Patent Publication Number: US-11381391-B2

Title: Pre-shared secret key capabilities in secure MAC layer communication protocols

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to key agreement protocols and, more specifically, to a streamlined method for use of pre-shared secret keys. 
     BACKGROUND 
     MACsec (Media Access Control security) is a secure MAC layer communication protocol. MACsec is a Layer 2 hop-by-hop encryption methodology that provides data confidentiality, integrity, and replay protection for media access-independent protocols. MACsec is described in the Institute of Electrical and Electronics Engineers (IEEE) 802.1AE standard, originally published in 2006 and revised in 2018. MACsec provides MAC layer encryption over networks by using out-of-band methods for encryption keying. MACsec encrypts all the data, except the source and destination MAC addresses of an Ethernet packet. Data can be secured on physical media using MACsec, which prevents data compromise at higher layers. As a result, MACsec encryption may take priority over any other encryption method, at higher layers. MACsec provides integrity for the entire frame including the source and destination MAC addresses. 
     Setting up a MACsec service utilizes a security association (SA) protocol, the MACsec Key Agreement (MKA) protocol. The MKA protocol is based on the IEEE 802.1x-2010 standard. The MKA protocol describes how session keys are provided and how encryption keys are managed. The MKA uses an Extensible Authentication Protocol Transport Layer Security (EAP-TLS) authentication method to mutually authenticate and get a Master Session Key (MSK) from which a Connectivity Association Key (CAK) is dynamically derived. The CAK is the root key for MKA key derivations. 
     A quantum attack on TLS, which is a public-key cryptography, may eventually compromise the MKA CAK and/or perhaps other keys, which would put the security of MACsec traffic at risk. However, it is thought that the MKA protocol may be made quantum secure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is set forth below with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical items. The systems depicted in the accompanying figures are not to scale and components within the figures may be depicted not to scale with each other. 
         FIG. 1  is a diagram illustrating a system including a first station and a second station that are configured to communicate according to a secure MAC layer communication protocol based on either a pre-shared secret key or a distributed shared key. 
         FIG. 2  is a diagram illustrating a system including a first station and a second station and shows detail of messages exchanged between the first station and the second station in the process of configuring for communication according to a secure MAC layer communication protocol. 
         FIG. 3  is a diagram illustrating a system including a first station and a second station, and the second station is not capable of communication according to a secure MAC layer communication protocol based on a pre-shared secret key. 
         FIG. 4  illustrates an example format of a basic parameter set of an Extensible Authentication Protocol over LAN (EAPOL)-announcement message, as modified to include an indication that a station is capable of secure MAC layer communication based on a pre-shared secret key. 
         FIG. 5  illustrates an example format of a basic parameter set of an EAPOL announcement message using a pre-shared secret key (PSK) capabilities TLV that is not explicitly described in IEEE Std 802.1X-2010. 
         FIG. 6  is a detailed diagram illustrating a configuration including a first station and a second station and shows details of messages exchanged between the first station and the second station in the process of configuring for communication according to a secure MAC layer communication protocol. 
         FIG. 7  is a flowchart for an example process by which a first computing node interoperates with a second computing node to establish communication according to a secure MAC layer communication protocol. 
         FIG. 8  is a flowchart for an example process  800  by which a second computing node interoperates with a first computing node to establish communication according to a secure MAC layer communication protocol. 
         FIG. 9  is a flowchart for an example process in which a second computing node is not capable of communicating with a first computing node according to a secure MAC layer communication protocol based on a pre-shared secret key. 
         FIG. 10  illustrates an example computer architecture for a computer capable of executing program components for implementing the functionality described herein. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     This disclosure describes a method for a first computing node to configure for communication with a second computing node according to a secure Media Access Layer (MAC) layer communication protocol. The first computing node transmits a first message, to the second computing node. The first message includes at least a first indication that the first computing node is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. The first computing node determines to communicate with the second computing node according to the secure MAC layer communication protocol based on one of a pre-shared secret key or a distributed shared key. The first computing node, at least in part based on the determining, transmits a second message to the second computing node according to the secure MAC layer communication protocol based on one of a pre-shared secret key or a distributed shared key. 
     The disclosure further describes a method for a second computing node to configure for communication with a first computing node according to a secure Media Access Layer (MAC) layer communication protocol. The second computing node receives a first message from the first computing node. The first message includes at least a first indication that the first computing node is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. The second computing node transmits a second message to the first computing node. The second message includes a second indication that the second computing node is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. The second computing node receives a third message from the first computing node according to the secure MAC layer communication protocol based on the pre-shared secret key. 
