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INTERNET-DRAFT G Brown
draft-petke-mech-00.txt CompuServe
Expires: 15-May-97 15 November 1996
Remote Passphrase Authentication
Part Two: The Mechanism
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its
areas, and its working groups. Note that other groups may also
distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-
Drafts as reference material or to cite them other than as
"work in progress."
To learn the current status of any Internet-Draft, please check
the "1id-abstracts.txt" listing contained in the Internet-
Drafts Shadow Directories on ftp.is.co.za (Africa),
nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast).
Abstract
Remote Passphrase Authentication provides a way to authenticate a
user to a service by using a pass phrase over an insecure network,
without revealing the pass phrase to eavesdroppers. In addition, the
service need not know and does not learn the user's pass phrase,
making this scheme useful in distributed environments where it would
be difficult or inappropriate to trust a service with a pass phrase
database or to allow the server to learn enough to masquerade as the
user in a future authentication attempt.
This draft is part two of a four part series and explains the
mechanism behind RPA. Part one of this series
(draft-petke-ext-intro-00.txt) provides an extended introduction to
the problems of authentication over insecure networks. Part three
(draft-petke-http-auth-scheme-00.txt) explains how to incorporate the
mechanism into HTTP. Part four
(draft-petke-serv-deity-protocol-00.txt) explains the protocol
between the service and deity.
This scheme was inspired by Dave Raggett's Mediated Digest
Authentication paper.
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Table of Contents
1. INTRODUCTION
2. TERMINOLOGY
3. DESIGN CRITERIA
4. THE MECHANISM
4.1 AUTHENTICATION
4.1.1 Values and their representation
4.1.2 The authentication process
4.2 REAUTHENTICATION
4.3 REAUTHENTICATION CHEATING
5. SECURITY CONSIDERATIONS
6. AUTHOR'S ADDRESS
1. Introduction
In this mechanism, we'll authenticate a user to a service and vice
versa. We'll use pass phrases--actually, they're 128-bit shared
secrets, but we'll define a way to use textual phrases--so the goal
is to prove to the service that you know your pass phrase, and vice
versa.
Of course, it's important not to reveal the pass phrase to an
eavesdropper. It is equally important not to reveal the pass phrase
to a spoofer.
Furthermore, the mechanism should work even if the service does not
know the user's pass phrase. In a distributed environment, with many
services that wish to authenticate the same set of users, it may be
difficult to make users' pass phrases available to all services. And
we might prefer not to do that, if we don't completely trust the
services. So, not only should the service not have to know the user's
pass phrase, but the service should not learn the user's pass phrase
during the authentication process.
On the other hand, the mechanism should be simple enough to apply
even in the traditional case where the service knows the user's pass
phrase; there's no need to use a different mechanism in that case.
Part one of this specification (draft-petke-ext-intro-00.txt)
contains an extended introduction that explains the problem and
various potential solutions and their problems, leading to this
mechanism. If you find yourself asking, "Why not just...," it might
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be worth reading part one to see if that explains it. However, it
contains only background material, so you needn't read part one
before reading the rest of this specification.
2. Terminology
Throughout this specification we'll speak of a "user" communicating
with a "service" that wishes to learn and authenticate the user's
identity. Often, the user is a "client" and the service is a
"server," but those terms refer to an implementation.
The "deity" knows the users' and services' pass phrases, and the
service talks to the deity during the authentication process.
Although the term "authentication server" is more conventional, we
call it a deity because it's got fewer syllables and the term
"server" is overloaded. If the service knows the pass phrases, then
it acts as its own deity, simplifying the implementation but
otherwise having no effect on the mechanism.
Identities exist in some "realm," and we use that term in its usual
sense. We often think of a realm as being a relatively large
collection of users, like compuserve.com or aol.com, but it might
well consist of a small set of users, e.g., user names and pass
phrases associated with an individual Web server. We allow the
service to specify a set of realms, to recognize an identity in any
of the realms in which it participates.
3. Design criteria
This authentication mechanism is intended to meet the following
criteria.
* The service learns and authenticates the user's identity.
* The user learns and authenticates the service's identity.
* The mechanism does not use public-key technology.
* The mechanism does not use encryption. (By encryption, we're
referring to reversible encryption, the ability to encrypt
something and later decrypt it. By avoiding encryption, we avoid
restrictions on exportability.)
