Internet Research Task Force (IRTF) F. Gont Internet-Draft SI6 Networks Intended status: Informational I. Arce Expires: November 15, 2020 Quarkslab May 14, 2020 On the Generation of Transient Numeric Identifiers draft-irtf-pearg-numeric-ids-generation-02 Abstract This document performs an analysis of the security and privacy implications of different types of "numeric identifiers" used in IETF protocols, and tries to categorize them 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 identifier category, while minimizing the security and privacy implications, thus providing guidance to protocol designers and protocol implementers. Finally, this describes a number of algorithms that have been employed in real implementations to generate transient numeric identifiers and analyzes their security and privacy properties. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 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 at https://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 November 15, 2020. Copyright Notice Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved. Gont & Arce Expires November 15, 2020 [Page 1] Internet-Draft Generation of Transient Numeric IDs May 2020 This document is subject to BCP 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 Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Threat Model . . . . . . . . . . . . . . . . . . . . . . . . 5 4. Issues with the Specification of Identifiers . . . . . . . . 5 5. Protocol Failure Severity . . . . . . . . . . . . . . . . . . 6 6. Categorizing Identifiers . . . . . . . . . . . . . . . . . . 6 7. Common Algorithms for Transient Numeric Identifier Generation 9 7.1. Category #1: Uniqueness (soft failure) . . . . . . . . . 9 7.2. Category #2: Uniqueness (hard failure) . . . . . . . . . 11 7.3. Category #3: Uniqueness, stable within context (soft failure) . . . . . . . . . . . . . . . . . . . . . . . . 12 7.4. Category #4: Uniqueness, monotonically increasing within context (hard failure) . . . . . . . . . . . . . . . . . 13 8. Common Vulnerabilities Associated with Transient Numeric Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . 19 8.1. Network Activity Correlation . . . . . . . . . . . . . . 19 8.2. Information Leakage . . . . . . . . . . . . . . . . . . . 20 8.3. Exploitation of Semantics of Transient Numeric Identifiers . . . . . . . . . . . . . . . . . . . . . . . 21 8.4. Exploitation of Collisions of Transient Numeric Identifiers . . . . . . . . . . . . . . . . . . . . . . . 21 8.5. Cryptanalysis . . . . . . . . . . . . . . . . . . . . . . 21 9. Vulnerability Analysis of Specific Transient Numeric Identifiers Categories . . . . . . . . . . . . . . . . . . . 22 9.1. Category #1: Uniqueness (soft failure) . . . . . . . . . 22 9.2. Category #2: Uniqueness (hard failure) . . . . . . . . . 23 9.3. Category #3: Uniqueness, stable within context (soft failure) . . . . . . . . . . . . . . . . . . . . . . . . 23 9.4. Category #4: Uniqueness, monotonically increasing within context (hard failure) . . . . . . . . . . . . . . . . . 23 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25 11. Security Considerations . . . . . . . . . . . . . . . . . . . 25 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 26 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 26 13.1. Normative References . . . . . . . . . . . . . . . . . . 26 13.2. Informative References . . . . . . . . . . . . . . . . . 27 Appendix A. Flawed Algorithms . . . . . . . . . . . . . . . . . 30 A.1. Predictable Linear Identifiers Algorithm . . . . . . . . 30 A.2. Random-Increments Algorithm . . . . . . . . . . . . . . . 32 Gont & Arce Expires November 15, 2020 [Page 2] Internet-Draft Generation of Transient Numeric IDs May 2020 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34 1. Introduction Network protocols employ a variety of numeric identifiers for different protocol entities, ranging from DNS Transaction IDs (TxIDs) to transport protocol ports (e.g. TCP ports) or IPv6 Interface Identifiers (IIDs). These identifiers usually have specific properties (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 properties 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 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 TCP Initial Sequence Numbers (ISNs) o Predictable transport protocol ephemeral port numbers o Predictable IPv4 or IPv6 Fragment Identifiers (Fragment IDs) o Predictable IPv6 Interface Identifiers (IIDs) o Predictable DNS Transaction Identifiers (TxIDs) Recent history indicates that when new protocols are standardized or new protocol implementations are produced, the security and privacy properties of the associated identifiers tend to be overlooked, and inappropriate algorithms to generate transient numeric identifiers are either suggested in the specification or selected by implementers. As a result, it should be evident that advice in this area is warranted. Gont & Arce Expires November 15, 2020 [Page 3] Internet-Draft Generation of Transient Numeric IDs May 2020 This document contains a non-exhaustive survey of 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 the associated 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. 2. 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 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 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 Gont & Arce Expires November 15, 2020 [Page 4] Internet-Draft Generation of Transient Numeric IDs May 2020 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. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 3. Threat Model Throughout this document, we assume an attacker does not have physical or logical access to the device(s) being attacked. We assume the attacker can simply send any traffic to the target device(s), to e.g. sample identifiers employed by such device(s). 4. Issues with the Specification of Identifiers While assessing protocol specifications regarding the use of identifiers, we found that most of the issues discussed in this document arise as a result of one of the following conditions: o Protocol specifications which under-specify the requirements for their identifiers o Protocol specifications that over-specify their 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 port numbers 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 {Src IP, Dst IP} could be achieved with other algorithms that do not result in negative security and privacy implications [RFC7739]. Gont & Arce Expires November 15, 2020 [Page 5] Internet-Draft Generation of Transient Numeric IDs May 2020 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]. 5. Protocol Failure Severity Section 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 application or protocol may not be the same as that of the protocol in question. For example, a TCP connection that is aborted may or may not result in a hard failure of the upper application protocol: 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 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 failure modes (soft or hard) Gont & Arce Expires November 15, 2020 [Page 6] Internet-Draft Generation of Transient Numeric IDs May 2020 +--------------+------------------------------------+---------------+ | Identifier | Interoperability Requirements | Failure | | | | Severity | +--------------+------------------------------------+---------------+ | IPv6 Frag ID | Uniqueness (for IP address pair) | Soft/Hard (1) | +--------------+------------------------------------+---------------+ | IPv6 IID | Uniqueness (and stable within IPv6 | Soft (3) | | | prefix) (2) | | +--------------+------------------------------------+---------------+ | TCP ISN | Monotonically-increasing | Hard (4) | +--------------+------------------------------------+---------------+ | TCP eph. | Uniqueness (for connection ID) | Hard | | port | | | +--------------+------------------------------------+---------------+ | IPv6 Flow | Uniqueness | None (5) | | Label | | | +--------------+------------------------------------+---------------+ | DNS TxID | Uniqueness | None (6) | +--------------+------------------------------------+---------------+ Table 1: Survey of Identifiers Notes: (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 the connection. However, a widespread Gont & Arce Expires November 15, 2020 [Page 7] Internet-Draft Generation of Transient Numeric IDs May 2020 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 sequence numbers allow for such optimization to work as expected [RFC6528], since otherwise such connections-establishment attempts would fail. (5) 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. (6) 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 | | | within context (hard failure) | | +-----+---------------------------------------+---------------------+ Table 2: Identifier Categories Gont & Arce Expires November 15, 2020 [Page 8] Internet-Draft Generation of Transient Numeric IDs May 2020 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. 7. 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. 7.1. Category #1: Uniqueness (soft failure) The requirement of uniqueness with a soft failure mode can be complied with a Pseudo-Random Number Generator (PRNG). In scenarios where ongoing use of previously selected numeric IDs is possible and desirable, 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, while Section 7.1.2 is an implementation of the later. We note that since the premise is that collisions of numeric identifiers of this category only leads to soft failures, in many (if not most) cases, the algorithm will not need to check the suitability of a selected identifier (i.e., check_suitable_id() would always be "true"). 7.1.1. Simple Randomization Algorithm Gont & Arce Expires November 15, 2020 [Page 9] Internet-Draft Generation of Transient Numeric IDs May 2020 /* Numeric ID selection function */ id_range = max_id - min_id + 1; next_id = min_id + (random() % id_range); count = next_id; do { if(check_suitable_id(next_id)) return next_id; if (next_id == max_id) { next_id = min_id; } else { next_id++; } count--; } while (count > 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 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 check_suitable_id() can check, when possible and desirable, whether this 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 been employed). When an identifier is found to be unsuitable, this algorithm selects the next available numeric identifier in sequence. All the variables (in this and all the algorithms discussed in this document) are unsigned integers. This algorithm does not suffer from any of the issues discussed in Section 8. Gont & Arce Expires November 15, 2020 [Page 10] Internet-Draft Generation of Transient Numeric IDs May 2020 7.1.2. Another Simple Randomization Algorithm The following pseudo-code illustrates another algorithm for selecting a random numeric identifier which, in the event a selected identifier is found to be unsuitable (e.g., already in use), another identifier is randomly selected: /* Numeric ID selection function */ id_range = max_id - min_id + 1; next_id = min_id + (random() % id_range); count = id_range; do { if(check_suitable_id(next_id)) return next_id; next_id = min_id + (random() % id_range); count--; } while (count > 0); return ERROR; This algorithm might be unable to select an identifier (i.e., return "ERROR") even if there are suitable identifiers available, in cases where a large number of identifiers are unsuitable (e.g. "in use"). The same considerations from Section 7.1.1 with respect to the properties of random() and the adaptation of its output length apply to this algorithm. This algorithm does not suffer from any of the issues discussed in Section 8. 