Certificates overview

Certificates overview

Certificates play a major role in authentication of clients connecting to network services via HTTPS, both for administrators and SSL VPN users. Certificate authentication is optional for IPsec VPN peers.

This section includes:

l Certificates and protocols l IPsec VPNs and certificates l Certificate types on the FortiGate unit

Certificates and protocols

There are a number of protocols that are commonly used with certificates including SSL and HTTPS, and other certificate-related protocols.

SSL and HTTPS

The secure HTTP (HTTPS) protocol uses SSL. Certificates are an integral part of SSL. When a web browser connects to the FortiGate unit via HTTPS, a certificate is used to verify the FortiGate unit’s identity to the client.

Certificates overview

Optionally, the FortiGate unit can require the client to authenticate itself in return.

By default, the FortiGate unit uses a self-signed security certificate to authenticate itself to HTTPS clients. When the certificate is offered, the client browser displays two security messages.

  • The first message prompts users to accept and optionally install the FortiGate unit’s self-signed security certificate. If the user does not accept the certificate, the FortiGate unit refuses the connection. When the user accepts the certificate, the FortiGate login page is displayed, and the credentials entered by the user are encrypted before they are sent to the FortiGate unit. If the user chooses to install the certificate, the prompt is not displayed again.
  • Just before the FortiGate login page is displayed, a second message informs users that the FortiGate certificate distinguished name differs from the original request. This message is displayed because the FortiGate unit redirects the connection (away from the distinguished name recorded in the self-signed certificate) and can be ignored.

Optionally, you can install an X.509 server certificate issued by a certificate authority (CA) on the FortiGate unit. You can then configure the FortiGate unit to identify itself using the server certificate instead of the self-signed certificate.

For more information, see the FortiOS Handbook SSL VPN guide.

After successful certificate authentication, communication between the client browser and the FortiGate unit is encrypted using SSL over the HTTPS link.

Certificate-related protocols

There are multiple protocols that are required for handling certificates. These include the Online Certificate Status Protocol (OCSP), Simple Certificate Enrollment Protocol (SCEP), Server-based Certificate Validation Protocol (SCVP), and Certificate Management Protocol (CMP).

Online Certificate Status Protocol

Online Certificate Status Protocol (OCSP) allows the verification of X.509 certificate expiration dates. This is important to prevent hackers from changing the expiry date on an old certificate to a future date.

Normally certificate revocation lists (CRLs) are used, but OCSP is an alternate method available. However a CRL is a public list, and some companies may want to avoid the public exposure of their certificate structure even if it is only invalid certificates.

The OSCP check on the certificate’s revocation status is typically carried out over HTTP with a request-response format. The authority responding can reply with a status of good, revoked, or unknown for the certificate in question.

Simple Certificate Enrollment Protocol

Simple Certificate Enrollment Protocol (SCEP) is an automated method of signing up for certificates. Typically this involves generating a request you send directly to the SCEP service, instead of generating a file request that may or may not be signed locally.

Server-based Certificate Validation Protocol

Server-based Certificate Validation Protocol (SCVP) is used to trace a certificate back to a valid root level certificate. This ensures that each step along the path is valid and trustworthy.

Certificates overview

Certificate Management Protocol version 2

Certificate Management Protocol version 2 (CMPv2) is an enrollment and revocation protocol for certificates.

IPsec VPNs and certificates

Certificate authentication is a more secure alternative to pre-shared key (shared secret) authentication for IPsec VPN peers. Unlike administrators or SSL VPN users, IPsec peers use HTTP to connect to the VPN gateway configured on the FortiGate unit. The VPN gateway configuration can require certificate authentication before it permits an IPsec tunnel to be established. See Authenticating IPsec VPN users with security certificates on page 126 .

Certificate types on the FortiGate unit

There are different types of certificates available that vary depending on their intended use. FortiOS supports local, remote, CA, and CRL certificates.

Local certificates

Local certificates are issued for a specific server, or web site. Generally they are very specific, and often for an internal enterprise network. For example a personal web site for John Smith at www.example.com (such as http://www.example.com/home/jsmith) would have its own local certificate.

These can optionally be just the certificate file, or also include a private key file and PEM passphrase for added security.

For information about generating a certificate request, see Generating a certificate signing request on page 115. For information about installing a local certificate, see Obtaining and installing a signed server certificate from an external CA on page 118

Remote certificates

Remote certificates are public certificates without a private key. For dynamic certificate revocation, you need to use an Online Certificate Status Protocol (OCSP) server. The OCSP is configured in the CLI only. Installed Remote (OCSP) certificates are displayed in the Remote Certificates list. You can select Import to install a certificate from the management PC.

