NIST Special Publication 800-38B

Recommendation for Block Cipher 

Modes of Operation:

The CMAC Mode for Authentication

Morris Dworkin

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C O M P U T E R S E C U R I T Y
NIST Special Publication 800-38B

Recommendation for Block Cipher 

Modes of Operation:

The CMAC Mode for Authentication

Morris Dworkin

Computer Security Division

Information Technology Laboratory

This publication is available free of charge from:

https://doi.org/10.6028/NIST.SP.800-38B

May 2005

INCLUDES UPDATES AS OF 10-06-2016: PAGE II

U.S. Department of Commerce

Penny Pritzker, Secretary

National Institute of Standards and Technology 

Willie May, Under Secretary of Commerce for Standards and Technology and Director
Authority

This publication has been developed by NIST in accordance with its statutory responsibilities under the 

Federal Information Security Modernization Act (FISMA) of 2014, 44 U.S.C. § 3551 et seq., Public Law

(P.L.) 113-283. NIST is responsible for developing information security standards and guidelines, including 

minimum requirements for federal information systems, but such standards and guidelines shall not apply 

to national security systems without the express approval of appropriate federal officials exercising policy 

authority over such systems. This guideline is consistent with the requirements of the Office of Management 

and Budget (OMB) Circular A-130.

Nothing in this publication should be taken to contradict the standards and guidelines made mandatory and 

binding on federal agencies by the Secretary of Commerce under statutory authority. Nor should these 

guidelines be interpreted as altering or superseding the existing authorities of the Secretary of Commerce, 

Director of the OMB, or any other federal official. This publication may be used by nongovernmental 

organizations on a voluntary basis and is not subject to copyright in the United States. Attribution would, 

however, be appreciated by NIST. 

National Institute of Standards and Technology Special Publication 800-38B

Natl. Inst. Stand. Technol. Spec. Publ. 800-38B, 21 pages (May 2005)

CODEN: NSPUE2

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Certain commercial entities, equipment, or materials may be identified in this document in order to describe an 

experimental procedure or concept adequately. Such identification is not intended to imply recommendation or 

endorsement by NIST, nor is it intended to imply that the entities, materials, or equipment are necessarily the best 

available for the purpose. 

There may be references in this publication to other publications currently under development by NIST in accordance 

with its assigned statutory responsibilities. The information in this publication, including concepts and methodologies, 

may be used by federal agencies even before the completion of such companion publications. Thus, until each 

publication is completed, current requirements, guidelines, and procedures, where they exist, remain operative. For 

planning and transition purposes, federal agencies may wish to closely follow the development of these new 

publications by NIST. 

Organizations are encouraged to review all draft publications during public comment periods and provide feedback to 

NIST. Many NIST cybersecurity publications, other than the ones noted above, are available at 

http://csrc.nist.gov/publications.

Comments on this publication may be submitted to:

National Institute of Standards and Technology

Attn: Computer Security Division, Information Technology Laboratory

100 Bureau Drive (Mail Stop 8930) Gaithersburg, MD 20899-8930

Email: encryptionmodes@nist.gov

All comments are subject to release under the Freedom of Information Act (FOIA).
NIST SP 800-38B BLOCK CIPHER MODES:

CMAC MODE FOR AUTHENTICATION

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Reports on Computer Systems Technology

The Information Technology Laboratory (ITL) at the National Institute of Standards and 

Technology (NIST) promotes the U.S. economy and public welfare by providing technical 

leadership for the Nation’s measurement and standards infrastructure. ITL develops tests, test 

methods, reference data, proof of concept implementations, and technical analyses to advance the 

development and productive use of information technology. ITL’s responsibilities include the 

development of management, administrative, technical, and physical standards and guidelines for 

the cost-effective security and privacy of other than national security-related information in federal 

information systems. The Special Publication 800-series reports on ITL’s research, guidelines, and 

outreach efforts in information system security, and its collaborative activities with industry, 

government, and academic organizations.

Abstract

This Recommendation specifies a message authentication code (MAC) algorithm based on a 

symmetric key block cipher. This block cipher-based MAC algorithm, called CMAC, may be 

used to provide assurance of the authenticity and, hence, the integrity of binary data.

