RFC 5776
Use of Timed Efficient Stream Loss-Tolerant Authentication (TESLA) in the Asynchronous Layered Coding (ALC) and NACK-Oriented Reliable Multicast (NORM) Protocols
4. Receiver Operations
This section describes the TESLA operations at a receiver.4.1. Verification of the Authentication Information
This section details the computation steps required to verify each of the three possible authentication information of an incoming packet. The verification MUST follow a strict order: o first of all, if the Group MAC is present and if the session uses this feature (e.g., if the G bit is set in the bootstrap information message), then verify the Group MAC. A packet that does not contain a Group MAC tag, whereas the session uses this feature, MUST be dropped immediately. On the opposite, if a packet contains a Group MAC tag whereas the session does not use this feature, this tag MUST be ignored; o then, verify the digital signature (with TESLA signaling packets) or enter the TESLA authentication process (with data packets).4.1.1. Processing the Group MAC Tag
Upon receiving a packet containing a Group MAC tag, the receiver recomputes the Group MAC and compares it to the value carried in the packet. If the check fails, the packet MUST be dropped immediately.
More specifically, recomputing the Group MAC requires saving the value of the "Group MAC" field, setting this field to 0, and doing the same computation as a sender does (see Section 3.3.3).4.1.2. Processing the Digital Signature
Upon receiving a packet containing a digital signature, the receiver verifies the signature as follows. The computation of the signature MUST include the ALC or NORM header (with the various header extensions) and the payload when applicable. The UDP/IP headers MUST NOT be included. During this computation, the "Signature" field MUST be set to 0 as well as the optional Group MAC, when present. From [RFC4359]: Digital signature verification is performed as described in [RFC3447], Section 8.2.2 (RSASSA-PKCS1-v1_5) and [RFC3447], Section 8.1.2 (RSASSA-PSS). Upon receipt, the digital signature is passed to the verification function as S. The authenticated portion of the packet is used as the message M, and the RSA public key is passed as (n, e). In summary (when SHA-256 is used), the verification function computes a SHA-256 hash of the authenticated packet bytes, decrypts the SHA-256 hash in the packet, and validates that the appropriate encoding was applied. The two SHA-256 hashes are compared, and if they are identical the validation is successful. It is assumed that the receivers have the possibility to retrieve the sender's public key required to check this digital signature (Section 2.2). This document does not specify how the public key of the sender is communicated reliably and in a secure way to all possible receivers.4.1.3. Processing the Authentication Tag
When a receiver wants to authenticate a packet using an authentication tag and when he has the key for the associated time interval (i.e., after the disclosing delay, d), the receiver recomputes the MAC and compares it to the value carried in the packet. If the check fails, the packet MUST be immediately dropped. More specifically, recomputing the MAC requires saving the value of the "MAC" field, setting this field to 0, and doing the same computation as a sender does (see Section 3.3.1).
4.2. Initialization of a Receiver
A receiver MUST be initialized before being able to authenticate the source of incoming packets. This can be done by an out-of-band mechanism or an in-band mechanism (Section 2.2). Let us focus on the in-band mechanism. Two actions must be performed: o receive and process a bootstrap information message, and o calculate an upper bound of the sender's local time. To that purpose, the receiver must perform time synchronization.4.2.1. Processing the Bootstrap Information Message
A receiver must first receive a packet containing the bootstrap information, digitally signed by the sender. Once the bootstrap information has been authenticated (see Section 4.1), the receiver can initialize its TESLA component. The receiver MUST then ignore the following bootstrap information messages, if any. There is an exception though: when a new key chain is used and if a receiver missed all the commitments for this new key chain, then this receiver MUST process one of the future bootstrap information messages (if any) in order to be able to authenticate the incoming packets associated to this new key chain. Before TESLA has been initialized, a receiver MUST discard incoming packets other than the bootstrap information message and direct time synchronization response.4.2.2. Performing Time Synchronization
First of all, the receiver must know whether the ALC or NORM session relies on direct or indirect time synchronization. This information is communicated by an out-of-band mechanism (for instance, when describing the various parameters of an ALC or NORM session). In some cases, both mechanisms might be available and the receiver can choose the preferred technique.4.2.2.1. Direct Time Synchronization
In the case of a direct time synchronization, a receiver MUST synchronize with the sender. To that purpose, the receiver sends a direct time synchronization request message. This message includes the local time (in NTP timestamp format) at the receiver when sending the message. This timestamp will be copied in the sender's response for the receiver to associate the response to the request.
