Thanks for your note and for thinking this through, James. Your comments were useful and made me think, as always. As I see it, there are several cases to consider.
If the producer implements the extension correctly, then as I wrote on the 21st, if the consumer doesn't understand it, then the signature will fail. That's the case I was writing about earlier So the case you're describing has to instead be one in which the producer is an attacker and intentionally not implementing the extension correctly. In that case, example you give is possible if the receiver doesn't understand the extension. I used to be concerned about this but now I'm not. Here are two reasons why: 1. The recipient is implementing an application that uses JWS - one property of which is a specification by the application of whether the extension is used or not. If the application uses the extension, then a correctly implemented application client will implement the extension. In that case, if the attacker uses "b64":false but signs as if "b64" were true (your described attack), then the signature verification will fail. Whereas, if you start with the premise that the recipient incorrectly implements the application, which your attack requires, then all bets are off in the first place. 2. The recipient is already making a trust decision dictated by the application's rules for using JWS about whether to trust the sender based on verifying its signature and therefore its identity within the application context. If the sender is actually an attacker and yet trusted by the recipient once its signature is verified, the recipient has bigger problems than the possibility of the sender being able to generate the same signature for both encoded and unencoded values. If the sender is an attacker trusted by the recipient, there are likely much worse ways for the sender to trick the recipient, for instance, by signing maliciously generated content, which the recipient then uses. Only if the application dynamically switches between b64:false and b64:true (something prohibited in https://tools.ietf.org/html/draft-ietf-jose-jws-signing-input-options-03#section-6), would it be necessary for the application to require the use of "crit":["b64"]. That would cause incorrectly implemented application clients to nonetheless reject content signed with b64:false when it doesn't implement "b64". I will add the thoughts above to the Security Considerations section. And similarly to argument 1 above, if the application specifies the use of b46:false with the compact serialization *and* the application will use non-baseurl-safe payload characters such as comma, then it already has to be prepared to accurately communicate those characters within the application context. There is no backwards compatibility problem for existing applications - only new options for new applications, if they choose to use the option. If the existence of the extension would break any existing applications or require any actions upon their part, then I would agree that this spec should be marked as updating RFC 7515. But as it is, it's just a spec using the IANA registry to register a new header parameter value. We certainly don't want to set the precedent that every new header parameter registration will also require updating RFC 7515. Jim, I believe that the thoughts above also address the two questions in your October 18th note in the same thread. Please let me know if you disagree. -- Mike From: Manger, James [mailto:[email protected]] Sent: Thursday, October 22, 2015 9:32 AM To: Jim Schaad; Mike Jones; [email protected] Subject: RE: [jose] JWS Unencoded Payload Option spec addressing WGLC comments Unfortunately Mike, the signature validation will not fail. By design draft-ietf-jose-jws-signing-input-options mimics RFC7515 in using "bytes before the 2nd dot" as the bytes to sign/verify. Consider a signer choosing the b64=false option from draft-ietf-jose-jws-signing-input-options to sign the 12-byte message "IOU_5000_USD". The header is {"alg":"HS256","b64":false} giving the JWS: eyJhbGciOiJIUzI1NiIsImI2NCI6ZmFsc2V9.IOU_5000_USD.xxxxxxxxxxxxxxxxxxxxxxxx A verifier that only understands RFC7515 JWS interprets this as a normal JWS with an unknown but ignorable header member. The signature is still over eyJ..USD so it still verifies. The verifier then base64url-decodes the IOU.. part to get the validly-signed 9-byte content (in hex) 20 E5 3F E7 4D 34 FD 44 83. Ouch! One JWS can be interpreted as a signed 12-byte or 9-byte message. One fix might be to make the b64=false signing input: BASE64URL(UTF8(JWS Protected Header)) || '..' || JWS Payload The two dots ensure the bytes signed when b64=false can never look like bytes signed according to RFC7515. draft-ietf-jose-jws-signing-input-options should probably do this in addition to setting the crit field (so you can get a meaningful error message of "unsupported variety of JWS", instead of "signature failure" that could have all sorts of causes). >From RFC7515 JWS, it would be perfectly reasonable to store a collection of >compact-serialized-JWSs as comma-separated values - since a RFC7515 >compact-serialized-JWS cannot contain a comma. >draft-ietf-jose-jws-signing-input-options