1. A digital watermark embedding apparatus comprising: an acquisition unit configured to acquire a topological invariant as digital watermark information, key information corresponding to the digital watermark information, and a target content in which the digital watermark information is to be embedded; a function generation unit configured to generate a topological function corresponding to the topological invariant; a randomizing-function generation unit configured to generate a randomizing function based on the key information, and compute a composite function by composition of the randomizing function and the topological function; and a function-embedding unit configured to embed the composite function in the target content.