X.509

History and usage

X.509 was initially issued on July 3, 1988 and was begun in association with the X.500 standard. It assumes a strict hierarchical system of certificate authorities (CAs) for issuing the certificates. This contrasts with web of trust models, like PGP, where anyone (not just special CAs) may sign and thus attest to the validity of others' key certificates. Version 3 of X.509 includes the flexibility to support other topologies like bridges and meshes. It can be used in a peer-to-peer, OpenPGP-like web of trust, but was rarely used that way as of 2004. The X.500 system has only been implemented by sovereign nations for state identity information sharing treaty fulfillment purposes, and the IETF's Public-Key Infrastructure (X.509), or PKIX, working group has adapted the standard to the more flexible organization of the Internet. In fact, the term X.509 certificate usually refers to the IETF's PKIX certificate and CRL Profile of the X.509 v3 certificate standard, as specified in RFC 5280, commonly called PKIX for Public Key Infrastructure (X.509).

Certificates

In the X.509 system, an organization that wants a signed certificate requests one via a certificate signing request (CSR).

To do this, it first generates a key pair, keeping the private key secret and using it to sign the CSR. This contains information identifying the applicant and the applicant's public key that is used to verify the signature of the CSR - and the Distinguished Name (DN) that the certificate is for. The CSR may be accompanied by other credentials or proofs of identity required by the certificate authority.

The certification authority issues a certificate binding a public key to a particular distinguished name.

An organization's trusted root certificates can be distributed to all employees so that they can use the company PKI system. Browsers such as Internet Explorer, Firefox, Opera, Safari and Chrome come with a predetermined set of root certificates pre-installed, so SSL certificates from major certificate authorities will work instantly; in effect the browsers' developers determine which CAs are trusted third parties for the browsers' users. For example, Firefox provides CSV file containing List of Included CAs.

X.509 also includes standards for certificate revocation list (CRL) implementations, an often neglected aspect of PKI systems. The IETF-approved way of checking a certificate's validity is the Online Certificate Status Protocol (OCSP). Firefox 3 enables OCSP checking by default, as do versions of Windows from at least Vista and later.

Structure of a certificate

The structure foreseen by the standards is expressed in a formal language, Abstract Syntax Notation One (ASN.1).

The structure of an X.509 v3 digital certificate is as follows:

Certificate

Version Number

Serial Number

Signature Algorithm ID

Issuer Name

Validity period

Not Before

Not After

Subject name

Subject Public Key Info

Public Key Algorithm

Subject Public Key

Issuer Unique Identifier (optional)

Subject Unique Identifier (optional)

Extensions (optional)

...

Certificate Signature Algorithm

Certificate Signature

Each extension has its own ID, expressed as object identifier, which is a set of values, together with either a critical or non-critical indication. A certificate-using system must reject the certificate if it encounters a critical extension that it does not recognize, or a critical extension that contains information that it cannot process. A non-critical extension may be ignored if it is not recognized, but must be processed if it is recognized.

The structure of version 1 is given in RFC 1422.

ITU-T introduced issuer and subject unique identifiers in version 2 to permit the reuse of issuer or subject name after some time. An example of reuse will be when a CA goes bankrupt and its name is deleted from the country's public list. After some time another CA with the same name may register itself, even though it is unrelated to the first one. However, IETF recommends that no issuer and subject names be reused. Therefore, version 2 is not widely deployed in the Internet.

Extensions were introduced in version 3. A CA can use extensions to issue a certificate only for a specific purpose (e.g. only for signing digital objects).

In all versions, the serial number must be unique for each certificate issued by a specific CA (as mentioned in RFC 2459).

Extensions informing a specific usage of a certificate

RFC 5280 (and its predecessors) defines a number of certificate extensions which indicate how the certificate should be used. Most of them are arcs from the joint-iso-ccitt(2) ds(5) id-ce(29) OID. Some of the most common, defined in section 4.2.1, are:

Basic Constraints, { id-ce 19 }, are used to indicate whether the certificate belongs to a CA.

Key Usage, { id-ce 15 }, provides a bitmap specifying the cryptographic operations which may be performed using the public key contained in the certificate; for example, it could indicate that the key should be used for signatures but not for encipherment.

