On the Generation of Transient Numeric Identifiers
draft-irtf-pearg-numeric-ids-generation-07
Internet Research Task Force (IRTF) F. Gont
Internet-Draft SI6 Networks
Intended status: Informational I. Arce
Expires: August 6, 2021 Quarkslab
February 2, 2021
On the Generation of Transient Numeric Identifiers
draft-irtf-pearg-numeric-ids-generation-07
Abstract
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
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This Internet-Draft will expire on August 6, 2021.
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Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Threat Model . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Issues with the Specification of Transient Numeric
Identifiers . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. Protocol Failure Severity . . . . . . . . . . . . . . . . . . 7
6. Categorizing Transient Numeric Identifiers . . . . . . . . . 7
7. Common Algorithms for Transient Numeric Identifier Generation 10
7.1. Category #1: Uniqueness (soft failure) . . . . . . . . . 10
7.2. 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 . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. Network Activity Correlation . . . . . . . . . . . . . . 21
8.2. Information Leakage . . . . . . . . . . . . . . . . . . . 22
8.3. 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 . . . . . . . . . . . . . . . . . . 24
8.7. Cryptanalysis . . . . . . . . . . . . . . . . . . . . . . 25
9. Vulnerability Assessment of Transient Numeric Identifiers . . 26
9.1. Category #1: Uniqueness (soft failure) . . . . . . . . . 26
9.2. 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) . . . . . . . . . . . . . . . . . 27
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
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11. Security Considerations . . . . . . . . . . . . . . . . . . . 30
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
13.1. Normative References . . . . . . . . . . . . . . . . . . 30
13.2. Informative References . . . . . . . . . . . . . . . . . 32
Appendix A. Algorithms and Techniques with Known Issues . . . . 37
A.1. Predictable Linear Identifiers Algorithm . . . . . . . . 37
A.2. Random-Increments Algorithm . . . . . . . . . . . . . . . 39
A.3. Re-using Identifiers Across Different Contexts . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
1. 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])
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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).
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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
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 assume
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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].
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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 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 Identifiers
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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)
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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.
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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.
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, while Section 7.1.2 is an
implementation of the later. Typically, the algorithm in
Section 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 in Section 7.1.2 may
require many more iterations than the algorithm in Section 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 in Section 7.1.1 over the algorithm in Section 7.1.2.
7.1.1. Simple Randomization Algorithm
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/* 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 ephemeral
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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 in Section 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 from Section 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 in Section 8.
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7.2. 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 in Section 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 in Section 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
from Section 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.
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/* 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 be
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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 in Section 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:
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/* 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.
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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 in Section 8.
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7.4.2. 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
from Section 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() in
Section 7.3.
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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 in
Section 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 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 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 from Section 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 keep
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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 from Section 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() in Section 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 appropriate
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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 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 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 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 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 in Section 7.4.2) and the predictable
linear identifiers (described in Appendix 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
in Section 8.
8. Common Vulnerabilities Associated with Transient Numeric Identifiers
8.1. 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 activity
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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 constant
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"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 (see
Section 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.
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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 from Section 7.3 will typically allow
fingerprinting within the context where the resulting identifiers are
stable. Similarly, the algorithms from Section 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 from Section 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-
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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 in Section 7.4.2 and Section 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.
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9. Vulnerability Assessment of Transient Numeric Identifiers
The following subsections analyze possible vulnerabilities associated
with the algorithms described in Section 7.
9.1. Category #1: Uniqueness (soft failure)
Possible vulnerabilities associated with the algorithms from
Section 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 in Section 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 in Section 9.4 also applies to
this category.
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Additionally, as noted in Section 7.2, some transient numeric
identifiers of this category might be able to use the algorithms from
Section 7.1, in which case the same considerations as in Section 9.1
would apply.
9.3. Category #3: Uniqueness, stable within context (soft failure)
Possible vulnerabilities associated with the algorithms from
Section 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 in Section 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 in Section 7.4.2 and Section 7.4.3 for
generating identifiers of Category #4, is as follows:
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/* 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.
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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 from
Section 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 in Section 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
(see Appendix 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 of
Section 9.3, with the addition of a monotonically-increasing
function, all the issues discussed in Section 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 in Section 7.4.3).
o Transient numeric identifiers generated with the algorithms
described in Section 7.4.2 and Section 7.4.3 will normally allow
for fingerprinting within CONTEXT since, for such context, the
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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. References
13.1. Normative References
[RFC0791] Postel, J., "Internet Protocol", STD 5, RFC 791,
DOI 10.17487/RFC0791, September 1981,
<https://www.rfc-editor.org/info/rfc791>.
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[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>.
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[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>.
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[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>.
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[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.u
senix.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_Vuln
erability.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>.
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[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>.
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[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/>.
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[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 in Section 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 see Section 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.:
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/* 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].
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A.2. Random-Increments Algorithm
This algorithm offers a middle ground between the algorithms that
generate randomized transient numeric identifiers (such as those
described in Section 7.1.1 and Section 7.1.2), and those that
generate identifiers with a predictable monotonically-increasing
function (see Appendix 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 is
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to a predicable, global linear ID generation algorithm (as the one in
Appendix A.1). 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 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' Addresses
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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
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