Public key infrastructure

We may try to use Symmetric Cryptography to help us to achieve our aims. It uses just one shared key, which is also called secret. The secret is used for encryption and for decryption of data. Let’s have a look how it can help us to archive our aims. Does it encrypt messages? Yes. Well, Confidentiality is solved, as soon as nobody else, except communicating parties, knows the secret. Does it provide Authentication? Mmm… I would say, no. If there are just two parties in conversation, is seems ok, but if there are hundreds, then should be hundreds secrets, which is hard to manage and distribute. What about Integrity?

Yes, it works fine – it’s very hard to modify encrypted message. As you can guess, symmetric cryptography has one big problem – and that problem is “shared secret”. These two words… they don’t even fit one to other. If something is known by more that one person, it is not a secret any more. Moreover, to be shared, that secret somehow has to be transferred and during that process there are too many way for secret to be stolen. This means that such type of cryptography hardly solves our problems. But it is still in use and works quite well for its purposes. It’s very fast and can be used for encryption/decryption of big amounts of data, e.g. you hard drive. Also, as far as it hundreds or even thousands times faster that asymmetric cryptography, it’s used in hybrid schemas (like TLS aka SSL), where asymmetric cryptography is used for just for transferring symmetric key and encryption/decryption is done by symmetric algorithm.

Let’s have a look at Asymmetric Cryptography. It was invented very recently about 40 years ago. The first paper (“New Directions in Cryptography”) was published in 1976 by Whitfield Diffie and Martin Hellman. Their work was influenced by Ralph Merkle, who believed to be the one who created the idea of Public Key Cryptography in 1974 (http://www.merkle.com/1974/) and suggested it as project to his mentor – Lance Hoffman, who rejected it. “New Directions in Cryptography” describes algorithm of key exchange known as “Diffie–Hellman key exchange”. Interesting fact that the same key exchange algorithm was invented earlier, in 1974 in Government Communication Headquarters, UK by Malcolm J. Williamson, but that information was classified and fact was disclosed just in 1997.

Asymmetric Cryptography uses pair of keys – one Private Key and one Public Key. Private Key has to be kept secret and not shared with anybody. Public Key can be available to public; it doesn’t need to be secret. Information encrypted with public key can be decrypted only with corresponding private key. As far as Private Key is not shared, there is no need to distribute it, and there is reasonably small chance that it will be compromised. So such way of exchanging information can solve Confidentiality problem. What about Authentication and Integrity? These problems are solvable as well and utilise mechanism called Digital Signature. The simplest variant if Digital Signature can use following scenario – subject creates a hash based on message, encrypt that hash with Private Key and attach it to message.

Now if recipient wants to verify the subject who created a message, he will encrypt that hash using subject’s public key (that’s Authentication) and compare it with hash generated on recipient side (Integrity). In reality hash is not exactly encrypted, instead it used in special signing algorithm, but the overall concept is the same. It’s important to notice that in Asymmetric Cryptography each pair of keys serves just one purpose, e.g. if pair is used for signing, it can’t be used for encryption.

Digital Signature, also, is the base for Digital Certificate AKA Public Key Certificate. Certificate is pretty much the same as your passport. It has identity information, which is similar to name, date of birth, etc. in passport. Owner of certificate has to have Private Key which matches Public Key stored in certificate, similar passport has photo of the owner, which matches owner’s face. And, finally, certificate has a signature, and its meaning is the same, as meaning of stamp in passport. Signature proves that certificate was issued by organization which made that signature. In Public Key Infrastructure world such organizations are called Certificate Authorities. If one system discovers that Certificate is signed by “trusted” Certificate Authority, it means that system will trust to information in certificate.

Last paragraph may not be obvious, especially “trust” part of it. What does “trust” mean in that context? Let have a look at simple example. Every site on Web which makes a use of encrypted connection does it via TLS (SSL) protocol, which is based on Certificates. When you go to https://www.amazon.co.uk and it sends its certificate back to your browser. In that certificate there is information about website and reference to Certificate Authority who signed that certificate. First browser will look at the name in certificate – it has to be exactly the same as website domain name, in our case, that’s “www.amazon.co.uk”. Then browser will verify that certificate is signed by Trusted Certificate Authority, which is VeriSing in case of Amazon. You browser already has a list of Certificate Authorities (this is just a list of certificates with public keys) which are known as trusted ones, so it can verify that certificate is issued by one of them. There are some other verification steps, that these two are the most important ones.

Assume in our case verification was successful (if it’s not browser will show is big red warning message, like that one) – certificate has proper name in it and was signed by Trusted Certificate Authority. What does it give to us? Just one thing – we know that we are on www.amazon.co.uk and the server behind that name is Amazon server, not some dodgy website, which just looks like Amazon. When we enter our credit card details and we can be relatively sure that they will be sent to Amazon, but not to hacker’s database. Our hope here based on assumption that such Certificate Authorities like VeriSign do not give dodgy certificates and Amazon server is not compromised. Well, better than nothing J

Another example are severs in organization, which use certificates to verify that they can trust one to other. The schema there is very similar to browser’s ones, except two differences: