The Shulhan Aruch (Orah Haim 128:5) writes that the Kohanim should not go to the “Duchan” (“stage” at the front of the synagogue) for the recitation of Birkat Kohanim while wearing shoes. This Halacha is based on the Gemara’s discussion in Masechet Sota, which tells that Rabbi Yohanan Ben Zakai made such an enactment, requiring the Kohanim to recite the blessing without wearing shoes.
One reason given for this enactment is that in ancient times, the shoes were held in place with straps, which occasionally became untied. The concern was that if a Kohen notices during Birkat Kohanim that a strap is untied, he might bend down to tie it in the middle of the blessing. Someone who sees him at that point might then mistakenly conclude that the Kohen is not reciting the blessing because he is not really a Kohen. Therefore, in order to avoid suspicion about a Kohen’s status, Rabban Yohanan enacted that Kohanim should not wear shoes during Birkat Kohanim. Another reason is that since the Kohanim lift their hands while reciting the blessing, their garments might rise and expose their shoes, which, in ancient times, were usually dirty. This would be disrespectful to the congregation, and so it was decided that Kohanim should not wear shoes while reciting the blessing.
The Mishna Berura (128:15) writes that although shoes worn in modern times do not have the same kind of straps, and are usually clean, nevertheless, “Lo Pelug” – we do not draw a Halachic distinction between our shoes and shoes in ancient times. Once the law was established that the Kohanim recite the Birkat Kohanim without shoes, we observe this provision even nowadays. The Aruch Ha’shulhan (Rav Yechiel Michel Epstein of Nevarduk, 1829-1908) adds that our Rabbinic leadership is not of the same stature as Rabban Yohanan Ben Zakai, and thus we cannot override his enactment. Therefore, even nowadays, Kohanim remove their shoes before reciting Birkat Kohanim.
The Poskim address the question of whether Kohanim are permitted to recite the Beracha with non-leather shoes, such as shoes made from cloth material. The Aruch Ha’shulhan ruled leniently in this regard, but others, including the Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909), in Parashat Tesaveh, ruled that no distinction is made in this regard. This is the view also of Hacham Ovadia Yosef, in Halichot Olam, and of his son, Hacham David Yosef, in Halacha Berura. Therefore, Kohanim must remove their shoes before Birkat Kohanim even if their shoes are not made from leather.
However, the Poskim allow Kohanim to recite Birkat Kohanim with socks on their feet.
As mentioned, the Shulhan Aruch writes that Kohanim should not “ascend the ‘Duchan’” with shoes – which seems to imply that this Halacha applies only when Kohanim recite the blessing from the elevated platform. The rationale would be that only when the Kohanim are elevated above the congregation does the concern arise that the Kohanim might feel embarrassed by an untied strap, or that the dirt on their shoes would be exposed. Indeed, Hacham David Yosef, in Halacha Berura (vol. 6, p. 355), notes that some congregations have the practice of allowing Kohanim to wear shoes during Birkat Kohanim when they stand on the floor, at the same level as the congregation. Hacham David notes that some Poskim indeed allow this, especially if requiring the Kohanim to remove their shoes would make them reluctant to recite Birkat Kohanim.
In truth, however, this does not matter much from a practical standpoint, because the Poskim write that it is proper for Kohanim to stand on a platform when reciting Birkat Kohanim. The Ben Ish Hai writes that the Sages speak of the Kohanim “ascending to the ‘Duchan’” to pronounce the blessing because this is where they should stand. He relates that there were Kohanim in Baghdad who, out of sheer laziness, remained on the ground level during Birkat Kohanim, but he was successful in instituting the policy that all the Kohanim should stand on the elevated area in front of the synagogue when reciting the blessing, which is the proper practice. Since the Shechina (divine presence) rests upon the hands of the Kohanim, it is proper that they stand in an elevated area out of respect for the Shechina.
Thus, regardless of whether Kohanim must remove their shoes when reciting the blessing on the ground level, they should not remain on the ground level for Birkat Kohanim, and should instead remove their shoes and ascend to the “Duchan.”
Summary: The Kohanim must remove their shoes before reciting Birkat Kohanim, but they may wear socks while reciting the blessing. They must remove even shoes which are not made from leather. Some Poskim permit the Kohanim to recite Birkat Kohanim with shoes if they recite the blessing on the ground level, without ascending to the elevated area in the front of the synagogue. Regardless, it is proper for the Kohanim to stand on the elevated area, so they should remove their shoes and then ascend to the platform for Birkat Kohanim.