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In cryptography, X.509 is an ITU-T standard for a public key infrastructure (PKI) and Privilege Management Infrastructure (PMI). X.509 specifies, amongst other things, standard formats for public key certificates, certificate revocation lists, attribute certificates, and a certification path validation algorithm.
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 ever 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 referred to as PKIX for Public Key Infrastructure (X.509).
In the X.509 system, a certification authority issues a certificate binding a public key to a particular distinguished name in the X.500 tradition, or to an alternative name such as an e-mail address or a DNS entry.
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 larger vendors will work instantly; in effect the browsers' developers determine which CAs are trusted third parties for the browsers' users.
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 along with versions of Windows including Vista and later.
The structure foreseen by the standards is expressed in a formal language, namely Abstract Syntax Notation One.
The structure of an X.509 v3 digital certificate is as follows:
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 object). Each extension can be critical or non-critical. If an extension is critical and the system processing the certificate does not recognize the extension or cannot process it, the system MUST reject the entire certificate. A non-critical extension, on the other hand, can be ignored while the system processes the rest of the certificate.
In all versions, the serial number MUST be unique for each certificate issued by a specific CA (as mentioned in RFC 2459).
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:
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.
Common filename extensions for X.509 certificates are:
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.
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:
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.
Although a single X.509 certificate can have only one issuer (cannot have more than one CA signature), different certificate chains can exist for the same target certificate because more than one certificate can exist containing the same public key. This is crucial for cross-certification between different PKIs and other applications. See the following examples.
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In addition to "cert2.2 → cert2" a second valid certificate chain exits for cert2.2: "cert2.2 → cert2.1 → cert1", which enables that cert2.2 (issued by PKI 2) can be trusted by PKI 1. Similarly, user certificates issued by PKI 1 (e.g. cert1.2) can be trusted by PKI 2.
Understanding Certification Path Construction. 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.
This is an example of a decoded X.509 certificate for www.freesoft.org, generated with OpenSSL—the actual certificate is about 1 kB in size. It was issued by Thawte (since acquired by VeriSign and now owned by Symantec), as stated in the Issuer field. Its subject contains many personal details, but the most important part is usually the common name (CN), as this is the part that must match the host being authenticated. Also included is an RSA public key (modulus and public exponent), followed by the signature, computed by taking a MD5 hash of the first part of the certificate and signing it (applying the encryption operation) using Thawte's RSA private key.
$ openssl x509 -in freesoft-certificate.pem -noout -text Certificate: Data: Version: 1 (0x0) Serial Number: 7829 (0x1e95) Signature Algorithm: md5WithRSAEncryption Issuer: C=ZA, ST=Western Cape, L=Cape Town, O=Thawte Consulting cc, OU=Certification Services Division, CN=Thawte Server CA/emailAddressemail@example.com Validity Not Before: Jul 9 16:04:02 1998 GMT Not After : Jul 9 16:04:02 1999 GMT Subject: C=US, ST=Maryland, L=Pasadena, O=Brent Baccala, OU=FreeSoft, CN=www.freesoft.org/emailAddressfirstname.lastname@example.org Subject Public Key Info: Public Key Algorithm: rsaEncryption RSA Public Key: (1024 bit) Modulus (1024 bit): 00:b4:31:98:0a:c4:bc:62:c1:88:aa:dc:b0:c8:bb: 33:35:19:d5:0c:64:b9:3d:41:b2:96:fc:f3:31:e1: 66:36:d0:8e:56:12:44:ba:75:eb:e8:1c:9c:5b:66: 70:33:52:14:c9:ec:4f:91:51:70:39:de:53:85:17: 16:94:6e:ee:f4:d5:6f:d5:ca:b3:47:5e:1b:0c:7b: c5:cc:2b:6b:c1:90:c3:16:31:0d:bf:7a:c7:47:77: 8f:a0:21:c7:4c:d0:16:65:00:c1:0f:d7:b8:80:e3: d2:75:6b:c1:ea:9e:5c:5c:ea:7d:c1:a1:10:bc:b8: e8:35:1c:9e:27:52:7e:41:8f Exponent: 65537 (0x10001) Signature Algorithm: md5WithRSAEncryption 93:5f:8f:5f:c5:af:bf:0a:ab:a5:6d:fb:24:5f:b6:59:5d:9d: 92:2e:4a:1b:8b:ac:7d:99:17:5d:cd:19:f6:ad:ef:63:2f:92: ab:2f:4b:cf:0a:13:90:ee:2c:0e:43:03:be:f6:ea:8e:9c:67: d0:a2:40:03:f7:ef:6a:15:09:79:a9:46:ed:b7:16:1b:41:72: 0d:19:aa:ad:dd:9a:df:ab:97:50:65:f5:5e:85:a6:ef:19:d1: 5a:de:9d:ea:63:cd:cb:cc:6d:5d:01:85:b5:6d:c8:f3:d9:f7: 8f:0e:fc:ba:1f:34:e9:96:6e:6c:cf:f2:ef:9b:bf:de:b5:22: 68:9f
To validate this certificate, one needs a second certificate that matches the Issuer (Thawte Server CA) of the first certificate. First, one verifies that the second certificate is of a CA kind; that is, that it can be used to issue other certificates. This is done by inspecting a value of the CA attribute in the X509v3 extension section. Then the RSA public key from the CA certificate is used to decode the signature on the first certificate to obtain a MD5 hash, which must match an actual MD5 hash computed over the rest of the certificate. An example CA certificate follows:
