wdiff rfc7636.original rfc7636.txt

OAuth Working Group
Internet Engineering Task Force (IETF) N. Sakimura, Ed.
Internet-Draft
Request for Comments: 7636 Nomura Research Institute
Intended status:
Category: Standards Track J. Bradley
Expires: January 9, 2016
ISSN: 2070-1721 Ping Identity
N. Agarwal
Google
July 8,
September 2015

Proof Key for Code Exchange by OAuth Public Clients
draft-ietf-oauth-spop-15

Abstract

OAuth 2.0 public clients utilizing the Authorization Code Grant are
susceptible to the authorization code interception attack. This
specification describes the attack as well as a technique to mitigate
against the threat through the use of Proof Key for Code Exchange
(PKCE, pronounced "pixy").

Status of This Memo

This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of six months this document, any errata,
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material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 9, 2016.
http://www.rfc-editor.org/info/rfc7636.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 ....................................................3
1.1. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . 5 ..............................................5
2. Notational Conventions . . . . . . . . . . . . . . . . . . . 6 ..........................................6
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7 .....................................................7
3.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 7 ..............................................7
4. Protocol . . . . . . . . . . . . . . . . . . . . . . . . . . 7 ........................................................8
4.1. Client creates Creates a code verifier . . . . . . . . . . . . . 7 Code Verifier .............................8
4.2. Client creates Creates the code challenge . . . . . . . . . . . . 8 Code Challenge ..........................8
4.3. Client sends Sends the code challenge Code Challenge with the authorization
request . . . . . . . . . . . . . . . . . . . . . . . . . 8
Authorization Request ......................................9
4.4. Server returns Returns the code . . . . . . . . . . . . . . . . . 9 Code ....................................9
4.4.1. Error Response . . . . . . . . . . . . . . . . . . . 9 ......................................9
4.5. Client sends Sends the Authorization Code and the Code
Verifier to the token endpoint . . . . . . . . . . . . . . . . . . 9 Token Endpoint ............................10
4.6. Server verifies Verifies code_verifier before returning Returning the tokens 10
Tokens ....................................................10
5. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . 10 ..................................................11
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 ............................................11
6.1. OAuth Parameters Registry . . . . . . . . . . . . . . . . 10 .................................11
6.2. PKCE Code Challenge Method Registry . . . . . . . . . . . 11 .......................11
6.2.1. Registration Template . . . . . . . . . . . . . . . . 11 ..............................12
6.2.2. Initial Registry Contents . . . . . . . . . . . . . . 12 ..........................13
7. Security Considerations . . . . . . . . . . . . . . . . . . . 12 ........................................13
7.1. Entropy of the code_verifier . . . . . . . . . . . . . . 12 ..............................13
7.2. Protection against eavesdroppers . . . . . . . . . . . . 13 Eavesdroppers ..........................13
7.3. Salting the code_challenge . . . . . . . . . . . . . . . 13 ................................14
7.4. OAuth security considerations . . . . . . . . . . . . . . 14 Security Considerations .............................14
7.5. TLS security considerations . . . . . . . . . . . . . . . 14 Security Considerations ...............................15
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
9. Revision History . . . . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
10.1. .....................................................15
8.1. Normative References . . . . . . . . . . . . . . . . . . 18
10.2. ......................................15
8.2. Informative References . . . . . . . . . . . . . . . . . 18 ....................................16
Appendix A. Notes on implementing base64url encoding Implementing Base64url Encoding without
padding . . . . . . . . . . . . . . . . . . . . . . 18
Padding .............................................17
Appendix B. Example for the S256 code_challenge_method . . . . . 19 ...........17
Acknowledgements ..................................................19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20 ................................................20

1. Introduction

OAuth 2.0 [RFC6749] public clients are susceptible to the
authorization code interception attack.

The

In this attack, the attacker thereby intercepts the authorization code
returned from the authorization endpoint within a communication path
not protected by
TLS, Transport Layer Security (TLS), such as inter-app inter-
application communication within the client's operating system of
the client. system.

Once the attacker has gained access to the authorization code code, it can
use it to obtain the access token.

