Network Working Group D. McGrew
Request for Comments: 5116 Cisco Systems, Inc.
Category: Standards Track January 2008
An Interface and Algorithms for Authenticated Encryption
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Abstract
This document defines algorithms for Authenticated Encryption with
Associated Data (AEAD), and defines a uniform interface and a
registry for such algorithms. The interface and registry can be used
as an application-independent set of cryptoalgorithm suites. This
approach provides advantages in efficiency and security, and promotes
the reuse of crypto implementations.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Scope . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Conventions Used in This Document . . . . . . . . . . . . 4
2. AEAD Interface . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Authenticated Encryption . . . . . . . . . . . . . . . . . 5
2.2. Authenticated Decryption . . . . . . . . . . . . . . . . . 7
2.3. Data Formatting . . . . . . . . . . . . . . . . . . . . . 7
3. Guidance on the Use of AEAD Algorithms . . . . . . . . . . . . 8
3.1. Requirements on Nonce Generation . . . . . . . . . . . . . 8
3.2. Recommended Nonce Formation . . . . . . . . . . . . . . . 9
3.2.1. Partially Implicit Nonces . . . . . . . . . . . . . . 10
3.3. Construction of AEAD Inputs . . . . . . . . . . . . . . . 11
3.4. Example Usage . . . . . . . . . . . . . . . . . . . . . . 11
4. Requirements on AEAD Algorithm Specifications . . . . . . . . 12
5. AEAD Algorithms . . . . . . . . . . . . . . . . . . . . . . . 14
5.1. AEAD_AES_128_GCM . . . . . . . . . . . . . . . . . . . . . 14
5.1.1. Nonce Reuse . . . . . . . . . . . . . . . . . . . . . 14
5.2. AEAD_AES_256_GCM . . . . . . . . . . . . . . . . . . . . . 15
5.3. AEAD_AES_128_CCM . . . . . . . . . . . . . . . . . . . . . 15
5.3.1. Nonce Reuse . . . . . . . . . . . . . . . . . . . . . 16
5.4. AEAD_AES_256_CCM . . . . . . . . . . . . . . . . . . . . . 16
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
7. Other Considerations . . . . . . . . . . . . . . . . . . . . . 17
8. Security Considerations . . . . . . . . . . . . . . . . . . . 18
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 18
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . . 19
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1. Introduction
Authenticated encryption [BN00] is a form of encryption that, in
addition to providing confidentiality for the plaintext that is
encrypted, provides a way to check its integrity and authenticity.
Authenticated Encryption with Associated Data, or AEAD [R02], adds
the ability to check the integrity and authenticity of some
Associated Data (AD), also called "additional authenticated data",
that is not encrypted.
1.1. Background
Many cryptographic applications require both confidentiality and
message authentication. Confidentiality is a security service that
ensures that data is available only to those authorized to obtain it;
usually it is realized through encryption. Message authentication is
the service that ensures that data has not been altered or forged by
unauthorized entities; it can be achieved by using a Message
Authentication Code (MAC). This service is also called data
integrity. Many applications use an encryption method and a MAC
together to provide both of those security services, with each
algorithm using an independent key. More recently, the idea of
providing both security services using a single cryptoalgorithm has
become accepted. In this concept, the cipher and MAC are replaced by
an Authenticated Encryption with Associated Data (AEAD) algorithm.
Several crypto algorithms that implement AEAD algorithms have been
defined, including block cipher modes of operation and dedicated
algorithms. Some of these algorithms have been adopted and proven
useful in practice. Additionally, AEAD is close to an 'idealized'
view of encryption, such as those used in the automated analysis of
cryptographic protocols (see, for example, Section 2.5 of [BOYD]).
The benefits of AEAD algorithms, and this interface, are outlined in
Section 1.3.
1.2. Scope
In this document, we define an AEAD algorithm as an abstraction, by
specifying an interface to an AEAD and defining an IANA registry for
AEAD algorithms. We populate this registry with four AEAD algorithms
based on the Advanced Encryption Standard (AES) in Galois/Counter
Mode [GCM] with 128- and 256-bit keys, and AES in Counter and CBC MAC
Mode [CCM] with 128- and 256-bit keys.
