hashcæ

version1

• no scaling of difficulty with amount of spam emails
• no way for email rcpt to require higher difficulty
• hashes can be computed ahead of time and then spammed later

X-Hashcash: <version = 1>:<difficulty = 20>:<date>:<email>::<random>:<nonce>

sequenceDiagram
Sender ->> Hashcash V1: difficulty, date, email recipient
loop Proof of Work
   Hashcash V1 ->> Hashcash V1: hash header
end
Hashcash V1 ->> Sender: X-Hashcash: 1:16:2022.05.14:email@example.com::ljyQKNT+Gxw=:xNz
Sender ->> Recipient: Send Email

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version2

X-Hashcash2: <salt>:<difficulty>:<recipient email>:<nonce>

sequenceDiagram
participant Sender
participant Hashcash V2
participant Relay Server
participant Recipient
Sender ->> Relay Server: sender's email, email recipient
Relay Server ->> Sender: salt, difficulty
Sender ->> Hashcash V2: difficulty, salt from server, recipient
loop Proof of Work
   Hashcash V2 ->> Hashcash V2: hash header
end
Hashcash V2 ->> Sender: X-Hashcash2: <difficulty>:<recipient email>:<salt>:<nonce>
Sender ->> Relay Server: header
Relay Server ->> Recipient: relays email after verification

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