Internet Research Task Force (IRTF) F. GontInternet-Draft SI6 NetworksIntended status: Informational I. ArceExpires: August 6, 2021 Quarkslab February 2, 2021On the Generation of Transient Numeric Identifiersdraft-irtf-pearg-numeric-ids-generation-07Abstract This document performs an analysis of the security and privacy implications of different types of "transient numeric identifiers" used in IETF protocols, and tries to categorize them based on their interoperability requirements and their associated failure severity when such requirements are not met. Subsequently, it provides advice on possible algorithms that could be employed to satisfy the interoperability requirements of each identifier category, while minimizing the negative security and privacy implications, thus providing guidance to protocol designers and protocol implementers. Finally, it describes a number of algorithms that have been employed in real implementations to generate transient numeric identifiers, and analyzes their security and privacy properties. This document is a product of the Privacy Enhancement and Assessment Research Group (PEARG) in the IRTF.Status of This Memo This Internet-Draft is submitted in full conformance with the provisions ofBCP 78 andBCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is athttps://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on August 6, 2021.Gont & Arce Expires August 6, 2021 [Page 1]
Internet-Draft Generation of Transient Numeric IDs February 2021Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject toBCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document.Table of Contents1. Introduction . . . . . . . . . . . . . . . . . . . . . . . .32. Terminology . . . . . . . . . . . . . . . . . . . . . . . . .53. Threat Model . . . . . . . . . . . . . . . . . . . . . . . .5 4. Issues with the Specification of Transient Numeric Identifiers . . . . . . . . . . . . . . . . . . . . . . . . .65. Protocol Failure Severity . . . . . . . . . . . . . . . . . .76. Categorizing Transient Numeric Identifiers . . . . . . . . .7 7. Common Algorithms for Transient Numeric Identifier Generation 107.1. Category #1: Uniqueness (soft failure) . . . . . . . . .107.2. Category #2: Uniqueness (hard failure) . . . . . . . . .13 7.3. Category #3: Uniqueness, stable within context (soft failure) . . . . . . . . . . . . . . . . . . . . . . . .13 7.4. Category #4: Uniqueness, monotonically increasing within context (hard failure) . . . . . . . . . . . . . . . . .15 8. Common Vulnerabilities Associated with Transient Numeric Identifiers . . . . . . . . . . . . . . . . . . . . . . . . .218.1. Network Activity Correlation . . . . . . . . . . . . . .218.2. Information Leakage . . . . . . . . . . . . . . . . . . .228.3. Fingerprinting . . . . . . . . . . . . . . . . . . . . .23 8.4. Exploitation of the Semantics of Transient Numeric Identifiers . . . . . . . . . . . . . . . . . . . . . . .24 8.5. Exploitation of Collisions of Transient Numeric Identifiers . . . . . . . . . . . . . . . . . . . . . . .24 8.6. Exploitation of Predictable Transient Numeric Identifiers for Injection Attacks . . . . . . . . . . . . . . . . . .248.7. Cryptanalysis . . . . . . . . . . . . . . . . . . . . . .259. Vulnerability Assessment of Transient Numeric Identifiers . .269.1. Category #1: Uniqueness (soft failure) . . . . . . . . .269.2. Category #2: Uniqueness (hard failure) . . . . . . . . .26 9.3. Category #3: Uniqueness, stable within context (soft failure) . . . . . . . . . . . . . . . . . . . . . . . .27 9.4. Category #4: Uniqueness, monotonically increasing within context (hard failure) . . . . . . . . . . . . . . . . .2710. IANA Considerations . . . . . . . . . . . . . . . . . . . . .30Gont & Arce Expires August 6, 2021 [Page 2]
Internet-Draft Generation of Transient Numeric IDs February 202111. Security Considerations . . . . . . . . . . . . . . . . . . .3012. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .3013. References . . . . . . . . . . . . . . . . . . . . . . . . .3013.1. Normative References . . . . . . . . . . . . . . . . . .3013.2. Informative References . . . . . . . . . . . . . . . . .32Appendix A. Algorithms and Techniques with Known Issues . . . .37A.1. Predictable Linear Identifiers Algorithm . . . . . . . .37A.2. Random-Increments Algorithm . . . . . . . . . . . . . . .39A.3. Re-using Identifiers Across Different Contexts . . . . .40 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . .401. Introduction Networking protocols employ a variety of transient numeric identifiers for different protocol objects, such as IPv4 and IPv6 Fragment Identifiers [RFC0791] [RFC8200], IPv6 Interface Identifiers (IIDs) [RFC4291], transport protocol ephemeral port numbers [RFC6056], TCP Initial Sequence Numbers (ISNs) [RFC0793], and DNS Transaction IDs (TxIDs) [RFC1035]. These identifiers usually have specific interoperability requirements (e.g. uniqueness during a specified period of time) that must be satisfied such that they do not result in negative interoperability implications, and an associated failure severity when such requirements are not met, ranging from soft to hard failures. For more than 30 years, a large number of implementations of the TCP/ IP protocol suite have been subject to a variety of attacks, with effects ranging from Denial of Service (DoS) or data injection, to information leakages that could be exploited for pervasive monitoring [RFC7258]. The root cause of these issues has been, in many cases, the poor selection of transient numeric identifiers in such protocols, usually as a result of insufficient or misleading specifications. While it is generally trivial to identify an algorithm that can satisfy the interoperability requirements of a given transient numeric identifier, empirical evidence exists that doing so without negatively affecting the security and/or privacy properties of the aforementioned protocols is prone to error [I-D.irtf-pearg-numeric-ids-history]. For example, implementations have been subject to security and/or privacy issues resulting from: o Predictable IPv4 or IPv6 Fragment Identifiers (see e.g. [Sanfilippo1998a], [RFC6274], and [RFC7739]) o Predictable IPv6 IIDs (see e.g. [RFC7721], [RFC7707], and [RFC7217])Gont & Arce Expires August 6, 2021 [Page 3]
Internet-Draft Generation of Transient Numeric IDs February 2021 o Predictable transport protocol ephemeral port numbers (see e.g. [RFC6056] and [Silbersack2005]) o Predictable TCP Initial Sequence Numbers (ISNs) (see e.g. [Morris1985], [Bellovin1989], and [RFC6528]) o Predictable initial timestamp in TCP timestamps Options (see e.g. [TCPT-uptime] and [RFC7323]) o Predictable DNS TxIDs (see e.g. [Schuba1993] and [Klein2007]) Recent history indicates that when new protocols are standardized or new protocol implementations are produced, the security and privacy properties of the associated transient numeric identifiers tend to be overlooked, and inappropriate algorithms to generate transient numeric identifiers are either suggested in the specifications or selected by implementers. As a result, it should be evident that advice in this area is warranted. We note that the use of cryptographic techniques may readily mitigate some of the issues arising from predictable transient numeric identifiers. For example, cryptographic integrity and authentication can readily mitigate data injection attacks even in the presence of predictable transient numeric identifiers (such as "sequence numbers"). However, use of flawed algorithms (such as global counters) for generating transient numeric identifiers could still result in information leakages even when cryptographic techniques are employed. This document contains a non-exhaustive survey of transient numeric identifiers employed in various IETF protocols, and aims to categorize such identifiers based on their interoperability requirements, and the associated failure severity when such requirements are not met. Subsequently, it provides advice on possible algorithms that could be employed to satisfy the interoperability requirements of each category, while minimizing negative security and privacy implications. Finally, it analyzes several algorithms that have been employed in real implementations to meet such requirements, and analyzes their security and privacy properties. This document represents the consensus of the Privacy Enhancement and Assessment Research Group (PEARG).Gont & Arce Expires August 6, 2021 [Page 4]
Internet-Draft Generation of Transient Numeric IDs February 20212. Terminology Transient Numeric Identifier: A data object in a protocol specification that can be used to definitely distinguish a protocol object (a datagram, network interface, transport protocol endpoint, session, etc.) from all other objects of the same type, in a given context. Transient numeric identifiers are usually defined as a series of bits, and represented using integer values. These identifiers are typically dynamically selected, as opposed to statically-assigned numeric identifiers (see e.g. [IANA-PROT]). We note that different transient numeric identifiers may have additional requirements or properties depending on their specific use in a protocol. We use the term "transient numeric identifier" (or simply "numeric identifier" or "identifier" as short forms) as a generic term to refer to any data object in a protocol specification that satisfies the identification property stated above. Failure Severity: The consequences of a failure to comply with the interoperability requirements of a given identifier. Severity considers the worst potential consequence of a failure, determined by the system damage and/or time lost to repair the failure. In this document we define two types of failure severity: "soft failure" and "hard failure". Soft Failure: A soft failure is a recoverable condition in which a protocol does not operate in the prescribed manner but normal operation can be resumed automatically in a short period of time. For example, a simple packet-loss event that is subsequently recovered with a packet-retransmission can be considered a soft failure. Hard Failure: A hard failure is a non-recoverable condition in which a protocol does not operate in the prescribed manner or it operates with excessive degradation of service. For example, an established TCP connection that is aborted due to an error condition constitutes, from the point of view of the transport protocol, a hard failure, since it enters a state from which normal operation cannot be resumed.3. Threat Model Throughout this document, we assume an attacker does not have physical or logical access to the system(s) being attacked, and cannot observe the packets being transferred between the sender and the receiver(s) of the target protocol (if any). However, we assumeGont & Arce Expires August 6, 2021 [Page 5]