7.2. Category #2: Uniqueness (hard failure) One of the most trivial approaches for achieving uniqueness for an identifier (with a hard failure mode) is to reduce the identifier reuse frequency by generating the numeric identifiers with a linear function. As a result, all of the algorithms described in Section 7.4 ("Category #4: Uniqueness, monotonically increasing within context (hard failure)") can be readily employed for complying with the requirements of this numeric identifier category. Gont & Arce Expires November 15, 2020 [Page 11] Internet-Draft Generation of Transient Numeric IDs May 2020 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 numeric identifier computed for each CONTEXT value, the following algorithm employs a calculated technique (as opposed to keeping state in memory) to generate a stable identifier for each given context. /* Numeric ID selection function */ id_range = max_id - min_id + 1; counter = 0; do { offset = F(CONTEXT, counter, secret_key); next_id = min_id + (offset % id_range); if(check_suitable_id(next_id)) return next_id; counter++; } while (counter <= MAX_RETRIES); return ERROR; In the following algorithm, the function F() provides a stateless and stable per-CONTEXT numeric identifier, 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 card (NIC) and SLAAC autoconfiguration prefix, the CONTEXT should be the concatenation of e.g. the interface index and the SLAAC autoconfiguration prefix (please see [RFC7217] for an implementation of this algorithm for generation of stable IPv6 IIDs). F() must be a cryptographically-secure hash function (e.g. SHA-256 [FIPS-SHS]), that is computed over the concatenation of its arguments. 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 Gont & Arce Expires November 15, 2020 [Page 12] Internet-Draft Generation of Transient Numeric IDs May 2020 this algorithm would not have the desired stability properties (i.e., stable for a given CONTEXT). In most cases, 'secret_key' can be selected with a PRNG (see [RFC4086] for recommendations on choosing secrets) at an appropriate time, and stored in stable or volatile storage 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 in Section 7.1.1. check_suitable_id() checks that the candidate identifier has suitable uniqueness properties. Collisions (i.e., an identifier that is not unique) are recovered by incrementing the 'counter' variable and recomputing F(). For obvious reasons, the transient network identifiers generated with this algorithm allow for network activity correlation 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 to achieve low identifier reuse frequency while still avoiding predictable sequences would be to employ a per-context counter, as opposed to a global counter. Such an algorithm could be described as follows: Gont & Arce Expires November 15, 2020 [Page 13] Internet-Draft Generation of Transient Numeric IDs May 2020 /* Initialization code */ id_inc = 1; /* Numeric ID selection function */ count = max_id - min_id + 1; if(lookup_counter(CONTEXT) == ERROR){ create_counter(CONTEXT); } next_id = lookup_counter(CONTEXT); do { if (next_id == max_id) { next_id = min_id; } else { next_id = next_id + id_inc; } if (check_suitable_id(next_id)){ store_counter(CONTEXT, next_id); return next_id; } count--; } while (count > 0); store_counter(CONTEXT, next_id); return ERROR; NOTE: lookup_counter() returns the current counter for a given context, or an error condition if such a counter does not exist. create_counter() creates a counter for a given context, and initializes such counter to a random value. store_counter() saves (updates) the current counter for a given context. check_suitable_id() is a function that checks whether the resulting identifier is acceptable (e.g., whether it is not already in use, etc.). Gont & Arce Expires November 15, 2020 [Page 14] Internet-Draft Generation of Transient Numeric IDs May 2020 Essentially, whenever a new identifier is to be selected, the algorithm checks whether there there is a counter for the corresponding context. If there is, such counter is incremented to obtain the new identifier, and the new identifier updates the corresponding counter. If there is no counter for such context, a new counter is created an initialized to a random value, and used as the new identifier. This algorithm produces a per-context counter, which results in one linear function for each context. Since each counter is initialized to a random value, the resulting values are unpredictable by an off-path attacker. This algorithm has the following drawbacks: o This algorithm 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 identifier value used for that context will be lost. Thus, if