| OAuth Working Group | N. Sakimura, Ed. |
| Internet-Draft | Nomura Research Institute |
| Intended status: Standards Track | J. Bradley |
| Expires: April 29, 2015 | Ping Identity |
| N. Agarwal | |
| October 26, 2014 |
Symmetric Proof of Possession for the OAuth Authorization Code Grant
draft-ietf-oauth-spop-03
The OAuth 2.0 public client utilizing Authorization Code Grant (RFC 6749 - 4.1) is susceptible to the code interception attack. This specification describes a mechanism that acts as a control against this threat.
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Public clients in OAuth 2.0 [RFC6749] are susceptible to the authorization code interception attack. A malicious client intercepts the authorization code returned from the authorization endpoint and uses it to obtain the access token. This is possible on a public client as there is no client secret associated for it to be sent to the token endpoint. This is especially true on Smartphone applications where the authorization code can be returned through custom URL Schemes where the same scheme can be registered by multiple applications. Under this scenario, the mitigation strategy stated in section 4.4.1 of [RFC6819] does not work as they rely on per-client instance secret or per client instance redirect URI.
To mitigate this attack, this extension utilizes a dynamically created cryptographically random key called 'code verifier'. The code verifier is created for every authorization 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' 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.
+--------+ +---------------+
| |--(A)-- Authorization Request --->| |
| | + t(code_verifier), t | Resource |
| | | Owner |
| |<-(B)--- Authorization Grant -----| |
| | +---------------+
| Client |
| | +---------------+
| |--(C)--- Access Token Request --->| |
| | + code_verifier | Authorization |
| | | Server |
| |<-(D)------ Access Token ---------| |
+--------+ +---------------+
Figure 1: 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.
An attacker who intercepts the Authorization Grant at (B) is unable to redeem it for an Access Token, as they are not in possession of the code_verifier secret.
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 words for use in RFCs to Indicate Requirement Levels [RFC2119]. If these words are used without being spelled in 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].
BASE64URL(OCTETS) denotes the base64url encoding of OCTETS, per Section 3 producing a [US-ASCII] STRING.
BASE64URL-DECODE(STRING) denotes the base64url decoding of STRING, per Section 3, producing a UTF-8 sequence of octets.
SHA256(STRING) denotes a SHA2 256bit hash [RFC4634] of STRING.
UTF8(STRING) denotes the octets of the UTF-8 [RFC3629] representation of STRING.
ASCII(STRING) denotes the octets of the ASCII [US-ASCII] representation of STRING.
The concatenation of two values A and B is denoted as A || B.
In addition to the terms defined in OAuth 2.0 [RFC6749], this specification defines the following terms:
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 cryptographic random [US-ASCII] sequence using the url and filename safe Alphabet [A-Z] / [a-z] / [0-9] / "-" / "_" from Sec 5 of RFC 4648 [RFC4648], with length less than 128 characters.
ABNF for code_verifier is as follows.
code_verifier = 42*128unreserved unreserved = [A-Z] / [a-z] / [0-9] / "-" / "_"
NOTE: 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 sequence is then BASE64URL encoded to produce a 42-octet URL safe string to use as the code verifier.
The client then creates a code challenge, code_challenge, derived from the code_verifier by using one of the following transformations on the code_verifier:
It is RECOMMENDED to use the S256 transformation when possible.
ABNF for code_challenge is as follows.
code_challenge = 42*128unreserved unreserved = [A-Z] / [a-z] / [0-9] / "-" / "_"
The client sends the code challenge as part of the OAuth 2.0 [RFC6749] Authorization Request (Section 4.1.1.) using the following additional parameters:
When the server issues the code in the Authorization Response, it MUST associate the code_challenge and code_challenge_method values with the 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, but could alternatively be stored on the 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.
