Biometric EncryptionTM is an algorithm which has been developed to securely link and retrieve a digital key using the interaction of a biometric image, such as a fingerprint, with a secure block of data, known as a BioscryptTM. The key can be used, for example, as an encryption/decryption key. The BioscryptTM comprises a filter function, which is calculated using an image processing algorithm, and other information which is required to first retrieve, and then verify the validity of, the key. The key is retrieved using information from an output pattern formed via the interaction of the biometric image with the filter function. Therefore, the filter function must be designed so that it produces a consistent output pattern (and thus, key). The filter function must also be designed to be secure (i.e. information about the fingerprint cannot be retrieved from the filter function). The consistency of the output pattern and the security of the filter function are the two topics discussed in this paper.