subsequently an identifier needs to be generated for the same context, that counter will need to be recreated and reinitialized to random value, thus possibly leading to reuse/collision of numeric identifiers. o An implementation may map more than one context to the same counter, such the amount of memory required to store counters is reduce, at the expense of a possible unnecessary increase in the numeric identifier reuse frequency. In such cases, if the identifiers are predictable by the destination system (e.g., the destination host represents the "context"), a vulnerable host might possibly leak to third parties the identifiers used by other hosts to send traffic to it (i.e., a vulnerable Host B could leak to Host C the identifier values that Host A is using to send packets to Host B). Appendix A of [RFC7739] describes one possible scenario for such leakage in detail. Otherwise, the identifiers produced by this algorithm do not suffer from the other issues discussed in Section 8. 7.4.2. Simple Hash-Based Algorithm The goal of this algorithm is to produce monotonically-increasing sequences, with a randomized initial value, for each given context. For example, if the identifiers being generated must be unique for each {src IP, dst IP} set, then each possible combination of {src IP, dst IP} should have a corresponding "next_id" value. Keeping one counter for each possible "context" may in many cases be considered too onerous in terms of memory requirements. As a workaround, the following algorithm employs a calculated technique Gont & Arce Expires November 15, 2020 [Page 15] Internet-Draft Generation of Transient Numeric IDs May 2020 (as opposed to keeping state in memory) to maintain the random offset for each possible context. In the following algorithm, the function F() provides a (stateless) unpredictable offset for each given context (as identified by 'CONTEXT'). /* Initialization code */ counter = 0; /* Numeric ID selection function */ id_range = max_id - min_id + 1; offset = F(CONTEXT, secret_key); count = id_range; do { next_id = min_id + (counter + offset) % id_range; counter++; if(check_suitable_id(next_id)) return next_id; count--; } while (count > 0); return ERROR; 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 in Section 7.1.1. This allows us to simply increment the 'counter' variable and rely on the unsigned integer to wrap around. The function F() should be a cryptographically-secure hash function (e.g. SHA-256 [FIPS-SHS]). 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. Gont & Arce Expires November 15, 2020 [Page 16] Internet-Draft Generation of Transient Numeric IDs May 2020 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., stable for a given CONTEXT). In most cases, 'secret_key' can be selected with a PRNG (see [RFC4086] for recommendations on choosing secrets) at an appropriate time, and stored in stable or volatile storage 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 produces an information leakage (as described in Section 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 machine (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 target host within that time period (up to wrap-around issues and five-tuple collisions, of course). 7.4.3. Double-Hash 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 keep an array of TABLE_LENGTH integers, which would provide a separation of the increment space into multiple buckets. This improvement could be incorporated into the algorithm from Section 7.4.2 as follows: Gont & Arce Expires November 15, 2020 [Page 17] Internet-Draft Generation of Transient Numeric IDs May 2020 /* Initialization code */ for(i = 0; i < TABLE_LENGTH; i++) table[i] = random(); id_inc = 1; /* Numeric ID selection function */ id_range = max_id - min_id + 1; offset = F(CONTEXT, secret_key1); index = G(CONTEXT, secret_key2) % TABLE_LENGTH; count = id_range; do { next_id = min_id + (offset + table[index]) % id_range; table[index] = table[index] + id_inc; if(check_suitable_id(next_id)) return next_id; count--; } while (count > 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() should be a cryptographically-secure hash functions (e.g. SHA-256 [FIPS-SHS]) computed over the concatenation of each of their respective arguments. Both F() and G() would employ the same CONTEXT (the concatenation of all the elements that define a given context), and would use separate secreted keys (secret_key1, and secret_key2, respectively). 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 numeric identifiers generated by this algorithm would not have the desired stability properties (i.e., stable for a given CONTEXT). In most cases, both secret keys can be selected with a PRNG (see [RFC4086] for recommendations on Gont & Arce Expires November 15, 2020 [Page 18] Internet-Draft Generation of Transient Numeric IDs May 2020 choosing secrets) at an appropriate time, and stored in stable or volatile storage for future use. The array 'table[]' assures that successive identifiers for a given context will be monotonically-increasing. However, the increments space is separated into TABLE_LENGTH different spaces, and thus identifier reuse frequency will be (probabilistically) lower than that of the algorithm in Section 7.4.2. That is, the generation of an identifier for one given context will not necessarily result in increments in the identifier sequence for 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 *increments* into TABLE_LENGTH different spaces. The identifier sequence will result from adding the corresponding entry of 'table[]' to the variable 'offset', which selects the actual identifier sequence (as in the algorithm from Section 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 node to generate identifiers where G(CONTEXT, secret_key2) identifies the target "increment space". However, the attacker's ability to perform traffic analysis is very reduced when compared to the predictable linear identifiers (described in Appendix A.1) and the hash-based identifiers (described in Section 7.4.2). 