CFRG Working Group Ted Krovetz
INTERNET-DRAFT CSU Sacramento
Expires October 2007 Wei Dai
Bitvise Limited
April 2007
VMAC: Message Authentication Code using Universal Hashing
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applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
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Abstract
This specification describes how to generate an authentication tag
using the VMAC message authentication algorithm. VMAC is designed to
have exceptional performance in software on 64-bit CPU architectures
while still performing well on 32-bit architectures. Measured speeds
are as fast as one-half CPU cycle per byte (cpb) on 64-bit
architectures, under five cpb on desktop 32-bit processors, and
around ten cpb on embedded 32-bit architectures.
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2 Notation and basic operations . . . . . . . . . . . . . . . . . . 4
2.1 Operations on strings . . . . . . . . . . . . . . . . . . . 4
2.2 Operations on integers . . . . . . . . . . . . . . . . . . . 5
2.3 String-Integer conversion operations . . . . . . . . . . . . 5
2.4 Mathematical operations on strings . . . . . . . . . . . . . 6
2.5 ENDIAN-SWAP: Adjusting endian orientation . . . . . . . . . 6
3 Key and pad derivation functions . . . . . . . . . . . . . . . . 7
3.1 Block cipher choice . . . . . . . . . . . . . . . . . . . . 7
3.2 KDF: Key-derivation function . . . . . . . . . . . . . . . . 7
3.3 PDF: Pad-derivation function . . . . . . . . . . . . . . . . 8
4 VMAC tag generation . . . . . . . . . . . . . . . . . . . . . . . 9
4.1 VMAC Algorithm . . . . . . . . . . . . . . . . . . . . . . . 9
4.2 VMAC-64 and VMAC-128 . . . . . . . . . . . . . . . . . . . . 10
5 VHASH: Universal hash function . . . . . . . . . . . . . . . . . 10
5.1 VHASH Constants . . . . . . . . . . . . . . . . . . . . . . 10
5.2 VHASH Algorithm . . . . . . . . . . . . . . . . . . . . . . 11
5.3 L1-HASH: First-layer hash . . . . . . . . . . . . . . . . . 11
5.4 L2-HASH: Second-layer hash . . . . . . . . . . . . . . . . . 13
5.5 L3-HASH: Third-layer hash . . . . . . . . . . . . . . . . . 14
6 Security considerations . . . . . . . . . . . . . . . . . . . . . 15
6.1 Resistance to cryptanalysis . . . . . . . . . . . . . . . . 15
6.2 Tag lengths and forging probability . . . . . . . . . . . . 16
6.3 Nonce considerations . . . . . . . . . . . . . . . . . . . . 17
6.4 Replay attacks . . . . . . . . . . . . . . . . . . . . . . . 18
7 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . . 18
Appendix - Test vectors . . . . . . . . . . . . . . . . . . . . . . 19
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Author contact information . . . . . . . . . . . . . . . . . . . . . 20
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . . 20
Intellectual Property . . . . . . . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . 21
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1 Introduction
VMAC is a message authentication code (MAC) algorithm designed for
high performance. It is backed by a formal analysis, and there are
no intellectual property claims made by any of the authors to any
ideas used in its design.
VMAC is a MAC in the style of Wegman and Carter [4, 8]. A fast
"universal" hash function is used to hash an input message M into a
short string. This short string is then combined by addition with a
pseudorandom pad, resulting in the VMAC tag. Security depends on the
sender and receiver sharing a randomly-chosen secret hash function
and pseudorandom pad. This is achieved by using keyed hash function
H and pseudorandom function F. A tag is generated by performing the
computation
Tag = H_K1(M) + F_K2(Nonce)
where K1 and K2 are secret random keys shared by sender and receiver,
and Nonce is a value that changes with each generated tag. The
receiver needs to know which nonce was used by the sender, so some
method of synchronizing nonces needs to be used. This can be done by
explicitly sending the nonce along with the message and tag, or
agreeing upon the use of some other non-repeating value such as a
sequence number. The nonce need not be kept secret, but care needs
to be taken to ensure that, over the lifetime of a VMAC key, a
different nonce is used with each message.