Extended Key Usage, { id-ce 37 }, is used, typically on a leaf certificate, to indicate the purpose of the public key contained in the certificate. It contains a list of OIDs, each of which indicates an allowed use. For example, { id-pkix 3 1 } indicates that the key may be used on the server end of a TLS or SSL connection; { id-pkix 3 4 } indicates that the key may be used to secure email.

In general, if a certificate has several extensions restricting its use, all restrictions must be satisfied for a given use to be appropriate. RFC 5280 gives the specific example of a certificate containing both keyUsage and extendedKeyUsage: in this case, both must be processed and the certificate can only be used if both extensions are coherent in specifying the usage of a certificate. For example, NSS uses both extensions to specify certificate usage.

Certificate filename extensions

There are several commonly used filename extensions for X.509 certificates. Unfortunately, some of these extensions are also used for other data such as private keys.

.pem – (Privacy-enhanced Electronic Mail) Base64 encoded DER certificate, enclosed between "-----BEGIN CERTIFICATE-----" and "-----END CERTIFICATE-----"

.cer, .crt, .der – usually in binary DER form, but Base64-encoded certificates are common too (see .pem above)

.p7b, .p7c – PKCS#7 SignedData structure without data, just certificate(s) or CRL(s)

.p12 – PKCS#12, may contain certificate(s) (public) and private keys (password protected)

.pfx – PFX, predecessor of PKCS#12 (usually contains data in PKCS#12 format, e.g., with PFX files generated in IIS)

PKCS#7 is a standard for signing or encrypting (officially called "enveloping") data. Since the certificate is needed to verify signed data, it is possible to include them in the SignedData structure. A .P7C file is a degenerated SignedData structure, without any data to sign.

PKCS#12 evolved from the personal information exchange (PFX) standard and is used to exchange public and private objects in a single file.

Certificate chains and cross-certification

A certificate chain (see the equivalent concept of "certification path" defined by RFC 5280) is a list of certificates (usually starting with an end-entity certificate) followed by one or more CA certificates (usually the last one being a self-signed certificate), with the following properties:

1. The Issuer of each certificate (except the last one) matches the Subject of the next certificate in the list.
2. Each certificate (except the last one) is supposed to be signed by the secret key corresponding to the next certificate in the chain (i.e. the signature of one certificate can be verified using the public key contained in the following certificate).
3. The last certificate in the list is a trust anchor: a certificate that you trust because it was delivered to you by some trustworthy procedure.

Certificate chains are used in order to check that the public key (PK) contained in a target certificate (the first certificate in the chain) and other data contained in it effectively belongs to its subject. In order to ascertain this, the signature on the target certificate is verified by using the PK contained in the following certificate, whose signature is verified using the next certificate, and so on until the last certificate in the chain is reached. As the last certificate is a trust anchor, successfully reaching it will prove that the target certificate can be trusted.

The description in the preceding paragraph is a simplified view on the certification path validation process as defined by RFC 5280, which involves additional checks, such as verifying validity dates on certificates, looking up CRLs, etc.

Examining how certificate chains are built and validated, it is important to note that a concrete certificate can be part of very different certificate chains (all of them valid). This is because several CA certificates can be generated for the same subject and public key signing them with different private keys (from different CAs or different private keys from the same CA). So, although a single X.509 certificate can have only one issuer and one CA signature, it can be validly linked to more than one certificate building completely different certificate chains. This is crucial for cross-certification between PKIs and other applications. See the following examples.

In these diagrams:

Each box represents a certificate, with its Subject in bold.

A → B means "A is signed by B" (or, more precisely, "A is signed by the secret key corresponding to the public key contained in B").

Certificates with the same color (that are not white) contain the same public key.

Example 1: Cross-certification at root CA level between two PKIs

In order to manage that user certificates existing in PKI 2 (like "User 2") are trusted by PKI 1, CA1 generates a certificate (cert2.1) containing the public key of CA2. Now both "cert2 and cert2.1 (in green) have the same subject and public key, so there are two valid chains for cert2.2 (User 2): "cert2.2 → cert2" and "cert2.2 → cert2.1 → cert1".

Similarly, CA2 can generate a certificate (cert1.1) containing the public key of CA1 so that user certificates existing in PKI 1 (like "User 1") are trusted by PKI 2.

