MSGID / Message-ID Conversion for Fido-Internet Gateways : RFC Message-ID to FTN MSGID Conversion

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## 5. RFC Message-ID to FTN MSGID Conversion

## 5.1 NOMSGID Message-IDs

Any Message-ID starting with <NOMSGID...@...> is NOT converted back to an FTN ^AMSGID or ^AREPLY!

## 5.2 Normal Message-IDs

    Message-ID: <id@do.main>
    -->
    ^AMSGID: <id@do.main> abcd1234

RFC Message-IDs are converted by putting the entire string into the origin address part of FTN MSGID and adding a hex serial number.

- For EchoMail, the `abcd1234' hex number is a checksum computed for the concatenation of the Message-ID string and the FTN area name (upper case letters). abcd1234 = CRC32("<id@do.main>" + "AREA")
- For NetMail the hex number is just the checksum computed for the Message-ID string. abcd1234 = CRC32("<id@domain>")

If the Message-ID contains the characters ` ' (space) or `"', the `<id@domain>' string must be quoted using "..." according to FTS-0009. Any `"' within this string must be doubled.

Putting the area name into the CRC32 value generates different MSGIDs for different areas, preventing nopes, if the FTN tosser does dupe checking on a global basis. Examples:

    Message-ID: <1991Aug9.034239.10837@bisun.nbg.sub.org>
    FTN Area: DE.COMM.GATEWAYS
    -->
    ^AMSGID: <1991Aug9.034239.10837@bisun.nbg.sub.org> 9dc743f7

    Message-ID: <IBNTXSD@methan.chemie.fu-berlin.de>
    FTN Area: GATEWAYS.GER
    -->
    ^AMSGID: <IBNTXSD@methan.chemie.fu-berlin.de> 22f000eb

    Message-ID: <junk" id "@illegal>
    FTN Area: JUNK
    -->
    ^AMSGID: "<junk"" id ""@illegal>" 22a75d09

## 5.3 Converted FTN Message-IDs

    Message-ID: <MSGID-mimeanything_abcd1234@ftn.domain>
    -->
    ^AMSGID: anything abcd1234

Reversing the rules of 3.3:

- `_' is converted to ` ' (space).
- `=XX' is converted to the character with hex code XX.

Examples:

    Message-ID: <MSGID_2=3A2452=2F110.99_fedcba98@fidonet.org>
    -->
    ^AMSGID: 2:2452/110.99 fedcba98

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