What Is PKI Certificate

PKI Certificate Tutorials - Herong's Tutorial Examples

∟What Is PKI Certificate

This section describes what is PKI certificate and what are basic data fields included in a PKI certificate.

What Is PKI Certificate? A PKI Certificate is a document, digitally signed by a PKI Certificate Authority (CA), that certifies the identity of a given entity, e.g. Website and email address, and its public key. In other words, a certificate is used to prove the ownership of a public key.

From a cryptographic point of view, a PKI certificate only needs 4 basic data fields:

Issuer - The CA who issued the certificate.
Subject - The entity who owns the public key included in the certificate.
Public Key - The public key owned by the "Subject".
Signature - The digital signature generated by CA's private key. It proves that the "Subject" owns the "Public Key".

However, a PKI certificate is required to contain other data fields to provide better protection. For example:

Validity - A period of time during which the certificate is assumed to be valid. Outside this period, the certificate should be considered as invalid.
Key Usage - A list of codes representing usage areas where the public key is restricted to. Using the public key in other areas is considered as invalid.
Authority Key Identifier - A fingerprint of CA's public key. It helps to retrieve the CA certificate that continas the CA's public key, which is needed to verify the Signature of the certificate.

Here is an example of a PKI certificate encoded in the Privacy Enhanced Mail (PEM) format:

Here is the content of the above certificated decoded and printed out by the OpenSSL tool:

Certificate:
  Data:
    Version: 3 (0x2)
    Serial Number:
      16:0e:df:b2:f2:06:81:53:49:0b:5a:cc:20:41:aa:04
    Signature Algorithm: sha256WithRSAEncryption
    Issuer: C=GB, ST=Greater Manchester, L=Salford, O=COMODO CA Limited,
      CN=COMODO RSA Client Authentication and Secure Email CA
    Validity
      Not Before: Sep 10 00:00:00 2018 GMT
      Not After : Sep 10 23:59:59 2019 GMT
    Subject: emailAddress=herong_yang@yahoo.com
    Subject Public Key Info:
      Public Key Algorithm: rsaEncryption
      RSA Public Key: (2048 bit)
        Modulus (2048 bit):
          00:9b:5a:13:a6:76:62:5e:76:d6:01:06:4d:e0:ff:
          1f:9b:1a:aa:83:65:f2:92:0c:61:17:ed:1d:7d:64:
          33:23:eb:7f:72:05:a1:f2:10:c8:68:b6:12:24:9f:
          ...
        Exponent: 65537 (0x10001)
    X509v3 extensions:
        X509v3 Authority Key Identifier:
          keyid:
            82:AF:6C:8C:F8:C5:FE:96:61:7C:E8:1F:3D:2B:71:48:5E:C4:8B:C0
        X509v3 Subject Key Identifier:
          37:32:0F:1C:36:A9:66:72:A1:07:81:30:4C:07:86:1E:44:F5:78:88
        X509v3 Key Usage: critical
          Digital Signature, Key Encipherment
        X509v3 Basic Constraints: critical
          CA:FALSE
        X509v3 Extended Key Usage:
          E-mail Protection, 1.3.6.1.4.1.6449.1.3.5.2
        Netscape Cert Type:
          S/MIME
        X509v3 Certificate Policies:
          Policy: 1.3.6.1.4.1.6449.1.2.1.1.1
            CPS: https://secure.comodo.net/CPS
        X509v3 CRL Distribution Points:
          URI:http://crl.comodoca.com/COMODORSAClient...EmailCA.crl
        Authority Information Access:
          CA Issuers - URI:http://crt.comodoca.com/COMODORSA...CA.crt
          OCSP - URI:http://ocsp.comodoca.com
        X509v3 Subject Alternative Name:
           email:herong_yang@yahoo.com
  Signature Algorithm: sha256WithRSAEncryption
      68:43:06:cd:62:ab:fa:98:2e:95:5a:1f:9b:51:08:bf:e7:b4:
      a1:fa:c6:d1:92:22:46:85:35:4f:3a:47:bd:52:f3:6c:8c:02:
      83:f5:ba:1a:7d:d1:ef:a7:25:a3:a9:56:41:a5:e8:d7:84:1c:
      ...

From the printout, we can easily locate certificate basic data fields mentioned earlier.

Table of Contents

About This Book

Introduction of PKI (Public Key Infrastructure)

►Introduction of PKI Certificate

PKI Certificate File Formats

OpenSSL - Cryptography Toolkit

"openssl ca" - CA (Certificate Authority) Tool

Java "keytool" Commands and KeyStore Files

PKI Certificate Store

PKCS12 Certificate Bundle File

PKCS7 Certificate Chain File

PKI Certificate Related Terminology

References

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