* The mechanism is based on shared secrets: "pass phrases," although
they can be arbitrary bit patterns rather than text.
* Neither the user nor the service needs to know the other's pass
phrase.
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* Neither the user nor the service nor eavesdroppers will learn the
other's pass phrase. However, if the pass phrase is based on text,
it's important to choose a "good" pass phrase to avoid a dictionary
attack.
* The mechanism is reasonably easy to implement in clients and does
not require the client to communicate with a third party nor to a
possess a reliable clock.
* The mechanism derives a shared secret that may be used as a session
key for subsequent authentication.
* The mechanism may be incorporated into almost any protocol. In
other words, the mechanism is not designed around a protocol; the
protocol is designed around the mechanism. But the mechanism must
be suitable for incorporation into protocols like HTTP.
* The mechanism provides the ability to accept an identity in any of
a set of realms in which the user and service are members.
4. The Mechanism
This authentication mechanism consists of three related processes:
authentication, reauthentication, and reauthentication cheating.
Authentication is the fundamental process by which a user and a
service mutually authenticate one another within one of a set of
realms, without revealing their pass phrases to one another.
Reauthentication is a process by which a user and service, having
recently authenticated one another, may again authenticate one
another. They could, of course, simply repeat the authentication
process, but that requires interaction with an authentication deity.
The reauthentication process is faster, requiring no communication
with a third party. Reauthentication is useful when multiple
connections between the user and service are established, whether
sequential as in HTTP or simultaneous. Each connection must be
authenticated, but the reauthentication process provides a shortcut.
Reauthentication cheating is a further optimization for HTTP, a
protocol that is quite unfriendly to challenge-response mechanisms.
Reauthentication cheating can be performed in parallel with an HTTP
transaction. True reauthentication is just as simple, but requires
two sequential requests because of the characteristics of HTTP. By
using reauthentication cheating, we create a "one-way" handshake.
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4.1 Authentication
There are three parties involved in the authentication process:
* the user;
* the service; and
* the authentication deity.
Each user has a name and a pass phrase in some realm of interest.
Similarly, each service has a name and a pass phrase in that realm.
The pass phrase isn't really text; it's a 128-bit (16-octet) string
of bits.
However, it's often useful to use pass phrases in the conventional,
textual sense, so we define a procedure for converting a textual
phrase to the 128-bit value used by the authentication mechanism. If
such a pass phrase is poorly chosen, it will be subject to dictionary
attack, and that's why we never use the word password in this
specification (well, except in this sentence)--use a phrase, not a
word.
The service may specify a list of realms, and the user chooses one in
which he has an identity. Thus, a service is not restricted to
authenticating identities in a single realm. The service must possess
a name and pass phrase in all realms it lists.
Each realm has an authentication deity, which knows the names and
pass phrases of its members. It's the service's responsibility to
know how to locate an authentication deity for each realm; the user
never communicates directly with an authentication deity. If the
service knows the user's pass phrase, it performs the role of the
authentication deity itself, but this does not affect the mechanism.
4.1.1 Values and their representation
Following is a glossary of the values involved in the authentication
process; we'll use these symbols in the following explanation.
As--Authentication deity's response to service; proves user's
identity
Au--Authentication deity's response to user; proves service's
identity
Cs--Challenge from service
Cu--Challenge from user
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Kus--Session key for user and service
Kuss--Session key obscured so visible only to service
Kusu--Session key obscured so visible only to user
Nr--Realm name
Ns--Service name
Nu--User name
Ps--Service's pass phrase, a 128-bit value
Pu--User's pass phrase, a 128-bit value
Rs--Service's response to challenge (during authentication
process, goes to authentication deity; during
reauthentication, goes to user)
Ru--User's response to challenge (during authentication process,
goes via service to authentication deity; during
reauthentication, goes to service)
Ts--Service's time stamp
Z--Padding consisting of 48 octets (384 bits) with all bits set
to zero
+--Concatenation of octet strings
xor--Bitwise exclusive or
Bit patterns for each value must be specified. Imagine, for example,
that one implementation uses ASCII, another EBCDIC, and another
Unicode for the user name. Or one implementation converts the name to
lowercase, another to all caps. Each would generate a different
result for the same calculation, and authentication would fail.
