CFRG S. Smyshlyaev, Ed.
InternetDraft CryptoPro
Intended status: Informational June 20, 2017
Expires: December 22, 2017
Rekeying Mechanisms for Symmetric Keys
draftirtfcfrgrekeying03
Abstract
If encryption is performed under a single key, there is a certain
maximum threshold amount of data that can be safely encrypted. This
amount is called key lifetime. This specification contains a
description of a variety of methods to increase the lifetime of
symmetric keys. It provides external and internal rekeying
mechanisms based on hash functions and on block ciphers that can be
used with such modes of operations as CTR, GCM, CCM, CBC, CFB, OFB
and OMAC.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used in This Document . . . . . . . . . . . . . . 5
3. Basic Terms and Definitions . . . . . . . . . . . . . . . . . 5
4. Principles of Choice of Constructions and Security Parameters 6
5. External Rekeying Mechanisms . . . . . . . . . . . . . . . . 8
5.1. Methods of Key Lifetime Control . . . . . . . . . . . . . 10
5.2. Parallel Constructions . . . . . . . . . . . . . . . . . 11
5.2.1. Parallel Construction Based on a KDF on a Block
Cipher . . . . . . . . . . . . . . . . . . . . . . . 12
5.2.2. Parallel Construction Based on HKDF . . . . . . . . . 12
5.3. Serial Constructions . . . . . . . . . . . . . . . . . . 13
5.3.1. Serial Construction Based on a KDF on a Block Cipher 13
5.3.2. Serial Construction Based on HKDF . . . . . . . . . . 14
6. Internal Rekeying Mechanisms . . . . . . . . . . . . . . . . 14
6.1. Methods of Key Lifetime Control . . . . . . . . . . . . . 17
6.2. Constructions that Do Not Require Master Key . . . . . . 17
6.2.1. ACPKM Rekeying Mechanisms . . . . . . . . . . . . . 17
6.2.2. CTRACPKM Encryption Mode . . . . . . . . . . . . . . 19
6.2.3. GCMACPKM Authenticated Encryption Mode . . . . . . . 21
6.2.4. CCMACPKM Authenticated Encryption Mode . . . . . . . 23
6.3. Constructions that Require Master Key . . . . . . . . . . 25
6.3.1. ACPKMMaster Key Derivation from the Master Key . . . 25
6.3.2. CTRACPKMMaster Encryption Mode . . . . . . . . . . 26
6.3.3. GCMACPKMMaster Authenticated Encryption Mode . . . 29
6.3.4. CCMACPKMMaster Authenticated Encryption Mode . . . 32
6.3.5. CBCACPKMMaster Encryption Mode . . . . . . . . . . 32
6.3.6. CFBACPKMMaster Encryption Mode . . . . . . . . . . 34
6.3.7. OFBACPKMMaster Encryption Mode . . . . . . . . . . 36
6.3.8. OMACACPKMMaster Mode . . . . . . . . . . . . . . . 37
7. Joint Usage of External and Internal Rekeying . . . . . . . 39
8. Security Considerations . . . . . . . . . . . . . . . . . . . 39
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 40
9.1. Normative References . . . . . . . . . . . . . . . . . . 40
9.2. Informative References . . . . . . . . . . . . . . . . . 40
Appendix A. Test examples . . . . . . . . . . . . . . . . . . . 41
Appendix B. Contributors . . . . . . . . . . . . . . . . . . . . 45
Appendix C. Acknowledgments . . . . . . . . . . . . . . . . . . 46
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 46
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1. Introduction
If encryption is performed under a single key, there is a certain
maximum threshold amount of data that can be safely encrypted. This
amount is called key lifetime and can be calculated from the
following considerations:
1. Methods based on the combinatorial properties of used encryption
mode
[Sweet32] is an example of attack that is based on such
methods. These methods do not depend on the used block cipher
permutation E_{K}. Common encryption modes restrictions
resulting from such methods are of order 2^{n/2}.
2. Methods based on sidechannel analysis issues
In most cases these methods do not depend on the used
encryption modes and weakly depend on the used block cipher
features. Restrictions resulting from these methods are
usually the strongest ones.
3. Methods based on the properties of the used block cipher
permutation E_{K}
The most common methods of this type are linear and
differential cryptanalysis [LDC]. In most cases these methods
do not depend on the used encryption modes. In case of secure
block ciphers, restrictions resulting from such methods are
roughly the same as the natural limitation 2^n and so can be
excluded from consideration as they become trivial.
Therefore, as soon as the total size of a plaintext processed with a
single key reaches the key lifetime limitation, that key must be
replaced. A specific value of the key lifetime is determined in
accordance with safety margin for protocol security and methods
outlined above.
Suppose L is a key lifetime limitation in some protocol P. For
simplicity, assume that all messages have the same length m. Hence
the number of messages q that can be processed with a single key K
should be such that m*q <= L. This can be depicted graphically as a
rectangle with sides m and q which is enclosed by area L (see
Figure 1).
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++
 L 
 +m+ 
 ================== 
 ================== 
 q==================  m*q <= L
 ================== 
 ================== 
 ++ 
++
Figure 1: Graphic display of the key lifetime limitation
Thus, with increasing one of the parameters m or q, the second
parameter should be reduced in proportion to the first.
In practice, such amount of data that corresponds to limitation L may
not be enough. The most simple and obvious way in this situation is
a regular renegotiation of a session key. However, this reduces the
total performance since it usually entails termination of application
data transmission, additional service messages, the use of random
number generator and many other additional calculations, including
resourceintensive asymmetric cryptography.
This specification presents two approaches that extend the key
lifetime for a single symmetric key while avoiding renegotiation:
external and internal rekeying. External rekeying is performed by
a protocol, and it is independent of block cipher, key size, and
mode. External rekeying can use parallel or serial construction.
In the first case subkeys K^1, K^2,... are generated directly from
the key K independently of each other, while in the second case every
next subkey depends on the state that is updated after each new
subkey generation. Internal rekeying is built into the mode, and it
depends heavily on the properties of the block cipher and key size.
The rekeying approaches extend the key lifetime for a single agreed
key by providing the possibility to strictly limit the key leakage
(to meet side channel limitations) and by improving combinatorial
properties of a used block cipher encryption mode.
As for practical issues, rekeying can be particularly useful in such
fields as protocols functioning in hostile environments (additional
side channel resistance against DPA or EMI style attacks) or
lightweight cryptography (usage of ciphers with small block size
leads to very strong combinatorial limitations). Moreover, many
mechanisms that use external and internal rekeying provide
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particular types of PFS security. Also rekeying can provide
additional security against possible future attacks on the used
ciphers (by limiting the number of plaintextciphertext pairs
collected by an adversary), however, it must not be used as a method
to prolong life of ciphers that are already known to be vulnerable.
2. 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].
3. Basic Terms and Definitions
This document uses the following terms and definitions for the sets
and operations on the elements of these sets:
(xor) exclusiveor of two binary vectors of the same length.
V* the set of all strings of a finite length (hereinafter
referred to as strings), including the empty string;
V_s the set of all binary strings of length s, where s is a non
negative integer; substrings and string components are
enumerated from right to left starting from one;
X the bit length of the bit string X;
AB concatenation of strings A and B both belonging to V*, i.e.,
a string in V_{A+B}, where the left substring in V_A is
equal to A, and the right substring in V_B is equal to B;
Z_{2^n} ring of residues modulo 2^n;
Int_s: V_s > Z_{2^s} the transformation that maps a string a =
(a_s, ... , a_1), a in V_s, into the integer Int_s(a) =
2^{s1}*a_s + ... + 2*a_2 + a_1;
Vec_s: Z_{2^s} > V_s the transformation inverse to the mapping
Int_s;
MSB_i: V_s > V_i the transformation that maps the string a = (a_s,
... , a_1) in V_s, into the string MSB_i(a) = (a_s, ... ,
a_{si+1}) in V_i;
LSB_i: V_s > V_i the transformation that maps the string a = (a_s,
... , a_1) in V_s, into the string LSB_i(a) = (a_i, ... ,
a_1) in V_i;
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Inc_c: V_s > V_s the transformation that maps the string a = (a_s,
... , a_1) in V_s, into the string Inc_c(a) = MSB_{a
c}(a)  Vec_c(Int_c(LSB_c(a)) + 1(mod 2^c)) in V_s;
a^s denotes the string in V_s that consists of s 'a' bits;
E_{K}: V_n > V_n the block cipher permutation under the key K in
V_k;
ceil(x) the least integer that is not less than x;
k the key K size (in bits), k is multiple of 8;
n the block size of the block cipher (in bits), n is multiple
of 8;
b the total number of data blocks in the plaintext (b = ceil(m/
n));
N the section size (the number of bits in a data section);
l the number of data sections in the plaintext;
phi_i: V_s > {0,1} the transformation that maps a string a = (a_s,
... , a_1) into the value phi_i(a) = a_i for all i in {1, ...
, s}.
A plaintext message P and a ciphertext C are divided into b =
ceil(P/n) blocks denoted as P = P_1  P_2  ...  P_b and C = C_1 
C_2  ...  C_b, where P_i and C_i are in V_n, for i = 1, 2, ... ,
b1, and P_b, C_b are in V_r, where r <= n if not otherwise stated.
4. Principles of Choice of Constructions and Security Parameters
External rekeying provides an approach, decording to which a key is
transformed after encrypting a limited number of messages. A
specific external rekeying method is chosen at the protocol level
regardless of a used block cipher or encryption mode. External re
keying approach is recommended for usage in protocols that process
quite small messages or in protocols that have a way to break a large
message into manageable parts. As a result of external rekeying,
the number of messages that can be processed with a single symmetric
key is substantially increased without loss in message length.
The use of external rekeying has the following advantages:
1. the lifetime of a negotiated key drastically increases by
increasing the number of messages processed with one key;
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2. it almost does not affect performance in case when a number of
messages processed with one key is sufficiently large;
3. provides forward and backward security of session keys for all
messages.
However, the use of external rekeying has the following
disadvantages:
1. in case of restrictive key lifetime limitations the message sizes
can become inconvenient due to impossibility of processing
sufficiently large messages, so it could be necessary to perform
additional fragmentation at the protocol level;
2. it is not transparent: procedures (like IVs generation) must be
handled separately.
Internal rekeying provides an approach according to which a key is
transformed during each separate message processing. Such approaches
are integrated into the base modes of operations so every internal
rekeying mechanism is defined for a particular mode and block cipher
(e.g. depending of block and key sizes). Internal rekeying approach
is recommended to be used in protocols that process large messages.
As a result of internal rekeying, the size of each single message
can be substantially increased without loss in number of messages
that can be processed with a single symmetric key.
The use of internal rekeying has the following advantages:
1. the lifetime of a negotiated key drastically increases by
increasing the size of messages processed with one key;
2. minimal impact on performance;
3. internal rekeying mechanisms without master key does not affect
short messages transformation at all;
4. transparent (works like any encryption mode): does not require
changes of IV's and restarting MACing.
However, the use of internal rekeying has the following
disadvantages:
1. a specific method must not be chosen independently of a mode of
operation;
2. internal rekeying mechanisms with master key provide backward
security of session keys only for one separate message;
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3. internal rekeying mechanisms without master key do not provide
backward security of session keys.
Any block cipher modes of operations with internal rekeying can be
jointly used with any external rekeying mechanisms. Such joint
usage increases both the number of messages processed with one key
and their maximum possible size.
The use of the same cryptographic primitives both for data processing
and rekeying transformation decreases the code size but can lead to
some possible vulnerabilities because the adversary always has access
to the data processing interface. This vulnerability can be
eliminated by using different primitives for data processing and re
keying, however, in this case the security of the whole scheme cannot
be reduced to standard notions like PRF or PRP so security
estimations become more difficult and unclear.
5. External Rekeying Mechanisms
This section presents an approach to increase the key lifetime by
using a transformation of a previously negotiated key after
processing a limited number of integral messages. It provides an
external parallel and serial rekeying mechanisms (see [AbBell]).
These mechanisms use an initial (negotiated) key as a master key,
which is never used directly for the data processing but is used for
key generation. Such mechanisms operate outside of the base modes of
operations and do not change them at all, therefore they are called
"external rekeying" mechanisms in this document.
External rekeying mechanisms are recommended for usage in protocols
that process quite small messages since the maximum gain in
increasing the key lifetime is achieved by increasing the number of
messages.
External rekeying increases the key lifetime through the following
approach. Suppose there is a protocol P with some mode of operation
(base encryption or authentication mode). Let L1 be a key lifetime
limitation induced by sidechannel analysis methods (sidechannel
limitation), let L2 be a key lifetime limitation induced by methods
based on the combinatorial properties of used mode of operation
(combinatorial limitation) and let q1, q2 be the total numbers of
messages of length m, that can be safely processed with a single key
K according to these limitations.
Let L = min(L1, L2), q = min (q1, q2), q*m <= L. As L1 limitation is
usually much stronger then L2 limitation (L1 < L2), the final key
lifetime restriction is equal to the most restrictive limitation L1.
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Thus, as displayed in Figure 2, without rekeying only q1 (q1*m <=
L1) messages can be safely processed.
<m>
++ ^ ^
================  
================  
K>================ q1
================  
==============L1  
++ v 
  
