a session is established the client binds requests to sessions
as described here.
There are two cases: HTTPS and HTTP. In both cases the client adds a
header
For the HTTPS case the client adds a "Session" header to its requests
with the following content: the session identifier assigned by the
server, a nonce generated by the client, and a MAC of the nonce and
the TLS channel bindings.
The value of the Session header consists of a base64-encoded 128-bit
nonce and a MAC, using the session's MAC algorithm, of the nonce and
the channel binding, each base64-encoded then concatenated in that
order:
CB = <base64-encoding of the channel bindings>
nonce = <base64-encoded 128-bit nonce>
new-state = ...
direction = "c2s" | "s2c"
prot-state = "protected" | "unprotected"
response-status = "" | "Invalid-MAC" |
| "Session-expired" "Session-unknown"
MAC-input = direction "," prot-state "," nonce
"," CB "," status "," [new-state]
MAC = <base64encoded MAC taken over MAC-input>
Session = nonce "," response-status "," [new-state] "," MAC
Figure 3: Session header
Where the response must carry a Session header, the form of the value
is the same as for requests.
The MAC is taken over a direction indicator, an indicator of whether
TLS is used, the nonce, the channel bindings, and so on, as shown in
Figure 3. Only the server may assert new session state, and only the
server indicates a response-status other than "" (empty string).
6.1. Session Validation and Error Handling
The receiver computes the same MAC using the sender's nonce (and new-
state, if present, when the receiver is the client) and compares the
resulting MAC to the MAC from the Session header.
If MAC validation of a request fails then the server MUST respond
with a 403 status code with a non-empty response-status int he
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Session-header. Error responses MUST include a Session header. If
403 response's Session header indicates "Invalid-MAC" then if the
client had used HTTPS then the client SHOULD warn the user, otherwise
the client SHOULD retry. If the 403 response's Session header
indicates "Session-expired" then the client SHOULD renew the session
(see Section 6.2). Otherwise the client must assume that the old
session has been destroyed (e.g., because of a logout or server state
data loss) and may establish a new session.
If MAC validation of a response fails the the client MUST act as
though a 400 (bad request) had been sent instead. If the request was
idempotent the client SHOULD retry, otherwise recovery is not
specified.
6.2. Session Expiration and Renewal
If the server decides that a session is no longer valid then the
server should respond with a 401 status code. The client should then
re-authenticate or establish a new unauthenticated session, using the
Previous-Session-URI and Previous-Session-State session-params of the
new Session-Init to indicate that the old session is being "renewed".
6.3. Alternative: Define Session Scheme for WWW-Authenticate
One possibility that has some appeal would be to define a new HTTP
authentication scheme called "Session" (say) and use that instead of
the "Session" header defined above. The primary advantage to the
WWW-Authenticate approach is that it fits the existing HTTP
authentication framework, allowing a server to present to an
application the user authentication information embedded in the
session state as if the user were re-authenticated in each request.
Session continuation can then be seen as a form of fast re-
authentication.
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7. Logout
To logout the client SHOULD perform a DELETE of the session URI.
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8. Inquiring Session Status
The client MAY do a GET of the session URI. The semantics of the
response body for this are not specified here. As explained in
Section 4, servers also may GET a session URI; see Section 4 for more
details.
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9. Analysis
Quite clearly this protocol meets requirements 1, 2, 3, 5, and 11
from [I-D.williams-websec-session-continue-prob].
The security requirements are also met:
requirement 4 The active cookie recovery attacks on TLS we consider
are adaptive chosen plaintext attacks. These attacks depend on
the cookies sent by the client being the same in every request.
This protocol uses MAC of at least channel bindings data (which
doesn't change for any one connection) salted (so to speak) with a
nonce. This use of nonces causes the MAC sent to be different for
each request, which defeats the known cookie recovery attacks on
TLS. Note that we assume confidentiality protection from TLS;
clients MUST NOT negotiate cipher suites that provide no
confidentiality protection.
requirement 6 This is clearly met by the use of a MAC keyed with a
session key not available to attackers. This clearly depends on
implementations having decent entropy sources, but this is no
different than for TLS. Note, however, that insecure session
initiation with key assertion is clearly insecure relative to
passive attackers, as well as active attackers that can redirect
packet flows so they can observe session initiation.
requirement_7 This is clearly met by prefixing an indicator of
whether TLS is used or not to the MAC input.
requirement_8 The use of channel bindings as an input to the MAC
meets this requirement.
requirement_9 This requirement is clearly met by having DELETE of a
session URI terminate a session. It is important that clients
promptly destroy any remnant of deleted sessions' state so that
servers get no benefit from not deleting sessions when the clients
demand it.
requirement_10 This is clearly met by using headers that proxies
should pass unmodified.
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10. IANA Considerations
This document creates a number of new HTTP request and response
headers. These headers will need to be added to the HTTP header
registry: <TBD>.
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11. Security Considerations
This session continuation protocol appears to meet the requirements
outlined in [I-D.williams-websec-session-continue-prob]. [XXX Add
analysis. In particular explain how MAC(CB + nonce) is sufficient to
defeat BEAST and CRIME.]
This proposal meets security requirements from the problem statement
[I-D.williams-websec-session-continue-prob]. See Section 9 for
details.
[...]
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12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5056] Williams, N., "On the Use of Channel Bindings to Secure
Channels", RFC 5056, November 2007.
[RFC5929] Altman, J., Williams, N., and L. Zhu, "Channel Bindings
for TLS", RFC 5929, July 2010.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869, May 2010.
[I-D.williams-websec-session-continue-prob]
Williams, N., "Hypertext Transport Protocol (HTTP) Session
Continuation: Problem Statement",
draft-williams-websec-session-continue-prob-00 (work in
progress), January 2013.
12.2. Informative References
[RFC2617] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A., and L. Stewart, "HTTP
Authentication: Basic and Digest Access Authentication",
RFC 2617, June 1999.
[RFC5849] Hammer-Lahav, E., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.
[I-D.ietf-oauth-v2]
Hardt, D., "The OAuth 2.0 Authorization Framework",
draft-ietf-oauth-v2-31 (work in progress), August 2012.
[I-D.hallambaker-httpintegrity]
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Hallam-Baker, P., "HTTP Integrity Header",
draft-hallambaker-httpintegrity-02 (work in progress),
November 2012.
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Author's Address
Nicolas Williams
Cryptonector, LLC
Email: nico@cryptonector.com
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