An excuse to mention cryptography

I needn’t tell Troppo readers that a few days of heady excitement are afoot in Canberra. Personally I doubt that the PM will go, but Swan might be in serious trouble.

A lot of argy-bargy has gone on about whether a “smoking gun” email, allegedly in the Coalition’s possession, is genuine. A lot of people have pointed out that it’s pretty easy to forge an email — after all, it’s just a bunch of text.

Enter the noble and mysterious art of cryptography. In particular, enter the mature but rarely-used technology of digital signatures.

Some Technical Talk

Digital signature schemes combine the two great pillars of modern cryptography: one-way hash functions and dual-key encryption.

A hash function turns any document into a fixed-length string. The clever part about hash functions is that they aren’t reversible. You can’t take the generated hash and run it backwards to get the original document. Hashes are widely used to store passwords or to check the integrity of downloads.

Dual-key encryption means that every person gets a private key, and they distribute a public key. If you possess the private key you can read items encrypted with the public key, which ensures that only the intended reader will see it. Alternatively, if you encrypt an item with your private key, people can ensure that it was you who did it by applying your public key.

In a digital signature scheme, these two things are combined. First, the system generates a hash for the document being signed. Then, it encrypts that hash with the sender’s private key.

At the other end, the receiver decrypts the signature with the public key, then computes the document hash themselves. If the received hash is the same as their own calculation, they know that the document was sent by the person claimed.

Some Practical Consequences

Digital signatures (and a related technology, HMACs, which I won’t discuss here) can quickly settle the question of who said what and when. If the sender’s private key is secure, the digital signature cannot be forged, cannot be reneged and cannot be refuted. It is, in actual fact, more reliable than a paper signature.

If the Federal government used digital signatures, it would be trivial to establish whether the Opposition possessed the genuine article. HMACs would further make it possible to determine, given a collection of correspondence, whether any items are missing from that collection.

Support for digital signatures is built into every modern email client and server. I reckon that the government could do worse than to roll out digital signature infrastructure throughout government. For centuries we’ve been relied on the integrity of the paper trail; it would be nice if the e-trail was as trustworthy.

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