  
  q2
  
  
  
  
  
  
  
  
 L2 
++ v
Figure 2: Basic principles of message processing without external rekeying
Suppose that the safety margin for the protocol P is fixed and the
external rekeying approach is applied. As the key is transformed
with an external rekeying mechanism, the leakage of a previous key
does not have any impact on the following one, so the side channel
limitation L1 goes off. Thus, the resulting key lifetime limitation
of the negotiated key K can be calculated on the basis of a new
combinatorial limitation L2'. It is proven (see [AbBell]) that the
security of the mode of operation that uses external rekeying leads
to an increase when compared to base mode without rekeying (thus, L2
< L2'). Hence, as displayed in Figure 3, the resulting key lifetime
limitation in case of using external rekeying can be increased up to
L2'.
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<m>
++
================
================
K> ================
 ================
 ==============L1
 ++
 ================
v ================
K^2> ================
 ================
 ==============L1
 ++
 ================
v ================
...  . . . 
 
 
 L2
++
 L2'
++
Figure 3: Basic principles of message processing with external rekeying
Note: the key transformation process is depicted in a simplified
form. A specific approach (parallel and serial) is described below.
Consider an example. Let the message size in protocol P be equal to
1 KB. Suppose L1 = 128 MB and L2 = 1 TB. Thus, if an external re
keying mechanism is not used, the key K must be renegotiated after
processing 128 MB / 1 KB = 131072 messages.
If an external rekeying mechanism is used, the key lifetime
limitation L1 goes off. Hence the resulting key lifetime limitation
in case of using external rekeying can be set to 1 TB (and even
more). Thus if an external rekeying mechanism is used, then 1 TB /
1 KB = 2^30 messages can be processed before the master key K is
renegotiated. This is 8192 times greater than the number of messages
that can be processed, when external rekeying mechanism is not used.
5.1. Methods of Key Lifetime Control
Suppose L is an amount of data that can be safely processed with one
key (without rekeying). For i in {1, 2, ..., t} the key K^i (see
Figure 1 and Figure 2) should be transformed after processing q_i
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integral messages, where q_i can be calculated in accordance with one
of the following two approaches:
o Explicit approach:
M^{i,1} + ... + M^{i,q_i} <= L, M^{i,1} + ... + M^{i,q_i +
1} > L.
This approach allows to use the key K^i in almost optimal way but
it cannot be applied in case when messages may be lost or
reordered (e.g. DTLS packets).
o Implicit approach:
q_i = L / m_max, i = 1, ... , t.
The amount of data processed with one key K^i is calculated under
the assumption that every message has the maximum length m_max.
Hence this amount can be considerably less than the key lifetime
limitation L. On the other hand this approach can be applied in
case when messages may be lost or reordered (e.g. DTLS packets).
5.2. Parallel Constructions
The main idea behind external rekeying with parallel construction is
presented in Figure 4:
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Maximum message size = m_max.
_____________________________________________________________
m_max
<>
M^{1,1} === 
M^{1,2} =============== 
+K^1> . . .
 M^{1,q_1} ======== 


