-----Original Message-----
From: oauth-boun...@ietf.org [mailto:oauth-boun...@ietf.org] On Behalf
Of Igor Faynberg
Sent: Tuesday, May 31, 2011 3:50 PM
To: Phil Hunt
Cc: OAuth WG
Subject: Re: [OAUTH-WG] Fwd: issues with token age element - MAC token
...Sorry to turn the question around so as to underline my pet peeve:
Have we *documented* all cases? (This is what the Use Cases document is
supposed to be all about.)
Just to clarify: I am not arguing with Phil's point now. I just stress that as
of
this moment we don't have anything to check against.
Igor
Phil Hunt wrote:
There seems to be a demonstrated need for both age and timestamp
tokens.
The list has 2 separate cases with 2 separate proposals that do not solve all
cases.
Can we at least agree that neither proposal works in all cases?
Phil
@independentid
www.independentid.com
phil.h...@oracle.com
On 2011-05-31, at 2:41 PM, Adam Barth wrote:
You haven't described a problem.
On Tue, May 31, 2011 at 1:46 AM, Skylar Woodward <sky...@kiva.org>
wrote:
First we should agree on a common understanding of the spec. The
reason why age works with unsynchronized clocks is that the client
determines issue_date based on the time when it receives the token over
the wire. This depends on the server and client both recording time this way
and for the transmission of the token to be be not longer than the margin of
error for validating age. Are we agreed on this understanding?
That's not correct.
The age allows the server to protect against replay attacks in
bounded memory. With unbounded memory, the server can just
remember
every nonce it has ever seen associated with a given key and reject
replays.
With bounded memory, the server eventually needs to evict some of
these nonces due to memory pressure. The age value lets the server
reject the nonces with the smallest age values first. The server
then rejects any messages with age values smaller than (or equal to)
the largest age value it has ever evicted for the given key.
Notice that neither clock synchronization nor transmission time plays
a role in that implementation.
The easiest case for me to explain here is client credentials because I
have to assume you've built and registered a Twitter app at some point (or
similar OAuth 1.0a app). You registered your app on the site and were issued
a client_id and client_secret (called consumer_key and consumer_secret in
1.0). You then embedded these values in your client (they were not issued
programmatically to your app). Assuming the MAC token algorithm is used
then for establishing client identity (originally one of the uses we, the
working group, hoped MAC would cover) how then will your client
determine issue date?
I recommend the date at which the developer obtained the credential
from Twitter because that is the date when the credential was issued.
After we can establish where you're at on the two above points I'll
continue with the explanation. But as a preview, the next points would be:
- If issue_date comes form the server, how is it translated to the client?
The issue_date does not come from the server.
- If you use a server provided issue_date, how do you then translate
that a value relative to the local unsyncronized clock?
The server does not provide the issue_date.
- If your answer to that is to also provide the current server time to the
client so the offset on the server provided issue_date can be calculated what
is the difference between all of these values and just using timestamp?
My answer is not to provide the current server time to the client.
Kind regards,
Adam
So don't get wrapped up in those 3 questions until we establish your
contextual understanding of the first two paragraphs. Feel free to also
respond to me off the list so we don't trouble everyone else with us getting
on the same page (the reason, I thought, why a Skype call would be more
efficient and painless). I do think my explanations all have been very clear
thus far. There must be a contextual confusion that is keeping us from a
common understanding of the terminology or the use cases.
skylar
On May 31, 2011, at 10:30 AM, Adam Barth wrote:
I'm not sure we need a Skype call. Can you explain the trouble
caused by age clearly? I didn't understand your previous
explanation. The more concrete you can be, the better.
Thanks,
Adam
On Tue, May 31, 2011 at 1:04 AM, Skylar Woodward <sky...@kiva.org>
wrote:
It seems we're failing to communicate. Or you're not understanding
my use cases. Age doesn't "just" work. It only works for a limited number of
uses cases that must include all of yours - and it is brittle at that. It
doesn't
work for any of our uses cases (where the client can't know issue_date w/o
the server telling it - in which case we have the equivalent problem as
timestamp).
If you'd like to talk this out over Skype I'm happy to do that, so I can
help you understand why age doesn't work.
On May 31, 2011, at 9:47 AM, Adam Barth wrote:
Timestamps don't work when the client doesn't have a synchronized
clock. It's true that a client could synchronize its clock with
the network, but our implementation experience is that many
clients don't for a variety of reasons. That means that age
better because, you know, it works.
Adam
On Mon, May 30, 2011 at 11:19 PM, Skylar Woodward
<sky...@kiva.org> wrote:
I don't think you read my first message on the topic (or I wrote too
much).
Age is fragile because if the clock changes between issue_date and
the time of submission, it will fail. We know many people don't have
synchronized clocks, but using age only solves this problem if two
assumptions hold true:
1) the client is able to guess the issue_date the server is
using based on the time the credential was issued
2) the client system clock doesn't change between issue date and
submission time.
