Re: [smime] PKCS#7 v1.5 vs. CMS / ContentInfo vs. EncapsulatedContentInfo based on version

Russ Housley <housley@vigilsec.com> Fri, 03 November 2017 18:30 UTC

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From: Russ Housley <housley@vigilsec.com>
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Date: Fri, 3 Nov 2017 14:30:36 -0400
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To: Martin Rex <mrex@sap.com>
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Subject: Re: [smime] PKCS#7 v1.5 vs. CMS / ContentInfo vs. EncapsulatedContentInfo based on version
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As Jim already said, that is not the point of the version number.  The point is to avoid decode errors.

Russ


> On Nov 3, 2017, at 1:01 PM, Martin Rex <mrex@sap.com>; wrote:
> 
> Russ,
> 
> It seems I wasn't sufficiently clear.
> 
> This is about a third-party specification refering to CMS/rfc5652.
> 
> That organization specifically refers to SignedData structure and
> members as defined in section 5.1 of RFC 5652, it quotes that structure
> definition, and describes contents for the structure elements.
> 
> For the version element, since they _not_ doing any of the fancy
> stuff, they came up with "version=1" from the pseudo-code.
> 
> 
>  https://tools.ietf.org/html/rfc5652#section-5.1
> 
>   5.1.  SignedData Type
> 
>   The following object identifier identifies the signed-data content
>   type:
> 
>      id-signedData OBJECT IDENTIFIER ::= { iso(1) member-body(2)
>         us(840) rsadsi(113549) pkcs(1) pkcs7(7) 2 }
> 
>   The signed-data content type shall have ASN.1 type SignedData:
> 
>      SignedData ::= SEQUENCE {
>        version CMSVersion,
>        digestAlgorithms DigestAlgorithmIdentifiers,
>        encapContentInfo EncapsulatedContentInfo,
>        certificates [0] IMPLICIT CertificateSet OPTIONAL,
>        crls [1] IMPLICIT RevocationInfoChoices OPTIONAL,
>        signerInfos SignerInfos }
> 
>      DigestAlgorithmIdentifiers ::= SET OF DigestAlgorithmIdentifier
> 
>      SignerInfos ::= SET OF SignerInfo
> 
> 
> However, the use of version=1 would IMHO imply the use of the original
> PKCS#7 v1.5 structure instead:
> 
>  https://tools.ietf.org/html/rfc2315#section-9.1
> 
> and more specifically, the use of the original PKCS#7 v1.5 ContentInfo rather
> than the CMS version 3 EncapsulatedContentInfo.
> 
> 
> The organization is currently assuming that CMS (rfc5652) would suggest
> combining "version=1" with CMS 3+ SignedData with EncapsulatedContentInfo.
> 
> To me, this looks "underspecified".  I.e. rfc5652 section 5.1 fails to
> describe what exactly version=1 means for the ContentInfo vs.
> EncapsulatedContentInfo encoding of SignedData.
> 
> -Martin
> 
> 
> Russ Housley <housley@vigilsec.com>; wrote:
>> The version is structure in this manner so that an implementation
>> that checks the version number and then does a decode will never
>> get a decode error on a properly constructed message.
>> 
>> If the only changes are (migrate from PKCS#1 v1.5 to RSA-PSS) and
>> (migrate from PKCS#1 v1.5 to PRS-OAEP), then the change should be
>> very straightforward.
>> 
>> If you are not using any version 1 attribute certificates, identifying
>> the signer with the subject public key identifier, or using a content
>> type other than id-data, then the version for Signed-Data should not change.
>> 
>> I am assuming that you are not mixing RSA-OAEP with other key management
>> algorithms.  If you are not using unprotect attributes or identifying
>> the signer with the subject public key identifier, then the version
>> number for Enveloped-Data should not change.
> 
> 
> 
>> 
>>> On Nov 3, 2017, at 12:04 PM, Martin Rex <mrex@sap.com>; wrote:
>>> 
>>> There is a somewhat confusing ruleset around the "SignedData" PDU
>>> version field in the CMS specification, and insufficient guidance
>>> about the ramifications for the Encoder/Decoder for ContentInfo
>>> when version 1 vs. 3 is chosen.
>>> 
>>> The organization responsible for certain legally mandated data exchanges
>>> in Germany is rev'ving their requirements, intoducing RSA-PSS signatures
>>> on certificates plus RSA-OAEP encryption for AppData.
>>> 
>>> Previously, they've been using PKCS#7 v1.5 PDUs with RSA PKCS#1 v1.5
>>> transforms.
>>> 
>>> The confusion I'm seeing is about the choice of the SignedData "version"
>>> field, and the resulting consequences for the (ASN.1) PDU encoder/decoder
>>> for the ContentInfo vs. EncapsulatedContentInfo in SignedData.
>>> 
>>> 
>>> For the encoder/decoder, the reasonable interpretation would be,
>>> that whenever version=1, then the PKCS#7 ContentInfo encoding will be
>>> used, and only for version>=3, the CMS EncapsulatedContentInfo encoding
>>> will be encoded or decoded.
>>> 
>>> 
>>> However the current reading of the CMS standard by that organization is
>>> that they want to specify version=1 in combination with EncapsulatedContentInfo
>>> encoding -- something that looks extremely weird to me, and would require
>>> significant contortions in the ASN.1 encoder and decoder.
>>> 
>>> For the encoder, it will require a laying violation from within the encoder,
>>> looking at later elements and semantics of higher level PDUs.
>>> 
>>> For the decoder, it essentially will require heuristics (trial-and-error)
>>> decoding if the PDU version will no longer matter, and the data determining
>>> which encoding is appropriate, has not been decoded yet at this point
>>> requiring a retroactive verification of whether the heuristically determined
>>> encoding was actually a _valid_ encoding.
>>> 
>>> 
>>> Any comments from folks more experienced with CMS ?
>>> 
>>> -Martin
>>> 
>>> _______________________________________________
>>> smime mailing list
>>> smime@ietf.org
>>> https://www.ietf.org/mailman/listinfo/smime
>>