Figure 1 shows the attack graphically. In step (1) (1), the native app
application running on the end device, such as a smart phone, smartphone, issues
an OAuth 2.0 Authorization Request via the browser/operating system.
The Redirection Endpoint URI in this case typically uses a custom URI
scheme. Step (1) happens through a secure API that cannot be
intercepted, though it may potentially be observed in advanced attack
scenarios. The request then gets forwarded to the OAuth 2.0
authorization server in step (2). Because OAuth requires the use of
TLS, this communication is protected by TLS, TLS and also cannot be
intercepted. The authorization server returns the authorization code
in step (3). In step (4), the Authorization Code is returned to the
requester via the Redirection Endpoint URI that was provided in step
(1).

Note that it is possible for a malicious app that has been designed to attack this native app has
previously registered register itself as a
handler for the custom URI scheme in addition to the legitimate OAuth 2.0
app. Once it does so, the malicious app is now able to intercept the Authorization Code
authorization code in step (4). This allows the attacker to request
and obtain an access token in steps (5) and (6), respectively.

+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
| End Device (e.g., Smart Phone) Smartphone) |
| |
| +-------------+ +----------+ | (6) Access Token +----------+
| |Legitimate | | Malicious|<--------------------| |
| |OAuth 2.0 App| | App |-------------------->| |
| +-------------+ +----------+ | (5) Authorization | |
| | ^ ^ | Grant | |
| | \ | | | |
| | \ (4) | | | |
| (1) | \ Authz| | | |
| Authz| \ Code | | | Authz |
| Request| \ | | | Server |
| | \ | | | |
| | \ | | | |
| v \ | | | |
| +----------------------------+ | | |
| | | | (3) Authz Code | |
| | Operating System/ |<--------------------| |
| | Browser |-------------------->| |
| | | | (2) Authz Request | |
| +----------------------------+ | +----------+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+

Figure 1: Authorization Code Interception Attack. Attack

A number of pre-conditions need to hold in order for this attack to work:

1. The attacker manages to register a malicious application on the
client device and registers a custom URI scheme that is also used
by another application. The operating systems must allow a custom
URI schemes scheme to be registered by multiple applications.
2)

2. The OAuth 2.0 authorization code grant is used.
3)

3. The attacker has access to the OAuth 2.0 [RFC6749] client_id "client_id" and
client_secret(if
"client_secret" (if provisioned). All OAuth 2.0 native app client-
instances
client-instances use the same client_id. "client_id". Secrets provisioned in
client binary applications cannot be considered confidential.
4a)

4. Either one of the following condition is met:

4a. The attacker (via the installed app) application) is able to
observe only the responses from the authorization endpoint. The plain
code_challenge_method mitigates
When "code_challenge_method" value is "plain", only this attack.
4b)
attack is mitigated.

4b. A more sophisticated attack scenario allows the attacker to
observe requests (in addition to responses) to the
authorization endpoint. The attacker is, however, not able to
act as a man-in-
the-middle. man in the middle. This has been was caused by leaking http
log information in the OS. To mitigate this the S256 code_challenge_method or
cryptographically secure code_challenge_method extension this,
"code_challenge_method" value must be
used. set either to "S256" or
a value defined by a cryptographically secure
"code_challenge_method" extension.

While this is a long list of pre-conditions pre-conditions, the described attack has
been observed in the wild and has to be considered in OAuth 2.0
deployments. While the OAuth 2.0 Threat Model Section threat model (Section 4.4.1 [RFC6819] of
[RFC6819]) describes mitigation techniques techniques, they are, unfortunately,
not applicable since they rely on a per-client instance secret or aper client a
per-client instance redirect URI.

To mitigate this attack, this extension utilizes a dynamically
created cryptographically random key called "code verifier". A
unique code verifier is created for every authorization request request, and
its transformed value, called "code challenge", is sent to the
authorization server to obtain the authorization code. The
authorization code obtained is then sent to the token endpoint with
the "code verifier" verifier", and the server compares it with the previously
received request code so that it can perform the proof of possession
of the "code verifier" by the client. This works as the mitigation
since the attacker would not know this one-time key, since it is sent
over TLS and cannot be intercepted.

1.1. Protocol Flow

+-------------------+
| Authz Server |
+--------+ | +---------------+ |
| |--(A)- Authorization Request ---->| | |
| | + t(code_verifier), t_m | | Authorization | |
| | | | Endpoint | |
| |<-(B)---- Authorization Code -----| | |
| | | +---------------+ |
| Client | | |
| | | +---------------+ |
| |--(C)-- Access Token Request ---->| | |
| | + code_verifier | | Token | |
| | | | Endpoint | |
| |<-(D)------ Access Token ---------| | |
+--------+ | +---------------+ |
+-------------------+

Figure 2: Abstract Protocol Flow
This specification adds additional parameters to the OAuth 2.0
Authorization and Access Token Requests, shown in abstract form in
Figure 1. 2.