     The disclosure further describes a method for a second computing node to configure for communication with a first computing node according to a secure Media Access Layer (MAC) layer communication protocol. The second computing node receives a first message from the first computing node, the first message includes at least a first indication that the first computing node is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. The second computing node receives a second message from the first computing node. The second message includes a pre-shared key identifier distributed by the first computing node. The second computing node receives a third message from the first computing node according to the secure MAC layer communication protocol based on the distributed shared key. 
     Additionally, the techniques described herein may be performed by a system and/or device having non-transitory computer-readable media storing computer-executable instructions that, when executed by one or more processors, performs the methods described herein. 
     Example Embodiments 
     A “distributed shared key” is a key that has been produced, such as by a software or hardware module, and a copy transported in an unspecified way from a location 1 to a location N. Therefore, stations in location 1 and N may use the same key, which is referred to as “distributed shared key”. A “pre-shared secret key” is a key that has been produced, such as by a software or hardware module, or is configured in a location 1 and N as a key value pair (key identifier and key). Using pre-shared secret keys can ameliorate the risk of quantum attack on encrypted communication. For example, each entity may separately build a session key, for encrypting communication between the two entities, by processing the pre-shared secret key using the same key derivation function. 
     In some examples, as a communication link is being established, a pre-shared key identifier is communicated from one entity to the other entity. The pre-shared key-identifier may be utilized, for example, by each entity as a seed for each entity to separately generate the same pre-shared secret key or separately securely obtain the pre-shared secret key from a key source. The pre-shared secret key may be separately generated or obtained by each entity and may then be used by each entity as the basis for building the session key to be used for secure communication between the two entities. 
     In establishing communications and/or communicating according to the MACsec protocol, a pre-shared key may be utilized if both to-be communicating entities are capable of establishing a communication link and/or communicating based on a pre-shared key. IEEE 802.1x-2010 standard describes how pre-shared keys may be utilized including for example, at Section 6.2. 
     The pre-shared key may include a connectivity association key name (CKN) and a connectivity association key (CAK). A pre-shared key may be exchanged between two devices at each end of a point-to-point link to enable MACsec using static CAK security mode. The MACsec Key Agreement (MKA) protocol is enabled after the pre-shared keys are successfully verified and exchanged. The pre-shared keys, the CKN and CAK, must match on both ends of a link in order for communication to be established. 
     In some examples according to the MACsec protocol, the MKA protocol is extended to allow a key server (KS) entity to distribute a post-quantum pre-shared key identifier (PSK_ID) to a non-key server (NKS) peer entity. This distribution of PSK_ID is in place of distributing an actual MACsec secret key (SAK) to MACsec peers. The PSK_ID may be negotiated between the KS and NKS entities. Both the KS and NKS entities may use the negotiated PSK_ID to separately obtain the same pre-shared secret key (PSK) from a quantum key source (QKS), which will serve as MACsec secret key (SAK) for securing communication between the KS and NKS entity or the basis for determining the SAK. 
     In some examples, one entity may be configured to establish a MAC layer communication link and/or communicate based on a post-quantum (or other) pre-shared secret key whereas another entity may not be capable of and/or configured to establish a MAC layer communication link and/or communicate based on a pre-shared secret key. For example, for entities that may communicate using MACsec, a KS entity may be configured to establish a MAC layer communication link and/or communicate based on a pre-shared secret key whereas an NKS entity may not be capable of and/or configured to establish a MAC layer communication link and/or communicate based on a pre-shared secret key. 
     It may be desirable to be backwards-compatible, so that an entity such as a station or a computing node that is capable of establishing a MAC layer communication link and/or securely communicating based on a pre-shared secret key may establish a MAC layer communication link and/or communicate with another entity instead based on a distributed shared key if, for example, the other entity is not capable of securely communicating based on a pre-shared secret key. 
     In some examples, an entity advertises its capability to communicate using a secure MAC layer communication protocol based on a pre-shared secret key. However, protocols for secure MAC layer communication may not inherently include functionality for an entity to advertise its ability to communicate using a secure MAC layer communication protocol based on a pre-shared secret key. An entity that receives such an advertisement may not actually have capability to communicate using a secure MAC layer communication protocol based on a pre-shared secret key. Moreover, an entity that receives such an advertisement may not respond to it or even know how to respond to it. 
