Secure communications in UWB based on cryptographic keys generated from channel measurements without a trusted third party have been developed. The fine time resolution of UWB allows high levels of mutual information to be obtained by a given A and B through independent characterizations of their shared communication channel. This mutual information determines the maximum secret key rate available to A and B. Since UWB channel gains change drastically with small antenna movements, it is inherently difficult for eavesdroppers to obtain channel measurements and reduce the secret key rate. In essence, UWB can provide spatial-temporal specific secret keys. Upper bounds on the secret key rate for standard UWB channels are calculated. It is demonstrated that high secure key generation rates are possible. It is shown that these key rates can be generated over a wide range of signal to noise ratios and channel synchronization errors.