blessing - Why aren't Purim and Channuka considered a violation of לֹא תֹסִפוּ?

It says in Devarim 4:2:

לֹא תֹסִפוּ עַל הַדָּבָר אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם וְלֹא תִגְרְעוּ מִמֶּנּוּ לִשְׁמֹר אֶת מִצְוֹת יְ־הֹוָ־ה אֱלֹהֵיכֶם אֲשֶׁר אָנֹכִי מְצַוֶּה אֶתְכֶם

Translated:

Do not add to the word which I command you, nor diminish from it, to observe the commandments of the Lord your God which I command you.

Now I understand that the rabbis have the ability to make halachot that are either guards on another halacha (e.g. the concept of muktza) or interpretations of a halacha in torah (e.g. not using electricity on shabbat, being that there was obviously no such thing as electricity when the torah was given). However Chanukah and Purim are not guards on mitzvot, nor are they interpretations of what a mitzva means. They are entirely new mitzvot! So why don't Purim and Chanukah fall into the category of לֹא תֹסִפוּ?

At this point, I am usually given the answer: "Purim and Chanukah are mitzvot derrabannun (from the rabbis) and since they don't claim that it is a mitzva deorita (a mitzva directly from torah) they were allowed to establish it".

Now the problem I have with the above statement is as follows:

There is a Gemara in Brachot (27b) that says (and I'm paraphrasing) that a person who quotes his rabbi on something that he never said is causing the Shechinah to depart from the Jewish people.

Now I quote the actual bracha that is said on Chanukah and Purim.

ברוך אתה ה' א‑לוהינו מלך העולם, אשר קדשנו במצותיו וצונו להדליק נר של חנוכה.‏

Translation:

"Blessed are You, Lord, our G-d, King of the universe, Who has sanctified us with His commandments and commanded us to kindle the Hanukkah lights.

The same is true for Purim:

ברוך אתה ה' א‑לוהינו מלך העולם, אשר קדשנו במצותיו וצונו על מקרא מגלה.‏

Translation:

"Blessed are You, Lord, our G-d, King of the universe, Who has sanctified us with His commandments and commanded us to read the Megillah.

Now according to the bracha, it is saying that G-d himself commanded us to light the Chanukah candles, which is in fact not true, as the torah was given far before the story of Chanukah.

Around this time, I usually get the answer: "It says in the torah that you have to listen to the rabbis, therefor, since the torah says listen to the rabbis, and the rabbis say light candles on Chanukah, then the torah is saying light candles on Chanukah".

Now I have a huge problem with this, because they could have very easily worded the bracha such that it said "Blessed are You, Lord, our G-d, King of the universe, Who has sanctified us with His commandments and commanded us to follow the ways of the rabbis, who have told us to kindle the Hanukkah lights."

Now I go back to the Gemara that I quoted earlier. If it considered such a horrible thing to misquote your rabbi, than Kal Vachomer wouldn't it be even worse to misquote G-d himself???

• Since most of the people here probably originated on Stack Overflow, I though I would try giving an analogy to programming. Let's say I take a programming class with teacher-X. Teacher-X learned how to program directly from Mark Zuckerberg himself. On the first day of class, Mark Zuckerberg comes in, and tells us that he had personally taught Teacher-X himself, and he really knows what he is talking about, so we should listen to everything he says. Teacher-X teaches me to program using a specific style which I go on to use when I code. One day, I am writing a program with my friend, and he notices the strange style I am coding with, so he asks why I am doing it that way. I tell him, Mark Zuckerberg taught me to do it. Well, in fact, Mark Zuckerberg did not teach me to do it that way, and saying that he did was is a false statement. I learned it from Teacher-X, Zuckerberg just told me to listen to him because he had trained him. Similarly, when saying the bracha on Chanukah candles, saying that G-d himself commanded me to light the Chanukah candles is a false statement. He simply told me to listen to someone who did. And I am now quoting G-d with something he never said. And while it may not in practice be as horrible as what is said in the Gemara, I would imagine it is still not a good thing to do.

So how were the Rabbanim able to create Channukah and Purim in the way they did? I don't want to start accusing the Rabbanim of directly violating the laws of the torah, however I'm having a hard time understanding how they could create these holidays in the way they did without doing so. So what did allow them to do it like this?

Answer

The mitzvos of Purim and Chanuka definitely fit the bill for the violation of lo sosifu according to the Ramban (vaeschanan 4:2). More specifically, the Yerushalmi quotes a different pasuk- These are the mitzvos that Hashem commanded Moshe. Lo sosifu refers to adding in general, but the former pasuk forbids adding even through prophecy.

Both the Bavli (Megila 14a) and the Yerushalmi (Megila 6b) take issue with Purim. The Bavli allows Purim based on a kal v'chomer drash from the Torah: If we establish a day of praise for Hashem freeing us from bondage, certainly we can do so for His saving our lives. The Ritva applies the same logic to Chanuka. The Yerushalmi says that they only allowed the writting of the megilla after finding "hints" in the Torah (ie, megilla was built into the Torah in advance).

I would guess that once the Holiday had a leg to stand on, any mitzvos that are ancillary to the holiday would be permitted as well.

As far as the Brachos, I'm not bothered by that question. Throughout the talmud, the sages try to keep things uniform so that the masses would not get confused. For example: Lo palug, nasata devarecha l'shiurim, and generally many decrees were based on avoiding confusion. As long as the sages are not instituting a text that is false (see Alex and Drew's answer), best practice is to keep the same format as other blessings and not confuse the masses by trying to be "more truthful".