The direct time synchronization request message format is the following: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + t_r (NTP timestamp) + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ Group MAC (optional) ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 8: Format of a Direct Time Synchronization Request The direct time synchronization request (Figure 8) contains the following information: "t_r" (NTP timestamp, 64 bits): "t_r" is a timestamp in NTP timestamp format that contains the receiver local time value when sending this direct time synchronization request message; "Group MAC" field (optional, variable length, multiple of 32 bits): This field contains the Group MAC, calculated with the group key, K_g, shared by all group members. The field length, in bits, is given by n_w, which is known once the Group MAC function type is known (Section 7). The receiver then awaits a response message (Section 3.4.2). Upon receiving this message, the receiver: checks that this response relates to the request, by comparing the "t_r" fields; checks the Group MAC if present; checks the signature; retrieves the t_s value and calculates D_t (Section 2.4.1). Note that in an ALC session, the direct time synchronization request message is sent to the sender by an out-of-band mechanism that is not specified by the current document.
4.2.2.2. Indirect Time Synchronization
With the indirect time synchronization method, the sender MAY provide out-of-band the URL or IP address of the NTP server(s) he trusts along with an OPTIONAL certificate for each NTP server. When several NTP servers are specified, a receiver MUST choose one of them. This document does not specify how the choice is made, but for the sake of scalability, the clients SHOULD NOT use the same server if several possibilities are offered. The NTP synchronization between the NTP server and the receiver MUST be authenticated, either using the certificate provided by the server or another certificate the client may obtain for this NTP server. Then the receiver computes the time offset between itself and the NTP server chosen. Note that the receiver does not need to update the local time, (which often requires root privileges), computing the time offset is sufficient. Since the offset between the server and the time reference, D^O_t, is indicated in the bootstrap information message (or communicated out- of-band), the receiver can now calculate an upper bound of the sender's local time (Section 2.4.2). Note that this scenario assumes that each client trusts the sender and accepts aligning its NTP configuration to that of the sender, using one of the NTP server(s) suggested. If this assumption does not hold, the client MUST NOT use the NTP indirect time synchronization method (Section 2.3.2).4.3. Authentication of Received Packets
The receiver can now authenticate incoming packets (other than bootstrap information and direct time synchronization response packets). To that purpose, he MUST follow different steps (see [RFC4082], Section 3.5): 1. The receiver parses the different packet headers. If none of the four TESLA authentication tags are present, the receiver MUST discard the packet. If the session is in "Single Key Chain" mode (e.g., when the "S" flag is set in the bootstrap information message), then the receiver MUST discard any packet containing an Authentication Tag with a New Key Chain Commitment or an Authentication Tag with a Last Key of Old Chain Disclosure. 2. Safe packet test: When the receiver receives packet P_j, it first records the local time T at which the packet arrived. The receiver then computes an upper bound t_j on the sender's clock at the time when the packet arrived: t_j = T + D_t. The receiver
then computes the highest interval the sender could possibly be
in: highest_i = floor((t_j - T_0) / T_int). He also retrieves
the "i" interval index from the authentication tag. The receiver
can now proceed with the "safe packet" test. If highest_i < i +
d, then the sender is not yet in the interval during which it
discloses the key K_i. The packet is safe (but not necessarily
authentic). If the test fails, the packet is unsafe, and the
receiver MUST discard the packet.
3. Group MAC test: if the optional Group MAC tag is present and if
the session uses this feature, then verify the Group MAC
(Section 4.1.1). If the verification fails, the packet MUST be
immediately dropped. A packet that does not contain a Group MAC
tag whereas the session uses this feature MUST be immediately
dropped. On the opposite, if a packet contains a Group MAC tag
whereas the session does not use this feature, this tag MUST be
ignored.