Should we leave such details to the underlying protocol? We could,
but that would make the service-to-deity protocol dependent on the
user-to-service protocol, so we couldn't have a single deity for each
realm. If we specify the bit patterns, we can allow any mixture of
user-to-service and service-to-deity protocols to operate on the same
data. Therefore, we adopt the following conventions.
Text strings are represented in the Unicode character set, in
big-endian byte order, without a trailing null character. Note that
ASCII can be converted to ISO 8859-1 by prefixing a single 0 bit, and
ISO 8859-1 can be converted to Unicode by prefixing eight 0 bits.
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Each 16-bit Unicode character is stored in two octets, with its
high-order 8 bits in the first octet. Representation of characters
with multiple encodings is for further study. For example, e-acute
has more than one representation. The form that uses combining
characters, in character-code order, is probably the most logical.
Note, by the way, that this specification refers only to values used
in the authentication calculations, not the underlying protocol. For
example, it's quite reasonable for a protocol to use ASCII for user
names, if that character set is adequate. Those ASCII characters must
be converted to Unicode before using them in authentication
calculations, but the protocol need not transmit Unicode characters.
* Names--Nr, Ns, Nu--are converted to lowercase Unicode. Note that
there is no trailing null character.
* Challenges--Cs, Cu--are arbitrary strings of octets, not text. They
may contain any bit patterns, including nulls, and must be at least
eight octets in length.
* The time stamp--Ts--is the ISO 646 (ASCII) textual representation
of the current universal time--UTC--in exactly 14 octets, using
24-hour time, with leading zeroes: 19950805011344.
* Pass phrases--Ps, Pu--are 16-octet quantities that contain
arbitrary bit patterns, including nulls. If the pass phrase is
based on a textual phrase, the textual phrase is converted to a
16-octet quantity by the following process.
* Convert the text string to a sequence of characters in either the
Unicode or ISO 8859-1 character sets, as appropriate for the
realm.
* Convert each character to its lowercase equivalent, or its
uppercase equivalent, or leave it alone, as appropriate for the
realm.
* Store the sequence of characters in an octet stream, with each
Unicode character in two consecutive octets in big-endian order,
or each ISO 8859-1 character in one octet. Do not append a
trailing null character.
* Take the MD5 digest of the resulting string of octets. The result
is the 128-bit value to use in the authentication calculations.
A realm will specify which of the preceding options--character set,
case conversion, and hash function--it uses for the text-to-128-bit
value transformation; the defaults are Unicode, convert to lowercase,
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and MD5. More options might be added in the future. The user-service
protocol should be designed to convey the appropriate options for
each realm from the service to the user, if other than the defaults
are to be supported, to avoid requiring the (human) user to manually
configure software.
4.1.2 The authentication process
Here we describe the individual steps. Taken literally, one might
envision many messages between the service and deity, but an actual
implementation within a protocol combines steps. [Perhaps we should
add a "sample protocol" section showing a three-way handshake
version.] For example, "The user sends a random challenge" is shown
as a separate step for clarity, but it needn't be a separate message
to the service, nor must it be sent at the point shown--if it makes
sense in the underlying protocol, the user's challenge might be
included with the user's response to the service.
* The service supplies a sequence of realms, with the service's name
in each realm, to the user. For example,
foo@compuserve.com bar@aol.com
means "Please identify yourself with a CIS user ID. If you don't
have one, your AOL ID will do." The service indicates its realm
preferences in most-preferred to least-preferred order; by
specifying only one realm, the service requires identification in
that realm.
* The user chooses a realm, Nr, and gives it and his name in that
realm, Nu, to the service. That, in turn, determines Ns, the
service's name in that realm. Note that a protocol might allow the
service to include a null realm name, meaning "I'll accept you as
an anonymous user if you wish." The user might make this choice by
supplying a null name; the process stops here, and no
authentication is performed.
* The service transmits a random challenge, Cs, and a time stamp, Ts.
The challenges are random values that make each authentication
unique. The time stamp is, in effect, a third challenge, which the
deity will ensure is recent. The user may examine it, but most
users lack an accurate source of universal time, so most users will
treat it as an opaque value.
* The user sends a random challenge, Cu.
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* The user calculates a response, Ru:
Ru = MD5(Pu + Z + Nu + Ns + Nr + Cu + Cs + Ts + Pu)
and sends it to the service.
