| CoRE Working Group | C. Amsuess |
| Internet-Draft | |
| Updates: 7252 (if approved) | J. Mattsson |
| Intended status: Standards Track | G. Selander |
| Expires: December 31, 2018 | Ericsson AB |
| June 29, 2018 |
Echo and Request-Tag
draft-ietf-core-echo-request-tag-02
This document specifies several security enhancements to the Constrained Application Protocol (CoAP). Two optional extensions are defined: the Echo option and the Request-Tag option. Each of these options provide additional features to CoAP and protects against certain attacks. The document also updates the processing requirements on the Token of [RFC7252]. The updated Token processing ensures secure binding of responses to requests.
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The initial Constrained Application Protocol (CoAP) suite of specifications ([RFC7252], [RFC7641], and [RFC7959]) was designed with the assumption that security could be provided on a separate layer, in particular by using DTLS ([RFC6347]). However, for some use cases, additional functionality or extra processing is needed to support secure CoAP operations. This document specifies several security enhancements to the Constrained Application Protocol (CoAP).
This document specifies two server-oriented CoAP options, the Echo option and the Request-Tag option, mainly addressing the security features request freshness and fragmented message body integrity, respectively. The Echo option enables a CoAP server to verify the freshness of a request, verify the aliveness of a client, synchronize state, or force a client to demonstrate reachability at its apparent network address. The Request-Tag option allows the CoAP server to match message fragments belonging to the same request, fragmented using the CoAP Block-Wise Transfer mechanism, which mitigates attacks and enables concurrent blockwise operations. These options in themselves do not replace the need for a security protocol; they specify the format and processing of data which, when integrity protected using e.g. DTLS ([RFC6347]), TLS ([RFC5246]), or OSCORE ([I-D.ietf-core-object-security]), provide the additional security features.
The document also updates the processing requirements on the Token. The updated processing ensures secure binding of responses to requests.
A CoAP server receiving a request is in general not able to verify when the request was sent by the CoAP client. This remains true even if the request was protected with a security protocol, such as DTLS. This makes CoAP requests vulnerable to certain delay attacks which are particularly incriminating in the case of actuators ([I-D.mattsson-core-coap-actuators]). Some attacks are possible to mitigate by establishing fresh session keys (e.g. performing the DTLS handshake) for each actuation, but in general this is not a solution suitable for constrained environments.
A straightforward mitigation of potential delayed requests is that the CoAP server rejects a request the first time it appears and asks the CoAP client to prove that it intended to make the request at this point in time. The Echo option, defined in this document, specifies such a mechanism which thereby enables the CoAP server to verify the freshness of a request. This mechanism is not only important in the case of actuators, or other use cases where the CoAP operations require freshness of requests, but also in general for synchronizing state between CoAP client and server and to verify aliveness of the client.
CoAP was designed to work over unreliable transports, such as UDP, and include a lightweight reliability feature to handle messages which are lost or arrive out of order. In order for a security protocol to support CoAP operations over unreliable transports, it must allow out-of-order delivery of messages using e.g. a sliding replay window such as described in Section 4.1.2.6 of DTLS ([RFC6347]).
The Block-Wise Transfer mechanism [RFC7959] extends CoAP by defining the transfer of a large resource representation (CoAP message body) as a sequence of blocks (CoAP message payloads). The mechanism uses a pair of CoAP options, Block1 and Block2, pertaining to the request and response payload, respectively. The blockwise functionality does not support the detection of interchanged blocks between different message bodies to the same resource having the same block number. This remains true even when CoAP is used together with a security protocol such as DTLS or OSCORE, within the replay window ([I-D.mattsson-core-coap-actuators]), which is a vulnerability of CoAP when using RFC7959.
A straightforward mitigation of mixing up blocks from different messages is to use unique identifiers for different message bodies, which would provide equivalent protection to the case where the complete body fits into a single payload. The ETag option [RFC7252], set by the CoAP server, identifies a response body fragmented using the Block2 option. This document defines the Request-Tag option for identifying the request body fragmented using the Block1 option, similar to ETag, but ephemeral and set by the CoAP client.