Example 2: CA certificate renewal

Understanding Certification Path Construction (PDF). PKI Forum. September 2002. To allow for graceful transition from the old signing key pair to the new signing key pair, the CA should issue a certificate that contains the old public key signed by the new private signing key and a certificate that contains the new public key signed by the old private signing key. Both of these certificates are self-issued, but neither is self-signed. Note that these are in addition to the two self-signed certificates (one old, one new). 

Since both cert1 and cert3 contain the same public key (the old one), there are two valid certificate chains for cert5: "cert5 → cert1" and "cert5 → cert3 → cert2", and analogously for cert6. This allows that old user certificates (such as cert5) and new certificates (such as cert6) can be trusted indifferently by a party having either the new root CA certificate or the old one as trust anchor during the transition to the new CA keys.

Sample X.509 certificates

This is an example of a decoded X.509 certificate for wikipedia.org and several other Wikipedia websites. It was issued by GlobalSign, as stated in the Issuer field. Its Subject field describes Wikipedia as an organization, and its Subject Alternative Name field describes the hostnames for which it can be used. The Subject Public Key Info field contains an ECDSA public key, while the signature at the bottom is generated by GlobalSign's RSA private key.

End-entity certificate

Certificate: Data: Version: 3 (0x2) Serial Number: 10:e6:fc:62:b7:41:8a:d5:00:5e:45:b6 Signature Algorithm: sha256WithRSAEncryption Issuer: C=BE, O=GlobalSign nv-sa, CN=GlobalSign Organization Validation CA - SHA256 - G2 Validity Not Before: Nov 21 08:00:00 2016 GMT Not After : Nov 22 07:59:59 2017 GMT Subject: C=US, ST=California, L=San Francisco, O=Wikimedia Foundation, Inc., CN=*.wikipedia.org Subject Public Key Info: Public Key Algorithm: id-ecPublicKey Public-Key: (256 bit) pub: 04:c9:22:69:31:8a:d6:6c:ea:da:c3:7f:2c:ac:a5: af:c0:02:ea:81:cb:65:b9:fd:0c:6d:46:5b:c9:1e: ed:b2:ac:2a:1b:4a:ec:80:7b:e7:1a:51:e0:df:f7: c7:4a:20:7b:91:4b:20:07:21:ce:cf:68:65:8c:c6: 9d:3b:ef:d5:c1 ASN1 OID: prime256v1 NIST CURVE: P-256 X509v3 extensions: X509v3 Key Usage: critical Digital Signature, Key Agreement Authority Information Access: CA Issuers - URI:http://secure.globalsign.com/cacert/gsorganizationvalsha2g2r1.crt OCSP - URI:http://ocsp2.globalsign.com/gsorganizationvalsha2g2 X509v3 Certificate Policies: Policy: 1.3.6.1.4.1.4146.1.20 CPS: https://www.globalsign.com/repository/ Policy: 2.23.140.1.2.2 X509v3 Basic Constraints: CA:FALSE X509v3 CRL Distribution Points: Full Name: URI:http://crl.globalsign.com/gs/gsorganizationvalsha2g2.crl X509v3 Subject Alternative Name: DNS:*.wikipedia.org, DNS:*.m.mediawiki.org, DNS:*.m.wikibooks.org, DNS:*.m.wikidata.org, DNS:*.m.wikimedia.org, DNS:*.m.wikimediafoundation.org, DNS:*.m.wikinews.org, DNS:*.m.wikipedia.org, DNS:*.m.wikiquote.org, DNS:*.m.wikisource.org, DNS:*.m.wikiversity.org, DNS:*.m.wikivoyage.org, DNS:*.m.wiktionary.org, DNS:*.mediawiki.org, DNS:*.planet.wikimedia.org, DNS:*.wikibooks.org, DNS:*.wikidata.org, DNS:*.wikimedia.org, DNS:*.wikimediafoundation.org, DNS:*.wikinews.org, DNS:*.wikiquote.org, DNS:*.wikisource.org, DNS:*.wikiversity.org, DNS:*.wikivoyage.org, DNS:*.wiktionary.org, DNS:*.wmfusercontent.org, DNS:*.zero.wikipedia.org, DNS:mediawiki.org, DNS:w.wiki, DNS:wikibooks.org, DNS:wikidata.org, DNS:wikimedia.org, DNS:wikimediafoundation.org, DNS:wikinews.org, DNS:wikiquote.org, DNS:wikisource.org, DNS:wikiversity.org, DNS:wikivoyage.org, DNS:wiktionary.org, DNS:wmfusercontent.org, DNS:wikipedia.org X509v3 Extended Key Usage: TLS Web Server Authentication, TLS Web Client Authentication X509v3 Subject Key Identifier: 28:2A:26:2A:57:8B:3B:CE:B4:D6:AB:54:EF:D7:38:21:2C:49:5C:36 X509v3 Authority Key Identifier: keyid:96:DE:61:F1:BD:1C:16:29:53:1C:C0:CC:7D:3B:83:00:40:E6:1A:7C Signature Algorithm: sha256WithRSAEncryption 8b:c3:ed:d1:9d:39:6f:af:40:72:bd:1e:18:5e:30:54:23:35: ...

