What Vidduy

The meforshim ask why is vidduy maaser called a vidduy, the person does the opposite of vidduy, he declares that he fulfilled all the mitzvot and obligations?  The Malbim in Vayikra (5:5) explains the word vidduy means גדר הוידוי הוא הפך הכפירה וההכחשה, והוא שמפרסם בפירוש דבר שטבע בני אדם להעלימהו.  
The Rambam Sefer Mitzvot #131 says להתוודות וידוי מעשר, שנאמר: "ואמרת לפני ה' אלוהיך ביערתי הקודש" (דברים כו,יג).  He doesn't mention the act of biur masser as the mitzvah, only the vidduy.  He repeats this understanding in in the Laws of Maaser Sheni Ch. 11  מִצְוַת עֲשֵׂה לְהִתְוַדּוֹת לִפְני ה' אַחַר שֶׁמּוֹצִיאִין כָּל הַמַּתָּנוֹת שֶׁבְּזֶרַע הָאָרֶץ וְזֶהוּ הַנִּקְרָא וִדּוּי מַעֲשֵׂר:  Similarly in the beginning of the Laws of Teshuva the Rambam calls the mitzvah of teshuva the vidduy.  Why?  Because it is the vidduy that demonstrates the action is done with complete intent.  By pronouncing out loud the vidduy of biur masser and one's regret of sins that proves that the biur masser and the teshuva is done properly (see Binyan Av volume 2 #47.)