PKIX Certificates

## 3 PKIX Certificates

### 3.1 Introduction to Certificates

Certificates were originally defined by ITU (CCITT) and the latest definitions are described in ITU-T X.509 , but those definitions are (as always) not working.

Working certificate definitions for the Internet Community are found in the the PKIX RFCs RFC 3279 and RFC 3280 . The parsing of certificates in the Erlang/OTP SSL application is based on those RFCS.

Certificates are defined in terms of ASN.1 (ITU-T X.680 ). For an introduction to ASN.1 see ASN.1 Information Site.

### 3.2 PKIX Certificates

Here we base the PKIX certificate definitions in RFCs RFC 3279 and RFC 3280 . We however present the definitions according to `SSL-PKIX.asn1` module, which is an amelioration of the `PKIX1Explicit88.asn1`, `PKIX1Implicit88.asn1`, and `PKIX1Algorithms88.asn1` modules. You find all these modules in the `pkix` subdirectory of SSL.

The Erlang terms that are returned by the functions `ssl:peercert/1/2`, `ssl_pkix:decode_cert/1/2`, and `ssl_pkix:decode_cert_file/1/2` when the option `ssl` is used in those functions, correspond the ASN.1 structures described in the sequel.

#### 3.2.1 Certificate and TBSCertificate

    Certificate  ::=  SEQUENCE  {
         tbsCertificate       TBSCertificate,
         signatureAlgorithm   SignatureAlgorithm,
         signature            BIT STRING }

    TBSCertificate  ::=  SEQUENCE  {
         version         [0]  Version DEFAULT v1,
         serialNumber         CertificateSerialNumber,
         signature            SignatureAlgorithm,
         issuer               Name,
         validity             Validity,
         subject              Name,
         subjectPublicKeyInfo SubjectPublicKeyInfo,
         issuerUniqueID  [1]  IMPLICIT UniqueIdentifier OPTIONAL,
                              -- If present, version MUST be v2 or v3
         subjectUniqueID [2]  IMPLICIT UniqueIdentifier OPTIONAL,
                              -- If present, version MUST be v2 or v3
         extensions      [3]  Extensions OPTIONAL
                              -- If present, version MUST be v3 --  }

    Version  ::=  INTEGER  {  v1(0), v2(1), v3(2)  }

    CertificateSerialNumber  ::=  INTEGER

    Validity ::= SEQUENCE {
         notBefore      Time,
         notAfter       Time  }

    Time ::= CHOICE {
         utcTime        UTCTime,
         generalTime    GeneralizedTime }

The meaning of the fields `version`, `serialNumber`, and `validity` are quite obvious given the type definitions above, so we do not go further into their details.

The `signatureAlgorithm` field of `Certificate` and the `signature` field of `TBSCertificate` contain the name and parameters of the algorithm used for signing the certificate. The values of these two fields must be equal.

The `signature` field of `Certificate` contains the value of the signature that the issuer computed by using the prescribed algorithm.

The `issuer<c> and <c>subject` fields can contain many different types av data, and is therefore considered in a separate section. The same holds for the `extensions` field. The `issuerUniqueID` and the `subjectUniqueID` fields are not considered further.

#### 3.2.2 TBSCertificate issuer and subject

    Name ::= CHOICE { -- only one possibility for now --
          rdnSequence  RDNSequence }

    RDNSequence ::= SEQUENCE OF RelativeDistinguishedName

    DistinguishedName ::=   RDNSequence

    RelativeDistinguishedName  ::=
                        SET SIZE (1 .. MAX) OF AttributeTypeAndValue

    AttributeTypeAndValue ::=  SEQUENCE {
            type    ATTRIBUTE-TYPE-AND-VALUE-CLASS.&id
                    ({SupportedAttributeTypeAndValues}),
            value   ATTRIBUTE-TYPE-AND-VALUE-CLASS.&Type
                    ({SupportedAttributeTypeAndValues}{@type}) }

    SupportedAttributeTypeAndValues ATTRIBUTE-TYPE-AND-VALUE-CLASS ::=
            { name | surname | givenName | initials | generationQualifier |
              commonName | localityName | stateOrProvinceName | organizationName |
              organizationalUnitName | title | dnQualifier | countryName |
              serialNumber | pseudonym | domainComponent | emailAddress }

#### 3.2.3 TBSCertificate extensions

The `extensions` field of a `TBScertificate` is a sequence of type `Extension`, defined as follows,

    Extension  ::=  SEQUENCE  {
         extnID      OBJECT IDENTIFIER,
         critical    BOOLEAN DEFAULT FALSE,
         extnValue   ANY }

Each extension has a unique object identifier. An extension with a `critical` value set to `TRUE` must be recognised by the reader of a certificate, or else the certificate must be rejected.

Extensions are divided into two groups: standard extensions and internet certificate extensions. All extensions listed in the table that follows are standard extensions, except for `authorityInfoAccess` and `subjectInfoAccess`, which are internet extensions.

Depending on the object identifier the `extnValue` is parsed into an appropriate welldefined structure.

The following table shows the purpose of each extension, but does not specify the structure. To see the structure consult the `PKIX1Implicit88.asn1` module.

PKIX Extensions
authorityKeyIdentifier | Used by to identify a certificate signed that has multiple signing keys.
subjectKeyIdentifier | Used to identify certificates that contain a public key. Must appear i CA certificates.
keyUsage | Defines the purpose of the certificate. Can be one or several of `digitalSignature`, `nonRepudiation`, `keyEncipherment`, `dataEncipherment`, `keyAgreement`, `keyCertSign`, `cRLSign`, `encipherOnly`, `decipherOnly`.
privateKeyUsagePeriod | Allows certificate issuer to provide a private key usage period to be short than the certificate usage period.
certificatePolicies | Contains one or more policy information terms indicating the policies under which the certificate has been issued.
policyMappings | Used i CA certificates.
subjectAltName | Allows additional identities to be bound the the subject.
issuerAltName | Allows additional identities to be bound the the issuer.
subjectDirectoryAttributes | Conveys identity attributes of the subject.
basicConstraints | Tells if the certificate holder is a CA or not.
nameConstraints | Used in CA certificates.
policyConstraints | Used in CA certificates.
extKeyUsage | Indicates for which purposed the public key may be used.
cRLDistributionPoints | Indicates how CRL (Certificate Revokation List) information is obtained.
inhibitAnyPolicy | Used i CA certificates.
freshestCRL | For CRLs.
authorityInfoAccess | How to access CA information of the isssuer of the certificate.
subjectInfoAccess | How to access CA information of the subject of the certificate.

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