CA root certificates

CA root certificates are similar to local certificates, however they apply to a broader range of addresses or to whole company; they are one step higher up in the organizational chain. Using the local certificate example, a CA root certificate would be issued for all of www.example.com instead of just the smaller single web page.

Certificate revocation list

Certificate revocation list (CRL) is a list of certificates that have been revoked and are no longer usable. This list includes certificates that have expired, been stolen, or otherwise compromised. If your certificate is on this list, it will not be accepted. CRLs are maintained by the CA that issues the certificates and includes the date and time when the next CRL will be issued as well as a sequence number to help ensure you have the most current version of the CRL.

 

Certificate signing

The trust in a certificate comes from the authority that signs it. For example if VeriSign signs your CA root certificate, it is trusted by everyone. While these certificates are universally accepted, it is cumbersome and expensive to have all certificates on a corporate network signed with this level of trust.

With self-signed certificates nobody, except the other end of your communication, knows who you are and therefore they do not trust you as an authority. However this level is useful for encryption between two points — neither point may care about who signed the certificate, just that it allows both points to communicate. This is very useful for internal networks and communications.

A general rule is that CA signed certificates are accepted and sometimes required, but it is easier to self-sign certificates when you are able.

For more on the methods of certificate signing see Generating a certificate signing request on page 115.

BIOS certificate compatibility

FortiOS supports backwards compatibility between BIOS version 4 and BIOS version 3.

BIOS V4 certificates:

  • Fortinet_CA l Fortinet_Sub_CA l Fortinet_Factory

BIOS V3 certificates:

  • Fortinet_CA_Backup l Fortinet_Factory_Backup

When FortiOS connects to FortiGuard, FortiCloud, FortiManager, FortiAnalyzer, FortiSandbox as a client, the

BIOS certificate Fortinet_Factory will be the default client certificate. When the server returns its certificate (chain) back, FortiOS looks up the issuer of the server certificate and either keeps client certificate as is or switches to the BIOS certificate Fortinet_Factory_Backup. This process occurs in one handshake.

When FortiOS connects to FortiCare, the BIOS certificate Fortinet_Factory is the only client certificate and Server Name Indication (SNI) is set. There is no switchover of certificate during SSL handshake.

When FortiOS acts as a server when connected by FortiExtender, FortiSwitch, FortiAP, etc., Fortinet_Factory is the default server certificate. FortiOS detects SNI in client hello, and if no SNI is found or if the CN in SNI is different from the CN of Fortinet_CA, it switches to use the Fortinet_Factory_Backup.

Managing X.509 certificates

Managing security certificates is required due to the number of steps involved in both having a certificate request signed, and then distributing the correct files for use.

You use the FortiGate unit or CA software such as OpenSSL to generate a certificate request. That request is a text file that you send to the CA for verification, or alternately you use CA software to self-validate. Once validated, the certificate file is generated and must be imported to the FortiGate unit before it can be used. These steps are explained in more detail later in this section.

This section provides procedures for generating certificate requests, installing signed server certificates, and importing CA root certificates and CRLs to the FortiGate unit.

For information about how to install root certificates, CRLs, and personal or group certificates on a remote client browser, refer to your browser’s documentation.

l Generating a certificate signing request l Generating certificates with CA software l Obtaining and installing a signed server certificate from an external CA l Installing a CA root certificate and CRL to authenticate remote clients l ExtendedKeyUsage for x.509 certificates

Generating a certificate signing request

Whether you create certificates locally with a software application or obtain them from an external certificate service, you will need to generate a certificate signing request (CSR).

When you generate a CSR, a private and public key pair is created for the FortiGate unit. The generated request includes the public key of the FortiGate unit and information such as the FortiGate unit’s public static IP address, domain name, or email address. The FortiGate unit’s private key remains confidential on the FortiGate unit.

After you submit the request to a CA, the CA will verify the information and register the contact information on a digital certificate that contains a serial number, an expiration date, and the public key of the CA. The CA will then sign the certificate, and you install the certificate on the FortiGate unit.

The Certificate Request Standard is a public key cryptography standard (PKCS) published by RSA, specifically PKCS10 which defines the format for CSRs. This is defined in RFC 2986.

To generate a certificate request in FortiOS – web-based manager:

  1. Go to System > Certificates.
  2. Select Generate.
  3. In the Certificate Name field, enter a unique meaningful name for the certificate request. Typically, this would be the hostname or serial number of the FortiGate unit or the domain of the FortiGate unit such as example.com.