In the following, we define the AEAD interface (Section 2), and then
provide guidance on the use of AEAD algorithms (Section 3), and
outline the requirements that each AEAD algorithm must meet
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(Section 4). Then we define several AEAD algorithms (Section 5), and
establish an IANA registry for AEAD algorithms (Section 6). Lastly,
we discuss some other considerations (Section 7).
The AEAD interface specification does not address security protocol
issues such as anti-replay services or access control decisions that
are made on authenticated data. Instead, the specification aims to
abstract the cryptography away from those issues. The interface, and
the guidance about how to use it, are consistent with the
recommendations from [EEM04].
1.3. Benefits
The AEAD approach enables applications that need cryptographic
security services to more easily adopt those services. It benefits
the application designer by allowing them to focus on important
issues such as security services, canonicalization, and data
marshaling, and relieving them of the need to design crypto
mechanisms that meet their security goals. Importantly, the security
of an AEAD algorithm can be analyzed independent from its use in a
particular application. This property frees the user of the AEAD of
the need to consider security aspects such as the relative order of
authentication and encryption and the security of the particular
combination of cipher and MAC, such as the potential loss of
confidentiality through the MAC. The application designer that uses
the AEAD interface need not select a particular AEAD algorithm during
the design stage. Additionally, the interface to the AEAD is
relatively simple, since it requires only a single key as input and
requires only a single identifier to indicate the algorithm in use in
a particular case.
The AEAD approach benefits the implementer of the crypto algorithms
by making available optimizations that are otherwise not possible to
reduce the amount of computation, the implementation cost, and/or the
storage requirements. The simpler interface makes testing easier;
this is a considerable benefit for a crypto algorithm implementation.
By providing a uniform interface to access cryptographic services,
the AEAD approach allows a single crypto implementation to more
easily support multiple applications. For example, a hardware module
that supports the AEAD interface can easily provide crypto
acceleration to any application using that interface, even to
applications that had not been designed when the module was built.
1.4. 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. AEAD Interface
An AEAD algorithm has two operations, authenticated encryption and
authenticated decryption. The inputs and outputs of these algorithms
are defined below in terms of octet strings.
An implementation MAY accept additional inputs. For example, an
input could be provided to allow the user to select between different
implementation strategies. However, such extensions MUST NOT affect
interoperability with other implementations.
2.1. Authenticated Encryption
The authenticated encryption operation has four inputs, each of which
is an octet string:
A secret key K, which MUST be generated in a way that is uniformly
random or pseudorandom.
A nonce N. Each nonce provided to distinct invocations of the
Authenticated Encryption operation MUST be distinct, for any
particular value of the key, unless each and every nonce is zero-
length. Applications that can generate distinct nonces SHOULD use
the nonce formation method defined in Section 3.2, and MAY use any
other method that meets the uniqueness requirement. Other
applications SHOULD use zero-length nonces.
A plaintext P, which contains the data to be encrypted and
authenticated.
The associated data A, which contains the data to be
authenticated, but not encrypted.
There is a single output:
A ciphertext C, which is at least as long as the plaintext, or
an indication that the requested encryption operation could not be
performed.
All of the inputs and outputs are variable-length octet strings,
whose lengths obey the following restrictions:
The number of octets in the key K is between 1 and 255. For each
AEAD algorithm, the length of K MUST be fixed.
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For any particular value of the key, either 1) each nonce provided
to distinct invocations of the Authenticated Encryption operation
MUST be distinct, or 2) each and every nonce MUST be zero-length.
If zero-length nonces are used with a particular key, then each
and every nonce used with that key MUST have a length of zero.
Otherwise, the number of octets in the nonce SHOULD be twelve
(12). Nonces with different lengths MAY be used with a particular
key. Some algorithms cannot be used with zero-length nonces, but
others can; see Section 4. Applications that conform to the
recommended nonce length will avoid having to construct nonces
with different lengths, depending on the algorithm that is in use.
This guidance helps to keep algorithm-specific logic out of
applications.
The number of octets in the plaintext P MAY be zero.
The number of octets in the associated data A MAY be zero.
The number of octets in the ciphertext C MAY be zero.
This specification does not put a maximum length on the nonce, the
plaintext, the ciphertext, or the additional authenticated data.
However, a particular AEAD algorithm MAY further restrict the lengths
of those inputs and outputs. A particular AEAD implementation MAY
further restrict the lengths of its inputs and outputs. If a
particular implementation of an AEAD algorithm is requested to
process an input that is outside the range of admissible lengths, or
an input that is outside the range of lengths supported by that
implementation, it MUST return an error code and it MUST NOT output
any other information. In particular, partially encrypted or
partially decrypted data MUST NOT be returned.
