Understanding X.509 Certificate Sructure

13 minute read

NOTE: If you are totally new to digital certificates and the ecosystem, please read my article introducing the basics of Digital certificates here before continuing with this article.

Introduction

Digital certificates are now prevalent and its significance is growing in the current market where an increasing number of embedded devices are deployed into secure networks.

Even with increasing significance of Digital certificates, engineers are largely unaware of underlying concepts and standards that dictate certificate formats and contents. This is primarily because of the availability of hardened and stable software like OpenSSL that abstracts many things under friendly top level APIs. However, with the new wave of embedded devices using these technologies, we need to understand and re-visit the underlying technology to optimize them for new-age requirements.

When it comes to parsing and understanding a X.509 certificate by hand, information is scattered all over the place. This article aims at bringing in the basics together without going too much into specification details so that you can use it as a starting point to understand where and what to look for.

Basics

First a little bit of glossary and background. Some seemingly disconnected terminology and concepts will be introduced in this section.

Abstract Syntax Notation One (ASN.1)

ASN.1 is an abstract syntax that defines a machine encoding independent way of representing ( encoding, transmitting and decoding) data. Being an abstract syntax, it only defines the structure of a data tree and leaves actual data representation (encoding) to application specific implementations. In other (simple) words, ASN.1 is a schema language. To give an idea of what this means, ASN.1 can be used to define the schema for formats like JSON, XML etc.

A tip for later: In the schema representation , a SEQUENCE models an ordered collection of variables of different type

Object identifier (OID)

In the context of digital certificates, OID refers to ITU-T maintained tree based object identifier hierarchy that allows unambiguous representation of information in the form of a dot separated number.

For example 2.5.4.8 is the doted OID representation of {joint-iso-itu-t(2) ds(5) attributeType(4) stateOrProvinceName(8)} . A certificate will use this OID as the tag of a string tthat represents the state or province of the entity to which a certificate was issued.

OID Repository is a good place to lookup entities in the OID tree.

OID encoding (for DER)

Special encoding rules have been defined to represent OIDs inside the ASN.1 tree . Note that ASN.1 by itself is agnostic of encoding rules. The rule illustrated below is part of DER specification . (More on DER in the next session)

For directly encoded OID components, the individual octets should have first bit as 0.

Consider a dummy OID 1.2.62329.4. OID binary encoding for octet representation is done using the following rules.

Step 1: The first two components (A.B) are encoded as 40*A+B . In this case, it would be 2A (Hex of 42)

Step 2: Since the third component (62329) has more than 7 bits in it, we need to split it into multiple octets with only leading octet having MSB =1.

For this, first convert the number into binary and split into groups of 7 bits (Pad 0s to form a leading set to form an octet) 000_0011 110_0110 111_1001

Set MSB of all octets except last to 1. Set MSB of last octet to 0. Now, convert resulting decimal into hex
1000_0011 1110_0110 0111_1001 = 0x83E679

Step 3: since the last component has a value with less than 7 bits, it can be converted to hex directly.

Final HEX encoded OID of 1.2.62329.4 would be 2A 83 E6 79 04

Distinguished Encoding Rules (DER)

DER defines rules for unambiguous encoding of data into ASN.1. This is one of the formats commonly used in cryptographic systems.

A portion an ASN.1 tree within a certificate is given below:

Here a SET contains a SEQUENCE that contains an IUT-T OID ObjectIdentifier and a “value” for the identified object. In the example here, all OIDs have been mapped to corresponding string notation as well. DER defines that for OID 2.3.4.6, the next element in SEQUENCE should be of type PrintableString and further goes on into definition of how a PrintableString appears in ASN.1 tree.

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SET
  SEQUENCE
    ObjectIdentifier countryName (2 5 4 6)
    PrintableString 'IN'

The HEX notation of this portion of the tree encoded as per DER would be:

31 0B 30 09 06 03 55 04 06 13 02 49 4E

This means:

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31 - Type is SET
0B - length of immediate contents is 11 bytes.
30 - Type is SEQUENCE
09 - Length is 9 bytes
06 - Type is ObjectIdentifier
03 - length is 3 bytes
55 04 06 - OID is 2.5.4.6 (see OID encoding section above for explanation) 
13 - Type is  PrintableString
02 - length is 2
49 4E - ASCII of "IN" 

Essentially ASN.1 DER encoding is a tag, length, value encoding system for each element in the certificate.