$ openssl x509 -in thawte-ca-certificate.pem -noout -text Certificate: Data: Version: 3 (0x2) Serial Number: 1 (0x1) Signature Algorithm: md5WithRSAEncryption Issuer: C=ZA, ST=Western Cape, L=Cape Town, O=Thawte Consulting cc, OU=Certification Services Division, CN=Thawte Server CA/emailAddressemail@example.com Validity Not Before: Aug 1 00:00:00 1996 GMT Not After : Dec 31 23:59:59 2020 GMT Subject: C=ZA, ST=Western Cape, L=Cape Town, O=Thawte Consulting cc, OU=Certification Services Division, CN=Thawte Server CA/emailAddressfirstname.lastname@example.org Subject Public Key Info: Public Key Algorithm: rsaEncryption RSA Public Key: (1024 bit) Modulus (1024 bit): 00:d3:a4:50:6e:c8:ff:56:6b:e6:cf:5d:b6:ea:0c: 68:75:47:a2:aa:c2:da:84:25:fc:a8:f4:47:51:da: 85:b5:20:74:94:86:1e:0f:75:c9:e9:08:61:f5:06: 6d:30:6e:15:19:02:e9:52:c0:62:db:4d:99:9e:e2: 6a:0c:44:38:cd:fe:be:e3:64:09:70:c5:fe:b1:6b: 29:b6:2f:49:c8:3b:d4:27:04:25:10:97:2f:e7:90: 6d:c0:28:42:99:d7:4c:43:de:c3:f5:21:6d:54:9f: 5d:c3:58:e1:c0:e4:d9:5b:b0:b8:dc:b4:7b:df:36: 3a:c2:b5:66:22:12:d6:87:0d Exponent: 65537 (0x10001) X509v3 extensions: X509v3 Basic Constraints: critical CA:TRUE Signature Algorithm: md5WithRSAEncryption 07:fa:4c:69:5c:fb:95:cc:46:ee:85:83:4d:21:30:8e:ca:d9: a8:6f:49:1a:e6:da:51:e3:60:70:6c:84:61:11:a1:1a:c8:48: 3e:59:43:7d:4f:95:3d:a1:8b:b7:0b:62:98:7a:75:8a:dd:88: 4e:4e:9e:40:db:a8:cc:32:74:b9:6f:0d:c6:e3:b3:44:0b:d9: 8a:6f:9a:29:9b:99:18:28:3b:d1:e3:40:28:9a:5a:3c:d5:b5: e7:20:1b:8b:ca:a4:ab:8d:e9:51:d9:e2:4c:2c:59:a9:da:b9: b2:75:1b:f6:42:f2:ef:c7:f2:18:f9:89:bc:a3:ff:8a:23:2e: 70:47
This is an example of a self-signed certificate, as the issuer and subject are the same. There's no way to verify this certificate except by checking it against itself; instead, these top-level certificates are manually stored by web browsers. Thawte is one of the root certificate authorities recognized by both Microsoft and Netscape. This certificate comes with the web browser and is trusted by default. As a long-lived, globally trusted certificate that can sign anything (as there are no constraints in the X509v3 Basic Constraints section), its matching private key has to be closely guarded.
A majority of Internet users, either business or social, currently lack the basic ability, knowledge and willingness to effectively use cryptographic applications in a way that can successfully deter imminent threats. The complexity of this task is one of the weaknesses of public key cryptography. A lack of user friendliness and overall usability thus affects solution efficacy. To deal with such issues, major software companies have included a bundle of root certificates, which have been audited for security purposes, into user browsers and operating systems. For the sake of user friendliness and interoperability, all web browsers and operating systems currently contain this audited Trusted Root Store of certificate issuing authorities. Certificates issued by these organizations, or their subordinate authorities, are transparently trusted by relying entities. These certificates are automatically deemed as secure and trustworthy, as opposed to those issued by “unknown” issuers, which a relying party is warned not to trust. This interprets into certificates published by all authorities that have not been included in the root store. This approach attempts to make the provision of system security automatic and transparent, and essentially removes from the end user the decision making process about the trustworthiness of web entities.
The X.509 standard was primarily designed to support the X.500 structure, but today's use cases center around the web. Many features are of little or no relevance today. The X.509 specification suffers[how?] from being over-functional and underspecified and the normative information is spread across many documents from different standardization bodies. Several profiles were developed to solve this, but these introduce interoperability issues and did not fix the problem.
Implementations suffer from design flaws, bugs, different interpretations of standards and lack of interoperability of different standards. Some problems are:
A certification authority (CA) is an entity which issues digital certificates for use by other parties. It is an example of a trusted third party. CAs are characteristic of many public key infrastructure (PKI) schemes.
There are many commercial CAs that charge for their services. Institutions and governments may have their own CAs, and there are free CAs.
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The The Public-Key Infrastructure (X.509) working group (PKIX) was a working group of the Internet Engineering Task Force dedicated to creating RFCs and other standard documentation on issues related to public key infrastructure based on X.509 certificates. PKIX was established in Autumn 1995 in conjunction with the National Institute of Standards and Technology. The Working Group was closed in November 2013.
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