17.2 YaST Modules for CA Management

YaST provides two modules for basic CA management. The primary management tasks with these modules are explained here.

17.2.1 Creating a Root CA

The first step when setting up a PKI is to create a root CA. Do the following:

  1. Start YaST and go to Security and Users > CA Management.

  2. Click Create Root CA.

  3. Enter the basic data for the CA in the first dialog, shown in YaST CA Module—Basic Data for a Root CA. The text fields have the following meanings:

    Figure 17-1 YaST CA Module—Basic Data for a Root CA

    CA Name

    Enter the technical name of the CA. Directory names, among other things, are derived from this name, which is why only the characters listed in the help can be used. The technical name is also displayed in the overview when the module is started.

    Common Name

    Enter the name for use in referring to the CA.

    E-Mail Addresses

    Several e-mail addresses can be entered that can be seen by the CA user. This can be helpful for inquiries.

    Country

    Select the country where the CA is operated.

    Organisation, Organisational Unit, Locality, State

    Optional values

    Proceed with Next.

  4. Enter a password in the second dialog. This password is always required when using the CA—when creating a sub-CA or generating certificates. The text fields have the following meaning:

    Key Length

    Key Length contains a meaningful default and does not generally need to be changed unless an application cannot deal with this key length. The higher the number the more secure your password is.

    Valid Period (days)

    The Valid Period in the case of a CA defaults to 3650 days (roughly ten years). This long period makes sense because the replacement of a deleted CA involves an enormous administrative effort.

    Clicking Advanced Options opens a dialog for setting different attributes from the X.509 extensions (Figure 17-4). These values have rational default settings and should only be changed if you are really sure of what you are doing. Proceed with Next.

  5. Review the summary. YaST displays the current settings for confirmation. Click Create. The root CA is created then appears in the overview.

HINT: In general, it is best not to allow user certificates to be issued by the root CA. It is better to create at least one sub-CA and create the user certificates from there. This has the advantage that the root CA can be kept isolated and secure, for example, on an isolated computer on secure premises. This makes it very difficult to attack the root CA.

17.2.2 Changing Password

If you need to change your password for your CA, proceed as follows:

  1. Start YaST and open the CA module.

  2. Select the required root CA and click Enter CA.

  3. Enter the password if you entered a CA the first time. YaST displays the CA key information in the Description tab (see YaST CA Module—Using a CA).

  4. Click Advanced and select Change CA Password. A dialog box opens.

  5. Enter the old and the new password.

  6. Finish with OK

17.2.3 Creating or Revoking a Sub-CA

A sub-CA is created in exactly the same way as a root CA.

NOTE: The validity period for a sub-CA must be fully within the validity period of the parent CA. A sub-CA is always created after the parent CA, therefore, the default value leads to an error message. To avoid this, enter a permissible value for the period of validity.

Do the following:

  1. Start YaST and open the CA module.

  2. Select the required root CA and click Enter CA.

  3. Enter the password if you are entering a CA for the first time. YaST displays the CA key information in the tab Description (see YaST CA Module—Using a CA).

    Figure 17-2 YaST CA Module—Using a CA

  4. Click Advanced and select Create SubCA. This opens the same dialog as for creating a root CA.

  5. Proceed as described in Section 17.2.1, Creating a Root CA.

    It is possible to use one password for all your CAs. Enable Use CA Password as Certificate Password to give your sub-CAs the same password as your root CA. This helps to reduce the amount of passwords for your CAs.

    NOTE: Check your Valid Period

    Take into account that the valid period must be lower than the valid period in the root CA.

  6. Select the Certificates tab. Reset compromised or otherwise unwanted sub-CAs here, using Revoke. Revocation alone is not enough to deactivate a sub-CA. You must also publish revoked sub-CAs in a CRL. The creation of CRLs is described in Section 17.2.6, Creating Certificate Revocation Lists (CRLs).

  7. Finish with OK

17.2.4 Creating or Revoking User Certificates

Creating client and server certificates is very similar to creating CAs in Section 17.2.1, Creating a Root CA. The same principles apply here. In certificates intended for e-mail signature, the e-mail address of the sender (the private key owner) should be contained in the certificate to enable the e-mail program to assign the correct certificate. For certificate assignment during encryption, it is necessary for the e-mail address of the recipient (the public key owner) to be included in the certificate. In the case of server and client certificates, the hostname of the server must be entered in the Common Name field. The default validity period for certificates is 365 days.

To create client and server certificates, do the following:

  1. Start YaST and open the CA module.

  2. Select the required root CA and click Enter CA.

  3. Enter the password if you are entering a CA for the first time. YaST displays the CA key information in the Description tab.

  4. Click Certificates (see Certificates of a CA).

    Figure 17-3 Certificates of a CA

  5. Click Add > Add Server Certificate and create a server certificate.

  6. Click Add > Add Client Certificate and create a client certificate. Do not forget to enter an e-mail address.

  7. Finish with OK

To revoke compromised or otherwise unwanted certificates, do the following:

  1. Start YaST and open the CA module.

  2. Select the required root CA and click Enter CA.

  3. Enter the password if you are entering a CA for the first time. YaST displays the CA key information in the Description tab.

  4. Click Certificates (see Section 17.2.3, Creating or Revoking a Sub-CA).

  5. Select the certificate to revoke and click Revoke.

  6. Choose a reason to revoke this certificate.

  7. Finish with OK.

NOTE: Revocation alone is not enough to deactivate a certificate. Also publish revoked certificates in a CRL. Section 17.2.6, Creating Certificate Revocation Lists (CRLs) explains how to create CRLs. Revoked certificates can be completely removed after publication in a CRL with Delete.

