To apply the 'normal' and commonly used in other contexts (eg contracts
and insurance) definitions.
The sender is the first party.
The recipient(s) jointly and individually are the second party.
Anyone else is a third party.
I have never heard anyone talk about 2nd party signatures.
S/MIME has 2nd party signatures, where you encrypt something using the
recipient's public key. ADSP has no equivalent.
And as Steve noted, DKIM has no Nth party signatures for any N, so
let's stop here.
R's,
John
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