On Fri, Jul 01, 2016, Stephan M?hlstrasser wrote:
> Hi,
>
> this message is related to another question that I sent with subject
> "Unable to decrypt CMS object encrypted with EC prime256v1
> certificate".
>
> Below I have included the full ASN.1 dump of the CMS object
> generated by a third-party application.
>
> The CMS object has two properties that I so far was not able to
> reproduce when creating CMS objects with OpenSSL:
>
> First the AlgorithmIdentifier includes the EC curve name:
>
> 40 19: SEQUENCE {
> 42 7: OBJECT IDENTIFIER ecPublicKey (1 2 840
> 10045 2 1)
> 51 8: OBJECT IDENTIFIER ansiX9p256r1 (1 2 840
> 10045 3 1 7)
> : }
>
> In CMS objects created with OpenSSL with the same recipient
> certificate, the curve name is always omitted. Is it possible to
> make OpenSSL emit the curve name as well?
>
No as this is a violation of the standards. From RFC3278:
originator MUST be the alternative originatorKey. The
originatorKey algorithm field MUST contain the id-ecPublicKey
object identifier (see Section 8.1) with NULL parameters. The
originatorKey publicKey field MUST contain the DER-encoding of a
value of the ASN.1 type ECPoint (see Section 8.2), which
represents the sending agent's ephemeral EC public key.
> Second the following:
>
> 129 10: [1] {
> 131 8: OCTET STRING B1 04 4A FD FC 8B 70 6D
> : }
>
> If I match this correctly to RFC 5652, this is
>
> ukm [1] EXPLICIT UserKeyingMaterial OPTIONAL
>
> inside the KeyAgreeRecipientInfo SEQUENCE (see
> https://tools.ietf.org/html/rfc5652#section-6.2.2).
>
> Can OpenSSL emit this optional element?
Yes but not using the command line utility. It would require a custom program
to set the parameter using the CMS API.
> What is the purpose of the "ukm" field?
>
It provides some additional optional random data used in the key encryption
key derivation algorithm.
Note that you can get a diagnistic dump using:
openssl cms -cmsout -inform DER -print -in cmd.der
Steve.
--
Dr Stephen N. Henson. OpenSSL project core developer.
Commercial tech support now available see: http://www.openssl.org