4. Disclosed Key processing: When the packet discloses a key (i.e.,
with a Standard Authentication Tag, or with an Authentication Tag
with a Last Key of Old Chain Disclosure), the following tests are
performed:
* New key index test: the receiver checks whether a legitimate
key already exists with the same index (i.e., i-d). If such a
legitimate key exists, the receiver compares its value with
the current disclosed key and if they are identical, skips the
"Unverifiable key test" and "Key verification test". If such
a legitimate key exists but the values differ, the receiver
MUST discard the packet.
* Unverifiable key test: when the disclosed key index is new, it
is possible that no earlier disclosed and legitimate key
exists for this key chain, thereby preventing the verification
of the disclosed key. This happens when the disclosed key
belongs to the old key chain and no commitment to this old key
chain has ever been received (e.g., because the first
bootstrap packet received by a latecomer is for the current
key chain, and therefore includes a commitment to the current
key chain, not the previous one). When this happens, the
receiver MUST ignore the disclosed key (anyway useless) and
skip the Key verification test.
* Key verification test: If the disclosed key index is new and
the key can be verified, the receiver checks the legitimacy of
K_{i-d} by verifying, for some earlier disclosed and
legitimate key K_v (with v < i-d), that K_v and F^{i-d-
v}(K_{i-d}) are identical. In other words, the receiver
checks the disclosed key by computing the necessary number of
PRF functions to obtain a previously disclosed and legitimate
(i.e., verified) key. If the key verification fails, the
receiver MUST discard the packet. If the key verification
succeeds, this key is said to be legitimate and is stored by
the receiver, as well as all the keys between indexes v and
i-d.
5. When applicable, the receiver performs any congestion control
related action (i.e., the ALC or NORM headers are used by the
associated congestion control building block, if any), even if
the packet has not yet been authenticated [RFC5651]. If this
feature leads to a potential DoS attack (the attacker can send a
faked packet with a wrong sequence number to simulate packet
losses), it does not compromise the security features offered by
TESLA and enables a rapid reaction in front of actual congestion
problems.
6. The receiver then buffers the packet for a later authentication,
once the corresponding key will be disclosed (after d time
intervals) or deduced from another key (if all packets disclosing
this key are lost). In some situations, this packet might also
be discarded later, if it turns out that the receiver will never
be able to deduce the associated key.
7. Authentication test: Let v be the smallest index of the
legitimate keys known by the receiver so far. For all the new
keys K_w, with v < w <= i-d, that have been either disclosed by
this packet (i.e., K_{i-d}) or derived by K_{i-d} (i.e., keys in
interval {v+1,.. i-d-1}), the receiver verifies the authenticity
of the safe packets buffered for the corresponding interval w.
To authenticate one of the buffered packets P_h containing
message M_h protected with a MAC that used key index w, the
receiver will compute K'_w = F'(K_w) from which it can compute
MAC( K'_w, M_h). If this MAC does not equal the MAC stored in
the packet, the receiver MUST discard the packet. If the two
MACs are equal, the packet is successfully authenticated and the
receiver continues processing it.
8. Authenticated new key chain commitment processing: If the
authenticated packet contains a new key chain commitment and if
no verified commitment already exists, then the receiver stores
the commitment to the new key chain. Then, if there are non-
authenticated packets for a previous chain (i.e., the key chain
before the current one), all these packets can be discarded
(Section 4.4).
9. The receiver continues the ALC or NORM processing of all the
packets authenticated during the authentication test.
In this specification, a receiver using TESLA MUST immediately drop
unsafe packets. But the receiver MAY also decide, at any time, to
continue an ALC or NORM session in unsafe (insecure) mode, ignoring
TESLA extensions. There SHOULD be an explicit user action to that
purpose.
4.3.1. Discarding Unnecessary Packets Earlier
Following strictly the above steps can lead to excessive processing
overhead in certain situations. This is the case when a receiver
receives packets for an unwanted object with the ALC or NORM
protocols, i.e., an object in which the application (or the end user)
explicitly mentioned it is not interested. This is also the case
when a receiver receives packets for an already decoded object, or
when this object has been partitioned in several blocks, for an
already decoded block. When such a packet is received, which is
easily identified by looking at the receiver's status for the
incoming ALC or NORM packet, the receiver MUST also check that the
packet is a pure data packet that does not contain any signaling
information of importance for the session.