     In some examples, after an entity advertises to another entity its capability to communicate according to a secure MAC layer communication protocol based on a pre-shared secret key, the entity may make a determination whether to establish communication with the other entity according to the secure MAC layer communication protocol based on a pre-shared secret key or to establish communication with the other entity according to the secure MAC layer communication protocol based on a distributed shared key. 
     For example, the entity may make the determination based at least in part on not receiving a response to the advertisement by the entity of its capability to communicate using a secure MAC layer communication protocol based on a pre-shared secret key. For example, the entity may wait a predetermined time for such a response and, if the response is not received, the entity may make a determination to establish communication with the other entity according to the secure MAC layer communication protocol based on a distributed shared key. For example, as part of establishing secure MAC layer communication with the other entity, the entity may distribute a key to the other entity to be shared for conducting secure MAC layer communication between the entity and the other entity based on the distributed shared key. 
     In some examples, the entity may receive a response, such as within the predetermined waiting time. The received response may be an advertisement of the other entity&#39;s capability to communicate according to a secure MAC layer communication protocol based on a pre-shared secret key. Based at least in part on the entity receiving such a response, the entity may make a determination to establish communication with the other entity according to the secure MAC layer communication protocol based on a pre-shared secret key. 
     In some examples, the entity may provide an indication of a pre-shared secret key to the other entity, and the other entity may respond with an indication that the other entity accepts or rejects the indication of the pre-shared secret key. Based at least partly on the indication of the other entity accepting or rejecting the indication of the pre-shared secret key, the entity may make a determination to establish communication with the other entity according to the secure MAC layer communication protocol based on a pre-shared secret key or to establish communication with the other entity according to the secure MAC layer communication protocol based on a distributed shared key. For example, for communication according to the secure MAC layer communication protocol based on a pre-shared secret key, the entity and the other entity may each operate to independently determine the pre-shared secret key based on the indication of the pre-shared secret key the entity provides to the other entity. 
     As mentioned above, the other entity may not even know how to respond to an advertisement from the entity of the entity&#39;s capability to communicate according to a secure MAC layer communication protocol based on a pre-shared secret key. The other entity may not respond to an advertisement the other entity receives indicating the entity&#39;s capability to communicate according to a secure MAC layer communication protocol based on a pre-shared secret key. The other entity may receive a shared key distributed from the entity, and the other entity may communicate with the entity according to the secure MAC layer communication protocol based on the distributed shared secret key. 
     The techniques described herein may be performed by a system and/or device having non-transitory computer-readable media storing computer-executable instructions that, when executed by one or more processors, performs the methods described herein. 
     Certain implementations and embodiments of the disclosure will now be described more fully below with reference to the accompanying figures, in which various aspects are shown. However, the various aspects may be implemented in many different forms and should not be construed as limited to the implementations set forth herein. The disclosure encompasses variations of the embodiments, as described herein. Like numbers refer to like elements throughout. 
       FIG. 1  is a diagram illustrating a configuration  100  including a first station  102  and a second station  104 . As illustrated in  FIG. 1 , the first station  102  includes a communication stack  106 . The communication stack  106  is configured to implement one or more protocols at each layer of the communication stack  106 . The layers may be thought of, for example, as layers of a communication stack according to the Open Systems Interconnection (OSI) model. The OSI model is a model that characterizes the communication functions of the first station  102  in a conceptual way. In the  FIG. 1  example, the communication stack  106  includes a physical (PHY) layer, a Media Access Control (MAC) layer and an Internet Protocol (IP) layer. The second station  104  similarly includes a communication stack  108 . The communication stack  108  also includes a PHY layer, a MAC layer and an IP layer. 
     As illustrated in  FIG. 1 , the first station  102  provides an indication  110  to the second station  104  that the first station  102  is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. As also illustrated in  FIG. 1 , the first station  102  and the second station  104  are communicating  112  at the MAC layer according to a secure MAC layer communication protocol based on the pre-shared secret key or a distributed shared key. That is, at least a portion of the communication  112  at the MAC layer is encrypted by the transmitting one of the first station  102  or the second station  104 , and the encrypted portion is decrypted by the receiving one of the first station  102  or the second station  104 . As mentioned above, the MACsec protocol is an example of a secure MAC layer communication protocol. 