Internet-Draft Generation of Transient Numeric IDs February 2021 the attacker can send any traffic to the target device(s), to e.g. sample transient numeric identifiers employed by such device(s).4. Issues with the Specification of Transient Numeric Identifiers While assessing protocol specifications regarding the use of transient numeric identifiers, we have found that most of the issues discussed in this document arise as a result of one of the following conditions: o Protocol specifications that under-specify the requirements for their transient numeric identifiers o Protocol specifications that over-specify their transient numeric identifiers o Protocol implementations that simply fail to comply with the specified requirements A number of protocol specifications (too many of them) have simply overlooked the security and privacy implications of transient numeric identifiers [I-D.irtf-pearg-numeric-ids-history]. Examples of them are the specification of TCP ephemeral ports in [RFC0793], the specification of TCP sequence numbers in [RFC0793], or the specification of the DNS TxID in [RFC1035]. On the other hand, there are a number of protocol specifications that over-specify some of their associated transient numeric identifiers. For example, [RFC4291] essentially overloads the semantics of IPv6 Interface Identifiers (IIDs) by embedding link-layer addresses in the IPv6 IIDs, when the interoperability requirement of uniqueness could be achieved in other ways that do not result in negative security and privacy implications [RFC7721]. Similarly, [RFC2460] suggested the use of a global counter for the generation of Fragment Identification values, when the interoperability properties of uniqueness per {IPv6 Source Address, IPv6 Destination Address} could be achieved with other algorithms that do not result in negative security and privacy implications [RFC7739]. Finally, there are protocol implementations that simply fail to comply with existing protocol specifications. For example, some popular operating systems (notably Microsoft Windows) still fail to implement transport protocol ephemeral port randomization, as recommended in [RFC6056].Gont & Arce Expires August 6, 2021 [Page 6]
Internet-Draft Generation of Transient Numeric IDs February 20215. Protocol Failure SeveritySection 2 defines the concept of "Failure Severity", along with two types of failure severities that we employ throughout this document: soft and hard. Our analysis of the severity of a failure is performed from the point of view of the protocol in question. However, the corresponding severity on the upper protocol (or application) might not be the same as that of the protocol in question. For example, a TCP connection that is aborted might or might not result in a hard failure of the upper application: if the upper application can establish a new TCP connection without any impact on the application, a hard failure at the TCP protocol may have no severity at the application level. On the other hand, if a hard failure of a TCP connection results in excessive degradation of service at the application layer, it will also result in a hard failure at the application.6. Categorizing Transient Numeric Identifiers This section includes a non-exhaustive survey of transient numeric identifiers, and proposes a number of categories that can accommodate these identifiers based on their interoperability requirements and their associated failure severity (soft or hard) +------------------+---------------------------------+--------------+ | Identifier | Interoperability Requirements | Failure | | | | Severity | +------------------+---------------------------------+--------------+ | IPv6 Frag ID | Uniqueness (for IP address | Soft/Hard | | | pair) | (1) | +------------------+---------------------------------+--------------+ | IPv6 IID | Uniqueness (and stable within | Soft (3) | | | IPv6 prefix) (2) | | +------------------+---------------------------------+--------------+ | TCP ISN | Monotonically-increasing (4) | Hard (4) | +------------------+---------------------------------+--------------+ | TCP initial | Monotonically-increasing (5) | Hard (5) | | timestamps | | | +------------------+---------------------------------+--------------+ | TCP eph. port | Uniqueness (for connection ID) | Hard | +------------------+---------------------------------+--------------+ | IPv6 Flow Label | Uniqueness | None (6) | +------------------+---------------------------------+--------------+ | DNS TxID | Uniqueness | None (7) | +------------------+---------------------------------+--------------+ Table 1: Survey of Transient Numeric IdentifiersGont & Arce Expires August 6, 2021 [Page 7]
Internet-Draft Generation of Transient Numeric IDs February 2021 NOTE: (1) While a single collision of Fragment ID values would simply lead to a single packet drop (and hence a "soft" failure), repeated collisions at high data rates might trash the Fragment ID space, leading to a hard failure [RFC4963]. (2) While the interoperability requirements are simply that the Interface ID results in a unique IPv6 address, for operational reasons it is typically desirable that the resulting IPv6 address (and hence the corresponding Interface ID) be stable within each network [RFC7217] [RFC8064]. (3) While IPv6 Interface IDs must result in unique IPv6 addresses, IPv6 Duplicate Address Detection (DAD) [RFC4862] allows for the detection of duplicate addresses, and hence such Interface ID collisions can be recovered. (4) In theory, there are no interoperability requirements for TCP Initial Sequence Numbers (ISNs), since the TIME-WAIT state and TCP's "quiet time" concept take care of old segments from previous incarnations of a connection. However, a widespread optimization allows for a new incarnation of a previous connection to be created if the ISN of the incoming SYN is larger than the last sequence number seen in that direction for the previous incarnation of the connection. Thus, monotonically-increasing TCP ISNs allow for such optimization to work as expected [RFC6528], and can help avoid connection-establishment failures. (5) Strictly speaking, there are no interoperability requirements for the *initial* TCP timestamp employed by a TCP instance (i.e., the TS Value (TSval) in a segment with the SYN bit set). However, some TCP implementations allow a new incarnation of a previous connection to be created if the TSval of the incoming SYN is larger than the last TSval seen in that direction for the previous incarnation of the connection (please see [RFC6191]). Thus, monotonically-increasing TCP initial timestamps (across connections to the same endpoint) allow for such optimization to work as expected [RFC6191], and can help avoid connection- establishment failures. (6)Gont & Arce Expires August 6, 2021 [Page 8]
Internet-Draft Generation of Transient Numeric IDs February 2021 The IPv6 Flow Label is typically employed for load sharing [RFC7098], along with the Source and Destination IPv6 addresses. Reuse of a Flow Label value for the same set {Source Address, Destination Address} would typically cause both flows to be multiplexed onto the same link. However, as long as this does not occur deterministically, it will not result in any negative implications. (7) DNS TxIDs are employed, together with the Source Address, Destination Address, Source Port, and Destination Port, to match DNS requests and responses. However, since an implementation knows which DNS requests were sent for that set of {Source Address, Destination Address, Source Port, and Destination Port, DNS TxID}, a collision of TxID would result, if anything, in a small performance penalty (the response would nevertheless be discarded when it is found that it does not answer the query sent in the corresponding DNS query). Based on the survey above, we can categorize identifiers as follows: +-----+---------------------------------------+---------------------+ | Cat | Category | Sample Proto IDs | | # | | | +-----+---------------------------------------+---------------------+ | 1 | Uniqueness (soft failure) | IPv6 Flow L., DNS | | | | TxIDs | +-----+---------------------------------------+---------------------+ | 2 | Uniqueness (hard failure) | IPv6 Frag ID, TCP | | | | ephemeral port | +-----+---------------------------------------+---------------------+ | 3 | Uniqueness, stable within context | IPv6 IIDs | | | (soft failure) | | +-----+---------------------------------------+---------------------+ | 4 | Uniqueness, monotonically increasing | TCP ISN, TCP | | | within context (hard failure) | initial timestamps | +-----+---------------------------------------+---------------------+ Table 2: Identifier Categories We note that Category #4 could be considered a generalized case of category #3, in which a monotonically increasing element is added to a stable (within context) element, such that the resulting identifiers are monotonically increasing within a specified context. That is, the same algorithm could be employed for both #3 and #4, given appropriate parameters.Gont & Arce Expires August 6, 2021 [Page 9]