To validate this end-entity certificate, one needs an intermediate certificate that matches its Issuer and Authority Key Identifier:

In a TLS connection, a properly-configured server would provide the intermediate as part of the handshake. However, it's also possible to retrieve the intermediate certificate by fetching the "CA Issuers" URL from the end-entity certificate.

Intermediate certificate

This is an example of an intermediate certificate belonging to a certificate authority. This certificate signed the end-entity certificate above, and was signed by the root certificate below. Note that the subject field of this intermediate certificate matches the issuer field of the end-entity certificate that it signed. Also, the "subject key identifier" field in the intermediate matches the "authority key identifier" field in the subject.

Certificate: Data: Version: 3 (0x2) Serial Number: 04:00:00:00:00:01:44:4e:f0:42:47 Signature Algorithm: sha256WithRSAEncryption Issuer: C=BE, O=GlobalSign nv-sa, OU=Root CA, CN=GlobalSign Root CA Validity Not Before: Feb 20 10:00:00 2014 GMT Not After : Feb 20 10:00:00 2024 GMT Subject: C=BE, O=GlobalSign nv-sa, CN=GlobalSign Organization Validation CA - SHA256 - G2 Subject Public Key Info: Public Key Algorithm: rsaEncryption Public-Key: (2048 bit) Modulus: 00:c7:0e:6c:3f:23:93:7f:cc:70:a5:9d:20:c3:0e: ... Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Key Usage: critical Certificate Sign, CRL Sign X509v3 Basic Constraints: critical CA:TRUE, pathlen:0 X509v3 Subject Key Identifier: 96:DE:61:F1:BD:1C:16:29:53:1C:C0:CC:7D:3B:83:00:40:E6:1A:7C X509v3 Certificate Policies: Policy: X509v3 Any Policy CPS: https://www.globalsign.com/repository/ X509v3 CRL Distribution Points: Full Name: URI:http://crl.globalsign.net/root.crl Authority Information Access: OCSP - URI:http://ocsp.globalsign.com/rootr1 X509v3 Authority Key Identifier: keyid:60:7B:66:1A:45:0D:97:CA:89:50:2F:7D:04:CD:34:A8:FF:FC:FD:4B Signature Algorithm: sha256WithRSAEncryption 46:2a:ee:5e:bd:ae:01:60:37:31:11:86:71:74:b6:46:49:c8: ...

Root certificate

This is an example of a self-signed root certificate representing a certificate authority. Its issuer and subject fields are the same, and its signature can be validated with its own public key. Validation of the trust chain has to end here. If the validating program has this root certificate in its trust store, the end-entity certificate can be considered trusted for use in a TLS connection. Otherwise, the end-entity certificate is considered untrusted.

Certificate: Data: Version: 3 (0x2) Serial Number: 04:00:00:00:00:01:15:4b:5a:c3:94 Signature Algorithm: sha1WithRSAEncryption Issuer: C=BE, O=GlobalSign nv-sa, OU=Root CA, CN=GlobalSign Root CA Validity Not Before: Sep 1 12:00:00 1998 GMT Not After : Jan 28 12:00:00 2028 GMT Subject: C=BE, O=GlobalSign nv-sa, OU=Root CA, CN=GlobalSign Root CA Subject Public Key Info: Public Key Algorithm: rsaEncryption Public-Key: (2048 bit) Modulus: 00:da:0e:e6:99:8d:ce:a3:e3:4f:8a:7e:fb:f1:8b: ... Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Key Usage: critical Certificate Sign, CRL Sign X509v3 Basic Constraints: critical CA:TRUE X509v3 Subject Key Identifier: 60:7B:66:1A:45:0D:97:CA:89:50:2F:7D:04:CD:34:A8:FF:FC:FD:4B Signature Algorithm: sha1WithRSAEncryption d6:73:e7:7c:4f:76:d0:8d:bf:ec:ba:a2:be:34:c5:28:32:b5: ...