VMAC uses a function, called VHASH (also specified in this document),
as the keyed hash function H and uses a pseudorandom function F whose
default implementation uses the AES block cipher. VMAC allows for
tag lengths of any 64-bit multiple up to the block size of the block
cipher in use. When using AES, this means VMAC can produce 64- or
128-bit tags.
The theory of Wegman-Carter MACs and the analysis of VMAC show that
if one "instantiates" VMAC with truly random keys and pads then the
probability that an attacker (even a computationally unbounded one)
produces a correct tag for messages of its choosing is less than
1/2^60 or 1/2^120 when the tags are of length 64 or 128 bits,
respectively (here the symbol ^ represents exponentiation). When an
attacker makes N forgery attempts the probability of getting one or
more tags right increases linearly to less than N/2^60 or N/2^120.
In a real implementation of VMAC, using AES to produce keys and pads,
these forgery probabilities increase by a small amount related to the
security of AES. As long as AES is secure, this small additive term
is insignificant for any practical attack. See Section 6.2 for more
details. Analysis relevant to VMAC security is in [5, 6].
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VMAC performs best in environments where 64-bit quantities are
efficiently read from memory "little-endian" and multiplied into
128-bit results. Performance on 32-bit architechtures suppporting
Intel's SSE2 instruction-set is also very good. On other 32-bit
architectures, each 64-bit multiplication is accomplished via four
32-bit multiplications, resulting in a corresponding slowdown. The
data in the following table were generated using the reference
implementation available at the VMAC website [7]. The table shows
sample performance on several architectures over message lengths of
64, 512 and 4096 bytes.
64-bit Tags 128-bit Tags
Bits/Endian/Architecture 64 512 4K 64 512 4K
---------------------------------+-----+----+-----+-----+----+----
64/LE/AMD Athlon 64 "Manchester" | 6.0 1.1 0.5 | 7.0 1.6 0.9
64/LE/Intel Core 2 "Merom" | 5.9 1.2 0.6 | 6.9 1.7 1.1
64/BE/IBM PowerPC 970FX | 10.1 2.5 1.6 | 11.4 3.8 3.0
32/LE/Intel Core 2 "Merom" | 8.3 2.2 1.4 | 11.1 3.6 2.8
32/LE/Intel NetBurst "Nocona" | 15.0 4.4 3.1 | 18.9 7.1 5.8
32/BE/Freescale PowerPC 7457 | 15.3 6.4 5.3 | 22.1 11.2 10.0
32/LE/Embedded ARM v5te core | 39.9 13.1 10.1 | 53.6 22.9 19.8
---------------------------------+-----+----+-----+-----+----+----
Table: Tag generation speed measured in CPU cycles per message
byte, for cache-resident messages of length 64, 512 and 4K bytes.
Architechtures are listed as register-size/endianness/model.
2 Notation and basic operations
The specification of VMAC involves the manipulation of strings and
numbers. String variables are denoted with an initial upper-case
letter, whereas numeric variables are denoted in all lower case. The
algorithms of VMAC are denoted in all upper-case letters. Simple
functions, such as for string-length, are written in all lower case.
Whenever a variable is followed by an underscore ("_"), the
underscore is intended to denote a subscript, with the subscripted
expression evaluated to resolve the meaning of the variable. For
example, if i=2, then M_{2 * i} refers to the variable M_4.
2.1 Operations on strings
Messages to be hashed are viewed as strings of bits. The following
notation is used to manipulate these strings.
bitlength(S): The length of string S in bits.
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zeros(n): The string made of n zero-bits.
S[i]: The i-th bit of the string S (indices begin at 1).
S[i...j]: The substring of S consisting of bits i through j.
S || T: The string S concatenated with string T.
zeropad(S,n): The string S, padded with zero-bits to the nearest
multiple of n bits in length. If S is empty or
already a multiple of n in length, nothing is
appended. Formally, zeropad(S,n) = S || T, where T
is the shortest string of zero-bits so that
bitlength(S || T) is a multiple of n.