Both confidentiality and message authentication are provided on the
plaintext P. When the length of P is zero, the AEAD algorithm acts
as a Message Authentication Code on the input A.
The associated data A is used to protect information that needs to be
authenticated, but does not need to be kept confidential. When using
an AEAD to secure a network protocol, for example, this input could
include addresses, ports, sequence numbers, protocol version numbers,
and other fields that indicate how the plaintext or ciphertext should
be handled, forwarded, or processed. In many situations, it is
desirable to authenticate these fields, though they must be left in
the clear to allow the network or system to function properly. When
this data is included in the input A, authentication is provided
without copying the data into the plaintext.
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The secret key K MUST NOT be included in any of the other inputs (N,
P, and A). (This restriction does not mean that the values of those
inputs must be checked to ensure that they do not include substrings
that match the key; instead, it means that the key must not be
explicitly copied into those inputs.)
The nonce is authenticated internally to the algorithm, and it is not
necessary to include it in the AD input. The nonce MAY be included
in P or A if it is convenient to the application.
The nonce MAY be stored or transported with the ciphertext, or it MAY
be reconstructed immediately prior to the authenticated decryption
operation. It is sufficient to provide the decryption module with
enough information to allow it to construct the nonce. (For example,
a system could use a nonce consisting of a sequence number in a
particular format, in which case it could be inferred from the order
of the ciphertexts.) Because the authenticated decryption process
detects incorrect nonce values, no security failure will result if a
nonce is incorrectly reconstructed and fed into an authenticated
decryption operation. Any nonce reconstruction method will need to
take into account the possibility of loss or reorder of ciphertexts
between the encryption and decryption processes.
Applications MUST NOT assume any particular structure or formatting
of the ciphertext.
2.2. Authenticated Decryption
The authenticated decryption operation has four inputs: K, N, A, and
C, as defined above. It has only a single output, either a plaintext
value P or a special symbol FAIL that indicates that the inputs are
not authentic. A ciphertext C, a nonce N, and associated data A are
authentic for key K when C is generated by the encrypt operation with
inputs K, N, P, and A, for some values of N, P, and A. The
authenticated decrypt operation will, with high probability, return
FAIL whenever the inputs N, P, and A were crafted by a nonce-
respecting adversary that does not know the secret key (assuming that
the AEAD algorithm is secure).
2.3. Data Formatting
This document does not specify any particular encoding for the AEAD
inputs and outputs, since the encoding does not affect the security
services provided by an AEAD algorithm.
When choosing the format of application data, an application SHOULD
position the ciphertext C so that it appears after any other data
that is needed to construct the other inputs to the authenticated
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decryption operation. For instance, if the nonce and ciphertext both
appear in a packet, the former value should precede the latter. This
rule facilitates efficient and simple hardware implementations of
AEAD algorithms.
3. Guidance on the Use of AEAD Algorithms
This section provides advice that must be followed in order to use an
AEAD algorithm securely.
If an application is unable to meet the uniqueness requirement on
nonce generation, then it MUST use a zero-length nonce. Randomized
or stateful algorithms, which are defined below, are suitable for use
with such applications. Otherwise, an application SHOULD use nonces
with a length of twelve octets. Since algorithms are encouraged to
support that length, applications should use that length to aid
interoperability.
3.1. Requirements on Nonce Generation
It is essential for security that the nonces be constructed in a
manner that respects the requirement that each nonce value be
distinct for each invocation of the authenticated encryption
operation, for any fixed value of the key. In this section, we call
attention to some consequences of this requirement in different
scenarios.
When there are multiple devices performing encryption using a single
key, those devices must coordinate to ensure that the nonces are
unique. A simple way to do this is to use a nonce format that
contains a field that is distinct for each one of the devices, as
described in Section 3.2. Note that there is no need to coordinate
the details of the nonce format between the encrypter and the
decrypter, as long the entire nonce is sent or stored with the
ciphertext and is thus available to the decrypter. If the complete
nonce is not available to the decrypter, then the decrypter will need
to know how the nonce is structured so that it can reconstruct it.