With ALC, a packet containing an "A" flag ("Close Session") or a "B"
flag ("Close Object") MUST NOT be discarded before having been
authenticated and processed normally. Otherwise, the receiver can
safely discard the incoming packet for instance just after step 1 of
Section 4.3. This optimization can dramatically reduce the
processing overhead by avoiding many useless authentication checks.
4.4. Flushing the Non-Authenticated Packets of a Previous Key Chain
In some cases, a receiver having experienced a very long
disconnection might have lost all the disclosures of the last key(s)
of a previous key chain. Let j be the index of this key chain for
which there remains non-authenticated packets. This receiver can
flush all the packets of the key chain j if he determines that:
o he has just switched to a chain of index j+2 (inclusive) or
higher;
o the sender has sent a commitment to the new key chain of index j+2
(Section 3.1.2.3). This situation requires that the receiver has
received a packet containing such a commitment and that he has
been able to check its integrity. In some cases, it might require
receiving a bootstrap information message for the current key
chain.
If one of the above two tests succeeds, the sender can discard all the awaiting packets since there is no way to authenticate them.5. Integration in the ALC and NORM Protocols
5.1. Authentication Header Extension Format
The integration of TESLA in ALC or NORM is similar and relies on the header extension mechanism defined in both protocols. More precisely, this document details the EXT_AUTH==1 header extension defined in [RFC5651]. Several fields are added in addition to the "HET" (Header Extension Type) and "HEL" (Header Extension Length) fields (Figure 9). 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HET (=1) | HEL | ASID | Type | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | | ~ ~ | Content | ~ ~ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 9: Format of the TESLA EXT_AUTH Header Extension The fields of the TESLA EXT_AUTH Header Extension are: "ASID" (Authentication Scheme IDentifier) field (4 bits): The "ASID" identifies the source authentication scheme or protocol in use. The association between the "ASID" value and the actual authentication scheme is defined out-of-band, at session startup. "Type" field (4 bits): The "Type" field identifies the type of TESLA information carried in this header extension. This specification defines the following types: * 0: Bootstrap information, sent by the sender periodically or after a direct time synchronization request; * 1: Standard Authentication Tag for the ongoing key chain, sent by the sender along with a packet;
* 2: Authentication Tag without Key Disclosure, sent by the
sender along with a packet;
* 3: Authentication Tag with a New Key Chain Commitment, sent by
the sender when approaching the end of a key chain;
* 4: Authentication Tag with a Last Key of Old Chain Disclosure,
sent by the sender some time after moving to a new key chain;
* 5: Direct time synchronization request, sent by a NORM
receiver. This type of message is invalid in the case of an
ALC session since ALC is restricted to unidirectional
transmissions. Yet, an external mechanism may provide the
direct time synchronization functionality;
* 6: Direct time synchronization response, sent by a NORM sender.
This type of message is invalid in the case of an ALC session
since ALC is restricted to unidirectional transmissions. Yet,
an external mechanism may provide the direct time
synchronization functionality.
"Content" field (variable length):
This is the TESLA information carried in the header extension,
whose type is given by the "Type" field.
5.2. Use of Authentication Header Extensions
Each packet sent by the session's sender MUST contain exactly one
TESLA EXT_AUTH Header Extension.
All receivers MUST recognize EXT_AUTH but MAY not be able to parse
its content, for instance, because they do not support TESLA. In
that case, these receivers MUST ignore the TESLA EXT_AUTH extensions.
In the case of NORM, the packets sent by receivers MAY contain a
direct synchronization request but MUST NOT contain any of the other
five TESLA EXT_AUTH Header Extensions.
5.2.1. EXT_AUTH Header Extension of Type Bootstrap Information
The "bootstrap information" TESLA EXT_AUTH (Type==0) MUST be sent in
a stand-alone control packet, rather than in a packet containing
application data. The reason for that is the large size of this
bootstrap information. By using stand-alone packets, the maximum
payload size of data packets is only affected by the (mandatory)
authentication information header extension.