Keywords

authentication; block cipher; cryptography; information security; integrity; message authentication 

code; mode of operation

Errata

The following changes have been incorporated into Special Publication 800-38B. Errata updates 

include corrections, clarifications, or other minor changes in the publication that are either 

editorial or substantive in nature. 

Date Type Change Pages

10-06-2016 Substantive Updated the document’s front matter (including 

pagination), and moved most of the Authority text (Sec. 

2) to the Authority statement on p. i. Renamed Sec. 2 

from “Authority” to “Conformance Testing.”

i-iv, 1

10-06-2016 Substantive The CMAC examples originally included in Appendix D 

have been removed and are now available at 

http://csrc.nist.gov/groups/ST/toolkit/examples.html. 

15

10-06-2016 Substantive Updated several references in Appendix E 

(Bibliography).

16
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Table of Contents

1 PURPOSE...........................................................................................................................................................1

2 CONFORMANCE TESTING...........................................................................................................................1

3 INTRODUCTION..............................................................................................................................................1

4 DEFINITIONS, ABBREVIATIONS, AND SYMBOLS.................................................................................2

4.1 DEFINITIONS AND ABBREVIATIONS..................................................................................................................2

4.2 SYMBOLS .........................................................................................................................................................3

4.2.1 Variables................................................................................................................................................3

4.2.2 Operations and Functions .....................................................................................................................4

5 PRELIMINARIES.............................................................................................................................................4

5.1 EXAMPLES OF OPERATIONS AND FUNCTIONS...................................................................................................5

5.2 BLOCK CIPHER.................................................................................................................................................5

5.3 SUBKEYS..........................................................................................................................................................6

5.4 MAC GENERATION AND VERIFICATION...........................................................................................................6

5.5 INPUT AND OUTPUT DATA ...............................................................................................................................6

6 CMAC SPECIFICATION.................................................................................................................................6

6.1 SUBKEY GENERATION......................................................................................................................................7

6.2 MAC GENERATION..........................................................................................................................................7

6.3 MAC VERIFICATION ........................................................................................................................................9

APPENDIX A: LENGTH OF THE MAC..............................................................................................................11

A.1 ASSURANCE AGAINST GUESSING ATTACKS ..............................................................................................11

A.2 SELECTION OF THE MAC LENGTH.............................................................................................................11

APPENDIX B: MESSAGE SPAN OF THE KEY .................................................................................................13

APPENDIX C: PROTECTION AGAINST REPLAY OF MESSAGES .............................................................14

APPENDIX D: EXAMPLES ...................................................................................................................................15

APPENDIX E: BIBLIOGRAPHY ..........................................................................................................................16

Figures

Figure 1: Illustration of the two cases of MAC Generation........................................................... 9
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1 Purpose

This publication is the second Part in a series of Recommendations regarding modes of operation 

of symmetric key block ciphers. 

2 Conformance Testing

Conformance testing for implementations of the mode of operation that is specified in this Part of 

the Recommendation will be conducted within the framework of the Cryptographic Module 

Validation Program (CMVP), a joint effort of NIST and the Communications Security 

Establishment of the Government of Canada. An implementation of a mode of operation must 

adhere to the requirements in this Recommendation in order to be validated under the CMVP. The 

requirements of this Recommendation are indicated by the word “shall.”

3 Introduction 

This Recommendation specifies a message authentication code (MAC) algorithm that is based on 

a symmetric key block cipher. This cipher-based MAC is abbreviated CMAC, analogous to the 

abbreviation for the hash function-based MAC, HMAC, that is standardized in FIPS Pub. 198 [4]. 

CMAC may be appropriate for information systems in which an approved block cipher is more 

readily available than an approved hash function. 

The basic Cipher Block Chaining MAC algorithm (CBC-MAC) has security deficiencies [9]. The 

core of the CMAC algorithm is a variation of CBC-MAC that Black and Rogaway proposed and 

analyzed under the name XCBC in Ref. [2] and submitted to NIST in Ref. [1]. The XCBC 

algorithm efficiently addresses the security deficiencies of CBC-MAC. Iwata and Kurosawa 

proposed an improvement of XCBC and named the resulting algorithm One-Key CBC-MAC 

(OMAC) in Ref. [6] and in Ref. [5], their initial submission to NIST; they later submitted OMAC1 

[7], a refinement of OMAC, and additional security analysis [8]. The OMAC1 variation efficiently 

reduces the key size of XCBC. CMAC is equivalent to OMAC1. 

