CORE Working Group Working Group P. Urien
Internet Draft Telecom Paris
Intended status: Experimental
December 15 2020
Expires: June 2021
Bijective MAC for Constraint Nodes
draft-urien-core-bmac-07.txt
Abstract
In this draft context, things are powered by micro controllers units
(MCU) comprising a set of memories such as static RAM (SRAM), FLASH
and EEPROM. The total memory size, ranges from 10KB to a few
megabytes. In this context code and data integrity are major
security issues, for the deployment of Internet of Things
infrastructure. The goal of the bijective MAC (bMAC) is to compute
an integrity value, which cannot be guessed by malicious software.
In classical keyed MACs, MAC is computing according to a fixed
order.
In the bijective MAC, the content of N addresses is hashed according
to a permutation P (i.e. bijective application).
The bijective MAC key is the permutation P.
The number of permutations for N addresses is N!. So the computation
of the bMAC requires the knowledge of the whole space memory; this
is trivial for genuine software, but could very difficult for
corrupted software, especially for time stamped bMAC.
Requirements Language
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 RFC 2119.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 2021.
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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warranty as described in the Simplified BSD License.
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Table of Contents
Abstract........................................................... 1
Requirements Language.............................................. 1
Status of this Memo................................................ 1
Copyright Notice................................................... 2
1 Overview......................................................... 4
2 Bijective MAC.................................................... 4
2.1 Memory space................................................ 4
2.2 Permutation................................................. 4
2.3 bMAC computation............................................ 5
2.4 Unused memory............................................... 5
2.5 Permutation entropy......................................... 5
2.6 Time-stamped bMAC........................................... 6
2.6.1 Rational ............................................. 6
2.6.2 Canonical time ....................................... 6
3. The Pq permutation family....................................... 7
3.1 How to find a generator..................................... 7
3.1.1 Method 1 ............................................. 7
3.1.2 Method 2 ............................................. 7
3.1.3 Method 3 ............................................. 8
3.2 How to compute generators................................... 8
3.2.1 Example 1 ............................................ 8
3.2.2 Example 2. ........................................... 8
3.2.3 Example 3. ........................................... 9
3.2.4 Example 4 ............................................ 9
3.2.5 Example 5 ............................................ 9
3.2.6 Example 6 ............................................ 9
3.3 Shifted permutation......................................... 9
3.4 Composition in Fq.......................................... 10
3.5 Code example............................................... 10
3.5.1 Example 1 ........................................... 10
3.5.2 Example 2 ........................................... 11
4 bMAC protocol................................................... 12
5 IANA Considerations............................................. 12
6 Security Considerations......................................... 12
7 References...................................................... 12
7.1 Normative References....................................... 12
7.2 Informative References..................................... 12
8 Authors' Addresses.............................................. 12
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1 Overview
In this draft context, things are powered by micro controllers units
(MCU) comprising a set of memories such as static RAM (SRAM), FLASH
and EEPROM. The total memory size ranges from 10KB to a few
megabytes.
In this context code and data integrity is a major security issue
for the deployment of Internet of Things infrastructure.
The goal of the bijective MAC (bMAC) is to compute an integrity
value, which cannot be guessed by malicious software.
In classical keyed MACs, MAC is computing according to a fixed
order.
In the bijective MAC, the content of N addresses (A[0]...A[N-1]) is
hashed according to a hash function H and a permutation P (i.e.
bijective application in [0,N-1])so that :
bMAC(A, P) = H( A[P(0)] || A[P(1)] ... || A[P(N-1)] )
The bijective MAC key is the permutation P. The number of
permutations for N addresses is N!, as an illustration 35! is
greater than 2**128. So the bMAC computation requires the knowledge
of the whole space memory. This is trivial for genuine software, but
could very difficult for corrupted software, especially for time
stamped bMAC.
2 Bijective MAC
2.1 Memory space
The memory space is represented by an application A, working with N
addresses, whose content is a byte value.
| [0,N-1] -> [0,255]
A |
| x -> A[x]
Non volatile memories (FLASH, EEPROM) MUST be included in the memory
space. A subset of SRAM is included in the memory, whose structure
relies on operational constraints (heap size, stack size,...).
2.2 Permutation
For practical reasons, permutation MAY use a range of M values,
greater than the size N of the memory space (M>=N).
| [0,M-1] -> [0,M-1]
P |
| x -> P(x)
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For example, given a N memory space, and q a prime number so that
q>N, and g a generator for the group Z/qZ, the P permutation (with
M= q-1) can computed as:
| [0,q-2] -> [0,q-2]
P |
| x -> (g**(1+x) mod q)-1
2.3 bMAC computation
We consider a one way hash function H (such as SHA2 or SHA3) with
three procedures, H.reset, H.update, and H.final.
