In fact, a document or a record can be digitally signed by typing some unique number or password at a place where the user otherwise would physically sign his name. However, a digital signature should be unique to the person using it and could be verified when attached to a document or file.

Digital signature are based on Crytography, the science of encoding (encryption) and decoding (decryption) messaging and information in a secret code. There are two methods of using digital signatures: symmetric encryption and assymetric encryption.

In symmetric encryption, a single key is used with a unique password for encrypting and decrypting. Though symmetric method is quite fast to execute, it does not provide high security to the transaction. The assymetric method works on two related keys named private key and public key which are used for encryption and decryption as a combination.These two keys are mathematically related so that a document encrypted by one key can not be decrypted with the other. Assymetric method is the one, which is used for digital signatures worldwide as a standard. The standard is also called PKI or Public Key Infrastructure System.

Whenever a subscriber decides to use digital signature, he has to have his own unique private key and public key. These keys are generated by specific software following different cryptology standards.

These keys are a complex sequence of symbols, represented in binary form as a string of bits, which control the encryption algorithm. It means that whenever a message or a document or a file is encrypted using this key, it follows the algorithm stored in the key in a binary form to create a encrypted form of that file. Because both the keys are inter-related and combination of each other, they create the same results when applied. While private key remains a secret and known to only its subscriber, public key is publicly available to all subscribers.

Before understanding how digital signatures are created and encrypted using the public and private key, we should know that digital signature should have three security functions - Integrity, which protect the data from tempering, Authentication to verify the message originator, and non-repudiation, which means the sender can not deny the involvement in transaction.

The following steps are done to create a digital signature.1. A unique number is computed after applying the algorithm of the user's private key on the document to be signed. It means that the private key formula is used to convert the entire document or file into a unique number. This number is called Hash result or mathematical fingerprint.

2 This Hash result is encrypted using sender's private key again. After this, the encrypted file is attached to the document, which can also contain a watermark, photo, image, or just text to display on the document to show that it is digitally signed.

3. The file along with digital signature is again encrypted using the receiver's public key to protect it further and is sent to the receiver.Receiver, on getting the encrypted file, will decrypt the file, verify the signature and ensure that the file has not been tempered after being digitally signed.

Here are steps :
1 The file will be decrypted by the receiver using his own private key.

2. The digital signature will be decrypted using the public key of the sender to find out Hash value of sent document.

3. A new Hash value will be calculated for the same file but this time by the receiver using the public key of the sender.

4.The new Hash value will be matched with the Hash value sent by the sender in encrypted digital signature to ascertain that the file has not been tempered.

The above process ensures that digital signature follows three security functions. Integrity is established by matching Hash values because if any change is made to the file, the new Hash value will not match the old value. The authentication is ascertained by using the public key to decrypt the signature. And, the document's Hash value embedded in digital signature establishes the connection between the sender and the document to make sure he can not deny the ownership of the file.

In India, the digital certificates will be granted by Certifying Authorities appointed by the Digital Signature Controller. It means, any subscriber, who will apply for digital signature, will get a unique code which will generate a public and private key of the subscriber through the software of the authority. The public key will be put on the network for the general access and private key will be known to the subscriber only.

The public key is also used to manage the hierarchy of certifying authorities. It means that the controller's public key is used to certify the authenticity of the certifying authorities and authorities' public keys are used to validate subscribers.