Internet DRAFT - draft-mcgrew-iv-gen
draft-mcgrew-iv-gen
Network Working Group D. McGrew
Internet-Draft Cisco Systems, Inc.
Intended status: Standards Track October 15, 2013
Expires: April 18, 2014
Generation of Deterministic Initialization Vectors (IVs) and Nonces
draft-mcgrew-iv-gen-03.txt
Abstract
Many cryptographic algorithms use deterministic IVs, including CTR,
GCM, CCM, GMAC. This type of IV is also called a (deterministic)
nonce. Deterministic IVs must be distinct, for each fixed key, to
guarantee the security of the algorithm. This note describes best
practices for the generation of such IVs, and summarizes how they are
generated and used in different protocols. Some problem areas are
highlighted, and test considerations are outlined. This note will be
useful to implementers of algorithms using deterministic IVs, and to
protocol or system designers using them.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on April 18, 2014.
Copyright Notice
Copyright (c) 2013 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
Provisions Relating to IETF Documents
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carefully, as they describe your rights and restrictions with respect
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to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions Used In This Document . . . . . . . . . . . . 3
2. Deterministic IVs in Algorithms . . . . . . . . . . . . . . . 4
3. Deterministic IVs in Protocols . . . . . . . . . . . . . . . . 5
4. Deterministic IVs in Standards . . . . . . . . . . . . . . . . 7
4.1. Recommended IV/Nonce Format . . . . . . . . . . . . . . . 7
4.2. Partially Implicit IV/Nonce Format . . . . . . . . . . . . 8
4.3. Alternative IV/Nonce Format . . . . . . . . . . . . . . . 9
4.4. Unpredictable IV/Nonce Format . . . . . . . . . . . . . . 10
4.5. ESP . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.6. IKE . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.7. TLS . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.8. SSH . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.9. SRTP . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.10. Summary . . . . . . . . . . . . . . . . . . . . . . . . . 12
5. Implementation . . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. IV Verification . . . . . . . . . . . . . . . . . . . . . 16
6. Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. Internal IV Generator . . . . . . . . . . . . . . . . . . 17
7. Issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Choice of Fixed-Distinct Field . . . . . . . . . . . . . . 18
7.2. Size of the Fixed-Distinct Field . . . . . . . . . . . . . 19
7.3. Security . . . . . . . . . . . . . . . . . . . . . . . . . 19
7.4. Bandwidth Use . . . . . . . . . . . . . . . . . . . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 21
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
10.2. Informative References . . . . . . . . . . . . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
This note describes deterministic IVs and nonces and how they are
used in cryptographic algorithms (Section 2), then describes their
use in protocols (Section 3), and then their use in standards
(Section 4). Considerations for implementation (Section 5) and
testing (Section 6) are presented. Issues and potential problems are
discussed (Section 7). The focus is on network security protocols,
rather than on the security of data at rest, though many of the same
considerations apply in both areas.
1.1. Conventions Used In This Document
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 [RFC2119].
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2. Deterministic IVs in Algorithms
Many cryptographic algorithms use Initialization Vectors, or IVs. An
IV is provided to an algorithm along with a message to be processed;
the IV initializes the algorithm to process the message. Typically,
there will be many IVs that are used with a single key. Some
algorithms, such as the Cipher Block Chaining (CBC) encryption mode,
require that the IVs that it uses are completely unpredictable. Such
IVs are typically called random IVs, and they must be generated by a
cryptographically strong random or pseudorandom process [RFC4086].
Another type of IVs are deterministic IVs. These IVs are generated
by a deterministic process. The classic example of an algorithm that
uses a deterministic IV is counter (CTR) mode encryption [CTR]. An
algorithm that uses deterministic IVs requires that each IV provided
as input to the algorithm be distinct, for a fixed key.
A deterministic IV is sometimes called a nonce, or a deterministic
nonce. In cryptography, a nonce is a value that is used only once.
Many cryptographic protocols include a nonce in a message to enable
its receiver to recognize whether or not the message has been
previously received and processed. From the point of view of a
cryptographic algorithm that uses deterministic IVs, calling the IV a
nonce emphasizes the role of the IV in the overall system or
protocol. Calling that value a deterministic IV emphasizes its role
in initializing the algorithm to process a new message. Nonetheless,
these are just different monikers for the same thing.
Authenticated Encryption is a symmetric encryption method that
provides for the authenticity and integrity of the data that it
protects, as well as its confidentiality [BN00] [R02]. An
authenticated encryption method that uses deterministic IVs will need
to make sure that the IVs used for encryption are distinct. However,
when performing the decryption operation, there is no need to ensure
that the IVs are distinct; the authenticated decryption operation
does not impose that requirement. The Authenticated Encryption
methods used in standards include Galois Counter Mode [GCM] and
Counter and CBC MAC mode [CCM].
