Just after Yaakov's fight with the angel has ended with the rising of the morning sun, Yaakov asks the angel what his name is. The angel replies 'why do you ask my name' (32;20) and blesses Yaakov there. What's going on in this strange exchange; why does Yaakov care what the angel's name is, and why does the angel refuse to reveal his name?
Rashi (32;25) quotes the mirdash which explains that this angel was the evil force of Esav, whose descendants would be responsible for future persecutions against us and the destruction of thesecond beis hamikdash. One must also realise that a name indicates one's mission and essance; the Arizal says that our judgment in Heaven will involve HaShem asking us what our name is, and whoever has not fulfilled their mission in life will not be able to respond. Therefore, Yaakov was asking the angel not just what his name was, but what his essence is, ie what tactics he will use against the Jewish people in the future. And Yaakov would reveal this to his descendants to guard thereselves. But the angel responds that he is not revealing his name, for (Rashi 32;30), his name changes constantly, ie there is no fixed tactic across the generations. Each generation has its own different spiritual battle and different spiritual challenges.