LDAP Implementation HOWTO
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# 9. Publishing digital certificates with LDAP

This section focuses on how to publish digital certificates into an ldap server. You need to publish digital certificates if you run a Certificaton Authority. Publishing to LDAP is a simple way to make this information available in the network .Also many certificate aware software uses LDAP as a preferred repository for user certificates.

This allows to keep users certificates with the rest of the user information avoiding useless replication of data.

To deal with certificates you need a cryptographic toolkit, the one used here is OpenSSL.

## 9.1. LDAP Server configuration

The LDAP server used here is OpenLDAP 2.0.x.

Your LDAP server must support objectclasses that allows attributes to store certificates. In particular you need to store in the LDAP server the Certification Authority certificate, the Certificate Revocation List, the Authority Revocation List and end users certificates.

The `certificationAuthority` objectclass implements the `authorityRevocationList`, `certificateRevocationList` and `cACertificate` attributes.

The `inetOrgPerson` objectclass supports the usercertificate (binary) attribute.

You can also use the mix-in objectclass `strongAuthenticationUser` to add certificates to non `inetOrgPerson` entries.

You can include required schemas to OpenLDAP including the following schemas into your `slapd.conf` file.

    include        /usr/local/etc/openldap/schema/core.schema
    include        /usr/local/etc/openldap/schema/cosine.schema
    include        /usr/local/etc/openldap/schema/inetorgperson.schema

## 9.2. Certificate Publishing

Certificates are encoded using ASN.1 DER (Distingushed Encoding Rules). So it must be published into the LDAP server as a binary piece of data (using BER encoding).

You can convert a pem certificate into der format using openssl

    openssl x509 -outform DER -in incert.pem  -out outcert.der

Then an LDIF file can be created using the `ldif` utility provided with OpenLDAP. The command:

    ldif -b "usercertificate;binary" < outcert.der   > cert.ldif

creates an usercertificate attribute encoded in BASE64. You can add this certificate to an LDIF entry and then use `ldapmodify` to add the certificate to an entry.

    ldapmodify -x -W -D "cn=Manager,dc=yourorg,dc=com" -f cert.ldif

Where `cert.ldif` contains something like:

    dn: cn=user,ou=people,dc=yourorg,dc=com
    changetype: modify
    add: usercertificate
    usercertificate;binary:: MIIC2TCCAkKgAwIBAgIBADANBgkqhkiG9w0BAQQFADBGMQswCQYD
     VQQGEwJJVDENMAsGA1UEChMESU5GTjESMBAGA1UECxMJQXV0aG9yaXR5MRQwEgYDVQQDEwtJTkZO
     IENBICgyKTAeFw05OTA2MjMxMTE2MDdaFw0wMzA4MDExMTE2MDdaMEYxCzAJBgNVBAYTAklUMQ0w
     CwYDVQQKEwRJTkZOMRIwEAYDVQQLEwlBdXRob3JpdHkxFDASBgNVBAMTC0lORk4gQ0EgKDIpMIGf
     MA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQCrHdRKJsobcjXz/OsGjyq8v73DbggG3JCGrQZ9f1Vm
     9RrIWJPwggczqgxwWL6JLPKglxbUjAtUxiZm3fw2kX7FGMUq5JaN/Pk2PT4ExA7bYLnbLGZ9jKJs
     Dh4bNOKrGRIxRO9Ff+YwmH8EQdoVpSRFbBpNnoDIkHLc4DtzB+B4wwIDAQABo4HWMIHTMAwGA1Ud
     EwQFMAMBAf8wHQYDVR0OBBYEFK3QjOXGc4j9LqYEYTn9WvSRAcusMG4GA1UdIwRnMGWAFK3QjOXG
     c4j9LqYEYTn9WvSRAcusoUqkSDBGMQswCQYDVQQGEwJJVDENMAsGA1UEChMESU5GTjESMBAGA1UE
     CxMJQXV0aG9yaXR5MRQwEgYDVQQDEwtJTkZOIENBICgyKYIBADALBgNVHQ8EBAMCAQYwEQYJYIZI
     AYb4QgEBBAQDAgAHMAkGA1UdEQQCMAAwCQYDVR0SBAIwADANBgkqhkiG9w0BAQQFAAOBgQCDs5b1
     jmbIYVq2epd5iDjQ109SJ/V7b6DFw2NIl8CWeDPOOjL1E5M8dnlmCDeTR2TlBxqUZaBBJZPqzFdv
     xpxqsHC0HfkCXAnUe5MaefFNAH9WbxoB/A2pkXtT6WGWed+QsL5wyKJaO4oD9UD5T+x12aGsHcsD
     Cy3EVEaGEOl+/A==

It is also possible to specify the certificate in the LDIF file as:

    userCertificate;binary:< file:///path/to/cert.der

## 9.3. LDAP Aware Clients

Once you stored certificates in the server you may wonder to retrieve them.

Among other clients, Netscape has supprt to retrieve certificates automatically from an LDAP server. Using the Security Panel-->User Certificates-->Search Directory; you can search for certificates in the LDAP dierctory and have them automatically installed in your Netscape certificate database.

Another client that has good support for certificates is web2ldap www.web2ldap.de

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