Internet-Draft Generation of Transient Numeric IDs February 20217. Common Algorithms for Transient Numeric Identifier Generation The following subsections describe some sample algorithms that can be employed for generating transient numeric identifiers for each of the categories above. All of the variables employed in the algorithms of the following subsections are of "unsigned integer" type, except for the "retry" variable, that is of (signed) "integer" type.7.1. Category #1: Uniqueness (soft failure) The requirement of uniqueness with a soft failure severity can be complied with a Pseudo-Random Number Generator (PRNG). We note that since the premise is that collisions of transient numeric identifiers of this category only leads to soft failures, in many cases, the algorithm might not need to check the suitability of a selected identifier (i.e., suitable_id() could always return "true"). In scenarios where e.g. simultaneous use of a given numeric ID is undesirable and the implementation detects such condition, an implementation may opt to select the next available identifier in the same sequence, or select another random number.Section 7.1.1 is an implementation of the former strategy, whileSection 7.1.2 is an implementation of the later. Typically, the algorithm inSection 7.1.2 results in a more uniform distribution of the generated transient numeric identifiers. However, for transient numeric identifiers where an implementation typically keeps local state about unsuitable/used identifiers, the algorithm inSection 7.1.2 may require many more iterations than the algorithm inSection 7.1.1 to generate a suitable transient numeric identifier. This will usually be affected by the current usage ratio of transient numeric identifiers (i.e., number of numeric identifiers considered suitable / total number of numeric identifiers) and other parameters. Therefore, in such cases many implementations tend to prefer the algorithm inSection 7.1.1 over the algorithm inSection 7.1.2.7.1.1. Simple Randomization AlgorithmGont & Arce Expires August 6, 2021 [Page 10]
Internet-Draft Generation of Transient Numeric IDs February 2021 /* Transient Numeric ID selection function */ id_range = max_id - min_id + 1; next_id = min_id + (random() % id_range); retry = id_range; do { if (suitable_id(next_id)) { return next_id; } if (next_id == max_id) { next_id = min_id; } else { next_id++; } retry--; } while (retry > 0); return ERROR; NOTE: random() is a function that returns a pseudo-random unsigned integer number of appropriate size. Note that the output needs to be unpredictable, and typical implementations of the POSIX random() function do not necessarily meet this requirement. See [RFC4086] for randomness requirements for security. Beware that "adapting" the length of the output of random() with a modulo operator (e.g., C language's "%") may change the distribution of the PRNG. The function suitable_id() can check, when possible and desirable, whether a selected transient numeric identifier is suitable (e.g. it is not already in use). Depending on how/where the numeric identifier is used, it may or may not be possible (or even desirable) to check whether the numeric identifier is in use (or whether it has been recently employed). When an identifier is found to be unsuitable, this algorithm selects the next available numeric identifier in sequence. Even when this algorithm selects numeric IDs randomly, it is biased towards the first available numeric ID after a sequence of unavailable numeric IDs. For example, if this algorithm is employed for transport protocol ephemeral port randomization [RFC6056] and the local list of unsuitable port numbers (e.g., registered port numbers that should not be used for ephemeralGont & Arce Expires August 6, 2021 [Page 11]
Internet-Draft Generation of Transient Numeric IDs February 2021 ports) is significant, an attacker may actually have a significantly better chance of guessing a port number. All the variables (in this and all the algorithms discussed in this document) are unsigned integers. Assuming the randomness requirements for the PRNG are met (see [RFC4086]), this algorithm does not suffer from any of the issues discussed inSection 8.7.1.2. Another Simple Randomization Algorithm The following pseudo-code illustrates another algorithm for selecting a random transient numeric identifier which, in the event a selected identifier is found to be unsuitable (e.g., already in use), another identifier is randomly selected: /* Transient Numeric ID selection function */ id_range = max_id - min_id + 1; retry = id_range; do { next_id = min_id + (random() % id_range); if (suitable_id(next_id)) { return next_id; } retry--; } while (retry > 0); return ERROR; This algorithm might be unable to select a transient numeric identifier (i.e., return "ERROR") even if there are suitable identifiers available, in cases where a large number of identifiers are found to be unsuitable (e.g. "in use"). The same considerations fromSection 7.1.1 with respect to the properties of random() and the adaptation of its output length apply to this algorithm. Assuming the randomness requirements for the PRNG are met (see [RFC4086]), this algorithm does not suffer from any of the issues discussed inSection 8.Gont & Arce Expires August 6, 2021 [Page 12]
Internet-Draft Generation of Transient Numeric IDs February 20217.2. Category #2: Uniqueness (hard failure) One of the most trivial approaches for generating unique transient numeric identifier (with a hard failure severity) is to reduce the identifier reuse frequency by generating the numeric identifiers with a monotonically-increasing function (e.g. linear). As a result, any of the algorithms described inSection 7.4 ("Category #4: Uniqueness, monotonically increasing within context (hard failure)") can be readily employed for complying with the requirements of this transient numeric identifier category. In cases where suitability (e.g. uniqueness) of the selected identifiers can be definitely assessed by the local system, any of the algorithms described inSection 7.1 ("Category #1: Uniqueness (soft failure)") can be readily employed for complying with the requirements of this numeric identifier category. NOTE: In the case of e.g. TCP ephemeral ports or TCP ISNs, a transient numeric identifier that might seem suitable from the perspective of the local system, might actually be unsuitable from the perspective of the remote system (e.g., because there is state associated with the selected identifier at the remote system). Therefore, in such cases it is not possible employ the algorithms fromSection 7.1 ("Category #1: Uniqueness (soft failure)").7.3. Category #3: Uniqueness, stable within context (soft failure) The goal of the following algorithm is to produce identifiers that are stable for a given context (identified by "CONTEXT"), but that change when the aforementioned context changes. In order to avoid storing in memory the transient numeric identifiers computed for each CONTEXT, the following algorithm employs a calculated technique (as opposed to keeping state in memory) to generate a stable transient numeric identifier for each given context.Gont & Arce Expires August 6, 2021 [Page 13]
Internet-Draft Generation of Transient Numeric IDs February 2021 /* Transient Numeric ID selection function */ id_range = max_id - min_id + 1; retry = 0; do { offset = F(CONTEXT, retry, secret_key); next_id = min_id + (offset % id_range); if (suitable_id(next_id)) { return next_id; } retry++; } while (retry <= MAX_RETRIES); return ERROR; In this algorithm, the function F() provides a stateless and stable per-CONTEXT offset, where CONTEXT is the concatenation of all the elements that define the given context. For example, if this algorithm is expected to produce IPv6 IIDs that are unique per network interface and SLAAC autoconfiguration prefix, the CONTEXT should be the concatenation of e.g. the network interface index and the SLAAC autoconfiguration prefix (please see [RFC7217] for an implementation of this algorithm for generation of stable IPv6 IIDs). F() is a pseudorandom function (PRF). It must not computable from the outside (without knowledge of the secret key). F() must also be difficult to reverse, such that it resists attempts to obtain the secret_key, even when given samples of the output of F() and knowledge or control of the other input parameters. F() should produce an output of at least as many bits as required for the transient numeric identifier. SipHash-2-4 (128-bit key, 64-bit output) [SipHash] and BLAKE3 (256-bit key, arbitrary-length output) [BLAKE3] are two possible options for F(). Alternatively, F() could be implemented with a keyed-hash message authentication code (HMAC) [RFC2104]. HMAC-SHA-256 [FIPS-SHS] would be one possible option for such implementation alternative. Note: Use of HMAC-MD5 [RFC1321] is not recommended for F() [RFC6151]. The result of F() is no more secure than the secret key, and therefore 'secret_key' must be unknown to the attacker, and must beGont & Arce Expires August 6, 2021 [Page 14]
Internet-Draft Generation of Transient Numeric IDs February 2021 of a reasonable length. 'secret_key' must remain stable for a given CONTEXT, since otherwise the numeric identifiers generated by this algorithm would not have the desired stability properties (i.e., stable for a given CONTEXT). In most cases, 'secret_key' should be selected with a PRNG (see [RFC4086] for recommendations on choosing secrets) at an appropriate time, and stored in stable or volatile storage (as necessary) for future use. The result of F() is stored in the variable 'offset', which may take any value within the storage type range, since we are restricting the resulting identifier to be in the range [min_id, max_id] in a similar way as in the algorithm described inSection 7.1.1. suitable_id() checks whether the candidate identifier has suitable uniqueness properties. Collisions (i.e., an identifier that is not unique) are recovered by incrementing the 'retry' variable and recomputing F(), up to a maximum of MAX_RETRIES times. However, recovering from collisions will usually result in identifiers that fail to remain constant for the specified context. This is normally acceptable when the probability of collisions is small, as in the case of e.g. IPv6 IIDs resulting from SLAAC [RFC7217] [RFC4941]. For obvious reasons, the transient numeric identifiers generated with this algorithm allow for network activity correlation and fingerprinting within "CONTEXT". However, this is essentially a design goal of this category of transient numeric identifiers.7.4. Category #4: Uniqueness, monotonically increasing within context (hard failure)7.4.1. Per-context Counter Algorithm One possible way of selecting unique monotonically-increasing identifiers (per context) is to employ a per-context counter. Such an algorithm could be described as follows:Gont & Arce Expires August 6, 2021 [Page 15]