 M^{2,1} ================
 M^{2,2} ===== 
KK^2> . . .
 M^{2,q_2} ========== 

...
 M^{t,1} ============ 
 M^{t,2} ============= 
+K^t> . . .
M^{t,q_t} ========== 
_____________________________________________________________
Figure 4: External parallel rekeying mechanisms
The key K^i, i = 1, ... , t1, is updated after processing a certain
amount of data (see Section 5.1).
5.2.1. Parallel Construction Based on a KDF on a Block Cipher
ExtParallelC rekeying mechanism is based on key derivation function
on a block cipher and is used to generate t keys for t sections as
follows:
K^1  K^2  ...  K^t = ExtParallelC(K, t*k) =
MSB_{t*k}(E_{K}(0)  E_{K}(1)  ...  E_{K}(R1)),
where R = ceil(t*k/n).
5.2.2. Parallel Construction Based on HKDF
ExtParallelH rekeying mechanism is based on HMAC key derivation
function HKDFExpand, described in [RFC5869], and is used to generate
t keys for t sections as follows:
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K^1  K^2  ...  K^t = ExtParallelH(K, t*k) = HKDFExpand(K,
label, t*k),
where label is a string (can be a zerolength string) that is defined
by a specific protocol.
5.3. Serial Constructions
The main idea behind external rekeying with serial construction is
presented in Figure 5:
Maximum message size = m_max.
_____________________________________________________________
m_max
<>
M^{1,1} === 
M^{1,2} =============== 
K*_1 = K K^1> . . .
 M^{1,q_1} ======== 


 M^{2,1} ================
v M^{2,2} ===== 
K*_2 K^2> . . .
 M^{2,q_2} ========== 

...
 M^{t,1} ============ 
v M^{t,2} ============= 
K*_t K^t> . . .
M^{t,q_t} ========== 
_____________________________________________________________
Figure 5: External serial rekeying mechanisms
The key K^i, i = 1, ... , t1, is updated after processing a certain
amount of data (see Section 5.1).
5.3.1. Serial Construction Based on a KDF on a Block Cipher
The key K^i is calculated using ExtSerialC transformation as follows:
K^i = ExtSerialC(K, i) = MSB_k(E_{K*_i}(0)  E_{K*_i}(1)  ... 
E_{K*_i}(J1)),
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where J = ceil(k/n), i = 1, ... , t, K*_i is calculated as follows:
K*_1 = K,
K*_{j+1} = MSB_k(E_{K*_j}(J)  E_{K*_j}(J+1)  ...  E_{K*_j}(2J
1)),
where j = 1, ... , t1.
5.3.2. Serial Construction Based on HKDF
The key K^i is calculated using ExtSerialH transformation as follows:
K^i = ExtSerialH(K, i) = HKDFExpand(K*_i, label1, k),
where i = 1, ... , t, HKDFExpand is an HMACbased key derivation
function, described in [RFC5869], K*_i is calculated as follows:
K*_1 = K,
K*_{j+1} = HKDFExpand(K*_j, label2, k), where j = 1, ... , t1,
where label1 and label2 are different strings (can be a zerolength
strings) that are defined by a specific protocol (see, for example,
TLS 1.3 updating traffic keys algorithm [TLSDraft]).
6. Internal Rekeying Mechanisms
This section presents an approach to increase the key lifetime by
using a transformation of a previously negotiated key during each
separate message processing.
It provides internal rekeying mechanisms called ACPKM (Advanced
cryptographic prolongation of key material) and ACPKMMaster that do
not use and use a master key respectively. Such mechanisms are
integrated into the base modes of operations and actually form new
modes of operation, therefore they are called "internal rekeying"
mechanisms in this document.
Internal rekeying mechanism is recommended to be used in protocols
that process large single messages (e.g. CMS messages) since the
maximum gain in increasing the key lifetime is achieved by increasing
the length of a message, while it almost does not affect performance
for increasing the number of messages.
Internal rekeying increases the key lifetime through the following
approach. Suppose there is a protocol P with some base mode of
operation. Let L1 and L2 be a side channel and combinatorial
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limitations respectively and for some fixed amount of messages q let
m1, m2 be the length of each separate message, that can be safely
processed with a single key K according to these limitations.
Thus, by analogy with the Section 5 without rekeying the final key
lifetime restriction, as displayed in Figure 6, is equal to L1 and
only q messages of the length m1 can be safely processed.
K

v
^ +++
 ==============L1 L2
 ================ 
q ================ 
 ================ 
 ================ 
v +++
<m1>
<m2>
Figure 6: Basic principles of message processing without internal rekeying
Suppose that the safety margin for the protocol P is fixed and
internal rekeying approach is applied to the base mode of operation.
Suppose further that for every message the key is transformed after
processing N bits of data, where N is a parameter. If q*N does not
exceed L1 then the side channel limitation L1 goes off and the
resulting key lifetime limitation of the negotiated key K can be
calculated on the basis of a new combinatorial limitation L2'. The
security of the mode of operation that uses external rekeying must
lead to an increase when compared to base mode of operation without
rekeying (thus, L2 < L2'). Hence, as displayed in Figure 7, the
resulting key lifetime limitation in case of using external rekeying
can be increased up to L2'.
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K > K^2 > . . .
 
v v
^ +++++
 ==============L1==============L1====== L2 L2'
 ======================================  
q ====================================== . . .  
 ======================================  
 ======================================  
v +++++
<N>
Figure 7: Basic principles of message processing with internal rekeying
Note: the key transformation process is depicted in a simplified
form. A specific approach (ACPKM and ACPKMMaster rekeying
mechanisms) is described below.
Since the performance of encryption can slightly decrease for rather
small values of N, the parameter N should be selected for a
particular protocol as maximum possible to provide necessary key
lifetime for the adversary models that are considered.
Consider an example. Suppose L1 = 128 MB and L2 = 10 TB. Let the
message size in the protocol be large/unlimited (may exhaust the
whole key lifetime L2'). The most restrictive resulting key lifetime
limitation is equal to 128 MB.
Thus, there is a need to put a limit on the maximum message size
m_max. For example, if m_max = 32 MB, it may happen that the
renegotiation of key K would be required after processing only four
messages.
If an internal rekeying mechanism with section size N = 1 MB (see
Figure 3 and Figure 4) is used, more then L1 / N = 128 MB / 1 MB =
128 messages can be processed before the renegotiation of key K
(instead of 4 messages in case when an internal rekeying mechanism
is not used). Note that only one section of each message is
processed with one key K^i, and, consequently, the key lifetime
limitation L1 goes off. Hence the resulting key lifetime limitation
in case of using external rekeying can be set to at least 10 TB (in
the case when the single large message is processed using the key K).
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6.1. Methods of Key Lifetime Control
Suppose L is an amount of data that can be safely processed with one
key (without rekeying), N is a section size (fixed parameter).
Suppose M^{i}_1 is the first section of message M^{i}, i = 1, ... , q
(see Figure 3 and Figure 4), then the parameter q can be calculated
in accordance with one of the following two approaches:
o Explicit approach:
M^{1}_1 + ... + M^{q}_1 <= L, M^{1}_1 + ... + M^{q+1}_1 >
L
This approach allows to use the key K^i in an almost optimal way
but it cannot be applied in case when messages may be lost or
reordered (e.g. DTLS packets).
o Implicit approach:
q = L / N.
The amount of data processed with one key K^i is calculated under
the assumption that the length of every message is equal or more
then section size N and so it can be considerably less than the
key lifetime limitation L. On the other hand this approach can be
applied in case when messages may be lost or reordered (e.g. DTLS
packets).
6.2. Constructions that Do Not Require Master Key
This section describes the block cipher modes that use the ACPKM re
keying mechanism, which does not use master key: an initial key is
used directly for the encryption of the data.
6.2.1. ACPKM Rekeying Mechanisms
This section defines periodical key transformation with no master key
which is called ACPKM rekeying mechanism. This mechanism can be
applied to one of the basic encryption modes (CTR and GCM block
cipher modes) for getting an extension of this encryption mode that
uses periodical key transformation with no master key. This
extension can be considered as a new encryption mode.
An additional parameter that defines the functioning of base
encryption modes with the ACPKM rekeying mechanism is the section
size N. The value of N is measured in bits and is fixed within a
specific protocol based on the requirements of the system capacity
and key lifetime (some recommendations on choice of N will be
provided in Section 8). The section size N MUST be divisible by the
block size n.
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The main idea behind internal rekeying with no master key is
presented in Figure 8:
Section size = const = N,
maximum message size = m_max.
____________________________________________________________________
ACPKM ACPKM ACPKM
K^1 = K > K^2 ...> K^{l_max1} > K^{l_max}
   
