1. A method for operating a digital data processor to obtain one or more valid signatures of an undesirable software entity, the digital data processor including a memory that is bidirectionally coupled to the digital data processor, the method comprising the steps of: storing in the memory a corpus of computer programs that are representative of computer programs that are likely to be infected by an undesirable software entity; inputting to the digital data processor at least one portion of the undesirable software entity, the at least one portion including a sequence of bytes of the undesirable software entity that are likely to remain substantially invariant from a first instance of the undesirable software entity to a second instance of the undesirable software entity; storing the at least one inputted portion in the memory; selecting at least one candidate signature of the undesirable software entity from the stored at least one portion of the undesirable software entity; constructing with the digital data processor a list of unique n-grams from the sequence of bytes, each of the unique n-grams being comprised of from one to a chosen maximal number of sequential bytes (B) of the sequence of bytes, the constructed list of unique n-grams being stored in the memory; for each of the unique n-grams of the stored list, estimating with the digital data processor a probability of an occurrence of the unique n-gram within sequences of bytes obtained from the stored corpus of computer programs; for each candidate signature that is comprised of one or more of the unique n-grams, estimating with the digital data processor a false-positive probability of an occurrence of the candidate signature within the sequences of bytes obtained from the corpus of computer programs; comparing the estimated false-positive probabilities of the candidate signatures with one another and with a set threshold probabilities, the threshold probabilities having values selected to reduce a likelihood of an occurrence of a false positive indication during the use of a signature; and outputting at least one signature for subsequent use in identifying an occurrence of the undesirable software entity or a modified version of the undesirable software entity, the outputted at least one signature being determined to exhibit a false alarm probability that is comparable to or less than a lowest false alarm probability of others of the candidate signatures.