Introduction to Digital Signatures

The Process & Validity behind Digital Signature Technology.

What is Public Key Infrastructure – PKI

cryptographic system that uses two keys, a public key known to everyone and a private key, the private key has full control to the key owner, and has to keep in secured environment. A unique element to the public key system is that the public and private keys are related in such a way that only the public key can be used to encrypt messages and only the corresponding private key can be used to decrypt them. Moreover, it is virtually impossible to deduce the private key if you know the public key.

When David wants to send a secure message to Donna, he uses Donna’s public key to encrypt the message. Donna then uses her private key to decrypt it.

Public key cryptography was invented in 1976 by Whitfield Diffie and Martin Hellman. It is also called asymmetric encryption because it uses two keys instead of one key (symmetric encryption).

Digital Signatures Process: An Explanation

We can use David and Donna to demonstrate how digital signatures work.

From David’s perspective, the process of digitally signing a document seems simple. However, several important steps happen behind the scenes.

Generating a Private and Public Key

Before David can digitally sign any documents, he must first obtain a Private Key and a corresponding Public Key. This is a one-time setup that is typically done by the Secured Signing Service when David first registers.

  • The Private Key is kept secret and is used only by David to sign documents. It is never shared.

  • The Public Key is made available to everyone (including Donna). It is used to validate the signer’s (David’s) digital signature.

Generating a Private and Public Key

Digitally Signing Document

This section details the steps David takes to sign a document and what happens when he sends it to Donna.

Create a Digital Signature

First, a unique cryptographic code called a hash is generated for the document using a mathematical algorithm. This hash is a short string of characters that represents the entire document.

Even a tiny change to the document will result in a completely different hash.

Add the Digital Signature to the Document

To create the final digital signature, the following elements are combined:

  • The document’s hash (from step 1).

  • David’s Digital Certificate (which contains his Public Key).

This is done by using David’s Private Key to encrypt the document’s hash.

The resulting digital signature is unique to both the document and David. Finally, this digital signature is embedded into the document.

David then sends the signed document to Donna.

Donna’s Validation Process

    1. Donna uses David’s Public Key (included in the Digital Certificate) to decrypt the digital signature. This action reveals the original document hash that David created when he signed the document.
    2. Donna compares the original hash (extracted from the signature) with the new hash (she just calculated).

    3. Determine Validity:

      • If the hashes match: The signature is valid, and the document has not been altered since David signed it.

      • If the hashes do not match: The document has been changed after signing, or the signature is invalid.

signing a secured signing document
verified signed document

What is a Certificate Authority (CA)

A Certificate Authority (CA) is a trusted third party that issues digital certificates.

  • Digital Certificates act like a digital ID card, confirming the identity of a signatory (user).

  • The CA issues a certificate after verifying the user’s information.

  • In a digital signature system (PKI), the CA uses its own authority to authenticate the user’s certificate, assuring others that the signatory is genuinely who they claim to be.