Applications SHOULD provide encryption engines with some freedom in
choosing their nonces; for example, a nonce could contain both a
counter and a field that is set by the encrypter but is not processed
by the receiver. This freedom allows a set of encryption devices to
more readily coordinate to ensure the distinctness of their nonces.
If a secret key will be used for a long period of time, e.g., across
multiple reboots, then the nonce will need to be stored in non-
volatile memory. In such cases, it is essential to use checkpointing
of the nonce; that is, the current nonce value should be stored to
provide the state information needed to resume encryption in case of
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unexpected failure. One simple way to provide a high assurance that
a nonce value will not be used repeatedly is to wait until the
encryption process receives confirmation from the storage process
indicating that the succeeding nonce value has already been stored.
Because this method may add significant latency, it may be desirable
to store a nonce value that is several values ahead in the sequence.
As an example, the nonce 100 could be stored, after which the nonces
1 through 99 could be used for encryption. The nonce value 200 could
be stored at the same time that nonces 1 through 99 are being used,
and so on.
Many problems with nonce reuse can be avoided by changing a key in a
situation in which nonce coordination is difficult.
Each AEAD algorithm SHOULD describe what security degradation would
result from an inadvertent reuse of a nonce value.
3.2. Recommended Nonce Formation
The following method to construct nonces is RECOMMENDED. The nonce
is formatted as illustrated in Figure 1, with the initial octets
consisting of a Fixed field, and the final octets consisting of a
Counter field. For each fixed key, the length of each of these
fields, and thus the length of the nonce, is fixed. Implementations
SHOULD support 12-octet nonces in which the Counter field is four
octets long.
<----- variable ----> <----------- variable ----------->
+---------------------+----------------------------------+
| Fixed | Counter |
+---------------------+----------------------------------+
Figure 1: Recommended nonce format
The Counter fields of successive nonces form a monotonically
increasing sequence, when those fields are regarded as unsigned
integers in network byte order. The length of the Counter field MUST
remain constant for all nonces that are generated for a given
encryption device. The Counter part SHOULD be equal to zero for the
first nonce, and increment by one for each successive nonce that is
generated. However, any particular Counter value MAY be skipped
over, and left out of the sequence of values that are used, if it is
convenient. For example, an application could choose to skip the
initial Counter=0 value, and set the Counter field of the initial
nonce to 1. Thus, at most 2^(8*C) nonces can be generated when the
Counter field is C octets in length.
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The Fixed field MUST remain 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 Fixed field, to ensure the uniqueness of the
nonces. Thus, at most 2^(8*F) distinct encrypters can share a key
when the Fixed field is F octets in length.
3.2.1. Partially Implicit Nonces
In some cases, it is desirable to not transmit or store an entire
nonce, but instead to reconstruct that value from contextual
information immediately prior to decryption. As an example,
ciphertexts could be stored in sequence on a disk, and the nonce for
a particular ciphertext could be inferred from its location, as long
as the rule for generating the nonces is known by the decrypter. We
call the portion of the nonce that is stored or sent with the
ciphertext the explicit part. We call the portion of the nonce that
is inferred the implicit part. When part of the nonce is implicit,
the following specialization of the above format is RECOMMENDED. The
Fixed field is divided into two sub-fields: a Fixed-Common field and
a Fixed-Distinct field. This format is shown in Figure 2. 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 nonces. The Fixed-Distinct field
and the Counter field MUST be in the explicit part of the nonce. The
Fixed-Common field MAY be in the implicit part of the nonce. These
conventions ensure that the nonce is easy to reconstruct from the
explicit data.
+-------------------+--------------------+---------------+
| Fixed-Common | Fixed-Distinct | Counter |
+-------------------+--------------------+---------------+
<---- implicit ---> <------------ explicit ------------>
Figure 2: Partially implicit nonce format
The rationale for the partially implicit nonce format is as
follows. This method of nonce construction incorporates the best
known practice; it is used by both GCM Encapuslating Security
Payload (ESP) [RFC4106] and CCM ESP [RFC4309], in which the Fixed
field contains the Salt value and the lowest eight octets of the
nonce are explicitly carried in the ESP packet. In GCM ESP, the
Fixed field must be at least four octets long, so that it can
contain the Salt value. In CCM ESP, the Fixed field must be at
least three octets long for the same reason. This nonce
generation method is also used by several counter mode variants
including CTR ESP.