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. Otherwise, this algorithm does not suffer from the issues discussed in Section 8. 8. Common Vulnerabilities Associated with Transient Numeric Identifiers 8.1. Network Activity Correlation An identifier that is predictable or stable 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 for the context in which that address 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 the same across networks) allows not only network activity correlation within the same network, but also across networks ("host tracking"). Gont & Arce Expires November 15, 2020 [Page 19] Internet-Draft Generation of Transient Numeric IDs May 2020 Similarly, a node that generates TCP ISNs with a global counter could allow network activity correlation across networks, since the communicating nodes could infer the identity of the node based on the TCP ISNs employed for subsequent communication instances. Similarly, a node that generates predictable IPv6 Fragment Identification values could be subject to network activity correlation (see e.g. [Bellovin2002]). 8.2. Information Leakage Transient numeric identifiers that are not randomized can leak out information to other communicating nodes. For example, it is common to generate identifiers like: ID = offset(CONTEXT_1) + linear(CONTEXT_2); This generic expression generates identifiers by adding a linear function to an offset. The offset is stable within a given context, whereas linear() is a linear function for a given context (possibly different to that of offset()). Identifiers generated with this expression will generally be predictable within CONTEXT_1. Thus, CONTEXT_1 essentially specifies the context within which information will be "leaked". When both CONTEXT_1 and CONTEXT_2 are a constant value, then all the corresponding transient numeric identifiers become predictable in all contexts. NOTE: If offset() has a global context and the specific value is known, the resulting identifiers may leak even more information. For example, the if Fragment Identification values are generated with the generic function above, and CONTEXT_1 is "global", then the corresponding identifiers will leak the number of fragmented datagrams sent for CONTEXT_2. If both CONTEXT_1 and CONTEXT_2 are "global", then Fragment Identification values would be generated with a global counter (initialized to offset()), and thus each generated Fragment Identification value would leak the number of fragmented datagrams transmitted by the node since it has been bootstrapped. On the other hand, linear() will be predictable within CONTEXT_2. The predictability of linear(), irrespective of the context and/or predictability of offset(), can leak out information that is of use to attackers. For example, a node that selects ephemeral port numbers on as in: ephemeral_port = offset(Dest_IP) + linear() that is, with a per-destination offset, but global linear() function (e.g., a global counter), will leak information about the number of Gont & Arce Expires November 15, 2020 [Page 20] Internet-Draft Generation of Transient Numeric IDs May 2020 outgoing connections that have been issued between any two issued outgoing connections. Similarly, a node that generates Fragment Identification values as in: Frag_ID = offset(Srd_IP, Dst_IP) + linear() will leak out information about the number of fragmented packets that have been transmitted between any two other transmitted fragmented packets. The vulnerabilities described in [Sanfilippo1998a], [Sanfilippo1998b], and [Sanfilippo1999] are all associated with the use of a global linear() function (i.e., a global CONTEXT_2). 8.3. Exploitation of 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 underlying network interface card. This fact may be leveraged by an attacker meaning to "guess" MAC addresses and 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 a number by traditional IID- generation algorithms that embed MAC addresses (now replaced by [RFC8064] with [RFC7217]). 8.4. 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]. Similarly, predictable TCP ISNs open the door to trivial connection-reset and data injection attacks (see e.g. [Joncheray1995]). 8.5. Cryptanalysis A number of algorithms discussed in this document (such as Section 7.4.2 and Section 7.4.3 rely on cryptographically-secure hash functions. Implementations that employ weak hash functions and keys of inappropriate size may be subject to cryptanalysis, where an Gont & Arce Expires November 15, 2020 [Page 21] Internet-Draft Generation of Transient Numeric IDs May 2020 attacker may be able to obtain the secret key employed for the hash algorithms, predict numeric identifiers, etc. Futhermore, an implementation that overloads the semantics of the secret key may result in more trivial cryptanalysis, possibly resulting in the leakage of the value employed for the secret key. NOTE: [IPID-DEV] describes two vulnerable numeric ID generators that employ cryptographically-weak hash functions. Additionally, one of such implementations employs a 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. 