A fundamental requirement of secure REST operations is that the client can bind a response to a particular request. In HTTPS this is assured by the ordered and reliable delivery as well as mandating that the server sends responses in the same order that the requests were received.
The same is not true for CoAP where the server can return responses in any order. Concurrent requests are instead differentiated by their Token. Unfortunately, CoAP [RFC7252] does not treat Token as a cryptographically important value and does not give stricter guidelines than that the tokens currently “in use” SHOULD (not SHALL) be unique. If used with security protocol not providing bindings between requests and responses (e.g. DTLS and TLS) token reuse may result in situations where a client matches a response to the wrong request (see e.g. Section 2.3 of [I-D.mattsson-core-coap-actuators]). Note that mismatches can also happen for other reasons than a malicious attacker, e.g. delayed delivery or a server sending notifications to an uninterested client.
A straightforward mitigation is to mandate clients to never reuse tokens until the traffic keys have been replaced. As there may be any number of responses to a request (see e.g. [RFC7641]), the easiest way to accomplish this is to implement the token as a counter and never reuse any tokens at all. This document updates the Token processing in [RFC7252] to always assure a cryptographically secure binding of responses to requests.
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
Unless otherwise specified, the terms “client” and “server” refers to “CoAP client” and “CoAP server”, respectively, as defined in [RFC7252].
The terms “payload” and “body” of a message are used as in [RFC7959]. The complete interchange of a request and a response body is called a (REST) “operation”. An operation fragmented using [RFC7959] is called a “blockwise operation”. A blockwise operation which is fragmenting the request body is called a “blockwise request operation”. A blockwise operation which is fragmenting the response body is called a “blockwise response operation”.
Two request messages are said to be “matchable” if they occur between the same endpoint pair, have the same code and the same set of options except for elective NoCacheKey options and options involved in bock-wise transfer (Block1, Block2 and Request-Tag). Two operations are said to be matchable if any of their messages are.
Two matchable blockwise operations are said to be “concurrent” if a block of the second request is exchanged even though the client still intends to exchange further blocks in the first operation. (Concurrent blockwise request operations are impossible with the options of [RFC7959] because the second operation’s block overwrites any state of the first exchange.).
The Echo and Request-Tag options are defined in this document.
The Echo option is a server-driven challenge-response mechanism for CoAP. The Echo option value is a challenge from the server to the client included in a CoAP response and echoed in one or more CoAP request.
The Echo Option is elective, safe-to-forward, not part of the cache-key, and not repeatable, see Figure 1, which extends Table 4 of [RFC7252]).
+-----+---+---+---+---+-------------+--------+--------+---------+---+
| No. | C | U | N | R | Name | Format | Length | Default | E |
+-----+---+---+---+---+-------------+--------+--------+---------+---+
| TBD | | | x | | Echo | opaque | 4-40 | (none) | x |
+-----+---+---+---+---+-------------+--------+--------+---------+---+
C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable,
E = Encrypt and Integrity Protect (when using OSCORE)
Figure 1: Echo Option Summary
[ Note to RFC editor: If this document is released before core-object-security, the following paragraph and the “E” column above need to move into OSCORE. ]
The Echo option value is generated by the server, and its content and structure are implementation specific. Different methods for generating Echo option values are outlined in Appendix A. Clients and intermediaries MUST treat an Echo option value as opaque and make no assumptions about its content or structure.
When receiving an Echo option in a request, the server MUST be able to verify that the Echo option value was generated by the server as well as the point in time when the Echo option value was generated.
The Echo option MAY be included in any request or response (see Section 2.3 for different applications), but the Echo option MUST NOT be used with empty CoAP requests (i.e. Code=0.00).