       FIG. 2  is a diagram illustrating a configuration  200  including a first station  202  and a second station  204 . The first station  202  transmits, to the second station  204 , an announcement message  206  concerning network access conditions. In one example, the announcement message  206  is an Extensible Authentication Protocol over LAN (EAPOL) announcement message such as is described at Section 11.12 of IEEE Std 802.1X-2010. The network access conditions indicated in the announcement message  206  may include an indication of a secure MAC layer communication protocol to use for communication between the first station  202  and the second station  204 . The announcement message  206  also includes an indication that the first station  202  is capable of secure MAC layer communication with the second station  204  according to the indicated secure MAC layer communication protocol, based on a pre-shared secret key. 
     If the announcement message  206  is an EAPOL announcement message, the indication of the secure MAC layer communication protocol may, for example, be encoded in a basic parameter set of the EAPOL announcement message as described in Table 11-7 and Figure 11-18 of IEEE Std 802.1X-2010. Furthermore, if the announcement message  206  is an EAPOL announcement message, the indication in the announcement message  206  that the first station  202  is capable of secure MAC layer communication with the second station  204  according to the indicated secure MAC layer communication protocol based on the pre-shared secret key may be included in a PSK capabilities TLV. The PSK capabilities TLV is not explicitly described in IEEE Std 802.1X-2010. The PSK capabilities TLV may be included as a TLV type that is reserved for future standardization, as described in Table 11-8 of IEEE Std 802.1X-2010 (“EAPOL-Announcement TLVs). Furthermore, by using a TLV type that is near or at the end of the reserved TLV types, such as TLV type  110 , there is minimal risk that the TLV type would be otherwise used in future standardization. As such, the PSK capabilities TLV may be implemented even without being included in future standardization. 
     The second station  204  transmits, to the first station  202 , an announcement message  208 . The announcement message  208  includes a confirmation, by the second station  204 , that the second station  204  is capable of secure MAC layer communication with the first station  202  according to the secure MAC layer communication protocol indicated by the first station  202  based on a pre-shared secret key. 
     In one example, the announcement message  208  is an EAPOL announcement message such as is described at Section 11.12 of IEEE Std 802.1X-2010. If the announcement message  206  is an EAPOL announcement message, the confirmation that the second station  204  is capable of secure MAC layer communication with the first station  202  based on the pre-shared secret key may, for example, be encoded in an existing parameter set such as the “MACsec SAK Use” parameter set described in Table 11-7 and Figure 11-10 of IEEE Std 802.1X-2010. For example, an otherwise reserved flag may be utilized to convey the indication. As another example, an exclusive parameter set may be defined to convey the indication. 
     One or more messages  210  are exchanged in the process of the first station  202  and the second station  204  agreeing on an indication of the pre-shared secret key. This may be, for example, as simple as the first station  202  transmitting to the second station  204  a single message  210  including an indication of the pre-shared secret key. In some examples, the first station  202  and the second station  204  exchange a series of messages  210  in a process of suggesting/negotiating and indicating agreement on a pre-shared secret key. 
     The first station  202  and the second station  204  independently determine or retrieve the pre-shared secret key based on the agreed-upon indication of the pre-shared secret key. The first station  202  and the second station  204  exchange one or more messages  212  comprising secure MAC layer communication based on the pre-shared secret key. In some examples, the secure MAC layer communication is based on the pre-shared secret key and, in some examples, the secure MAC layer communication is based on a key, such as a session key, that is based upon the pre-shared secret key. 
       FIG. 3  is a diagram illustrating a configuration  300  including a first station  302  and a second station  304 . Similar to the  FIG. 2  configuration  200 , the first station  302  transmits, to the second station  304 , an announcement message  306  concerning network access conditions. In one example, the announcement message  306  is an EAPOL announcement message such as is described at Section 11.12 of IEEE Std 802.1X-2010. The network access conditions indicated in the announcement message  306  may include an indication of a secure MAC layer communication protocol to use for communication between the first station  302  and the second station  304 . The announcement message  306  also includes an indication that the first station  302  is capable of secure MAC layer communication with the second station  304  according to the indicated secure MAC layer communication protocol based on the pre-shared secret key. 
     The dashed line  308  denotes that the second station  304  is not capable of secure MAC layer communication with the first station  202  based on a pre-shared secret key. For example, the second station  304  may not even be configured to interpret the indication of pre-shared secret key capability transmitted by the first station  302  to the second station  304 . In such a case, the second station  304  may not respond to the indication. As another example, the second station may be generally capable of secure may layer communication based on a pre-shared secret key, but the first station  302  and the second station  304  may not be able to agree on a pre-shared secret key or indication of pre-shared secret key to use. 
