I glad there’s no such thing as a hash collision

ISPs pressed to become child porn cops – Security- msnbc.com

# A law enforcement agency would make available a list of files known to contain child pornography. Such files are commonly discovered in law enforcement raids, in undercover operations and in Internet searches that start with certain keywords (such as “pre-teens hard core”). Police officers have looked at those files, making a judgment that the children are clearly under age and that the files are illegal in their jurisdiction, before adding them to the list. Each digital file has a unique digital signature, called a hash value, that can be recognized no matter what the file is named, and without having to open the file again. The company calls this list of hash values its Global File Registry.
# Whenever an Internet user searched the Web, attached a file to an e-mail or examined a menu of files using file-sharing software on a peer-to-peer network, the software would compare the hash values of those files against the file registry. It wouldn’t be “reading” the content of the files — it couldn’t tell a love note from a recipe — but it would determine whether a file is digitally identical to one on the child-porn list.

The article mentions that there are other investigative methods that don’t raise the same universal-search issues, but it doesn’t really talk about how easy it would be to defeat such hashing systems (either with false positives or false negatives) by making trivial changes to the contents of files.

It does, however, mention that the man-in-the-middle software they’re talking about deploying can intercept requests for an encrypted file and turn them into requests for a compressed (and thus easily-decodable) version. Gosh, I wouldn’t mind having a proxy that took all of someone’s https: requests and turned them into http: instead; it sure wouldn’t be a gaping security hole, now would it?

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