   
v v v v
M^{1} ==================== ... =================: 
M^{2} ==================== ... ===  : 
. . . . . . :
: : : : : : :
M^{q} ==================== ... =============== : 
section :
<> m_max
N bit
___________________________________________________________________
l_max = ceil(m_max/N).
Figure 8: Internal rekeying with no master key
During the processing of the input message M with the length m in
some encryption mode that uses ACPKM key transformation of the key K
the message is divided into l = ceil(m/N) sections (denoted as M =
M_1  M_2  ...  M_l, where M_i is in V_N for i = 1, 2, ... , l1
and M_l is in V_r, r <= N). The first section of each message is
processed with the initial key K^1 = K. To process the (i+1)th
section of each message the K^{i+1} key value is calculated using
ACPKM transformation as follows:
K^{i+1} = ACPKM(K^i) = MSB_k(E_{K^i}(D_1)  ...  E_{K^i}(D_J)),
where J = ceil(k/n), parameter c is fixed by a specific encryption
mode which uses ACPKM key transformation and D_1, D_2, ... , D_J are
in V_n and are calculated as follows:
D_1  D_2  ...  D_J = MSB_{J*n}(D),
where D is the following constant in V_{1024}:
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D = ( 80  81  82  83  84  85  86  87
 88  89  8a  8b  8c  8d  8e  8f
 90  91  92  93  94  95  96  97
 98  99  9a  9b  9c  9d  9e  9f
 a0  a1  a2  a3  a4  a5  a6  a7
 a8  a9  aa  ab  ac  ad  ae  af
 b0  b1  b2  b3  b4  b5  b6  b7
 b8  b9  ba  bb  bc  bd  be  bf
 c0  c1  c2  c3  c4  c5  c6  c7
 c8  c9  ca  cb  cc  cd  ce  cf
 d0  d1  d2  d3  d4  d5  d6  d7
 d8  d9  da  db  dc  dd  de  df
 e0  e1  e2  e3  e4  e5  e6  e7
 e8  e9  ea  eb  ec  ed  ee  ef
 f0  f1  f2  f3  f4  f5  f6  f7
 f8  f9  fa  fb  fc  fd  fe  ff )
N o t e : The constant D is such that D_1, ... , D_J are pairwise
different for any allowed n, k values.
N o t e : The constant D is such that phi_c(D_t) = 1 for any allowed
n, k, c and t in {1, ... , J}. This condition is important, as it
allows to prevent collisions of blocks of transformed key and block
cipher permutation inputs.
6.2.2. CTRACPKM Encryption Mode
This section defines a CTRACPKM encryption mode that uses internal
ACPKM rekeying mechanism for the periodical key transformation.
The CTRACPKM mode can be considered as the extended by the ACPKM re
keying mechanism basic encryption mode CTR (see [MODES]).
The CTRACPKM encryption mode can be used with the following
parameters:
o 64 <= n <= 512;
o 128 <= k <= 512;
o the number of bits c in a specific part of the block to be
incremented is such that 16 <= c <= 3/4 n, c is multiple of 8.
The CTRACPKM mode encryption and decryption procedures are defined
as follows:
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++
 CTRACPKMEncrypt(N, K, ICN, P) 

 Input: 
  Section size N, 
  key K, 
  initial counter nonce ICN in V_{nc}, 
  plaintext P = P_1  ...  P_b, P < n * 2^{c1}. 
 Output: 
  Ciphertext C. 

 1. CTR_1 = ICN  0^c 
 2. For j = 2, 3, ... , b do 
 CTR_{j} = Inc_c(CTR_{j1}) 
 3. K^1 = K 
 4. For i = 2, 3, ... , ceil(P/N) 
 K^i = ACPKM(K^{i1}) 
 5. For j = 1, 2, ... , b do 
 i = ceil(j*n / N), 
 G_j = E_{K^i}(CTR_j) 
 6. C = P (xor) MSB_{P}(G_1  ...  G_b) 
 7. Return C 
++
++
 CTRACPKMDecrypt(N, K, ICN, C) 

 Input: 
  Section size N, 
  key K, 
  initial counter nonce ICN in V_{nc}, 
  ciphertext C = C_1  ...  C_b, C < n * 2^{c1}. 
 Output: 
  Plaintext P. 

 1. P = CTRACPKMEncrypt(N, K, ICN, C) 
 2. Return P 
++
The initial counter nonce ICN value for each message that is
encrypted under the given key must be chosen in a unique manner.
The message size MUST NOT exceed n * 2^{c1} bits.
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6.2.3. GCMACPKM Authenticated Encryption Mode
This section defines GCMACPKM authenticated encryption mode that
uses internal ACPKM rekeying mechanism for the periodical key
transformation.
The GCMACPKM mode can be considered as the basic authenticated
encryption mode GCM (see [GCM]) extended by the ACPKM rekeying
mechanism.
The GCMACPKM authenticated encryption mode can be used with the
following parameters:
o n in {128, 256};
o 128 <= k <= 512;
o the number of bits c in a specific part of the block to be
incremented is such that 32 <= c <= 3/4 n, c is multiple of 8.;
o authentication tag length t.
The GCMACPKM mode encryption and decryption procedures are defined
as follows:
++
 GHASH(X, H) 

 Input: 
  Bit string X = X_1  ...  X_m, X_i in V_n for i in 1, ... , m.
 Output: 
  Block GHASH(X, H) in V_n. 

 1. Y_0 = 0^n 
 2. For i = 1, ... , m do 
 Y_i = (Y_{i1} (xor) X_i) * H 
 3. Return Y_m 
++
++
 GCTR(N, K, ICB, X) 

 Input: 
  Section size N, 
  key K, 
  initial counter block ICB, 
  X = X_1  ...  X_b, X_i in V_n for i = 1, ... , b1 and 
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 X_b in V_r, where r <= n. 
 Output: 
  Y in V_{X}. 

 1. If X in V_0 then return Y, where Y in V_0 
 2. GCTR_1 = ICB 
 3. For i = 2, ... , b do 
 GCTR_i = Inc_c(GCTR_{i1}) 
 4. K^1 = K 
 5. For j = 2, ... , ceil(l*n / N) 
 K^j = ACPKM(K^{j1}) 
 6. For i = 1, ... , b do 
 j = ceil(i*n / N), 
 G_i = E_{K_j}(GCTR_i) 
 7. Y = X (xor) MSB_{X}(G_1  ...  G_b) 
 8. Return Y. 
++
++
 GCMACPKMEncrypt(N, K, IV, P, A) 

 Input: 
  Section size N, 
  key K, 
  initial counter nonce ICN in V_{nc}, 
  plaintext P, P <= n*(2^{c1}  2), P = P_1  ...  P_b, 
  additional authenticated data A. 
 Output: 
  Ciphertext C, 
  authentication tag T. 

 1. H = E_{K}(0^n) 
 2. If c = 32, then ICB_0 = ICN  0^31  1 
 if c!= 32, then s = n * ceil(ICN / n)  ICN, 
 ICB_0 = GHASH(ICN  0^{s+n64}  Vec_64(ICN), H) 
 3. C = GCTR(N, K, Inc_32(ICB_0), P) 
 4. u = n*ceil(C / n)  C 
 v = n*ceil(A / n)  A 
 5. S = GHASH(A  0^v  C  0^u  0^{n128}  Vec_64(A)  
  Vec_64(C), H) 
 6. T = MSB_t(E_{K}(ICB_0) (xor) S) 
 7. Return C  T 
++
++
 GCMACPKMDecrypt(N, K, IV, A, C, T) 

 Input: 
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  Section size N, 
  key K, 
  initial counter block ICB, 
  additional authenticated data A. 
  ciphertext C, C <= n*(2^{c1}  2), C = C_1  ...  C_b, 
  authentication tag T 
 Output: 
  Plaintext P or FAIL. 