Some Message Authentication Code (MACs) use deterministic IVs,
including GMAC [GCM] and UMAC [RFC4118]. The considerations for
Authenticated Encryption also apply to these MAC algorithms: the IVs
used in the generation of an authentication tag must be distinct, but
there is no need to verify the distinctness of an IV prior to
inputting that IV to a tag verification algorithm.
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3. Deterministic IVs in Protocols
The simplest way to implement a deterministic IV or nonce is to use a
counter: initialize an integer variable to zero, then each time that
an IV is needed, output the integer value, then store the incremented
value after checking to make sure that no integer overflow occurred,
so that no counter value is used twice. The simplicity of this
method has made it popular in practice, and recommended by standards.
The straightforward method of using a counter is not sufficient when
there are multiple encryption engines that are using the same
encryption key. This can be the case when encryption is distributed
across multiple processors, or across multiple software threads,
processes, or virtual machines. It can also happen in cases where a
protocol allows group keys. In these cases, some mechanism is needed
that ensures that IVs are distinct across all encryption engines that
use the same key. This is easily accomplished by including a fixed
field in the IV that is distinct for each distinct encrypter. (This
is detailed in Section 4.1.)
When a deterministic IV is used to encrypt and/or authenticate a
message, the receiver(s) of that message needs to know that IV in
order to decrypt it and/or verify its authenticity. A deterministic
IV can be sent along with a message, which makes it plain to the
receiver(s), or it can be left out of a message if the receiver(s)
have enough information to reconstruct it. Leaving the IV out of the
message reduces the amount of data that must be communicated, which
is advantageous. On the other hand, if the IV is included in the
message, the receiver(s) need not be aware of the method by which the
sender has chosen the IVs.
In practice, some protocols have split the difference between the
implicit method (in which the IV is absent and a receiver infers its
value) and the explicit method (in which the entire IV is included
with the message). The IV is constructed out of two fields: an
explicit field, which is conveyed along with the message, and an
implicit field, which is coordinated between the encrypter and the
decrypter using an "out of band" method. (This is detailed in
Section 4.2.) In most cases, the key management protocol that
establishes the encryption key can also establish the implicit field.
In a block cipher mode of operation that use deterministic IVs, the
inputs to each of the block cipher invocations during the encryption
process are determined by the IV provided to that process. It is
desirable to make the inputs to the block cipher unpredictable to an
attacker, to the extent that is possible, to make cryptanalytic
attacks more difficult and costly to attackers. This is true for
several types of attacks, including time-memory tradeoff attacks and
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key collision attacks [MF00], which are generic attacks that can be
applied to any cipher, and cipher-specific attacks such as integral
cryptanalysis [KW02]. (It is worth noting that counter mode gives an
attacker exactly what they want for integral cryptanalysis: a
complete set of block cipher inputs that differ only in some bit
positions.) The cost of these attacks can be significantly increased
by making the deterministic IV unpredictable to potential attackers.
This security benefit is one motivation for why the implicit field of
the deterministic IV is kept secret in some protocols.
It is not hard to adapt the simple methods for constructing
deterministic IVs so that they produce IVs that are unpredictable.
An easy way to do that is to have a secret value that is bitwise
exclusive-ored into the IV after all of the other processing is done.
(This is detailed in Section 4.4.) This secret value must be known
to all encrypters and decrypters, and be established via some "out of
band" mechanism. In practice, it is typically established by the key
management system.
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4. Deterministic IVs in Standards
Many different protocols use deterministic IVs, including ESP
[RFC4106], TLS [RFC5288], SSH [RFC5647], and SRTP [RFC3711]. The way
that these protocols define their IVs is outlined in this section and
is summarized in Table 1.
4.1. Recommended IV/Nonce Format
RFC 5116 defines the interface for Authenticated Encryption, which is
the most common use of deterministic IVs at present. That RFC
recommends an IV format that is used by ESP, IKE, TLS, and SSH. The
recommended format has a total length of 12 octets, and consists of a
Fixed Field and a Counter field, and is structured as in Figure 1.
(See Section 3.2 of [RFC5116] for the precise normative description.)
+-------------------------------+------------------------+
| Fixed | Counter |
+-------------------------------+------------------------+
Figure 1: Recommended IV/Nonce format.
Fixed Counter
<------><-------------->
1st 5DAD87F80000000000000001
2nd 5DAD87F80000000000000002
3rd 5DAD87F80000000000000003
4th 5DAD87F80000000000000004
5th 5DAD87F80000000000000005
... ...
Figure 2: An example output of recommended IV/nonce format, showing
successive IVs where the Fixed field is 5DAD87F8.
The Fixed field remains constant for all nonces that are generated
for a given encryption device. If different devices are performing
encryption with a single key, then each distinct device MUST use a
distinct value for the Fixed field, to ensure the uniqueness of the
nonces that it generates.