9. Vulnerability Analysis of Specific Transient Numeric Identifiers Categories The following subsections analyze common vulnerabilities associated with the generation of identifiers for each of the categories identified in Section 6. 9.1. Category #1: Uniqueness (soft failure) Possible vulnerabilities associated with identifiers of this category are: o Use of trivial algorithms (e.g. global counters) that generate predictable identifiers o Use of flawed PRNGs (please see e.g. [Zalewski2001], [Zalewski2002] and [Klein2007]) Since the only interoperability requirement for these identifiers is uniqueness (with an associated soft failure), the obvious approach to generate them is to employ a PRNG. An implementer should consult [RFC4086] regarding randomness requirements for security, and consult relevant documentation when employing a PRNG provided by the underlying system. Use of algorithms other than PRNGs for generating identifiers of this category is discouraged. Gont & Arce Expires November 15, 2020 [Page 22] Internet-Draft Generation of Transient Numeric IDs May 2020 9.2. Category #2: Uniqueness (hard failure) As noted in Section 7.2 this category typically employs the same algorithms as Category #4, since a monotonically-increasing sequence tends to minimize the identifier reuse frequency. Therefore, the vulnerability analysis of Section 9.4 applies to this category. 9.3. Category #3: Uniqueness, stable within context (soft failure) There are three main vulnerabilities that may be associated with identifiers of this category: 1. Use algorithms or sources that result in predictable identifiers 2. Use cryptographically-weak hash functions, or inappropriate secret key sizes that allow for cryptanalysis 3. Employing the same identifier across contexts in which stability is not required (overloading the numeric identifier) At times, an implementation or specification may be tempted to employ a source for the numeric identifiers which is known to provide unique values, that may have other properties such as being predictable or leaking information about the node in question. For example, as noted in [RFC7721], embedding link-layer addresses for generating IPv6 IIDs not only results in predictable values, but also leaks information about the manufacturer of the network interface card. Employing cryptographically-weak hash functions or inappropriate secret key sizes may allow for cryptanalysis, which may eventually be exploited by an attacker to predict future numeric identifiers and perform a variety of attacks. On the other hand, using an identifier across contexts where stability is not required can be leveraged for correlation of activities. On of the most trivial examples of this is the use of IPv6 IIDs that are stable across networks (such as IIDs that embed the underlying link-layer address). 9.4. Category #4: Uniqueness, monotonically increasing within context (hard failure) A simple way to generalize algorithms employed for generating identifiers of Category #4 would be as follows: Gont & Arce Expires November 15, 2020 [Page 23] Internet-Draft Generation of Transient Numeric IDs May 2020 /* Numeric ID selection function */ count = max_id - min_id + 1; do { linear(CONTEXT_2)= linear(CONTEXT_2) + increment(); next_id = offset(CONTEXT_1) + linear(CONTEXT_2); if(check_suitable_id(next_id)) return next_id; count--; } while (count > 0); return ERROR; Essentially, an identifier (next_id) is generated by adding a linear function (linear()) to an offset value, which is unknown to the attacker, and stable for given context (CONTEXT_1). 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-path attacker. While the resulting sequence will be monotonically-increasing, the use of an offset value that is unknown to the attacker makes the resulting values unknown to the attacker. o The most straightforward "stateless" implementation of offset would be that in which offset() is the result of a cryptographically-secure hash-function that takes the values that identify the context and a "secret_key" (not shown in the figure above) as arguments. o Another possible (but stateful) approach would be to simply generate a random "per-context" "counter" and store it in memory, and then look-up the corresponding context when a new identifier is to be selected, and increment the counter to obtain the transient numeric identifier. The algorithm in Section 7.4.1 is essentially an implementation of this type. o The linear function is incremented according to increment(). In the most trivial case increment() could always return the constant "1". But it could also possibly return small random integers such the increments are unpredictable. Gont & Arce Expires November 15, 2020 [Page 24] Internet-Draft Generation of Transient Numeric IDs May 2020 Considering the generic algorithm illustrated above we can identify the following possible vulnerabilities: o All the vulnerabilities discussed in Section 9.3 ("Category #3: Uniqueness, stable within context (soft failure)") since the algorithms for this category are similar to those of Section 9.3, with the addition of a linear function. o The function linear() could be seen as representing the number of identifiers that have so far been generated for a given context (CONTEXT_2). If