 1. H = E_{K}(0^n) 
 2. If c = 32, then ICB_0 = ICN  0^31  1 
 if c!= 32, then s = n*ceil(ICN/n)ICN, 
 ICB_0 = GHASH(ICN  0^{s+n64}  Vec_64(ICN), H) 
 3. P = GCTR(N, K, Inc_32(ICB_0), C) 
 4. u = n*ceil(C / n)C 
 v = n*ceil(A / n)A 
 5. S = GHASH(A  0^v  C  0^u  0^{n128}  Vec_64(A)  
  Vec_64(C), H) 
 6. T' = MSB_t(E_{K}(ICB_0) (xor) S) 
 7. If T = T' then return P; else return FAIL 
++
The * operation on (pairs of) the 2^n possible blocks corresponds to
the multiplication operation for the binary Galois (finite) field of
2^n elements defined by the polynomial f as follows (by analogy with
[GCM]):
n = 128: f = a^128 + a^7 + a^2 + a^1 + 1.
n = 256: f = a^256 + a^10 + a^5 + a^2 + 1.
The initial vector IV value for each message that is encrypted under
the given key must be chosen in a unique manner.
The plaintext size MUST NOT exceed n*(2^{c1}  2) bits.
The key for computing values E_{K}(ICB_0) and H is not updated and is
equal to the initial key K.
6.2.4. CCMACPKM Authenticated Encryption Mode
This section defines a CCMACPKM authenticated encryption block
cipher mode that uses internal ACPKM rekeying mechanism for the
periodical key transformation.
The CCMACPKM mode can be considered as the extended by the ACPKM re
keying mechanism basic authenticated encryption mode CCM (see
[RFC3610]).
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Since [RFC3610] defines CCM authenticated encryption mode only for
128bit block size, the CCMACPKM authenticated encryption mode can
be used only with the parameter n = 128. However, the CCMACPKM
design principles can easily be applied to other block sizes, but
these modes will require their own specifications.
The CCMACPKM authenticated encryption mode differs from CCM mode in
keys that are used for encryption during CBCMAC calculation (see
Section 2.2 of [RFC3610]) and key stream blocks generation (see
Section 2.3 of [RFC3610]).
The CCM mode uses the same initial key K block cipher encryption
operations, while the CCMACPKM mode uses the keys K^1, K^2, ...,
which are generated from the key K as follows:
K^1 = K,
K^{i+1} = ACPKM( K^i ).
The keys K^1, K^2, ..., which are used as follows.
CBCMAC calculation: under a separate message processing during the
first N/n block cipher encryption operations the key K^1 is used, the
key K^2 is used for the next N/n block cipher encryption operations
and so on. For example, if N = 2n, then CBCMAC calculation for a
sequence of t blocks B_0, B_1, ..., B_t is as follows:
X_1 = E( K^1, B_0 ),
X_2 = E( K^1, X_1 XOR B_1 ),
X_3 = E( K^2, X_2 XOR B_2 ),
X_4 = E( K^2, X_3 XOR B_3 ),
X_5 = E( K^3, X_4 XOR B_4 ),
...
T = firstMbytes( X_t+1 )
The key stream blocks generation: under a separate message processing
during the first N/n block cipher encryption operations the key K^1
is used, the key K^2 is used for the next N/n block cipher encryption
operations and so on. For example, if N = 2n, then the key stream
blocks are generated as follows:
S_0 = E( K^1, A_0 ),
S_1 = E( K^1, A_1 ),
S_2 = E( K^2, A_2 ),
S_3 = E( K^2, A_3 ),
S_4 = E( K^3, A_4 ),
...
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6.3. Constructions that Require Master Key
This section describes the block cipher modes that uses the ACPKM
Master rekeying mechanism, which use the initial key K as a master
key K, so K is never used directly for the data processing but is
used for key derivation.
6.3.1. ACPKMMaster Key Derivation from the Master Key
This section defines periodical key transformation with master key K
which is called ACPKMMaster rekeying mechanism. This mechanism can
be applied to one of the basic modes of operation (CTR, GCM, CBC,
CFB, OFB, OMAC modes) for getting an extension of this modes of
operations that uses periodical key transformation with master key.
This extension can be considered as a new mode of operation .
Additional parameters that defines the functioning of basic modes of
operation with the ACPKMMaster rekeying mechanism are the section
size N and change frequency T* of the key K. The values of N and T*
are measured in bits and are fixed within a specific protocol based
on the requirements of the system capacity and key lifetime (some
recommendations on choosing N and T* will be provided in Section 8).
The section size N MUST be divisible by the block size n. The key
frequency T* MUST be divisible by n.
The main idea behind internal rekeying with master key is presented
in Figure 9:
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Change frequency T*,
section size N,
maximum message size = m_max.
__________________________________________________________________________________
ACPKM ACPKM
K*_1 = K> K*_2 ...> K*_l_max
______ ______ ______
     
v ... v v ... v v ... v
K[1] K[t] K[t+1] K[2t] K[(l_max1)t+1] K[l_max*t]
     
     
v v v v v v
M^{1}========...================...========...========...== : 
M^{2}========...================...========...========...======: 
...           : 
M^{q}========...============ ... ... ... : 
section :
<> :
N bit m_max
__________________________________________________________________________________
K[i] = d,
t = T*/d,
l_max = ceil(m_max/N).
Figure 9: Internal rekeying with master key
During the processing of the input message M with the length m in
some mode of operation that uses ACPKMMaster key transformation with
the master key K and key frequency T* the message M is divided into l
= ceil(m/N) sections (denoted as M = M_1  M_2  ...  M_l, where M_i
is in V_N for i in {1, 2, ... , l1} and M_l is in V_r, r <= N). The
jth section of each message is processed with the key material K[j],
j in {1, ... ,l}, K[j] = d, that has been calculated with the
ACPKMMaster algorithm as follows:
K[1]  ...  K[l] = ACPKMMaster(T*, K, d*l) = CTRACPKMEncrypt
(T*, K, 1^{n/2}, 0^{d*l}).
6.3.2. CTRACPKMMaster Encryption Mode
This section defines a CTRACPKMMaster encryption mode that uses
internal ACPKMMaster rekeying mechanism for the periodical key
transformation.
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The CTRACPKMMaster encryption mode can be considered as the
extended by the ACPKMMaster rekeying mechanism basic encryption
mode CTR (see [MODES]).
The CTRACPKMMaster encryption mode can be used with the following
parameters:
o 64 <= n <= 512;
o 128 <= k <= 512;
o the number of bits c in a specific part of the block to be
incremented is such that 32 <= c <= 3/4 n, c is multiple of 8.
The key material K[j] that is used for one section processing is
equal to K^j, K^j = k bits.
The CTRACPKMMaster mode encryption and decryption procedures are
defined as follows:
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++
 CTRACPKMMasterEncrypt(N, K, T*, ICN, P) 

 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initial counter nonce ICN in V_{nc}, 
  plaintext P = P_1  ...  P_b, P <= 2^{n/21}*n*N / k. 
 Output: 
  Ciphertext C. 

 1. CTR_1 = ICN  0^c 
 2. For j = 2, 3, ... , b do 
 CTR_{j} = Inc_c(CTR_{j1}) 
 3. l = ceil(b*n / N) 
 4. K^1  ...  K^l = ACPKMMaster(T*, K, k*l) 
 5. For j = 1, 2, ... , b do 
 i = ceil(j*n / N), 
 G_j = E_{K^i}(CTR_j) 
 6. C = P (xor) MSB_{P}(G_1  ... G_b) 
 7. Return C 
+
++
 CTRACPKMMasterDecrypt(N, K, T*, ICN, C) 

 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initial counter nonce ICN in V_{nc}, 
  ciphertext C = C_1  ...  C_b, C <= 2^{n/21}*n*N / k. 
 Output: 
  Plaintext P. 

