Described embodiments include an apparatus, comprising a communication interface and a processor. The processor is configured to obtain an NT Local Area Network Manager (NTLM) authentication token, which authenticates a client device to a service using an NTLM authentication protocol. The processor is further configured to, subsequently to obtaining the NTLM authentication token, receive, via the communication interface, from another processor that belongs to the client device, a challenge that was sent to the client device by the service in response to a request, from the client device, to access the service. The processor is further configured to, using the NTLM authentication token, compute a response to the received challenge, and to communicate the computed response to the client device, without exposing the NTLM authentication token to the client device. Other embodiments are also described.