Because CMAC is based on an approved symmetric key block cipher, such as the Advanced 

Encryption Standard (AES) algorithm that is specified in Federal Information Processing Standard 

(FIPS) Pub. 197 [3], CMAC can be considered a mode of operation of the block cipher. CMAC 

is also an approved mode of the Triple Data Encryption Algorithm (TDEA) [10]; however, as 

discussed in Appendix B, the recommended default message span for TDEA is much more 

restrictive than for the AES algorithm, due to the smaller block size of TDEA. 

CMAC, like any well-designed MAC algorithm, provides stronger assurance of data integrity than 

a checksum or an error detecting code. The verification of a checksum or an error detecting code 

is designed to detect only accidental modifications of the data, while CMAC is designed to detect 

intentional, unauthorized modifications of the data, as well as accidental modifications.
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4 Definitions, Abbreviations, and Symbols 

4.1 Definitions and Abbreviations 

AES Advanced Encryption Standard.

Approved FIPS approved or NIST recommended: an algorithm or technique that 

is either 1) specified in a FIPS or a NIST Recommendation, or 2) 

adopted in a FIPS or a NIST Recommendation.

Authenticity The property that data originated from its purported source.

Bit A binary digit: 0 or 1.

Bit String A finite, ordered sequence of bits.

Block For a given block cipher, a bit string whose length is the block size of 

the block cipher.

Block Cipher An algorithm for a parameterized family of permutations on bit strings 

of a fixed length. 

Block Size For a given block cipher, the fixed length of the input (or output) bit 

strings.

CBC Cipher Block Chaining.

Collision For a given function, a pair of distinct input values that yield the same 

output value.

Exclusive-OR The bitwise addition, modulo 2, of two bit strings of equal length. 

FIPS Federal Information Processing Standard.

Forward Cipher

Function 

A permutation on blocks that is determined by the choice of a key for 

a given block cipher. 

Integrity The property that received data has not been altered.

Inverse Cipher Function The inverse function of the forward cipher function for a given block 

cipher key.

Key

(Block Cipher Key)

The parameter of the block cipher that determines the selection of the 

forward cipher function from the family of permutations. 
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Least Significant Bit(s) The right-most bit(s) of a bit string.

Message

Authentication Code

(MAC)

A bit string of fixed length, computed by a MAC generation algorithm, 

that is used to establish the authenticity and, hence, the integrity of a 

message.

MAC Generation 

(Generation)

An algorithm that computes a MAC from a message and a key.

MAC Verification

(Verification)

An algorithm that verifies if a purported MAC is valid for a given 

message and key.

Mode of Operation

(Mode)

An algorithm for the cryptographic transformation of data that features 

a symmetric key block cipher. 

Most Significant Bit(s) The left-most bit(s) of a bit string.

NIST National Institute of Standards and Technology.

Permutation An invertible function.

Subkey A secret string that is derived from the key.

Subkey Generation An algorithm that derives subkeys from a key.

TDEA Triple Data Encryption Algorithm.

4.2 Symbols 

4.2.1 Variables

b The bit length of a block.

Rb The constant string for subkey generation for a cipher with block size b.

K The block cipher key.

K1 The first subkey.

K2 The second subkey.

Key1 The first component of a TDEA key.

Key2 The second component of a TDEA key.
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Key3 The third component of a TDEA key.

M The message.

Mi The ith block of the formatted message.

Mn

* The final block, possibly a partial block, of the formatted message.

Mlen The bit length of the message.

n The number of blocks in the formatted message.

T The MAC. 

Tlen The bit length of the MAC.

4.2.2 Operations and Functions

x The least integer that is not less than the real number x.

X || Y The concatenation of two bit strings X and Y.

X ⊕Y The bitwise exclusive-OR of two bit strings X and Y of the same length.

CIPHK(X) The output of the forward cipher function of the block cipher under the key K

applied to the block X.

LSBs(X) The bit string consisting of the s right-most bits of the bit string X.

MSBs(X) The bit string consisting of the s left-most bits of the bit string X.

X << 1 The bit string that results from discarding the leftmost bit of the bit string X and

appending a ‘0’ bit on the right.

lg(x) The base 2 logarithm of the positive real number x. 