Given a space memory N, a permutation P with M values, the bMAC,
according to C like notation, is computed as:
H.reset() ;
for (i=0; i< M; i++)
{ if (P(i) < N)
H.update(A[P[i]);
}
bMAC= H.final();
2.4 Unused memory
Unused memory MAY be filled by pseudo random values, before
performing the bMAC computation.
2.5 Permutation entropy
A family of Pk permutations is a subset of M! permutations of M
elements, which is computed according to dedicated algorithms.
We note #Pk the number of elements of a Pk family.
The entropy is the integer e, such as 2**e is closed to #Pk:
2**e <= #Pk < 2**(e+1)
The entropy of a family may be increased by the composition of Pk
functions so that :
P(k1,k2,...,kn) = Pkn o ... o Pk2 o Pk1
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2.6 Time-stamped bMAC
2.6.1 Rational
The main idea is to detect corrupted software that uses a code
compression algorithm.
+-------------------------+ +-------------------------------+
| | | +-+ Genuine Code Compressed |
| | +-|-|---------------------------+
| | | | | Code Compression Algo. |
| Genuine Code | +-|-|---------------------------+
| | | V + Malicious bMAC + ^ |
| | +------------------------|-|----+
| | | Genuine Code +-+ |
+-------------------------+ +-------------------------------+
| bMAC | | MALWARE |
+-------------------------+ +-------------------------------+
The basic principle of the time stamped bMAC is that the code
compression algorithm modifies the time needed for the bMAC
computing. Furthermore we assume that the time required by the bMAC
computing is dependent on the permutation.
Below is an illustration of C code that returns the content of a
corrupted address:
if ((Adr >= Adr-Min) && (Adr <= Adr-Max))
v =decompress(Adr);
else
v= read(Adr);
Many computing cycles are added to the genuine code (read(Adr)) due
to Program Counter jumps and execution of the decompression
procedure.
2.6.2 Canonical time
We assume that the bMAC computing time (T) ranges between the values
Tmin and Tmax
Tmin <= T <= Tmax
If the computing time is fixed (Tmin=Tmax) then the Canonical Time
(cT) is the computing time T.
If Tmin#Tmax we define the following values:
Range = Tmax-Tmin+1
Delta = Tmin modulo Range
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For a given computing time T, we define the canonical computing time
cT as:
cT = (T-Delta)/Range
For every T value, cT has a fix value equal to the quotient of
Tmin/Range.
The main interest of the canonical time is that it works as a secret
value, deduced from the bMAC computing but not stored in the
software memory image.
The time-stamped bMAC is computed from an exor operation between the
bMAC and the canonical time:
Time-Stamped bMAC = bMAC exor cT
3. The Pq permutation family
We consider a N memory space, and q a prime number so that q>N.
Z/qZ is a monogenous group with n=phi(q-1) generators (g), phi being
the Euler number. Generators (g) in Z/qZ can be used to build a
permutation family Pq= = {Pg1, Pg2,.., Pgn}, so that:
| [1,q-1] -> [1,q-1]
Pg(x) |
| x -> g**x mod q
Given a P permutation working in the [1,q-1] range (such as Pg), we
use the P*(P) permutation in order to enforce compatibility with the
memory space A(x) starting at the zero address :
| [0,q-2] -> [0,q-2]
P* |
| x -> P*(x) = P(1+x)-1
3.1 How to find a generator
3.1.1 Method 1
Given x in [2, q-1],
If x**k mod q # 1 for all k in [1, q-2], then g is a generator.
3.1.2 Method 2
Factorize q-1 into primes: q-1 = q1**k1...qi**ki...qn**kn
Find n integers ai (a1...an) of order qi**ki, in Z/qZ (phi(qi**ki)
elements)
The product of the n elements a1 x...x an, is a generator.
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3.1.3 Method 3
q being a safe prime, q = 2*p+1 with p prime (p is the Sophie
Germain prime),and q = 7 mod 8.
phi(q-1) = phi(2p) = p-1
1 generator of order 2, i.e. q-1
p-1 generators of order p, i.e. 2**k mod q with k in [1,p-1]
p-1 generators gk of order q-1.
The generators gk are the product of (q-1).2**k mod q, for k in
[1,p-1]. In other words the generators gk are equal to q-(2**k mod
q), for k in [1,p-1]
3.2 How to compute generators
Find a generator g.
There are phi(q-1) generators g**k, with k prime with q-1.
GCD(k,q-1)=1, GCD being the Greatest Common Divisor of two integers.
