Cryptography Applications - public key infrastructure

Certificate Authority (CA)

A trusted third-party agency that is responsible for issuing digital certificates. The entity that's responsible for storing, issuing, and signing certificates

X.509 standard

Defines specific items that must be part of any certificate for use on the Internet. what defines the format of digital certificates.

A certificate that has no authority as a CA is referred to as an

End Entity or Leaf Certificate. it's the end of the tree structure and can be considered the opposite of the roots.

digital certificate

Is a file that proves that an entity owns a certain public key.

PKI (Public Key Infrastructure)

Is a system that defines the creation, storage and distribution of digital certificates.

Usually people who are interested in establishing web of trust will organize what are called

Key Signing Parties where participants performed the same verification and signing.

PKI

Public Key Infrastructure

This chain of trust has to start somewhere and that starts with the

Root Certificate Authority.

Root Certificate Authority

These root certificates are self signed because they are the start of the chain of trust. So there's no higher authority that can sign on their behalf.

Self Sign Certificate

This certificate has been signed by the same entity that issued the certificate. This would basically be signing your own public key using your private key. Unless you already trusted this key, this certificate would fail to verify.

SSL/TLS certificates

This is a certificate that a web server presents to a client as part of the initial secure setup of an SSL, TLS connection.

SSL or TLS client certificate

This is an optional component of SSL, TLS connections and is less commonly seen than server certificates. these are certificates that are bound to clients and are used to authenticate the client to the server, allowing access control to a SSL, TLS service.

Registration Authority (RA)

This performs certificate registration services on behalf of a CA. that's responsible for verifying the identities of any entities requesting certificates to be signed and stored with the CA. This role is usually lumped together with the CA.

An alternative to the centralized PKI model of establishing trust and binding identities is what's called the

Web of trust

Fields defined in X.509 - Serial number

a unique identifier for their certificate assigned by the CA which allows the CA to manage and identify individual certificates.

If the root CA signs a certificate and sets a field in the certificate called CA to true, this marks a certificate as

an intermediary or subordinate CA.

code signing certificates

are used for signing executable programs. This allows users of these signed applications to verify the signatures and ensure that the application was not tampered with.

In some cases, a wild card certificate can be issued where the host name is replaced with an

asterisk, denoting validity for all host names within a domain.

In the X.509 standard. there are also certificate fingerprints which aren't actually fields in the certificate itself, but are computed by

clients when validating or inspecting certificates. These are just hash digests of the whole certificate.

The X.509 standard also defines It also defines a certificate revocation list or CRL which is means to

distribute a list of certificates that are no longer valid.

Bootstrapping a chain of trust requires

distributing root CA certificates via alternative channels. Each major OS vendor ships a large number of trusted root CA certificates with their OS. And they typically have their own programs to facilitate distribution of root CA certificates. Most browsers will then utilize the OS provided store of root certificates.

PKI is very much dependent on trust relationships between

entities, and building a network or chain of trust.

At the end of the Key Signing party everyone's public key should have been signed by

every other participant establishing a web of trust.

An intermediary CA can also sign other

intermediate CAs

central repository

is needed to securely store and index keys and a certificate management system of some sort makes managing access to storage certificates and issuance of certificates easier.

Web of Trust

is where individuals, instead of certificate authorities, sign other individuals' public keys.

SSL or TLS client certificates are different from server certificates in that the client certificates aren't

issued by a public CA. Usually the service operator would have their own internal CA which issues and manages client certificates for their service.

It's possible for a certificate to be valid for

multiple host names

In Web of Trust the process of verifying and vouching for a person would be

reciprocal, meaning both parties would sign each other's keys.

Fields defined in X.509 - Certificate signature algorithm

same as the Subject Public Key Info field, these two fields must match.

This Root Certificate Authority can now use the self-signed certificate and the associated private key to begin

signing other public keys and issuing certificates.

Code signing certificates also lets them verify that the application came from the

software author and is not a malicious twin.

When the Root Certificate Authority begins signing other public keys and issuing certificates, it begins to build a

sort of tree structure with the root private key at the top of the structure.

With a SSL/TLS certificate, after the client verify the subject of a certificate it then also verify

the certificate is signed by a certificate authority that the client trusts.

Fields defined in X.509 - Certificate Signature Value

the digital signature data itself

intermediary or subordinate CA.

the entity that this certificate was issued to can now sign other certificates. And this CA has the same trust as the root CA.

In Web of Trust, before an individual signs a key, they should first verify

the person's identity through an agreed upon mechanism. Usually by checking some form of identification, driver's license, passport, etc.

A certificate contains information about

the public key, the entity it belongs to and a digital signature from another party that has verified this information.

When a SSL/TLS certificate is presented to a client as part of the setup of the connection, The client, usually a web browser, will then verify that

the subject of the certificate matches the host name of the server the client is trying to connect to.

Fields defined in X.509 - Subject Public Key Info

these two subfields define the algorithm of the public key along with the public key itself.

Fields defined in X.509 - Validity

this contains two subfields, Not Before and Not After, which define the dates when the certificate is valid for.

Fields defined in X.509 - Subject

this field contains identifying information about the entity the certificate was issued to.

Fields defined in X.509 - Issuer Name

this field contains information about the authority that signed the certificate.

Fields defined in X.509 - Certificate Signature Algorithm

this field indicates what public key algorithm is used for the public key and what hashing algorithm is used to sign the certificate.

In the future when one of these participants in the initial key signing party establishes trust with a new member, the web of trust extends to include

this new member and other individuals they also trust. This allows separate webs of trust to be bridged by individuals and allows the network of trust to grow.

On the digital certificate, If the signature is valid and we trust the entity that signed the certificate, then we can

trust the public key to be used to securely communicate with the entity that owns it.

In order to bootstrap this chain of trust, you have to trust a root CA certificate, otherwise the whole chain is

untrusted

The X.509 standard was first issued in 1988 and the current modern version of the standard is

version 3.

In Web of Trust, once they determine the person is who they claim to be, signing their public key is basically

vouching for this person. You're saying that you trust that this public key belongs to this individual.

Fields defined in X.509 - version

what version of the X.509 standard certificate adheres to.