2.2 Operations on integers
Standard notation is used for most mathematical operations, such as
"*" for multiplication, "+" for addition and "mod" for modular
reduction. Some less standard notations are defined here.
a^i: The integer a raised to the i-th power.
ceil(x): The smallest integer not less than x.
floor(x): The largest integer not greater than x.
a div b: The largest integer i for which b * i <= a.
2.3 String-Integer conversion operations
Conversion between strings and integers is done using the following
functions. Each function treats initial bits as more significant
than later ones.
str2uint(S): The non-negative integer whose binary representation
is the string S. More formally, if S is t bits long
then str2uint(S) = 2^{t-1} * S[1] + 2^{t-2} * S[2] +
... + 2^{1} * S[t-1] + S[t].
uint2str(n,i): The i-bit string S so that str2uint(S) = n.
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2.4 Mathematical operations on strings
One of the primary operations in VMAC is addition and multiplication
of strings. The operations "+_64", "+_128" and "*_128" are defined
"S +_64 T" as uint2str(str2uint(S) + str2uint(T) mod 2^64, 64),
"S +_128 T" as uint2str(str2uint(S) + str2uint(T) mod 2^128, 128),
"S *_128 T" as uint2str(str2uint(S) * str2uint(T) mod 2^128, 128).
On many 64-bit architectures, these operations can each be
implemented with one or two assembly-language instructions.
2.5 ENDIAN-SWAP: Adjusting endian orientation
Message data is normally read little-endian to speed tag generation
on little-endian computers.
2.5.1 ENDIAN-SWAP Algorithm
Input:
S, string with bitlength divisible by 64.
Output:
T, string S with each 64-bit substring endian-reversed.
Compute T using the following algorithm.
//
// Partition S into 64-bit substrings
//
n = bitlength(S) / 64
Let S_1, S_2, ..., S_n be strings of length 64 bits
so that S_1 || S_2 || ... || S_n = S.
//
// Endian-reverse each, and build-up T
//
T =
for i = 1 to n do
Let W_1, W_2, ..., W_8 be strings of length 8 bits
so that W_1 || W_2 || ... || W_8 = S_i
SReversed_i = W_8 || W_7 || ... || W_1
T = T || SReversed_i
end for
Return T
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3 Key and pad derivation functions
Pseudorandom bits are needed internally by VHASH and at the time of
tag generation. The functions listed in this section use a block
cipher to generate these bits.
3.1 Block cipher choice
VMAC uses the services of a block cipher. The selection of a block
cipher defines the following constants and functions.
BLOCKLEN The length, in bits, of the plaintext block on which
the block cipher operates.
KEYLEN The block cipher's key length, in bits.
ENCIPHER(K,P) The application of the block cipher on P (a string of
BLOCKLEN bits) using key K (a string of KEYLEN bits).
As an example, if AES is used with 192-bit keys, then BLOCKLEN would
equal 128 (because AES employs 128-bit blocks), KEYLEN would equal
192, and ENCIPHER would refer to the AES block encryption function
for 192-bit AES keys.
Unless specified otherwise, AES with 128-bit keys shall be assumed to
be the chosen block cipher for VMAC. In any case, BLOCKLEN must be
at least 128. AES is defined in another document [1].
3.2 KDF: Key-derivation function
The key-derivation function generates pseudorandom bits used by the
hash function.
3.2.1 KDF Algorithm
Input:
K, string of length KEYLEN bits.
index, an integer in the range 0...255.
numbits, a non-negative integer.
Output:
Y, string of length numbits bits.
Compute Y using the following algorithm.
//
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// Calculate number of block cipher iterations
//
n = ceil(numbits / BLOCKLEN)
//
// Build Y using block cipher in a counter mode
//
Y =
for i = 0 to (n-1) do
T = uint2str(index, 8) || uint2str(i, BLOCKLEN-8)
Y = Y || ENCIPHER(K, T)
end for
Y = Y[1...numbits]
Return Y
3.3 PDF: Pad-derivation function
This function takes a key and a nonce and returns a pseudorandom pad
for use in tag generation. The length of the pad can be any positive
multiple of 64 bits, up to BLOCKLEN bits. Notice that when the
block-cipher block-length is twice as long as the pad, nonces that
differ only in their last bit are derived from the same block cipher
encryption. This allows caching and sharing a single block cipher
invocation for sequential nonces.
