Do you mean a browser plugin? Probably the biggest issue currently is that browser plugins tend to be written in JS. See this for why it's a problem: http://ww

Do you mean a browser plugin? Probably the biggest issue currently is that browser plugins tend to be written in JS. See this for why it's a problem: http://www.matasano.com/articles/javascript-cryptography/ http://www.matasano.com/articles/javascript-cryptography/ If we someday have an ecosystem of C browser extensions, in-browser crypto may be much more promising. Or not. I suppose there could be other problems besides the language.