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Author Topic: Understanding Digital Signatures  (Read 3056 times)

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Vineesh K P

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Understanding Digital Signatures
« on: January 27, 2018, 04:27:11 pm »
Handwritten signatures have long been used as a proof of authorship of, or at least agreement with, the contents of a document. Digital signatures can provide the same functionality as handwritten signatures, and much more.

It works on the basis of asymmetric encryption and hashing. We know that asymmetric encryption works using public and private key pair. The private key is kept secret by the owner while the public key is available to others. Files encrypted with either of them will need the other one to decrypt. If we encrypt a file with our private key, anyone with our public key can decrypt it and they can verify that it was encrypted by us. But as asymmetric encryption is computationally expensive it is not always ideal. Digital signatures solve this issue by incorporating hashing process with asymmetric encryption.

Digital signatures use a hashing algorithm to produce a much smaller fingerprint of the original data. This fingerprint is then encrypted with the sender's private key to produce the digital signature. After that, the document and the digital signature are delivered together. The digital signature is validated by taking the document and running it through the hash algorithm to produce its fingerprint. The signature is then decrypted with the sender’s public key. If the decrypted signature and the computed hash match, then the document is identical to what was originally signed by the signer.

Usually asymmetric algorithms, such as RSA and DSA are used for digital signatures.

Digital signatures provide three basic security services in secure communications:

    Authenticity of digitally signed data: Digital signatures authenticate a source, proving that a certain party has seen and has signed the data in question.

    Integrity of digitally signed data: Digital signatures guarantee that the data has not changed from the time it was signed.

    Nonrepudiation of the transaction: The recipient can take the data to a third party, and the third party accepts the digital signature as a proof that this data exchange did take place. The signing party cannot repudiate that it has signed the data.

To achieve these goals, digital signatures have the following properties:

    The signature is authentic: The signature convinces the recipient of the document that the signer signed the document.

    The signature is not forgeable: The signature is proof that the signer, and no one else, signed the document.

    The signature is not reusable: The signature is a part of the document and cannot be moved to a different document.

    The signature is unalterable: After a document is signed, it cannot be altered.

    The signature cannot be repudiated: Signers cannot claim later that they did not sign it.