X.509

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In cryptography, X.509 is an ITU-T standard for public key infrastructure (PKI). X.509 specifies, amongst other things, standard formats for public key certificates and a certification path validation algorithm.

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[edit] History and usage

X.509 was initially issued in 1988 and was begun in association with the X.500 standard and assumed 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 is rarely used that way as of 2004. The X.500 system has never been fully implemented, 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 3280, commonly referred to as PKIX for Public Key Infrastructure (X.509).

[edit] Certificates

In the X.509 system, a CA 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, Netscape/Mozilla and Opera come with root certificates pre-installed, so SSL certificates from larger vendors who have paid for the privilege of being pre-installed will work instantly; in effect the browsers' owners determine which CAs are trusted third parties for the browsers' users. Although these root certificates can be removed or disabled, users rarely do so.

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). Popular browsers like Internet Explorer and Firefox don't check for certificate revocation by default. The time lag for performing the checking could be one of the reasons.

[edit] Structure of a certificate

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

  • Certificate
    • Version
    • Serial Number
    • Algorithm ID
    • Issuer
    • Validity
      • Not Before
      • Not After
    • Subject
    • 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

Issuer and subject unique identifiers were introduced in Version 2, Extensions in Version 3.

[edit] Certificate file extensions

Common filename extensions for X.509-certificates are:

  • .CER - CER encoded certificate, sometimes sequence of certificates
  • .DER - DER encoded certificate
  • .PEM - (Privacy Enhanced Mail) Base64 encoded certificate, enclosed between "-----BEGIN CERTIFICATE-----" and "-----END CERTIFICATE-----"
  • .P7B - See .p7c
  • .P7C - PKCS#7 SignedData structure without data, just certificate(s) or CRL(s)
  • .PFX - See .p12
  • .P12 - PKCS#12, may contain certificate(s) (public) and private keys (password protected)

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 just a degenerated SignedData structure, without any data to sign.

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

A .PEM-file may contain certificate(s) or private key(s), enclosed between the appropriate BEGIN/END-lines (CERTIFICATE or RSA PRIVATE KEY). .

[edit] Sample X.509 certificates

As an example of an X.509 certificate, here's a decode (generated with openssl) of one of www.freesoft.org's old certificates; the actual certificate is about 1KB in size. It was issued (signed) by Thawte (since acquired by Verisign), as stated in its Issuer field. Its subject contains a lot of personal information, but the most important part is the common name (CN) of www.freesoft.org - this is the part that must match the host being authenticated. Next comes an RSA public key (modulus and public exponent), followed by the signature, computed by taking an MD5 hash of the first part of the certificate and encrypting it with Thawte's RSA private key.

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/emailAddress=server-certs@thawte.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/emailAddress=baccala@freesoft.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, we need another certificate, one that matches the Issuer (Thawte Server CA) in the first certificate. Then we take the RSA public key from the second (CA) certificate, use it to decode the signature on the first certificate to obtain an MD5 hash, which must match an actual MD5 hash computed over the rest of the certificate. Here's the CA cert:

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/emailAddress=server-certs@thawte.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/emailAddress=server-certs@thawte.com
       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; note that the issuer and subject are the same. There's no way to verify this certificate except by checking it against itself; we've reached the top of the certificate chain. So how does this certificate become trusted? Simple - it's manually configured. Thawte is one of the root certificate authorities recognized by both Microsoft and Netscape. This certificate comes with the web browser (you can probably find it listed as "Thawte Server CA" in the security settings); it's trusted by default. As a long-lived (note the expiration date), globally trusted certificate that can sign pretty much anything (note the lack of any constraints), its matching private key has to be one of the most closely guarded in the world.

[edit] Security

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. [1]

[edit] Certificate authority

Main article: Certificate authority

A certificate authority or 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.

[edit] Public-Key Infrastructure (X.509) Working Group

The Public-Key Infrastructure (X.509) working group (PKIX) is a working group of the Internet Engineering Task Force dedicated to creating RFCs and other standards documentation on issues related to public key infrastructure based on X.509 certificates. PKIX was established in Autumn 1995.

[edit] See also

[edit] Protocols and Standards supporting X.509 certificates

[edit] References

  • [ITU-T Recommendation X.509][2] (2005): Information Technology - Open Systems Interconnection - The Directory: Authentication Framework, 08/05.
  • Housley, R., W. Ford, W. Polk and D. Solo, "Internet X.509 Public Key Infrastructure: Certificate and CRL Profile", RFC 2459, January 1999.
  • C. Adams, S. Farrell, "Internet X.509 Public Key Infrastructure: Certificate Management Protocols", RFC 2510, March 1999
  • Housley, R., W. Ford, W. Polk and D. Solo, "Internet X.509 Public Key Infrastructure: Certificate and CRL Profile", RFC 3280, April 2002.
  • Arjen Lenstra, Xiaoyun Wang and Benne de Weger, On the possibility of constructing meaningful hash collisions for public keys, full version, with an appendix on colliding X.509 certificates, 2005 [3] (see also [4]).

[edit] External links

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