3.3.1 PDF Algorithm
Input:
K, string of length KEYLEN bits.
Nonce, string of length less than BLOCKLEN bits.
taglen, positive multiple of 64, no greater than BLOCKLEN.
Output:
Y, string of length taglen bits.
Compute Y using the following algorithm.
//
// Extract and zero low bits of Nonce if needed.
// If BLOCKLEN/taglen < 2, this step does nothing but set index=0
//
Let i be the smallest integer for which BLOCKLEN/taglen <= 2^i
index = str2uint(Nonce) mod 2^i
Nonce = Nonce[1...bitlength(Nonce)-i] || zeros(i)
//
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// Make Nonce BLOCKLEN bits by prepending zeros.
// At least one zero bit is prepended here.
//
Nonce = zeros(BLOCKLEN - bitlength(Nonce)) || Nonce
//
// Encipher and extract indexed substring
//
T = ENCIPHER(K, Nonce)
Y = T[index * taglen + 1 ... index * taglen + taglen ]
Return Y
4 VMAC tag generation
Tag generation for VMAC proceeds by using VHASH (defined in the next
section) to hash the message, applying the PDF to the nonce and then
computing the addition of the resulting strings. The length of the
pad and hash can be any positive multiple of 64 bits, up to BLOCKLEN
bits.
4.1 VMAC Algorithm
Input:
K, string of length KEYLEN bits.
M, string of length up to 2^64 bits.
Nonce, string of length less than BLOCKLEN bits.
taglen, positive multiple of 64, no greater than BLOCKLEN.
Output:
Tag, string of length taglen bits.
Compute Tag using the following algorithm.
HashedMessage = VHASH(K, M, taglen)
Pad = PDF(K, Nonce, taglen)
Tag =
for i = 0 to (taglen/64 - 1) do
T = Pad [1 + 64 * i ... 64 * (i + 1)] +_64
HashedMessage[1 + 64 * i ... 64 * (i + 1)]
Tag = Tag || T
end for
Return Tag
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4.2 VMAC-64 and VMAC-128
The preceding VMAC definition has a parameter "taglen" which
specifies the length of tag generated by the algorithm. The
following aliases define names that make tag length explicit in the
name.
VMAC-64(K, M, Nonce) = VMAC(K, M, Nonce, 64)
VMAC-128(K, M, Nonce) = VMAC(K, M, Nonce, 128)
5 VHASH: Universal hash function
VHASH is a keyed hash function, which takes as input a string and
produces a string output with length that is a multiple of 64 bits.
VHASH is a three-layered hash function. A message is first hashed by
L1-HASH, its output is then hashed by L2-HASH, whose output is then
hashed by L3-HASH. This process is done once for each 64 bits of
output.
Note that VHASH has certain combinatoric properties making it
suitable for Wegman-Carter message authentication. VHASH is not a
cryptographic hash function and is not a suitable general replacement
for functions like SHA-1.
VHASH is presented here in a top-down manner. First VHASH is
described, then each of its component hashes are presented.
5.1 VHASH Constants
The following constants are referred to in the definition of VHASH.
L1KEYLEN defines how many bits of key material are generated
internally for the first layer of hashing. FAVOR-ENDIAN determines
which endian orientation is used to read messages.
L1KEYLEN = 1024
FAVOR-ENDIAN = LITTLE
One could change L1KEYLEN to any positive multiple of 128 or change
FAVOR-ENDIAN to BIG. A larger L1KEYLEN improves the speed of the
algorithms at the cost of increased memory usage, and changing FAVOR-
ENDIAN to BIG improves the speed of the algorithms on big-endian
machines at the cost of decreased speed on little-endian machines.
The resulting algorithms would be incompatible with the VMAC and
VHASH algorithms defined here, but might be useful in custom
applications.
