How does raven post work? A raven knows to fly to one particular castle. But what happens after that? Kept at its home castle, a raven is useless.
The only solution I can think of is that ravens are redistributed by ground, carted in cages (or shipped by sea). That'd work, although slowly—Westeros takes weeks to traverse. This'd explain why raven post is so costly.
Is this ever discussed in the books? Of all the characters travelling between castles, I don't recall any moving ravens, or seeing others doing it.
The system above is vulnerable to man-in-the-middle attack. Suppose a man arrived at Winterfell with a trout on his coat and a cage of ravens "from Riverrun". It's entirely possible that the man and the ravens came from Casterly Rock, and that letters sent with them find their way to Tywin Lannister.
Answer
According to a sample chapter from the Winds of Winter, most ravens can only fly to one place, although there are rare exceptions:
"A maester's raven flies to one place, and one place only. Is that correct?"
The maester mopped sweat from his brow with his sleeve. "N-not entirely, Your Grace. Most, yes. Some few can be taught to fly between two castles. Such birds are greatly prized. And once in a very great while, we find a raven who can learn the names of three or four or five castles, and fly to each upon command. Birds as clever as that come along only once in a hundred years."
Other than those rare birds, presumably once a raven is back at its home castle it would then have to be transported to a new location in a cage in order for it to be of any practical use. The clearest example of this is, as Zoe mentioned, when the Night's Watch took a cart of ravens with them on the Great Ranging. There may be other examples as well.
Regarding a "man-in-the-middle" attack. To prevent such an attack you would either have to cipher your messages using a code only the intended recipient knows how to decode, or you would have to know/trust the person(s) bringing you the ravens.
Neither contingency is discussed in the books, although in chapter 6 of GOT Catelyn mentions that a letter sent by Lysa was written using a secret language only the two of them shared. And it is not unreasonable that the people bringing/receiving ravens would expect to know each other.
In "our" universe (IE outside of the books), RFC 1149 briefly discusses security considerations for secure transmission of data via avian carriers:
Security is not generally a problem in normal operation, but special measures must be taken (such as data encryption) when avian carriers are used in a tactical environment.
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