A. The client creates and records a secret named the "code_verifier", "code_verifier"
and derives a transformed version "t(code_verifier)" (referred to
as the "code_challenge") "code_challenge"), which is sent in the OAuth 2.0
Authorization Request, Request along with the transformation method "t_m".

B. The Authorization Endpoint responds as usual, usual but records
"t(code_verifier)" and the transformation method.

C. The client then sends the authorization code in the Access Token
Request as usual, usual but includes the "code_verifier" secret generated
at (A).

D. The authorization server transforms "code_verifier" and compares
it to "t(code_verifier)" from (B). Access is denied if they are
not equal.

An attacker who intercepts the Authorization Grant authorization code at (B) is unable to
redeem it for an Access Token, access token, as they are not in possession of the
"code_verifier" secret.

2. Notational Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in Key
"Key words for use in RFCs to Indicate Requirement Levels Levels" [RFC2119].
If these words are used without being spelled in uppercase uppercase, then they
are to be interpreted with their normal natural language meanings.

This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234].

STRING denotes a sequence of zero or more ASCII [RFC0020] characters.

OCTETS denotes a sequence of zero or more octets.

ASCII(STRING) denotes the octets of the ASCII [RFC0020]
representation of STRING where STRING is a sequence of zero or more
ASCII characters.

BASE64URL-ENCODE(OCTETS) denotes the base64url encoding of OCTETS,
per Section 3 Appendix A, producing a STRING.

BASE64URL-DECODE(STRING) denotes the base64url decoding of STRING,
per Section 3, Appendix A, producing a sequence of octets.

SHA256(OCTETS) denotes a SHA2 256bit 256-bit hash [RFC6234] of OCTETS.

3. Terminology

In addition to the terms defined in OAuth 2.0 [RFC6749], this
specification defines the following terms:

code verifier
A cryptographically random string that is used to correlate the
authorization request to the token request.

code challenge
A challenge derived from the code verifier that is sent in the
authorization request, to be verified against later.

code challenge method
A method that was used to derive code challenge.

Base64url Encoding
Base64 encoding using the URL- and filename-safe character set
defined in Section 5 of [RFC4648], with all trailing '='
characters omitted (as permitted by Section 3.2 of [RFC4648]) and
without the inclusion of any line breaks, whitespace, or other
additional characters. (See Appendix A for notes on implementing
base64url encoding without padding.)

3.1. Abbreviations

ABNF Augmented Backus-Naur Form

Authz Authorization

PKCE Proof Key for Code Exchange

MITM Man-in-the-middle

MTI Mandatory To Implement

4. Protocol

4.1. Client creates Creates a code verifier Code Verifier

The client first creates a code verifier, "code_verifier", for each
OAuth 2.0 [RFC6749] Authorization Request, in the following manner:

code_verifier = high entropy high-entropy cryptographic random STRING using the
Unreserved Characters
unreserved characters [A-Z] / [a-z] / [0-9] / "-" / "." / "_" / "~"
from Sec Section 2.3 of [RFC3986], with a minimum length of 43 characters
and a maximum length of 128 characters.

ABNF for "code_verifier" is as follows.

code-verifier = 43*128unreserved
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
ALPHA = %x41-5A / %x61-7A
DIGIT = %x30-39

NOTE: The code verifier SHOULD have enough entropy to make it
impractical to guess the value. It is RECOMMENDED that the output of
a suitable random number generator be used to create a 32-octet
sequence. The
Octet octet sequence is then base64url encoded base64url-encoded to produce a
43-octet URL safe string to use as the code verifier.

4.2. Client creates Creates the code challenge Code Challenge

The client then creates a code challenge derived from the code
verifier by using one of the following transformations on the code
verifier:

plain
code_challenge = code_verifier

S256
code_challenge = BASE64URL-ENCODE(SHA256(ASCII(code_verifier)))

If the client is capable of using "S256", it MUST use "S256", as
"S256" is Mandatory To Implement (MTI) on the server. Clients are
permitted to use "plain" only if they cannot support "S256" for some
technical reason and know via out of band out-of-band configuration that the
server supports "plain".

The plain transformation is for compatibility with existing
deployments and for constrained environments that can't use the S256
transformation.