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5.2 VHASH Algorithm
Input:
K, string of length KEYLEN bits.
M, string of length up to 2^64 bits.
taglen, positive multiple of 64.
Output:
Y, string of length taglen bits.
Compute Y using the following algorithm.
Y =
for i = 0 to (taglen/64 - 1) do
A = L1-HASH(K, M, i)
B = L2-HASH(K, A, bitlength(M), i)
Y = Y || L3-HASH(K, B, i)
end for
Return Y
5.3 L1-HASH: First-layer hash
The first-layer hash breaks the message into blocks, each of length
up to L1KEYLEN (normally defined as 1024 bits), and hashes each with
a function called NH. Concatenating the results forms a string which
is shorter than the original (unless the original length was no
greater than 128 bits).
5.2.1 L1-HASH Algorithm
Input:
K, string of length KEYLEN bits.
M, string of any length.
iter, non-negative integer.
Output:
Y, string of length ceil(bitlength(M)/L1KEYLEN) * 128 bits.
Compute Y using the following algorithm.
//
// Set subkey for L1-HASH
//
T = KDF(K, 128, L1KEYLEN + 128 * iter)
K = T[1 + 128 * iter ... L1KEYLEN + 128 * iter]
Y =
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if bitlength(M) > 0 then
//
// Break M into L1KEYLEN-bit segments (last one may be shorter)
//
t = ceil(bitlength(M) / L1KEYLEN)
Let M_1, M_2, ..., M_t be strings so that M_1 || M_2 || ...
|| M_t = M, and bitlength(M_i) = L1KEYLEN for all 0 < i < t.
//
// For each segment: pad, endian-adjust, NH hash, and use
// results to build output Y. Note that padding only effects
// the final segment because all other segment lengths are
// already a multiple of 128.
//
for i = 1 to t do
M_i = zeropad(M_i, 128)
if FAVOR-ENDIAN = LITTLE then
ENDIAN-SWAP(M_i)
end if
Y = Y || NH(K, M_i)
end for
end if
Return Y
5.2.2 NH Algorithm
Because this routine is applied directly to every bit of input data,
an optimized implementation of it yields great benefit.
Input:
K, string with length a multiple of 128 bits.
M, string with length a multiple of 128 bits, but no longer than K.
Output:
Y, string of length 128 bits.
Compute Y using the following algorithm.
//
// Partition M and K into 64-bit substrings
//
t = bitlength(M) / 64
Let M_1, M_2, ..., M_t be 64-bit strings
so that M = M_1 || M_2 || ... || M_t.
Let K_1, K_2, ..., K_t be 64-bit strings
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so that K_1 || K_2 || ... || K_t is a prefix of K.
//
// Perform NH hash on each.
//
Y = zeros(128)
i = 1
while (i < t) do
Y = Y +_128 ((M_i +_64 K_i) *_128 (M_{i+1} +_64 K_{i+1}))
i = i + 2
end while
Y = zeros(2) || Y[3...128] // Zero two bits (ie, mod 2^126)
Return Y
5.4 L2-HASH: Second-layer hash
The second-layer rehashes the L1-HASH output using a polynomial hash.
5.3.1 L2-HASH Algorithm
Input:
K, string of length KEYLEN bits.
M, string with length a multiple of 128 bits.
len, non-negative integer.
iter, non-negative integer.
Output:
Y, string of length 128 bits.
Compute y using the following algorithm.
//
// Create subkey - the (iter+1)'st 128-bit chunk of the
// string generated by KDF(K, 192)
//
T = KDF(K, 192, 128 * (iter + 1))
T = T[1 + 128 * iter ... 128 * (iter + 1)]
k = str2uint(zeros(3) || T[ 4...32] || zeros(3) || T[ 36... 64] ||
zeros(3) || T[68...96] || zeros(3) || T[100...128])
n = bitlength(M) / 128
if n > 0 then
//
// Partition M into 128-bit substrings and polynomial hash
//
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Let M_1, M_2, ..., M_n be strings of length 128 bits
so that M = M_1 || M_2 || ... || M_n
p127 = 2^127 - 1 // 0x7FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF in hex
y = 1
for i = 1 to n do
m_i = str2uint(M_i)
y = (y * k + m_i) mod p127
end for
else
//
// M is an empty string; handled as a special case
//
y = k
end if
y = (y + (len mod L1KEYLEN) * 2^64) mod p127
Y = uint2str(y, 128)
Return Y
5.5 L3-HASH: Third-layer hash
The output from L2-HASH is 128 bits long. This final hash function
hashes the 128-bit string to a fixed length of 64 bits.
