Typically the application will contain an option to point to an extension section. Each line of the extension section takes the form:
If critical is present then the extension will be critical.
The format of extension_options depends on the value of extension_name.
There are four main types of extension: string extensions, multi-valued extensions, raw and arbitrary extensions.
String extensions simply have a string which contains either the value itself or how it is obtained.
nsComment="This is a Comment"
Multi-valued extensions have a short form and a long form. The short form is a list of names and values:
The long form allows the values to be placed in a separate section:
basicConstraints=critical,@bs_section [bs_section] CA=true pathlen=1
Both forms are equivalent.
The syntax of raw extensions is governed by the extension code: it can for example contain data in multiple sections. The correct syntax to use is defined by the extension code itself: check out the certificate policies extension for an example.
If an extension type is unsupported then the arbitrary extension syntax must be used, see the ARBITRART EXTENSIONS section for more details.
basicConstraints=CA:TRUE basicConstraints=CA:FALSE basicConstraints=critical,CA:TRUE, pathlen:0
A CA certificate must include the basicConstraints value with the CA field set to TRUE. An end user certificate must either set CA to FALSE or exclude the extension entirely. Some software may require the inclusion of basicConstraints with CA set to FALSE for end entity certificates.
The pathlen parameter indicates the maximum number of CAs that can appear below this one in a chain. So if you have a CA with a pathlen of zero it can only be used to sign end user certificates and not further CAs.
The supporte names are: digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment, keyAgreement, keyCertSign, cRLSign, encipherOnly and decipherOnly.
keyUsage=digitalSignature, nonRepudiation keyUsage=critical, keyCertSign
These can either be object short names of the dotted numerical form of OIDs. While any OID can be used only certain values make sense. In particular the following PKIX, NS and MS values are meaningful:
Value Meaning ----- ------- serverAuth SSL/TLS Web Server Authentication. clientAuth SSL/TLS Web Client Authentication. codeSigning Code signing. emailProtection E-mail Protection (S/MIME). timeStamping Trusted Timestamping msCodeInd Microsoft Individual Code Signing (authenticode) msCodeCom Microsoft Commercial Code Signing (authenticode) msCTLSign Microsoft Trust List Signing msSGC Microsoft Server Gated Crypto msEFS Microsoft Encrypted File System nsSGC Netscape Server Gated Crypto
If the keyid option is present an attempt is made to copy the subject key identifier from the parent certificate. If the value ``always'' is present then an error is returned if the option fails.
The issuer option copies the issuer and serial number from the issuer certificate. This will only be done if the keyid option fails or is not included unless the ``always'' flag will always include the value.
The email option include a special 'copy' value. This will automatically include and email addresses contained in the certificate subject name in the extension.
The IP address used in the IP options can be in either IPv4 or IPv6 format.
The value of dirName should point to a section containing the distinguished name to use as a set of name value pairs. Multi values AVAs can be formed by preceeding the name with a + character.
otherName can include arbitrary data associated with an OID: the value should be the OID followed by a semicolon and the content in standard ASN1_generate_nconf() format.
subjectAltName=email:copy,email:email@example.com,URI:http://my.url.here/ subjectAltName=IP:192.168.7.1 subjectAltName=IP:13::17 subjectAltName=email:firstname.lastname@example.org,RID:220.127.116.11 subjectAltName=otherName:18.104.22.168;UTF8:some other identifier subjectAltName=dirName:dir_sect [dir_sect] C=UK O=My Organization OU=My Unit CN=My Name
issuserAltName = issuer:copy
authorityInfoAccess = OCSP;URI:http://ocsp.my.host/ authorityInfoAccess = caIssuers;URI:http://my.ca/ca.html
Currently each option will set a new DistributionPoint with the fullName field set to the given value.
Other fields like cRLissuer and reasons cannot currently be set or displayed: at this time no examples were available that used these fields.
If you follow the PKIX recommendations and just using one OID then you just include the value of that OID. Multiple OIDs can be set separated by commas, for example:
certificatePolicies= 22.214.171.124, 126.96.36.199
If you wish to include qualifiers then the policy OID and qualifiers need to be specified in a separate section: this is done by using the @section syntax instead of a literal OID value.
The section referred to must include the policy OID using the name policyIdentifier, cPSuri qualifiers can be included using the syntax:
userNotice qualifiers can be set using the syntax:
The value of the userNotice qualifier is specified in the relevant section. This section can include explicitText, organization and noticeNumbers options. explicitText and organization are text strings, noticeNumbers is a comma separated list of numbers. The organization and noticeNumbers options (if included) must BOTH be present. If you use the userNotice option with IE5 then you need the 'ia5org' option at the top level to modify the encoding: otherwise it will not be interpreted properly.
certificatePolicies=ia5org,188.8.131.52,184.108.40.206.8,@polsect [polsect] policyIdentifier = 220.127.116.11 CPS.1="http://my.host.name/" CPS.2="http://my.your.name/" userNotice.1=@notice [notice] explicitText="Explicit Text Here" organization="Organisation Name" noticeNumbers=1,2,3,4
The ia5org option changes the type of the organization field. In RFC2459 it can only be of type DisplayText. In RFC3280 IA5Strring is also permissible. Some software (for example some versions of MSIE) may require ia5org.
policyConstraints = requireExplicitPolicy:3
inhibitAnyPolicy = 2
nameConstraints=permitted;IP:192.168.0.0/255.255.0.0 nameConstraints=permitted;email:.somedomain.com nameConstraints=excluded;email:.com
nsComment = "Some Random Comment"
Other supported extensions in this category are: nsBaseUrl, nsRevocationUrl, nsCaRevocationUrl, nsRenewalUrl, nsCaPolicyUrl and nsSslServerName.
Acceptable values for nsCertType are: client, server, email, objsign, reserved, sslCA, emailCA, objCA.
There are two ways to encode arbitrary extensions.
The first way is to use the word ASN1 followed by the extension content using the same syntax as ASN1_generate_nconf(). For example:
18.104.22.168=critical,ASN1:UTF8String:Some random data 22.214.171.124=ASN1:SEQUENCE:seq_sect [seq_sect] field1 = UTF8:field1 field2 = UTF8:field2
It is also possible to use the word DER to include the raw encoded data in any extension.
The value following DER is a hex dump of the DER encoding of the extension Any extension can be placed in this form to override the default behaviour. For example:
The DER and ASN1 options should be used with caution. It is possible to create totally invalid extensions if they are not used carefully.
will produce an error but the equivalent form:
subjectAltName=@subject_alt_section [subject_alt_section] subjectAltName=URI:ldap://somehost.com/CN=foo,OU=bar
Due to the behaviour of the OpenSSL conf library the same field name can only occur once in a section. This means that:
subjectAltName=@alt_section [alt_section] email=steve@here email=steve@there
will only recognize the last value. This can be worked around by using the form:
[alt_section] email.1=steve@here email.2=steve@there
Policy mappings, inhibit any policy and name constraints support was added in OpenSSL 0.9.8
The directoryName and otherName option as well as the ASN1 option for arbitrary extensions was added in OpenSSL 0.9.8