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How the Private and Public Key Pair Works

Created on Apr 13 2015, 4:28 PM by Charlotte Pommier

Did you know this month was “couple appreciation month”? Let’s use this as an opportunity to explain in simple words how the security of an online transaction relies on a happy, inseparable couple: a public key and a private key.

Public keys and private keys are part of a general structure we call PKI – Public Key Infrastructure. The SSL and TLS protocols, which are globally used to secure not only websites, but also emails and web applications, are based on this structure. So we might as well say that there are thousands and thousands of public and private keys in operation right now around the world!

Keys are used in algorithms to encrypt and decrypt data. You may think the same key is used to encrypt and decrypt, but there’s a twist: there are algorithms in this world which are able to encrypt data with one key… and decrypt it only with the help of another key! Magical, isn’t it? (For those who don’t believe in magic, you can read more about trapdoor functions here). In the case of SSL, one key – the public key - is used to encrypt data; only the corresponding private key can decrypt it. What a lovely (and useful) couple.

Couple_Appreciation_1.png

In the SSL protocol, public keys and private keys are generated by servers. The private key remains locked and secure in the server, while the public key is pinned to the server’s SSL certificate. Whenever a browser connects to the server, the server sends its SSL certificate which contains the public key. The browser can then use this public key to encrypt data and send it to the server, which is now the only one able to decrypt such data thanks to its private key.

Both keys are inseparable, and of course each pair is unique: the public key belongs to its corresponding private key and only to this one.

Couple_Appreciation_2.png

Public and private keys are essential to the security of our exchanges. Thanks to them, we don’t have to worry about someone eavesdropping on our conversations. But there is still a major issue: what if a hacker intercepts the server’s public key, and sends their own public key instead?

What guarantees the browser that the public key received is actually the public key from the server it wanted to reach?  This is why Certification Authorities like Symantec play an essential role: CAs authenticate servers and their public key through a unique document called the SSL certificate!

If you’re curious about SSL and more specifically about how SSL certificates work, you can find more

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