Re: [Cfrg] Question from JOSE working group

Russ Housley <> Mon, 02 July 2012 19:43 UTC

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Date: Mon, 02 Jul 2012 15:43:20 -0400
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Subject: Re: [Cfrg] Question from JOSE working group
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In the CMS signature processing, the hash algorithm is covered by the signature.  The hash algorithm is a signed attribute, and the collection of signed attributes is hashed as part of the signature process.

Other parameters of the signature algorithm, if any, are not integrity protected.  The thinking is that the signature algorithm is seriously flawed is a change to the parameters will make an invalid signature seem valid.


Begin forwarded message:

> From: "Richard L. Barnes" <>
> Subject: [Cfrg] Question from JOSE working group
> Date: July 2, 2012 2:00:11 PM EDT
> To:
> Dear CFRG,
> This is a request for input on a crypto question that has come up in the JOSE working group.
> The question is whether there is any benefit to applying integrity protection to cryptographic parameters attached to a signed/authenticated/encrypted object.  By "cryptographic parameters" here, I mean public keys, wrapped symmetric keys, algorithm identifiers, etc.
> For historical background:
> CMS does not provide parameter protection [RFC5652].  The content that is signed / authenticated / encrypted is precisely the protected content (eContent).  Optionally, an application may add signed attributes.  In the case of SignedData and AuthenticatedData, if signed attributes are present, then they must include the message digest that was signed or authenticated; not a parameter, but an intermediate result.
> IKE and TLS does not provide integrity protection for parameters [RFC4306][RFC5246].  Algorithm negotiation is done in parallel with key agreement in the initial phase of IKEv2   Algorithm negotiation in TLS is done in the ClientHello / ServerHello messages, which are sent before the secure channel is established.  When TLS client certificates are used, the client demonstrates its possession of the relevant private key by signing the handshake messages, which provides an integrity check on cryptographic parameters as a side-effect.  However, the parameters for this signature (the public key and signature/hash algorithms) are not protected.
> PKIX is an intermediate case.  The signature algorithm used to sign / verify the certificate is included in the certificate body (under the signature) as well as ouside the signature.
> Back to today:
> The current draft specifications for JOSE [JWS][JWE] require integrity protection for parameters.  The creator of an object constructs a "header" value including the cryptographic parameters used to generate the object.  This header value is prepended to the protected content before the content is signed / authenticated (JWS), or attached as "Associated Data" to the encrypted content (JWE).  (JWE only allows AEAD algorithms.)
> So the questions for this group are the following: 
> -- Does applying integrity protection to cryptographic parameters result in a meaningful increase the security of an secure object format?  
> -- What are the classes of attack that this integrity protection would address?
> -- Does parameter protection create any additional security vulnerabilities?
> Any input would be very helpful for the group to decide on a way forward.
> Thanks a lot,
> --Richard
> [RFC5652] <>
> [RFC4306] <>
> [RFC5246] <>
> [RFC5280] <>
> [JWS] <>
> [JWE] <>
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