Gemara on today’s daf relates that Bar Koziva ruled for two-and-a-half years and then approached the Sages declaring himself to be the Messiah. The Sages replied that the Messiah was expected to be able to judge based on smell (see Yeshayahu 11:1-3), i.e. that he would have a unique sense of truth and justice. Finding that he was unable to do so, the Sages rejected his claim and he was killed.
According to the simple reading of the Gemara it sounds as if the Sages themselves killed Bar Koziva, in contrast with the story that appears in various midrashim (Eikha and others) that report that he was killed by the Roman enemy. The Ra’avad accepts the simple version of our Gemara, but the Rambam rejects it entirely – either because he relies on the other midrashim or, if he accepts our Gemara, views the actions of the Sages as badly mistaken. Thus, the Rambam presents Bar Koziva as the model of what we look for in the Messiah. Some suggest that the Rambam may interpret our Gemara differently, understanding that the Sages rejected him as the Messiah, and without their support he was ultimately killed by the enemy.
The individual who is known to the Gemara as Bar Koziva is the same person who is known to us – as he was called by Rabbi Akiva – as Bar Kokheva. In letters written by him that have been unearthed in archaeological excavations, we find that he signed his name Shimon bar Kosba. Apparently, the other names that he had “played off” of his actual name. His supporters called him Bar Kohkeva – “the son of the star” – basing themselves on the passage recited by the prophet, Bilam (Bamidbar 24:17), darakh kokhav mi-Yaakov. Those who opposed his revolt – especially after it failed – called him Bar Koziva – “the son of falsehood.”