Token-based key generation

1. A method performed by a first computing device, the method comprising:

engaging in a handshake procedure with a remote second computing device to establish a secure channel;

generating a first encryption key using a first token having a secret seed, the first encryption key being the same as a second encryption key generated by the second computing device using a second token having the same secret seed; and

using the first encryption key to engage in encrypted communications with the second computing device,

wherein:

generating the first encryption key using the first token includes cryptographically combining the secret seed with a current time value;

engaging in the handshake procedure includes:

receiving a remote clock value within a handshake message from the second computing device, the remote clock value indicating a current clock value of a clock device in the possession of the second computing device;

obtaining the current time value from a local clock device in the possession of the first computing device; and

calculating a clock skew between the remote clock value and the current time value; and

cryptographically combining the secret seed with the current time value includes offsetting the current time value by the clock skew.

2. The method of claim 1 wherein the method further comprises:

generating a revised encryption key after a period of time by cryptographically combining the secret seed with a time value indicative of passage of the period of time; and

after generating the revised encryption key, using the revised encryption key instead of the first encryption key to engage in encrypted communications with the second computing device.

3. The method of claim 2 wherein the period of time is a fixed interval.

4. The method of claim 2 wherein the method further includes:

estimating an average amount of time it would take an attacker to guess the first encryption key using a key cracking procedure; and

setting the period of time to be less than the estimated average amount of time.

5. The method of claim 2 wherein the method further includes:

estimating an average amount of time it would take an attacker to guess the first encryption key using a key cracking procedure; and

setting the period of time to be an order of magnitude less than the estimated average amount of time.

6. The method of claim 2 wherein the method further includes:

estimating an average amount of time it would take an attacker to guess the first encryption key using a key cracking procedure; and

setting the period of time to be a factor less than the estimated average amount of time, a magnitude of the factor depending on a degree of sensitivity of the secure channel.

7. The method of claim 1 wherein

the handshake message from the second computing device is a Transport Layer Security (TLS) ServerKeyExchange message; and

receiving the remote clock value within the handshake message from the second computing device includes receiving the remote clock value within a key identity hint field of the TLS ServerKeyExchange message.

8. The method of claim 1 wherein both the first token and the second token are hardware tokens.

9. The method of claim 8 wherein the method further comprises, prior to engaging in the handshake procedure:

determining, by a central authority, that there is a need for the first computing device and the second computing device to securely communicate with each other; and

in response to determining, pre-provisioning, by the central authority, the first hardware token to the first computing device and the second hardware token to the second computing device.

10. The method of claim 1 wherein receiving the remote clock value within the handshake message from the second computing device includes receiving the remote clock value within a key identity hint field of the handshake message.

11. The method of claim 10 wherein the handshake message from the second computing device is a Transport Layer Security (TLS) ServerKeyExchange message.

12. A system comprising:

a first computing device;

a second computing device;

a network connecting the first computing device and the second computing device;

a first token device assigned to the first computing device, the first token device being programmed with a secret seed; and

a second token device assigned to the second computing device, the second token device being programmed with the same secret seed as the first token device;

wherein the first computing device is configured to:

engage in a handshake procedure with the second computing device to establish a secure channel;

generate a first encryption key using the first token device, the first encryption key being the same as a second encryption key generated by the second computing device using the second token; and

use the first encryption key to engage in encrypted

communications with the second computing device over the network;

wherein:

generating the first encryption key using the first token includes cryptographically combining the secret seed with a current time value;

engaging in the handshake procedure includes:

obtaining the current time value from a local clock device in the possession of the first computing device; and

sending the current time value within a handshake message to the second computing device for the second computing device to use in correcting a remote clock value indicating a current clock value of a clock device in the possession of the second computing device, wherein sending the current time value within the handshake message to the second computing device includes embedding the current time value within a key identity field of the handshake message.

13. The system of claim 12 wherein the secret seed is shared only by the first token device and the second token device, the secret seed not being shared by any other token device.

14. The system of claim 12 wherein:

the secret seed is shared by a third token device in addition the first token device and the second token device; and

engaging in the handshake procedure with the second computing device to establish the secure channel further includes:

sending a first unique identifier of the first computing device to the second computing device; and

receiving a second unique identifier of the second computer from the second computer; and

generating the first encryption key using the first token device further includes cryptographically combining the secret seed with both the first unique identifier and the second unique identifier.

15. The system of claim 12 wherein the handshake message sent to the second computing device is a Transport Layer Security (TLS) ClientKeyExchange message.

16. An apparatus comprising:

a clock;

a network interface for communicating with a remote computing device over a network;

a processor; and

memory, the memory storing a set of instructions which, when executed by the processor, cause the apparatus to perform the operations of:

engaging in a handshake procedure with the remote computing device to establish a secure channel;

generating a first encryption key using a first token having a secret seed, the first token being under the control of the apparatus, the first encryption key being the same as a second encryption key generated by the remote computing device using a second token having the same secret seed, the second token under the control of the remote computing apparatus; and

using the first encryption key to engage in encrypted communications with the remote computing device;

wherein:

generating the first encryption key using the first token includes cryptographically combining the secret seed with a current time value;

engaging in the handshake procedure includes:

receiving a remote clock value within a handshake message from the remote computing device, the remote clock value indicating a current clock value of a clock device in the possession of the remote computing device;

obtaining the current time value from the clock; and

calculating a clock skew between the remote clock value and the current time value; and

cryptographically combining the secret seed with the current time value includes offsetting the current time value by the clock skew.

17. The apparatus of claim 16 wherein receiving the remote clock value within the handshake message from the remote computing device includes receiving the remote clock value within a key identity hint field of the handshake message.

18. The apparatus of claim 17 wherein the handshake message from the remote computing device is a Transport Layer Security (TLS) ServerKeyExchange message.