This format is suggested, but not required, by [CTR].
With this format, the Counter fields of successive nonces form a
monotonically increasing sequence, when those fields are regarded as
unsigned integers in network byte order.
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4.2. Partially Implicit IV/Nonce Format
The case in which the recommended format is used with Partially
Implicit Nonces has further details. In that case, the IV is
structured as in Figure 3.
+--------------+----------------+------------------------+
| Fixed-Common | Fixed-Distinct | Counter |
+--------------+----------------+------------------------+
<- implicit -> <--------------- explicit -------------->
Figure 3: Partially implicit IV/Nonce format
Fixed Fixed
Common Distinct Counter
<------><--><---------->
1st 5DAD87F81E0E000000000001
2nd 5DAD87F81E0E000000000002
3rd 5DAD87F81E0E000000000003
4th 5DAD87F81E0E000000000004
5th 5DAD87F81E0E000000000005
... ...
Figure 4: An example output of Partially Implicit IV/Nonce format,
showing successive IVs where the Fixed-Common field is 5DAD87F8 and
the Fixed-Distinct field is 1E0E.
The portion of the IV that is stored or sent with the ciphertext is
the explicit part. The portion of the IV that is not sent with the
ciphertext is the implicit part.
The Fixed field is divided into two sub-fields: a Fixed-Common field
and a Fixed-Distinct field.
If different devices are performing encryption with a single key,
then each distinct device MUST use a distinct Fixed-Distinct field.
The Fixed-Common field is common to all IVs. The Fixed-Distinct
field and the Counter field MUST be in the explicit part of the IV.
The Fixed-Common field MAY be in the implicit part of the IV.
ESP, IKE, TLS, and SSH conform to the alternative IV/nonce format,
though in practice the partially implicit format is often used.
Those standards do not require that the "Changing" field actually be
a counter (instead, "anything that guarantees uniqueness can be
used"), but in practice a counter is convenient.
The partially implicit format can save on bandwidth or data storage
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requirements, because it avoids sending or storing the implicit part
of the IV. However, it limits the number of IVs that can be
generated, because the implicit part is fixed, and it adds complexity
to the system, by making the system coordinate the implicit part
through out-of-band means. Thus, new protocol and system designs
SHOULD NOT use the partially implicit format unless a review of all
of the issues shows that the bandwidth or storage savings are worth
the complexity. (An alternative strategy for bandwidth savings is
discussed in Section 7.4.)
4.3. Alternative IV/Nonce Format
In some cases, it may be desirable to avoid the use of a network byte
order monotonically increasing counter. This would be especially
true in a protocol that has the security goal of obscuring the
relationship between messages, so that an attacker cannot infer that
particular messages belong to the same flow, and cannot infer the
order of messages within a flow. In other cases, there may be a data
element available that meets the requirement of being distinct for
each invocation of the encryption operation, but is not monotonically
increasing in network byte order. In such situations, it makes sense
to use an alternative nonce format that replaces the Counter field,
as it is used above, with a field that is distinct for each IV/nonce
that is generated, but which is not a counter. We call this field
the Changing field. The alternative format is shown in Figure 5, and
an example output is shown in Figure 6.
IVs/Nonces that are in the partially implicit format also happen to
conform to the alternative format as well.
+--------------+----------------+------------------------+
| Fixed-Common | Fixed-Distinct | Changing |
+--------------+----------------+------------------------+
<- implicit -> <--------------- explicit -------------->
Figure 5: Alternative IV/Nonce format
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Fixed
Common Changing
<------><-------------->
1st 88be20d4600ced63f924ff5b
2nd 88be20d458a0169be27d661f
3rd 88be20d43ece0f6a4061eca8
4th 88be20d46016f0a41c1cbc27
5th 88be20d4ba7f697eb00aee67
... ...
Figure 6: An example output of Alternative IV/Nonce format, showing
successive IVs where the Fixed-Common field is 5DAD87F8 and the
Fixed-Distinct field is zero length (or equivalently, is not
present).
4.4. Unpredictable IV/Nonce Format
This method is shown in Figure 7, in which the symbol (+) denotes the
bitwise exclusive-or operation. (Here the Fixed field consists of
the Fixed-Common field followed by the Fixed-Distinct field.) This
format uses a Randomizer, which is an octet string that is combined
with the other fields to make the IVs unpredictable. The length of
the Randomizer must be no greater than the sum of the lengths of the
Fixed and Counter fields.