Base64 encoding

Base64 is an encoding scheme used to represent binary data as an ASCII string using radix64 (just like binary representation uses radix2). Characters chosen to represent encoding depends on the standard used . For example, MIME uses A-Z, a-z , 0-9 and +/.

There are well deifined methods to handle corner cases and there are variants (like URL safe Base64) to handle different scenarios.

Privacy Enhanced Mail format (PEM)

Though the e-mail securing proposal in PEM was not a success, PEMs certificate and key encoding format described by PEM is one of the most widely used for certificate storage and transmission. PEM files are used for storage of single certificates, complete certificate chain or just keys.

Essentially PEM is a BASE64 encoded DER file.

Key identifiers and usage

X.509 PKI certificate format defines some methods to generate unique identifiers for a public key. This is especially useful to identify the relevant keys when a CA has multiple active Public keys that can be used to sign certificates. These IDs appear in two forms in a certificate as part of certificate extensions.

  • authorityKeyIdentifier - key ID of CA public key used to sign the certificate.
  • subjectKeyIdentifier - key ID of public key present in the certificate. This is a mandatory field for a certificate marked as CA

Two recommended methods for generating unique key IDs from public keys as per the x.509 RFC are:

  • compute 160 bit SHA-1 hash of the BIT STRING value of subjectPublicKey (excluding tag and length)
  • type indicator 0100 followed by least significant 60 bits of subjectPublicKey (excluding tag and length)

Other methods for generating unique IDs can also be used.

Each key will be marked with a usage bitmap in the “extension” portion of the certificate. The defined bitmap and meanings are :

Bit position Name Description
0 digitalSignature subject public key is used for verifying digital signatures (except CRL and its own)
1 nonRepudiation used to provide a non-repudiation service that protects against the signing entity falsely denying some action
2 keyEncipherment subject public key is used for enciphering private or secret keys
3 dataEncipherment subject public key is used for directly enciphering raw user data without the use of an intermediate symmetric cipher
4 keyAgreement subject public key is used for key agreement (like in the case of D-H
5 keyCertSign subject public key is used for verifying signatures on public key certificates. This also mandates marking the certificate as a CA certificate
6 cRLSign subject public key is used for verifying signatures on certificate revocation lists
7 encipherOnly When the encipherOnly bit is asserted and the keyAgreement bit is also set, the subject public key may be used only for enciphering data while performing key agreement
8 decipherOnly When the decipherOnly bit is asserted and the keyAgreement bit is also set, the subject public key may be used only for deciphering data while performing key agreement.

So, for exmple, 0xa0 for keyUsage (2.5.29.15) would mean that the key can be used for digitalSignature and keyEncipherment

Public Key Cryptography Standards (PKCS)

PKCS is a set of asymmetric crypto standards initially published by RSA and later enhanced by Microsoft. It describes a series of Public key infrastructure (PKI) assisting standards for items like certificate syntax, token interface, certificate signing request, private key storage etc.

The pkcs12 standard defines a password protected container format for storing private and public keys.

Key formats

Each cryptographic algorithm requires a set of instance specific parameters to be fed into it for operation. The generalized term used for these set of parameters is called a “Key”.

In case of certificates and keys stored in certificates, there are well defined formats for storing key parameters for every supported cipher suite .

The generalised ASN.1 Schema for public key storage in a certificate as per TLS 1.2 RFC is :

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SubjectPublicKeyInfo ::= SEQUENCE {
    algorithm AlgorithmIdentifier,
    publicKey BIT STRING }

For example, ECCDSA public key is essentially a set of two points in the curve. These points are encoded as an octet string and placed inside the certificate. Section 2.3.4 (OctetString-to-EllipticCurvePoint Conversion) of SEC 1: Elliptic Curve Cryptography V1 spec defines how to decode an octet string representation of the public key (that is generally seen within the certificate) into curve points (a,b).

In a sample certificate using this algorithm, the following will appear in its ASN.1 tree:

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SEQUENCE
  ObjectIdentifier ecPublicKey (1 2 840 10045 2 1)
  ObjectIdentifier secp256r1 (1 2 840 10045 3 1 7)
  BITSTRING 0004664fca46ef4c681b74760bd89534588794d177252788432e63f8a7d81d287d9e474e5402beb90a75ae4b2759ff35799223e1ff14aa00c7489bb68c21142edccd

DER has defined rules for decoding BIT STRING which is essentially a padded ASN.1 representation. In this case, it will be decoded into an octet string that can further be decoded using the aforementioned SEC 1 spec.

