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EB/U2 Topic 4 Digital signature, Digital Certificates


A digital signature is a mathematical scheme for demonstrating the authenticity of digital messages or documents. A valid digital signature gives a recipient reason to believe that the message was created by a known sender (authentication), that the sender cannot deny having sent the message (non-repudiation), and that the message was not altered in transit (integrity).

Digital signatures are a standard element of most cryptographic protocol suites, and are commonly used for software distribution, financial transactions, contract management software, and in other cases where it is important to detect forgery or tampering.

Digital signatures are often used to implement electronic signatures, a broader term that refers to any electronic data that carries the intent of a signature, but not all electronic signatures use digital signatures. In some countries, including the United States, Algeria, Turkey, India, Brazil, Indonesia, Mexico, Saudi Arabia, Uruguay, Switzerland and the countries of the European Union, electronic signatures have legal significance.

A digital signature (not to be confused with a digital certificate) is a mathematical technique used to validate the authenticity and integrity of a message, software or digital document.


Digital signatures are based on public key cryptography, also known as asymmetric cryptography. Using a public key algorithm such as RSA, one can generate two keys that are mathematically linked: one private and one public. To create a digital signature, signing software (such as an email program) creates a one-way hash of the electronic data to be signed. The private key is then used to encrypt the hash. The encrypted hash — along with other information, such as the hashing algorithm — is the digital signature. The reason for encrypting the hash instead of the entire message or document is that a hash function can convert an arbitrary input into a fixed length value, which is usually much shorter.


A digital certificate is an electronic “passport” that allows a person, computer or organization to exchange information securely over the Internet using the public key infrastructure (PKI). A digital certificate may also be referred to as a public key certificate.

An attachment to an electronic message used for security purposes. The most common use of a digital certificate is to verify that a user sending a message is who he or she claims to be, and to provide the receiver with the means to encode a reply.

An individual wishing to send an encrypted message applies for a digital certificate from a Certificate Authority (CA). The CA issues an encrypted digital certificate containing the applicant’s public key and a variety of other identification information. The CA makes its own public key readily available through print publicity or perhaps on the Internet.

Digital certificates function similarly to identification cards such as passports and drivers’ licenses. Identification cards are issued by recognized government authorities. When someone requests an identification card, a government authority verifies the identity of the requester, certifies that the requester meets all requirements to receive the card, and then issues the card. When an identification card such as a driver’s license is presented to others, they can verify the identify of its owner because the card provides the following security benefits:-

  • It contains personal information to help identify and trace the owner.
  • It contains the photograph and the signature of the rightful owner to enable positive identification.
  • It contains the information that is required to identify and contact the issuing authority.
  • It is designed to be tamper resistant and difficult to counterfeit.
  • It is issued by an authority that can revoke the identification card at any time (for example, if the card is misused or stolen).
  • It can be checked for revocation by contacting the issuing authority.


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