Only the real user can generate the correct response, because it
depends on the user's pass phrase, Pu. No one can determine the
user's pass phrase from a captured response, because it's generated
by a one-way function, although there is the risk of a dictionary
attack if Pu is based on a poorly chosen pass phrase.
* The service calculates a response, Rs:
Rs = MD5(Ps + Z + Nu + Ns + Nr + Cu + Cs + Ts + Ru + Ps)
This response is not sent to the user; it would do no harm if the
user saw it, but the user won't need it.
* The service sends a request to the authentication deity for the
realm in question. The request contains
- The realm name, Nr (included so the same deity can serve more
than one realm)
- The user's name, Nu
- The service's name, Ns
- The user's challenge, Cu
- The service's challenge, Cs
- The time stamp, Ts
- The user's response, Ru
- The service's response, Rs
* The deity verifies the time stamp per previously agreed upon
criteria. In some applications, one might require it within a few
minutes; in others, one might want to allow 25 hours to eliminate
problems of misconfigured time zones. Beware of overzealousness,
though; this time stamp went from the service to the user, then
back to the service, then to the deity, perhaps with human
interaction--typing a pass phrase--introducing further delay. The
deity might implement a replay cache.
* The deity uses Nr, Ns, and Nu to look up the user's and service's
pass phrases.
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* The deity uses the values in the request, plus the service's pass
phrase, Ps, to verify Rs. If it is incorrect, the deity returns a
negative response; this request apparently did not come from a
legitimate service.
* Having verified the requesting service's identity, the deity uses
the values in the request, plus the user's pass phrase, Pu, to
verify Ru. If it is incorrect, the deity returns a failure response
to the service; the user does not know the correct pass phrase.
[Think about the effects of replaying the authentication
request. I think the answer is that there's no problem
because it reveals no new information.]
* Having verified both the user's and service's identity, the deity
creates a random, 128-bit session key, Kus, for use by the user and
service. They might use it for session encryption; in addition, it
will be used in the reauthentication process described later.
* The deity generates two obscured copies of the session key:
- Kuss = Kus xor MD5(Ps + Z + Ns + Nu + Nr + Cs + Cu + Ts + Ps)
- Kusu = Kus xor MD5(Pu + Z + Ns + Nu + Nr + Cs + Cu + Ts + Pu)
The obscuring masks resemble Ru and Rs, but differ, of course, so
an eavesdropper cannot recover Kus.
* The deity generates a pair of authentication "proofs":
- Au = MD5(Pu + Z + Ns + Nu + Nr + Kusu + Cs + Cu + Ts + Kus + Pu)
- As = MD5(Ps + Z + Ns + Nu + Nr + Kuss + Cs + Cu + Ts + Kus + M +
Ps)
Here "M" is the message transmitted from the deity to the
service; it is included in the calculation to authenticate the
response to the service. Refer to part four of this
specification (draft-petke-serv-deity-protocol-00.txt) for more
details.
* The deity sends the four values Kuss, Kusu, As, and Au to the
service.
* The service extracts its copy of the session key from Kuss by
calculating the obscuring mask value and XORing. (The service can
determine the user's key-obscuring value by calculating Kus xor
Kusu; and if the user sees Kuss, it can do likewise. But the
obscuring masks reveal nothing.)
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* The service verifies As by performing the same calculation and
comparing the result. If it matches, the service knows that someone
who knows its pass phrase--the deity--replied, having verified that
the user is who he claims to be.
* The service forwards Kusu and Au to the user.
* The user extracts its copy of the session key from Kusu by
calculating the mask value and XORing.
* The user verifies Au by computing it and comparing. If it matches,
the user knows that someone who knows his pass phrase--the
deity--replied, having verified that the service is who it claims
to be. Of course, if the service itself knows the user's pass
phrase, it can assert any service identity; but this is the case
where the service is trusted and acts as its own deity.
Now the user and service are confident of each others' identities,
and the two parties share a session key that they may use for
encryption, if they so choose.
[Perhaps we should add another value to the authentication
calculations, opaque to the mechanism, provided by the
protocol in which this mechanism is embedded. This value
would, of course, have to be added to the service-to-deity
protocol, and its generation and interpretation would be up
to the lower-level protocol. For example, HTTP might choose
to include the Web server's IP address and, perhaps, the
URL in the authentication calculations, making it harder to
do a man-in-the-middle attack. (Of course, that problem
cannot be completely solved without using the session key
to authenticate data, which is a protocol issue outside the
scope of this mechanism.)]
