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Add intermediate CAs capability to S/MIME email signature

What does this MR do?

Add new configuration parameter to enable extra CAs to be embedded in S/MIME signatures. This will typically be intermediate CAs.

The main issue is that OpenSSL::X509::Certificate does not support multiple certificates in files, it will only read the first entry in a bundle file and silently ignore the others, so we need to manually parse the rest if we want to include them in S/MIME signatures.

Since the extra CAs to be included in the signatures have to be provided as a different parameter to OpenSSL::PKCS7::sign, we've added a new configuration parameter for a file including the CA. The alternative is to keep using a single file with all the signatures, but users would have to make sure that the first entry is always the certificate.

Corresponding MR to Omnibus to add the new config param: omnibus-gitlab!4085 (merged)

Related to gitlab-foss!30644 (merged)

🛠 with at Siemens

/cc @bufferoverflow @rpaik

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Does this MR meet the acceptance criteria?

Conformity

Availability and Testing

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

  • Label as security and @ mention @gitlab-com/gl-security/appsec
  • The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
  • Security reports checked/validated by a reviewer from the AppSec team
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