Internet-Draft Generation of Transient Numeric IDs February 2021 /* Transient Numeric ID selection function */ id_range = max_id - min_id + 1; retry = id_range; id_inc = increment() % id_range; if( (next_id = lookup_counter(CONTEXT)) == ERROR){ next_id = min_id + random() % id_range; } do { if ( (max_id - next_id) >= id_inc){ next_id = next_id + id_inc; } else { next_id = min_id + id_inc - (max_id - next_id); } if (suitable_id(next_id)){ store_counter(CONTEXT, next_id); return next_id; } retry = retry - id_inc; } while (retry > 0); return ERROR; NOTE: increment() returns a small integer that is employed to increment the current counter value to obtain the next transient numeric identifier. This value must be much smaller than the number of possible values for the numeric IDs (i.e., "id_range"). Most implementations of this algorithm employ a constant increment of 1. Using a value other than 1 can help mitigate some information leakages (please see below), at the expense of a possible increase in the numeric ID reuse frequency. The code above makes sure that the increment employed in the algorithm (id_inc) is always smaller than the number of possible values for the numeric IDs (i.e., "max_id - min_d + 1"). However, as noted above, this value must also be much smaller than the number of possible values for the numeric IDs. lookup_counter() is a function that returns the current counter for a given context, or an error condition if that counter does not exist.Gont & Arce Expires August 6, 2021 [Page 16]
Internet-Draft Generation of Transient Numeric IDs February 2021 store_counter() is a function that saves a counter value for a given context. suitable_id() is a function that checks whether the resulting identifier is acceptable (e.g., whether it is not already in use, etc.). Essentially, whenever a new identifier is to be selected, the algorithm checks whether a counter for the corresponding context exists. If does, the value of such counter is incremented to obtain the new transient numeric identifier, and the counter is updated. If no counter exists for such context, a new counter is created and initialized to a random value, and used as the selected transient numeric identifier. This algorithm produces a per-context counter, which results in one monotonically-increasing function for each context. Since each counter is initialized to a random value, the resulting values are unpredictable by an off-path attacker. The choice of id_inc has implications on both the security and privacy properties of the resulting identifiers, but also on the corresponding interoperability properties. On one hand, minimizing the increments generally minimizes the identifier reuse frequency, albeit at increased predictability. On the other hand, if the increments are randomized, predictability of the resulting identifiers is reduced, and the information leakage produced by global constant increments is mitigated. However, using larger increments than necessary can result in higher numeric ID reuse frequency. This algorithm has the following drawbacks: o It requires an implementation to store each per-CONTEXT counter in memory. If, as a result of resource management, the counter for a given context must be removed, the last transient numeric identifier value used for that context will be lost. Thus, if subsequently an identifier needs to be generated for the same context, the corresponding counter will need to be recreated and reinitialized to a random value, thus possibly leading to reuse/ collision of numeric identifiers. o Keeping one counter for each possible "context" may in some cases be considered too onerous in terms of memory requirements. Otherwise, the identifiers produced by this algorithm do not suffer from the other issues discussed inSection 8.Gont & Arce Expires August 6, 2021 [Page 17]
Internet-Draft Generation of Transient Numeric IDs February 20217.4.2. Simple PRF-Based Algorithm The goal of this algorithm is to produce monotonically-increasing transient numeric identifiers (for each given context), with a randomized initial value. For example, if the identifiers being generated must be monotonically-increasing for each {IP Source Address, IP Destination Address} set, then each possible combination of {IP Source Address, IP Destination Address} should have a separate monotonically-increasing sequence, that starts at a different random value. Instead of maintaining a per-context counter (as in the algorithm fromSection 7.4.1), the following algorithm employs a calculated technique to maintain a random offset for each possible context. /* Initialization code */ counter = 0; /* Transient Numeric ID selection function */ id_range = max_id - min_id + 1; id_inc = increment() % id_range; offset = F(CONTEXT, secret_key); retry = id_range; do { next_id = min_id + (offset + counter) % id_range; counter = counter + id_inc; if (suitable_id(next_id)) { return next_id; } retry = retry - id_inc; } while (retry > 0); return ERROR; In the algorithm above, the function F() provides a (stateless) unpredictable offset for each given context (as identified by 'CONTEXT'). F() is a PRF, with the same properties as those specified for F() inSection 7.3.Gont & Arce Expires August 6, 2021 [Page 18]
Internet-Draft Generation of Transient Numeric IDs February 2021 CONTEXT is the concatenation of all the elements that define a given context. For example, if this algorithm is expected to produce identifiers that are monotonically-increasing for each set (Source IP Address, Destination IP Address), CONTEXT should be the concatenation of these two IP addresses. The function F() provides a "per-CONTEXT" fixed offset within the numeric identifier "space". Both the 'offset' and 'counter' variables may take any value within the storage type range since we are restricting the resulting identifier to be in the range [min_id, max_id] in a similar way as in the algorithm described inSection 7.1.1. This allows us to simply increment the 'counter' variable and rely on the unsigned integer to wrap around. The result of F() is no more secure than the secret key, and therefore 'secret_key' must be unknown to the attacker, and must be of a reasonable length. 'secret_key' must remain stable for a given CONTEXT, since otherwise the numeric identifiers generated by this algorithm would not have the desired stability properties (i.e., monotonically-increasing for a given CONTEXT). In most cases, 'secret_key' should be selected with a PRNG (see [RFC4086] for recommendations on choosing secrets) at an appropriate time, and stored in stable or volatile storage (as necessary) for future use. It should be noted that, since this algorithm uses a global counter ("counter") for selecting identifiers (i.e., all counters share the same increments space), this algorithm results in an information leakage (as described inSection 8.2). For example, if this algorithm were used for selecting TCP ephemeral ports, and an attacker could force a client to periodically establish a new TCP connection to an attacker-controlled system (or through an attacker- observable routing path), the attacker could subtract consecutive source port values to obtain the number of outgoing TCP connections established globally by the victim host within that time period (up to wrap-around issues and five-tuple collisions, of course). This information leakage could be partially mitigated by employing small random values for the increments (i.e., increment() function), instead of having increment() return the constant "1". We nevertheless note that an improved mitigation of this information leakage could be more successfully achieved by employing the algorithm fromSection 7.4.3, instead.7.4.3. Double-PRF Algorithm A trade-off between maintaining a single global 'counter' variable and maintaining 2**N 'counter' variables (where N is the width of the result of F()), could be achieved as follows. The system would keepGont & Arce Expires August 6, 2021 [Page 19]
Internet-Draft Generation of Transient Numeric IDs February 2021 an array of TABLE_LENGTH values, which would provide a separation of the increment space into multiple buckets. This improvement could be incorporated into the algorithm fromSection 7.4.2 as follows: /* Initialization code */ for(i = 0; i < TABLE_LENGTH; i++) { table[i] = random(); } /* Transient Numeric ID selection function */ id_range = max_id - min_id + 1; id_inc = increment() % id_range; offset = F(CONTEXT, secret_key1); index = G(CONTEXT, secret_key2) % TABLE_LENGTH; retry = id_range; do { next_id = min_id + (offset + table[index]) % id_range; table[index] = table[index] + id_inc; if (suitable_id(next_id)) { return next_id; } retry = retry - id_inc; } while (retry > 0); return ERROR; 'table[]' could be initialized with random values, as indicated by the initialization code in the pseudo-code above. Both F() and G() are PRFs, with the same properties as those required for F() inSection 7.3. The results of F() and G() are no more secure than their respective secret keys ('secret_key1' and 'secret_key2', respectively), and therefore both secret keys must be unknown to the attacker, and must be of a reasonable length. Both secret keys must remain stable for the given CONTEXT, since otherwise the transient numeric identifiers generated by this algorithm would not have the desired stability properties (i.e., monotonically-increasing for a given CONTEXT). In most cases, both secret keys should be selected with a PRNG (see [RFC4086] for recommendations on choosing secrets) at an appropriateGont & Arce Expires August 6, 2021 [Page 20]