Parsing a certificate

In “Server Certificate” stage of TLS handshake, the server presents a “certificate_list” to the client. As per RFC5246 (TLS 1.2), a certificate list is a sequence (chain) of certificates. The sender’s certificate MUST come first in the list. Each following certificate MUST directly certify the one preceding it. This chain transmission happens in ASN.1 schema defined in the rfc and can have up to a maximum of 2^24-1 certificates in the chain.

The root certificate can be omitted as it does not make sense to validate a chain with a root sent in the chain.

When exported, certificates are generally stored between string tags:

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-----BEGIN CERTIFICATE-----
<encoded certificate>
-----END CERTIFICATE-----

We will use the certificate of https://example.com to understand the basic structure of an X.509 certificate.

Following command will fetch the certificate chain:

openssl s_client -showcerts -connect www.example.com:443 </dev/null

This will show the servers certificate as well as the intermediate CAs certificate . We will focus on the first certificate (server certificate) for this exercise

The certificate contents is :

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-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----

This is in base64 encoded format. This can be decoded into the follwoing hex string:

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

When this is passed through an ASN.1 decoder, we get the following tree:

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SEQUENCE
  SEQUENCE
	[0]
	  INTEGER 02
	INTEGER 0e64c5fbc236ade14b172aeb41c78cb0
	SEQUENCE
	  ObjectIdentifier SHA256withRSA (1 2 840 113549 1 1 11)
	  NULL
	SEQUENCE
	  SET
		SEQUENCE
		  ObjectIdentifier countryName (2 5 4 6)
		  PrintableString 'US'
	  SET
		SEQUENCE
		  ObjectIdentifier organization (2 5 4 10)
		  PrintableString 'DigiCert Inc'
	  SET
		SEQUENCE
		  ObjectIdentifier organizationalUnit (2 5 4 11)
		  PrintableString 'www.digicert.com'
	  SET
		SEQUENCE
		  ObjectIdentifier commonName (2 5 4 3)
		  PrintableString 'DigiCert SHA2 High Assurance Server CA'
	SEQUENCE
	  UTCTime 151103000000Z
	  UTCTime 181128120000Z
	SEQUENCE
	  SET
		SEQUENCE
		  ObjectIdentifier countryName (2 5 4 6)
		  PrintableString 'US'
	  SET
		SEQUENCE
		  ObjectIdentifier stateOrProvinceName (2 5 4 8)
		  PrintableString 'California'
	  SET
		SEQUENCE
		  ObjectIdentifier locality (2 5 4 7)
		  PrintableString 'Los Angeles'
	  SET
		SEQUENCE
		  ObjectIdentifier organization (2 5 4 10)
		  PrintableString 'Internet Corporation for Assigned Names and Numbers'
	  SET
		SEQUENCE
		  ObjectIdentifier organizationalUnit (2 5 4 11)
		  PrintableString 'Technology'
	  SET
		SEQUENCE
		  ObjectIdentifier commonName (2 5 4 3)
		  PrintableString 'www.example.org'
	SEQUENCE
	  SEQUENCE
		ObjectIdentifier rsaEncryption (1 2 840 113549 1 1 1)
		NULL
	  BITSTRING 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
	[3]
	  SEQUENCE
		SEQUENCE
		  ObjectIdentifier authorityKeyIdentifier (2 5 29 35)
		  OCTETSTRING, encapsulates
			SEQUENCE
			  [0] 5168ff90af0207753cccd9656462a212b859723b
		SEQUENCE
		  ObjectIdentifier subjectKeyIdentifier (2 5 29 14)
		  OCTETSTRING, encapsulates
			OCTETSTRING a64f601e1f2dd1e7f123a02a9516e4e89aea6e48
		SEQUENCE
		  ObjectIdentifier subjectAltName (2 5 29 17)
		  OCTETSTRING, encapsulates
			SEQUENCE
			  [2] www.example.org
			  [2] example.com
			  [2] example.edu
			  [2] example.net
			  [2] example.org
			  [2] www.example.com
			  [2] www.example.edu
			  [2] www.example.net
		SEQUENCE
		  ObjectIdentifier keyUsage (2 5 29 15)
		  BOOLEAN TRUE
		  OCTETSTRING, encapsulates
			BITSTRING 05a0
		SEQUENCE
		  ObjectIdentifier extKeyUsage (2 5 29 37)
		  OCTETSTRING, encapsulates
			SEQUENCE
			  ObjectIdentifier serverAuth (1 3 6 1 5 5 7 3 1)
			  ObjectIdentifier clientAuth (1 3 6 1 5 5 7 3 2)
		SEQUENCE
		  ObjectIdentifier cRLDistributionPoints (2 5 29 31)
		  OCTETSTRING, encapsulates
			SEQUENCE
			  SEQUENCE
				[0]
				  [0]
					[6] http://crl3.digicert.com/sha2-ha-server-g4.crl
			  SEQUENCE
				[0]
				  [0]
					[6] http://crl4.digicert.com/sha2-ha-server-g4.crl
		SEQUENCE
		  ObjectIdentifier certificatePolicies (2 5 29 32)
		  OCTETSTRING, encapsulates
			SEQUENCE
			  SEQUENCE
				ObjectIdentifier (2 16 840 1 114412 1 1)
				SEQUENCE
				  SEQUENCE
					ObjectIdentifier (1 3 6 1 5 5 7 2 1)
					IA5String 'https://www.digicert.com/CPS'
			  SEQUENCE
				ObjectIdentifier (2 23 140 1 2 2)
		SEQUENCE
		  ObjectIdentifier authorityInfoAccess (1 3 6 1 5 5 7 1 1)
		  OCTETSTRING, encapsulates
			SEQUENCE
			  SEQUENCE
				ObjectIdentifier (1 3 6 1 5 5 7 48 1)
				[6] http://ocsp.digicert.com
			  SEQUENCE
				ObjectIdentifier (1 3 6 1 5 5 7 48 2)
				[6] http://cacerts.digicert.com/DigiCertSHA2HighAssuranceServerCA.crt
		SEQUENCE
		  ObjectIdentifier basicConstraints (2 5 29 19)
		  BOOLEAN TRUE
		  OCTETSTRING, encapsulates
			SEQUENCE {}
  SEQUENCE
	ObjectIdentifier SHA256withRSA (1 2 840 113549 1 1 11)
	NULL
  BITSTRING 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

