X.509v3¶

links: SPA TOC - Certificates and PKI - Index

This topic was already handled in AC2 X.509 - X.509 Certificates. In this document we will summarize the direct talking point from the module SPA.

X.509 Certificate¶

International Telecommunication Unit (ITU) standard, defining the format of public key certificates
Assumes a strict hierarchical system of certificate authorities (CAs) to issue certificates.
Version 3 of X.509 include the support of other topologies like bridges and meshes
Common PKI installation use LDAP or AD server to store certificates
Certificates should be provided with the full chain of trust.
- Root, top level (self-signed) CA certificates
- End-entity, end-user, leaf certificates
- Intermediate, subordinate CA certificates

X.509v3¶

Structure¶

X.509v3 certificates are ASN.1 encoded. Followed are the mandatory fields:

Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number:
            5b:34:0f:bf:66:b8:79:83:70:bb:db:18:b7:28:ad:7d:0f:dc:00:cc
        Signature Algorithm: sha256WithRSAEncryption
        Issuer: C = CH, O = QuoVadis Trustlink Schweiz AG, CN = QuoVadis Swiss Advanced CA G4
        Validity
            Not Before: Jul 26 10:20:10 2021 GMT
            Not After : Jul 26 10:30:00 2024 GMT
        Subject: C = CH, ST = Bern, L = Bern, O = Berner Fachhochschule, CN = Benjamin Fehrensen
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
            Public-Key: (4096 bit)
            Modulus:
                00:a4:c1:8e:ec:4c:81:12:8f:ca:9a:57:b0:73:6e:
                ...
            Exponent: 65537 (0x10001)
    Signature Algorithm: sha256WithRSAEncryption
    Signature Value:
69:54:b9:34:b2:1f:b0:d3:e8:5c:88:f2:5b:8b:2f:ea:7e:d5:
...

Distinguished Name (DN)¶

The Subject and Issuer fields contain DN. A DN consists of Relative Distinguished Names (RDN) see table below. Special characters such as äöé might lead to interoperability issues.

Acronym	RDN	Example
C	Country	C=CH
ST	State or Province	ST=Bern
L	Location	L=Bern
O	Organization	O=Berner Fachhochschule
OU	Organizational Unit	OU=TI
CN	CommonName Unit	CN=www.bfh.ch
G	GivenName	G=Alice
SN	SerialNumber	SN=987654321
Email	Email (deprecated)	Email=alice@bfh.ch
...	Arbitrary RDN	jurisdictionC = CH

Extensions¶

There are many optional extensions, each with its own unique id, expressed as object identifier (OID). http://www.oid-info.com

Extensions can be critical or non-critical. Critical extensions can't be ignored, if it cannot be recognized or processed the certificate has to be rejected. Non-critical may be ignored.

Examples

X509v3 extensions:
    X509v3 Authority Key Identifier:
        95:01:E3:59:D7:55:73:E4:C2:36:0D:A8:3B:92:4B:D9:E6:1E:10:7E
    Authority Information Access:
        CA Issuers - URI:http://trust.quovadisglobal.com/quovadisswissadvancedcag4.crt
        OCSP - URI:http://ocsp.quovadisglobal.com
    X509v3 Subject Alternative Name:
        email:benjamin.fehrensen@bfh.ch
    X509v3 Certificate Policies:
        Policy: 1.3.6.1.4.1.8024.1.200
        CPS: http://www.quovadisglobal.com/repository
    X509v3 Extended Key Usage:
        TLS Web Client Authentication, E-mail Protection
    X509v3 CRL Distribution Points:
        Full Name:
            URI:http://crl.quovadisglobal.com/quovadisswissadvancedcag4.crl
    X509v3 Subject Key Identifier:
        03:FC:B3:79:65:06:A7:4D:B1:C8:70:3F:F5:ED:9F:A1:39:5C:8B:25
    X509v3 Key Usage: critical
        Digital Signature, Key Encipherment
    ...

subjectAltName¶

SubjectAltName type	Notation
ipAddress	IP: 11.22.33.44
dNSName	DNS:gateway.example.com
rfc822Name	email:alice@example.com
uniformResourceIdentifier	URI:https://www.example.com
directoryName	dirName: CN=Alice
...

X509v3 certificate extensions may contain multiple subjectAltName entries:

subjectAltName=DNS:gateway.example.com,IP:11.22.33.44
subjectAltName=email:alice.example.com,email:alice2@example.com

Certificate Encoding¶

Binary DER format(.der, .cer)
Base64 encoded PEM format (.pem, .cer, .crt)
PKCS#12 Container either DER or PEM encoded (.p12, .pfx)
- Certificate belongs to a private key
- Private key is encrypted using a symmetric key derived from a passphrase
- Optional: Might contain the full key chain

With the openssl command line tool you are able to convert between the encodings:

#Convert PEM to DER
openssl x509 -inform PEM -outform DER -in cert.pem -out cert.der

#Dump ASN1 structure
openssl asn1pare -inform der -in qvrca1g3.der

Chain of Trust Overview¶

PKCS standards¶

PKCS

Revoke Certificates¶

There are multiple mechanisms to revoke certificates;

Certificate Revocation List (CRL)
Online Certificate Status Protocol (OCSP)
(CRL) Distribution Points (DP's)
Authority revocation list (ARL)

Certificate Types¶

SSL certificates
- Domain Validation DV (single domain)
- Organization Validation OV (single domain)
- Wildcard (multi-domain, DV, OV)
- Extended validation (EV)
Email certificates
Authentication certificates
Signing / timestamping certificates

Certificate Transparency (CT)¶

Internet security standard for monitoring and auditing the issuance digital TLS server certificate
CT originated after the 2011 attack on DigiNoter (CA)
Before a CA can log a Certificate it needs an Signed Certificate Timestamp (SCT). To get an SCT the certificate has to be submitted to a log.
Each entry of this log references the hash of the previous one forming a Merkle tree
- Append-only. Certificates can only be added to a log
- Merkle trees to prevent tempering and misbehaviour
- Publicly auditable
Most browser enforce CT. At least 2 SCTs are required

Deliver SCTs to the Browser¶

CT offers 3 ways to deliver SCTs to a browser:

In the TLS extension
In stapled OCSP
In the X509v3 certificate CT extensions

Precertificate¶

PreCertificates are issued by CAs with a "poisoned critical exteions" OID (1.3.6.1.4.1.11129.2.4.3). It will be submitted to the CT Log afterward. If a browser catches a poisoned certificate it would rise an alert. Because there is an extension marked as critical but is unknown.

links: SPA TOC - Certificates and PKI - Index