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Introduction to cyber security: stay safe online
Introduction to cyber security: stay safe online

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1.4 Asymmetric or public key cryptography

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Asymmetric cryptography, better known as public key cryptography, side-steps the key distribution problem as each user creates their own keys:

  • the private key which they keep safe and never distribute
  • the public key which can be sent to anyone with whom they want exchange encrypted information.

Together the two keys are known as a key pair, which is what was used by Alice and Bob.

Unlike symmetric encryption, the two keys behave differently; the public key is the only key that can decrypt ciphertext encrypted using the corresponding private key and the private key is the only key capable of decrypting files encrypted with the corresponding public key. Crucially, the value of one key cannot easily be determined from the other, so even if the public key falls into hostile hands, the value of the private key cannot be determined.

Public keys can be distributed using email attachments or through public key chain servers which act as distributors for large numbers of public keys. The creator of a public key uploads their key to the key chain server and it is freely available to anyone who wants to use it.

Although the mathematics behind public key cryptography is incredibly complex, the process of using it is relatively simple. To send a message using public key cryptography is simple. The sender obtains a copy of the recipient’s public key, either by email or from a key chain server, and uses it to encrypt the message. The resulting ciphertext is then sent to the recipient who uses their corresponding private key to restore the original plaintext.

Public key cryptography is popular because there does not have to be any initial secure exchange of secret keys for an encrypted message to be sent (remember, users only ever exchange their public keys). However, it is generally far slower than symmetric encryption; and because of a quirk in the underlying mathematics, traditional public key cryptographic techniques require far longer keys to offer the same level of protection as symmetric encryption.

A newer type of public key cryptography, known as ‘elliptic curve cryptography’, can be just as secure as symmetric encryption using similar key lengths.

In the next section you’ll discover why these encryption methods aren’t used to keep the internet more secure.