5.4.1 L3-HASH Algorithm
Input:
K, string of length KEYLEN bits.
M, string of length 128 bits.
iter, non-negative integer.
Output:
Y, string of length 64 bits.
Compute Y using the following algorithm.
//
// Create subkey - the (iter+1)'st 128-bit chunk of the
// string generated by KDF(K, 224) that passes a test
//
p64 = 2^64 - 257 // 0xFFFFFFFFFFFFFEFF in hex
i = 0
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need = iter + 1
repeat
T = KDF(K, 224, 128 * (i + 1))
T = T[1 + 128 * i ... 128 * (i + 1)]
k_1 = str2uint(T[ 0... 64])
k_2 = str2uint(T[65...128])
i = i + 1
if (k_1 < p64) and (k_2 < p64) then
need = need - 1
end if
until (need = 0)
//
// Transform M into two integers less than p64 and hash
//
m_1 = str2uint(M) div (2^64 - 2^32)
m_2 = str2uint(M) mod (2^64 - 2^32)
y = ((m_1 + k_1) * (m_2 + k_2)) mod p64
Y = uint2str(y, 64)
Return Y
6 Security considerations
Here we describe some security considerations important for the
proper understanding and use of VMAC.
6.1 Resistance to cryptanalysis
The strength of VMAC depends on the strength of its underlying
cryptographic functions: the key-derivation function (KDF) and the
pad-derivation function (PDF). In this specification both operations
are implemented using a block cipher, by default the Advanced
Encryption Standard (AES). However, the core of the VMAC design, the
VHASH function, does not depend on cryptographic assumptions: its
strength is specified by a purely mathematical property stated in
terms of collision probability, and this property is proven
unconditionally [5, 6]. This means the strength of VHASH is
guaranteed regardless of advances in cryptanalysis and that an
adversarial attack on VMAC that forges with probability significantly
exceeding the established collision probability of VHASH will give
rise to an attack of comparable complexity which breaks the block
cipher, in the sense of distinguishing the block cipher from a family
of random permutations. This design approach essentially obviates
the need for cryptanalysis on VMAC: cryptanalytic efforts might as
well focus on the block cipher.
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6.2 Tag lengths and forging probability
A MAC algorithm is used to authenticate messages between two parties
that share a secret MAC key K. An authentication tag is computed for
a message using K and, in some MAC algorithms such as VMAC, a nonce.
Messages transmitted between parties are accompanied by their tag
and, possibly, nonce. Breaking the MAC means that the attacker is
able to generate, on its own, with no knowledge of the key K, a new
message M (ie, one not previously transmitted between the legitimate
parties) and to compute on M a correct authentication tag under the
key K. This is called a forgery. Note that if the authentication
tag is specified to be of length t then the attacker can trivially
break the MAC with probability 1/2^t. For this the attacker can just
generate any message of its choice and try a random tag; obviously,
the tag is correct with probability 1/2^t. By repeated guesses the
attacker can increase linearly its probability of success.
In the case of VMAC-64, for example, the above guessing-attack
strategy is close to optimal. An adversary can correctly guess a
64-bit VMAC tag with probability 1/2^64 by simply guessing a random
value. The theory of Wegman-Carter MACs and results of [5, 6] show
that no attack strategy can produce a correct tag with probability
better than 1/2^60 if VMAC were to use a random function in its work
rather than AES. Another result shows that so long as AES is secure
as a pseudorandom permutation, it can be used instead of a random
function without significantly increasing the 1/2^60 forging
probability, assuming that no more than 2^64 messages are
authenticated with the same key [2]. Similarly for VMAC-128, the
per-message forgery probability, when using a random function rather
than AES to instantiate VMAC is no more than 1/2^120.