4.2 Reauthentication
Reauthentication is a process by which a user and service, having
recently authenticated each other, may again mutually authenticate
without talking to a deity. This is useful with protocols like HTTP,
which involve a sequence of connections that must be independently
authenticated. It's also useful with parallel connections--imagine a
scheme in which a user and service are connected, and wish to
establish a second connection.
To reauthenticate one another, the user and service prove to each
other that they both possess a secret 128-bit key--the session key,
Kus, derived during the authentication process. The reauthentication
process is essentially an ordinary challenge-response mechanism in
which the session key is used as a pass phrase.
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* The service sends a challenge, Cs, to the user.
* The user sends a challenge, Cu, to the service.
* The user calculates
Ru = MD5(Kus + Z + Ns + Nu + Nr + Cs + Cu + Kus)
and sends it to the service.
* The service verifies the result. If correct, it calculates
Rs = MD5(Kus + Z + Nu + Ns + Nr + Cu + Cs + Kus)
and sends it to the user. Both responses involve the same set of
values, but they're used in a different order, so the responses are
different.
* The user verifies the result.
4.3 Reauthentication cheating
In HTTP, one can shortcut the reauthentication process by cheating,
for an increase in efficiency.
A naive approach allows the user to repeat its authentication data,
presumably in the form of an Authorization header. If the service
recognizes the same Authorization header, it presumes that it's
talking to the previously authenticated user; essentially, we pretend
that we reauthenticated with the same challenges. But this approach
is vulnerable to replay attacks during the period of time the service
considers the data valid. The service can check the user's IP address
to reduce the risk, but IP addresses mean surprisingly little. Even
neglecting address spoofing, multiple users share an IP address when
they're on the same host or routed through a proxy or SOCKS server.
There's a better solution. We begin by noting why it's
desirable--from an efficiency, not security, point of view--to allow
the Authorization header to be replayed. To embed a
challenge-response mechanism in HTTP, we require at least two HTTP
transactions for authentication, because we cannot send a challenge
and receive a response in one HTTP transaction. If we could challenge
the user without sending a challenge to the user, we could
authenticate in one HTTP transaction. And we can do exactly that by
treating the URI as a challenge.
* The first time, the user and service perform the authentication
process.
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* The user and service remember the session key (Kus), challenges (Cu
and Cs), and timestamp (Ts).
* When the user generates an HTTP request, he includes an
Authorization header containing a response calculated as
MD5(Kus + Z + Ns + Nu + Nr + Cs + Cu + Ts + method + URI + Kus)
The method and URI are canonicalized by taking the big-endian Unicode
representation and converting all characters to lowercase; the URI
should not include the scheme://host:port. It always begins with a
slash; for "http://www.foo.com" the one-character string "/" would be
used.
Now the authentication response is unique for each URI, and
calculable only by the authenticated user, even without a unique
challenge. This doesn't completely eliminate the risk of replay, of
course, but an attacker can replay only a previously referenced URI
during the window in which the service considers the session key to
be valid. Is that acceptable?
Sometimes. If we're reading Web pages, and the only impact of replay
is that the attacker could re-read the page, it might be
acceptable--after all, the attacker saw the page, anyway, when he
captured it along with the original request. On the other hand, if
we're charging the user per page, or if the request "did" something,
replay might not be so harmless.
One strategy is to maintain some history. In its simplest form, the
service sets a flag for this session when it does something for which
replay would be harmful. If the user tries reauthentication cheating,
and the flag is set, the service forces reauthentication. Because the
cheating response is based on Cu and Cs, and those values change
during reauthentication, the correct response for a given URI changes
after reauthentication. Thus, reauthentication creates a boundary
after which previous requests cannot be replayed.
Or the service can maintain a history of URIs for which replay would
be harmful, and force reauthentication only if the user tries
reauthentication cheating on one of those URIs.
5. Security Considerations
This entire document is about security.
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6. Author's Address
Gary S. Brown
CompuServe Incorporated
5000 Britton Rd
P.O. Box 5000
Hilliard OH 43026-5000
USA
+1 614 723 1127
<gsb@csi.compuserve.com>
This Internet-Draft expired on May 15, 1997.
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