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Re: [lamps] smile.p7m as attachments in MUA

Russ Housley <housley@vigilsec.com> Sat, 18 June 2022 15:37 UTC

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From: Russ Housley <housley@vigilsec.com>
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Date: Sat, 18 Jun 2022 11:37:14 -0400
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Cc: LAMPS <spasm@ietf.org>, Tawhidul Islam <tislam20@gmu.edu>, Eric Osterweil <eoster@gmu.edu>
To: Pavan Kumar Dinesh <pdinesh@gmu.edu>
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Subject: Re: [lamps] smile.p7m as attachments in MUA
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Please see Section 3.2.1 of RFC 8551.

Russ

> On Jun 18, 2022, at 10:41 AM, Pavan Kumar Dinesh <pdinesh@gmu.edu> wrote:
> 
> TLDR: Is "smime.p7m" a file that is attached to emails, or a reserved name used by recipient MUAs that represents the source MIME of the email? 
> 
> Hello,
> 
> We are students working on an MUA extension that sends SMIME emails as attachments. 
> A file containing the "multipart/signed" or "application/pkcs7-mime" MIME structures is attached to the email being sent. 
> 
> From our understanding, this should be similar to how MUAs sometimes show SMIME messages as an attachment called "smime.p7m". RFC 8551 describes .p7m as the extension for "application/pkcs7-mime", and examples include Content-Disposition: attachment (Section 3.3).
> 
> However, when calling the attachment "smime.p7m" (Or really anything with the ".p7m" extension) , certain MUA (Apple Mail) has odd behavior on the recipient side, renaming the attachment to "Mail Attachment.eml" and showing security warnings for user before message displayed. This occurs no matter the content of the attached file. This makes us suspect sending emails with *.p7m named attachments is not widely compatible (perhaps a conflict with reserved file names for the MUA). 
> 
> Any words of wisdom on what "smime.p7m" actually is when mail clients show it as an attachment? Is that something we can send as an attachment at all?
> 
> Thank you!
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