The next IV in sequence is computed as follows. The Fixed field and
the Counter field are concatenated. If the length of the Randomizer
is less than the combined length of the Fixed and Counter fields,
then the Randomizer is padded on the right with enough zeros so that
the padded value has a length that exactly matches that of the Fixed
and Counter fields together. The concatenated Fixed and Counter
field is bitwise exclusive-ored with the padded Randomizer, and the
resulting value is the IV. The Counter is incremented, treating it
as an unsigned integer with the most significant byte on the left,
and the stored Counter field is set to the incremented value. Then
the IV is returned. This is the method used by SRTP [RFC3711],
wherein the Randomizer field is called "Salt". (We use the term
Randomizer instead of Salt, because the latter term is used with
slightly different meanings in some other specifications, such as
[RFC4309].)
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+-----------------+-----------------+
| Fixed | Counter |---+
+-----------------+-----------------+ |
|
+-----------------------------------+ v
| Randomizer |->(+)
+-----------------------------------+ |
|
+-----------------------------------+ |
| Initialization Vector |<--+
+-----------------------------------+
Figure 7: Unpredictable IV/Nonce Format.
Fixed Fixed
Common Distinct Counter IV
<--><------><----------> <---------------------->
1st 000097B4AE8F000000000001 0C81C77A5DDB678EE16FA2D0
2nd 000097B4AE8F000000000002 0C81C77A5DDB678EE16FA2D3
3rd 000097B4AE8F000000000003 0C81C77A5DDB678EE16FA2D2
4th 000097B4AE8F000000000004 0C81C77A5DDB678EE16FA2D5
5th 000097B4AE8F000000000005 0C81C77A5DDB678EE16FA2D4
... ...
Figure 8: An example output of the Unpredictable IV/nonce format,
showing successive IVs where the Fixed-Distinct field has the value
97B4AE8F and the Salt has value 0C8150CEF354678EE16FA2D1.
4.5. ESP
In the IP Encapsulating Security Payload (ESP)
[RFC3686][RFC4106][RFC4309] the implicit and explicit parts are four
and eight bytes long, respectively. The exception is [RFC4309], for
which the implicit part is three bytes in length. The Fixed-Common
field is four bytes, and its value is set by the Internet Key
Exchange (IKE). (This field is named inconsistently, being called
Nonce in [RFC3686], and Salt in [RFC4106] and [RFC4309].) When ESP
is used with IKE, there is exactly one entity performing encryption,
and the Fixed-Distinct part is usually not present (or equivalently,
is has a length of zero bytes). When ESP is used with a group key
management protocol such as GDOI, the Fixed-Distinct field may be two
or four bytes in length, and the value of the Fixed-Distinct field to
be used by an encrypter is established by the group key management
protocol [RFC6054]. The case in which IKE is used with ESP and there
are multiple encryption engines is not specifically addressed by the
standards, but it can be handled by the use of a nonzero Fixed-
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Distinct field.
4.6. IKE
The Internet Key Exchange (IKE) [RFC5282] uses the recommended IV/
nonce format. The Fixed-Common field is four bytes in length, and
its value is set from the IKE Keying Material. The Fixed-Distinct
part is usually zero bytes, but it may be any number of bytes if
there are multiple encrypters in use.
4.7. TLS
In Transport Layer Security (TLS) [RFC5288], the Fixed-Common field
is four bytes in length, and the Fixed-Distinct part is usually zero
bytes, but it may be any number of bytes when there are multiple
encrypters in use. Section 6.2 of [RFC5288] gives an example of TLS
deterministic IV formation.
4.8. SSH
In the Secure Shell (SSH) protocol [RFC5647] the Fixed-Common field
is not present, the Fixed-Distinct field is four bytes long, and the
Counter field is eight bytes in length. The implicit part is not
present, and the explicit part contains the entire 12 byte IV.
4.9. SRTP
The Secure Real-time Transport Protocol (SRTP) [RFC3711] and
draft-ietf-avt-srtp-aes-gcm-01 use the unpredictable format, which is
a bit more complex than RFC 5116. It is essentially RFC 5116 format
with the additional step of performing a bitwise exclusive-or
operation with a Randomizer value. (This step provides additional
strength against cryptographic attacks that rely on predicting all or
most of the IV.) draft-ietf-avt-srtp-aes-gcm-01 uses a 12-byte IV,
though RFC 3711 uses a 14-byte IV.
4.10. Summary
The following table gives a synopsis of how standard protocols use
deterministic IVs.