0s The bit string that consists of s ‘0’ bits.

5 Preliminaries

The elements of CMAC and the associated notation are introduced in the five sections below. 

Examples of operations and functions are given in Sec. 5.1. The underlying block cipher and key 

are discussed in Sec. 5.2. The two subkeys that are derived from the key are discussed in Sec. 5.3. 

MAC generation and verification are discussed in Sec. 5.4. The input and output data for MAC 

generation are discussed in Sec. 5.5.
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5.1 Examples of Operations and Functions

Given a positive integer s, 0s denotes the string that consists of s ‘0’ bits. For example, 08 = 

00000000. 

Given a real number x, the ceiling function, denoted x, is the least integer that is not less than x. 

For example, 2.1 = 3, and 4 = 4.

The concatenation operation on bit strings is denoted ||; for example, 001 || 10111 = 00110111.

Given bit strings of equal length, the exclusive-OR operation, denoted ⊕, specifies the addition, 

modulo 2, of the bits in each bit position, i.e., without carries. For example, 10011 ⊕ 10101 =

00110.

Given a bit string X, the functions LSBs(X) and MSBs(X) return the s least significant (i.e., rightmost) bits and the s most significant (i.e., left-most) bits, respectively, of X. For example, 

LSB3(111011010) = 010, and MSB4(111011010) = 1110.

Given a bit string X that consists of Xlen bits, the (single) left-shift function, denoted X << 1, is 

LSBXlen(X || 0). For example, 1101110 << 1 = 1011100.

Given a positive real number x, its base 2 logarithm is denoted lg(x). For example, lg(210) = 10. 

5.2 Block Cipher

The CMAC algorithm depends on the choice of an underlying symmetric key block cipher. The 

CMAC algorithm is thus a mode of operation (a mode, for short) of the block cipher. The CMAC 

key is the block cipher key (the key, for short).

For any given key, the underlying block cipher of the mode consists of two functions that are 

inverses of each other. The choice of the block cipher includes the designation of one of the two 

functions of the block cipher as the forward function/transformation, and the other as the inverse 

function, as in the specifications of the AES algorithm and TDEA in Ref. [3] and Ref. [10], 

respectively. The CMAC mode does not employ the inverse function. 

The forward cipher function is a permutation on bit strings of a fixed length; the strings are called 

blocks. The bit length of a block is denoted b, and the length of a block is called the block size. 

For the AES algorithm, b = 128; for TDEA, b = 64. The key is denoted K, and the resulting 

forward cipher function of the block cipher is denoted CIPHK.

The underlying block cipher shall be approved, and the key shall be generated uniformly at 

random, or close to uniformly at random, i.e., so that each possible key is (nearly) equally likely 

to be generated. The key shall be secret and shall be used exclusively for the CMAC mode of the 

chosen block cipher. The message span of the key is discussed in Appendix B. To fulfill the 

requirements on the key, the key should be established among the parties to the information within 

an approved key management structure; the details of the establishment and management of keys 
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are outside the scope of this Recommendation. 

5.3 Subkeys

The block cipher key is used to derive two additional secret values, called the subkeys, denoted 

K1 and K2. The length of each subkey is the block size. The subkeys are fixed for any invocation 

of CMAC with the given key. Consequently, the subkeys may be precomputed and stored with 

the key for repeated use; alternatively, the subkeys may be computed anew for each invocation. 

Any intermediate value in the computation of the subkey, in particular, CIPHK(0b), shall also be 

secret. This requirement precludes the system in which CMAC is implemented from using this 

intermediate value publicly for some other purpose, for example, as an unpredictable value or as 

an integrity check value on the key.

One of the elements of the subkey generation process is a bit string, denoted Rb, that is completely 

determined by the number of bits in a block. In particular, for the two block sizes of the currently 

approved block ciphers, R128 = 012010000111, and R64 = 05911011.

In general, Rb is a representation of a certain irreducible binary polynomial of degree b, namely, 

the lexicographically first among all such polynomials with the minimum possible number of 

nonzero terms. If this polynomial is expressed as ub+cb-1ub-1+...+c2u2+c1u+c0, where the 

coefficients cb-1, cb-2, ..., c2, c1, c0 are either 0 or 1, then Rb is the bit string cb-1cb-2...c2c1c0. 

