I have a situation where B2B and B2C communication needs to be sent securely and not visible to most SMTP hijackers. I don't care about conspiracy theories or NSA style attacks, but want to provide reasonable security for individuals who don't want their PII data exposed to less capable attackers.
This business requirement comes from Massachusetts data privacy laws that require PII for residents to be sent "encrypted" without much further elaboration of the technical requirements.
Our client's business relies heavily on email and the ability to send PII for life insurance, health insurance and other financial products.
To that end, I intend to use TLS to provide this security due to its ubiquity, ease of use, and that it co-operates well with financial compliance requirements. I envision creating a direct TLS tunnel between partner's MTA and ours. (Forced TLS not opportunistic)
The problem is that TLS "security" is buried in the SMTP headers, difficult to understand, and the borders of administration are hard to delineate. e.g.
company1 ----> MSFT Hosted Relay --> [TLS between providers] ----> Google Hosted --> Company 2 company1 ----> Proofpoint --> [TLS between providers] ----> Google Hosted --> Company 2
Assume an insurance company needs to send a SSN in an email message (body or attachment). The next hop MTA is a Gmail, Yahoo, or other trusted private MTA.
How can I give the recipient confidence that the message was sent securely over TLS?
What alternative technical solutions or RFC would assist in giving me this assurance? (Perhaps a variant of DMARC/DKIM + TLS?)