If the server receives a request which has freshness requirements, the request does not contain a fresh Echo option value, and the server cannot verify the freshness of the request in some other way, the server MUST NOT process the request further and SHOULD send a 4.01 Unauthorized response with an Echo option.
The application decides under what conditions a CoAP request to a resource is required to be fresh. These conditions can for example include what resource is requested, the request method and other data in the request, and conditions in the environment such as the state of the server or the time of the day.
The server may also include the Echo option in a response to verify the aliveness of a client, to synchronize state, or to force a client to demonstrate reachability at their apparent network address.
Upon receiving a 4.01 Unauthorized response with the Echo option, the client SHOULD resend the original request with the addition of an Echo option with the received Echo option value. The client MAY send a different request compared to the original request. Upon receiving any other response with the Echo option, the client SHOULD echo the Echo option value in the next request to the server. The client MAY include the same Echo option value in several different requests to the server.
Upon receiving a request with the Echo option, the server determines if the request has freshness requirement. If the request does not have freshness requirements, the Echo option MAY be ignored. If the request has freshness requirements and the server cannot verify the freshness of the request in some other way, the server MUST verify that the Echo option value was generated by the server; otherwise the request is not processed further. The server MUST then calculate the round-trip time RTT = (t1 - t0), where t1 is the request receive time and t0 is the transmit time of the response that included the specific Echo option value. The server MUST only accept requests with a round-trip time below a certain threshold T, i.e. RTT < T, otherwise the request is not processed further, and an error message MAY be sent. The threshold T is application specific, its value depends e.g. on the freshness requirements of the request. An example message flow is illustrated in Figure 2.
Client Server | | +------>| Code: 0.03 (PUT) | PUT | Token: 0x41 | | Uri-Path: lock | | Payload: 0 (Unlock) | | |<------+ t0 Code: 4.01 (Unauthorized) | 4.01 | Token: 0x41 | | Echo: 0x437468756c687521 | | +------>| t1 Code: 0.03 (PUT) | PUT | Token: 0x42 | | Uri-Path: lock | | Echo: 0x437468756c687521 | | Payload: 0 (Unlock) | | |<------+ Code: 2.04 (Changed) | 2.04 | Token: 0x42 | |
Figure 2: Example Echo Option Message Flow
When used to serve freshness requirements (including client aliveness and state synchronizing), CoAP requests containing the Echo option MUST be integrity protected, e.g. using DTLS, TLS, or OSCORE ([I-D.ietf-core-object-security]). When used to demonstrate reachability at their apparent network address, the Echo option MAY be used without protection.
Note that the server does not have to synchronize the time used for the Echo timestamps with any other party. If the server loses time synchronization, e.g. due to reboot, it MUST reject all Echo values that was created before time synchronization was lost.
CoAP-CoAP proxies MUST relay the Echo option unmodified. The CoAP server side of CoAP-HTTP proxies MAY request freshness, especially if they have reason to assume that access may require it (e.g. because it is a PUT or POST); how this is determined is out of scope for this document. The CoAP client side of HTTP-CoAP-Proxies SHOULD respond to Echo challenges themselves if they know from the recent establishing of the connection that the HTTP request is fresh. Otherwise, they SHOULD respond with 503 Service Unavailable, Retry-After: 0 and terminate any underlying Keep-Alive connection. They MAY also use other mechanisms to establish freshness of the HTTP request that are not specified here.
The Request-Tag is intended for use as a short-lived identifier for keeping apart distinct blockwise request operations on one resource from one client. It enables the receiving server to reliably assemble request payloads (blocks) to their message bodies, and, if it chooses to support it, to reliably process simultaneous blockwise request operations on a single resource. The requests must be integrity protected in order to protect against interchange of blocks between different message bodies.
In essence, it is an implementation of the “proxy-safe elective option” used just to “vary the cache key” as suggested in [RFC7959] Section 2.4.
The Request-Tag option is not critical, is safe to forward, repeatable, and part of the cache key, see Figure 3, which extends Table 4 of [RFC7252]).
