The boarding pass format spec linked in the article [1] shows the support for signing (page 49, fields 25 onwards)

The boarding pass data is still plaintext as explained, but a signature is appended to validate that the content has not been tempered with, and who generated it.

I think boarding pass signing is mandatory on all U.S. airlines at least but I have no source for that

[1] http://www.iata.org/whatwedo/stb/documents/bcbp_implementati...