The construction of the Mishkan it says, זהב וכסף ונחשת, putting zahav (gold) before kesef (silver). In Bereishis, by the creation of the world, during the first day it says Yehi Ohr. Ohr is chesed (which is represented by silver). The second day represents gevura, which in turn is represented by gold.
The reason why during the creation of the world the order is ‘silver’ (chesed) then ‘gold (gevura), is because at the time of creation there was pure chesed, as it says (Tehillim 89:3), “The world was built on chesed,” therefore, chesed, which is represented in kesef, precedes gevura, which is represented by zahav.
By the Mishkan, however, Hashem had to, so-to-speak, contract (tzimtzum) the Shechina (Divine Presence) to dwell in the Mishkan, and any form of constriction (tzimtzum) is associated with Gevura. This is why zahav precedes kesef in the building of the Mishkan.
Still, the word zahav written there has the nekuda (vowel sound) of a פַ תַּ ח (it has a pasach instead of the usual kometz), it says זְ הַ ב הַ תְּ נוּפָ ה (Shemos 38:24), and that is the nekuda of chesed – the nekuda of chochma. And פתח (the vowel) also means an opening as in פתח ושער - from there comes all the kindness.
Putting it all together, when we say in Shir Hashirim, וֹתּרֵ י זָ הָ ב נַ ﬠֲשׂ ֶ ה לָּ ﬠִ ם נְ קֻ וֹדּת הַ כָּ סֶ ף, this means that the Mishkan was made with zahav - the aspect of tzimtzum, but with the nekuda of kesef, meaning the that word “zahav”, instead of having the usual vowel, kometz, is written with the nekuda of pasach, which is associated with chesed.
The Shabbos after Pesach always coincides with the second week of the sefira which is gevura, the aspect of zahav, except that it is menukad with kesef – the nekuda of pasach, which represent chesed. On this Shabbos we make the image of a key on the challah, since every gate has a מפתח (key), in way of beseeching Hashem to open up all the gates of blessings.
5) The previous four reasons are all brought by the Ohev Yisrael in Shabbos Achar Pesach and Likutim Parshas Shmini. There is a fifth reason brought by the Ohev Yisrael, also based on the posuk וֹתּרֵ י זָ הָ ב נַ ﬠֲשׂ ֶ ה לָּ ﬠִ ם נְ קֻ וֹדּת הַ כָּ סֶ ף, connecting the Written and Oral Torah to Challah. (See Ohev Yisrael, Shabbos AchAr Pesach).
6) The matzah we ate on Pesach is supposed to instill in us Yiras Hashem. The Gemara in Mesechtas Shabbos (31a-b) compares Yiras Shomayim to a key, the Gemara says, “Rabbah bar Rav Huna said: Any person that has Torah but doesn’t have Yiras Shomayim is comparable to a treasurer who has the keys to the inner chambers (of the treasure house) but the keys to the outer area were not handed to him. How can he get to the inner chambers (if he can’t first get into the outer parts)?”
Therefore, we put a key in the challah the Shabbos after Pesach to show we want the Yirah obtained from the matzos to stay with us, because if one has Yirah then he will succeed in his Torah learning (Yismach Yisrael).
7) At the beginning of Hilchos Chometz U’Matza, the Rambam lists 8 mitzvos (3 positive & 5 negative) involved in chometz on Pesach. The key we put in the challah alludes to this Rambam. The letters of מפתח (key) stand for ]פ״תפ״ת׳צות מ׳ ח bread, representing the chometz and מ׳ is for matza, these allude to Hilchos Chometz U’Matza, and the ח׳ is the 8 mitzvos involved] (Tzvi LaTzaddik).
8) The Shabbos after Pesach we make challos that resemble matzos, as an allusion to the matzos that were eaten on Pesach Sheini. We place a key in the challah to allude to the “gates” being open until Pesach Sheini (Imrei Pinchos).
9) One minhag is to put keys in the challah and make them in the form of matzos. The reason is that in these seven weeks of the sefira, we are supposed to work on our Avodas Hashem until re-attain the level we reached on first night of Pesach. The way to do this is to put the “left into the right”, meaning mix the trait of ahava (right side) with yirah (left side). In the second week of sefira, we have these two traits in our hands, since the first week of sefira is chesed - ahava, and the second week is gevura - yirah. That is why we make the challah look like matza. Matza represents the Yetzer Tov, the right, and chometz represents the Yetzer Hora, the left. Now, we have challos which are true chometz, in the form of matza; “the left is in the right”, chometz in matza (Shearis L’Pinchos).
10) There are many reasons given for the shlissel challah, and I say that the shlissel challos are the keys to parnasa (Segulos HaBeShT V’Talmidov quoting Nachlas Yaakov).