Internet-Draft Generation of Transient Numeric IDs February 2021 time, and stored in stable or volatile storage (as necessary) for future use. The 'table[]' array assures that successive transient numeric identifiers for a given context will be monotonically-increasing. Since the increments space is separated into TABLE_LENGTH different spaces, the identifier reuse frequency will be (probabilistically) lower than that of the algorithm inSection 7.4.2. That is, the generation of an identifier for one given context will not necessarily result in increments in the identifier sequence of other contexts. It is interesting to note that the size of 'table[]' does not limit the number of different identifier sequences, but rather separates the *increment space* into TABLE_LENGTH different spaces. The selected transient numeric identifier sequence will be obtained by adding the corresponding entry from 'table[]' to the value in the 'offset' variable, which selects the actual identifier sequence space (as in the algorithm fromSection 7.4.2). An attacker can perform traffic analysis for any "increment space" (i.e., context) into which the attacker has "visibility" -- namely, the attacker can force a system to generate identifiers for G(CONTEXT, secret_key2), where the result of G() identifies the target "increment space". However, the attacker's ability to perform traffic analysis is very reduced when compared to the simple PRF- based identifiers (described inSection 7.4.2) and the predictable linear identifiers (described inAppendix A.1). Additionally, an implementation can further limit the attacker's ability to perform traffic analysis by further separating the increment space (that is, using a larger value for TABLE_LENGTH) and/or by randomizing the increments (i.e., increment() returning a small random number as opposed to the constant "1"). Otherwise, this algorithm does not suffer from the issues discussed inSection 8.8. Common Vulnerabilities Associated with Transient Numeric Identifiers8.1. Network Activity Correlation An identifier that is predictable within a given context allows for network activity correlation within that context. For example, a stable IPv6 Interface Identifier allows for network activity to be correlated within the context in which the Interface Identifier is stable [RFC7721]. A stable-per-network IPv6 Interface Identifier (as in [RFC7217]) allows for network activity correlation within a network, whereas a constant IPv6 Interface Identifier (that remains constant across networks) allows not only network activityGont & Arce Expires August 6, 2021 [Page 21]
Internet-Draft Generation of Transient Numeric IDs February 2021 correlation within the same network, but also across networks ("host tracking"). Similarly, an implementation that generates TCP ISNs with a global counter could allow for fingerprinting and network activity correlation across networks, since an attacker could passively infer the identity of the victim based on the TCP ISNs employed for subsequent communication instances. Similarly, an implementation that generates predictable IPv6 Fragment Identification values could be subject to fingerprinting attacks (see e.g. [Bellovin2002]).8.2. Information Leakage Transient numeric identifiers that result in specific patterns can produce an information leakage to other communicating entities. For example, it is common to generate transient numeric identifiers with an algorithm such as: ID = offset(CONTEXT) + mono(CONTEXT); This generic expression generates identifiers by adding a monotonically-increasing function (e.g. linear) to a randomized offset. offset() is constant within a given context, whereas mono() produces a monotonically-increasing sequence for the given context. Identifiers generated with this expression will generally be predictable within CONTEXT. The predictability of mono(), irrespective of the predictability of offset(), can leak information that may be of use to attackers. For example, a node that selects ephemeral port numbers as in: ephemeral_port = offset(Dest_IP) + mono() that is, with a per-destination offset, but a global mono() function (e.g., a global counter), will leak information about total number of outgoing connections that have been issued by the vulnerable implementation. Similarly, a node that generates Fragment Identification values as in: Frag_ID = offset(IP_src_addr, IP_dst_addr) + mono() will leak out information about the total number of fragmented packets that have been transmitted by the vulnerable implementation. The vulnerabilities described in [Sanfilippo1998a], [Sanfilippo1998b], and [Sanfilippo1999] are all associated with the use of a global mono() function (i.e., with a global and constantGont & Arce Expires August 6, 2021 [Page 22]
Internet-Draft Generation of Transient Numeric IDs February 2021 "context") -- particularly when it is a linear function (constant increments of 1). Predicting transient numeric identifiers can be of help for other types of attacks. For example, predictable TCP ISNs can open the door to trivial connection-reset and data injection attacks (seeSection 8.6).8.3. Fingerprinting Fingerprinting is the capability of an attacker to identify or re- identify a visiting user, user agent or device via configuration settings or other observable characteristics. Observable protocol objects and characteristics can be employed to identify/re-identify a variety of entities, ranging from the underlying hardware or Operating System (vendor, type and version), to the user itself (i.e. his/her identity). [EFF] illustrates web browser-based fingerprinting, but similar techniques can be applied at other layers and protocols, whether alternatively or in conjunction with it. Transient numeric identifiers are one of the observable protocol components that could be leveraged for fingerprinting purposes. That is, an attacker could sample transient numeric identifiers to infer the algorithm (and its associated parameters, if any) for generating such identifiers, possibly revealing the underlying Operating System (OS) vendor, type, and version. This information could possibly be further leveraged in conjunction with other fingerprinting techniques and sources. Evasion of protocol-stack fingerprinting can prove to be a very difficult task: most systems make use of a wide variety of protocols, each of which have a large number of parameters that can be set to arbitrary values or generated with a variety of algorithms with multiple parameters. NOTE: General protocol-based fingerprinting is discussed in [RFC6973], along with guidelines to mitigate the associated vulnerability. [Fyodor1998] and [Fyodor2006] are classic references on Operating System detection via TCP/IP stack fingerprinting. Nmap [nmap] is probably the most popular tool for remote OS identification via active TCP/IP stack fingerprinting. p0f [Zalewski2012], on the other hand, is a tool for performing remote OS detection via passive TCP/IP stack fingerprinting. Finally, [TBIT] is a TCP fingerprinting tool that aims at characterizing the behaviour of a remote TCP peer based on active probes, and which has been widely used in the research community.Gont & Arce Expires August 6, 2021 [Page 23]
Internet-Draft Generation of Transient Numeric IDs February 2021 Algorithms that, from the perspective of an observer (e.g., the legitimate communicating peer), result in specific values or patterns, will allow for at least some level of fingerprinting. For example, the algorithm fromSection 7.3 will typically allow fingerprinting within the context where the resulting identifiers are stable. Similarly, the algorithms fromSection 7.4 will result in a monotonically-increasing sequences within a given context, thus allowing for at least some level of fingerprinting (when the other communicating entity can correlate different sampled identifiers as belonging to the same monotonically-increasing sequence). Thus, where possible, algorithms fromSection 7.1 should be preferred over algorithms that result in specific values or patterns.8.4. Exploitation of the Semantics of Transient Numeric Identifiers Identifiers that are not semantically opaque tend to be more predictable than semantically-opaque identifiers. For example, a MAC address contains an OUI (Organizationally-Unique Identifier) which identifies the vendor that manufactured the corresponding network interface card. This can be leveraged by an attacker trying to "guess" MAC addresses, who has some knowledge about the possible NIC vendor. [RFC7707] discusses a number of techniques to reduce the search space when performing IPv6 address-scanning attacks by leveraging the semantics of the IIDs produced by traditional SLAAC algorithms (eventually replaced by [RFC7217]) that embed MAC addresses in the IID of IPv6 addresses.8.5. Exploitation of Collisions of Transient Numeric Identifiers In many cases, the collision of transient network identifiers can have a hard failure severity (or result in a hard failure severity if an attacker can cause multiple collisions deterministically, one after another). For example, predictable Fragment Identification values open the door to Denial of Service (DoS) attacks (see e.g. [RFC5722].).8.6. Exploitation of Predictable Transient Numeric Identifiers for Injection Attacks Some protocols rely on "sequence numbers" for the validation of incoming packets. For example, TCP employs sequence numbers for reassembling TCP segments, while IPv4 and IPv6 employ Fragment Identification values for reassembling IPv4 and IPv6 fragments (respectively). Lacking built-in cryptographic mechanisms for validating packets, these protocols are therefore vulnerable to on-Gont & Arce Expires August 6, 2021 [Page 24]
