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X.509 Certificates

issued by a Certification Authority (CA), containing:, CA Hierarchy Use

X.509 Certificates

1. Overview:

issued by a Certification Authority (CA), containing:


version (1, 2, or 3)

serial number (unique within CA) identifying certificate

signature algorithm identifier

issuer X.500 name (CA)

period of validity (from - to dates)

subject X.500 name (name of owner)

subject public-key info (algorithm, parameters, key)

issuer unique identifier (v2+)

subject unique identifier (v2+)

extension fields (v3)

signature (of hash of all fields in certificate)


notation CA<<A>> denotes certificate for A signed by CA


X.509 defines a framework for the provision of authentication services by the X.500 directory to


its users. The directory may serve as a repository of public-key certificates. Each certificate contains the public key of a user and is signed with the private key of a trusted certification authority. In addition, X.509 defines alternative authentication protocols based on the use of public-key certificates.


X.509 is an important standard because the certificate structure and authentication protocols


defined in X.509 are used in a variety of contexts. For example, the X.509 certificate format is


used in S/MIME), IP Security and SSL/TLS and SET


2. Certificates


The heart of the X.509 scheme is the public-key certificate associated with each user.


These user certificates are assumed to be created by some trusted certification authority (CA) and


placed in the directory by the CA or by the user.




Differentiates among successive versions of the certificate format; the default is version 1. If the Issuer Unique Identifier or Subject Unique Identifier are present, the value must be version 2. If one or more extensions are present, the version must be version 3.


Serial number:  


An integer value, unique within the issuing CA, that is unambiguously associated with this certificate.


4.  Signature algorithm identifier:  


The algorithm used to sign the certificate, together with any associated parameters. Because this information is repeated in the Signature field at the end of the certificate, this field has little, if any, utility.


4.  Issuer name:  


X.500 name of the CA that created and signed this certificate.


Period of validity:  

Consists of two dates: the first and last on which the certificate is valid.




The name of the user to whom this certificate refers. That is, this certificate certifies the public key of the subject who holds the corresponding private key.


Subject's public-key information:  


The public key of the subject, plus an identifier of the algorithm for which this key is to be used, together with any associated parameters.


Issuer unique identifier:  


An optional bit string field used to identify uniquely the issuing CA in the event the X.500 name has been reused for different entities.


Subject unique identifier:  


An optional bit string field used to identify uniquely the subject in the event the X.500 name has been reused for different entities.




A set of one or more extension fields. Extensions were added in version 3 and are discussed later in this section.




Covers all of the other fields of the certificate; it contains the hash code of the other fields, encrypted with the CA's private key. This field includes the signature algorithm identifier.


The standard uses the following notation to define a certificate:


CA<<A>> = CA {V, SN, AI, CA, TA, A, Ap}


Where, Y <<X>> = the certificate of user X issued by certification authority Y


Y {I} = the signing of I by Y. It consists of I with an encrypted hash code appended


The CA signs the certificate with its private key. If the corresponding public key is known to a user, then that user can verify that a certificate signed by the CA is valid.


3. Obtaining a User's Certificate


User certificates generated by a CA have the following characteristics:


o   Any user with access to the public key of the CA can verify the user public key that was certified.  


o   No party other than the certification authority can modify the certificate without this being detected.  


Because certificates are unforgeable, they can be placed in a directory without the need for the directory to make special efforts to protect them.


If all users subscribe to the same CA, then there is a common trust of that CA. All user certificates can be placed in the directory for access by all users.


If there is a large community of users, it may not be practical for all users to subscribe to the same CA. Because it is the CA that signs certificates, each participating user must have a copy of the CA's own public key to verify signatures. This public key must be provided to each user in an absolutely secure (with respect to integrity and authenticity) way so that the user has confidence in the associated certificates. Thus, with many users, it may be more practical for there to be a number of CAs, each of which securely provides its public key to some fraction of the users.


Now suppose that A has obtained a certificate from certification authority X1 and B has obtained a certificate from CA X2. If A does not securely know the public key of X 2, then B's certificate, issued by X2, is useless to A.


A can read B's certificate, but A cannot verify the signature. However, if the two CAs have securely exchanged their own public keys, the following procedure will enable A to obtain B's public key:


o   A obtains, from the directory, the certificate of X2 signed by X1. Because A securely knows X1's public key, A can obtain X2's public key from its certificate and verify it by means of X1's signature on the certificate.


o   A then goes back to the directory and obtains the certificate of B signed by X2 Because A now has a trusted copy of X2's public key, A can verify the signature and securely obtain B's public key.


A has used a chain of certificates to obtain B's public key. In the notation of X.509, this chain is


expressed as


X1<<X2>> X2 <<B>>


In the same fashion, B can obtain A's public key with the reverse chain:


X2<<X1>> X1 <<A>>


This scheme need not be limited to a chain of two certificates. An arbitrarily long path of CAs can be followed to produce a chain. A chain with N elements would be expressed as


X1<<X2>> X2 <<X3>>... XN<<B>>


In this case, each pair of CAs in the chain (Xi, Xi+1) must have created certificates for each other.


All these certificates of CAs by CAs need to appear in the directory, and the user needs to know how they are linked to follow a path to another user's public-key certificate. X.509 suggests that CAs be arranged in a hierarchy so that navigation is straightforward.


From X.509, is an example of such a hierarchy. The connected circles indicate the hierarchical relationship among the CAs; the associated boxes indicate certificates maintained in the directory for each CA entry. The directory entry for each CA includes two types of certificates:


o   Forward certificates: Certificates of X generated by other CAs  

o   Reverse certificates: Certificates generated by X that are the certificates of other CAs  


4. CA Hierarchy Use


In the example given below , user A can acquire the following certificates from the directory to establish a certification path to B:


X<<W>> W <<V>> V <<Y>> <<Z>> Z <<B>>


When A has obtained these certificates, it can unwrap the certification path in sequence to recover a


trusted copy of B's public key. Using this public key, A can send encrypted messages to B. If A wishes to receive encrypted messages back from B, or to sign messages sent to B, then B will require A's public key, which can be obtained from the following certification path:


Z<<Y>> Y <<V>> V <<W>> W <<X>>X <<A>>


B can obtain this set of certificates from the directory, or A can provide them as part of its initial message to B.



Certificate Revocation


o   certificates have a period of validity


o   may need to revoke before expiry, for the following reasons eg:


§  user's private key is compromised

§  user is no longer certified by this CA

§  CA's certificate is compromised


o   CA‟s maintain list of revoked certificates


§  the Certificate Revocation List (CRL)


o   users should check certs with CA‟s CRL

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