5.4 MAC Generation and Verification

As for any MAC algorithm, an authorized party applies the MAC generation process to the data to 

be authenticated to produce a MAC for the data. Subsequently, any authorized party can apply the 

verification process to the received data and the received MAC. Successful verification provides 

assurance of data authenticity, as discussed in Appendix A, and, hence, of integrity.

5.5 Input and Output Data

For a given block cipher and key, the input to the MAC generation function is a bit string called 

the message, denoted M. The bit length of M is denoted Mlen. The value of Mlen is not an essential 

input for the MAC generation algorithm if the implementation has some other means of identifying 

the last block in the partition of the message, as discussed in Sec. 6.2. Thus, in such a case, the 

computation of the MAC may begin “on-line” before the entire message is available. In principle, 

there is no restriction on the lengths of messages. In practice, however, the system in which CMAC 

is implemented may restrict the length of the input messages to the MAC generation function. 

The output of the MAC generation function is a bit string called the MAC, denoted T. The length 

of T, denoted Tlen, is a parameter that shall be fixed for all invocations of CMAC with the given 

key. The requirements for the selection of Tlen are given in Appendix A.

6 CMAC Specification
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Subkey generation, MAC generation, and MAC verification are specified in Sections 6.1, 6.2, 

and 6.3 below. The specifications include the inputs, the outputs, a suggested notation for the 

function, the steps, and a summary; for MAC generation, a diagram is also given. The inputs 

that are typically fixed across many invocations of CMAC are called the prerequisites. The 

prerequisites and the other inputs shall meet the requirements in Sec. 5. The suggested notation 

does not include the block cipher.

6.1 Subkey Generation

The following is a specification of the subkey generation process of CMAC: 

Prerequisites:

block cipher CIPH with block size b;

key K.

Output: 

subkeys K1, K2.

Suggested Notation:

SUBK(K).

Steps:

1. Let L = CIPHK(0b). 

2. If MSB1(L) = 0, then K1 = L << 1;

Else K1 = (L << 1) ⊕ Rb; see Sec. 5.3 for the definition of Rb.

3. If MSB1(K1) = 0, then K2 = K1 << 1;

Else K2 = (K1 << 1) ⊕ Rb.

4. Return K1, K2.

In Step 1, the block cipher is applied to the block that consists entirely of ‘0’ bits. In Step 2, the 

first subkey is derived from the resulting string by a left shift of one bit, and, conditionally, by 

XORing a constant that depends on the block size. In Step 3, the second subkey is derived in the 

same manner from the first subkey.1 As discussed in Sec. 5.3, any intermediate value in the 

computation of the subkey, in particular, CIPHK(0b

), shall be secret. 

6.2 MAC Generation

The following is a specification of the MAC generation process of CMAC:

Prerequisites:

block cipher CIPH with block size b;

key K;

MAC length parameter Tlen.

 1 As detailed in Ref. [5], the generation of K1 and K2 is essentially equivalent to multiplication by u and u2

, 

respectively, within the Galois field that is determined by the irreducible polynomial that is represented by Rb, which 

is discussed in Sec. 5.3.
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Input:

message M of bit length Mlen.

Output: 

MAC T of bit length Tlen.

Suggested Notation:

CMAC(K, M, Tlen) or, if Tlen is understood from the context, CMAC(K, M).

Steps:

1. Apply the subkey generation process in Sec. 6.1 to K to produce K1 and K2.

2. If Mlen = 0, let n = 1; else, let n = Mlen/b.

3. Let M1, M2, ... , Mn-1, Mn

* denote the unique sequence of bit strings such that M = 

M1 || M2 || ... || Mn-1 || Mn

*

, where M1, M2,..., Mn-1 are complete blocks.2

4. If Mn

* is a complete block, let Mn = K1 ⊕ Mn*

; else, let Mn = K2 ⊕ (Mn

*

||10j), where 

j = nb-Mlen-1. 