+-----+---+---+---+---+-------------+--------+--------+---------+---+---+
| No. | C | U | N | R | Name | Format | Length | Default | E | U |
+-----+---+---+---+---+-------------+--------+--------+---------+---+---+
| TBD | | | | x | Request-Tag | opaque | 0-8 | (none) | x | x |
+-----+---+---+---+---+-------------+--------+--------+---------+---+---+
C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable,
E = Encrypt and Integrity Protect (when using OSCORE)
Figure 3: Request-Tag Option Summary
[ Note to RFC editor: If this document is released before core-object-security, the following paragraph and the “E”/”U” columns above need to move into OSCORE. ]
Request-Tag, like the block options, is both a class E and a class U option in terms of OSCORE processing (see Section 4.1 of [I-D.ietf-core-object-security]): The Request-Tag MAY be an inner or outer option. The inner option is encrypted and integrity protected between client and server, and provides message body identification in case of end-to-end fragmentation of requests. The outer option is visible to proxies and labels message bodies in case of hop-by-hop fragmentation of requests.
The Request-Tag option is only used in the request messages of blockwise operations.
The Request-Tag mechanism can be applied independently on the server and client sides of CoAP-CoAP proxies as are the block options, though given it is safe to forward, a proxy is free to just forward it when processing an operation. CoAP-HTTP proxies and HTTP-CoAP proxies can use Request-Tag on their CoAP sides; it is not applicable to HTTP requests.
The Request-Tag option does not require any particular processing on the server side: As it varies the set of options that distinguish blockwise operations (ie. is neither Block1 or Block2 nor elective NoCacheKey), the server can not treat their messages as belonging to the same operation.
To keep utilizing the cache, a server (including proxies) MAY discard the Request-Tag option from an assembled block-wise request when consulting its cache, as the option describes the individual blocks but not the operation as a whole. For example, a FETCH request with the same body can have a fresh response even if they were requested using different request tags. (This is similar to the situation about ETag in that it is formally part of the cache key, but implementations that are aware of its meaning can cache more efficiently, see [RFC7252] Section 5.4.2).
A server receiving a Request-Tag MUST treat it as opaque and make no assumptions about its content or structure.
Two messages carrying the same Request-Tag is a necessary but not sufficient condition for being part of the same operation. They can still be treated as independent messages by the server (e.g. when it sends 2.01/2.04 responses for every block), or initiate a new operation (overwriting kept context) when the later message carries Block1 number 0.
[ The following paragraph might be better placed in lwig-coap, but was left here until lwig-coap has decided on its fate there. ]
As it has always been, a server that can only serve a limited number of block-wise operations at the same time can delay the start of the operation by replying with 5.03 (Service unavailable) and a Max-Age indicating how long it expects the existing operation to go on, or it can forget about the state established with the older operation and respond with 4.08 (Request Entity Incompelte) to later blocks on the first operation.
Especially, that is the case for any correctly implemented proxy that does not know how to use Request-Tag in requests and has only one client endpoint. When it receives concurrent incoming requests on the same resource, it needs to make that very choice: either send a 5.03 with Max-Age (holding off the second operation), or to commence the second operation and reject any further requests on the first operation with 4.08 Request Entity Incompelte errors without forwarding them. (Alternatively, it could spool the second request, but the unpredictable nature of the timeouts involved often makes that an unsuitable choice.)
For each separate blockwise request operation, the client can choose a Request-Tag value, or choose not to set a Request-Tag. Starting a request operation matchable to a previous operation and even using the same Request-Tag value is called request tag recycling. Clients MUST NOT recycle a request tag unless the first operation has concluded. What constitutes a concluded operation depends on the application, and is outlined individually in Section 3.4.
When Block1 and Block2 are combined in an operation, the Request-Tag of the Block1 phase is set in the Block2 phase as well for otherwise the request would have a different set of options and would not be recognized any more.
Clients are encouraged to generate compact messages. This means sending messages without Request-Tag options whenever possible, and using short values when the absent option can not be recycled.
