Public Key Technology

The public key technology is simple to understand if you are familiar with padlocks using two keys. For a given key pair, these padlocks use one key for locking and the corresponding other key for unlocking.

Suppose Alice and Bob each have a pair of such keys for protecting their house. We will call Alice’s keys A1 and A2. We will call Bob’s keys B1 and B2.

Now, if Alice locks her door with A1, she will be able to open it only with A2. She will not be able to unlock the door with say B1, B2 or even A1. Similarly, if she had locked her key using A2, she will be able to open it only with A1 and not B1, B2 or A2.

The same applies to Bob. If he had locked his door with B1, he will be able to open it only with B2 and vice versa.

The public key technology uses the same concept of a padlock. Here, Alice and Bob each have a pair of keys. The keys here refer to a number that is very lengthy to remember or use for manual computation. Of the two keys, one is called the private key and the other is called the public key. The two keys are complementary in the sense that if a message is encrypted with one key, it can be decrypted only using the other. The private and public keys are together referred to as a 'key pair'.

Normally, the public key can be shared with all people who may wish to encrypt and send a message to the owner of the key pair. The private key is kept confidential by the owner. Thus only the owner of a key pair can decrypt and understand the encrypted message. In this way, the public key technology helps in ensuring confidentiality of a message.

The public key technology also helps in authenticating the sender of a message. This is called digital signature. To produce a digital signature, the sender of a message encrypts the message using his private key. If the message decrypts using the corresponding public key, then it can be assumed that the sender of the message is the owner of the key pair.