AES has undergone extensive study and is assumed to be very secure as
a pseudorandom permutation. If we assume that no attacker with
feasible computational power can distinguish randomly keyed AES from
a randomly chosen permutation with probability delta (more precisely,
delta is a function of the computational resources of the attacker
and of its ability to sample the function), then we obtain that no
such attacker can forge messages in VMAC with probability greater
than about 1/2^60 or 1/2^120, plus delta. Over N forgery attempts,
forgery occurs with probability no more than N/^60 or N/2^120, plus
delta. The value delta could possibly be greater than 1/2^60 or
1/2^120, in which case the probability of VMAC forging is dominated
by a term representing the security of AES.
With VMAC, off-line computation aimed at exceeding the forging
probability is hopeless as long as the underlying cipher is not
broken. An attacker attempting to forge VMAC tags will need to
interact with the entity that verifies message tags and try a large
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number of forgeries before one is likely to succeed. The system
architecture will determine the extent to which this is possible. In
a well-architected system there should not be any high-bandwidth
capability for presenting forged MACs and determining if they are
valid.
Let us reemphasize: a forging probability of 1/2^60 does not mean
that there is an attack that runs in 2^60 time; to the contrary, as
long as the block cipher in use is not broken there is no such attack
for VMAC. Instead, a 1/2^60 forging probability means that if an
attacker could have N forgery attempts, then the attacker would have
no more than N/2^60 probability of getting one or more of them right.
It should be pointed out that once an attacker knows that an
attempted forgery is successful, it is possible, in principle, that
subsequent messages under this key may be more easily forged. This
is important to understand in gauging the severity of a successful
forgery, even though no such attack on VMAC is known to date. Due to
the short-lived nature of most authentication sessions, 64-bit tags
are appropriate for many security architectures and applications.
If, however, one wants a more conservative option, at a cost of about
double the computation, VMAC's 128-bit tags may be more appropriate.
6.3 Nonce considerations
VMAC requires a nonce with length less than BLOCKLEN bits. All
nonces in an authentication session must be unique and equal in
length.
The security of VMAC depends on the assumption that no nonce is ever
used to generate tags for more than one message under the same key.
If an attacker is able to observe two VMAC tags that were generated
using the same key, the same nonce, and different messages, he may be
able to easily forge other VMAC tags. While such an attack is not
known to date, VMAC was not designed to offer any protection in this
scenario, and nonce reuse must be prevented through appropriate
system architecture.
To authenticate messages over a duplex channel (where two parties
send messages to each other), a different key could be used for each
direction. If the same key is used in both directions, then it is
crucial that all nonces be distinct. For example, one party can use
even nonces while the other party uses odd ones. The receiving party
must verify that the sender is using a nonce of the correct form.
This specification does not indicate how nonce values are created,
updated, or communicated between the entity producing a tag and the
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entity verifying a tag, but there are many possibilities. Nonce
values could be randomly generated, could come from an incrementing
counter, or could be co-opted from some non-repeating part of the
messages being authenticated (such as a sequence number). The nonce
can then be sent along with the message if necessary, or if the
receiver is able to deduce the nonce in use, the nonce need not be
sent. We emphasize that the nonce need not be kept secret, but that
no nonce should be used more than once in any session by either
sender or receiver.
Designers of systems and applications that use VMAC should be aware
that modern virtual machine software such as VMware may allow the
user of a virtual machine to roll back its state (both persistent
storage and volatile memory) to earlier snapshots or checkpoints.
This rollback may be part of an attack, or simply due to an unrelated
decision by an authorized user. In any case, if VMAC is used in such
a virtual machine, a rollback may cause a nonce to be reused,
intentionally or unintentionally, thus violating an important
security assumption. The system or application must either prevent
the occurrence of a state rollback, or be designed to ensure that
nonces are not reused on different messages even when state rollbacks
are possible. For example, one possible design is to generate a
fresh random string as the nonce for each message, after the content
of that message has been fixed.