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+-----------+---------+--------------+----------------+-------------+
| Protocol | IV | Fixed-Common | Fixed-Distinct | Counter |
| | (bytes) | (bytes) | (bytes) | (bytes) |
+-----------+---------+--------------+----------------+-------------+
| ESP | 12 | 4 | 0,1,2,[4] | 8,7,6,[4] |
| | | | | |
| | | Not on wire | On wire | On wire |
| | | | | |
| | | Set by IKE | | |
| | | | | |
| ESP | 11 | 3 | 0,1,2,[4] | 8,7,6,[4] |
| | | | | |
| [RFC4309] | | Not on wire | On wire | On wire |
| | | | | |
| | | Set by IKE | | |
| | | | | |
| IKE | 12 | 4 | Unspecified | Unspecified |
| | | | | |
| | | Not on wire | On wire | On wire |
| | | | | |
| | | Set from KM | | |
| | | | | |
| TLS | 12 | 4 | 0-8 | 8-0 |
| | | | | |
| | | Not on wire | On wire | On wire |
| | | | | |
| | | Set by TLS | | |
| | | | | |
| SSH | 12 | 0 | 4 | 8 |
| | | | | |
| | | | On wire | On wire |
| | | | | |
| | | | Unspecified | |
| | | | | |
| SRTP-CTR | 14 | 4 | 4 | 6 |
| | | | | |
| | | Not on wire | Not on wire | Not on wire |
| | | | | |
| | | Set by KM | | |
| | | | | |
| SRTP-GCM | 12 | 2 | 4 | 6 |
| | | | | |
| | | Not on wire | Not on wire | Not on wire |
| | | | | |
| | | Set by KM | | |
+-----------+---------+--------------+----------------+-------------+
Table 1: Fields in Deterministic IVs, by Protocol.
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5. Implementation
A cryptographic implementation typically consists of a self-contained
and testable module that implements all of the essential
functionality that it needs. This functionality should include the
generation of deterministic IVs.
Because of the variety of ways in which IVs are formed in different
protocols, implementers may be tempted to put the generation of the
IV under the control of the protocol implementation. That is, from
the point of view of the application making use of the encryption
algorithm, the IV is an input to that algorithm, as shown in
Figure 9. Regardless, it is not good for security to have the IV be
generated outside the crypto module. It is possible to implement an
IV Generator that can be used with all of the protocols outlined
above and use it inside of a cryptographic module. In the following
we outline how that can be done.
+----------------------+
| +--------------+ | IV +-------------+
| | |<-------------------| |
| | Encryption | | Plaintext | |
| | Algorithm |<-------------------| Application |
| | | | Ciphertext | |
| | |------------------->| |
| +--------------+ | +-------------+
| |
| Cryptographic Module |
+----------------------+
Figure 9: Architecture with IV generation outside of the
cryptographic module, showing how the IV is entered into the
cryptographic module during an encryption operation.
The internal IV generator architecture is illustrated in Figure 10.
The cryptographic module contains an IV Generator sub-module that
understands the IV formats outlined in Section 3. To initialize the
IV generator, the application inputs the parameter values to be used.
Once initialized, the IV generator will produce successive IVs on
request, and send these values to the algorithm and to the calling
application. The encryption algorithm will need the entire IV, but
if the partially implicit IV format is in use, only the explicit part
of the IV needs to be provided to the application. The IV generator
is responsible for ensuring the distinctness of all of the IVs that
it generates.
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+----------------------+
| +--------------+ |
| | IV Generator |-----------+
| +--------------+ | | IV (explicit part)
| | IV | |
| v | |
| +--------------+ | | +-------------+
| | | +------->| |
| | Encryption | | Plaintext | |
| | Algorithm |<-------------------| Application |
| | | | Ciphertext | |
| | |------------------->| |
| +--------------+ | +-------------+
| |
| Cryptographic Module |
+----------------------+
Figure 10: Architecture with IV generation inside of the
cryptographic module, showing how the IV is generated internally
during an encryption operation.
More formally, an IV generator supports the operations of Initialize
and Output Next IV. The Initialize operation prepares an IV
Generator for use with a particular set of parameters. It takes the
following inputs:
A nonnegative integer indicating the number of bytes in the IV to
be generated. All of the IVs output from the Generator will have
the same length.
An octet string indicating the Fixed part of the IV; this value
will be used as the initial part in each IV that is generated.
A nonnegative integer indicating the number of bytes in the Fixed
part of the IV. This value must be no greater than the number of
bytes in the IV.
An octet string indicating the salt value to be exclusive-ored
with the other fields of the IV. If no salt is to be used when
Generating IVs, then this parameter must not be present.
A nonnegative integer indicating the number of bytes in the salt
value. If no salt value is used, this parameter must be zero. If
a salt value is used, this parameter must be no greater than the
number of bytes in the IV.
The Fixed field consists of the Fixed-Common field, followed by the
Fixed-Distinct field. The Fixed field and Salt field are stored when
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the IV generator is initialized; at that time, the Counter field is
initialized to zero. The length of the Counter field is equal to the
length of the IV less the length of the Fixed field. If the Salt
field is shorter than the IV, then it is padded on the right with
zeroes. If no Salt is to be used, this is conceptually equivalent to
having a Salt value that is the all-zero value.
The Output Next IV operation returns the next IV in sequence, or it
returns an indication that there are no more IVs that are available.