With ALC, the "bootstrap information" TESLA EXT_AUTH MUST be sent in a control packet, i.e., containing no encoding symbol. With NORM, the "bootstrap information" TESLA EXT_AUTH MUST be sent in a NORM_CMD(APPLICATION) message.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- | HET (=1) | HEL (=46) | ASID | 0 | 0 | 0 |0|1|0| ^ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | d | 2 | 2 | 2 | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | 1 | 3 | 128 | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | 0 (reserved) | T_int | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | | + T_0 (NTP timestamp format) + | 5 | | | 2 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | N (Key Chain Length) | | b +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | y | Current Interval Index i | | t +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | e | | | s + + | | | | + Current Key Chain Commitment + | | (20 bytes) | | + + | | | | + + | | | v +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- | | ^ 1 + + | 2 | | | 8 . . | . Signature . | b . (128 bytes) . | y | | | t + + | e | | v s +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ --- | Group MAC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: Example: Format of the Bootstrap Information Message (Type 0) Using SHA-256/1024-Bit Signatures, the Default HMAC-SHA-256, and a Group MAC
For instance, Figure 10 shows the bootstrap information message when using the HMAC-SHA-256 transform for the PRF, MAC, and Group MAC functions, along with SHA-256/128 byte (1024 bit) key digital signatures (which also means that the "Signature" field is 128 bytes long). The TESLA EXT_AUTH Header Extension is then 184 bytes long (i.e., 46 words of 32 bits).5.2.2. EXT_AUTH Header Extension of Type Authentication Tag
The four "authentication tag" TESLA EXT_AUTH Header Extensions (Type 1, 2, 3, and 4) MUST be attached to the ALC or NORM packet (data or control packet) that they protect. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HET (=1) | HEL (=10) | ASID | 1 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | i (Interval Index of K'_i) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | | + Disclosed Key K_{i-d} + | (20 bytes) | + + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | MAC(K'_i, M) | + (16 bytes) + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 11: Example: Format of the Standard Authentication Tag (Type 1) Using the Default HMAC-SHA-256
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | HET (=1) | HEL (=5) | ASID | 2 | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | i (Interval Index of K'_i) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | + + | MAC(K'_i, M) | + (16 bytes) + | | + + | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 12: Example: Format of the Authentication Tag without Key Disclosure (Type 2) Using the Default HMAC-SHA-256 For instance, Figures 11 and 12 show the format of the authentication tags, respectively with and without the K_{i-d} key disclosure, when using the (default) HMAC-SHA-256 transform for the PRF and MAC functions. In these examples, the Group MAC feature is not used.5.2.3. EXT_AUTH Header Extension of Type Direct Time Synchronization Request
With NORM, the "direct time synchronization request" TESLA EXT_AUTH (Type==7) MUST be sent by a receiver in a NORM_CMD(APPLICATION) NORM packet. With ALC, the "direct time synchronization request" TESLA EXT_AUTH cannot be included in an ALC packet, since ALC is restricted to unidirectional transmissions, from the session's sender to the receivers. An external mechanism must be used with ALC for carrying direct time synchronization requests to the session's sender. In the case of direct time synchronization, it is RECOMMENDED that the receivers spread the transmission of direct time synchronization requests over the time (Section 2.3.1).5.2.4. EXT_AUTH Header Extension of Type Direct Time Synchronization Response
With NORM, the "direct time synchronization response" TESLA EXT_AUTH (Type==8) MUST be sent by the sender in a NORM_CMD(APPLICATION) message.