5. Let C0 = 0b. 

6. For i = 1 to n, let Ci = CIPHK(Ci-1 ⊕ Mi).

7. Let T = MSBTlen(Cn).

8. Return T.

In Step 1, the subkeys are generated from the key. In Steps 2–4, the input message is formatted 

into a sequence of complete blocks in which the final block has been masked by a subkey. There 

are two cases: 

• If the message length is a positive multiple of the block size, then the message is partitioned 

into complete blocks. The final block is masked with the first subkey; in other words, the 

final block in the partition is replaced with the exclusive-OR of the final block with the 

first subkey. The resulting sequence of blocks is the formatted message.

• If the message length is not a positive multiple of the block size, then the message is 

partitioned into complete blocks to the greatest extent possible, i.e., into a sequence of 

complete blocks followed by a final bit string whose length is less than the block size. A 

padding string is appended to this final bit string, in particular, a single ‘1’ bit followed by 

the minimum number of ‘0’ bits, possibly none, that are necessary to form a complete 

block. The complete final block is masked, as described in the previous bullet, with the 

second subkey. The resulting sequence of blocks is the formatted message. 

In Steps 5 and 6, the cipher block chaining (CBC) technique, with the zero block as the 

initialization vector, is applied to the formatted message. In Steps 7 and 8, the final CBC output 

block is truncated according to the MAC length parameter that is associated with the key, and the 

result is returned as the MAC. 

 2 Consequently, if Mlen ≤ b, then M = M1

*

.
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Equivalent sets of steps, i.e., procedures that yield the correct output from the same input, are 

permitted. For example, it is not necessary to complete the formatting of the entire message (Steps 

3 and 4) prior to the cipher block chaining (Steps 5 and 6). Instead, the iterations of Step 5 may 

be executed “on the fly,” i.e., on each successive block of the message as soon as it is available for 

processing. Step 4 may be delayed until the final bit string in the partition is available; the 

appropriate case, and value of j, if necessary, can be determined from the length of the final bit 

string. In such an implementation, the determination in Step 2 of the total number of blocks in the 

formatted message may be omitted, assuming that the implementation has another way to identify 

the final string in the partition.

Similarly, the subkeys need not be computed anew for each invocation of CMAC with a given 

key; instead, they may be precomputed and stored along with the key as algorithm inputs.

Figure 1: Illustration of the two cases of MAC Generation.

The two cases of MAC Generation are illustrated in Figure 1 above. On the left is the case 

where the message length is a positive multiple of the block size; on the right is the case where 

the message length is not a positive multiple of the block length.

 

6.3 MAC Verification

The following is a specification of the MAC verification process of CMAC:

Prerequisites:

block cipher CIPH with block size b;

key K;

subkeys K1, K2; 

MAC length Tlen.

Input:

message M of bit length Mlen;

...

K1

MSBTlen

T

CIPHK CIPHK

M1 M2

⊕

CIPHK

Mn

*

⊕

...

K2

MSBTlen

T

CIPHK CIPHK

M1 M2

⊕

CIPHK

Mn

*

⊕

... 10...0

K1

MSBTlen

T

CIPHK CIPHK

M1 M2

⊕

CIPHK

Mn

*

⊕

...

K2

MSBTlen

T

CIPHK CIPHK

M1 M2

⊕

CIPHK

Mn

*

⊕

10...0
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received MAC T'.

Output: 

VALID or INVALID.

Suggested Notation:

VER(K, M, T').

Steps:

1. Apply the MAC generation process in Sec. 6.2 to M to produce T.

2. If T = T', return VALID; else, return INVALID. 

In Step 1, the MAC generation process in Sec. 6.2 is applied to the message, and, in Step 2, the 

resulting MAC is compared with the received MAC to determine its validity. 
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Appendix A: Length of the MAC 

The length, Tlen, of the MAC is an important security parameter. The role of this parameter in 

resisting guessing attacks is outlined in Sec. A.1, and guidance in the selection of Tlen is given in 

Sec. A.2.

A.1 Assurance Against Guessing Attacks

The verification process determines whether the purported MAC on a message is the valid output 

of the MAC generation process applied to the message. The output of the MAC verification 

determines the assurance that the receiver of the message obtains:

• If the output is INVALID, then the message is definitely not authentic, i.e., it did not 

originate from a source that executed the generation process on the message to produce the 

purported MAC. 

• If the output is VALID, then the design of the mode provides assurance that the message 

is authentic and, hence, was not corrupted in transit; however, this assurance, as for any 

MAC algorithm, is not absolute.