 1. P = CTRACPKMMasterEncrypt(N, K, T*, ICN, C) 
 1. Return P 
++
The initial counter nonce ICN value for each message that is
encrypted under the given key must be chosen in a unique manner. The
counter (CTR_{i+1}) value does not change during key transformation.
The message size MUST NOT exceed (2^{n/21}*n*N / k) bits.
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6.3.3. GCMACPKMMaster Authenticated Encryption Mode
This section defines a GCMACPKMMaster authenticated encryption mode
that uses internal ACPKMMaster rekeying mechanism for the
periodical key transformation.
The GCMACPKMMaster authenticated encryption mode can be considered
as the extended by the ACPKMMaster rekeying mechanism basic
authenticated encryption mode GCM (see [GCM]).
The GCMACPKMMaster authenticated encryption mode can be used with
the following parameters:
o n in {128, 256};
o 128 <= k <= 512;
o the number of bits c in a specific part of the block to be
incremented is such that 32 <= c <= 3/4 n, c is multiple of 8;
o authentication tag length t.
The key material K[j] that is used for one section processing is
equal to K^j, K^j = k bits, that is calculated as follows:
K^1  ...  K^j  ...  K^l = ACPKMMaster(T*, K, k*l).
The GCMACPKMMaster mode encryption and decryption procedures are
defined as follows:
++
 GHASH(X, H) 

 Input: 
  Bit string X = X_1  ...  X_m, X_i in V_n for i in {1, ... ,m}
 Output: 
  Block GHASH(X, H) in V_n 

 1. Y_0 = 0^n 
 2. For i = 1, ... , m do 
 Y_i = (Y_{i1} (xor) X_i)*H 
 3. Return Y_m 
++
++
 GCTR(N, K, T*, ICB, X) 

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 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initial counter block ICB, 
  X = X_1  ...  X_b, X_i in V_n for i = 1, ... , b1 and 
 X_b in V_r, where r <= n. 
 Output: 
  Y in V_{X}. 

 1. If X in V_0 then return Y, where Y in V_0 
 2. GCTR_1 = ICB 
 3. For i = 2, ... , b do 
 GCTR_i = Inc_c(GCTR_{i1}) 
 4. l = ceil(b*n / N) 
 5. K^1  ...  K^l = ACPKMMaster(T*, K, k*l) 
 6. For j = 1, ... , b do 
 i = ceil(j*n / N), 
 G_j = E_{K^i}(GCTR_j) 
 7. Y = X (xor) MSB_{X}(G_1  ...  G_b) 
 8. Return Y 
++
++
 GCMACPKMMasterEncrypt(N, K, T*, IV, P, A) 

 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initial counter nonce ICN in V_{nc}, 
  plaintext P, P <= n*(2^{c1}  2). 
  additional authenticated data A. 
 Output: 
  Ciphertext C, 
  authentication tag T. 

 1. K^1 = ACPKMMaster(T*, K, k) 
 2. H = E_{K^1}(0^n) 
 3. If c = 32, then ICB_0 = ICN  0^31  1 
 if c!= 32, then s = n*ceil(ICN/n)  ICN, 
 ICB_0 = GHASH(ICN  0^{s+n64}  Vec_64(ICN), H) 
 4. C = GCTR(N, K, T*, Inc_32(J_0), P) 
 5. u = n*ceil(C / n)  C 
 v = n*ceil(A / n)  A 
 6. S = GHASH(A  0^v  C  0^u  0^{n128}  Vec_64(A)  
  Vec_64(C), H) 
 7. T = MSB_t(E_{K^1}(J_0) (xor) S) 
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 8. Return C  T 
++
++
 GCMACPKMMasterDecrypt(N, K, T*, IV, A, C, T) 

 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initial counter nonce ICN in V_{nc}, 
  additional authenticated data A. 
  ciphertext C, C <= n*(2^{c1}  2), 
  authentication tag T, 
 Output: 
  Plaintext P or FAIL. 

 1. K^1 = ACPKMMaster(T*, K, k) 
 2. H = E_{K^1}(0^n) 
 3. If c = 32, then ICB_0 = ICN  0^31  1 
 if c!= 32, then s = n*ceil(ICN / n)  ICN, 
 ICB_0 = GHASH(ICN  0^{s+n64}  Vec_64(ICN), H) 
 4. P = GCTR(N, K, T*, Inc_32(J_0), C) 
 5. u = n*ceil(C / n)  C 
 v = n*ceil(A / n)  A 
 6. S = GHASH(A  0^v  C  0^u  0^{n128}  Vec_64(A)  
  Vec_64(C), H) 
 7. T' = MSB_t(E_{K^1}(ICB_0) (xor) S) 
 8. IF T = T' then return P; else return FAIL. 
++
The * operation on (pairs of) the 2^n possible blocks corresponds to
the multiplication operation for the binary Galois (finite) field of
2^n elements defined by the polynomial f as follows (by analogy with
[GCM]):
n = 128: f = a^128 + a^7 + a^2 + a^1 + 1.
n = 256: f = a^256 + a^10 + a^5 + a^2 + 1.
The initial vector IV value for each message that is encrypted under
the given key must be chosen in a unique manner.
The plaintext size MUST NOT exceed (2^{n/21}*n*N / k) bits.
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6.3.4. CCMACPKMMaster Authenticated Encryption Mode
This section defines a CCMACPKMMaster authenticated encryption mode
of operations that uses internal ACPKMMaster rekeying mechanism for
the periodical key transformation.
The CCMACPKMMaster authenticated encryption mode is differed from
CCMACPKM mode in the way the keys K^1, K^2, ... are generated. For
CCMACPKMMaster mode the keys are generated as follows: K^i = K[i],
where K^i=k and K[1]K[2]...K[l] = ACPKMMaster( T*, K, k*l ).
6.3.5. CBCACPKMMaster Encryption Mode
This section defines a CBCACPKMMaster encryption mode that uses
internal ACPKMMaster rekeying mechanism for the periodical key
transformation.
The CBCACPKMMaster encryption mode can be considered as the
extended by the ACPKMMaster rekeying mechanism basic encryption
mode CBC (see [MODES]).
The CBCACPKMMaster encryption mode can be used with the following
parameters:
o 64 <= n <= 512;
o 128 <= k <= 512.
In the specification of the CBCACPKMMaster mode the plaintext and
ciphertext must be a sequence of one or more complete data blocks.
If the data string to be encrypted does not initially satisfy this
property, then it MUST be padded to form complete data blocks. The
padding methods are outside the scope of this document. An example
of a padding method can be found in Appendix A of [MODES].
The key material K[j] that is used for one section processing is
equal to K^j, K^j = k bits.
We will denote by D_{K} the decryption function which is a
permutation inverse to the E_{K}.
The CBCACPKMMaster mode encryption and decryption procedures are
defined as follows:
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++
 CBCACPKMMasterEncrypt(N, K, T*, IV, P) 

 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initialization vector IV in V_n, 
  plaintext P = P_1  ...  P_b, P <= 2^{n/21}*n*N / k, 
 P_b = n. 
 Output: 
  Ciphertext C. 

 1. l = ceil(b*n/N) 
 2. K^1  ...  K^l = ACPKMMaster(T*, K, k*l) 
 3. C_0 = IV 
 4. For j = 1, 2, ... , b do 
 i = ceil(j*n / N), 
 C_j = E_{K^i}(P_j (xor) C_{j1}) 
 5. Return C = C_1  ...  C_b 
+
++
 CBCACPKMMasterDecrypt(N, K, T*, IV, C) 

 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initialization vector IV in V_n, 
  ciphertext C = C_1  ...  C_b, C <= 2^{n/21}*n*N/k, 
 C_b = n. 
 Output: 
  Plaintext P. 

 1. l = ceil(b*n / N) 
 2. K^1  ...  K^l = ACPKMMaster(T*, K, k*l) 
 3. C_0 = IV 
 4. For j = 1, 2, ... , b do 
 i = ceil(j*n/N) 
 P_j = D_{K^i}(C_j) (xor) C_{j1} 
 5. Return P = P_1  ...  P_b 
++
The initialization vector IV for each message that is encrypted under
the given key need not to be secret, but must be unpredictable.
The message size MUST NOT exceed (2^{n/21}*n*N / k) bits.
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6.3.6. CFBACPKMMaster Encryption Mode
This section defines a CFBACPKMMaster encryption mode that uses
internal ACPKMMaster rekeying mechanism for the periodical key
transformation.
The CFBACPKMMaster encryption mode can be considered as the
extended by the ACPKMMaster rekeying mechanism basic encryption
mode CFB (see [MODES]).
The CFBACPKMMaster encryption mode can be used with the following
parameters:
o 64 <= n <= 512;
o 128 <= k <= 512.
The key material K[j] that is used for one section processing is
equal to K^j, K^j = k bits.
The CFBACPKMMaster mode encryption and decryption procedures are
defined as follows:
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++
 CFBACPKMMasterEncrypt(N, K, T*, IV, P) 

 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initialization vector IV in V_n, 
  plaintext P = P_1  ...  P_b, P <= 2^{n/21}*n*N / k. 
 Output: 
  Ciphertext C. 