Internet-Draft Generation of Transient Numeric IDs February 2021 path data (see e.g. [Joncheray1995]) and/or control-information (see e.g. [RFC4953] and [RFC5927]) injection attacks. The extent to which these protocols may resist off-path (i.e. "blind") injection attacks depends on whether the associated "sequence numbers" are predictable, and effort required to successfully predict a valid "sequence number" (see e.g. [RFC4953] and [RFC5927]). We note that the use of unpredictable "sequence numbers" is a completely-ineffective mitigation for on-path injection attacks, and also a mostly-ineffective mitigation for off-path (i.e. "blind") injection attacks. However, many legacy protocols (such as TCP) do not natively incorporate cryptographic mitigations, but rather only as optional features (see e.g. [RFC5925]), if at all available. Additionally, ad-hoc use of cryptographic mitigations might not be sufficient to relieve a protocol implementation of generating appropriate transient numeric identifiers. For example, use of the Transport Layer Security (TLS) protocol [RFC8446] with TCP will protect the application protocol, but will not help to mitigate e.g. TCP-based connection-reset attacks (see e.g. [RFC4953]). Similarly, use of SEcure Neighbor Discovery (SEND) [RFC3971] will still imply reliance on the successful reassembly of IPv6 fragments in those cases where SEND packets do not fit into the link Maximum Transmission Unit (MTU) (see [RFC6980]).8.7. Cryptanalysis A number of algorithms discussed in this document (such as those described inSection 7.4.2 andSection 7.4.3) rely on PRFs. Implementations that employ weak PRFs or keys of inappropriate size can be subject to cryptanalysis, where an attacker can obtain the secret key employed for the PRF, predict numeric identifiers, etc. Furthermore, an implementation that overloads the semantics of the secret key can result in more trivial cryptanalysis, possibly resulting in the leakage of the value employed for the secret key. NOTE: [IPID-DEV] describes two vulnerable transient numeric ID generators that employ cryptographically-weak hash functions. Additionally, one of such implementations employs 32-bits of a kernel address as the secret key for a hash function, and therefore successful cryptanalysis leaks the aforementioned kernel address, allowing for Kernel Address Space Layout Randomization (KASLR) [KASLR] bypass.Gont & Arce Expires August 6, 2021 [Page 25]
Internet-Draft Generation of Transient Numeric IDs February 20219. Vulnerability Assessment of Transient Numeric Identifiers The following subsections analyze possible vulnerabilities associated with the algorithms described inSection 7.9.1. Category #1: Uniqueness (soft failure) Possible vulnerabilities associated with the algorithms fromSection 7.1 include: o Use of flawed PRNGs (please see e.g. [Zalewski2001], [Zalewski2002] and [Klein2007]), o Inadvertently affecting the distribution of an otherwise suitable PRNG. An implementer should consult [RFC4086] regarding randomness requirements for security, and consult relevant documentation when employing a PRNG provided by the underlying system. When employing a PRNG, many implementations "adapt" the length of its output with a modulo operator (e.g., C language's "%"), possibly changing the distribution of the output of the PRNG. For example, consider an implementation that employs the following code: id = random() % 50000; This example implementation means to obtain a transient numeric identifier in the range 0-49000. If random() produces e.g. a pseudorandom number of 16 bits (with uniform distribution), the selected numeric ID will have a non-uniform distribution with the numbers in the range 0-15535 having double-frequency as the numbers in the range 15536-49000. This effect is reduced if the PRNG produces an output that is much longer than the length implied by the modulo operation. Use of algorithms other than PRNGs for generating identifiers of this category is discouraged.9.2. Category #2: Uniqueness (hard failure) As noted inSection 7.2, this category can employ the same algorithms as Category #4, since a monotonically-increasing sequence tends to minimize the transient numeric identifier reuse frequency. Therefore, the vulnerability analysis inSection 9.4 also applies to this category.Gont & Arce Expires August 6, 2021 [Page 26]
Internet-Draft Generation of Transient Numeric IDs February 2021 Additionally, as noted inSection 7.2, some transient numeric identifiers of this category might be able to use the algorithms fromSection 7.1, in which case the same considerations as inSection 9.1 would apply.9.3. Category #3: Uniqueness, stable within context (soft failure) Possible vulnerabilities associated with the algorithms fromSection 7.3 are: 1. Use of weak PRFs, or inappropriate secret keys (whether inappropriate selection or inappropriate size) could allow for cryptanalysis, which could eventually be exploited by an attacker to predict future transient numeric identifiers. 2. Since the algorithm generates a unique and stable identifier within a specified context, it may allow for network activity correlation and fingerprinting within the specified context.9.4. Category #4: Uniqueness, monotonically increasing within context (hard failure) The algorithm described inSection 7.4.1 for generating identifiers of Category #4 will result in an identifiable pattern (i.e. a monotonically-increasing sequence) for the transient numeric identifiers generated for each CONTEXT, and thus will allow for fingerprinting and network activity correlation within each CONTEXT. On the other hand, a simple way to generalize and analyze the algorithms described inSection 7.4.2 andSection 7.4.3 for generating identifiers of Category #4, is as follows:Gont & Arce Expires August 6, 2021 [Page 27]
Internet-Draft Generation of Transient Numeric IDs February 2021 /* Transient Numeric ID selection function */ id_range = max_id - min_id + 1; retry = id_range; id_inc = increment() % id_range; do { update_mono(CONTEXT, id_inc); next_id = min_id + (offset(CONTEXT) + \ mono(CONTEXT)) % id_range; if (suitable_id(next_id)) { return next_id; } retry = retry - id_inc; } while (retry > 0); return ERROR; NOTE: increment() returns a small integer that is employed to generate a monotonically-increasing function. Most implementations employ a constant value for "increment()" (usually 1). The value returned by increment() must be much smaller than the value computed for "id_range". update_mono(CONTEXT, id_inc) increments the counter corresponding to CONTEXT by "id_inc". mono(CONTEXT) reads the counter corresponding to CONTEXT. Essentially, an identifier (next_id) is generated by adding a monotonically-increasing function (mono()) to an offset value, unknown to the attacker and stable for given context (CONTEXT). The following aspects of the algorithm should be considered: o For the most part, it is the offset() function that results in identifiers that are unpredictable by an off-patch attacker. While the resulting sequence is known to be monotonically- increasing, the use of a randomized offset value makes the resulting values unknown to the attacker.Gont & Arce Expires August 6, 2021 [Page 28]
Internet-Draft Generation of Transient Numeric IDs February 2021 o The most straightforward "stateless" implementation of offset() is with a PRF that takes the values that identify the context and a "secret_key" (not shown in the figure above) as arguments. o One possible implementation of mono() would be to have mono() internally employ a single counter (as in the algorithm fromSection 7.4.2), or map the increments for different contexts into a number of counters/buckets, such that the number of counters that need to be maintained in memory is reduced (as in the algorithm from algorithm inSection 7.4.3). o In all cases, a monotonically increasing function is implemented by incrementing the previous value of a counter by increment() units. In the most trivial case, increment() could return the constant "1". But increment() could also be implemented to return small random integers such that the increments are unpredictable (seeAppendix A of [RFC7739]). This represents a tradeoff between the unpredictability of the resulting transient numeric IDs and the transient numeric ID reuse frequency. Considering the generic algorithm illustrated above, we can identify the following possible vulnerabilities: o Since the algorithms for this category are similar to those ofSection 9.3, with the addition of a monotonically-increasing function, all the issues discussed inSection 9.3 ("Category #3: Uniqueness, stable within context (soft failure)") also apply to this case. o mono() can be correlated to the number of identifiers generated for a given context (CONTEXT). Thus, if mono() spans more than the necessary context, the "increments" could be leaked to other parties, thus disclosing information about the number of identifiers that have been generated by the algorithm for all contexts. This is information disclosure becomes more evident when an implementation employs a constant increment of 1. For example, an implementation where mono() is actually a single global counter, will unnecessarily leak information the number of identifiers that have been generated by the algorithm (globally, for all contexts). [Fyodor2003] is one example of how such information leakages can be exploited. We note that limiting the span of the increments space will require a larger number of counters to be stored in memory (i.e., a larger value for the TABLE_LENGTH parameter of the algorithm inSection 7.4.3). o Transient numeric identifiers generated with the algorithms described inSection 7.4.2 andSection 7.4.3 will normally allow for fingerprinting within CONTEXT since, for such context, theGont & Arce Expires August 6, 2021 [Page 29]