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3.3. Construction of AEAD Inputs
If the AD input is constructed out of multiple data elements, then it
is essential that it be unambiguously parseable into its constituent
elements, without the use of any unauthenticated data in the parsing
process. (In mathematical terms, the AD input must be an injective
function of the data elements.) If an application constructs its AD
input in such a way that there are two distinct sets of data elements
that result in the same AD value, then an attacker could cause a
receiver to accept a bogus set by substituting one set for the other.
The requirement that the AD be uniquely parseable ensures that this
attack is not possible. This requirement is trivially met if the AD
is composed of fixed-width elements. If the AD contains a variable-
length string, for example, this requirement can be met by also
including the length of the string in the AD.
Similarly, if the plaintext is constructed out of multiple data
elements, then it is essential that it be unambiguously parseable
into its constituent elements, without using any unauthenticated data
in the parsing process. Note that data included in the AD may be
used when parsing the plaintext, though of course since the AD is not
encrypted there is a potential loss of confidentiality when
information about the plaintext is included in the AD.
3.4. Example Usage
To make use of an AEAD algorithm, an application must define how the
encryption algorithm's inputs are defined in terms of application
data, and how the ciphertext and nonce are conveyed. The clearest
way to do this is to express each input in terms of the data that
form it, then to express the application data in terms of the outputs
of the AEAD encryption operation.
For example, AES-GCM ESP [RFC4106] can be expressed as follows. The
AEAD inputs are
P = RestOfPayloadData || TFCpadding || Padding || PadLength ||
NextHeader
N = Salt || IV
A = SPI || SequenceNumber
where the symbol "||" denotes the concatenation operation, and the
fields RestOfPayloadData, TFCpadding, Padding, PadLength, NextHeader,
SPI, and SequenceNumber are as defined in [RFC4303], and the fields
Salt and IV are as defined in [RFC4106]. The field RestOfPayloadData
contains the plaintext data that is described by the NextHeader
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field, and no other data. (Recall that the PayloadData field
contains both the IV and the RestOfPayloadData; see Figure 2 of
[RFC4303] for an illustration.)
The format of the ESP packet can be expressed as
ESP = SPI || SequenceNumber || IV || C
where C is the AEAD ciphertext (which in this case incorporates the
authentication tag). Please note that here we have not described the
use of the ESP Extended Sequence Number.
4. Requirements on AEAD Algorithm Specifications
Each AEAD algorithm MUST only accept keys with a fixed key length
K_LEN, and MUST NOT require any particular data format for the keys
provided as input. An algorithm that requires such structure (e.g.,
one with subkeys in a particular parity-check format) will need to
provide it internally.
Each AEAD algorithm MUST accept any plaintext with a length between
zero and P_MAX octets, inclusive, where the value P_MAX is specific
to that algorithm. The value of P_MAX MUST be larger than zero, and
SHOULD be at least 65,536 (2^16) octets. This size is a typical
upper limit for network data packets. Other applications may use
even larger values of P_MAX, so it is desirable for general-purpose
algorithms to support higher values.
Each AEAD algorithm MUST accept any associated data with a length
between zero and A_MAX octets, inclusive, where the value A_MAX is
specific to that algorithm. The value of A_MAX MUST be larger than
zero, and SHOULD be at least 65,536 (2^16) octets. Other
applications may use even larger values of A_MAX, so it is desirable
for general-purpose algorithms to support higher values.
Each AEAD algorithm MUST accept any nonce with a length between N_MIN
and N_MAX octets, inclusive, where the values of N_MIN and N_MAX are
specific to that algorithm. The values of N_MAX and N_MIN MAY be
equal. Each algorithm SHOULD accept a nonce with a length of twelve
(12) octets. Randomized or stateful algorithms, which are described
below, MAY have an N_MAX value of zero.
An AEAD algorithm MAY structure its ciphertext output in any way; for
example, the ciphertext can incorporate an authentication tag. Each
algorithm SHOULD choose a structure that is amenable to efficient
processing.
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An Authenticated Encryption algorithm MAY incorporate or make use of
a random source, e.g., for the generation of an internal
initialization vector that is incorporated into the ciphertext
output. An AEAD algorithm of this sort is called randomized; though
note that only encryption is random, and decryption is always
deterministic. A randomized algorithm MAY have a value of N_MAX that
is equal to zero.