In the second case, an attacker, i.e., a party without access to the key or to the MAC generation 

process, may have simply guessed the correct MAC for the message. In particular, if the attacker 

selects a MAC at random from the set of strings of length Tlen bits, then the probability is 1 in 

2Tlen that the MAC will be valid. Consequently, larger values of Tlen provide greater protection 

against such an event. Of course, an attacker may attempt to systematically guess many different 

MACs for a message, or for different messages, and thereby increase the probability that one (or 

more) of them will be accepted as valid. For this reason, a system should limit the number of 

unsuccessful verification attempts for each key. 

A.2 Selection of the MAC Length

Larger values of Tlen provide greater assurance against guessing attacks. The performance 

tradeoff is that larger values of Tlen require more bandwidth/storage for the MAC.

For most applications, a value for Tlen that is at least 64 should provide sufficient protection 

against guessing attacks. A value of Tlen that is less than 64 shall only be used in conjunction 

with a careful analysis of the risks of accepting an inauthentic message as authentic. 

In particular, a value of Tlen smaller than 64 should not be used unless the controlling protocol or 

system sufficiently restricts the number of times that the verification process can return INVALID, 

across all implementations with any given key. For example, the short duration of a session or, 

more generally, the low bandwidth of the communication channel may preclude many repeated 

trials. 
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This guidance can be quantified in terms of the following two bounds: 1) the highest acceptable 

probability for an inauthentic message to pass the verification process, and 2) a limit on the number 

of times that the output is the error message INVALID before the key is retired, across all 

implementations of the verification process for the key. Given estimates of these quantities, 

denoted Risk and MaxInvalids, respectively, Tlen should satisfy the following inequality:

Tlen ≥ lg(MaxInvalids/ Risk) .

For example, suppose that the MAC verification process(es) within a system will not output 

INVALID for more than 1024 messages before the key is retired (i.e., MaxInvalids = 210), and that 

the users can tolerate about a one in a million chance that the system will accept an inauthentic 

message (i.e., Risk = 2-20). In this case, any value of Tlen that is greater than or equal to 30 satisfies 

the inequality. 
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Appendix B: Message Span of the Key 

The message span of a key is the total number of messages for which MACs are generated across 

all implementations of CMAC with that key. The message span of the key affects the security of 

the system against attacks that are based on the detection of a pair of distinct messages with the 

same MAC before its truncation3. Such a pair is called a collision4 in this appendix. As with other 

block cipher-based MAC algorithms, an attacker may be able to exploit a collision to produce the 

valid MAC for a new message, the content of which may be largely of the attacker's choosing. 

Such an event would be a fundamental breach of the expected authentication assurance.

 

In principle, collisions must exist because there are many more possible messages than MACs; in 

practice, however, collisions may not occur among the messages for which MACs are actually 

generated during the lifetime of the key. The probability that at least one collision will occur 

depends mostly on the message span of the key relative to the block size, b, of the underlying block 

cipher. For example, a collision is expected to exist among a set of 2b/2 arbitrary messages; in 

other words, 264 messages for the AES algorithm, and 232 messages for TDEA. This property was 

one of the motivations to develop the AES with a block size of 128 bits.

For any system in which CMAC is implemented, the risk that an attacker can detect and exploit a 

collision shall be limited to a level that is appropriate to the value of the data. A simple and prudent 

method to achieve this goal is to establish and enforce an appropriate limit on the message span of 

any CMAC key, which in turn limits the probability that a collision will even occur. For generalpurpose applications, the default recommendation is to limit the key to no more than 248 messages 

when the block size of the underlying block cipher is 128 bits, as with the AES algorithm, and 221

messages when the block size is 64 bits, as with TDEA. Within these limits, the probability that 

a collision will occur is expected to be less than one in a billion for the AES algorithm, and less 

than one in a million for TDEA.

For applications where higher confidence in the security is required, the message span of a key 

may be measured in terms of the total number of message blocks. The recommendation in this 

case is to limit the key to no more than 248 message blocks (222 Gbytes) when the block size is 128 

bits, and 221 message blocks (16 Mbytes) when the block size is 64 bits. Within these limits, the 

probability that a collision will occur is proved to be less than one in a billion for the AES 

algorithm, and less than one in a million for TDEA, assuming that the underlying block cipher has 

no weakness, as modeled in Ref. [6].

