# Patent

## Title
One-way key fob and vehicle pairing verification, retention, and revocation

## Abstract

Embodiments of the invention provide methods for key fob to control unit verification, retention, and revocation. After an initial pairing between a key fob and a control unit, the devices share a secret operation key (OpKey). For verification, the key fob sends the 8 lowest-order bits of a 128-bit counter and some bits of an AES-128, OpKey encrypted value of the counter to the control unit. For key revocation and retention, the control unit is prompted to enter an OpKey retention and revocation mode. Subsequently, each of the remaining or new key fobs is prompted by the user to send a verification message to the control unit. When the control unit is prompted to exit the OpKey retention and revocation mode, it retains the OpKeys of only the key fobs that sent a valid verification message immediately before entering and exiting the OpKey retention and revocation mode.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of the filing date of U.S. Provisional Patent Application No. 61/695,155, which is titled “One-Way Keyfob—Vehicle Key Verification, Retention, and Revocation” and was filed on Aug. 30, 2012, the disclosure of which is hereby incorporated by reference herein in its entirety.


TECHNICAL FIELD
Embodiments of the invention are directed, in general, to security and, more specifically, to methods for key fob vehicle operation key verification, retention, and revocation.
BACKGROUND
Key fobs may be paired with a control unit, such as vehicle control unit, to perform well-known actions such as opening/closing and locking/unlocking vehicle doors. The key fob may be capable of transmitting only, which limits the authentication processes available through a challenge message sent from the control unit to verify the commands sent by a key fob due to the inability for two-way communications. A control unit must be able to verify the validity of received commands and to reject unauthorized commands, including replays of earlier transmissions from a valid key fob.
Occasionally, a key fob will be lost or its holder may no longer be authorized to access the control unit. In this situation, there must be a process that allows the control unit to identify which key fobs are still valid and which should be ignored.
SUMMARY OF THE INVENTION
Embodiments of the invention provide methods for key fob to control unit verification, retention, and revocation. After an initial pairing, a key fob and a vehicle control unit share a secret operation key (OpKey). For verification, the key fob sends an identifier, which may be the 8 lowest-order bits of a 128-bit counter, and some bits of the AES-128, OpKey-encrypted value of the counter to the control unit.
After one or more key fobs are paired with a control unit, such as vehicle control unit, the key fobs each have their own OpKey secretly shared with the control unit. A key fob needs to verify possession of a shared OpKey to the control unit in order for the control unit to take a prescribed action, such as unlocking/locking or opening/closing vehicle doors. This invention solves this problem even though key fobs are capable of transmitting but not receiving. Additionally, the embodiments described herein prevent third parties from impersonating a legitimate key fob by replaying an earlier transmitted message. In addition, after a key fob is lost, its OpKey may be revoked while the OpKey of the remaining or new key fobs may be retained.
For OpKey revocation and retention, a remaining or new key fob is prompted by a legitimate control unit user to send a verification message to the control unit. The control unit is then prompted to enter an OpKey retention and revocation mode. Subsequently, each of the remaining or new key fobs is prompted by the user to send a verification message to the control unit. The control unit is finally prompted to exit the OpKey retention and revocation mode, retaining only the OpKeys of the key fobs from which it received a valid verification message immediately before entering and during the OpKey retention and revocation mode, respectively.



BRIEF DESCRIPTION OF THE DRAWINGS
Having thus described the invention in general terms, reference will now be made to the accompanying drawings, wherein:
FIG. 1 illustrates a normal operation of a key fob and a control unit.
FIG. 2 illustrates a key fob retention and revocation process that can be used to deactivate a key fob in one embodiment.
FIG. 3 is a flowchart illustrating normal operation of a key fob and control unit using OpKey verification according to one embodiment.
FIG. 4 is a flowchart illustrating a process for retention and revocation of OpKeys.
FIG. 5 is a block diagram of an example key fob according to one embodiment.
FIG. 6 is a block diagram of an example control unit according to one embodiment.