An Authenticated Encryption algorithm MAY incorporate internal state
information that is maintained between invocations of the encrypt
operation, e.g., to allow for the construction of distinct values
that are used as internal nonces by the algorithm. An AEAD algorithm
of this sort is called stateful. This method could be used by an
algorithm to provide good security even when the application inputs
zero-length nonces. A stateful algorithm MAY have a value of N_MAX
that is equal to zero.
The specification of an AEAD algorithm MUST include the values of
K_LEN, P_MAX, A_MAX, N_MIN, and N_MAX defined above. Additionally,
it MUST specify the number of octets in the largest possible
ciphertext, which we denote C_MAX.
Each AEAD algorithm MUST provide a description relating the length of
the plaintext to that of the ciphertext. This relation MUST NOT
depend on external parameters, such as an authentication strength
parameter (e.g., authentication tag length). That sort of dependence
would complicate the use of the algorithm by creating a situation in
which the information from the AEAD registry was not sufficient to
ensure interoperability.
EACH AEAD algorithm specification SHOULD describe what security
degradation would result from an inadvertent reuse of a nonce value.
Each AEAD algorithm specification SHOULD provide a reference to a
detailed security analysis. This document does not specify a
particular security model, because several different models have been
used in the literature. The security analysis SHOULD define or
reference a security model.
An algorithm that is randomized or stateful, as defined above, SHOULD
describe itself using those terms.
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5. AEAD Algorithms
This section defines four AEAD algorithms; two are based on AES GCM,
two are based on AES CCM. Each pair includes an algorithm with a key
size of 128 bits and one with a key size of 256 bits.
5.1. AEAD_AES_128_GCM
The AEAD_AES_128_GCM authenticated encryption algorithm works as
specified in [GCM], using AES-128 as the block cipher, by providing
the key, nonce, and plaintext, and associated data to that mode of
operation. An authentication tag with a length of 16 octets (128
bits) is used. The AEAD_AES_128_GCM ciphertext is formed by
appending the authentication tag provided as an output to the GCM
encryption operation to the ciphertext that is output by that
operation. Test cases are provided in the appendix of [GCM]. The
input and output lengths are as follows:
K_LEN is 16 octets,
P_MAX is 2^36 - 31 octets,
A_MAX is 2^61 - 1 octets,
N_MIN and N_MAX are both 12 octets, and
C_MAX is 2^36 - 15 octets.
An AEAD_AES_128_GCM ciphertext is exactly 16 octets longer than its
corresponding plaintext.
A security analysis of GCM is available in [MV04].
5.1.1. Nonce Reuse
The inadvertent reuse of the same nonce by two invocations of the GCM
encryption operation, with the same key, but with distinct plaintext
values, undermines the confidentiality of the plaintexts protected in
those two invocations, and undermines all of the authenticity and
integrity protection provided by that key. For this reason, GCM
should only be used whenever nonce uniqueness can be provided with
assurance. The design feature that GCM uses to achieve minimal
latency causes the vulnerabilities on the subsequent uses of the key.
Note that it is acceptable to input the same nonce value multiple
times to the decryption operation.
The security consequences are quite serious if an attacker observes
two ciphertexts that were created using the same nonce and key
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values, unless the plaintext and AD values in both invocations of the
encrypt operation were identical. First, a loss of confidentiality
ensues because he will be able to reconstruct the bitwise
exclusive-or of the two plaintext values. Second, a loss of
integrity ensues because the attacker will be able to recover the
internal hash key used to provide data integrity. Knowledge of this
key makes subsequent forgeries trivial.
5.2. AEAD_AES_256_GCM
This algorithm is identical to AEAD_AES_128_GCM, but with the
following differences:
K_LEN is 32 octets, instead of 16 octets, and
AES-256 GCM is used instead of AES-128 GCM.
5.3. AEAD_AES_128_CCM
The AEAD_AES_128_CCM authenticated encryption algorithm works as
specified in [CCM], using AES-128 as the block cipher, by providing
the key, nonce, associated data, and plaintext to that mode of
operation. The formatting and counter generation function are as
specified in Appendix A of that reference, and the values of the
parameters identified in that appendix are as follows:
the nonce length n is 12,
the tag length t is 16, and
the value of q is 3.
An authentication tag with a length of 16 octets (128 bits) is used.
