Missing support for NMEA NavTex messages (alerts)

[mgrouch]

Specs are here:

https://www.pentestpartners.com/security-blog/hacking-navtex-maritime-warning-messages/

Example:

Source messages look like this:

    $CRNRX,007,001,00,TD02,1,135600,27,06,2001,241,3,A,==========================*09
    $CRNRX,007,002,00,,,,,,,,,,========^0D^0AISSUED ON SATURDAY 06 JANUARY 2001.*29
    $CRNRX,007,003,00,,,,,,,,,,^0D^0AINSHORE WATERS FORECAST TO 12 MILES^0D^0AOFF*0D
    $CRNRX,007,004,00,,,,,,,,,,SHORE FROM 1700 UTC TO 0500 UTC.^0D^0A^0D^0ANORT*70
    $CRNRX,007,005,00,,,,,,,,,,H FORELAND TO SELSEY BILL.^0D^0A12 HOURS FOREC*16
    $CRNRX,007,006,00,,,,,,,,,,AST:^0D^0A^0ASHOWERY WINDS, STRONGEST IN NORTH.^0D*15
    $CRNRX,007,007,00,,,,,,,,,,^0A^0A*79

The syntax is as follows:

$CRNRX <number of lines in message>,<message identifier (B1B2B3B4)>,<line number>, <frequency>, <day of month>,<month>,<year>,<time in UTC>,<error rate> then a 2 byte XOR checksum

Preceded by ZCZC and terminated with NNNN

The Message Identifier is made up of

    B1: transmitter identity
    B2: subject indicator
    B3&B4: serial number of the subject indicator

Thanks