Internet-Draft Generation of Transient Numeric IDs February 2021 resulting identifiers will have an identifiable pattern (i.e. a monotonically-increasing sequence).10. IANA Considerations This document has no IANA actions.11. Security Considerations The entire document is about the security and privacy implications of transient numeric identifiers. [I-D.gont-numeric-ids-sec-considerations] recommends that protocol specifications specify the interoperability requirements of their transient numeric identifiers, perform a vulnerability assessment of their transient numeric identifiers, and suggest an algorithm for generating each of their transient numeric identifiers. This document analyzes possible algorithms (and their implications) that could be employed to comply with the interoperability properties of most common categories of transient numeric identifiers, while minimizing the associated negative security and privacy implications.12. Acknowledgements The authors would like to thank (in alphabetical order) Bernard Aboba, Jean-Philippe Aumasson, Steven Bellovin, Luis Leon Cardenas Graide, Guillermo Gont, Joseph Lorenzo Hall, Gre Norcie, Shivan Sahib, Rich Salz, Martin Thomson, and Michael Tuexen, for providing valuable comments on earlier versions of this document. The authors would like to thank Shivan Sahib and Christopher Wood, for their guidance during the publication process of this document. The authors would like to thank Jean-Philippe Aumasson and Mathew D. Green (John Hopkins University) for kindly answering a number of questions. The authors would like to thank Diego Armando Maradona for his magic and inspiration.13. References13.1. Normative References [RFC0791] Postel, J., "Internet Protocol", STD 5,RFC 791, DOI 10.17487/RFC0791, September 1981, <https://www.rfc-editor.org/info/rfc791>.Gont & Arce Expires August 6, 2021 [Page 30]
Internet-Draft Generation of Transient Numeric IDs February 2021 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,RFC 793, DOI 10.17487/RFC0793, September 1981, <https://www.rfc-editor.org/info/rfc793>. [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13,RFC 1035, DOI 10.17487/RFC1035, November 1987, <https://www.rfc-editor.org/info/rfc1035>. [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm",RFC 1321, DOI 10.17487/RFC1321, April 1992, <https://www.rfc-editor.org/info/rfc1321>. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification",RFC 2460, DOI 10.17487/RFC2460, December 1998, <https://www.rfc-editor.org/info/rfc2460>. [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security",BCP 106,RFC 4086, DOI 10.17487/RFC4086, June 2005, <https://www.rfc-editor.org/info/rfc4086>. [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture",RFC 4291, DOI 10.17487/RFC4291, February 2006, <https://www.rfc-editor.org/info/rfc4291>. [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration",RFC 4862, DOI 10.17487/RFC4862, September 2007, <https://www.rfc-editor.org/info/rfc4862>. [RFC4941] Narten, T., Draves, R., and S. Krishnan, "Privacy Extensions for Stateless Address Autoconfiguration in IPv6",RFC 4941, DOI 10.17487/RFC4941, September 2007, <https://www.rfc-editor.org/info/rfc4941>. [RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments",RFC 5722, DOI 10.17487/RFC5722, December 2009, <https://www.rfc-editor.org/info/rfc5722>. [RFC5925] Touch, J., Mankin, A., and R. Bonica, "The TCP Authentication Option",RFC 5925, DOI 10.17487/RFC5925, June 2010, <https://www.rfc-editor.org/info/rfc5925>. [RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport- Protocol Port Randomization",BCP 156,RFC 6056, DOI 10.17487/RFC6056, January 2011, <https://www.rfc-editor.org/info/rfc6056>.Gont & Arce Expires August 6, 2021 [Page 31]
Internet-Draft Generation of Transient Numeric IDs February 2021 [RFC6151] Turner, S. and L. Chen, "Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms",RFC 6151, DOI 10.17487/RFC6151, March 2011, <https://www.rfc-editor.org/info/rfc6151>. [RFC6191] Gont, F., "Reducing the TIME-WAIT State Using TCP Timestamps",BCP 159,RFC 6191, DOI 10.17487/RFC6191, April 2011, <https://www.rfc-editor.org/info/rfc6191>. [RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence Number Attacks",RFC 6528, DOI 10.17487/RFC6528, February 2012, <https://www.rfc-editor.org/info/rfc6528>. [RFC7217] Gont, F., "A Method for Generating Semantically Opaque Interface Identifiers with IPv6 Stateless Address Autoconfiguration (SLAAC)",RFC 7217, DOI 10.17487/RFC7217, April 2014, <https://www.rfc-editor.org/info/rfc7217>. [RFC7323] Borman, D., Braden, B., Jacobson, V., and R. Scheffenegger, Ed., "TCP Extensions for High Performance",RFC 7323, DOI 10.17487/RFC7323, September 2014, <https://www.rfc-editor.org/info/rfc7323>. [RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu, "Recommendation on Stable IPv6 Interface Identifiers",RFC 8064, DOI 10.17487/RFC8064, February 2017, <https://www.rfc-editor.org/info/rfc8064>. [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86,RFC 8200, DOI 10.17487/RFC8200, July 2017, <https://www.rfc-editor.org/info/rfc8200>.13.2. Informative References [Bellovin1989] Bellovin, S., "Security Problems in the TCP/IP Protocol Suite", Computer Communications Review, vol. 19, no. 2, pp. 32-48, 1989, <https://www.cs.columbia.edu/~smb/papers/ipext.pdf>. [Bellovin2002] Bellovin, S., "A Technique for Counting NATted Hosts", IMW'02 Nov. 6-8, 2002, Marseille, France, 2002, <https://www.cs.columbia.edu/~smb/papers/fnat.pdf>.Gont & Arce Expires August 6, 2021 [Page 32]
Internet-Draft Generation of Transient Numeric IDs February 2021 [BLAKE3] O'Connor, J., Aumasson, J., Neves, S., and Z. Wilcox- O'Hearn, "BLAKE3: one function, fast everywhere", 2020, <https://blake3.io/>. [CPNI-TCP] Gont, F., "Security Assessment of the Transmission Control Protocol (TCP)", United Kingdom's Centre for the Protection of National Infrastructure (CPNI) Technical Report, 2009, <https://www.gont.com.ar/papers/tn-03-09-security-assessment-TCP.pdf>. [EFF] EFF, "Cover your tracks: See how trackers view your browser", 2020, <https://coveryourtracks.eff.org/>. [FIPS-SHS] NIST, "Secure Hash Standard (SHS)", Federal Information Processing Standards Publication 180-4, August 2015, <https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf>. [Fyodor1998] Fyodor, "Remote OS Detection via TCP/IP Stack Fingerprinting", Phrack Magazine, Volume 9, Issue 54, 1998, <http://www.phrack.org/archives/issues/54/9.txt>. [Fyodor2003] Fyodor, "Idle scanning and related IP ID games", 2003, <https://nmap.org/presentations/CanSecWest03/CD_Content/idlescan_paper/idlescan.html>. [Fyodor2006] Lyon, G., "Chapter 8. Remote OS Detection", 2006, <https://nmap.org/book/osdetect.html>. [I-D.gont-numeric-ids-sec-considerations] Gont, F. and I. Arce, "Security Considerations for Transient Numeric Identifiers Employed in Network Protocols",draft-gont-numeric-ids-sec-considerations-06 (work in progress), December 2020. [I-D.irtf-pearg-numeric-ids-history] Gont, F. and I. Arce, "Unfortunate History of Transient Numeric Identifiers",draft-irtf-pearg-numeric-ids-history-06 (work in progress), January 2021. [IANA-PROT] IANA, "Protocol Registries", <https://www.iana.org/protocols>.Gont & Arce Expires August 6, 2021 [Page 33]
Internet-Draft Generation of Transient Numeric IDs February 2021 [IPID-DEV] Klein, A. and B. Pinkas, "From IP ID to Device ID and KASLR Bypass (Extended Version)", June 2019, <https://arxiv.org/pdf/1906.10478.pdf>. [Joncheray1995] Joncheray, L., "A Simple Active Attack Against TCP", Proc. Fifth Usenix UNIX Security Symposium, 1995, <https://www.usenix.org/legacy/publications/library/proceedings/security95/full_papers/joncheray.pdf>. [KASLR] PaX Team, "Address Space Layout Randomization", <https://pax.grsecurity.net/docs/aslr.txt>. [Klein2007] Klein, A., "OpenBSD DNS Cache Poisoning and Multiple O/S Predictable IP ID Vulnerability", 2007, <https://dl.packetstormsecurity.net/papers/attack/OpenBSD_DNS_Cache_Poisoning_and_Multiple_OS_Predictable_IP_ID_Vulnerability.pdf>. [Morris1985] Morris, R., "A Weakness in the 4.2BSD UNIX TCP/IP Software", CSTR 117, AT&T Bell Laboratories, Murray Hill, NJ, 1985, <https://pdos.csail.mit.edu/~rtm/papers/117.pdf>. [nmap] nmap, "Nmap: Free Security Scanner For Network Exploration and Audit", 2020, <https://www.insecure.org/nmap>. [RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- Hashing for Message Authentication",RFC 2104, DOI 10.17487/RFC2104, February 1997, <https://www.rfc-editor.org/info/rfc2104>. [RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander, "SEcure Neighbor Discovery (SEND)",RFC 3971, DOI 10.17487/RFC3971, March 2005, <https://www.rfc-editor.org/info/rfc3971>. [RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",RFC 4953, DOI 10.17487/RFC4953, July 2007, <https://www.rfc-editor.org/info/rfc4953>. [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates",RFC 4963, DOI 10.17487/RFC4963, July 2007, <https://www.rfc-editor.org/info/rfc4963>.Gont & Arce Expires August 6, 2021 [Page 34]