Timestamp has neither of these issues because the client can
always inquire about network time and can effectively correct for
inaccuracies in the device timekeeping system.
Regarding the first assumption, this fails when a token might be re-
issued between devices. An example is that we use MAC token for the client
credentials, which are issued when a developer registers an application. The
client has no way of determining on its own when the value was actually
issued (unless we communicate that on the developer website and force
users to embed that with client_id, which adds usability issues of users
copying and entering string date values correctly). The same is actually true
for all of our OAuth access tokens because we reissue tokens to the same
client_id if they were previously issued and are still valid. (The client would
thus think the issue_date was now() when if fact it was the time of the first
issue for client_id+scope+grantor_id). Thus, age is really just a convoluted
way of trying to communicate the device system offset:
local_offset = current_server_time - current_device_time
age = current_device_time -
(server_issue_date-local_offset)
Since the protocol doesn't currently allow for server_issue_date to
be given with tokens, thus age currently can't have the resilience that
timestamp does. It also forces servers to issue new tokens on demand just
to make the convoluted age system work (rather than reuse existing valid
tokens). Or, you have to modify the protocol to add server_issue_date and
current_server_time into the token-issue exchange - eg, more complexity.
Timestamp is simpler, proven, it and it has a solution for your use
case of unsyncronized clocks.
skylar
On May 30, 2011, at 9:08 AM, Adam Barth wrote:
I can't speak for Mozilla, but I can tell you that many folks
don't have synchronized clocks, for a wide variety of reasons.
I guess I don't really understand why you view age as
problematic. You reference "fragility of using
time-since-credentials-issued" but you don't say what exactly
is fragile about it. There's nothing particularly complex
about age, especially when using the monotonic clock provided by
all modern operating systems.
Adam
On Mon, May 30, 2011 at 12:03 AM, Skylar Woodward
<sky...@kiva.org> wrote:
But see, now you are specializing the use of MAC token even
more - now it's becoming a service mainly for user-agents on home
desktops? This is further for the original goal of making MAC as flexible is
possible. In this case you should change the spec name to
MAC_TOKEN_FOR_BROWSER_COOKIE_REPLACEMENT_IN_AGENTS_LIKE_FI
REFOX - or MTBCRLF for short.
Sarcasm aside, my point is that timestamp is just as good as your
offset technique and is more: reliable, straightforward, flexible.
User agents that care about creating robust behavior for home
desktops or mobiles (presumably of users and OS not yet sophisticated
enough to check network time on their own) should be advised to do clock
correction on their own (by pinging a time service) and trusting the device
clock alone.
Please change the spec back to using timestamp rather than age.
I'd also like to hear a convincing argument from the Mozilla co-
authors about why they think that age is more resilient than the above (I
believe it is not) and further more why they would find the above
unattractive or difficult to implement in a modern user-agent.
Thanks,
skylar
... -.- -.-- .-.. .- .-. - .-- --- --- -.. .-- .- .-. -.. - ... -.- -.-- .-..
.- .-. - .-
- --- --- -.. .-- .- .-. -..
skylar woodward
Kiva Developer Program / build.kiva.org / @buildkiva
On May 30, 2011, at 7:54 AM, Eran Hammer-Lahav wrote:
Any kind of clock sync requirement for user-agents (basically,
home desktops) it completely impractical. The added complexity pales in
comparison to the difficulty of trying to use timestamps and any kind of clock
sync. No window will be big enough as experience shows some users have
closes that are off by more than an hour and a half.
The issue here is who is this being optimized for. Server-to-
server communication should simply use TLS for privacy and MITM protection
on top of MAC instead of using nonces to prevent replay. The whole point of
this kind of replay protection is when TLS is not available.
I think a better approach is to simply make checking the nonce
optional when TLS is used.
EHL
-----Original Message-----
From: oauth-boun...@ietf.org [mailto:oauth-
boun...@ietf.org]
On Behalf Of Peter Wolanin
Sent: Sunday, May 29, 2011 6:53 PM
To: Skylar Woodward
Cc: OAuth WG
Subject: Re: [OAUTH-WG] Fwd: issues with token age
element -
MAC token
I am also concerned by the fragility of using
time-since-credentials-issued, and also the added complexity
of specifying this construction.
I think it would be preferable to always require a timestamp
as part of the authorization header, and maybe even include
in the spec a maximum time difference between client and
server (e.g. 900 seconds) that can be tolerated. This makes
generating the nonce easier also, since the value need to
longer be unique over all time.
We have such rules in place for an HMAC-based
authentication
system we use. Once in a while a client has a local clock
so far out of sync that there is an issue, but it's rare.
-Peter
On Mon, May 23, 2011 at 9:16 PM, Skylar Woodward
<sky...@kiva.org>
wrote:
Resending to the list from my subscribed account...
