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Add certificate serial number (sn) to mDNS TXT record requirements #360

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Add certificate serial number (sn) to mDNS TXT record requirements #360

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7 changes: 6 additions & 1 deletion network.bs
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Expand Up @@ -175,6 +175,11 @@ keys and values:
:: An alphanumeric, unguessable token consisting of characters from the set
`[A-Za-z0-9+/]`.

: sn
:: The [=certificate serial number=] of the advertising agent, encoded as a
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@baylesj baylesj Sep 25, 2025

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Why is the agent fingerprint insufficent for computing the agent hostname? I also don't see any other reference to TLS SNI in this file, and it's not entirely clear to me why we even need Server Name Indication for this protocol. Can you please provide some additional context?

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@backkem backkem Sep 27, 2025 •
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Please see the comment below for an improved motivation/overview.

base64 string according to [[!RFC4648]]. This is required for listening
agents to compute the [=agent hostname=] for TLS SNI.

Note: `at` prevents off-LAN parties from attempting authentication; see
[[#remote-active-mitigations]]. `at` should have at least 32 bits of true
entropy to make brute force attacks impractical.
Expand Down Expand Up @@ -613,7 +618,7 @@ considered public:

1. IP addresses and ports used by the Open Screen Network Protocol.
1. Data advertised through mDNS, including the display name prefix, the
certificate fingerprint and serial number, and the metadata version.
certificate fingerprint, certificate serial number, and the metadata version.

### Cross Origin State Considerations ### {#cross-origin-state}

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