With ALC, the "direct time synchronization response" TESLA EXT_AUTH can be sent in an ALC control packet (i.e., containing no encoding symbol) or through the external mechanism used to carry the direct time synchronization request.6. Security Considerations
[RFC4082] discusses the security of TESLA in general. These considerations apply to the present specification, namely: o great care must be taken in the timing aspects. In particular, the D_t parameter is critical and must be correctly initialized; o if the sender realizes that the key disclosure schedule is not appropriate, then the current session MUST be closed and a new one created. Indeed, Section 3.1.3 requires that these parameters be fixed during the whole session. o when the verifier that authenticates the incoming packets and the application that uses the data are two different components, there is a risk that an attacker located between these components inject faked data. Similarly, when the verifier and the secure timing system are two different components, there is a risk that an attacker located between these components inject faked timing information. For instance, when the verifier reads the local time by means of a dedicated system call (e.g., gettimeofday()), if an attacker controls the host, he may catch the system call and return a faked time information. The current specification discusses additional aspects with more details.6.1. Dealing with DoS Attacks
TESLA introduces new opportunities for an attacker to mount DoS attacks. For instance, an attacker can try to saturate the processing capabilities of the receiver (faked packets are easy to create but checking them requires computing a MAC over the packet or sometimes checking a digital signature as with the bootstrap and direct time synchronization response messages). An attacker can also try to saturate the receiver's memory (since authentication is delayed and non-authenticated packets will accumulate), or to make the receiver believe that a congestion has happened (since congestion control MUST be performed before authenticating incoming packets, Section 4.3).
In order to mitigate these attacks, it is RECOMMENDED to use the Group MAC scheme (Section 3.3.3). No mitigation is possible if a group member acts as an attacker with Group MAC. Generally, it is RECOMMENDED that the amount of memory used to store incoming packets waiting to be authenticated be limited to a reasonable value.6.2. Dealing With Replay Attacks
Replay attacks, whereby an attacker stores a valid message and replays it later, can have significant impacts, depending on the message type. Two levels of impacts must be distinguished: o within the TESLA protocol, and o within the ALC or NORM protocol.6.2.1. Impacts of Replay Attacks on TESLA
Replay attacks can impact the TESLA component itself. We review here the potential impacts of such an attack depending on the TESLA message type: o bootstrap information: Since most parameters contained in a bootstrap information message are static, replay attacks have no consequences. The fact that the "i" and "K_i" fields can be updated in subsequent bootstrap information messages does not create a problem either, since all "i" and "K_i" fields sent remain valid. Finally, a receiver that successfully initialized its TESLA component MUST ignore the following messages (see Section 4.2.1 for an exception to this rule), which voids replay attacks, unless he missed all the commitments to a new key chain (e.g., after a long disconnection) (Section 3.2.1). o direct time synchronization request: If the Group MAC scheme is used, an attacker that is not a member of the group can replay a packet and oblige the sender to respond, which requires digitally signing the response, a time-consuming process. If the Group MAC scheme is not used, an attacker can easily forge a request anyway. In both cases, the attack will not compromise the TESLA component, but might create a DoS. If this is a concern, it is RECOMMENDED, when the Group MAC scheme is used, that the sender verify the "t_r" NTP timestamp contained in the request and respond only if this value is strictly larger than the previous one received from this receiver. When the Group MAC scheme is not used, this attack can be mitigated by limiting the number of requests per second that will be processed.
o direct time synchronization response: Upon receiving a response, a
receiver who has no pending request MUST immediately drop the
packet. If this receiver has previously issued a request, he
first checks the Group MAC (if applicable), then the "t_r" field,
to be sure it is a response to his request, and finally the
digital signature. A replayed packet will be dropped during these
verifications, without compromising the TESLA component.
o other messages, containing an authentication tag: Replaying a
packet containing a TESLA authentication tag will never compromise
the TESLA component itself (but perhaps the underlying ALC or NORM
component, see below).
To conclude, TESLA itself is robust in front of replay attacks.
6.2.2. Impacts of Replay Attacks on NORM
We review here the potential impacts of a replay attack on the NORM
component. Note that we do not consider here the protocols that
could be used along with NORM, for instance, the congestion control
protocols.
First, let us consider replay attacks within a given NORM session.
NORM defines a "sequence" field that can be used to protect against
replay attacks [RFC5740] within a given NORM session. This
"sequence" field is a 16-bit value that is set by the message
originator (sender or receiver) as a monotonically increasing number
incremented with each NORM message transmitted. It is RECOMMENDED
that a receiver check this "sequence" field and drop messages
considered as replayed. Similarly, it is RECOMMENDED that a sender
check this sequence, for each known receiver, and drop messages
considered as replayed. In both cases, checking this "sequence"
field SHOULD be done before TESLA processing of the packet: if the
"sequence" field has not been corrupted, the replay attack will
immediately be identified; otherwise, the packet will fail the TESLA
authentication test. This analysis shows that NORM itself is robust
in front of replay attacks within the same session.