     The first station  302  transmits one more messages  310  to distribute a secret key. For example, the one or more messages  310  may be an EAPOL-MKA PDU such as is described at Section 11.11 of IEEE Std 802.1X-2010. If the one or more messages  310  is an EAPOL-MKA PDU, the secret key may be encoded in the “Distributed SAK” parameter set described in Table 11-7, Figure 11-11 and Figure 11-12 of IEEE Std 802.1X-2010. 
     The first station  302  and the second station  304  exchange one or more messages  312  comprising secure MAC layer communication based on the distributed shared key. In some examples, the secure MAC layer communication uses the distributed shared key and, in some examples, the secure MAC layer communication uses a key, such as a session key, that is based upon the distributed shared key. 
       FIG. 4  illustrates an example format  400  of a distributed SAK parameter set of the EAPOL MKPDU as described in Table 11-7 and Figure 11-18 of IEEE Std 802.1X-2010, as modified to be used by a first station, such as the first station  202  or the first station  302 , to include an indication that the first station is capable of secure MAC layer communication with the second station, such as the second station  204  or the second station  304 , based on a pre-shared secret key. The format  400  for the distributed SAK parameter set includes a field, in octet 1, to indicate that it is of Parameter set Type=4. 
     A parameter set of Parameter set Type=4 is described, in IEEE Std 802.1X-2010, as a “Distributed SAK parameter set (GCM-AES-128)” or a “Distributed SAK parameter set (other MACsec Cipher Suites).” The format  400  is the second type, as it includes a “MACsec Cipher Suite” field in octet 9 to octet 16. In some examples, the techniques described herein may use a format of the first type. Bit  4  of octet 2, as described in IEEE Std 802.1X-2010, is a reserved bit. By reserved, it is meant that the bit has no meaning, i.e., is “don&#39;t care” for a station that is receiving a message including a parameter set having Parameter set Type=4. 
     In accordance with the techniques described herein, bit  4  of octet 2 may be given meaning and considered as the indication that the first station is capable of secure MAC layer communication with the second station based on a pre-shared secret key. For example, the first station may set bit  4  of octet 2 to indicate that the first station is capable of secure MAC layer communication with the second station based on a pre-shared secret key. Otherwise, the first station may clear bit  4  of oct 2 to indicate that the first station will communicate with the second station by secure MAC layer communication based on a distributed shared key. 
     In the  FIG. 4  example format, the message including the indication that the first station is capable of secure MAC layer communication with the second station based on a pre-shared secret key also includes an indication of the pre-shared secret key. That is, in the  FIG. 4  example format  400 , the octets 17-40 include an AES Key Wrap of the indication of the pre-shared key. In the  FIG. 4  example format  400 , the indication of the pre-shared secret key is a PSK_ID, as described above. “AES Key Wrap” means that the indication of the pre-shared key identifier is encrypted as described in RFC 3394, entitled “Advanced Encryption Standard (AES) Key Wrap Algorithm” and dated September 2002. Otherwise, in the  FIG. 4  example format, for a message including an indication that the first station will communicate with the second station by secure MAC layer communication based on a distributed SAK, the octets 17-40 include an AES key wrap of the distributed SAK. 
       FIG. 5  illustrates an example format  500  of a distributed pre-shared key-identifier parameter set of an EAPOL MKPDU that is not explicitly described in IEEE Std 802.1X-2010. The distributed pre-shared key identifier parameter set may be used by a first station, such as the first station  202  or the first station  302 , to provide an indication that the first station is capable of secure MAC layer communication with the second station, such as the second station  204  or the second station  304  based on a pre-shared secret key. The Parameter set Type is shown in  FIG. 5  as xxx but, as discussed above, there may be advantages in some instances to using a Parameter set Type=110. The  FIG. 5  example format  500  is similar in many respect to the “Distributed SAK” parameter set described in Table 11-7 and Figure 11-12 of IEEE Std 802.1X-2010. One difference is that, in the  FIG. 5  example format  500 , octet 17 is a “PSK_ID Type” field. PSK_ID Type can be of PSK_ID_OPAQUE (for those implementations that choose not to disclose the type of PSK) or PSK_ID_FIXED (for potential interoperability). See, for example, Section 5.1 of Internet Engineering Task Force, Internet Draft entitled “Mixing Preshared Keys in IKEv2 for Post-quantum Security” dated Jan. 14, 2020. Referring still to the  FIG. 5  example format  500 , the octets 17-40 include an AES Key Wrap of the indication of the pre-shared secret key. In the  FIG. 5  example format  500 , the indication of the pre-shared secret key is a PSK_ID, as described above. 
