How do I make rapidly changing text in chat and signs
Solution 1:
This is using the "obfuscated" JSON option. It does not require a server or external program as LucidiousXIV said, it does however require access to commands, which I assume you already have if you're making an adventure map.
To /tellraw it into chat:
/tellraw @a {"obfuscated":"true","text":"This text will not be seen"}
To make a sign with it:
/setblock ~ ~ ~ standing_sign 0 replace {Text1:"{\"text\":\"This text will not be seen\",obfuscated:\"true\"}"}