ABNF for "code_challenge" is as follows.

code-challenge = 43*128unreserved
unreserved = ALPHA / DIGIT / "-" / "." / "_" / "~"
ALPHA = %x41-5A / %x61-7A
DIGIT = %x30-39

4.3. Client sends Sends the code challenge Code Challenge with the authorization request Authorization Request

The client sends the code challenge as part of the OAuth 2.0
Authorization Request (Section 4.1.1 of [RFC6749].) [RFC6749]) using the
following additional parameters:

code_challenge
REQUIRED. Code challenge.

code_challenge_method
OPTIONAL, defaults to "plain" if not present in the request. Code
verifier transformation method, method is "S256" or "plain".

4.4. Server returns Returns the code Code

When the server issues the authorization code in the authorization
response, it MUST associate the "code_challenge" and
"code_challenge_method" values with the authorization code so it can
be verified later.

Typically, the "code_challenge" and "code_challenge_method" values
are stored in encrypted form in the "code" itself, itself but could
alternatively be stored on the server, server associated with the code. The
server MUST NOT include the "code_challenge" value in client requests
in a form that other entities can extract.

The exact method that the server uses to associate the
"code_challenge" with the issued "code" is out of scope for this
specification.

4.4.1. Error Response

If the server requires Proof Key for Code Exchange (PKCE) by OAuth
Public Clients,
public clients and the client does not send the "code_challenge" in
the request, the authorization endpoint MUST return the authorization
error response with the "error" value set to "invalid_request". The
"error_description" or the response of "error_uri" SHOULD explain the
nature of error, e.g., code challenge required.

If the server supporting PKCE does not support the requested
transform,
transformation, the authorization endpoint MUST return the
authorization error response with "error" value set to
"invalid_request". The "error_description" or the response of
"error_uri" SHOULD explain the nature of error, e.g., transform
algorithm not supported.

4.5. Client sends Sends the Authorization Code and the Code Verifier to the
token endpoint
Token Endpoint

Upon receipt of the Authorization Code, the client sends the Access
Token Request to the token endpoint. In addition to the parameters
defined in the OAuth 2.0 Access Token Request (Section 4.1.3 of
[RFC6749]), it sends the following parameter:

code_verifier
REQUIRED. Code verifier

The code_challenge_method "code_challenge_method" is bound to the Authorization Code when
the Authorization Code is issued. That is the method that the token
endpoint MUST use to verify the code_verifier. "code_verifier".

4.6. Server verifies Verifies code_verifier before returning Returning the tokens Tokens

Upon receipt of the request at the Access Token token endpoint, the server
verifies it by calculating the code challenge from the received
"code_verifier" and comparing it with the previously associated
"code_challenge", after first transforming it according to the
"code_challenge_method" method specified by the client.

If the "code_challenge_method" from Section 4.2 4.3 was "S256", the
received "code_verifier" is hashed by SHA-256, then base64url
encoded, base64url-encoded, and
then compared to the "code_challenge". i.e.,

BASE64URL-ENCODE(SHA256(ASCII("code_verifier" ))) "code_challenge", i.e.:

BASE64URL-ENCODE(SHA256(ASCII(code_verifier))) == "code_challenge" code_challenge

If the "code_challenge_method" from Section 4.2 4.3 was "plain", they are
compared directly. i.e.,

"code_verifier" directly, i.e.:

code_verifier == "code_challenge". code_challenge.

If the values are equal, the Access Token token endpoint MUST continue processing
as normal (as defined by OAuth 2.0 [RFC6749]). If the values are not
equal, an error response indicating "invalid_grant" as described in section
Section 5.2 of [RFC6749] MUST be returned.

5. Compatibility

Server implementations of this specification MAY accept OAuth2.0
Clients
clients that do not implement this extension. If the "code_verifier"
is not received from the client in the Authorization Request, servers
supporting backwards compatibility revert to a normal the OAuth 2.0 [RFC6749] protocol.
protocol without this extension.

As the OAuth 2.0 [RFC6749] server responses are unchanged by this
specification, client implementations of this specification do not
need to know if the server has implemented this specification or not, not
and SHOULD send the additional parameters as defined in Section 3. 4 to
all servers.

6. IANA Considerations

This specification makes a registration request as follows:

IANA has made the following registrations per this document.