During that operation, the IV is computed as follows. First, the
stored Counter value MUST be checked to determine if an IV can be
generated; an IV can only be generated if the value of Counter + 1
does not exceed the maximum allowable value of the Counter field. If
an IV cannot be generated, then the operation returns an indication
that there are no more IVs that are available. Otherwise, the Fixed
field and the Counter field are concatenated, then they are bitwise
exclusive-ored with the Salt field, and the resulting value is the
IV. The Counter is incremented, treating it as an unsigned integer
with the most significant byte on the left, and the stored Counter
field is set to the incremented value. Then the IV is returned.
The IV generator should also be able to output the length of the
explicit field, so that an algorithm can output only the explicit
part, when that is appropriate.
5.1. IV Verification
In some protocols, the IV is constructed out of fields in the
protocol in such a way that it is difficult to have the IVs generated
inside of the cryptographic module, without requiring that module to
contain protocol-specific logic. In this case, assurance of the
uniqueness of IVs can be provided by having the IVs be generated by
the protocol, but checked by the cryptographic module.
This approach is taken by many implementations of Secure RTP
[RFC3711]. The IV in that protocol is constructed in a way that
incorporates a sender identifier (the SSRC field) and the protocol's
sequence number. To check the sequence number for uniqueness, an
implementation can make use of the anti-replay checking that the
protocol uses to check inbound packet. An encrypter can use this
approach as well, to make sure that the sequence number used to
construct the IV is unique. (Of course, it is necessary to have an
IV construction method such that the uniqueness of the sequence
number ensures the uniqueness of the IV.) Since many cryptographic
protocols contain a function to perform anti-replay check based on a
sequence number, this is a convenient strategy.
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6. Testing
The testing of a cryptographic module is an important step in
assessing the assurance of that module. The IV Generator defined in
Section 5 can be tested by an external system to verify that it is
operating correctly.
Any IV format can be tested by verifying that all of the IVs are
distinct. There are many ways that this can be done; for instance,
the command "sort | uniq -d" on POSIX systems can be used to detect
repeated lines in a file.
The recommended format can be tested by verifying that the Counter
field consists of monotonically increasing values.
An important aspect of an IV generator is that, when it has an N byte
Counter or Changing field, it should not generate more than (256)^N
IVs. This property should be tested for small values of N (at least
1, 2, and 3), by calling the Output Next IV operation M times, for
some M > (256)^N. Note that some implementations may produce fewer
than (256)^N IVs, e.g. due to their handling of the all-zero IV.
That would not affect security.
6.1. Internal IV Generator
When a cryptographic module uses an internal IV generator, only the
explicit part of the IV needs to be output from the module. It is
possible to test this use of the IV generator by interacting with an
encryption algorithm that uses it (or an Authenticated Encryption
algorithm, or a MAC).
The encryption operation takes as input a plaintext, and returns a
ciphertext and the explicit part of the IV. To test that the IV
generator is working properly, call the encryption operation
repeatedly, each time with the same plaintext value, and verify that
1) all of the ciphertexts returned are distinct, and 2) all of the
explicit parts that are returned are distinct. The plaintext must be
at least 32 bytes long, in order to avoid false positives.
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7. Issues
7.1. Choice of Fixed-Distinct Field
When considering what data should go into a Fixed-Distinct field, it
is tempting to use system values such as network addresses because
they appear to meet the criteria of uniqueness. However, there are
several significant problems with this idea. System values that are
taken from outside the cryptographic module may not actually be
distinct, especially if an attacker can influence the system. System
values can also change over time; even if they are actually distinct,
they may not be fixed. Lastly, the cryptographic system should have
the freedom to put distinct data into the Fixed-Distinct fields, so
that it can accommodate multiple encryption engines when they occur.
Internet Protocol (IP) version four addresses are four bytes in
length, and thus can fit into the Fixed-Distinct field of a 12-byte
IV. However, an IP address is highly unsuitable for this purpose.
Most networked devices use dynamically assigned IP addresses, with
address assignment via an automatic configuration protocol such as
the Dynamic Host Configuration Protocol (DHCP). The addresses are
determined by an external system and are communicated over an
insecure protocol; furthermore, a DHCP address is only valid for a
particular period of time, and may change after that lease has
expired. Even when an automatic configuration protocol is not in
use, IP addresses are determined by the networking subsystem, and are
not under the control of the cryptographic module. Network Address
Translation (NAT, [RFC1361]) is commonly used to modify the IP
addresses of packets as they traverse a network boundary, for
instance between a private address space [RFC1918] and the Internet.
