Typically the application will contain an option to point to an extension section. Each line of the extension section takes the form:
extension_name=[critical,] extension_options
If ccrriittiiccaall is present then the extension will be critical.
The format of eexxtteennssiioonn__ooppttiioonnss depends on the value of eexxtteennssiioonn__nnaammee.
There are four main types of extension: _s_t_r_i_n_g extensions, _m_u_l_t_i_-_v_a_l_u_e_d extensions, _r_a_w and _a_r_b_i_t_r_a_r_y extensions.
String extensions simply have a string which contains either the value itself or how it is obtained.
For example:
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:
basicConstraints=critical,CA:true,pathlen:1
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 _a_r_b_i_t_r_a_r_y extension syntax must be used, see the ARBITRARY EXTENSIONS section for more details.
BBaassiicc CCoonnssttrraaiinnttss..
This is a multi valued extension which indicates whether a certificate is a CA certificate. The first (mandatory) name is CCAA followed by TTRRUUEE or FFAALLSSEE. If CCAA is TTRRUUEE then an optional ppaatthhlleenn name followed by an non-negative value can be included.
For example:
basicConstraints=CA:TRUE
basicConstraints=CA:FALSE
basicConstraints=critical,CA:TRUE, pathlen:0
A CA certificate mmuusstt 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.
KKeeyy UUssaaggee..
Key usage is a multi valued extension consisting of a list of names of the permitted key usages.
The supporte names are: digitalSignature, nonRepudiation, keyEncipherment, dataEncipherment, keyAgreement, keyCertSign, cRLSign, encipherOnly and decipherOnly.
Examples:
keyUsage=digitalSignature, nonRepudiation
keyUsage=critical, keyCertSign
EExxtteennddeedd KKeeyy UUssaaggee..
This extensions consists of a list of usages indicating purposes for which the certificate public key can be used for,
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
Examples:
extendedKeyUsage=critical,codeSigning,1.2.3.4 extendedKeyUsage=nsSGC,msSGC
SSuubbjjeecctt KKeeyy IIddeennttiiffiieerr..
This is really a string extension and can take two possible values. Either the word hhaasshh which will automatically follow the guidelines in RFC3280 or a hex string giving the extension value to include. The use of the hex string is strongly discouraged.
Example:
subjectKeyIdentifier=hash
AAuutthhoorriittyy KKeeyy IIddeennttiiffiieerr..
The authority key identifier extension permits two options. keyid and issuer: both can take the optional value "always".
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.
Example:
authorityKeyIdentifier=keyid,issuer
SSuubbjjeecctt AAlltteerrnnaattiivvee NNaammee..
The subject alternative name extension allows various literal values to be included in the configuration file. These include eemmaaiill (an email address) UURRII a uniform resource indicator, DDNNSS (a DNS domain name), RRIIDD (a registered ID: OBJECT IDENTIFIER), IIPP (an IP address), ddiirrNNaammee (a distinguished name) and otherName.
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 IIPP options can be in either IPv4 or IPv6 format.
The value of ddiirrNNaammee 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 _A_S_N_1___g_e_n_e_r_a_t_e___n_c_o_n_f(3) format.
Examples:
subjectAltName=email:copy,email:my@other.address,URI:http://my.url.here/ subjectAltName=IP:192.168.7.1 subjectAltName=IP:13::17 subjectAltName=email:my@other.address,RID:1.2.3.4 subjectAltName=otherName:1.2.3.4;UTF8:some other identifier
subjectAltName=dirName:dir_sect
[dir_sect] C=UK O=My Organization OU=My Unit CN=My Name
IIssssuueerr AAlltteerrnnaattiivvee NNaammee..
The issuer alternative name option supports all the literal options of subject alternative name. It does nnoott support the email:copy option because that would not make sense. It does support an additional issuer:copy option that will copy all the subject alternative name values from the issuer certificate (if possible).
Example:
issuserAltName = issuer:copy
AAuutthhoorriittyy IInnffoo AAcccceessss..
The authority information access extension gives details about how to access certain information relating to the CA. Its syntax is accessOID;location where _l_o_c_a_t_i_o_n has the same syntax as subject alternative name (except that email:copy is not supported). accessOID can be any valid OID but only certain values are meaningful, for example OCSP and caIssuers.
Example:
authorityInfoAccess = OCSP;URI:http://ocsp.my.host/ authorityInfoAccess = caIssuers;URI:http://my.ca/ca.html
CCRRLL ddiissttrriibbuuttiioonn ppooiinnttss..
This is a multi-valued extension whose options can be either in name:value pair using the same form as subject alternative name or a single value representing a section name containing all the distribution point fields.
For a name:value pair a new DistributionPoint with the fullName field set to the given value both the cRLissuer and reasons fields are omitted in this case.
In the single option case the section indicated contains values for each field. In this section:
If the name is "fullname" the value field should contain the full name of the distribution point in the same format as subject alternative name.
If the name is "relativename" then the value field should contain a section name whose contents represent a DN fragment to be placed in this field.
The name "CRLIssuer" if present should contain a value for this field in subject alternative name format.
If the name is "reasons" the value field should consist of a comma separated field containing the reasons. Valid reasons are: "keyCompromise", "CACompromise", "affiliationChanged", "superseded", "cessationOfOperation", "certificateHold", "privilegeWithdrawn" and "AACompromise".