6.1. OAuth Parameters Registry

This specification registers the following parameters in the IANA
OAuth Parameters
"OAuth Parameters" registry defined in OAuth 2.0 [RFC6749].

o Parameter name: code_verifier
o Parameter usage location: token request
o Change controller: IESG
o Specification document(s): this document RFC 7636 (this document)

o Parameter name: code_challenge
o Parameter usage location: authorization request
o Change controller: IESG
o Specification document(s): this document RFC 7636 (this document)

o Parameter name: code_challenge_method
o Parameter usage location: authorization request
o Change controller: IESG
o Specification document(s): this document RFC 7636 (this document)

6.2. PKCE Code Challenge Method Registry

This specification establishes the PKCE "PKCE Code Challenge Method Methods"
registry. The new registry should be a sub-registry of OAuth
Parameters the "OAuth
Parameters" registry.

Additional code_challenge_method "code_challenge_method" types for use with the
authorization endpoint are registered using the Specification
Required policy [RFC5226], which includes review of the request by
one or more Designated Experts. Experts (DEs). The DEs will ensure that there
is at least a two-week review of the request on the oauth-ext-review@ietf.org oauth-ext-
review@ietf.org mailing list, list and that any discussion on that list
converges before they respond to the request. To allow for the
allocation of values prior to publication, the Designated Expert(s)
may approve registration once they are satisfied that an acceptable
specification will be published.

Registration requests and discussion on the oauth-ext-review@ietf.org
mailing list should use an appropriate subject, such as "Request for
PKCE code_challenge_method: example").

The Designated Expert(s) should consider the discussion on the
mailing list, as well as the overall security properties of the
challenge Method method when evaluating registration requests. New methods
should not disclose the value of the code_verifier in the request to
the Authorization endpoint. Denials should include an explanation
and, if applicable, suggestions as to how to make the request
successful.

6.2.1. Registration Template

Code Challenge Method Parameter Name:
The name requested (e.g., "example"). Because a core goal of this
specification is for the resulting representations to be compact,
it is RECOMMENDED that the name be short -- not to exceed 8
characters without a compelling reason to do so. This name is
case-sensitive. Names may not match other registered names in a
case-insensitive manner unless the Designated Expert(s) state states
that there is a compelling reason to allow an exception in this
particular case.

Change Controller:
For Standards Track RFCs, state "IESG". For others, give the name
of the responsible party. Other details (e.g., postal address,
email address, and home page URI) may also be included.

Specification Document(s):
Reference to the document(s) that specify specifies the parameter,
preferably including URI(s) that can be used to retrieve copies of
the document(s). An indication of the relevant sections may also
be included but is not required.

6.2.2. Initial Registry Contents

This specification registers

Per this document, IANA has registered the Code Challenge Method
Parameter
names Names defined in Section 4.2 in this registry.

o Code Challenge Method Parameter Name: "plain" plain
o Change Controller: IESG
o Specification Document(s): Section 4.2 of [[ this document ]] RFC 7636 (this document)

o Code Challenge Method Parameter Name: "S256" S256
o Change Controller: IESG
o Specification Document(s): Section 4.2 of [[ this document ]] RFC 7636 (this document)

7. Security Considerations

7.1. Entropy of the code_verifier

The security model relies on the fact that the code verifier is not
learned or guessed by the attacker. It is vitally important to
adhere to this principle. As such, the code verifier has to be
created in such a manner that it is cryptographically random and has
high entropy that it is not practical for the attacker to guess.

The client SHOULD create a code_verifier "code_verifier" with a minimum of 256bits 256 bits
of entropy. This can be done by having a suitable random number
generator create a 32-octet sequence. The Octet octet sequence can then be
base64url encoded
base64url-encoded to produce a 43-octet URL safe string to use as a
code_challenge
"code_challenge" that has the required entropy.

7.2. Protection against eavesdroppers Eavesdroppers

Clients MUST NOT downgrade to "plain" after trying the "S256" method.
Servers that support PKCE are required to support "S256", and servers
that do not support PKCE will simply ignore the unknown
"code_verifier" OAuth 2.0 (see Section 3.2 of [RFC6749].
"code_verifier". Because of
that, this, an error when "S256" is presented
can only mean that the server is faulty or that a MITM attacker is
trying a downgrade attack.

The "S256" method protects against eavesdroppers observing or
intercepting the "code_challenge", because the challenge cannot be
used without the verifier. With the "plain" method, there is a
chance that "code_challenge" will be observed by the attacker on the
device,
device or in the http request. Since the code challenge is the same
as the code verifier in this case, the "plain" method does not
protect against the eavesdropping of the initial request.

