1. A method of marking an original text document &lsqb; 300 &rsqb;, said original text document comprised of words &lsqb; 125 &rsqb; separated by inter-word intervals, said inter-word intervals including one &lsqb; 110 &rsqb; or more &lsqb; 115 &rsqb; blank characters, said method consisting in altering the numbers of said blank characters, said method comprising the steps of: applying a reversible transform &lsqb; 305 &rsqb; over said original text document in order that all said inter-word intervals become exclusively comprised of odd numbers of said blank characters &lsqb; 425 &rsqb;; splitting &lsqb; 310 &rsqb; transformed said original text document into a first subset &lsqb; 520 &rsqb; and a second subset &lsqb; 530 &rsqb; of said words including the trailing said inter-word intervals of said words; and, over said first subset: computing &lsqb; 315 &rsqb;, from said original text document and a secret-key &lsqb; 312 &rsqb;, an authentication pattern &lsqb; 610 &rsqb; that fits the number of said intervals of said first subset; adding &lsqb; 320 &rsqb; inter-word blank characters in positions corresponding to said authentication pattern; generating &lsqb; 325 &rsqb; canonical form of said first subset; computing &lsqb; 330 &rsqb;, from said canonical form of said first subset and said secret-key, a blurring pattern &lsqb; 630 &rsqb; that fits the number of said intervals of said first subset; modifying &lsqb; 335 &rsqb; the numbers of inter-word blank characters according to said blurring pattern; and, over said second subset: generating &lsqb; 345 &rsqb; canonical form of said second subset; computing &lsqb; 350 &rsqb;, from said canonical form of said second subset and said secret-key, a blurring pattern that fits the number of said intervals of said second subset; modifying &lsqb; 355 &rsqb; the numbers of inter-word blank characters according to said blurring pattern; recombining &lsqb; 340 &rsqb; said first subset and said second subset thereby, obtaining a marked text for authentication.