  Quantum states cannot be cloned. I show how to extend this property to
classical messages encoded using quantum states, a task I call "uncloneable
encryption." An uncloneable encryption scheme has the property that an
eavesdropper Eve not only cannot read the encrypted message, but she cannot
copy it down for later decoding. She could steal it, but then the receiver Bob
would not receive the message, and would thus be alerted that something was
amiss. I prove that any authentication scheme for quantum states acts as a
secure uncloneable encryption scheme. Uncloneable encryption is also closely
related to quantum key distribution (QKD), demonstrating a close connection
between cryptographic tasks for quantum states and for classical messages.
Thus, studying uncloneable encryption and quantum authentication allows for
some modest improvements in QKD protocols. While the main results apply to a
one-time key with unconditional security, I also show uncloneable encryption
remains secure with a pseudorandom key. In this case, to defeat the scheme, Eve
must break the computational assumption behind the pseudorandom sequence before
Bob receives the message, or her opportunity is lost. This means uncloneable
encryption can be used in a non-interactive setting, where QKD is not
available, allowing Alice and Bob to convert a temporary computational
assumption into a permanently secure message.
