There was a good discussion on this kind of service a while back--check out "Javascript Cryptography Considered Harmful"
HN: http://news.ycombinator.com/item?id=2935220 Direct link: http://www.matasano.com/articles/javascript-cryptography/
"Javascript Cryptography Considered Harmful" has been posted on here and lays things out nicely, as e1ven mentions.
Here's the original discussion: https://news.ycombinator.com/item?id=2935220
There was a good discussion on this kind of service a while back--check out "Javascript Cryptography Considered Harmful"
HN: http://news.ycombinator.com/item?id=2935220 Direct link: http://www.matasano.com/articles/javascript-cryptography/