The AEAD_AES_128_CCM ciphertext is formed by appending the
authentication tag provided as an output to the CCM encryption
operation to the ciphertext that is output by that operation. Test
cases are provided in [CCM]. The input and output lengths are as
follows:
K_LEN is 16 octets,
P_MAX is 2^24 - 1 octets,
A_MAX is 2^64 - 1 octets,
N_MIN and N_MAX are both 12 octets, and
C_MAX is 2^24 + 15 octets.
McGrew Standards Track [Page 15]RFC 5116 Authenticated Encryption January 2008
An AEAD_AES_128_CCM ciphertext is exactly 16 octets longer than its
corresponding plaintext.
A security analysis of AES CCM is available in [J02].
5.3.1. Nonce Reuse
Inadvertent reuse of the same nonce by two invocations of the CCM
encryption operation, with the same key, undermines the security for
the messages processed with those invocations. A loss of
confidentiality ensues because an adversary will be able to
reconstruct the bitwise exclusive-or of the two plaintext values.
5.4. AEAD_AES_256_CCM
This algorithm is identical to AEAD_AES_128_CCM, but with the
following differences:
K_LEN is 32 octets, instead of 16, and
AES-256 CCM is used instead of AES-128 CCM.
6. IANA Considerations
The Internet Assigned Numbers Authority (IANA) has defined the "AEAD
Registry" described below. An algorithm designer MAY register an
algorithm in order to facilitate its use. Additions to the AEAD
Registry require that a specification be documented in an RFC or
another permanent and readily available reference, in sufficient
detail that interoperability between independent implementations is
possible. Each entry in the registry contains the following
elements:
a short name, such as "AEAD_AES_128_GCM", that starts with the
string "AEAD",
a positive number, and
a reference to a specification that completely defines an AEAD
algorithm and provides test cases that can be used to verify the
correctness of an implementation.
Requests to add an entry to the registry MUST include the name and
the reference. The number is assigned by IANA. These number
assignments SHOULD use the smallest available positive number.
Submitters SHOULD have their requests reviewed by the IRTF Crypto
McGrew Standards Track [Page 16]RFC 5116 Authenticated Encryption January 2008
Forum Research Group (CFRG) at cfrg@ietf.org. Interested applicants
that are unfamiliar with IANA processes should visit
http://www.iana.org.
The numbers between 32,768 (binary 1000000000000000) and 65,535
(binary 1111111111111111) inclusive, will not be assigned by IANA,
and are reserved for private use; no attempt will be made to prevent
multiple sites from using the same value in different (and
incompatible) ways [RFC2434].
IANA has added the following entries to the AEAD Registry:
+------------------+-------------+--------------------+
| Name | Reference | Numeric Identifier |
+------------------+-------------+--------------------+
| AEAD_AES_128_GCM | Section 5.1 | 1 |
| AEAD_AES_256_GCM | Section 5.2 | 2 |
| AEAD_AES_128_CCM | Section 5.3 | 3 |
| AEAD_AES_256_CCM | Section 5.4 | 4 |
+------------------+-------------+--------------------+
An IANA registration of an AEAD does not constitute an endorsement of
that algorithm or its security.
7. Other Considerations
Directly testing a randomized AEAD encryption algorithm using test
cases with fixed inputs and outputs is not possible, since the
encryption process is non-deterministic. However, it is possible to
test a randomized AEAD algorithm using the following technique. The
authenticated decryption algorithm is deterministic, and it can be
directly tested. The authenticated encryption algorithm can be
tested by encrypting a plaintext, decrypting the resulting
ciphertext, and comparing the original plaintext to the post-
decryption plaintext. Combining both of these tests covers both the
encryption and decryption algorithms.
The AEAD algorithms selected reflect those that have been already
adopted by standards. It is an open question as to what other AEAD
algorithms should be added. Many variations on basic algorithms are
possible, each with its own advantages. While it is desirable to
admit any algorithms that are found to be useful in practice, it is
also desirable to limit the total number of registered algorithms.
The current specification requires that a registered algorithm
provide a complete specification and a set of validation data; it is
hoped that these prerequisites set the admission criteria
appropriately.
McGrew Standards Track [Page 17]RFC 5116 Authenticated Encryption January 2008
It may be desirable to define an AEAD algorithm that uses the generic
composition with the encrypt-then-MAC method [BN00], combining a
common encryption algorithm, such as CBC [MODES], with a common
message authentication code, such as HMAC-SHA1 [RFC2104] or AES CMAC
[CMAC]. An AEAD algorithm of this sort would reflect the best
current practice, and might be more easily supported by crypto
modules that lack support for other AEAD algorithms.