Because of NAT, the IP address associated with a particular device
will not be consistent throughout the network. Multiple devices can
use the same addresses; this technique is utilized in order to
provide redundancy or load sharing (see the Virtual Router Redundancy
Protocol [RFC3768] for instance). Lastly, IPv4 is currently being
replaced by version six of that protocol. IPv6 addresses are sixteen
bytes long; this is too long for inclusion in an IV, and the
coexistence of both versions on the Internet is likely to increase
the use of NAT for protocol translation [RFC6146]. In summary, IP
addresses are neither fixed nor distinct, and should not be used in a
Fixed-Distinct field.
Similar considerations hold for link layer addresses, Domain Name
System (DNS) names, and TCP, UDP, and SCTP ports.
A much better solution is to have the Fixed-Distinct field be
assigned by the security system. For instance, if a cryptographic
module has multiple encrypters, it can assign that field
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appropriately for each encrypter.
7.2. Size of the Fixed-Distinct Field
Deterministic IVs typically have an explicit part that is eight bytes
in length. (This size is natural to use with a block cipher that has
a 16 byte block width, because no more than (256)^8 packets can be
encrypted under a single key without encountering security
degradation due to the birthday paradox.) Because the Fixed-Distinct
field must appear in the explicit part, larger Fixed-Distinct fields
will reduce the number of IVs that can be generated. This can be
problematic, especially for high throughput situations. For
instance, the ESP protocol allows for up to 2^64 packets to be
encrypted under a single key, so it is desirable to use a Counter
field that is close to eight bytes in length; this is why [RFC6054]
encourages the use of short values in the Fixed-Distinct field.
Table 2 presents the lifetimes of a single key that can encrypt 2^32
packets, i.e. a key being used with a four-byte Counter field. At
high data rates, keys must be replaced quickly.
+----------+-------------------+------------------+-----------------+
| | Best Case | Typical Case | Worst Case |
+----------+-------------------+------------------+-----------------+
| | 9000 byte packets | 850 byte packets | 64 byte packets |
| | | | |
| 1 Gbps | 3 days | 8.6 hours | 66 minutes |
| | | | |
| 10 Gbps | 8.6 hours | 52 minutes | 6.6 minutes |
| | | | |
| 40 Gbps | 22 minutes | 13 minutes | 1.6 minutes |
| | | | |
| 100 Gbps | 8.9 minutes | 5.2 minutes | 0.7 minutes |
+----------+-------------------+------------------+-----------------+
Table 2: Key Lifetimes with a four-byte Counter field
7.3. Security
As long as each deterministic IV is distinct, for each key, then
security is assured. However, when deterministic IVs are not
distinct, security suffers.
The number of deterministic IVs is limited, regardless of how those
IVs are generated. What does an encrypter do when no more IVs are
available? It should retire the key that it is currently using, and
establish another one. This is the reason that the IETF Guidelines
for Cryptographic Key Management [RFC4107] require that automated key
management be used for algorithms with deterministic IVs. For
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network security protocols, this has proven to be an effective
strategy.
Particular care must be taken in Virtual Machine (VM) environments,
because the VM cloning and rollback processes can cause inadvertent
re-use of deterministic IVs. This is just one of many security
problems that can result from uncritical application of VM mechanisms
when cryptography is in use [GR05].
7.4. Bandwidth Use
An implicit or partially implicit IV uses less bandwidth than a full-
sized IV. But as noted above, the (partially) implicit IV format
reduces the number of IVs that can be generated and adds complexity
to the system.
An alternative approach to bandwidth savings in a protocol design is
to use a predictable IV format, such as that of Section 4.1, and then
apply header compression to the IV. Header compression is often used
on bandwidth-constrained links, and it can be applied to encrypted
packets [RFC3095]. The format of Section 4.1 can easily be handled
by header compression. This approach has several benefits: it makes
IV generation simpler, it allows bandwidth savings for environments
in which it matters while putting the complexity burden onto the
systems that opt to realize those savings, and it increases the
number of IVs that can be used. Specifications that use this design
alternative SHOULD require the use of the IV format in Section 4.1.
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8. Security Considerations
Cryptographic algorithms that rely on deterministic IVs or nonces
must ensure the uniqueness of those values. The recommendations in
this note aim to help implementers achieve that goal.
Implementations should use the nonce formats described in Section 3.
The way in which these formats are used in standards is summarized in
Table 1.
Implementations should use the internal IV generator described in
Section 5.
Almost all cryptographic systems can implement counter-based
deterministic IVs. In many cases, it is straightforward to generate
deterministic IVs associated with a short-term key in use by a single
encrypter, as in a typical point-to-point protocol. Complications
can arise, however, when there are multiple encrypters, or when a key
is used for an extended period of time. Cryptographic systems that
cannot ensure IV distinctness should not use deterministic IVs, and
should instead use a misuse-resistant mode of operation such as the
Synthetic Initialization Vector (SIV) Authenticated Encryption mode
of operation [RFC5295], or a randomized algorithm such as the CBC
mode of operation (though an additional authentication mechanism must
be used with that option). If authentication but not encryption is
required, then it is possible to use an algorithm that does not
require an IV, such as HMAC [RFC2104].