3.2.1 Example 1
q=11, phi(10)= 4
10= 2x5, phi(2)=1, phi(5)=4
prime numbers with 10= {1,3,7,9}
k 1 2 3 4 5 6 7 8 9 10
x**k 1
2 4 8 5 10 9 7 3 6 1
3 9 5 4 1
4 5 9 3 1
5 3 4 9 1
6 3 7 9 10 5 8 4 2 1
7 5 2 3 10 4 6 9 8 1
8 9 6 4 10 3 2 5 7 1
9 4 3 5 1
10 1
10 has an order 2
3, 4, 5, 9 have order 5
10*3= 8, 4*10= 7, 5*10=6, 9*10=2 are generators
2 is a generator
2**3 = 8 is a generator
2**7 = 7 is a generator
2**9 = 6 is a generator
3.2.2 Example 2.
q= 23 = 2x11 + 1, p=11, q is a safe prime with q mod 8 =7
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power of 2 mod 23 = {2**k, k in [1,10]}= {2,4,8,16,9,18,13,3,6,12}
10 generators gk of order 22 = {21,19,15,7,14,5,10,20,17,11}
3.2.3 Example 3.
Memory space N = 512B EEPROM + 8192B FLASH + 1024B SRAM = 9728B
Nearest prime number q = 9733
q-1 = 9732= 811 x 4 x 3
phi(9732) = 3240
2 is a generator
generators are numbers 2**k mod q, with k less than q-1, and k prime
with 811, 4 and 3.
3.2.4 Example 4
Memory space N = 512B EEPROM + 8192B FLASH + 1024B SRAM = 9728B
Safe prime = 9887
4943 generators
3.2.5 Example 5
Memory space N = 4096B EEPROM + 262144B FLASH + 1024B SRAM= 274432
prime number q = 278543
q-1= 278542 = 2 x 11**2 x 1151
phi(278542) = 126500
5 is a generator
generators are numbers, 5**k mod q, with k less than q-1, prime with
2, 11, and 1151
3.2.6 Example 6
Memory space N = 4096B EEPROM + 262144B FLASH + 1024B SRAM= 274432
Safe prime = 275447
137723 generators
3.3 Shifted permutation
Given an integer s in the range [0, q-1], the shifted permutation
P(g,s) is defined as
| [1,q-1] -> [1,q-1]
P(g,s)(x) |
| x -> s.g**x mod q
In other words P(g,s)(x) = s x Pg(x).
Because s can be written in the form s = g**d, s.g = g**(x+d), which
leads to a right shift.
The number of shifted permutations is (q-1)*phi(q-1).
The benefit of shifted permutation is to increase, with a low cost
computation, the bMAC entropy.
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3.4 Composition in Fq
Given a set of k ptuples {(g1,s1), (g2,s2),..., (gk,sk)} and
associated shifted permutations P(gi,si), a permutation P(q,k) is
computed according to the relation :
P(q,k) = P(gk,sk) o ... o P(g2,s2) o P(g1,s1)
3.5 Code example
The bMAC is computed with a permutation P= P(g2) o P(g1,s1)
The pseudo code is written in a C like way.
H is a SHA3-256 KECCAK hash function.
3.5.1 Example 1
In this example 32 bits integers are used.
The prime number q is 9733.
The address space is N= 9664.
For a 8 bits processor, 12MHz clock, the bMAC is computed in about
10s, i.e. 1ms per byte.
uint32-t x,y,bitn,v,gi[14];
uint32-t PRIME, g1=a-generator, s1=a-value, g2=a-generator;
bool tohash;
PRIME =9733;
H.reset();
gi[0]= g2;
for (int n=1;n<=13;n++)
gi[n] = (gi[n-1] * gi[n-1]) % PRIME;
x= s1;
for(int i=1;i>1;
}
v = (y-1);
// if address v exists, read the v address content A(v)
// tohash=true ;
if (tohash) H.update(A(v));
}
H.dofinal();
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3.5.2 Example 2
In this example 64 bits and 32 bits integers are used.
The prime number q is 278543.
The address space is N= 271360.
For a 8 bits processor, 16MHz clock, the bMAC is computed in about
320s, i.e. 1.1 ms per byte.
uint32-t bitn,v;
uint64-t x,y,gi[19];
uint32-t PRIME, g1=a-generator, s1=a-value, g2=a-generator;
bool tohash;
PRIME = 278543;
H.reset();
gi[0]=(uint64-t)g2;
for (n=1;n<=18;n++)
{ gi[n] = gi[n-1] * gi[n-1];
gi[n] = gi[n] % PRIME;
}
x= s1;
for(i=1;i>1;
}
v = (uint32-t)(y-1);
// if address v exists, read the v address content A(v)
// tohash=true ;
if (tohash) H.update(A(v));
}
H.final();
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4 bMAC protocol
A bMAC protocol involves a bMAC requester and a bMAC provider.
The requester sends to the bMAC provider the parameters needed for
the P permutation.
The bMAC provider computes the bMAC according to the P permutation
and returns the result.
If the bMAC provider has access to internet, the requester
(typically a gateway) SHOULD control its internet access in order to
avoid side channel attack.
5 IANA Considerations
TODO
6 Security Considerations
TODO
7 References
7.1 Normative References
7.2 Informative References
8 Authors' Addresses
Pascal Urien
Telecom Paris
19 Place Marguerite Perey
91120 Palaiseau
France
Phone: NA
Email: Pascal.Urien@telecom-paris.fr
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