Concepts of Digital Signatures ​

How Digital Signatures Work ​

Principle of Signature:

A hash value of the data to be encrypted is obtained through a hash function (a unique fingerprint of the data. Any tampering with the data content will result in a different hash). The hash value is encrypted using the signer's private key to obtain the digital signature. The signed data will be generated after attaching the digital signature to the data.

Verification Principle:

Separate the signature from the data, and obtain the hash value of the data through the same hash function used by the signer. Decrypt the hash value using the signer's public key to get the signer's hash value. By comparing the two values, we can confirm whether the file has been tampered with.

Digital Signatures vs Electronic Signatures ​

An electronic signature is essentially an annotation within a document. Apart from the customizable appearance of the signature, it lacks identifiable information about the creator and cannot verify whether the document has been altered.

However, a digital signature uses complex encryption algorithms to create a unique identifier that is linked to both the document's content and the creator's information. Any modification to the document's content results in a failed digital signature verification, ensuring the uniqueness and legitimacy of the signer's identity.

What Is a Digital Certificate ​

A digital certificate is a digital authentication that marks the identity information of the parties in Internet communication. It can be used online to identify the identity of the other party, hence, it is also known as a digital ID. The format of the digital certificate typically adopts the X.509 international standard. It will generally include the certificate's public key, user information, the validity period of the public key, the name of the certificate authority, the serial number of the digital certificate, and the digital signature of the issuing organization.

Digital certificates provide the transmission of information and data in an encrypted or decrypted form during communication between network users, ensuring the integrity and security of information and data.

Support PKCS12 Certificate ​

PKCS12 (Or PKCS #12) is one of the family of standards called Public-Key Cryptography Standards (PKCS) published by RSA Laboratories. ComPDFKit supports signing PDFs with PKCS12 files which are with ".p12" or ".pfx" file extensions.

What Is Certificate Chain ​

A Certificate Chain (Chain of Trust), is an ordered collection of digital certificates used to verify the authenticity and trustworthiness of a digital certificate. Certificate chains are typically employed to establish trust, ensuring that both the public key and the identity of entities are legitimate and trustworthy.

Here are some key concepts within a certificate chain:

• Root Certificate

  The starting point of a certificate chain is the Root Certificate. Root certificates are top-level certificates issued by trusted Certificate Authorities (CAs) and are often built into operating systems or applications. These root certificates serve as the foundation of trust because they are considered inherently trustworthy.

• Intermediate Certificates

  Intermediate certificates, also known as issuer certificates or sub-certificates, are issued by root certificate authorities and are used to issue certificates for end entities. Intermediate certificates form an intermediate link within the certificate chain.

• End Entity Certificate

  An end entity certificate is the certificate of the subject of a digital signature (typically an individual, server, or device). These certificates are issued by intermediate certificate authorities and contain the public key and relevant identity information.

• Trust Establishment

  Trust is established through the certificate chain, passing trust from the root certificate to the end entity certificate. If the root certificate is trusted, then the end entity certificate is also trusted, as the trust chain between them is continuous.

Certificate Authority (CA) ​

A digital certificate issuing authority is an authoritative body responsible for issuing and managing digital certificates, and as a trusted third party in e-commerce transactions, it bears the responsibility for verifying the legality of public keys in the public key system.

The CA center issues a digital certificate to each user who uses a public key, the function of the digital certificate is to prove that the user listed in the certificate legally owns the public key listed in the certificate. The CA is responsible for issuing, certifying, and managing issued certificates. It needs to formulate policies and specific steps to verify and identify user identities and sign user certificates to ensure the identity of the certificate holder and the ownership of the public key.

Whether a Digital Signature Needs a CA ​

It is not necessary. When there is not a third-party notary, a CA is not needed, and we can use a self-signed certificate. With ComPDFKit, you can manually set to trust self-signed certificates, which is very useful for trusted parties to sign and check files. However, since there is no digital certificate issuing authority for certification, self-signed digital identity cards cannot guarantee the validity of identity information, and they may not be accepted in some use cases.

How to Confirm the Identity of the Digital Certificate Creator ​

Subject contains identity information about the certificate holder, commonly including fields such as C (Country), ST (Province), L (Locality), O (Organization), OU (Organizational Unit), CN (Common Name), and others. These details help identify who the certificate holder is. DN (Distinguished Name) represents the complete and hierarchical representation of the "Subject" field. It includes all the information from the "Subject" field and organizes it in a structured manner.

The X.509 standard specifies a specific string format for describing DN, for example:

CN=Alan, OU=RD Department, O=ComPDFKit, C=SG, Email=xxxxxx@example.com