 1. l = ceil(b*n / N) 
 2. K^1  ...  K^l = ACPKMMaster(T*, K, k*l) 
 3. C_0 = IV 
 4. For j = 1, 2, ... , b do 
 i = ceil(j*n / N) 
 C_j = E_{K^i}(C_{j1}) (xor) P_j 
 5. Return C = C_1  ...  C_b. 
+
++
 CFBACPKMMasterDecrypt(N, K, T*, IV, C) 

 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initialization vector IV in V_n, 
  ciphertext C = C_1  ...  C_b, C <= 2^{n/21}*n*N / k.
 Output: 
  Plaintext P. 

 1. l = ceil(b*n / N) 
 2. K^1  ...  K^l = ACPKMMaster(T*, K, k*l) 
 3. C_0 = IV 
 4. For j = 1, 2, ... , b do 
 i = ceil(j*n / N), 
 P_j = E_{K^i}(C_{j1}) (xor) C_j 
 5. Return P = P_1  ...  P_b 
++
The initialization vector IV for each message that is encrypted under
the given key need not to be secret, but must be unpredictable.
The message size MUST NOT exceed 2^{n/21}*n*N/k bits.
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6.3.7. OFBACPKMMaster Encryption Mode
This section defines an OFBACPKMMaster encryption mode that uses
internal ACPKMMaster rekeying mechanism for the periodical key
transformation.
The OFBACPKMMaster encryption mode can be considered as the
extended by the ACPKMMaster rekeying mechanism basic encryption
mode OFB (see [MODES]).
The OFBACPKMMaster encryption mode can be used with the following
parameters:
o 64 <= n <= 512;
o 128 <= k <= 512.
The key material K[j] used for one section processing is equal to
K^j, K^j = k bits.
The OFBACPKMMaster mode encryption and decryption procedures are
defined as follows:
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++
 OFBACPKMMasterEncrypt(N, K, T*, IV, P) 

 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initialization vector IV in V_n, 
  plaintext P = P_1  ...  P_b, P <= 2^{n/21}*n*N / k. 
 Output: 
  Ciphertext C. 

 1. l = ceil(b*n / N) 
 2. K^1  ...  K^l = ACPKMMaster(T*, K, k*l) 
 3. G_0 = IV 
 4. For j = 1, 2, ... , b do 
 i = ceil(j*n / N), 
 G_j = E_{K_i}(G_{j1}) 
 5. Return C = P (xor) MSB_{P}(G_1  ...  G_b) 
+
++
 OFBACPKMMasterDecrypt(N, K, T*, IV, C) 

 Input: 
  Section size N, 
  master key K, 
  change frequency T*, 
  initialization vector IV in V_n, 
  ciphertext C = C_1  ...  C_b, C <= 2^{n/21}*n*N / k. 
 Output: 
  Plaintext P. 

 1. Return OFBACPKMMasterEncrypt(N, K, T*, IV, C) 
++
The initialization vector IV for each message that is encrypted under
the given key need not be unpredictable, but it must be a nonce that
is unique to each execution of the encryption operation.
The message size MUST NOT exceed 2^{n/21}*n*N / k bits.
6.3.8. OMACACPKMMaster Mode
This section defines an OMACACPKMMaster message authentication code
calculation mode that uses internal ACPKMMaster rekeying mechanism
for the periodical key transformation.
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The OMACACPKMMaster mode can be considered as the extended by the
ACPKMMaster rekeying mechanism basic message authentication code
calculation mode OMAC, which is also known as CMAC (see [RFC4493]).
The OMACACPKMMaster message authentication code calculation mode
can be used with the following parameters:
o n in {64, 128, 256};
o 128 <= k <= 512.
The key material K[j] that is used for one section processing is
equal to K^j  K^j_1, where K^j = k and K^j_1 = n.
The following is a specification of the subkey generation process of
OMAC:
++
 Generate_Subkey(K1, r) 

 Input: 
  Key K1, 
 Output: 
  Key SK. 

 1. If r = n then return K1 
 2. If r < n then 
 if MSB_1(K1) = 0 
 return K1 << 1 
 else 
 return (K1 << 1) (xor) R_n 
 
++
Where R_n takes the following values:
o n = 64: R_{64} = 0^{59}  11011;
o n = 128: R_{128} = 0^{120}  10000111;
o n = 256: R_{256} = 0^{145}  10000100101.
The OMACACPKMMaster message authentication code calculation mode is
defined as follows:
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++
 OMACACPKMMaster(K, N, T*, M) 

 Input: 
  Section size N, 
  master key K, 
  key frequency T*, 
  plaintext M = M_1  ...  M_b, M <= 2^{n/2}*n^2*N / (k + n). 
 Output: 
  message authentication code T. 