Internet-Draft Generation of Transient Numeric IDs February 2021 [RFC5927] Gont, F., "ICMP Attacks against TCP",RFC 5927, DOI 10.17487/RFC5927, July 2010, <https://www.rfc-editor.org/info/rfc5927>. [RFC6274] Gont, F., "Security Assessment of the Internet Protocol Version 4",RFC 6274, DOI 10.17487/RFC6274, July 2011, <https://www.rfc-editor.org/info/rfc6274>. [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., Morris, J., Hansen, M., and R. Smith, "Privacy Considerations for Internet Protocols",RFC 6973, DOI 10.17487/RFC6973, July 2013, <https://www.rfc-editor.org/info/rfc6973>. [RFC6980] Gont, F., "Security Implications of IPv6 Fragmentation with IPv6 Neighbor Discovery",RFC 6980, DOI 10.17487/RFC6980, August 2013, <https://www.rfc-editor.org/info/rfc6980>. [RFC7098] Carpenter, B., Jiang, S., and W. Tarreau, "Using the IPv6 Flow Label for Load Balancing in Server Farms",RFC 7098, DOI 10.17487/RFC7098, January 2014, <https://www.rfc-editor.org/info/rfc7098>. [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack",BCP 188,RFC 7258, DOI 10.17487/RFC7258, May 2014, <https://www.rfc-editor.org/info/rfc7258>. [RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6 Networks",RFC 7707, DOI 10.17487/RFC7707, March 2016, <https://www.rfc-editor.org/info/rfc7707>. [RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy Considerations for IPv6 Address Generation Mechanisms",RFC 7721, DOI 10.17487/RFC7721, March 2016, <https://www.rfc-editor.org/info/rfc7721>. [RFC7739] Gont, F., "Security Implications of Predictable Fragment Identification Values",RFC 7739, DOI 10.17487/RFC7739, February 2016, <https://www.rfc-editor.org/info/rfc7739>. [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3",RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>.Gont & Arce Expires August 6, 2021 [Page 35]
Internet-Draft Generation of Transient Numeric IDs February 2021 [Sanfilippo1998a] Sanfilippo, S., "about the ip header id", Post to Bugtraq mailing-list, Mon Dec 14 1998, <http://seclists.org/bugtraq/1998/Dec/48>. [Sanfilippo1998b] Sanfilippo, S., "Idle scan", Post to Bugtraq mailing-list, 1998, <https://github.com/antirez/hping/raw/master/docs/SPOOFED_SCAN.txt>. [Sanfilippo1999] Sanfilippo, S., "more ip id", Post to Bugtraq mailing- list, 1999, <https://github.com/antirez/hping/raw/master/docs/MORE-FUN-WITH-IPID>. [Schuba1993] Schuba, C., "ADDRESSING WEAKNESSES IN THE DOMAIN NAME SYSTEM PROTOCOL", 1993, <http://ftp.cerias.purdue.edu/pub/papers/christoph-schuba/schuba-DNS-msthesis.pdf>. [Shimomura1995] Shimomura, T., "Technical details of the attack described by Markoff in NYT", Message posted in USENET's comp.security.misc newsgroup Message-ID: <3g5gkl$5j1@ariel.sdsc.edu>, 1995, <https://www.gont.com.ar/docs/post-shimomura-usenet.txt>. [Silbersack2005] Silbersack, M., "Improving TCP/IP security through randomization without sacrificing interoperability", EuroBSDCon 2005 Conference, 2005, <https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.91.4542&rep=rep1&type=pdf>. [SipHash] Aumasson, J. and D. Bernstein, "SipHash: a fast short- input PRF", 2012, <https://github.com/veorq/SipHash>. [TBIT] TBIT, "TBIT, the TCP Behavior Inference Tool", 2001, <https://www.icir.org/tbit/>. [TCPT-uptime] McDanel, B., "TCP Timestamping - Obtaining System Uptime Remotely", March 2001, <https://securiteam.com/securitynews/5np0c153pi/>.Gont & Arce Expires August 6, 2021 [Page 36]
Internet-Draft Generation of Transient Numeric IDs February 2021 [Zalewski2001] Zalewski, M., "Strange Attractors and TCP/IP Sequence Number Analysis", 2001, <https://lcamtuf.coredump.cx/oldtcp/tcpseq.html>. [Zalewski2002] Zalewski, M., "Strange Attractors and TCP/IP Sequence Number Analysis - One Year Later", 2001, <https://lcamtuf.coredump.cx/newtcp/>. [Zalewski2012] Zalewski, M., "p0f v3 (version 3.09b)", 2012, <https://lcamtuf.coredump.cx/p0f.shtml>.Appendix A. Algorithms and Techniques with Known Issues The following subsections discuss algorithms and techniques with known negative security and privacy implications. NOTE: As discussed inSection 1, the use of cryptographic techniques might allow for the safe use of some of these algorithms and techniques. However, this should be evaluated on a case by case basis.A.1. Predictable Linear Identifiers Algorithm One of the most trivial ways to achieve uniqueness with a low identifier reuse frequency is to produce a linear sequence. This type of algorithm has been employed in the past to generate identifiers of Categories #1, #2, and #4 (please seeSection 6 for an analysis of these categories). For example, the following algorithm has been employed (see e.g. [Morris1985], [Shimomura1995], [Silbersack2005] and [CPNI-TCP]) in a number of operating systems for selecting IP fragment IDs, TCP ephemeral ports, etc.:Gont & Arce Expires August 6, 2021 [Page 37]
Internet-Draft Generation of Transient Numeric IDs February 2021 /* Initialization code */ next_id = min_id; id_inc= 1; /* Transient Numeric ID selection function */ id_range = max_id - min_id + 1; retry = id_range; do { if (next_id == max_id) { next_id = min_id; } else { next_id = next_id + id_inc; } if (suitable_id(next_id)) { return next_id; } retry--; } while (retry > 0); return ERROR; NOTE: suitable_id() is a function that checks whether the resulting identifier is acceptable (e.g., whether it's in use, etc.). For obvious reasons, this algorithm results in predictable sequences. Since a global counter is used to generate the transient numeric identifiers ("next_id" in the example above), an entity that learns one numeric identifier can infer past numeric identifiers and predict future values to be generated by the same algorithm. Since the value employed for the increments is known (such as "1" in this case), an attacker can sample two values, and learn the number of identifiers that have been were generated in-between the two sampled values. Furthermore, if the counter is initialized e.g. when the system its bootstrapped to some known value, the algorithm will leak additional information, such as the number of transmitted fragmented datagrams in the case of an IP ID generator [Sanfilippo1998a], or the system uptime in the case of TCP timestamps [TCPT-uptime].Gont & Arce Expires August 6, 2021 [Page 38]
Internet-Draft Generation of Transient Numeric IDs February 2021A.2. Random-Increments Algorithm This algorithm offers a middle ground between the algorithms that generate randomized transient numeric identifiers (such as those described inSection 7.1.1 andSection 7.1.2), and those that generate identifiers with a predictable monotonically-increasing function (seeAppendix A.1). /* Initialization code */ next_id = random(); /* Initialization value */ id_rinc = 500; /* Determines the trade-off */ /* Transient Numeric ID selection function */ id_range = max_id - min_id + 1; retry = id_range; do { /* Random increment */ id_inc = (random() % id_rinc) + 1; if ( (max_id - next_id) >= id_inc){ next_id = next_id + id_inc; } else { next_id = min_id + id_inc - (max_id - next_id); } if (suitable_id(next_id)) { return next_id; } retry = retry - id_inc; } while (retry > 0); return ERROR; This algorithm aims at producing a global monotonically-increasing sequence of transient numeric identifiers, while avoiding the use of fixed increments, which would lead to trivially predictable sequences. The value "id_inc" allows for direct control of the trade-off between unpredictability and identifier reuse frequency. The smaller the value of "id_inc", the more similar this algorithm isGont & Arce Expires August 6, 2021 [Page 39]
Internet-Draft Generation of Transient Numeric IDs February 2021 to a predicable, global linear ID generation algorithm (as the one inAppendix A.1). The larger the value of "id_inc", the more similar this algorithm is to the algorithm described inSection 7.1.1 of this document. When the identifiers wrap, there is a risk of collisions of transient numeric identifiers (i.e., identifier reuse). Therefore, "id_inc" should be selected according to the following criteria: o It should maximize the wrapping time of the identifier space. o It should minimize identifier reuse frequency. o It should maximize unpredictability. Clearly, these are competing goals, and the decision of which value of "id_inc" to use is a trade-off. Therefore, the value of "id_inc" is at times a configurable parameter so that system administrators can make the trade-off for themselves. We note that the alternative algorithms discussed throughout this document offer better interoperability, security and privacy properties than this algorithm, and hence implementation of this algorithm is discouraged.A.3. Re-using Identifiers Across Different Contexts Employing the same identifier across contexts in which stability is not required (i.e. overloading the semantics of transient numeric identifier) usually has negative security and privacy implications. For example, in order to generate transient numeric identifiers of Category #2 or Category #3, an implementation or specification might be tempted to employ a source for the numeric identifiers which is known to provide unique values, but that may also be predictable or leak information related to the entity generating the identifier. This technique has been employed in the past for e.g. generating IPv6 IIDs by re-using the MAC address of the underlying network interface. However, as noted in [RFC7721] and [RFC7707], embedding link-layer addresses in IPv6 IIDs not only results in predictable values, but also leaks information about the manufacturer of the underlying network interface card, allows for network activity correlation, and makes address-based scanning attacks feasible.Authors' AddressesGont & Arce Expires August 6, 2021 [Page 40]
Internet-Draft Generation of Transient Numeric IDs February 2021 Fernando Gont SI6 Networks Evaristo Carriego 2644 Haedo, Provincia de Buenos Aires 1706 Argentina Email: fgont@si6networks.com URI:https://www.si6networks.com Ivan Arce Quarkslab Email: iarce@quarkslab.com URI:https://www.quarkslab.comGont & Arce Expires August 6, 2021 [Page 41]
Datatracker
draft-irtf-pearg-numeric-ids-generation-07
This is an older version of an Internet-Draft that was ultimately published asRFC 9415.
| Document | Document type | This is an older version of an Internet-Draft that was ultimately published asRFC 9415. Expired & archived This document is an Internet-Draft (I-D) that has been submitted to the Internet Research Task Force (IRTF) stream. This I-D isnot endorsed by the IETF and hasno formal standing in theIETF standards process. | |
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| Authors | Fernando Gont,Ivan Arce | ||
| Replaces | draft-gont-numeric-ids-generation draft-gont-predictable-protocol-ids | ||
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| Additional resources | Mailing list discussion |