Upon receipt of the code, the client sends the Access Token Request to the token endpoint. In addition to the parameters defined in OAuth 2.0 [RFC6749] Access Token Request (Section 4.1.3.), it sends the following parameter:
Upon receipt of the request at the Access Token endpoint, the server verifies it by calculating the code challenge from 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 3.2 was S256, the received code_verifier is first hashed with SHA-256 then compared to the base64url decoded code_challenge. i.e.,
SHA256(code_verifier ) == BASE64URL-DECODE(code_challenge).
If the code_challenge_method from 3.2 was none, they are compared directly. i.e.,
code_challenge == code_verifier.
If the values are equal, the Access 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 5.2 of OAuth 2.0 [RFC6749] MUST be returned.
Server implementations of this specification MAY accept OAuth2.0 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 SHOULD revert to a normal OAuth 2.0 [RFC6749] protocol.
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, and SHOULD send the additional parameters as defined in Section 3. to all servers.
This specification makes a registration request as follows:
This specification registers the following parameters in the IANA OAuth Parameters registry defined in OAuth 2.0 [RFC6749].
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. It is RECOMMENDED that the output of a suitable random number generator be used to create a 32-octet sequence.
Unless there is a compelling reason, implementations SHOULD use S256 method to protect against eavesdroppers intercepting the code_challenge. If the no transformation algorithm, which is the default algorithm, is used, the client SHOULD make sure that the authorization request is adequately protected from an eavesdropper. If code_challenge is to be returned inside authorization code, it has to be encrypted in such a manner that only the server can decrypt and extract it.
Before starting the authorization process, the client SHOULD check if the server supports this specification. Confirmation of the server support may be obtained out-of-band or through some other mechanisms such as the discovery document in OpenID Connect Discovery [OpenID.Discovery]. The exact mechanism on how the client obtains this information, or the action it takes as a result is out of scope of this specification.
All the OAuth security analysis presented in [RFC6819] applies so readers SHOULD carefully follow it.
The initial draft of this specification was created by the OpenID AB/Connect Working Group of the OpenID Foundation, most notably by the following people:
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| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. |
| [RFC3629] | Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003. |
| [RFC4634] | Eastlake, D. and T. Hansen, "US Secure Hash Algorithms (SHA and HMAC-SHA)", RFC 4634, July 2006. |
| [RFC4648] | Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, October 2006. |
| [RFC5234] | Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008. |
| [RFC6749] | Hardt, D., "The OAuth 2.0 Authorization Framework", RFC 6749, October 2012. |
| [US-ASCII] | American National Standards Institute, "Coded Character Set -- 7-bit American Standard Code for Information Interchange", ANSI X3.4, 1986. |
| [OpenID.Discovery] | Sakimura, N., Bradley, J., Jones, M. and E. Jay, "OpenID Connect Discovery 1.0", February 2014. |
| [RFC6819] | Lodderstedt, T., McGloin, M. and P. Hunt, "OAuth 2.0 Threat Model and Security Considerations", RFC 6819, January 2013. |
This appendix describes how to implement base64url encoding and decoding functions without padding based upon standard base64 encoding and decoding functions that do use 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;
}
static byte [] base64urldecode(string arg)
{
string s = arg;
s = s.Replace('-', '+'); // 62nd char of encoding
s = s.Replace('_', '/'); // 63rd char of encoding
switch (s.Length % 4) // Pad with trailing '='s
{
case 0: break; // No pad chars in this case
case 2: s += "=="; break; // Two pad chars
case 3: s += "="; break; // One pad char
default: throw new System.Exception(
"Illegal base64url string!");
}
return Convert.FromBase64String(s); // Standard base64 decoder
}
As per the example code above, the number of '=' padding characters that needs to be added to the end of a base64url encoded string without padding to turn it into one with padding is a deterministic function of the length of the encoded string. Specifically, if the length mod 4 is 0, no padding is added; if the length mod 4 is 2, two '=' padding characters are added; if the length mod 4 is 3, one '=' padding character is added; if the length mod 4 is 1, the input is malformed.
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