DETAILED DESCRIPTION
The invention now will be described more fully hereinafter with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. One skilled in the art may be able to use the various embodiments of the invention.
Embodiments of the invention enable a key fob capable of transmitting, but not receiving, to verify its possession of a secret OpKey to a vehicle control unit, while preventing a third party from impersonating a legitimate key fob by replaying a message sent out earlier by the key fob to the control unit for verification. In addition, the invention also allows a legitimate vehicle user to revoke the OpKey of a lost or expired key fob but retain the OpKey of each remaining valid key fob.
FIG. 1 illustrates a normal operation of a key fob 101 and a control unit 102. After an initial pairing, key fob 101 and a control unit 102 share a secret OpKey. For example, key fob 101 and control unit 102 may be paired using the systems and methods disclosed in pending U.S. patent application Ser. No. 13/969,154, filed Aug. 16, 2013, title “One-Way Key Fob and Vehicle Pairing,” the disclosure of which is hereby incorporated by reference herein in its entirety.
In addition to the OpKey that is shared between both key fob 101 and control unit 102, both devices have a 128-bit counter 103, 104. In other embodiments, counters of different sizes may be used. In normal operation, key fob 101 creates an AES-128, OpKey-encrypted value of counter 103. Key fob 101 then transmits (105) the 8 lowest-order bits of 128-bit counter 103 and some predetermined bits of the AES-128, OpKey-encrypted value of counter 103 to control unit 102. The key fob increments its counter value by one after each transmission, starting from an initial counter value, such as one. The transmission of message 105 itself may represent a command from key fob 101, such as unlock/lock vehicle doors. Alternatively, a separate command data field may be included in message 105 to identify the desired command from key fob 101.
Upon receiving message 105, control unit 102 uses the 8 counter bits received from key fob 101 to set the 8 lowest-order bits of 128-bit counter 104, and increments the value of the remaining bits of counter 104 by one if the value of the received 8 bits is not larger than the value of the 8 lowest-order bits of counter 104. Additionally, control unit 102 creates an AES-128, OpKey encrypted value of counter 104. Control unit 102 then compares predetermined bits from its OpKey encrypted value of counter 104 to the bits representing an OpKey-encrypted value of counter 103. Control unit 102 verifies the message 105 and hence the OpKey if these bits match.
If the verification fails, control unit 102 restores counter 104 to the value before the change.
If an unauthorized or fake key fob 106 attempts to send a message 107 to control unit 102 without being paired, control unit 102 will reject the message 107. Fake key fob 106 does not have a valid OpKey for control unit 102. Additionally, fake key fob 106 does not know the proper counter value for control unit 102 to use for a valid message.
FIG. 2 illustrates a key fob retention and revocation process that can be used to deactivate a key fob in one embodiment. In this example, three key fobs 201-203 are paired with the same control unit 204. Each paired key fob 201-203 has a unique, secret OpKey (OpKey1, OpKey2, OpKey3) that is shared with control unit 204. Additionally, each key fob 201-203 has its own counter 205-207. Control unit 204 maintains a separate counter 208-210 for each key fob 201-203.
When a key fob 203 is lost or needs to be revoked, a user may perform the following steps. First, the user prompts control unit 204 to enter an OpKey revocation mode. The OpKey revocation mode may be triggered by a message from a remaining key fob 201, 202 or/and by some other input to control unit 204.
While the control unit 204 is in the OpKey revocation mode, the user prompts each remaining key fob 201, 202 to perform a normal operation with control unit 204. For example, each key fob 201, 202 will send a message derived from its OpKey to control unit 204, such as message 105 (FIG. 1). After each remaining key fob 201, 202 has sent its message or performed an operation with control unit 204, the user prompts the control unit to exit the OpKey retention mode. Because key fob 203 is lost or no longer authorized, it will not send a message during the revocation mode.
The control unit 204 retains only the OpKeys that were received before exiting the revocation mode. In one embodiment, the control unit 204 retains the last OpKey received before entering revocation mode and all OpKeys received during revocation mode. In other embodiments, control unit 204 retains only the OpKeys received during the revocation mode. All other OpKeys (e.g., OpKey 3) are deleted by control unit 204. This prevents the lost or unauthorized from operating with control unit 204 after the revocation procedure.