     Similar formats may be used for a response message from a second station, like the second station such as the second  204  or the second station  304 , to respond to an indication that the first station, such as the first station  202  or the second station  302 , is capable of secure MAC layer communication with the second station based on a pre-shared secret key. For example, for such a response message, an existing parameter set like “MACsec SAK Use,” as described in Table 11-7 and Figure 11-10 of IEEE Std 802.1X-2010, may be modified by considering a reserved flag to convey the PSK_ID status. As another example, analogous to the  FIG. 5  example format  500  of a distributed pre-shared key-identifier parameter set of an EAPOL MKPDU that is not explicitly described in IEEE Std 802.1X-2010, a format for a PSK_ID Status Parameter Set may be defined. The format may include a PSK_ID status field. As with the  FIG. 5  example format  500 , the format for the PSK_ID Status Parameter Set is not explicitly described in IEEE Std 802.1X-2010. 
     In the response message, whatever the format, the second station may indicate that the second station is capable of secure MAC layer communication with the first station based on a pre-shared secret key. If such a response message is not received within a particular time frame, for example, the first station may proceed with distributing a shared key to the second station, to configure the first station and the second station to communicate by secure MAC layer communication based on the distributed shared key and not based on a pre-shared secret key. Thus, the first station may be backwards compatible to interoperate with stations that are not capable of secure MAC layer communication based on a pre-shared secret key. 
     Otherwise, for example, a first station may provide a PSK_ID to a second station, such as by a message having the  FIG. 4  format  400  or the  FIG. 5  format. The second station may not recognize the provided PSK_ID or the provided PSK_ID may otherwise not be available for use by the second station. In this situation, the second station may provide a message to the first station indicating the PSK_ID status as “reject.” 
     In an example, the first station will continue to transmit different PSK IDs to the second station, until the first station receives a PSK_ID status of “accept” from the second station. For example, the first station may transmit a first PSK_ID to the second station and wait for a responsive message from the second station indicating a PSK_ID status of “accept” or “reject.” If the PSK_ID status is “reject,” the first station may transmit a second PSK_ID to the second station, and wait for a responsive message from the second station. 
     This cycle of the first station transmitting a PSK_ID and waiting for a responsive message indicating a PSK_ID status of “accept” or “reject” may continue until a responsive message indicates a PSK_ID status of “accept.” In some examples, the process may also be terminated if a configured lifetime expires, such as six seconds, or the first station has transmitted a PSK_ID a configured number of times. In some examples, this configured lifetime is referred to as the MKA Life Time. In some examples, the configured number of times is two, including the original PSK_ID transmission. If the PSK_ID transmission cycle terminates, such as described above, the first station may proceed with distributing a shared key to the second station, to configure the first station and the second station to communicate by secure MAC layer communication based on the distributed shared key and not based on a pre-shared secret key. 
       FIG. 6  is a diagram illustrating a configuration  600  including a first station  602  and a second station  604 .  FIG. 6  includes messages passed between the first station  602  and the second station  604 , as well as processing at each of the first station  602  and the second station  604 , for secure MAC layer communication between the first station  602  and the second station  604 . 
     Turning now to  FIG. 6 , at  606 , the first station  602  provides a message to the second station  604  indicating that the first station  602  is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. At  608 , the second station  604  provides a responsive announcement indicating that the second station  604  is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. If the second station  604  does not provide the responsive announcement at  608 , such as within a predetermined amount of time, then the first station  602  may make a determination to communicate with the second station  604  according to the secure MAC layer communication protocol based on a distributed shared key and not based on a pre-shared secret key. While not shown in  FIG. 6 , in such an instance, the first station  602  may distribute a key to the second station  604  to use for secure MAC layer communication between the first station  602  and the second station  604  based on the distributed shared key. 
     After  608 , the second station  604  is live. See, for example, IEEE Std 802.1X-2010, section 9.4.3, entitled “Determining liveness.” At  610 , the first station  602  accesses a quantum key store  612  and provides a pre-shared key identification PSK_ID to the second station  604 . The second station  604  accesses a quantum key store  614  and determines whether the second station  604  can use the PSK_ID to configure for secure MAC layer communication with the first station  602  based on a pre-shared secret key. If so, the second station  604  configures with a pre-shared secret key based on the PSK_ID. At  616 , the second station  604  provides a PSK_ID status indicating whether the second station  604  can use the PSK_ID (“accept” or “reject”). If the PSK_ID status indicates “accept,” then the first station  602  also configures with the pre-shared secret key based on the PSK_ID. 