linear() spans more than the necessary context, the "increments" could be leaked to other parties, thus disclosing information about the number of identifiers that have so far been generated. For example, an implementation in which linear() is implemented as a single global counter will unnecessarily leak information the number of identifiers that have been produced. [Fyodor2004] is one example of how such information leakages can be exploited. However, limiting the span of the increments space will require a larger number of counters to be stored in memory (i.e., a larger size for the TABLE_LENGTH parameter of the algorithm in Section 7.4.3. o increment() determines the increments of linear() for each identifier that is selected. In the most trivial case, increment() will return the integer "1". However, an implementation may have increment() return a "small" random integer value such that even if the current value employed by the generator is guessed (see Appendix A of [RFC7739]), the exact next identifier to be selected will be slightly harder to identify. 10. IANA Considerations There are no IANA registries within this document. The RFC-Editor can remove this section before publication of this document as an RFC. 11. Security Considerations The entire document is about the security and privacy implications of transient numeric identifiers. [I-D.gont-numeric-ids-sec-considerations] formally requires protocol specifications to include an appropriate analysis of the interoperability, security, and privacy implications of the transient numeric identifiers they specify and employ, while this document analyzes possible algorithms (and their implications) that could be employed to comply with the interoperability properties of a transient numeric identifier, while mitigating the possible security and privacy implications. Gont & Arce Expires November 15, 2020 [Page 25] Internet-Draft Generation of Transient Numeric IDs May 2020 12. Acknowledgements The authors would like to thank (in alphabetical order) Steven Bellovin, Joseph Lorenzo Hall, Gre Norcie, Shivan Sahib, and Martin Thomson, for providing valuable comments on earlier versions of this document. The authors would like to thank Diego Armando Maradona for his magic and inspiration. 13. References 13.1. Normative References [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, DOI 10.17487/RFC0793, September 1981, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998, . [RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker, "Randomness Requirements for Security", BCP 106, RFC 4086, DOI 10.17487/RFC4086, June 2005, . [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing Architecture", RFC 4291, DOI 10.17487/RFC4291, February 2006, . [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless Address Autoconfiguration", RFC 4862, DOI 10.17487/RFC4862, September 2007, . [RFC5722] Krishnan, S., "Handling of Overlapping IPv6 Fragments", RFC 5722, DOI 10.17487/RFC5722, December 2009, . [RFC6528] Gont, F. and S. Bellovin, "Defending against Sequence Number Attacks", RFC 6528, DOI 10.17487/RFC6528, February 2012, . Gont & Arce Expires November 15, 2020 [Page 26] Internet-Draft Generation of Transient Numeric IDs May 2020 [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, . [RFC8064] Gont, F., Cooper, A., Thaler, D., and W. Liu, "Recommendation on Stable IPv6 Interface Identifiers", RFC 8064, DOI 10.17487/RFC8064, February 2017, . 13.2. Informative References [Bellovin2002] Bellovin, S., "A Technique for Counting NATted Hosts", IMW'02 Nov. 6-8, 2002, Marseille, France, 2002. [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, . [FIPS-SHS] FIPS, "Secure Hash Standard (SHS)", Federal Information Processing Standards Publication 180-4, August 2015, . [Fyodor2004] Fyodor, "Idle scanning and related IP ID games", 2004, . [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-04 (work in progress), July 2019. [I-D.irtf-pearg-numeric-ids-history] Gont, F. and I. Arce, "Unfortunate History of Transient Numeric Identifiers", draft-irtf-pearg-numeric-ids- history-02 (work in progress), April 2020. [IANA-PROT] IANA, "Protocol Registries", . Gont & Arce Expires November 15, 2020 [Page 27] Internet-Draft Generation of Transient Numeric IDs May 2020 [IPID-DEV] Klein, A. and B. Pinkas, "From IP ID to Device ID and KASLR Bypass (Extended Version)", June 2019, . [Joncheray1995] Joncheray, L., "A Simple Active Attack Against TCP", Proc. Fifth Usenix UNIX Security Symposium, 1995. [KASLR] PaX Team, "Address Space Layout Randomization", . [Klein2007] Klein, A., "OpenBSD DNS Cache Poisoning and Multiple O/S Predictable IP ID Vulnerability", 2007, . [Morris1985] Morris, R., "A Weakness in the 4.2BSD UNIX TCP/IP Software", CSTR 117, AT&T Bell Laboratories, Murray Hill, NJ, 1985, . [RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987, . [RFC4963] Heffner, J., Mathis, M., and B. Chandler, "IPv4 Reassembly Errors at High Data Rates", RFC 4963, DOI 10.17487/RFC4963, July 2007, . [RFC6056] Larsen, M. and F. Gont, "Recommendations for Transport- Protocol Port Randomization", BCP 156, RFC 6056, DOI 10.17487/RFC6056, January 2011, . [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, . [RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May 2014, . Gont & Arce Expires November 15, 2020 [Page 28] Internet-Draft Generation of Transient Numeric IDs May 2020 [RFC7707] Gont, F. and T. Chown, "Network Reconnaissance in IPv6 Networks", RFC 7707, DOI 10.17487/RFC7707, March 