Now let us consider replay attacks across several NORM sessions.
Since the key chain used in each session MUST differ, a packet
replayed in a subsequent session will be identified as unauthentic.
Therefore, NORM is robust in front of replay attacks across different
sessions.
6.2.3. Impacts of Replay Attacks on ALC
We review here the potential impacts of a replay attack on the ALC component. Note that we do not consider here the protocols that could be used along with ALC, for instance, the layered or wave-based congestion control protocols. First, let us consider replay attacks within a given ALC session: o Regular packets containing an authentication tag: a replayed message containing an encoding symbol will be detected once authenticated, thanks to the object/block/symbol identifiers, and will be silently discarded. This kind of replay attack is only penalizing in terms of memory and processing load, but does not compromise the ALC behavior. o Control packets containing an authentication tag: ALC control packets, by definition, do not include any encoding symbol and therefore do not include any object/block/symbol identifier that would enable a receiver to identify duplicates. However, a sender has a very limited number of reasons to send control packets. More precisely: * At the end of the session, a "Close Session" ("A" flag) packet is sent. Replaying this packet has no impact since the receivers already left. * Similarly, replaying a packet containing a "Close Object" ("B" flag) has no impact since this object is probably already marked as closed by the receiver. This analysis shows that ALC itself is robust in front of replay attacks within the same session. Now let us consider replay attacks across several ALC sessions. Since the key chain used in each session MUST differ, a packet replayed in a subsequent session will be identified as unauthentic. Therefore, ALC is robust in front of replay attacks across different sessions.6.3. Security of the Back Channel
As specified in Section 1.1, this specification does not consider the packets that may be sent by receivers, for instance, NORM's feedback packets. When a back channel is used, its security is critical to the global security, and an appropriate security mechanism MUST be used. [RMT-SIMPLE-AUTH] describes several techniques that can be used to that purpose. However, the authentication and integrity
verification of the packets sent by receivers on the back channel, if any, is out of the scope of this document.7. IANA Considerations
IANA has registered the following attributes according to this document. The registries are provided by [RFC4442] under the "Timed Efficient Stream Loss-tolerant Authentication (TESLA) Parameters" registry [TESLA-REG]. Following the policies outlined in [RFC4442], the values in the range up to 240 (including 240) for the following attributes are assigned after expert review by the MSEC working group or its designated successor. The values in the range from 241 to 255 are reserved for private use. Cryptographic Pseudo-Random Function, TESLA-PRF: All implementations MUST support HMAC-SHA-256 (default). +------------------------+-------+ | PRF name | Value | +------------------------+-------+ | HMAC-SHA1 | 0 | | HMAC-SHA-224 | 1 | | HMAC-SHA-256 (default) | 2 | | HMAC-SHA-384 | 3 | | HMAC-SHA-512 | 4 | +------------------------+-------+ Cryptographic Message Authentication Code (MAC) Function, TESLA-MAC: All implementations MUST support HMAC-SHA-256 (default). These MAC schemes are used both for the computing of regular MAC and the Group MAC (if applicable). +------------------------+-------+ | MAC name | Value | +------------------------+-------+ | HMAC-SHA1 | 0 | | HMAC-SHA-224 | 1 | | HMAC-SHA-256 (default) | 2 | | HMAC-SHA-384 | 3 | | HMAC-SHA-512 | 4 | +------------------------+-------+ Furthermore, IANA has created two new registries. Here also, the values in the range up to 240 (including 240) for the following attributes are assigned after expert review by the MSEC working group or its designated successor. The values in the range from 241 to 255 are reserved for private use.
Signature Encoding Algorithm, TESLA-SIG-ALGO: All implementations
MUST support RSASSA-PKCS1-v1_5 (default).
+-----------------------------+-------+
| Signature Algorithm Name | Value |
+-----------------------------+-------+
| INVALID | 0 |
| RSASSA-PKCS1-v1_5 (default) | 1 |
| RSASSA-PSS | 2 |
+-----------------------------+-------+
Signature Cryptographic Function, TESLA-SIG-CRYPTO-FUNC: All
implementations MUST support SHA-256 (default).