     If the PSK_ID status indicates “reject,” then the first station  602  and the second station  604  may, in some examples, repeat  610  and  616  with at least one different PSK_ID. Upon a second rejection (or more rejections, if so configured), the first station  602  may make a determination to communicate with the second station  604  according to the secure MAC layer communication protocol based on a distributed shared key and not based on the pre-shared secret key. While not shown in  FIG. 6 , in such a situation, the first station  602  may distribute a shared key to the second station  604 , to use for secure MAC layer communication between the first station  602  and the second station  604  based on the distributed shared key. 
     Otherwise, at  618 , the first station  602  and the second station  604  communicate according to a secure MAC layer communication protocol based on the pre-shared secret key. At  620 , the first station  602  and the second station  604  continue to exchange announcement messages to maintain the communication  618  according to the secure MAC layer communication protocol based on the pre-shared secret key. 
       FIG. 7  is a flowchart for an example process  700  by which a first computing node interoperates with a second computing node to establish communication according to a secure MAC layer communication protocol. At  702 , a first computing node transmits a first message, from the first computing node to the second computing node. The first message includes at least a first indication that the first computing node is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. At  704 , the first computing node determines to communicate with the second computing node according to the secure MAC layer communication protocol based on one of a pre-shared secret key or a distributed shared key. At  706 , the first computing node, at least in part based on the determining, transmits a second message to the second computing node according to the secure MAC layer communication protocol based on the one of the pre-shared secret key or a distributed shared key. 
     For example, the first computing node may receive a message from the second computing node indicating that the second computing node is capable of communicating transmitting a first message. Further, for example, the first computing node and the second computing node may agree on an indication of a pre-shared secret key. The first computing node may determine to communicate with the second computing node according to the secure MAC layer communication protocol based on the pre-shared secret key. The first computing node may therefore communicate with the second computing node according to the secure MAC layer communication protocol based on the pre-shared secret key. Otherwise, the first computing node may be backward compatible, and may communicate with the second computing node according to the secure MAC layer communication protocol based on a distributed shared key. 
       FIG. 8  is a flowchart for an example process  800  by which a second computing node interoperates with a first computing node to establish communication according to a secure MAC layer communication protocol. That is,  FIG. 8  is a flowchart for a process from the point of view of the second station, whereas the  FIG. 7  flowchart is for a process from the point of view of the first station. 
     At  802 , the second computing node receives a first message from the first computing node. The first message includes at least a first indication that the first computing node is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. At  804 , the second computing node transmits a second message to the first computing node. The second message includes at least a second indication that the second computing node is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. At  806 , the second computing node receives a third message from the first computing node according to the secure MAC layer communication protocol based on the pre-shared secret key. 
     Thus, for example, if the second computing node is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key, the second computing node notifies the first computing node. As a result, the first computing node communicates with the second computing node according to the secure MAC layer communication protocol based on a pre-shared secret key. 
       FIG. 9  is a flowchart for an example process  900 . In accordance with the process, a second computing node is not capable of communicating with a first computing node according to a secure MAC layer communication protocol based on a pre-shared secret key. At  902 , the second computing node receives a first message, from the first computing node. The first message includes at least a first indication that the first computing node is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key. At  904 , the second computing nodes receives a second message from the first computing node. The second message includes a shared key distributed by the first computing node. At  906 , the second computing node receives a third message from the first computing node according to the secure MAC layer communication protocol based on the distributed shared key. Thus, for example, if the second computing node does not indicate the second computing node is capable of communicating according to the secure MAC layer communication protocol based on a pre-shared secret key, the first computing node may revert to communicating with the second computing node according to the secure MAC layer communication protocol based on a distributed shared key. 
       FIG. 10  illustrates an example computer architecture for a computer  1000  capable of executing program components for implementing the functionality described above. The computer architecture shown in  FIG. 10  illustrates an architecture of a server computer, workstation, desktop computer, laptop, tablet, network appliance, e-reader, smartphone, network switch, or other computing device, and can be utilized to execute any of the software components presented herein. The computer  1000  may, in some examples, correspond to a network infrastructure device discussed herein. 
