A key distribution scheme is provided, which is useful for establishing, distributing, and maintaining security associations in a Mobile IP network. An authentication server performs an initial validation of a new session and generates a root key which it delivers to the initial access gateway and to the home agent. The initial access gateway and the home agent each independently compute a derivative key available only to themselves. The initial access gateway, acting as proxy for the mobile station, uses the derivative key to sign the Mobile IP registration or binding update transactions, and sends the signed registration or binding update to the home agent for validation. Once the session is established between the mobile station and the home agent, the access gateways act as proxies on behalf of the mobile station to maintain the session mobility. In handoff, the new access gateway acquires the root key as part of the transferred session context. The new access gateway, acting as proxy for the mobile station, computes a new derivative key from the root key and uses it to sign a binding update.