OpenSSL X509V3 extension configuration: preliminary documentation.

INTRODUCTION.

For OpenSSL 0.9.2 the extension code has be considerably enhanced. It is now
possible to add and print out common X509 V3 certificate and CRL extensions.

For more information about the meaning of extensions see:

http://www.imc.org/ietf-pkix/
http://home.netscape.com/eng/security/certs.html

PRINTING EXTENSIONS.

Extension values are automatically printed out for supported extensions.

x509 -in cert.pem -text
crl -in crl.pem -text

will give information in the extension printout, for example:


        X509v3 extensions:
            X509v3 Basic Constraints: 
                CA:TRUE
            X509v3 Subject Key Identifier: 
                73:FE:F7:59:A7:E1:26:84:44:D6:44:36:EE:79:1A:95:7C:B1:4B:15
            X509v3 Authority Key Identifier: 
                keyid:73:FE:F7:59:A7:E1:26:84:44:D6:44:36:EE:79:1A:95:7C:B1:4B:15, DirName:/C=AU/ST=Some-State/O=Internet Widgits Pty Ltd/Email=email@1.address/Email=email@2.address, serial:00
            X509v3 Key Usage: 
                Certificate Sign, CRL Sign
            X509v3 Subject Alternative Name: 
                email:email@1.address, email:email@2.address

CONFIGURATION FILES.

The OpenSSL utilities 'ca' and 'req' can now have extension sections listing
which certificate extensions to include. In each case a line:

x509_extensions = extension_section

indicates which section contains the extensions. In the case of 'req' the
extension section is used when the -x509 option is present to create a
self signed root certificate.

EXTENSION SYNTAX.

Extensions have the basic form:

extension_name=[critical,] extension_options

the use of the critical option makes the extension critical. Extreme caution
should be made when using the critical flag. If an extension is marked
as critical then any client that does not understand the extension should
reject it as invalid. Some broken software will reject certificates which
have *any* critical extensions (these violates PKIX but we have to live
with it).

There are three main types of extension, string extensions, multi valued
extensions, and raw extensions.

String extensions simply have a string which defines the value of the 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. However it should be noted that in some cases the
same name can appear multiple times, for example,

subjectAltName=email:steve@here,email:steve@there

in this case an equivalent long form is:

subjectAltName=@alt_section

[alt_section]

email.1=steve@here
email.2=steve@there

This is because the configuration file code cannot handle the same name
occurring twice in the same extension.

Raw extensions allow arbitrary data to be placed in an extension. For
example

1.2.3.4=critical,RAW:01:02:03:04
1.2.3.4=RAW:01020304

The value following RAW is a hex dump of the extension contents. Any extension
can be placed in this form to override the default behaviour. For example:

basicConstraints=critical,RAW:00:01:02:03

WARNING: raw extensions should be used with caution. It is possible to create
totally invalid extensions unless care is taken.

CURRENTLY SUPPORTED EXTENSIONS.

Literal String extensions.

In each case the 'value' of the extension is placed directly in the extension.
Currently supported extensions in this category are: nsBaseUrl, nsRevocationUrl
nsCaRevocationUrl, nsRenewalUrl, nsCaPolicyUrl, nsSslServerName and
nsComment.

For example:

nsComment="This is a test comment"

Bit Strings.

Bit string extensions just consist of a list of suppported bits, currently
two extensions are in this category: PKIX keyUsage and the Netscape specific
nsCertType.

nsCertType (netscape certificate type) takes the flags: client, server, email,
objsign, reserved, sslCA, emailCA, objCA.

keyUsage (PKIX key usage) takes the flags: digitalSignature, nonRepudiation,
keyEncipherment, dataEncipherment, keyAgreement, keyCertSign, cRLCertSign,
encipherOnly, decipherOnly.

For example:

nsCertType=server

keyUsage=critical, digitalSignature, nonRepudiation


Basic Constraints.

Basic constraints is a multi valued extension that supports a CA and an
optional pathlen option. The CA option takes the values true and false and
pathlen takes an integer. Note if the CA option is false the pathlen option
should be omitted.

Examples:

basicConstraints=CA:TRUE
basicConstraints=critical,CA:TRUE, pathlen:10

NOTE: for a CA to be considered valid it must have the CA option set to
TRUE. An end user certificate MUST NOT have the CA value set to true.
According to PKIX recommendations it should exclude the extension entirely
however some software may require CA set to FALSE for end entity certificates.

Subject Key Identifier.

This is really a string extension and can take two possible values. Either
a hex string giving details of the extension value to include or the word
'hash' which then automatically follow PKIX guidelines in selecting and
appropriate key identifier. The use of the hex string is strongly discouraged.

Example: subjectKeyIdentifier=hash

Authority Key Identifier.

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. Normally this will only be done if the keyid option fails or
is not included: the "always" flag will always include the value.

Subject Alternative Name.

The subject alternative name extension allows various literal values to be
included in the configuration file. These include "email" (an email address)
"URI" a uniform resource indicator, "DNS" (a DNS domain name), RID (a
registered ID: OBJECT IDENTIFIER) and IP (and IP address).

Also the email option include a special 'copy' value. This will automatically
include and email addresses contained in the certificate subject name in
the extension.

Examples:

subjectAltName=email:copy,email:my@other.address,URL:http://my.url.here/
subjectAltName=email:my@other.address,RID:1.2.3.4

Issuer Alternative Name.

The issuer alternative name option supports all the literal options of
subject alternative name. It does *not* support the email:copy option because
that would not make sense. It does support and additional issuer:copy option
that will copy all the subject alternative name values from the issuer 
certificate (if possible).

Display only extensions.

Some extensions are only partially supported and currently are only displayed
but cannot be set. These include private key usage period, CRL number, and
CRL reason.