The use of "S256" protects against disclosure of the "code_verifier"
value to an attacker.

Because of this, "plain" SHOULD NOT be used, used and exists only for
compatibility with deployed implementations where the request path is
already protected. The "plain" method SHOULD NOT be used in new
implementations, unless they cannot support "S256" for some technical
reason.

The "S256" code_challenge_method code challenge method or other cryptographically secure
code_challenge_method
code challenge method extension SHOULD be used. The plain
code_challenge_method "plain" code
challenge method relies on the operating system and transport
security not to disclose the request to an attacker.

If the code_challenge_method code challenge method is plain, "plain" and the "code_challenge" code challenge is to
be returned inside authorization "code" to achieve a stateless
server, it MUST be encrypted in such a manner that only the server
can decrypt and extract it.

7.3. Salting the code_challenge

In order to

To reduce implementation complexity Salting complexity, salting is not used in the
production of the code_challenge, code challenge, as the code_verifier code verifier contains
sufficient entropy to prevent brute force brute-force attacks. Concatenating a
publicly known value to a code_verifier code verifier (containing 256 bits of
entropy) and then hashing it with SHA256 to produce a code_challenge code challenge
would not increase the number of attempts necessary to brute force a
valid value for code_verifier. code verifier.

While the S256 "S256" transformation is like hashing a password password, there are
important differences. Passwords tend to be relatively low entropy low-entropy
words that can be hashed offline and the hash looked up in a
dictionary. By concatenating a unique though public value to each
password prior to hashing, the dictionary space that an attacker
needs to search is greatly expanded.

Modern graphics processors now allow attackers to calculate hashes in
real time faster than they could be looked up from a disk. This
eliminates the value of the salt in increasing the complexity of a
brute force
brute-force attack for even low entropy low-entropy passwords.

7.4. OAuth security considerations Security Considerations

All the OAuth security analysis presented in [RFC6819] applies so
readers SHOULD carefully follow it.

7.5. TLS security considerations

Curent security considerations can be found in Recommendations for
Secure Use of TLS and DTLS [BCP195]. This supersedes the TLS version
recommendations in OAuth 2.0 [RFC6749].

8. Acknowledgements

The initial draft of this specification was created by the OpenID AB/
Connect Working Group of the OpenID Foundation.

This specification is the work of the OAuth Working Group, which
includes dozens of active and dedicated participants. In particular,
the following individuals contributed ideas, feedback, and wording
that shaped and formed the final specification:

Anthony Nadalin, Microsoft
Axel Nenker, Deutsche Telekom
Breno de Medeiros, Google
Brian Campbell, Ping Identity
Chuck Mortimore, Salesforce
Dirk Balfanz, Google
Eduardo Gueiros, Jive Communications
Hannes Tschonfenig, ARM
James Manger, Telstra
John Bradley, Ping Identity
Justin Richer, MIT Kerberos
Josh Mandel, Boston Children's Hospital
Lewis Adam, Motorola Solutions
Madjid Nakhjiri, Samsung
Michael B. Jones, Microsoft
Nat Sakimura, Nomura Research Institute
Naveen Agarwal, Google
Paul Madsen, Ping Identity
Phil Hunt, Oracle
Prateek Mishra, Oracle
Ryo Ito, mixi
Scott Tomilson, Ping Identity
Sergey Beryozkin
Takamichi Saito
Torsten Lodderstedt, Deutsche Telekom
William Denniss, Google

9. Revision History

-15

o Addressed Barry's IESG comments around IANA Registration
o Addressed Barry's IESG comments around Sec 7.2 downgrade attack
o fix a typo for William and make a small change to Fig 1.1
clarifying t_m
o more wording changes to sec 7.2 re Barry
o made the two SHOULD NOT use plain recommendations consistent.
o slightly cleaned up grammer in Sec 7.2

-14

o #38. Expanded Section 7.2 to explain why plain should not be
used.
o #39. Modified Section 4.4.1 to discourage the use of plain.
o #40. Modified Intro text to explain the attack better.
o #41. Added explanation that the token request is protected in the
Last paragraph of the Introduction.
o #42. Sec 4.2: Removed redundant double quotes caused by spanx.
o #43. Sec 4.4: Replaced code with authorization code.
o #44. Sec 4.5: say "code_verifier" rather than "secret"
o #45. Sec 4.4.1: Expanded PKCE.
o #46. Sec 5: SHOULD in para 1 removed.
o Added abbreviations section.

