A method of marking an original text document in which existing inter-word blank characters are altered is disclosed. A reversible transform is first applied so that inter-word intervals become exclusively comprised of odd numbers of blank characters. Transformed original text is then split in two subsets and an authentication pattern is merged into first subset by adding inter-word blank characters. A blurring pattern is computed which further modify the number of blank characters so as to hide the authentication pattern. Second subset is blurred too before subsets are recombined to obtain a marked text susceptible of authentication. A method of authenticating a text document marked according to the here above method is also disclosed. The system and the methods of the invention permit that a text document be authenticable while authentication pattern is imbedded, and deeply hidden, into the text document itself. Also, the exact original format, including the numbers of inter-word blanks, can be restored by the recipient.