Security

There are a number of publications about PKI problems by Bruce Schneier, Peter Gutmann and other security experts.

Architectural weaknesses

Use of blacklisting invalid certificates (using CRLs and OCSP),

If the client only trusts certificates when CRLs are available, then they lose the offline capability that makes PKI attractive. So most clients do trust certificates when CRLs are not available, but in that case an attacker that controls the communication channel can disable the CRLs. Adam Langley of Google has said CRLs are like a safety belt that works except when you are having an accident.

CRLs are notably a poor choice because of large sizes and convoluted distribution patterns,

Ambiguous OCSP semantics and lack of historical revocation status,

Revocation of root certificates is not addressed,

Aggregation problem: Identity claims (authenticate with an identifier), attribute claims (submit a bag of vetted attributes), and policy claims are combined in a single container. This raises privacy, policy mapping, and maintenance issues,

Delegation problem: CAs cannot technically restrict subordinate CAs from issuing certificates outside a limited namespaces or attribute set; this feature of X.509 is not in use. Therefore, a large number of CAs exist on the Internet, and classifying them and their policies is an insurmountable task. Delegation of authority within an organization cannot be handled at all, as in common business practice.

Federation problem: Certificate chains that are the result of subordinate CAs, bridge CAs, and cross-signing make validation complex and expensive in terms of processing time. Path validation semantics may be ambiguous. The hierarchy with a third-party trusted party is the only model. This is inconvenient when a bilateral trust relationship is already in place.

Issuance of an Extended Validation (EV) certificate for a hostname doesn't prevent issuance of a lower-validation certificate valid for the same hostname, which means that the higher validation level of EV doesn't protect against man-in-the-middle attacks.

Problems with certificate authorities

The subject, not the relying party, purchases certificates. The subject will often utilize the cheapest issuer, so quality is not being paid for in the competing market. This is partly addressed by Extended Validation certificates.

Certification authorities deny almost all warranties to the user (including subject or even relying parties).

The expiration date should be used to limit the time the key strength is deemed sufficient. This parameter is abused by certification authorities to charge the client an extension fee. This places an unnecessary burden on the user with key roll-over.

"Users use an undefined certification request protocol to obtain a certificate which is published in an unclear location in a nonexistent directory with no real means to revoke it."

Like all businesses, CAs are subject to the legal jurisdiction(s) of their site(s) of operation, and may be legally compelled to compromise the interests of their customers and their users. Intelligence agencies have also made use of false certificates issued through extralegal compromise of CAs, such as DigiNotar, to carry out man-in-the-middle attacks.

Implementation issues

Implementations suffer from design flaws, bugs, different interpretations of standards and lack of interoperability of different standards. Some problems are:

Many implementations turn off revocation check:

Seen as obstacle, policies are not enforced

If it was turned on in all browsers by default, including code signing, it would probably crash the infrastructure.

DNs are complex and little understood (lack of canonicalization, internationalization problems, ..)

rfc822Name has two notations

Name and policy constraints hardly supported

Key usage ignored, first certificate in a list being used

Enforcement of custom OIDs is difficult

Attributes should not be made critical because it makes clients crash.

Unspecified length of attributes lead to product-specific limits

There are implementation errors with X.509 that allow e.g. falsified subject names using null-terminated strings or code injections attacks in certificates.