 1. C_0 = 0^n 
 2. l = ceil(b*n / N) 
 3. K^1  K^1_1  ...  K^l  K^l_1 = ACPKMMaster(T*, K, (k+n)*l 
 4. For j = 1, 2, ... , b1 do 
 i = ceil(j*n / N), 
 C_j = E_{K^i}(M_j (xor) C_{j1}) 
 5. SK = Generate_Subkey(K^l_1, M_b) 
 6. If M_b = n then M*_b = M_b 
 else M*_b = M_b  1  0^{n  1 M_b} 
 7. T = E_{K^l}(M*_b (xor) C_{b1} (xor) SK) 
 8. Return T 
++
The message size MUST NOT exceed 2^{n/2}*n^2*N / (k + n) bits.
7. Joint Usage of External and Internal Rekeying
Any mechanism described in Section 5 can be used with any mechanism
described in Section 6.
8. Security Considerations
Rekeying should be used to increase "a priori" security properties
of ciphers in hostile environments (e.g. with sidechannel
adversaries). If some nonnegligible attacks are known for a cipher,
it must not be used. So rekeying cannot be used as a patch for
vulnerable ciphers. Base cipher properties must be well analyzed,
because security of rekeying mechanisms is based on security of a
block cipher as a pseudorandom function.
Rekeying is not intended to solve any postquantum security issues
for symmetric crypto since the reduction of security caused by
Grover's algorithm is not connected with a size of plaintext
transformed by a cipher  only a negligible (sufficient for key
uniqueness) material is needed and the aim of rekeying is to limit a
size of plaintext transformed on one key.
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Rekeying can provide backward security only if the previous traffic
keys are securely deleted by all parties that have the keys.
9. References
9.1. Normative References
[GCM] McGrew, D. and J. Viega, "The Galois/Counter Mode of
Operation (GCM)", Submission to NIST
http://csrc.nist.gov/CryptoToolkit/modes/proposedmodes/
gcm/gcmspec.pdf, January 2004.
[MODES] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Methods and Techniques", NIST Special
Publication 80038A, December 2001.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBCMAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
2003, .
[RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AESCMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
2006, .
[RFC5869] Krawczyk, H. and P. Eronen, "HMACbased ExtractandExpand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
.
[TLSDraft]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", 2017, .
9.2. Informative References
[AbBell] Michel Abdalla and Mihir Bellare, "Increasing the Lifetime
of a Key: A Comparative Analysis of the Security of Re
keying Techniques", ASIACRYPT2000, LNCS 1976, pp. 546559,
2000.
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[LDC] Howard M. Heys, "A Tutorial on Linear and Differential
Cryptanalysis", 2017,
.
[Sweet32] Karthikeyan Bhargavan, Gaetan Leurent, "On the Practical
(In)Security of 64bit Block Ciphers. Collision Attacks
on HTTP over TLS and OpenVPN", 2016,
.
Appendix A. Test examples
CTRACPKM mode with AES256
*********
c = 64
k = 256
N = 256
n = 128
D_1
80 81 82 83 84 85 86 87 88 89 8A 8B 8C 8D 8E 8F
D_2
90 91 92 93 94 95 96 97 98 99 9A 9B 9C 9D 9E 9F
Key K:
88 99 AA BB CC DD EE FF 00 11 22 33 44 55 66 77
FE DC BA 98 76 54 32 10 01 23 45 67 89 AB CD EF
Plain text P:
11 22 33 44 55 66 77 00 FF EE DD CC BB AA 99 88
00 11 22 33 44 55 66 77 88 99 AA BB CC EE FF 0A
11 22 33 44 55 66 77 88 99 AA BB CC EE FF 0A 00
22 33 44 55 66 77 88 99 AA BB CC EE FF 0A 00 11
33 44 55 66 77 88 99 AA BB CC EE FF 0A 00 11 22
44 55 66 77 88 99 AA BB CC EE FF 0A 00 11 22 33
55 66 77 88 99 AA BB CC EE FF 0A 00 11 22 33 44
ICN:
12 34 56 78 90 AB CE F0
ACPKM's iteration 1
Process block 1
Input block (ctr)
12 34 56 78 90 AB CE F0 00 00 00 00 00 00 00 00
Output block (ctr)
FD 7E F8 9A D9 7E A4 B8 8D B8 B5 1C 1C 9D 6D D0
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Plain text
11 22 33 44 55 66 77 00 FF EE DD CC BB AA 99 88
Cipher text
EC 5C CB DE 8C 18 D3 B8 72 56 68 D0 A7 37 F4 58
Process block 2
Input block (ctr)
12 34 56 78 90 AB CE F0 00 00 00 00 00 00 00 01
Output block (ctr)
19 98 C5 71 76 37 FB 17 11 E4 48 F0 0C 0D 60 B2
Plain text
00 11 22 33 44 55 66 77 88 99 AA BB CC EE FF 0A
Cipher text
19 89 E7 42 32 62 9D 60 99 7D E2 4B C0 E3 9F B8
Input block (ctr)
80 81 82 83 84 85 86 87 88 89 8A 8B 8C 8D 8E 8F
Output block (ctr)
F6 80 D1 21 2F A4 3D F4 EC 3A 91 DE 2A B1 6F 1B
Input block (ctr)
90 91 92 93 94 95 96 97 98 99 9A 9B 9C 9D 9E 9F
Output block (ctr)
36 B0 48 8A 4F C1 2E 09 98 D2 E4 A8 88 E8 4F 3D
Updated key:
F6 80 D1 21 2F A4 3D F4 EC 3A 91 DE 2A B1 6F 1B
36 B0 48 8A 4F C1 2E 09 98 D2 E4 A8 88 E8 4F 3D
ACPKM's iteration 2
Process block 1
Input block (ctr)
12 34 56 78 90 AB CE F0 00 00 00 00 00 00 00 02
Output block (ctr)
E4 88 89 4F B6 02 87 DB 77 5A 07 D9 2C 89 46 EA
Plain text
11 22 33 44 55 66 77 88 99 AA BB CC EE FF 0A 00
Cipher text
F5 AA BA 0B E3 64 F0 53 EE F0 BC 15 C2 76 4C EA
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Process block 2
Input block (ctr)
12 34 56 78 90 AB CE F0 00 00 00 00 00 00 00 03
Output block (ctr)
BC 4F 87 23 DB F0 91 50 DD B4 06 C3 1D A9 7C A4
Plain text
22 33 44 55 66 77 88 99 AA BB CC EE FF 0A 00 11
Cipher text
9E 7C C3 76 BD 87 19 C9 77 0F CA 2D E2 A3 7C B5
Input block (ctr)
80 81 82 83 84 85 86 87 88 89 8A 8B 8C 8D 8E 8F
Output block (ctr)
8E B9 7E 43 27 1A 42 F1 CA 8E E2 5F 5C C7 C8 3B
Input block (ctr)
90 91 92 93 94 95 96 97 98 99 9A 9B 9C 9D 9E 9F
Output block (ctr)
1A CE 9E 5E D0 6A A5 3B 57 B9 6A CF 36 5D 24 B8
Updated key:
8E B9 7E 43 27 1A 42 F1 CA 8E E2 5F 5C C7 C8 3B
1A CE 9E 5E D0 6A A5 3B 57 B9 6A CF 36 5D 24 B8
ACPKM's iteration 3
Process block 1
Input block (ctr)
12 34 56 78 90 AB CE F0 00 00 00 00 00 00 00 04
Output block (ctr)
68 6F 22 7D 8F B2 9C BD 05 C8 C3 7D 22 FE 3B B7
Plain text
33 44 55 66 77 88 99 AA BB CC EE FF 0A 00 11 22
Cipher text
5B 2B 77 1B F8 3A 05 17 BE 04 2D 82 28 FE 2A 95
Process block 2
Input block (ctr)
12 34 56 78 90 AB CE F0 00 00 00 00 00 00 00 05
Output block (ctr)
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C0 1B F9 7F 75 6E 12 2F 80 59 55 BD DE 2D 45 87
Plain text
44 55 66 77 88 99 AA BB CC EE FF 0A 00 11 22 33
Cipher text
84 4E 9F 08 FD F7 B8 94 4C B7 AA B7 DE 3C 67 B4
Input block (ctr)
80 81 82 83 84 85 86 87 88 89 8A 8B 8C 8D 8E 8F
Output block (ctr)
C5 71 6C C9 67 98 BC 2D 4A 17 87 B7 8A DF 94 AC
Input block (ctr)
90 91 92 93 94 95 96 97 98 99 9A 9B 9C 9D 9E 9F
Output block (ctr)
E8 16 F8 0B DB BC AD 7D 60 78 12 9C 0C B4 02 F5
Updated key:
C5 71 6C C9 67 98 BC 2D 4A 17 87 B7 8A DF 94 AC
E8 16 F8 0B DB BC AD 7D 60 78 12 9C 0C B4 02 F5
ACPKM's iteration 4
Process block 1
Input block (ctr)
12 34 56 78 90 AB CE F0 00 00 00 00 00 00 00 06
Output block (ctr)
03 DE 34 74 AB 9B 65 8A 3B 54 1E F8 BD 2B F4 7D
Plain text
55 66 77 88 99 AA BB CC EE FF 0A 00 11 22 33 44
Cipher text
56 B8 43 FC 32 31 DE 46 D5 AB 14 F8 AC 09 C7 39
Input block (ctr)
80 81 82 83 84 85 86 87 88 89 8A 8B 8C 8D 8E 8F
Output block (ctr)
74 1E B5 88 D6 AB DA B6 89 AA FD BA A9 3E A2 46
Input block (ctr)
90 91 92 93 94 95 96 97 98 99 9A 9B 9C 9D 9E 9F
Output block (ctr)
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16 3A A6 C2 3C E7 C3 74 CD 38 BF C6 FE 8C C5 FF
Updated key:
74 1E B5 88 D6 AB DA B6 89 AA FD BA A9 3E A2 46
16 3A A6 C2 3C E7 C3 74 CD 38 BF C6 FE 8C C5 FF
Encrypted src
EC 5C CB DE 8C 18 D3 B8 72 56 68 D0 A7 37 F4 58
19 89 E7 42 32 62 9D 60 99 7D E2 4B C0 E3 9F B8
F5 AA BA 0B E3 64 F0 53 EE F0 BC 15 C2 76 4C EA
9E 7C C3 76 BD 87 19 C9 77 0F CA 2D E2 A3 7C B5
5B 2B 77 1B F8 3A 05 17 BE 04 2D 82 28 FE 2A 95
84 4E 9F 08 FD F7 B8 94 4C B7 AA B7 DE 3C 67 B4
56 B8 43 FC 32 31 DE 46 D5 AB 14 F8 AC 09 C7 39
Appendix B. Contributors
o Russ Housley
Vigil Security, LLC
housley@vigilsec.com
o Mihir Bellare
University of California
mihir@eng.ucsd.edu
o Evgeny Alekseev
CryptoPro
alekseev@cryptopro.ru
o Ekaterina Smyshlyaeva
CryptoPro
ess@cryptopro.ru
o Daniel Fox Franke
Akamai Technologies
dfoxfranke@gmail.com
o Lilia Ahmetzyanova
CryptoPro
lah@cryptopro.ru
o Ruth Ng
University of California, San Diego
ring@eng.ucsd.edu
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o Shay Gueron
University of Haifa, Israel
Intel Corporation, Israel Development Center, Israel
shay.gueron@gmail.com
Appendix C. Acknowledgments
We thank Scott Fluhrer, Dorothy Cooley, Yoav Nir, Jim Schaad and Paul
Hoffman for their useful comments.
Author's Address
Stanislav Smyshlyaev (editor)
CryptoPro
18, Suschevsky val
Moscow 127018
Russian Federation
Phone: +7 (495) 9954820
Email: svs@cryptopro.ru
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