I might be wrong, but I'm pretty sure that in practice, whenever someone signs some content, they actually sign a hash instead of the entire content so that the

I might be wrong, but I'm pretty sure that in practice, whenever someone signs some content, they actually sign a hash instead of the entire content so that the signature is a sane size. If you can create a hash collision between two pieces of content and then get someone to sign one of the pieces of content, then effectively you've gotten them to sign the other piece of content too without them knowing it. This happened a few years ago when SSL certificates still allowed MD5: http://www.zdnet.com/article/ssl-broken-hackers-create-rogue-ca-certificate-using-md5-collisions/ http://www.zdnet.com/article/ssl-broken-hackers-create-rogue...