plain text watermarking and watermark recovery

 Snowdrop provides reliable, difficult to remove steganographic watermarking of
 text documents (internal memos, draft research papers, advisories and other
 writing) and C sources (limited distribution software, licensed software,
 or freely available code) so that:
  (1) leaks can be identified if the data goes public.
  (2) original source can be determined and demonstrated if part of the
      document is claimed by somebody else, copied without permission, etc.
 Snowdrop uses redundant steganography using four different logical
 channels, and should be proof to many modifications, including reformatting,
 spell checking and so on.
 .
 This package is useful in forensic investigations and security actions. It
 provides three commands: sd-eng, sd-engf and sd-c. sd-eng and sd-engf provide
 watermarking for draft-quality and fine-quality English language text
 documents, respectively, whereas sd-c provides experimental C source code
 watermarking.
 .
 Warning: Snowdrop is currently in beta, and may produce bad or corrupted
 results, especially when run on C source code.