When a client fragments a request body into multiple message payloads, even if the individual messages are integrity protected, it is still possible for a man-in-the-middle to maliciously replace a later operation’s blocks with an earlier operation’s blocks (see Section 2.5 of [I-D.mattsson-core-coap-actuators]). Therefore, the integrity protection of each block does not extend to the operation’s request body.
In order to gain that protection, use the Request-Tag mechanism as follows:
Authors of other documents (e.g. [I-D.ietf-core-object-security]) are invited to mandate this behavior for clients that execute blockwise interactions over secured transports. In this way, the server can rely on a conforming client to set the Request-Tag option when required, and thereby conclude on the integrity of the assembled body.
Note that this mechanism is implicitly implemented when the security layer guarantees ordered delivery (e.g. CoAP over TLS [RFC8323]). This is because with each message, any earlier message can not be replayed any more, so the client never needs to set the Request-Tag option unless it wants to perform concurrent operations.
CoAP clients, especially CoAP proxies, may initiate a blockwise request operation to a resource, to which a previous one is already in progress, which the new request should not cancel. A CoAP proxy would be in such a situation when it forwards operations with the same cache-key options but possibly different payloads.
For those cases, Request-Tag is the proxy-safe elective option suggested in [RFC7959] Section 2.4 last paragraph.
When initializing a new blockwise operation, a client has to look at other active operations:
The Block options were defined to be unsafe to forward because a proxy that woud forward blocks as plain messages would risk mixing up clients’ requests.
The Request-Tag option provides a very simple way for a proxy to keep them separate: if it appends a Request-Tag that is particular to the requesting endpoint to all request carrying any Block option, it does not need to keep track of any further block state. [I-D.ietf-lwig-coap] Section TBD provides further details.
[ Note to reviewers and co-authors: That section was so far only syggested in input for lwig-coap. If it does not get into the document, we should drop it here (for I don’t want to explain all this case’s details and security considerations here), but if the reference works, this section shows why Request-Tag has become repeatable. ]
[ This section needs to be reworked after assuming our RFC7959 interpretation. ]
The Request-Tag option can be elective, because to servers unaware of the Request-Tag option, operations with differing request tags will not be matchable.
The Request-Tag option can be safe to forward but part of the cache key, because to proxies unaware of the Request-Tag option will consider operations with differing request tags unmatchable but can still forward them.
In earlier versions of this draft, the Request-Tag option used to be critical and unsafe to forward. That design was based on an erroneous understanding of which blocks could be composed according to [RFC7959].
An alternative that was considered to the Request-Tag option for coping with the problem of fragmented message body integrity (Section 3.4.1) was to update [RFC7959] to say that blocks could only be assembled if their fragments’ order corresponded to the sequence numbers.
That approach would have been difficult to roll out reliably on DTLS where many implementations do not expose sequence numbers, and would still not prevent attacks like in [I-D.mattsson-core-coap-actuators] Section 2.5.2.
The same security properties as in Section 3.4.1 can be obtained for blockwise response operations. The threat model here is not an attacker (because the response is made sure to belong to the current request by the security layer), but blocks in the client’s cache.
Rules stating that response body reassembly is conditional on matching ETag values are already in place from Section 2.4 of [RFC7959].
To gain equivalent protection to Section 3.4.1, a server MUST use the Block2 option in conjunction with the ETag option ([RFC7252], Section 5.10.6), and MUST NOT use the same ETag value for different representations of a resource.
This section updates the Token processing in Section 5.3.1 of [RFC7252] by adding the following text:
When CoAP is used with a security protocol not providing bindings between requests and responses, the client MUST NOT reuse tokens until the traffic keys have been replaced. The easiest way to accomplish this is to implement the Token as a counter, this approach SHOULD be followed.
