author | Dremov Kirill (Nokia-D-MSW/Tampere) <kirill.dremov@nokia.com> |
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/** |
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@page overview_Certificate_Validation_in_PKIX Certificate Validation in PKIX |
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<hr> |
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@section certificate_chain Certificate Chain |
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Certificate validation is a recursive process. It begins with the need to verify the @ref digital_signature "digital signature" on some data presented by |
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an @ref EE "End Entity" (EE). This involves checking that the key pair is actually owned by that @ref EE. To do this, the |
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public signing key of that EE is acquired by getting its @ref certificate. That @ref certificate would have been |
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signed by the EE's @ref CA "certification authority" (CA), so the @ref digital_signature "digital signature" on the @ref certificate can be |
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verified by getting the @ref CA's public signing key. In turn, the @ref CA's certificate may need verifying, in which case |
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the process is repeated until it bottoms out when an entity that is already trusted is reached; that entity is |
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usually self signed. This process is illustrated in the figure below: |
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@image html Certman_X509_Certificate_Validation_chain.png |
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\n\n |
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The set of @ref certificates from an @ref EE up to a trusted root @ref CA_certificate "CA certificate" is called a certificate chain. |
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Once a certificate chain has been constructed, the @ref EE's key pair at the start can validated. |
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<hr> |
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@section input_to_certificate_validation Input to Certificate Validation |
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The following inputs need to be supplied to the validation process: |
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@li @ref End_Entity_and_Intermediate_Certificates |
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@li @ref Trusted_Root_Certificates |
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@li @ref Validation_Time |
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@li @ref Acceptable_Policies |
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@subsection End_Entity_and_Intermediate_Certificates End Entity & Intermediate Certificates |
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A set of @ref certificates, from the entity requesting @ref authentication up to, but not including, one already trusted |
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by the @ref relying_party "relying party". |
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Where these @ref certificates come from is potentially a difficult problem, if certificate management is expected to search |
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for intermediate certificates in remote repositories; however for @ref TLS, at least servers are required to supply a |
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complete, ordered set of certificates in the form of @ref DER encoded @ref ASN "ASN.1"; so client code can just pass this into |
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the certificate chain object. |
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@subsection Trusted_Root_Certificates Trusted Root Certificates |
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@ref Authentication cannot be done entirely by software: there must be a point at which the user confirms that they trust |
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a particular entity. The validation algorithm can only ascertain that if the user trusts certificate X then they may also |
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trust certificate Y. Certificates that the user trusts directly are called @ref root_certificate "root certificates" because they are the root |
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of the validation chain; they are usually self-signed. |
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It is likely that different applications will have different requirements about which @ref certificates may be considered |
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trust roots, and for the level of protection they require for trust roots. To this end, applications will pass in a unique |
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ID (\c TUid) which Certificate Management will use to identify the application, and so work out which @ref certificates |
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can be considered trusted for that application. |
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For additional flexibility, an overload is provided enabling clients to pass a set of @ref root_certificate "root certificates" directly into |
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the validation function. |
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For more detailed discussion of the storage and management of root certificates |
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see Root Certificate Management and Storage. |
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@subsection Validation_Time Validation Time |
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The time for which validation will be performed. |
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@subsection Acceptable_Policies Acceptable Policies |
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A set of @ref OID "OID"s, each of which represents a certification policy acceptable to the application. This enables client |
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code to restrict the @ref certificates that may occur in a valid certificate chain. Client code does not have to specify |
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any preferred policies here. An empty set is interpreted as any policy. |
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<hr> |
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@section chain_validation Chain Validation |
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@subsection chain_construction Chain Construction |
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The chain object parses the encoded set of certificates. The first certificate is considered to be the @ref EE |
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certificate, and subsequent @ref certificates must each certify the preceding one. The chain object then searches the set |
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of trusted @ref root_certificate "root certificates" for a certificate trusted by the client and whose subject name matches the issuer name |
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in the last certificate in the decoded set. If two or more match, it will attempt to resolve this be comparing the |
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authority key ID in the last certificate with the subject key ID in each candidate root. If this extension is not present, |
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it will attempt to find a single root by signature verification. If no root is found, validation will fail immediately |
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with an error. |
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@subsection initialisation_of_chain_validation_algorithm Initialisation of Chain Validation Algorithm |
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The certificate chain initialises the following state variables: |
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@li Validation Time: time for which validation is calculated. Initialised to the time supplied by client code. |
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@li Initial Policies: set of policy @ref OIDs, initialised to the policies supplied by client code |
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@li Acceptable Policies: set of X.509 policy information objects, initially any policy |
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@li Mapped Policies: set of policy @ref OIDs, initially empty |
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@li Excluded Subtrees: set of X.500 General Name objects, initially empty |
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@li Permitted Subtrees: set of X.500 General Name objects, initially any subtree |
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@li Max Path Length: integer representing the maximum path length. Initially the actual path length |
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@li Inhibit Policy Mapping: integer whose value is the number of certificates that may appear after the current one before |
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policy mapping is no longer permitted. Initially the chain length |
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@li Require Explicit Policy: integer whose value is the number of certificates that may appear after the current one |
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before an acceptable policy @ref OID must appear in the certificate. Initially the chain length |
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@li Current Cert: integer whose value is the position of the current certificate in the chain. Initially the chain length -1 |
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<hr> |
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@section validation_algorithm Validation Algorithm |
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Validation of a certificate chain starts at the root and ends at the @ref EE "End Entity" (EE). It returns a result object which |
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includes: |
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@li a single error value, which consists of a reason for the error and an integer identifying which @ref certificate the |
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error is associated with. If the reason is anything other than \c EValidatedOK then validation has failed. |
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@li a set of warning values, each consisting of a reason for the warning and an integer identifying which @ref certificate the |
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warning is associated with. |
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@li a set of policies accepted in the course of validating the chain. |
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The reason the warnings are present is that it is sometimes impossible for Certificate Management to work out if an |
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irregularity is an error or not, because it depends on the context of the @ref certificate (e.g. the use to which its key |
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will be put) which is only known by client code. Warnings give client code a chance to evaluate these irregularities. |
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For each @ref certificate the actions described below are performed: |
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@li Signature Verification & Name Chaining -- Each @ref certificate must be signed by, and its subject name should match the |
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issuer name in, the certificate above it in the chain. The only exception is the @ref root_certificate "root certificate", which, if it |
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claims to be self signed (i.e. its subject and issuer names match) must really be self signed; otherwise its signature |
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is not verified, but a warning is generated. |
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@li Validity Period Checking -- For each certificate, the Validation Time must lie within the validity period in the |
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certificate. |
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@li Extension Processing -- Extensions can be marked critical; all critical extensions must be 'recognized and processed' |
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or the certificate must be rejected. For some extensions it is either unclear what this means, or it clearly makes no |
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sense, and in these cases a common response is to reject the certificate if the extension is marked critical. It is |
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impossible to guarantee support for some extensions. |
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@li Revocation Checking |
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Additionally, for each certificate: |
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@li the \em Max \em Path \em Length, \em Inhibit \em Policy \em Mapping and \em Require \em Explicit \em Policy variables |
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are decremented, |
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@li and Current Cert must be less than or equal to \em Max \em Path \em Length. |
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When validation is complete the \em Acceptable \em Policies variable will be copied into the set of policies in the result |
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object. |
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Move the Security package to EPL, and add the implementations of the cryptographic algorithms
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*/ |