The basic tree structure and relevent information in this tree are annoted below:

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TLS version (02 for V03)
Serial Number (0e64c5fbc236ade14b172aeb41c78cb0)
Certificate signature Algorithm (SHA256withRSA)
Issuer
	countryName [CN] - (US)
	organization [O] - (DigiCert Inc)
	organizationalUnit [O] - (www.digicert.com)
	commonName [CN] - (DigiCert SHA2 High Assurance Server CA)
Validity
	notBefore (151103000000Z -Tuesday, November 03, 2015 5:30:00 AM)
	notAfter (181128120000Z -  Wednesday, November 28, 2018 5:30:00 PM)
Subject
	countryName [C] - (US)
	stateOrProvinceName [ST] - (California)
	locality [L] - (Los Angeles)
	organization [O] - (Internet Corporation for Assigned Names and Numbers)
    organizationalUnit [OU] - (Technology)
	commonName [CN] - (www.example.org)
SubjectPublicKeyInfo
	Subject Public Key Algorithm - (rsaEncryption)
	Subject Public Key - (represented as BITSTRING)
Extensions
	AuthorityKeyIdentifier  - (5168ff90af0207753cccd9656462a212b859723b)
	subjectKeyIdentifier  -  (a64f601e1f2dd1e7f123a02a9516e4e89aea6e48)
	subjectAltName - (list of identities bound to the same subject entity)
	keyUsage - (digitalSignature and keyEncipherment)
	extended key usage - (serverAuth and clientAuth)
	cRLDistributionPoints - (2 links to get the revoked serial numbers)
	certificatePolicies
		ssl-server-certificates
			certificate policy statement link provide by CA
		organization-validated (legal)
	authorityInfoAccess
		Online Certificate Status Protocol (OCSP)  - (CA OCSP URL)
		Certificate Authority Issuers (caIssuers)  - (link pointing to CA cert for verification)
	basicConstraints (used to identify whether this is a CA . Not a CA in this case)
certificate signature algotighm - (SHA256withRSA)
certificate signature value - (bitstring)

You can see the same set of information in Firefox certificate viewer.

example.com cert as parsed by Firefox

example.com cert as parsed by Firefox