     The computer  1000  includes a baseboard  1002 , or “motherboard,” which may be a printed circuit board to which a multitude of components or devices can be connected by way of a system bus or other electrical communication paths. In one illustrative configuration, one or more central processing units (“CPUs”)  1004  operate in conjunction with a chipset  1006 . The CPUs  1004  can be, for example, standard programmable processors that perform arithmetic and logical operations necessary for the operation of the computer  1000 . 
     The CPUs  1004  perform operations by transitioning from one discrete, physical state to the next through the manipulation of switching elements that differentiate between and change these states. Switching elements generally include electronic circuits that maintain one of two binary states, such as flip-flops, and electronic circuits that provide an output state based on the logical combination of the states of one or more other switching elements, such as logic gates. These basic switching elements can be combined to create more complex logic circuits, including registers, adders-subtractors, arithmetic logic units, floating-point units, and the like. 
     The chipset  1006  provides an interface between the CPUs  1004  and the remainder of the components and devices on the baseboard  1002 . The chipset  1006  can provide an interface to a RAM  1008 , used as the main memory in the computer  1000 . The chipset  1006  can further provide an interface to a computer-readable storage medium such as a read-only memory (“ROM”)  1010  or non-volatile RAM (“NVRAM”) for storing basic routines that help to startup the computer  1000  and to transfer information between the various components and devices. The ROM  1010  or NVRAM can also store other software components necessary for the operation of the computer  1000  in accordance with the configurations described herein. As illustrated in  FIG. 10 , the ROM  1010  or NVRAM can also store data usable by the computer  1000  to generate and/or process attestation information in messages exchanged among the computer  1000  and other devices. In other examples, this data may be stored elsewhere, such as in RAM  1008 . 
     The computer  1000  can operate in a networked environment using logical connections to remote computing devices and computer systems through a network. For example, the chipset  1006  can include functionality for providing network connectivity through a Network Interface Controller (NIC)  1012 , such as a gigabit Ethernet adapter. The NIC  1012  can connect the computer  1000  to other computing devices over a network. It should be appreciated that multiple NICs  1012  can be present in the computer  1000 , connecting the computer to other types of networks and remote computer systems. In some instances, the NICs  1012  may include at least one ingress port and/or at least one egress port. An input/output controller  1016  may be provided for other types of input/output. 
     The computer  1000  can be connected to a storage device  1018  that provides non-volatile storage for the computer. The storage device  1018  can store an operating system  1020 , programs  1022 , and data, for example. The storage device  1018  can be connected to the computer  1000  through a storage controller  1014  connected to the chipset  1006 . The storage device  1018  can include one or more physical storage units. The storage controller  1014  can interface with the physical storage units through a serial attached SCSI (“SAS”) interface, a serial advanced technology attachment (“SATA”) interface, a fiber channel (“FC”) interface, or other type of interface for physically connecting and transferring data between computers and physical storage units. 
     The computer  1000  can store data on the storage device  1018  by transforming the physical state of the physical storage units to reflect the information being stored. The specific transformation of physical state can depend on various factors, in different embodiments of this description. Examples of such factors can include, but are not limited to, the technology used to implement the physical storage units, whether the storage device  1018  is characterized as primary or secondary storage, and the like. For example, the computer  1000  can store information to the storage device  1018  by issuing instructions through the storage controller  1014  to alter the magnetic characteristics of a particular location within a magnetic disk drive unit, the reflective or refractive characteristics of a particular location in an optical storage unit, or the electrical characteristics of a particular capacitor, transistor, or other discrete component in a solid-state storage unit. Other transformations of physical media are possible without departing from the scope and spirit of the present description, with the foregoing examples provided only to facilitate this description. The computer  1000  can further read information from the storage device  1018  by detecting the physical states or characteristics of one or more particular locations within the physical storage units. 
     In addition to the storage device  1018  described above, the computer  1000  can have access to other computer-readable storage media to store and retrieve information, such as program modules, data structures, or other data, including data to generate and/or process attestation information. It should be appreciated by those skilled in the art that computer-readable storage media is any available media that provides for the non-transitory storage of data and that can be accessed by the computer  1000 . 
     While the invention is described with respect to the specific examples, it is to be understood that the scope of the invention is not limited to these specific examples. Since other modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the example chosen for purposes of disclosure and covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
     Although the application describes embodiments having specific structural features and/or methodological acts, it is to be understood that the claims are not necessarily limited to the specific features or acts described. Rather, the specific features and acts are merely illustrative of some embodiments that fall within the scope of the claims of the application.