Prior to FortiOS 5.4, passwords for local certificates that were generated via either SCEP or CLI could not have their passwords reset. Passwords can be set in the CLI using the following command:

config vpn certificate local edit <name> set password <password>

next end

  1. Enter values in the Subject Information area to identify the FortiGate unit:
  • If the FortiGate unit has a static IP address, select Host IPand enter the public IP address of the FortiGate unit. If the FortiGate unit does not have a public IP address, use an email address (or fully qualified domain name (FQDN) if available) instead.
  • If the FortiGate unit has a dynamic IP address and subscribes to a dynamic DNS service, use a FQDN if available to identify the FortiGate unit. If you select Domain Name, enter the FQDN of the FortiGate unit. Do not include the protocol specification (http://) or any port number or path names.

If a domain name is not available and the FortiGate unit subscribes to a dynamic DNS service, an “unable to verify certificate” type message may be displayed in the user’s browser whenever the public IP address of the FortiGate unit changes.

  • If you select E-Mail, enter the email address of the owner of the FortiGate unit.
  1. Enter values in the Optional Information area to further identify the FortiGate unit.
Organization Unit Name of your department. You can enter a series of OUs up to a maximum of 5. To add or remove an OU, use the plus (+) or minus (-) icon.
Organization Legal name of your company or organization.
Locality (City) Name of the city or town where the FortiGate unit is installed.
State/Province Name of the state or province where the FortiGate unit is installed.
Country Select the country where the FortiGate unit is installed.
e-mail Contact email address.
Subject Alternative Name Optionally, enter one or more alternative names for which the certificate is also valid. Separate names with a comma. A name can be:

l e-mail address l IP address l URI l DNS name (alternatives to the Common Name) l directory name (alternatives to the Distinguished Name)

You must precede the name with the name type. Examples:

IP:1.1.1.1 email:test@fortinet.com email:my@other.address

URI:http://my.url.here/

Password for private key Option to export local certificate and its private key in password protected p12.
  1. From the Key Type list, select RSA or Elliptic Curve.
  2. From the Key Size list, select 1024 Bit, 1536 Bit, 2048 Bit, 4096 Bit or secp256r1, secp384r1, secp521r1 Larger keys are slower to generate but more secure.
  3. In Enrollment Method, you have two methods to choose from. Select File Based to generate the certificate request, or Online SCEP to obtain a signed SCEP-based certificate automatically over the network. For the SCEP method, enter the URL of the SCEP server from which to retrieve the CA certificate, and the CA server challenge password.
  4. Select OK.
  5. The request is generated and displayed in the Local Certificates list with a status of PENDING.
  6. Select the Download button to download the request to the management computer.
  7. In the File Download dialog box, select Save and save the Certificate Signing Request on the local file system of the management computer.
  8. Name the file and save it on the local file system of the management computer. The certificate request is ready for the certificate authority to be signed.

Generating certificates with CA software

CA software allows you to generate unmanaged certificates and CA certificates for managing other certificates locally without using an external CA service. Examples of CA software include ssl-ca from OpenSSL (available for Linux, Windows, and Mac) or gensslcert from SuSE, MS Windows Server 2000 and 2003 come with a CA as part of their certificate services, and in MS Windows 2008 CA software can be installed as part of the Active Directory installation. See Example — Generate and Import CA certificate with private key pair on OpenSSL on page 128.

The general steps for generating certificates with CA software are

  1. Install the CA software as a stand-alone root CA.
  2. Provide identifying information for your self-administered CA.

While following these steps, the methods vary slightly when generating server certificates, CA certificates, and PKI certificates.

Server certificate

  1. Generate a Certificate Signing Request (CSR) on the FortiGate unit.
  2. Copy the CSR base-64 encoded text (PKCS10 or PKCS7) into the CA software and generate the certificate. PKCS10 is the format used to send the certificate request to the signing authority. PKCS7 is the format the signing authority can use for the newly signed certificate.
  3. Export the certificate as a X.509 DER encoded binary file with .CER extension
  4. Upload the certificate file to the FortiGate unit Local Certificates page (type is Certificate).

CA certificate

  1. Retrieve the CA Certificate from the CA software as a DER encoded file.
  2. Import the CA certificate file to the FortiGate unit at System > Certificates and select Import > Certificates.

PKI certificate

  1. Generate a Certificate Signing Request (CSR) on the FortiGate unit.
  2. Copy the CSR base-64 encoded text (PKCS#10 or PKCS#7) into the CA software and generate the certificate. PKCS10 is the format used to send the certificate request to the signing authority. PKCS7 is the format the signing authority can use for the newly signed certificate.
  3. Export the certificate as a X.509 DER encoded binary file with .CER extension.
  4. Install the certificate in the user’s web browser or IPsec VPN client as needed.