FIG. 3 is a flowchart illustrating a process for normal operation of a key fob and control unit using OpKey verification. In step 301, the key fob reads the 8 lowest-order bits of a 128-bit counter. In step 302, the key fob generates an AES-128, OpKey-encrypted value of the key fob counter. The 8 lowest-order bits and some selected bits from the AES-128, OpKey-encrypted value of the key fob counter are sent to the control unit in step 303. This information may be associated with a particular command or request by the key fob.
In step 304, the control unit updates a control unit counter based on the 8 lowest-order bits received from the key fob. According to one embodiment, the update is done by setting the 8 lowest-order bits of a control unit counter to the received 8 lowest-order bits of the key fob counter, and by incrementing the value of the remaining bits of the control unit counter by one if the value of the received bits of the key fob counter is not larger than the value of the corresponding bits of the control unit counter. In step 305, the control unit generates an AES-128, OpKey-encrypted value of the updated control unit counter. The control unit compares selected bits of the AES-128, OpKey-encrypted value of the control unit counter to the selected bits of the AES-128, OpKey-encrypted value of the key fob counter that were received from the key fob.
The control unit verifies the command or request from the key fob if the selected bits match, which indicates that both devices used the same OpKey and counter value.
FIG. 4 is a flowchart illustrating a process for retention and revocation of OpKeys. In step 401, immediately after a remaining (i.e., not lost) or new key fob completes a normal operation, the user prompts the control unit to enter an OpKey revocation mode. The user then prompts each remaining or approved key fob to perform a normal operation with the control unit. The normal operation may involve a transmission as illustrated in FIGS. 1 and 3 or any operation that allows the key fob to send an OpKey encrypted value to the control unit.
In step 403, the user prompts the control unit to exit the OpKey revocation mode after all of the remaining or approved key fobs have completed a normal operation. For example, the user may activate an “end” button to exit the revocation mode, or the revocation mode may end after a set period of time.
In step 404, the control unit deletes all OpKeys except the OpKey associated with the last key fob that operated before entering the OpKey revocation mode and any OpKey associated with a key fob used before the revocation mode ended. Because a lost or unapproved key fob would be unlikely to operate during the brief revocation mode duration, the OpKeys for the lost or unapproved devices would be deleted from the control unit. As a result, the lost and unapproved devices are no longer paired with the control unit and could no longer be used to send commands to the control unit. In another embodiment, only the OpKeys associated with key fobs that operate during the revocation mode are retained, and all other OpKeys that do not perform an operation during the revocation mode period are deleted.
FIGS. 5 and 6 are block diagrams of an example key fob 500 and control unit 600, respectively. The key fob 400 and control unit 600 each comprise a processor 501, 601, a memory 502, 602, and a transceiver 603 or transmitter 503. The processors 501, 601 of the devices may be used to perform normal operations such as maintaining and updating counters, generating OpKey encrypted values, and comparing such values to verify that only OpKeys from paired devices are used. The processors may be a standard CPU, a microcontroller, a low-power digital signal processor, etc. and may be capable of performing complex calculations in a short time.
The memories 502, 602 of the devices may be used to store OpKeys, counter values, encrypted values, and other bits exchanged between the key fob and control unit. The memories may be a non-volatile storage device such as a flash memory or an EEPROM.
The key fob transmitter 503 and control unit transceiver 603 may be wired (not shown), wireless, or capable of both. The transceiver and transmitter may be used by the devices to communicate counter values, OpKey-encrypted data, and other bits during normal operation and during a revocation mode. The key fob allows for remote entry and control of vehicles or other devices and may use wireless technology, such as Bluetooth, LF, or UHF, for those transmissions. The key fob transmitter 503 is capable of transmitting only and does not receive signals from the control unit 600.
Many modifications and other embodiments of the invention will come to mind to one skilled in the art to which this invention pertains having the benefit of the teachings presented in the foregoing descriptions, and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

