[Enigmail] Hello Enigmail, new user here!

[Robert J. Hansen]

Dogssup wrote:
> John: Could you define what you mean by altered? The packets within 
> the key have integrity mechanisms built-in that will keep a key from 
> importing if it's altered/mangled in transit.

Welcome to modulo mathematics.  You've seen it before: your third-grade 
teacher probably called it "clock time".  If you tell your wife you'll 
be home by ten o'clock and you're six hours late, when do you get in, 
and how much will she take you for in the divorce?

You can do math on letters.  E.g., if I take the letter 'Z' (the 26th 
letter) and add 'A' (the first letter), we go one letter past 'Z' and 
loop back to 'A'.

Now imagine:

NOWISTHETIMEFORALLGOODMENTOCOMETOTHEAIDOFTHEPARTY	P


... Hmm.  If you take 'N' + 'O' + 'W' + 'I' + 'S' ... and so on and so 
on and so on, and loop around from Z to A whenever you run off the end, 
you come up with P.

Now imagine there's an error in transmission:

NOWOSTHETIMEFORALLGOODMENTOCOMETOTHEAIDOFTHEPARTY	P

You do the computation on 'N' + 'O' + 'W' + 'O' + 'S' ... and so on and 
so on, and you get a value different from P.  This means that either the 
text has an error, or else the message integrity code (the letter 'P' at 
the end) is in error.  Either way, there's a problem and the message 
should be discarded.

Real schemes are much more complex than this, but this is the basic idea.