Begin forwarded message:
From: Skylar Woodward <sky...@larw.com>
Date: May 23, 2011 6:14:00 PM PDT
To: Skylar Woodward <sky...@kiva.org>
Cc: Eran Hammer-Lahav <e...@hueniverse.com>, OAuth
WG
<oauth@ietf.org>
Subject: Re: [OAUTH-WG] issues with token age element -
MAC token
So after discussing this today and reflecting on it a bit,
I would suggest that
nonce simply be the "unique value" (as it is so named)
without further requirements. It might be suggested that
this be composed of an
random+timestamp (not age) value, but that seems more of a
random+MAY or
"recommended" practice. If the expectation is that very few
if any providers would actually check the timestamp (or
moreover, the nonce itself), why add terminology in the
draft that requires it? Developers are doing extra
housekeeping (and perhaps for a perceived benefit) but with
no payoff or added security.
I'm sending this feedback based on having implemented
the
v3-5 changes
last night (for both client credentials and requests w/
access tokens). After the changes, the nonce creation is now
the most complicated part of the normalized request string
and yet these changes offer the least benefit.
What's most important is that nonces are unique on each
request for an ID.
There are issues with age as well:
- As Bill mentioned, if the client stores the issue time
based on receipt, then the internal clock changes
(presumably w/o knowledge of the software storing the
dates) then time will also fail. Assuming that a user with
a bad clock (of by hours or more) will never fix it and
actually encourages bad user behavior (don't fix your
clock or Twitterbot will stop working!). Though we say
that timezones won't bring about the situation of changed
clock, I'd be to differ. Many users aren't savvy enough to
change time zone, but just adjust the time to local time
anyway. Users who are more likely to get it right already
have auto clock sync enabled (via web, mobile, etc.)
- What if the token wasn't originally issued
programmatically? In this case,
the issue time has to be obtained from the server and stored
on the client then you have the same problem as with a
timestamp - the client clock is not sync'd to the server
clock and there is no adjustment. You want this to apply to
uses outside of just OAuth, but now requiring the client to
be able to determine an issue time based on when it receives
an HTTP request necessarily limits the types of token flows for which this can
be used.
- It's one more detail to store. Hardly an issue for a
developer, but it is
inelegant. It's like having a double ID. Yet it's not an ID,
it is actually more of a recording of "my personal clock
offset value" but obfuscated several times over (one for each
token) as issue_date.
- This implementation assumes software programs use the
computer
internal clock exclusively for timestamp. A robust program
that is dependent on accurate timestamps would ping the
origin server (or similar trusted time
authority) to ask it the current time. Then it could store a
"my device clock offset" value for the lifetime of the
program execution. All requests needing timestamp would be
adjusted accordingly. For age, if the clock is changed since the
stored issue_date, the problem can't be corrected in this manner.
Thus, a significant advantage for timestamp.
All in all, this seems like a misguided but
well-intentioned attempt to get
around end-user issues of mis-set clocks. It feels like a
hack and it certainly isn't a foolproof solution. The more I
think about the implications of the age parameter, the less
I like it. Timestamp has been used for many years in the
industry and with reasonable success in relevant
applications. If we change to a new way of trying to sync on
time I think we run a greater risk of stumbling upon unforeseen issues, such
as those outlined above.
I recommend the requirement of an age (or timestamp for
that matter)
be dropped from the nonce construction. For providers that
deem it valuable, timestamp can be an optional value (either
as part of the nonce or the overall header, as before).
skylar
On May 23, 2011, at 2:11 AM, Skylar Woodward wrote:
You may have noticed, on page 8 the host is listed as
"example.net"
- should be example.com, I believe. (draft v5)
All in all, I'm in support of the changes in v2.
Certainly addresses my
hesitations from v2.
skylar
On May 9, 2011, at 12:36 PM, Eran Hammer-Lahav wrote:
(Please discuss this draft on the Apps-Discuss
<apps-disc...@ietf.org> mailing list)
http://tools.ietf.org/html/draft-hammer-oauth-v2-mac-
tok
en
While this document has moved to the Apps-Discuss
mailing list for the
time being, I wanted to give a quick update to those who
have been following this draft which originated on this list.
The major changes since -02 are:
* Removed OAuth terminology and association. The
draft
is now a
general purpose HTTP authentication scheme. It does include
an OAuth 2.0 binding which is described in less than a page.
One suggestion would be to move section 5.1 into the OAuth
specification and drop all the OAuth 2.0 text from the MAC
draft.
* Added 'Set-Cookie' extension for using MAC with
session cookies.
* Removed request URI query normalization. The new
draft
uses the
raw request URI unchanged.
* Replaced timestamps with credentials age to remove
the
need for
clock sync.
* Added a placeholder for extension, allowing random
text to be
included in the request and MAC.
* Added issuer attribute for identifying the source of
the credentials as
an additional protection.
Draft -04 is not compatible with previous drafts.
EHL
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