17.2.5 Changing Default Values

The previous sections explained how to create sub-CAs, client certificates, and server certificates. Special settings are used in the extensions of the X.509 certificate. These settings have been given rational defaults for every certificate type and do not normally need to be changed. However, it may be that you have special requirements for these extensions. In this case, it may make sense to adjust the defaults. Otherwise, start from scratch every time you create a certificate.

  1. Start YaST and open the CA module.

  2. Enter the required root CA, as described in Section 17.2.3, Creating or Revoking a Sub-CA.

  3. Click Advanced > Edit Defaults.

  4. Choose the type the settings to change. The dialog for changing the defaults, shown in Figure 17-4, then opens.

    Figure 17-4 YaST CA Module—Extended Settings

  5. Change the associated value on the right side and set or delete the critical setting with critical.

  6. Click Next to see a short summary.

  7. Finish your changes with Save.

NOTE: All changes to the defaults only affect objects created after this point. Already-existing CAs and certificates remain unchanged.

17.2.6 Creating Certificate Revocation Lists (CRLs)

If compromised or otherwise unwanted certificates need to be excluded from further use, they must first be revoked. The procedure for this is explained in Section 17.2.3, Creating or Revoking a Sub-CA (for sub-CAs) and Section 17.2.4, Creating or Revoking User Certificates (for user certificates). After this, a CRL must be created and published with this information.

The system maintains only one CRL for each CA. To create or update this CRL, do the following:

  1. Start YaST and open the CA module.

  2. Enter the required CA, as described in Section 17.2.3, Creating or Revoking a Sub-CA.

  3. Click CRL. The dialog that opens displays a summary of the last CRL of this CA.

  4. Create a new CRL with Generate CRL if you have revoked new sub-CAs or certificates since its creation.

  5. Specify the period of validity for the new CRL (default: 30 days).

  6. Click OK to create and display the CRL. Afterwards, you must publish this CRL.

NOTE: Applications that evaluate CRLs reject every certificate if the CRL is not available or has expired. As a PKI provider, it is your duty always to create and publish a new CRL before the current CRL expires (period of validity). YaST does not provide a function for automating this procedure.

17.2.7 Exporting CA Objects to LDAP

The executing computer should be configured with the YaST LDAP client for LDAP export. This provides LDAP server information at runtime that can be used when completing dialog fields. Otherwise (although export may be possible), all LDAP data must be entered manually. You must always enter several passwords (see Table 17-3).

Table 17-3 Passwords during LDAP Export

Password

Meaning

LDAP Password

Authorizes the user to make entries in the LDAP tree.

Certificate Password

Authorizes the user to export the certificate.

New Certificate Password

The PKCS12 format is used during LDAP export. This format forces the assignment of a new password for the exported certificate.

Certificates, CAs, and CRLs can be exported to LDAP.

Exporting a CA to LDAP

To export a CA, enter the CA as described in Section 17.2.3, Creating or Revoking a Sub-CA. Select Extended > Export to LDAP in the subsequent dialog, which opens the dialog for entering LDAP data. If your system has been configured with the YaST LDAP client, the fields are already partly completed. Otherwise, enter all the data manually. Entries are made in LDAP in a separate tree with the attribute caCertificate.

Exporting a Certificate to LDAP

Enter the CA containing the certificate to export then select Certificates. Select the required certificate from the certificate list in the upper part of the dialog and select Export > Export to LDAP. The LDAP data is entered here in the same way as for CAs. The certificate is saved with the corresponding user object in the LDAP tree with the attributes userCertificate (PEM format) and userPKCS12 (PKCS12 format).

Exporting a CRL to LDAP

Enter the CA containing the CRL to export and select CRL. If desired, create a new CRL and click Export. The dialog that opens displays the export parameters. You can export the CRL for this CA either once or in periodical time intervals. Activate the export by selecting Export to LDAP and enter the respective LDAP data. To do this at regular intervals, select the Repeated Recreation and Export radio button and change the interval, if appropriate.

17.2.8 Exporting CA Objects as a File

If you have set up a repository on the computer for administering CAs, you can use this option to create the CA objects directly as a file at the correct location. Different output formats are available, such as PEM, DER, and PKCS12. In the case of PEM, it is also possible to choose whether a certificate should be exported with or without key and whether the key should be encrypted. In the case of PKCS12, it is also possible to export the certification path.

Export a file in the same way for certificates, CAs as with LDAP, described in Section 17.2.7, Exporting CA Objects to LDAP, except you should select Export as File instead of Export to LDAP. This then takes you to a dialog for selecting the required output format and entering the password and filename. The certificate is stored at the required location after clicking OK.

For CRLs click Export, select Export to file, choose the export format (PEM or DER) and enter the path. Proceed with OK to save it to the respective location.

HINT: You can select any storage location in the file system. This option can also be used to save CA objects on a transport medium, such as a USB stick. The /media directory generally holds any type of drive except the hard drive of your system.

17.2.9 Importing Common Server Certificates

If you have exported a server certificate with YaST to your media on an isolated CA management computer, you can import this certificate on a server as a common server certificate. Do this during installation or at a later point with YaST.

NOTE: You need one of the PKCS12 formats to import your certificate successfully.

The general server certificate is stored in /etc/ssl/servercerts and can be used there by any CA-supported service. When this certificate expires, it can easily be replaced using the same mechanisms. To get things functioning with the replaced certificate, restart the participating services.

HINT: If you select Import here, you can select the source in the file system. This option can also be used to import certificates from a transport medium, such as a USB stick.

To import a common server certificate, do the following:

  1. Start YaST and open Common Server Certificate under Security and Users

  2. View the data for the current certificate in the description field after YaST has been started.

  3. Select Import and the certificate file.

  4. Enter the password and click Next. The certificate is imported then displayed in the description field.

  5. Close YaST with Finish.