<P> The classical one - time pad of espionage used actual pads of minuscule, easily concealed paper, a sharp pencil, and some mental arithmetic . The method can be implemented now as a software program, using data files as input (plaintext), output (ciphertext) and key material (the required random sequence). The XOR operation is often used to combine the plaintext and the key elements, and is especially attractive on computers since it is usually a native machine instruction and is therefore very fast . However, it is difficult to ensure that the key material is actually random, is used only once, never becomes known to the opposition, and is completely destroyed after use . The auxiliary parts of a software one - time pad implementation present real challenges: secure handling / transmission of plaintext, truly random keys, and one - time - only use of the key . </P> <P> To continue the example from above, suppose Eve intercepts Alice's ciphertext: "EQNVZ". If Eve had infinite time, she would find that the key "XMCKL" would produce the plaintext "HELLO", but she would also find that the key "TQURI" would produce the plaintext "LATER", an equally plausible message: </P> <P> In fact, it is possible to "decrypt" out of the ciphertext any message whatsoever with the same number of characters, simply by using a different key, and there is no information in the ciphertext which will allow Eve to choose among the various possible readings of the ciphertext . </P> <P> One - time pads are "information - theoretically secure" in that the encrypted message (i.e., the ciphertext) provides no information about the original message to a cryptanalyst (except the maximum possible length of the message). This is a very strong notion of security first developed during WWII by Claude Shannon and proved, mathematically, to be true for the one - time pad by Shannon about the same time . His result was published in the Bell Labs Technical Journal in 1949 . Properly used, one - time pads are secure in this sense even against adversaries with infinite computational power . </P>

What are the two problems with one-time pad