Re: [Trans] [trans] #79 (rfc6962-bis): Precertificate signaturemust be over something other than just the TBSCertificate
Erwann Abalea <eabalea@gmail.com> Fri, 12 June 2015 15:28 UTC
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Date: Fri, 12 Jun 2015 17:27:57 +0200
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From: Erwann Abalea <eabalea@gmail.com>
To: Rob Stradling <rob.stradling@comodo.com>
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Cc: "trans@ietf.org" <trans@ietf.org>, Stephen Kent <kent@bbn.com>
Subject: Re: [Trans] [trans] #79 (rfc6962-bis): Precertificate signaturemust be over something other than just the TBSCertificate
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Bonjour, 2015-06-12 15:30 GMT+02:00 Rob Stradling <rob.stradling@comodo.com>: > On 11/06/15 22:42, Stephen Kent wrote: > >> what does it mean to not be "an X509 signature"? >> >> I thought the intent was to use a CMS object, and thus the signature >> would be defined by that (profiled) CMS object. >> > > Hi Steve. That's correct. > > At the top of ticket #79 I wrote: > "If I understand the CMS spec correctly, then we're currently defining a > Precertificate to be a CMS structure that contains a TBSCertificate and a > signature over just that TBSCertificate. > That means that the components of a Precertificate can be trivially > rearranged into an X.509 certificate with a valid signature!" > > It turns out that I didn't understand correctly. :-) > > Ben added some text to help clarify the situation: > "Note that, because of the structure of CMS, the signature on the CMS > object will not be a valid X.509v3 signature and so cannot be used to > construct a certificate from the precertificate." > And this is true if and only if the SignedData.encapContentInfo.eContentType field is set to a value different from id-data (id-data is the OID { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs7(7) 1 }). If the eContentType is set to id-data, the CMS producer can omit the SignedAttributes, the signature is performed over the eContent (which contains the TBSCertificate), and the resulting signed precertificate can be manipulated to become a valid certificate: - take the TBSCertificate and the signature from the CMS/precertificate, - build a SEQUENCE containing the TBSCertificate, a properly formatted SignatureAlgorithm, and the signature (reencode it from OCTET STRING to BIT STRING) - you have a valid certificate To avoid this, just make sure that the TBD in the current rfc6962-bis is NOT id-data. Or require the presence of the SignedAttributes. #79: Precertificate signature must be over something other than just the >>> TBSCertificate >>> >>> Changes (by benl@google.com): >>> >>> * milestone: => review >>> >>> >>> Comment: >>> >>> After discussion with Rob, core point is that the CMS signature is >>> not an X509 signature. >>> >>> Fixed at https://github.com/google/certificate-transparency- >>> rfcs/commit/546e6e9451186e96ddd7b54ca02f17c8d86f951e. >>> >> This commit has nothing to do with the ticket ;)
- [Trans] [trans] #79 (rfc6962-bis): Precertificate… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… Erwann Abalea
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… Stephen Kent
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… Rob Stradling
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… Erwann Abalea
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… Rob Stradling
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… Stephen Kent
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker
- Re: [Trans] [trans] #79 (rfc6962-bis): Precertifi… trans issue tracker