OTP Security (Score 1)

One-time pad's are secure if the key is used only once (hence the name, one-time pad). The key needs to be as long as the original message, which makes this method unpractical in most real-life situations. If you use a smaller key than your plaintext, your encrypted message is compromised. If you re-use the key, then all your encrypted messages are compromised.

There's a very nice visual representation of this property here:

http://www.cryptosmith.com/archives/70

I like this example as it provides a visual representation of the leak. If you encrypt two different images with the same key using OTP and if you have access to both encrypted images, then you can XOR the encrypted images together to get information from the original images. It's surprising how much information is actually leaked when you re-use a key in OTP.

In the case of WWII pigeons, if anyone re-used an OTP key and an attacker captured two pigeons carrying messages encrypted with the same key, these messages would all be compromised.