In some cases, a limit on the message span of a key may be established and enforced within a key 

management infrastructure by an appropriate constraint on the time span during which the key 

remains in effect, i.e., its cryptoperiod. 

 3 The MAC before truncation is denoted Cm in Sec. 6.2. 4 The standard definition of a collision, in Ref. [9], for example, is more general: for a given function, a collision is a 

pair of distinct input values that yield the same output value. 
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Appendix C: Protection Against Replay of Messages 

As described in Appendix A, the successful verification of a MAC for a message gives assurance 

that the source of the message executed the MAC generation algorithm to create the MAC; 

however, the party that presented the message and MAC for verification may not be the original 

source of the message. Therefore, the CMAC algorithm does not inherently prevent an attacker 

from intercepting a legitimate message and its MAC and “replaying” them for verification at a 

later time, for example, in an attempt to impersonate a party that has access to the key. In some 

protocols an attacker may even be able to present to a verifier a message-MAC pair that the verifier 

itself generated earlier in the protocol. 

The controlling protocol or application may protect against such an event by incorporating certain 

identifying information into the initial bits of every message. Examples of such information 

include a sequential message number, a timestamp, or a nonce. Upon successful verification of 

the message, this information may provide a means for the detection of replayed messages, outof-sequence messages, or missing messages.
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Appendix D: Examples

Examples for CMAC are available at the examples page on NIST’s Computer Security Resource 

Center (CSRC) website: http://csrc.nist.gov/groups/ST/toolkit/examples.html.
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Appendix E: Bibliography

[1] J. Black, P. Rogaway, A Suggestion for Handling Arbitrary-Length Messages with the 

CBC MAC, Natl. Inst. Stand. Technol. [Web page], 

http://csrc.nist.gov/groups/ST/toolkit/BCM/workshops.html#01.

[2] J. Black, P. Rogaway, “CBC MACs for arbitrary-length messages: The three-key 

constructions,” in Advances in Cryptology—Crypto 2000, Lecture Notes in Computer 

Science, Vol. 1880, Mihir Bellare, ed., Springer-Verlag (2000), pp. 197–215.

https://doi.org/10.1007/3-540-44598-6_12. 

[3] FIPS Publication 197, The Advanced Encryption Standard (AES), U.S. DoC/NIST, 

November 26, 2001. http://csrc.nist.gov/publications/fips/fips197/fips-197.pdf. 

[4] FIPS Publication 198-1, The Keyed-Hash Message Authentication Code (HMAC), U.S. 

DoC/NIST, July 2008. http://csrc.nist.gov/publications/fips/fips198-1/FIPS-198-

1_final.pdf. 

[5] T. Iwata, K. Kurosawa, OMAC: One-Key CBC MAC, Natl. Inst. Stand. Technol. [Web 

page], http://csrc.nist.gov/groups/ST/toolkit/BCM/modes_development.html.

[6] T. Iwata, K. Kurosawa, “OMAC: One-Key CBC MAC,” in Fast Software Encryption, 

10th International Workshop, FSE 2003, Lecture Notes in Computer Science, Vol. 2887, 

Thomas Johansson, ed., Springer-Verlag (2003), pp. 129–153.

https://doi.org/10.1007/978-3-540-39887-5_11. 

[7] T. Iwata, K. Kurosawa, OMAC: One-Key CBC MAC—Addendum, Natl. Inst. Stand. 

Technol. [Web page], 

http://csrc.nist.gov/groups/ST/toolkit/BCM/modes_development.html.

[8] T. Iwata, K. Kurosawa, Stronger Security Bounds for OMAC, TMAC, and XCBC, Natl. 

Inst. Stand. Technol. [Web page], 

http://csrc.nist.gov/groups/ST/toolkit/BCM/comments.html.

[9] A. Menezes, P. van Oorschot, S. Vanstone, Handbook of Applied Cryptography, CRC 

Press, Inc., Boca Raton (1996).

[10] NIST Special Publication 800-67 Revision 1, Recommendation for the Triple Data 

Encryption Algorithm (TDEA) Block Cipher, January 2012, Natl. Inst. Stand. Technol. 

[Web page], https://doi.org/10.6028/NIST.SP.800-67r1.