Simple examples:
crlDistributionPoints=URI:http://myhost.com/myca.crl crlDistributionPoints=URI:http://my.com/my.crl,URI:http://oth.com/my.crl
Full distribution point example:
crlDistributionPoints=crldp1_section
[crldp1_section]
fullname=URI:http://myhost.com/myca.crl CRLissuer=dirName:issuer_sect reasons=keyCompromise, CACompromise
[issuer_sect] C=UK O=Organisation CN=Some Name
IIssssuuiinngg DDiissttrriibbuuttiioonn PPooiinntt
This extension should only appear in CRLs. It is a multi valued extension whose syntax is similar to the "section" pointed to by the CRL distribution points extension with a few differences.
The names "reasons" and "CRLissuer" are not recognized.
The name "onlysomereasons" is accepted which sets this field. The value is in the same format as the CRL distribution point "reasons" field.
The names "onlyuser", "onlyCA", "onlyAA" and "indirectCRL" are also accepted the values should be a boolean value (TRUE or FALSE) to indicate the value of the corresponding field.
Example:
issuingDistributionPoint=critical, @idp_section
[idp_section]
fullname=URI:http://myhost.com/myca.crl indirectCRL=TRUE onlysomereasons=keyCompromise, CACompromise
[issuer_sect] C=UK O=Organisation CN=Some Name
CCeerrttiiffiiccaattee PPoolliicciieess..
This is a _r_a_w extension. All the fields of this extension can be set by using the appropriate syntax.
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= 1.2.4.5, 1.1.3.4
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:
CPS.nnn=value
userNotice qualifiers can be set using the syntax:
userNotice.nnn=@notice
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.
Example:
certificatePolicies=ia5org,1.2.3.4,1.5.6.7.8,@polsect
[polsect]
policyIdentifier = 1.3.5.8 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 iiaa55oorrgg option changes the type of the _o_r_g_a_n_i_z_a_t_i_o_n 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.
PPoolliiccyy CCoonnssttrraaiinnttss
This is a multi-valued extension which consisting of the names rreeqquuiirreeEExxpplliicciittPPoolliiccyy or iinnhhiibbiittPPoolliiccyyMMaappppiinngg and a non negative intger value. At least one component must be present.
Example:
policyConstraints = requireExplicitPolicy:3
IInnhhiibbiitt AAnnyy PPoolliiccyy
This is a string extension whose value must be a non negative integer.
Example:
inhibitAnyPolicy = 2
NNaammee CCoonnssttrraaiinnttss
The name constraints extension is a multi-valued extension. The name should begin with the word ppeerrmmiitttteedd or eexxcclluuddeedd followed by a ;;. The rest of the name and the value follows the syntax of subjectAltName except email:copy is not supported and the IIPP form should consist of an IP addresses and subnet mask separated by a //.
Examples:
nameConstraints=permitted;IP:192.168.0.0/255.255.0.0
nameConstraints=permitted;email:.somedomain.com
nameConstraints=excluded;email:.com issuingDistributionPoint = idp_section
OOCCSSPP NNoo CChheecckk
The OCSP No Check extension is a string extension but its value is ignored.
Example:
noCheck = ignored
NNeettssccaappee SSttrriinngg eexxtteennssiioonnss..
Netscape Comment (nnssCCoommmmeenntt) is a string extension containing a comment which will be displayed when the certificate is viewed in some browsers.
Example:
nsComment = "Some Random Comment"
Other supported extensions in this category are: nnssBBaasseeUUrrll,
nnssRReevvooccaattiioonnUUrrll, nnssCCaaRReevvooccaattiioonnUUrrll, nnssRReenneewwaallUUrrll, nnssCCaaPPoolliiccyyUUrrll
and nnssSSssllSSeerrvveerrNNaammee.
NNeettssccaappee CCeerrttiiffiiccaattee TTyyppee
This is a multi-valued extensions which consists of a list of flags to be included. It was used to indicate the purposes for which a certificate could be used. The basicConstraints, keyUsage and extended key usage extensions are now used instead.
Acceptable values for nsCertType are: cclliieenntt, sseerrvveerr, eemmaaiill, oobbjjssiiggnn, rreesseerrvveedd, ssssllCCAA, eemmaaiillCCAA, oobbjjCCAA.
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 _A_S_N_1___g_e_n_e_r_a_t_e___n_c_o_n_f(3). For example:
1.2.3.4=critical,ASN1:UTF8String:Some random data
1.2.3.4=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.
1.2.3.4=critical,DER:01:02:03:04 1.2.3.4=DER:01020304
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:
basicConstraints=critical,DER:00:01:02:03
The DER and ASN1 options should be used with caution. It is possible to create totally invalid extensions if they are not used carefully.
subjectAltName=URI:ldap://somehost.com/CN=foo,OU=bar
will produce an error but the equivalent form:
subjectAltName=@subject_alt_section
[subject_alt_section] subjectAltName=URI:ldap://somehost.com/CN=foo,OU=bar
is valid.
Due to the behaviour of the OpenSSL ccoonnff 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 ddiirreeccttoorryyNNaammee and ootthheerrNNaammee option as well as the AASSNN11 option for arbitrary extensions was added in OpenSSL 0.9.8