Oh, it's not like a password (where you'd want to prevent echo). The value is actually a digest of a public key -- one instance of the program prints the digest, and the user has to enter it into the other instance of the program. This is for setting up a secure P2P connection.

I may have misstated the problem above. What I really need to ensure is that a malicious output can't say "the digest is abc123" and trick the user into pasting that into the other program (when the real digest is def456).