-13

o Fix the parameter usage locations for the OAuth Parameters
Registry per Hannes response.
o Clarify for IANA that the new registry is a sub-registry of OAuth
Parameters registry
o aded text on why the code_challenge_method is not sent to the
token endpoint.

-12
o clarify that the client secret we are talking about in the
Introduction is a OAuth 2 client_secret.
o Update salting security consideration based on Ben's feedback

-11

o add spanx for plain in sec 4.4 RE Kathleen's comment
o Add security consideration on TLS and reference BCP195
o Update to make clearer that plain can only be used for backwards
compatibility and constrained environments

-10

o re #33 specify lower limit to code_verifier in prose
o remove base64url decode from draft, all steps now use encode only
o Expanded MTI
o re #33 change length of 32 octet base64url encoded string back to
43 octets

-09

o clean up some external references so they don't point at internal
sections

-08

o changed BASE64URL to BASE64URL-ENCODE to be more consistent with
appendix A Fixed lowercase base64url in appendix B
o Added appendix B as an example of S256 processing
o Change reference for unreserved characters to RFC3986 from
base64URL

-07

o removed unused discovery reference and UTF8
o re #32 added ASCII(STRING) to make clear that it is the byte array
that is being hashed
o re #2 Remove discovery requirement section.
o updated Acknowledgement
o re #32 remove unneeded UTF8(STRING) definition, and define STRING
for ASCII(STRING)
o re #32 remove unneeded utf8 reference from BASE64URL-
DECODE(STRING) def
o resolves #31 unused definition of concatenation
o re #30 Update figure text call out the endpoints
o re #30 Update figure to call out the endpoints
o small wording change to the introduction
-06

o fix date
o replace spop with pkce for registry and other references
o re #29 change name again
o re #27 removed US-ASCII reference
o re #27 updated ABNF for code_verifier
o resolves #24 added applies, so
readers SHOULD carefully follow it.

7.5. TLS Security Considerations

Current security consideration for salting
o resolves #29 Changed title
o updated reference to RFC4634 to RFC6234 re #27
o changed reference for US-ASCII to RFC20 re #27
o resolves #28 added Acknowledgements
o resolves #27 updated ABNF
o resolves #26 updated abstract and added Hannes figure

-05

o Added IANA registry considerations can be found in "Recommendations for code_challenge_method + fixed some broken
internal references.

-04

o Added error response to authorization response.

-03

o Added an abstract protocol diagram
Secure Use of Transport Layer Security (TLS) and explanation

-02

o Copy edits

-01

o Specified exactly two supported transformations
o Moved discovery steps to security considerations.
o Incorporated readability comments by Eduardo Gueiros.
o Changed MUST Datagram Transport
Layer Security (DTLS)" [BCP195]. This supersedes the TLS version
recommendations in 3.1 to SHOULD.

-00

o Initial IETF version.

10. OAuth 2.0 [RFC6749].

8. References

10.1.

8.1. Normative References

[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, May 2015. 2015,
<http://www.rfc-editor.org/info/bcp195>.

[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969. 1969,
<http://www.rfc-editor.org/info/rfc20>.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997. 1997,
<http://www.rfc-editor.org/info/rfc2119>.

[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, DOI 10.17487/RFC3986, January 2005. 2005,
<http://www.rfc-editor.org/info/rfc3986>.

[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006. 2006,
<http://www.rfc-editor.org/info/rfc4648>.

[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008. 2008,
<http://www.rfc-editor.org/info/rfc5226>.

[RFC5234] Crocker, D. D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008. 2008,
<http://www.rfc-editor.org/info/rfc5234>.

[RFC6234] Eastlake, Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011. 2011,
<http://www.rfc-editor.org/info/rfc6234>.

[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012.

10.2. 2012,
<http://www.rfc-editor.org/info/rfc6749>.

8.2. Informative References

[RFC6819] Lodderstedt, T., Ed., McGloin, M., and P. Hunt, "OAuth 2.0
Threat Model and Security Considerations", RFC 6819,
DOI 10.17487/RFC6819, January 2013. 2013,
<http://www.rfc-editor.org/info/rfc6819>.

Appendix A. Notes on implementing base64url encoding Implementing Base64url Encoding without padding Padding

This appendix describes how to implement a base64url encoding base64url-encoding
function without padding padding, based upon the standard base64 encoding base64-encoding
function that uses padding.

