Section 2(1)(p) of the Information Technology Act, 2000 (or the IT Act) defines it as authentication of any electronic record by a subscriber by means of an electronic method or procedure in accordance with the provisions of section 3
Before going into the provisions of section 3, it is pertinent to explain a few basics of cryptography. Public key cryptography is the method recognized by the IT Act for the safeguarding of computer documents.

Public key cryptography is a form of cryptography, which generally allows users to communicate securely without having prior access to a shared secret key. This is done by using a pair of cryptographic keys designated as public key and private key. A public key is essentially like an email address, and a private key, like the email address password. The public key is sent to the receiver, while the private key is not disclosed to anybody. They are related mathematically. What has been encrypted by the first key can only be decrypted by the second – and vice versa. Hence, if A wants to send a secure email to B, A must encrypt it with B’s public key, so that when B receives the encrypted email, he can decrypt it using his own private key.