Obtaining and installing a signed server certificate from an external CA

To obtain a signed server certificate for a FortiGate unit, you must send a request to a CA that provides digital certificates that adhere to the X.509 standard. The FortiGate unit provides a way for you to generate the request.

To submit the certificate signing request (file-based enrollment):

  1. Using the web browser on the management computer, browse to the CA web site.
  2. Follow the CA instructions for a base-64 encoded PKCS#10 certificate request and upload your certificate request.
  3. Follow the CA instructions to download their root certificate and CRL.

When you receive the signed server certificate from the CA, install the certificate on the FortiGate unit.

To install or import the signed server certificate – web-based manager

  1. On the FortiGate unit, go to System > Certificates and select Import > Local Certificates.
  2. From Type, select Local Certificate.
  3. Select Browse, browse to the location on the management computer where the certificate was saved, select the certificate, and then select Open.
  4. Select OK, and then select Return.

Installing a CA root certificate and CRL to authenticate remote clients

When you apply for a signed personal or group certificate to install on remote clients, you can obtain the corresponding root certificate and CRL from the issuing CA. When you receive the signed personal or group certificate, install the signed certificate on the remote client(s) according to the browser documentation. Install the corresponding root certificate (and CRL) from the issuing CA on the FortiGate unit according to the procedures given below.

To install a CA root certificate

  1. After you download the root certificate of the CA, save the certificate on the management computer. Or, you can use online SCEP to retrieve the certificate.
  2. On the FortiGate unit, go to System > Certificates and select Import > CA Certificates.
  3. Do one of the following: l To import using SCEP, select SCEP. Enter the URL of the SCEP server from which to retrieve the CA certificate. Optionally, enter identifying information of the CA, such as the filename.

l To import from a file, select Local PC, then select Browse and find the location on the management computer where the certificate has been saved. Select the certificate, and then select Open.

  1. Select OK, and then select Return.

The system assigns a unique name to each CA certificate. The names are numbered consecutively (CA_Cert_1, CA_Cert_2, CA_Cert_3, and so on).

To import a certificate revocation list

A Certificate Revocation List (CRL) is a list of the CA certificate subscribers paired with certificate status information. The list contains the revoked certificates and the reason(s) for revocation. It also records the certificate issue dates and the CAs that issued them.

When configured to support SSL VPNs, the FortiGate unit uses the CRL to ensure that the certificates belonging to the CA and remote peers or clients are valid. The CRL has an “effective date” and a “next update” date. The interval is typically 7 days (for Microsoft CA). FortiOS will update the CRL automatically. Also, there is a CLI command to specify an “update-interval” in seconds. Recommendation should be 24 hours (86400 seconds) but depends on company security policy.

  1. After you download the CRL from the CA web site, save the CRL on the management computer.
  2. Go to System > Certificates and select Import > CRL.
  3. Do one of the following:
    • To import using an HTTP server, select HTTP and enter the URL of the HTTP server.
    • To import using an LDAP server see this KB article.
    • To import using an SCEP server, select SCEP and select the Local Certificate from the list. Enter the URL of

the SCEP server from which the CRL can be retrieved.

  • To import from a file, select Local PC, then select Browse and find the location on the management computer where the CRL has been saved. Select the CRL and then select Open.
  1. Select OK, and then select Return.

To import a PKCS12 certificate from the CLI

The following CLI syntax can be entered to import a local certificate file:

execute vpn certificate local import tftp <file name> <tftp ip address> <file type> <Enter for ‘cer’>|<password for ‘p12’>

For example:

execute vpn certificate local import tftp FGTF-extern.p12 10.1.100.253 p12 123456

 

Troubleshooting

In addition, the following CLI syntax can be entered to update certificate bundles from an FTP or TFTP server:

execute vpn certificate ca import bundle <file-name.pkg> <ftp/tftp-server-ip>

ExtendedKeyUsage for x.509 certificates

As per Network Device Collaborative Protection Profile (NDcPP) v1.0 requirements, server certificates used for TLS connections between FortiGate and FortiAnalyzer have the “Server Authentication” and “Client Authentication” extendedKeyUsage fields in FIPS/CC mode.

The following CLI command is available under log fortianalyzer setting to allow you to specify the certificate used to communicate with FortiAnalyzer.

CLI syntax

config log fortianalyzer setting set certificate <name>

end


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