2016, . [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, . [RFC7739] Gont, F., "Security Implications of Predictable Fragment Identification Values", RFC 7739, DOI 10.17487/RFC7739, February 2016, . [Sanfilippo1998a] Sanfilippo, S., "about the ip header id", Post to Bugtraq mailing-list, Mon Dec 14 1998, . [Sanfilippo1998b] Sanfilippo, S., "Idle scan", Post to Bugtraq mailing-list, 1998, . [Sanfilippo1999] Sanfilippo, S., "more ip id", Post to Bugtraq mailing- list, 1999, . [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, . [Silbersack2005] Silbersack, M., "Improving TCP/IP security through randomization without sacrificing interoperability", EuroBSDCon 2005 Conference, 2005, . [TCPT-uptime] McDanel, B., "TCP Timestamping - Obtaining System Uptime Remotely", March 2001, . Gont & Arce Expires November 15, 2020 [Page 29] Internet-Draft Generation of Transient Numeric IDs May 2020 [Zalewski2001] Zalewski, M., "Strange Attractors and TCP/IP Sequence Number Analysis", 2001, . [Zalewski2002] Zalewski, M., "Strange Attractors and TCP/IP Sequence Number Analysis - One Year Later", 2001, . Appendix A. Flawed Algorithms The following subsections document algorithms with known negative security and privacy implications. 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. 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 November 15, 2020 [Page 30] Internet-Draft Generation of Transient Numeric IDs May 2020 /* Initialization code */ next_id = min_id; id_inc= 1; /* Numeric ID selection function */ count = max_id - min_id + 1; do { if (next_id == max_id) { next_id = min_id; } else { next_id = next_id + id_inc; } if (check_suitable_id(next_id)) return next_id; count--; } while (count > 0); return ERROR; Note: check_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 predicable sequences. If a global counter is used (such as "next_id" in the example above), a node that learns one numeric identifier can also learn or guess values employed by past and future protocol instances. On the other hand, when the value of increments is known (such as "1" in this case), an attacker can sample two values, and learn the number of identifiers that were generated in-between. Furthermore, if the counter is initialized e.g. when the system its bootstrapped to some known value, it will likely leak information (for example, the number of transmitted in the case of an IP ID generator [Sanfilippo1998a], or the system uptime in the case of TCP timestamps [TCPT-uptime]). Where identifier reuse would lead to a hard failure, one typical approach to generate unique identifiers (while minimizing the security and privacy implications of predictable identifiers) is to obfuscate the resulting numeric identifiers by either: Gont & Arce Expires November 15, 2020 [Page 31] Internet-Draft Generation of Transient Numeric IDs May 2020 o Replacing the global counter with multiple counters (initialized to a random value) o Randomizing the "increments" Avoiding global counters essentially means that learning one identifier for a given context (e.g., one TCP ephemeral port for a given {src IP, Dst IP, Dst Port}) is of no use for learning or guessing identifiers for a different context (e.g., TCP ephemeral ports that involve other peers). However, this may imply keeping one additional variables/counter per contexts, which may be prohibitive in some environments. 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 (as in "id_inc = 1" in our case) 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 identifier reuse frequency. A.2. Random-Increments Algorithm This algorithm offers a middle ground between the algorithms that select numeric identifiers randomly (such as those described in Section 7.1.1 and Section 7.1.2), and those that offer obfuscation but no randomization (such as those described in Section 7.4.2 and Section 7.4.3). Gont & Arce Expires November 15, 2020 [Page 32] Internet-Draft Generation of Transient Numeric IDs May 2020 /* Initialization code */ next_id = random(); /* Initialization value */ id_inc = 500; /* Determines the trade-off */ /* Numeric ID selection function */ id_range = max_id - min_id + 1; count = id_range; do { /* Random increment */ next_id = next_id + (random() % id_inc) + 1; /* Keep the identifier within acceptable range */ next_id = min_id + (next_id % id_range); if(check_suitable_id(next_id)) return next_id; count--; } while (count > 0); return ERROR; This algorithm aims at producing a monotonically-increasing sequence of 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 the level of obfuscation and the identifier reuse frequency. The smaller the value of "id_inc", the more similar this algorithm is to a predicable, global monotonically-increasing ID generation algorithm. The larger the value of "id_inc", the more similar this algorithm is to the algorithm described in Section 7.1.1 of this document. When the identifiers wrap, there is the risk of collisions of 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 obfuscation. Gont & Arce Expires November 15, 2020 [Page 33] Internet-Draft Generation of Transient Numeric IDs May 2020 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" should be configurable 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 implications than this algorithm, and hence implementation of this algorithm is discouraged. Authors' Addresses 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.com Gont & Arce Expires November 15, 2020 [Page 34]