breaks that perfectly reasonable >choice as now an unencoded non-detached JWS can have any bytes and still be >called a compact-serialized-JWS. Saying >draft-ietf-jose-jws-signing-input-options updates RFC7515 is the minimum >required to note this, though that isn't really adequate. A better solution is >to not allow an unencoded non-detached message to be called a >compact-serialized-JWS: either don't define this combination, or call it >something else (eg JWS raw serialization). -- James Manger From: Jim Schaad [mailto:[email protected]] Sent: Thursday, 22 October 2015 9:53 AM To: 'Mike Jones'; Manger, James; [email protected]<mailto:[email protected]> Subject: RE: [jose] JWS Unencoded Payload Option spec addressing WGLC comments Does making 'crit' not required open one up to the possibility of an attack along the following lines: 1. Create a JWS with a b64=true header 2. Sign it using the b64=false construction 3. Send it to a validator that does not understand the b64 header. 4. Claim that the validator should have failed validation and not performed the signed command Jim From: jose [mailto:[email protected]] On Behalf Of Mike Jones Sent: Wednesday, October 21, 2015 2:16 PM To: Manger, James <[email protected]<mailto:[email protected]>>; [email protected]<mailto:[email protected]> Subject: Re: [jose] JWS Unencoded Payload Option spec addressing WGLC comments As I see it, explicitly updating JWS isn't necessary, since JWS established the JSON Web Signature and Encryption Header Parameters Registry to allow for additional Header Parameters to be defined, and so implementers are expected to refer to the registry and gracefully handle the possibility of extensions registered there. The JWS Unencoded Payload Option specification registers such an extension. As to whether "crit" is required, "crit" is only necessary if an explicit directive is required that the validation must fail if the header parameter is not understood. However, in this case, if "b64" is not understood and simply ignored, the validation will fail without needing to use "crit", since the signature validation will fail. Thus, the use of "crit" is unnecessary for "b64". -- Mike From: Manger, James [mailto:[email protected]] Sent: Tuesday, October 13, 2015 7:55 PM To: Mike Jones <[email protected]<mailto:[email protected]>>; [email protected]<mailto:[email protected]> Subject: RE: JWS Unencoded Payload Option spec addressing WGLC comments Shouldn't draft-ietf-jose-jws-signing-input-options update RFC 7515 "JWS"? That seems quite important as draft-ietf-jose-jws-signing-input-options changes the meaning of valid JWS messages (new "b64" field that cannot be ignored, but is not listed in "crit"), and allows a bunch of previously invalid chars in JWS Compact Serializations (invalidating the JWS definition of Compact Serialization as a "URL-safe string"). -- James Manger From: jose [mailto:[email protected]] On Behalf Of Mike Jones Sent: Wednesday, 14 October 2015 10:49 AM To: [email protected]<mailto:[email protected]> Subject: [jose] JWS Unencoded Payload Option spec addressing WGLC comments Draft -03 of the JWS Unencoded Payload Option specification addresses the working group last call comments received. Thanks to Jim Schaad, Vladimir Dzhuvinov, John Bradley, and Nat Sakimura for the useful comments. Changes were: * Allowed the ASCII space character and all printable ASCII characters other than period ('.') in non-detached unencoded payloads using the JWS Compact Serialization. * Updated the abstract to say that that the spec updates RFC 7519. * Removed unused references. * Changed the change controller to IESG. The specification is available at: * https://tools.ietf.org/html/draft-ietf-jose-jws-signing-input-options-03<https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2ftools.ietf.org%2fhtml%2fdraft-ietf-jose-jws-signing-input-options-03&data=01%7c01%7cMichael.Jones%40microsoft.com%7c67566ac2856449dd329b08d2d442d2c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=cwfExLlgEK11IEBTdvKI63EI6xNBi1JTV0KVipTf8JU%3d> An HTML formatted version is also available at: * http://self-issued.info/docs/draft-ietf-jose-jws-signing-input-options-03.html<https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fself-issued.info%2fdocs%2fdraft-ietf-jose-jws-signing-input-options-03.html&data=01%7c01%7cMichael.Jones%40microsoft.com%7c67566ac2856449dd329b08d2d442d2c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=5nAlXMo6uPDM600pp0Kf1JQliQ4maLZc5eCMKfzCdQ8%3d> -- Mike P.S. This note was also published at http://self-issued.info/?p=1465<https://na01.safelinks.protection.outlook.com/?url=http%3a%2f%2fself-issued.info%2f%3fp%3d1465&data=01%7c01%7cMichael.Jones%40microsoft.com%7c67566ac2856449dd329b08d2d442d2c8%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=L6oZmQ6tOl1eW%2fmh9zyorKeY4ouQZTGMn4o9Zid5snk%3d> and as @selfissued<https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2ftwitter.com%2fselfissued&data=01%7c01%7cmichael.jones%40microsoft.com%7c3a69db7b8b6c4d47da0f08d2937a3d82%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=ggurSMkRVW%2bR8Nv93Mnbsf16CmVGqfjB9lW8SV5gAKM%3d>.
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