## Claims

1. A one way paired key fob device, comprising:
a transmitter configured to transmit signals to a control unit but not receive signals from the control unit; a memory configured to store a key fob counter and an operation key (OpKey); and a processor coupled to said transmitter and memory, the processor configured to:
generate an AES-128 OpKey-encrypted value of the key fob counter; transmit a message comprising a predetermined number of lowest-order bits of the key fob counter and a predetermined number of bits of the AES-128 OpKey-encrypted value of the key fob counter to the control unit; and
transmitting to the control unit a command to enter a revocation procedure, wherein OpKeys stored in the control unit that were not recorded during a period are deleted.

2. The one way paired key fob device of claim 1, wherein the predetermined number of lowest-order bits of the key fob counter is 8 bits.

3. The one way paired key fob device of claim 1, wherein the key fob counter is a 128-bit counter.

4. The one way paired key fob device of claim 1, the processor further configured to:
increment the key fob counter by one after transmitting the message to the control unit.

5. The one way paired key fob device of claim 1, wherein the message further comprises a data field that identifies a command to be executed by the control unit.

6. A control unit device, comprising:
a receiver configured to receive signals from a one way paired key fob;
a memory configured to store a control unit counter and an operation key (OpKey); and a processor coupled to said receiver and memory, the processor configured to:
receive, from the one way paired key fob, a message comprising a predetermined number of bits of an AES-128 OpKey-encrypted value of a key fob counter; generate an AES-128 OpKey-encrypted value of the control unit counter;
verify whether or not the predetermined number of bits of an AES-128 OpKey-encrypted value of a key fob counter matches a predetermined number of bits from the AES-128 OpKey-encrypted value of the control unit counter;
entering an OpKey revocation procedure, comprising:

begin recording OpKeys associated with messages received from a plurality of one way paired key fobs when the command to initiate the OpKey revocation procedure is detected; end recording OpKeys associated with messages received from the plurality of one way paired key fobs when a command to terminate an OpKey revocation procedure is detected; and
delete any stored OpKeys that were not recorded during a period between when the command to initiate the OpKey revocation procedure is detected and the command to terminate the OpKey revocation procedure is detected.

7. The control unit device of claim 6, the processor further configured to:
initiate a control operation if the predetermined number of bits of an AES-128 OpKey-encrypted value of the key fob counter matches the predetermined number of bits from the AES-128 OpKey-encrypted value of the control unit counter.

8. The control unit device of claim 6, wherein the control unit counter is a 128-bit counter.

9. The control unit device of claim 6, wherein the message further comprises a data field that identifies a command to be executed by the control unit.

10. A control unit device, comprising:
a receiver configured to receive signals from a one way paired key fob;
a memory configured to store a control unit counter and an operation key (OpKey); and a processor coupled to said receiver and memory, the processor configured to:
receive, from the one way paired key fob, a message comprising a predetermined number of bits of an AES-128 OpKey-encrypted value of a key fob counter:
generate an AES-128 OpKey-encrypted value of the control unit counter; verify whether or not the predetermined number of bits of an AES-128 OpKey-encrypted value of a key fob counter matches a predetermined number of bits from the AES-128 OpKey-encrypted value of the control unit counter; entering an OpKey revocation procedure, comprising:
begin recording OpKeys associated with messages received from a plurality of one way paired key fobs when the command to initiate the OpKey revocation procedure is detected;
end recording OpKeys associated with messages received from the plurality of one way paired key fobs when a command to terminate an OpKey revocation procedure is detected; and
delete any stored OpKeys that were not recorded during a period between when the command to initiate the OpKey revocation procedure is detected and the command to terminate the OpKey revocation procedure is detected or not recorded immediately before the command to initiate the OpKey revocation procedure is detected.

11. A method for selecting key fob operation keys (OpKey) to be retained at a control unit, comprising:
receiving a command, by the control unit, to initiate an OpKey revocation procedure; begin recording of OpKeys associated with messages received from a plurality of one way paired key fobs; receiving a command, by the control unit, to terminate an OpKey revocation procedure; end recording of OpKeys associated messages received from key fobs; and
delete any stored OpKeys that were not recorded during a period between when the command to initiate the OpKey revocation procedure is received and when the command to terminate the OpKey revocation procedure is received.

12. The method of claim 11, further a method for selecting key fob operation keys (OpKey) to be retained at a control unit, comprising:
receiving a command, by the control unit, to initiate an OpKey revocation procedure; begin recording of OpKeys associated with messages received from a plurality of one way paired key fobs; receiving a command, by the control unit, to terminate an OpKey revocation procedure;
end recording of OpKeys associated messages received from key fobs; and delete any stored OpKeys that were not recorded during a period between when the command to initiate the OpKey revocation procedure is received and when the command to terminate the OpKey revocation procedure is received or not recorded immediately before the command to initiate the OpKey revocation procedure is detected.