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9. Acknowledgments
Thanks to Greg Zaverucha and Peter Gutmann for comments.
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10. References
10.1. Normative References
[CCM] Dworkin, M., "NIST Special Publication 800-38C: The CCM
Mode for Authentication and Confidentiality", U.S.
National Institute of Standards and Technology http://
csrc.nist.gov/publications/nistpubs/800-38C/SP800-38C.pdf.
[GCM] Dworkin, M., "NIST Special Publication 800-38D:
Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) and GMAC.", U.S. National
Institute of Standards and Technology http://
csrc.nist.gov/publications/nistpubs/800-38D/SP800-38D.pdf.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008.
[RFC5647] Igoe, K. and J. Solinas, "AES Galois Counter Mode for the
Secure Shell Transport Layer Protocol", RFC 5647,
August 2009.
10.2. Informative References
[BN00] Bellare, M. and C. Namprempre, "Authenticated encryption:
Relations among notions and analysis of the generic
composition paradigm", Proceedings of ASIACRYPT 2000,
Springer-Verlag, LNCS 1976, pp. 531-545 http://
www-cse.ucsd.edu/users/mihir/papers/oem.html.
[CTR] Dworkin, M., "NIST Special Publication 800-38:
Recommendation for Block Cipher Modes of Operation", U.S.
National Institute of Standards and Technology http://
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csrc.nist.gov/publications/nistpubs/800-38a/sp800-38a.pdf.
[GR05] Garfinkel, T. and M. Rosenblum, "When Virtual is Harder
than Real: Security Challenges in Virtual Machine Based
Computing Environments", Proceedings of the 10th Workshop
on Hot Topics in Operating Systems http://
www.stanford.edu/~talg/papers/HOTOS05/
virtual-harder-hotos05.pdf.
[KW02] Knudsen, L. and D. Wagner, "Integral Cryptanalysis", 9th
International Workshop on Fast Software Encryption (FSE
'02) http://eprint.iacr.org/2004/193, December 2001.
[MF00] McGrew, D. and S. Fluhrer, "Attacks on Additive Encryption
of Redundant Plaintext and Implications on Internet
Security", Proceedings of the Seventh Annual Workshop on
Selected Areas in Cryptography (SAC 2000) Spinger-Verlag.
[R02] Rogaway, P., "Authenticated encryption with Associated-
Data", ACM Conference on Computer and Communication
Security (CCS'02), pp. 98-107, ACM Press,
2002. http://www.cs.ucdavis.edu/~rogaway/papers/ad.html.
[RFC1361] Mills, D., "Simple Network Time Protocol (SNTP)",
RFC 1361, August 1992.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC3095] Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
Compression (ROHC): Framework and four profiles: RTP, UDP,
ESP, and uncompressed", RFC 3095, July 2001.
[RFC3686] Housley, R., "Using Advanced Encryption Standard (AES)
Counter Mode With IPsec Encapsulating Security Payload
(ESP)", RFC 3686, January 2004.
[RFC3768] Hinden, R., "Virtual Router Redundancy Protocol (VRRP)",
RFC 3768, April 2004.
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[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
[RFC4118] Yang, L., Zerfos, P., and E. Sadot, "Architecture Taxonomy
for Control and Provisioning of Wireless Access Points
(CAPWAP)", RFC 4118, June 2005.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM
Mode with IPsec Encapsulating Security Payload (ESP)",
RFC 4309, December 2005.
[RFC5282] Black, D. and D. McGrew, "Using Authenticated Encryption
Algorithms with the Encrypted Payload of the Internet Key
Exchange version 2 (IKEv2) Protocol", RFC 5282,
August 2008.
[RFC5295] Salowey, J., Dondeti, L., Narayanan, V., and M. Nakhjiri,
"Specification for the Derivation of Root Keys from an
Extended Master Session Key (EMSK)", RFC 5295,
August 2008.
[RFC6054] McGrew, D. and B. Weis, "Using Counter Modes with
Encapsulating Security Payload (ESP) and Authentication
Header (AH) to Protect Group Traffic", RFC 6054,
November 2010.
[RFC6146] Bagnulo, M., Matthews, P., and I. van Beijnum, "Stateful
NAT64: Network Address and Protocol Translation from IPv6
Clients to IPv4 Servers", RFC 6146, April 2011.
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Author's Address
David A. McGrew
Cisco Systems, Inc.
13600 Dulles Technology Drive
Herndon, VA 20171
US
Phone: (408) 525 8651
Email: mcgrew@cisco.com
URI: http://www.mindspring.com/~dmcgrew/dam.htm
McGrew Expires April 18, 2014 [Page 26]