+-----------------------------+-------+
| Cryptographic Function Name | Value |
+-----------------------------+-------+
| INVALID | 0 |
| SHA-1 | 1 |
| SHA-224 | 2 |
| SHA-256 (default) | 3 |
| SHA-384 | 4 |
| SHA-512 | 5 |
+-----------------------------+-------+
8. Acknowledgments
The authors are grateful to Yaron Sheffer, Brian Weis, Ramu
Panayappan, Ran Canetti, David L. Mills, Brian Adamson, and Lionel
Giraud for their valuable comments while preparing this document.
The authors are also grateful to Brian Weis for the digital signature
details.
9. References
9.1. Normative References
[RFC1305] Mills, D., "Network Time Protocol (Version 3)
Specification, Implementation", RFC 1305,
March 1992.
[RFC2119] Bradner, S., "Key words for use in RFCs to
Indicate Requirement Levels", BCP 14, RFC 2119,
March 1997.
[RFC4082] Perrig, A., Song, D., Canetti, R., Tygar, J., and B. Briscoe, "Timed Efficient Stream Loss-Tolerant Authentication (TESLA): Multicast Source Authentication Transform Introduction", RFC 4082, June 2005. [RFC5651] Luby, M., Watson, M., and L. Vicisano, "Layered Coding Transport (LCT) Building Block", RFC 5651, October 2009. [RFC5740] Adamson, B., Bormann, C., Handley, M., and J. Macker, "NACK-Oriented Reliable Multicast (NORM) Transport Protocol", RFC 5740, November 2009. [RFC5775] Luby, M., Watson, M., and L. Vicisano, "Asynchronous Layered Coding (ALC) Protocol Instantiation", RFC 5775, April 2010. [TESLA-REG] "TESLA Parameters IANA Registry", http://www.iana.org.9.2. Informative References
[NTP-NTPv4] Burbank, J., Kasch, W., Martin, J., Ed., and D. Mills, "The Network Time Protocol Version 4 Protocol And Algorithm Specification", Work in Progress, October 2009. [Perrig04] Perrig, A. and J. Tygar, "Secure Broadcast Communication in Wired and Wireless Networks", Kluwer Academic Publishers ISBN 0-7923-7650-1, 2004. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-Hashing for Message Authentication", RFC 2104, February 1997. [RFC3447] Jonsson, J. and B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC 3447, February 2003. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004.
[RFC4330] Mills, D., "Simple Network Time Protocol (SNTP) Version 4 for IPv4, IPv6 and OSI", RFC 4330, January 2006. [RFC4359] Weis, B., "The Use of RSA/SHA-1 Signatures within Encapsulating Security Payload (ESP) and Authentication Header (AH)", RFC 4359, January 2006. [RFC4383] Baugher, M. and E. Carrara, "The Use of Timed Efficient Stream Loss-Tolerant Authentication (TESLA) in the Secure Real-time Transport Protocol (SRTP)", RFC 4383, February 2006. [RFC4442] Fries, S. and H. Tschofenig, "Bootstrapping Timed Efficient Stream Loss-Tolerant Authentication (TESLA)", RFC 4442, March 2006. [RMT-FLUTE] Paila, T., Walsh, R., Luby, M., Lehtonen, R., and V. Roca, "FLUTE - File Delivery over Unidirectional Transport", Work in Progress, August 2009. [RMT-SIMPLE-AUTH] Roca, V., "Simple Authentication Schemes for the ALC and NORM Protocols", Work in Progress, October 2009.
Authors' Addresses
Vincent Roca INRIA 655, av. de l'Europe Inovallee; Montbonnot ST ISMIER cedex 38334 France EMail: vincent.roca@inria.fr URI: http://planete.inrialpes.fr/~roca/ Aurelien Francillon INRIA 655, av. de l'Europe Inovallee; Montbonnot ST ISMIER cedex 38334 France EMail: aurelien.francillon@inria.fr URI: http://planete.inrialpes.fr/~francill/ Sebastien Faurite INRIA 655, av. de l'Europe Inovallee; Montbonnot ST ISMIER cedex 38334 France EMail: faurite@lcpc.fr