A system adapted to condition access to a network over an IPsec session to clients providing a proper one-time-password, even though the network access control uses IKEv1, which does not support one-time-passwords. An authentication service receives from a client an access request including the one-time-password, and provides the one-time-password to a service that checks the password. The one-time-password service returns a cookie when the password is successfully validated and the client is properly authenticated. The cookie is passed on to the client computer, which uses the cookie as part of a request for a certificate. A certificate authority generates a certificate if a request for a certificate is received from an authenticated client, which in turn may be used to form the IPsec session for access to the network.