6.4 Replay attacks
A replay attack occurs when an attacker repeats a message, nonce, and
authentication tag. If the replay of a previously authenticated
message would have negative consequences, then the receiver should
identify repeated message-nonce pairs and ignore them. One way to do
this is to look for a nonce that has already been used to
authenticate a prior message, and ignore it. On a reliable
connection, when the nonce is a counter, this is trivial. On an
unreliable connection, when the nonce is a counter, one would
normally cache some window of recent nonces. Out-of-order message
delivery in excess of what the window allows will result in rejecting
otherwise valid authentication tags. We emphasize that it is up to
the receiver to determine when a given (message, nonce, tag) triple
will be deemed authentic. Certainly the tag should be valid for the
message and nonce, as determined by VMAC, but the message may still
be deemed inauthentic because the nonce is detected to be a replay.
7 IANA Considerations
This document has no actions for IANA.
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Appendix - Test vectors
Following are some sample VMAC outputs over a collection of input
values, using AES with 128-bit keys. Let key K and nonce N be
defined by the following ASCII strings.
K = "abcdefghijklmnop" // A 128-bit VMAC key
N = "bcdefghi" // A 64-bit nonce
The tags generated by VMAC using key K and nonce N are:
Message 64-bit Tag 128-bit Tag
------- ---------- -----------
2576BE1C56D8B81B 472766C70F74ED23481D6D7DE4E80DAC
'abc' * 1 2D376CF5B1813CE5 4EE815A06A1D71EDD36FC75D51188A42
'abc' * 16 E8421F61D573D298 09F2C80C8E1007A0C12FAE19FE4504AE
'abc' * 100 4492DF6C5CAC1BBE 66438817154850C61D8A412164803BCB
'abc' * 1000000 09BA597DD7601113 2B6B02288FFC461B75485DE893C629DC
The first column lists a small sample of messages which are strings
of repeated ASCII 'abc' strings. The remaining columns give in
hexadecimal the tags generated when VMAC is called with the
corresponding message, nonce N and key K.
References
Normative References
[1] FIPS-197, "Advanced Encryption Standard (AES)", National
Institute of Standards and Technology, 2001.
Informative References
[2] D. Bernstein, "Stronger security bounds for permutations",
unpublished manuscript, 2005. This work refines "Stronger
security bounds for Wegman-Carter-Shoup authenticators",
Advances in Cryptology - EUROCRYPT 2005, LNCS vol. 3494, pp.
164-180, Springer-Verlag, 2005.
[3] J. Black, S. Halevi, A. Hevia, H. Krawczyk, T. Krovetz, and P.
Rogaway, "UMAC: Message authentication code using universal
hashing", RFC 4418, IETF, 2006.
[4] L. Carter and M. Wegman, "Universal classes of hash functions",
Journal of Computer and System Sciences, 18 (1979), pp.
143-154.
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[5] W. Dai and T. Krovetz, "VMAC high-speed message
authentication", in progress.
[6] T. Krovetz, "Message auhentication on 64-bit architectures",
Selected Areas in Cryptography - SAC 2006, Springer-Verlag,
2006.
[7] VMAC Website, http://fastcrypto.com/vmac, as seen April 2007.
[8] M. Wegman and L. Carter, "New hash functions and their use in
authentication and set equality", Journal of Computer and
System Sciences, 22 (1981), pp. 265-279.
Author contact information
Author Addresses
Ted Krovetz
Department of Computer Science
California State University
Sacramento CA 95819
USA
Email: tdk@acm.org
Wei Dai
Bitvise Limited
Email: rfc@weidai.com
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Acknowledgments
This document borrows much text from RFC 4418 [3]. That document
describes another message authentication scheme, UMAC, and was co-
written by John Black, Shai Halevi, Alejandro Hevia, Hugo Krawczyk,
Ted Krovetz and Phillip Rogaway. Funding for the RFC Editor function
is currently provided by the Internet Society.
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