Certificates

In cryptography, a public key certificate (also known as a digital certificate or identity certificate) is an electronic document which uses a digital signature to bind a public key with an identity - information such as the name of a person or an organization, their address, and so forth. The certificate can be used to verify that a public key belongs to an individual. In a typical public key infrastructure (PKI) scheme (e.g. X.509) the signature will be of a certificate authority (CA). In a web of trust scheme (e.g. PGP), the signature is of either the user (a self-signed certificate) or other users ("endorsements"). In either case, the signatures on a certificate are attestations by the certificate signer that the identity information and the public key belong together. A self-signed certificate is a certificate where the signer/issuer and subject are the same.

X.509 v3 certificate example (http://tools.ietf.org/html/rfc5280):

Certificate:
   Data:
       Version: 1 (0x0)
       Serial Number: 7829 (0x1e95)
       Signature Algorithm: md5WithRSAEncryption
       Issuer: C=ZA [...] 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 [...] 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
   Signature Value:
       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

Where:

• Version valid values are 1, 2 or 3
• Serial Number is a unique number for each certificate issued by a specific CA
• Issuer and Subject are non-empty distinguished names (DN)

VeriSign uses the concept of classes for different types of digital certificates:

• Class 1 for individuals, intended for email
• Class 2 for organizations, for which proof of identity is required
• Class 3 for servers and software signing, for which independent verification and checking of identity and authority is done by the issuing certificate authority
• Class 4 for online business transactions between companies
• Class 5 for private organizations or governmental security

Other vendors may choose to use different classes or no classes at all as this is not specified in the SSL protocol, though, most do opt to use classes in some form.

In cryptography, PKCS refers to a group of public-key cryptography standards devised and published by RSA Security.

In public key infrastructure systems, a certificate signing request (also CSR or certification request) is a message sent from an applicant to a certificate authority in order to apply for a digital identity certificate. The most common format for CSRs is the PKCS#10 specification and another is the Signed Public Key and Challenge Spkac format generated by some web browsers. The CSR contains information identifying the applicant (such as a distinguished name in the case of an X.509 certificate), and the public key chosen by the applicant. The PKCS#10 standard defines a binary format for encoding CSRs for use with X.509. It is expressed in ASN.1. A CSR may be represented as a Base64 encoded PKCS#10, e.g.:

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

Public key algorithms (with variable number of bits, typically 1024 or 2048):

• RSA (PKCS#1, Rivest, Shamir, Adleman)
• DSA (Digital Signature Algorithm)
• KEA (Key Exchange Algorithm)
• ECDSA (Elliptic Curve mathematical analog of DSA)
• ECDH (Elliptic Curve mathematical analog of Diffie-Hellman)
• Diffie-Hellman
• ...

Signature algorithms:

• RSA with MD2, MD5 or SHA-1 hash functions
• DSA with SHA-1 hash function
• ECDSA with SHA-1 hash function

MD2 and MD5 were developed by Ron Rivest for RSA Security. RSA Security has placed the MD2 and MD5 algorithms in the public domain. SHA-1 was developed by the U.S. Government.

The Online Certificate Status Protocol (OCSP) is an Internet protocol used for obtaining the revocation status of an X.509 digital certificate. It was created as an alternative to certificate revocation lists (CRL), specifically addressing certain problems associated with using CRLs in a public key infrastructure (PKI). Messages communicated via OCSP are encoded in ASN.1 and are usually communicated over HTTP.

Certificate file formats:

• DER (Distinguished Encoding Rules) is widely used for digital certificates such as X.509. DER is a subset of BER (Basic Encoding Rules) providing for exactly one way to encode an ASN.1 value. DER is intended for situations when a unique encoding is needed, such as in cryptography and ensures that a data structure that needs to be digitally signed produces a unique serialized representation. DER can be seen as a canonical form of BER. DER files may also use the .CER or .CRT extensions
• PEM (Privacy Enhanced Mail): Base64 encoded DER certificate, enclosed between "-----BEGIN CERTIFICATE-----" and "-----END CERTIFICATE-----"
• The PKCS #7 format enables the transfer of a certificate and all the certificates in its certification path from one computer to another, or from a computer to removable media. PKCS #7 files typically use the .P7B extension, and are compatible with the X.509 standard. PKCS #7 allows for attributes, such as countersignatures, to be associated with signatures, and attributes such as signing time can be authenticated along with message content
• PKCS #12 defines a file format commonly used to store private keys with accompanying public key certificates, protected with a password-based symmetric key. PFX is a predecessor to PKCS#12. This container format can contain multiple embedded objects, such as multiple certificates. Usually protected/encrypted with a password. Usable as a format for the Java key store and to establish client authentication certificates in Mozilla Firefox