To be concrete, example C# code implementing these functions is shown
below. Similar code could be used in other languages.

static string base64urlencode(byte [] arg)
{
string s = Convert.ToBase64String(arg); // Regular base64 encoder
s = s.Split('=')[0]; // Remove any trailing '='s
s = s.Replace('+', '-'); // 62nd char of encoding
s = s.Replace('/', '_'); // 63rd char of encoding
return s;
}

An example correspondence between unencoded and encoded values
follows. The octet sequence below encodes into the string below,
which when decoded, reproduces the octet sequence.

3 236 255 224 193

A-z_4ME

Appendix B. Example for the S256 code_challenge_method

The client uses output of a suitable random number generator to
create a 32-octet sequence. The octets representing the value in
this example (using JSON array notation) are:" are:

[116, 24, 223, 180, 151, 153, 224, 37, 79, 250, 96, 125, 216, 173,
187, 186, 22, 212, 37, 77, 105, 214, 191, 240, 91, 88, 5, 88, 83,
132, 141, 121]

Encoding this octet sequence as a Base64url base64url provides the value of the
code_verifier:

dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

The code_verifier is then hashed via the SHA256 hash function to
produce:

[19, 211, 30, 150, 26, 26, 216, 236, 47, 22, 177, 12, 76, 152, 46,
8, 118, 168, 120, 173, 109, 241, 68, 86, 110, 225, 137, 74, 203,
112, 249, 195]
Encoding this octet sequence as a base64url provides the value of the
code_challenge:

E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM

The authorization request includes:

code_challenge=E9Melhoa2OwvFrEMTJguCHaoeK1t8URWbuGJSstw-cM
&code_challange_method=S256
&code_challenge_method=S256

The Authorization authorization server then records the code_challenge and
code_challenge_method along with the code that is granted to the
client.

In the request to the token_endpoint token_endpoint, the client includes the code
received in the authorization response as well as the additional
paramater:
parameter:

code_verifier=dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk

The Authorization authorization server retrieves the information for the code
grant. Based on the recorded code_challange_method code_challenge_method being S256, it
then hashes and base64url encodes base64url-encodes the value of code_verifier.
BASE64URL-ENCODE(SHA256(ASCII("code_verifier" ))) code_verifier:

BASE64URL-ENCODE(SHA256(ASCII(code_verifier)))

The calculated value is then compared with the value of
"code_challenge":

BASE64URL-ENCODE(SHA256(ASCII("code_verifier" )))

BASE64URL-ENCODE(SHA256(ASCII(code_verifier))) == code_challenge

If the two values are equal equal, then the Authorization authorization server can
provide the tokens as long as there are no other errors in the
request. If the values are not equal equal, then the request must be
rejected, and an error returned.

Acknowledgements

The initial draft version of this specification was created by the
OpenID AB/Connect Working Group of the OpenID Foundation.

Anthony Nadalin, Microsoft
Axel Nenker, Deutsche Telekom
Breno de Medeiros, Google
Brian Campbell, Ping Identity
Chuck Mortimore, Salesforce
Dirk Balfanz, Google
Eduardo Gueiros, Jive Communications
Hannes Tschonfenig, ARM
James Manger, Telstra
Justin Richer, MIT Kerberos
Josh Mandel, Boston Children's Hospital
Lewis Adam, Motorola Solutions
Madjid Nakhjiri, Samsung
Michael B. Jones, Microsoft
Paul Madsen, Ping Identity
Phil Hunt, Oracle
Prateek Mishra, Oracle
Ryo Ito, mixi
Scott Tomilson, Ping Identity
Sergey Beryozkin
Takamichi Saito
Torsten Lodderstedt, Deutsche Telekom
William Denniss, Google

Authors' Addresses

Nat Sakimura (editor)
Nomura Research Institute
1-6-5 Marunouchi, Marunouchi Kitaguchi Bldg.
Chiyoda-ku, Tokyo 100-0005
Japan

Phone: +81-3-5533-2111
Email: n-sakimura@nri.co.jp
URI: http://nat.sakimura.org/

John Bradley
Ping Identity
Casilla 177, Sucursal Talagante
Talagante, RM
Chile

Phone: +44 20 8133 3718
Email: ve7jtb@ve7jtb.com
URI: http://www.thread-safe.com/

Naveen Agarwal
Google
1600 Amphitheatre Pkwy Parkway
Mountain View, CA 94043
USA
United States

Phone: +1 650-253-0000
Email: naa@google.com
URI: http://google.com/