By using illegal 0x80 padded subidentifiers of object identifiers, wrong implementations or by using integer-overflows of the client's browsers, an attacker can include an unknown attribute in the CSR, which the CA will sign, which the client wrongly interprets as "CN" (OID=2.5.4.3). Dan Kaminsky at the 26th Chaos Communication Congress "Black OPs of PKI"

Cryptographic weaknesses

Digital signature systems depend on secure cryptographic hash functions to work. When a public key infrastructure allows the use of a hash function that is no longer secure, an attacker can exploit weaknesses in the hash function to forge certificates. Specifically, if an attacker is able to produce a hash collision, they can convince a CA to sign a certificate with innocuous contents, where the hash of those contents is identical to the hash of another, malicious set of certificate contents, created by the attacker with values of their choosing. The attacker can then append the CA-provided signature to their malicious certificate contents, resulting in a malicious certificate that appears to be signed by the CA. Because the malicious certificate contents are chosen solely by the attacker, they can have different validity dates or hostnames than the innocuous certificate. The malicious certificate can even contain a "CA: true" field making it able to issue further trusted certificates.

MD2-based certificates were used for a long time and were vulnerable to preimage attacks. Since the root certificate already had a self-signature, attackers could use this signature and use it for an intermediate certificate.

In 2005, Arjen Lenstra and Benne de Weger demonstrated "how to use hash collisions to construct two X.509 certificates that contain identical signatures and that differ only in the public keys", achieved using a collision attack on the MD5 hash function.

In 2008, Alexander Sotirov and Marc Stevens presented at the Chaos Communication Congress a practical attack that allowed them to create a rogue Certificate Authority, accepted by all common browsers, by exploiting the fact that RapidSSL was still issuing X.509 certificates based on MD5.

In April 2009 at the Eurocrypt Conference, Australian Researchers of Macquarie University presented "Automatic Differential Path Searching for SHA-1". The researchers were able to deduce a method which increases the likelihood of a collision by several orders of magnitude.

In February 2017, a group of researchers produced a SHA-1 collision, demonstrating SHA-1's weakness.

Mitigations for cryptographic weaknesses

Exploiting a hash collision to forge X.509 signatures requires that the attacker be able to predict the data that the certificate authority will sign. This can be somewhat mitigated by the CA generating a random component in the certificates it signs, typically the serial number. The CA/Browser Forum has required serial number entropy in its Baseline Requirements Section 7.1 since 2011.

As of January 1, 2016, the Baseline Requirements forbid issuance of certificates using SHA-1. As of early 2017, Chrome and Firefox reject certificates that use SHA-1, with Edge and Safari planning to end support soon. Non-browser X.509 validators don't yet reject SHA-1 certificates.

PKI standards for X.509

PKCS7 (Cryptographic Message Syntax Standard — public keys with proof of identity for signed and/or encrypted message for PKI).

Transport Layer Security (TLS) and its predecessor SSL — cryptographic protocols for Internet secure communications.

Online Certificate Status Protocol (OCSP) / certificate revocation list (CRL) — this is to check certificate revocation status.

PKCS12 (Personal Information Exchange Syntax Standard) — used to store a private key with the appropriate public key certificate.

PKIX Working Group

In 1995, the Internet Engineering Task Force in conjunction with the National Institute of Standards and Technology formed the Public-Key Infrastructure (X.509) working group. The working group, concluded in June 2014, is commonly referred to as "PKIX." It produced RFCs and other standards documentation on using deploying X.509 in practice. In particular it produced RFC 3280 and its successor RFC 5280, which define how to use X.509 in Internet protocols.

Major protocols and standards using X.509 certificates

TLS/SSL and HTTPS use the RFC 5280 profile of X.509, as do S/MIME (Secure Multipurpose Internet Mail Extensions) and the EAP-TLS method for WiFi authentication. Any protocol that uses TLS, such as SMTP, POP, IMAP, LDAP, XMPP, and many more, inherently uses X.509. IPsec uses its own profile of X.509, defined in RFC 4945.

The OpenCable security specification defines its own profile of X.509 for use in the cable industry.

Devices like smart cards and TPMs often carry certificates to identify themselves or their owners. These certificates are in X.509 form.

The WS-Security standard defines authentication either through TLS or through its own certificate profile. Both methods use X.509.

The Microsoft Authenticode code signing system uses X.509 to identify authors of computer programs.

The OPC UA industrial automation communication standard uses X.509.

SSH generally uses a Trust On First Use security model and doesn't have need for certificates. However, the popular OpenSSH implementation does support a CA-signed identity model based on its own non-X.509 certificate format.