This document adds the following option numbers to the “CoAP Option Numbers” registry defined by [RFC7252]:
+--------+-------------+------------+ | Number | Name | Reference | +--------+-------------+------------+ | TBD1 | Echo | [RFC XXXX] | | | | | | TBD2 | Request-Tag | [RFC XXXX] | +--------+-------------+------------+
Figure 4: CoAP Option Numbers
Implementations SHOULD NOT put any privacy sensitive information in the Echo or Request-Tag option values. Unencrypted timestamps MAY reveal information about the server such as its wall clock time or location. Servers MUST use a monotonic clock to generate timestamps and compute round-trip times. Servers SHOULD NOT use wall clock time for timestamps, as wall clock time is not monotonic, may reveal that the server will accept expired certificates, or reveal the server’s location. Use of non-monotonic clocks is not secure as the server will accept expired Echo option values if the clock is moved backward. The server will also reject fresh Echo option values if the clock is moved forward. An attacker may be able to affect the server’s wall clock time in various ways such as setting up a fake NTP server or broadcasting false time signals to radio-controlled clocks. Servers MAY use the time since reboot measured in some unit of time. Servers MAY reset the timer periodically. When resetting the timer, the server MUST reject all Echo values that was created before the reset.
The availability of a secure pseudorandom number generator and truly random seeds are essential for the security of the Echo option. If no true random number generator is available, a truly random seed must be provided from an external source.
An Echo value with 64 (pseudo-)random bits gives the same theoretical security level against forgeries as a 64-bit MAC (as used in e.g. AES_128_CCM_8). In practice, forgery of an Echo option value is much harder as an attacker must also forge the MAC in the security protocol. The Echo option value MUST contain 32 (pseudo-)random bits that are not predictable for any other party than the server, and SHOULD contain 64 (pseudo-)random bits. A server MAY use different security levels for different uses cases (client aliveness, request freshness, state synchronization, network address reachability, etc.).
The security provided by the Echo and Request-Tag options depends on the security protocol used. CoAP and HTTP proxies require (D)TLS to be terminated at the proxies. The proxies are therefore able to manipulate, inject, delete, or reorder options or packets. The security claims in such architectures only hold under the assumption that all intermediaries are fully trusted and have not been compromised.
Servers that use the List of Cached Random Values and Timestamps method described in Appendix A may be vulnerable to resource exhaustion attacks. On way to minimizing state is to use the Integrity Protected Timestamp method described in Appendix A.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC7252] | Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014. |
| [RFC7959] | Bormann, C. and Z. Shelby, "Block-Wise Transfers in the Constrained Application Protocol (CoAP)", RFC 7959, DOI 10.17487/RFC7959, August 2016. |
| [RFC8174] | Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017. |
The content and structure of the Echo option value are implementation specific and determined by the server. Use of one of the mechanisms outlined in this section is RECOMMENDED.
Different mechanisms have different tradeoffs between the size of the Echo option value, the amount of server state, the amount of computation, and the security properties offered.
Echo option value: timestamp t0, MAC(k, t0)
Server State: secret key k
Echo option value: random value r
Server State: random value r, timestamp t0
A server MAY use different methods and security levels for different uses cases (client aliveness, request freshness, state synchronization, network address reachability, etc.).
In absence of concurrent operations, the Request-Tag mechanism for body integrity (Section 3.4.1) incurs no overhead if no messages are lost (more precisely: in OSCORE, if no operations are aborted due to repeated transmission failure; in DTLS, if no packages are lost), or when blockwise request operations happen rarely (in OSCORE, if there is always only one request blockwise operation in the replay window).
In those situations, no message has any Request-Tag option set, and that can be recycled indefinitely.
When the absence of a Request-Tag option can not be recycled any more within a security context, the messages with a present but empty Request-Tag option can be used (1 Byte overhead), and when that is used-up, 256 values from one byte long options (2 Bytes overhead) are available.
In situations where those overheads are unacceptable (e.g. because the payloads are known to be at a fragmentation threshold), the absent Request-Tag value can be made usable again:
[ The editor is asked to remove this section before publication. ]