What is an X.509 certificate?

An X.509 certificate is an electronic document that proves the ownership of a cryptographic public key. The certificate includes information about the key, its owner (subject), issuer, and the digital signature of the issuer that verifies the content of the certificate. If the certificate signature is valid and the software trusts the issuer, then the public key can be used to communicate securely with the certificate subject. The most common format for a certificate is defined in the X.509 standard and further detailed in RFC 5280.

How are certificates issued?

In public key infrastructure (PKI), the certificate is issued by a certificate authority (CA) that is usually a company that charges its customers for issuing certificates for them. Certificates can also be issued by individuals in a web of trust scheme.

What are the uses of a certificate?

A certificate can be used for Transport Layer Security (TLS) over HTTPS, email encryption, code signing, digital signatures and other purposes. A certificate can have one or more purposes.

What is the structure of a certificate?

• Certificate
  • Version
  • Serial Number
  • Signature Algorithm
  • Issuer
  • Validity period
    • Not Before
    • Not After
  • Subject
  • Subject Public Key Info
    • Subject Public Key Algorithm
    • Subject Public Key
  • Issuer Unique Identifier (optional, v2+)
  • Subject Unique Identifier (optional, v2+)
  • Extensions (optional, v3)
    • ...
• Certificate Signature Algorithm
• Certificate Signature

Note: Thumbprints are not part of the encoded certificate. They are generated by calculating the hash of the encoded certificate.

Following is the description of each field:

• Version: The version number of the certificate. For SSL/TLS certificates, version 3 is used for its introduction of the extensions fields.
• Serial Number: A positive integer assigned by the issuing CA to each certificate. Must be unique for each certificate issued by a given CA.
• Signature Algorithm: Contains the algorithm identifier for the algorithm used by the CA to sign the certificate.
• Issuer: Identifies the entity that issued and signed the certificate represented as a distinguished name (DN).
• Validity Period: The time interval during which the CA warrants that it will maintain information about the status of the certificate.
• Subject: Identifies the entity associated with the public key in the certificate represented as a distinguished name (DN). The subject name(s) can also be included in the subjectAltName extension. The certificate is self-signed if the subject is the same as the issuer (i.e. certificate signed by the same entity).
• Subject Public Key Algorithm: Identifies the algorithm used to generate the public key (e.g., RSA, DSA, or Diffie-Hellman).
• Subject Public Key: The subject's public key.
• Issuer/Subject Unique Identifier: These fields are optional and might only appear if the certificate version is 2 or 3. They should contain the subject and the issuer unique identifiers.
• Extensions: Contains a list of extensions associating additional attributes to the certificate. This field must only appear if the certificate version is 3.
• Certificate Signature Algorithm: Contains the signature algorithm identifier used by the issuer to sign the certificate. Its value must match the Signature Algorithm field contained within the Certificate fields.
• Certificate Signature: The digital signature of the certificate fields encoded in ASN.1 DER.

How is a certificate encoded?

An X.509 certificate is a data structure in binary form encoded in Abstract Syntax Notation One (ASN.1) based on Distinguished Encoding Rules (DER). ASN.1 defines the serialization format for each of the fields within a certificate while DER defines the structure of these fields. The DER is specified in X.690 ITU-T standard.

A certificate can be either distributed in its raw binary form or further encapsulated into a Privacy-Enhanced Mail (PEM) file which includes the base64 encoding of the DER binary message preceded by "-----BEGIN CERTIFICATE-----" and followed by "-----END CERTIFICATE-----". The PEM file is usually stored with .pem, .cer or .crt file extensions.

The PEM file may contain multiple certificates. For example, an operating system might provide a file containing the list of trusted CA certificates, or a web server might be configured with a certificate chain file that contains the end-entity certificate plus the list of intermediate certificates.

Useful links:

• Wikipedia - Public key certificate
• Wikipedia - X.509
• RFC 5280 - Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile
• Wikipedia - Abstract Syntax Notation One (ASN.1)
• Wikipedia - X.690
• Wikipedia - Privacy-Enhanced Mail (PEM)