Name this book? its been bugging me?

im on the hunt for titles i read in middle school.

this book was set during the civil war. it was about a girl who dressed up as a boy in order to leave and deliver a messege to other patriots. i beleive the messege was hidden in a loaf of bread. at some point she ends up on a loyalist ship on her way to deliver the messege and almost gets caught when a soldiers tries to make her give him the bread.

thanks for the help and check out my other book search questions =]

thank you