Digital signature

Digital signature is a sequence of bits that are calculated mathematically while signing a given document. Since it is easy to copy, alter or move a file on computers without leaving any trail, one needs to be very careful in designing a signature scheme. In keeping with the properties of a handwritten signature, a digital signature should depend on some secret known only to the signer and on the content of the message should be able to distinguish between a forgery and a valid signature without requiring the signer to reveal any secret information. A general digital scheme consists of three algorithms:
1. A key generation algorithm
2. A signing algorithm
3. A verification algorithm
Cryptography: The process or skill of communicating in or deciphering secret writings or ciphers.
Working of digital signature:
Public key cryptography gives reliable method for digital signing and signature verification system based on public/private key pairs. A person can sign a given digital message with private key. The two steps involved are:
1. Calculate the hash value
In the first step of the process, a hash value of the message is calculated by applying some cryptographic hashing algorithm. The calculated hash value is a sequence of bits, usually with affixed length, extracted in some manner from the message. All reliable algorithms for message digest calculation apply such mathematical that when just a single bit from the input message is changed, a completely different digest is obtained.
2. Calculate the digital signature
In the second step of digitally signing a message, the information obtained in the first step hash-value of the message is encrypted with the private key of the person who signs the message and thus an encrypted hash-value, also called digital signature is obtained.
Verifying signed data
A digital signature is associated with a X.509 certificate which contains the sender’s public key. This key is used to decrypt the digital signature into the original hash value on the recipient’s computer. To verify the digital signature, the same hashing algorithm is used to generate a hash value based on the original data. The decrypted hash value is compared to the generated hash value. If the values match, the digital signature is valid.
1. Calculate the current hash value
2. Calculate the original hash value
3. Compare the current and the original hash-value
Benefits of digital signature:
Authenticity
Although digital signatures alone cannot prevent the content from being manipulated during delivery, using digital signatures provides a mechanism to detect tampering of it occurs. If the data is altered in any way after being digitally signed, the recipient can tell via properties of the signature that the data sent does not match the data received.

Acknowledgement
Data can be signed by the recipient as well as the sender. When a recipient signs the data and returns it to the sender, this signature is an acknowledgement. Digital signatures used in this way also provide no repudiation; the ability to prove that the data was sent by the signer.