8. Security Considerations
This document describes authenticated encryption algorithms, and
provides guidance on their use. While these algorithms make it
easier, in some ways, to design a cryptographic application, it
should be borne in mind that strong cryptographic security is
difficult to achieve. While AEAD algorithms are quite useful, they
do nothing to address the issues of key generation [RFC4086] and key
management [RFC4107].
AEAD algorithms that rely on distinct nonces may be inappropriate for
some applications or for some scenarios. Application designers
should understand the requirements outlined in Section 3.1.
A software implementation of the AEAD encryption operation in a
Virtual Machine (VM) environment could inadvertently reuse a nonce
due to a "rollback" of the VM to an earlier state [GR05].
Applications are encouraged to document potential issues to help the
user of the application and the VM avoid unintentional mistakes of
this sort. The possibility exists that an attacker can cause a VM
rollback; threats and mitigations in that scenario are an area of
active research. For perspective, we note that an attacker who can
trigger such a rollback may have already succeeded in subverting the
security of the system, e.g., by causing an accounting error.
An IANA registration of an AEAD algorithm MUST NOT be regarded as an
endorsement of its security. Furthermore, the perceived security
level of an algorithm can degrade over time, due to cryptanalytic
advances or to "Moore's Law", that is, the diminishing cost of
computational resources over time.
9. Acknowledgments
Many reviewers provided valuable comments on earlier drafts of this
document. Some fruitful discussions took place on the email list of
the Crypto Forum Research Group in 2006.
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RFC 5116 Authenticated Encryption January 2008
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, November 2007,
<http://csrc.nist.gov/publications/nistpubs/800-38D/
SP-800-38D.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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, 2002.
[BOYD] Boyd, C. and A. Mathuria, "Protocols for Authentication
and Key Establishment", Springer 2003.
[CMAC] "NIST Special Publication 800-38B", <http://csrc.nist.gov/
publications/nistpubs/800-38B/SP_800-38B.pdf>.
[EEM04] Bellare, M., Namprempre, C., and T. Kohno, "Breaking and
provably repairing the SSH authenticated encryption
scheme: A case study of the Encode-then-Encrypt-and-MAC
paradigm", ACM Transactions on Information and
System Security,
<http://www-cse.ucsd.edu/users/tkohno/papers/TISSEC04/>.
[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>.
McGrew Standards Track [Page 19]RFC 5116 Authenticated Encryption January 2008
[J02] Jonsson, J., "On the Security of CTR + CBC-MAC",
Proceedings of the 9th Annual Workshop on Selected Areas
on Cryptography, 2002, <http://csrc.nist.gov/groups/ST/
toolkit/BCM/documents/proposedmodes/ccm/ccm-ad1.pdf>.
[MODES] Dworkin, M., "NIST Special Publication 800-38:
Recommendation for Block Cipher Modes of Operation", U.S.
National Institute of Standards and Technology,
<http://csrc.nist.gov/publications/nistpubs/800-38a/
sp800-38a.pdf>.
[MV04] McGrew, D. and J. Viega, "The Security and Performance of
the Galois/Counter Mode (GCM)", Proceedings of
INDOCRYPT '04, December 2004,
<http://eprint.iacr.org/2004/193>.
[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>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[RFC4086] Eastlake, D., Schiller, J., and S. Crocker, "Randomness
Requirements for Security", BCP 106, RFC 4086, June 2005.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[RFC4107] Bellovin, S. and R. Housley, "Guidelines for Cryptographic
Key Management", BCP 107, RFC 4107, June 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM
Mode with IPsec Encapsulating Security Payload (ESP)",
RFC 4309, December 2005.
McGrew Standards Track [Page 20]RFC 5116 Authenticated Encryption January 2008
Author's Address
David A. McGrew
Cisco Systems, Inc.
510 McCarthy Blvd.
Milpitas, CA 95035
US
Phone: (408) 525 8651
EMail: mcgrew@cisco.com
URI: http://www.mindspring.com/~dmcgrew/dam.htm
McGrew Standards Track [Page 21]
RFC 5116 Authenticated Encryption January 2008
Full Copyright Statement
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McGrew Standards Track [Page 22]
