This is virtually identical to the text in Massekhet Soferim, paragraph 12, until the last line. Before the second "Blessed are you", Soferim contains the line: "And in his days may Judah be made safe, and Israel to dwell securely, and he shall be called, ' is our vindicator'." This line remained in Romaniot liturgy. Instead of "Shield of David", Soferim has "who brings to fruition the mighty salvation of his people Israel." But by the 3rd century, "shield of David" was the text in use, predating Soferim. "He" and "his" refer to the Messiah, a descendant of King David. The lines "let no stranger sit on his throne" and "others continue to usurp his glory" might date back to the earliest Talmudic times, when the Hasmoneans and Herodians, rather than true descendants of the royal house of David, were rulers of the Holy Land.