Re-keying Mechanisms for Symmetric Keys
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General
CFRGre-keying, key, key lifetime, encryption mode, mode of
operation
A certain maximum amount of data can be safely encrypted when
encryption is performed under a single key. This amount is
called the "key lifetime".
This specification describes a variety of methods for increasing the lifetime of symmetric keys.
It provides two types of re-keying mechanisms based on hash functions and block ciphers
that can be used with modes of operations such as CTR, GCM, CBC, CFB, and OMAC.
This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF.
Introduction
A certain maximum amount of data can be safely encrypted when
encryption is performed under a single key. Hereinafter, this
amount will be REFERRED to as the "key lifetime".
The need for such a limitation is dictated by the following methods of cryptanalysis:
Methods based on the combinatorial properties of the used
block cipher mode of operation
These methods do not depend on the underlying block
cipher. Common mode restrictions derived from such methods
are of order 2^{n/2}, where n is a block size defined in
. includes an
example of an attack that is based on such methods.
Methods based on side-channel analysis issues
In most cases, these methods do not depend on the used
encryption modes and weakly depend on the used cipher
features. Limitations resulting from these considerations
are usually the most restrictive ones. is an example of an attack that is based on
such methods.
Methods based on the properties of the used block cipher
The most common methods of this type are linear and
differential cryptanalysis . In most
cases, these methods do not depend on the used modes of
operation. In the case of secure block ciphers, bounds
resulting from such methods are roughly the same as the
natural bounds of 2^n and are dominated by the other bounds
above. Therefore, they can be excluded from the
considerations here.
As a result, it is important to replace a key when the total size of the processed plaintext under that key
approaches the lifetime limitation. A specific value of the key lifetime should be determined in accordance with some safety margin for protocol security and the 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 enclosed by area L (see ).
In practice, the amount of data that corresponds to limitation L may not be enough.
The simplest and obvious way in this situation is a regular renegotiation of an initial key after processing this threshold amount of data L.
However, this reduces the total performance, since it usually entails
termination of application data transmission, additional service messages,
the use of a random number generator, and many other additional calculations,
including resource-intensive public key cryptography.
For protocols based on block ciphers or stream ciphers, a more efficient way to increase the key lifetime is to use various re-keying mechanisms.
This specification considers only the case of re-keying mechanisms for block ciphers, while re-keying mechanisms typical for stream ciphers
(e.g., , ) case go beyond the scope of this document.
Re-keying mechanisms can be applied at the different protocol
levels: the block cipher level (this approach is known as fresh re-keying and is described, for instance, in ;
the block cipher mode of operation level (see );
and the protocol level above the block cipher mode of operation (see ).
The usage of the first approach is highly inefficient due to the key changing after processing each message block.
Moreover, fresh re-keying mechanisms can change the block
cipher internal structure and, consequently, can require an additional security analysis for each particular block cipher.
As a result, this approach depends on particular primitive
properties and cannot be applied to any arbitrary block cipher
without additional security analysis. Therefore, fresh re-keying mechanisms go beyond the scope of this document.
Thus, this document contains the list of recommended re-keying mechanisms that can be used in the symmetric encryption schemes based on the block ciphers.
These mechanisms are independent from the particular block cipher specification, and their security properties rely only on the standard block cipher security assumption.
This specification presents two basic approaches to extending
the lifetime of a key while avoiding renegotiation, which were introduced in :
1. External re-keying
External re-keying is performed by a protocol, and it is independent of the underlying block cipher and the mode of operation.
External re-keying can use parallel and serial constructions.
In the parallel case, data processing keys K^1, K^2, ... are generated directly from the initial key K independently of each other.
In the serial case, every data-processing key depends on the state that is updated after the generation of each new data-processing key.
As a generalization of external parallel re-keying, an external tree-based mechanism can be considered.
It is specified in and can
be viewed as the tree generalization in . Similar constructions are used in the one-way tree mechanism () and standard.
2. Internal re-keying
Internal re-keying is built into the mode, and it depends heavily on the properties of the mode of operation and the block size.
The re-keying approaches extend the key lifetime for a single initial key by providing the possibility
to limit the leakages (via side channels)
and by improving combinatorial properties of the used block cipher mode of operation.
In practical applications, re-keying can be useful for protocols that need to operate in hostile environments
or under restricted resource conditions (e.g., those that require lightweight cryptography, where ciphers have a small block size, that imposes strict combinatorial limitations).
Moreover, mechanisms that use external or internal re-keying may provide some protection against possible future attacks (by limiting the number of plaintext-ciphertext pairs that an adversary can collect)
and some properties of forward or backward security (meaning
that past or future data-processing keys remain secure even if
the current key is compromised; see
for more details).
External or internal re-keying can be used in network protocols as well as in the systems for data-at-rest encryption.
Depending on the concrete protocol characteristics, there might be situations in which both external and internal re-keying mechanisms (see ) can be applied.
For example, a similar approach was used in Taha's tree construction (see ).
Note that there are key-updating (key regression) algorithms (e.g., and ) that are called "re-keying" as well,
but they pursue the goal different from increasing key lifetime. Therefore, key regression algorithms are excluded from the considerations here.
This document represents the consensus of the Crypto Forum Research Group (CFRG).
Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be interpreted as
described in BCP 14
when, and only when, they appear in all capitals, as shown here.
Basic Terms and Definitions
This document uses the following terms and definitions for the sets and operations
on the elements of these sets:
V*
the set of all bit strings of a finite length
(hereinafter referred to as strings), including the empty
string;
V_s
the set of all bit strings of length s, where s is a non-negative integer;
|X|
the bit length of the bit string X;
A | B
the 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;
(xor)
the exclusive-or of two bit strings of the same length;
Z_{2^n}
the ring of residues modulo 2^n;
Int_s: V_s -> Z_{2^s}
the transformation that maps the string a = (a_s, ... , a_1) in V_s
into the integer Int_s(a) = 2^{s-1} * a_s + ... + 2 * a_2 + a_1
(the interpretation of the binary string as an integer);
Vec_s: Z_{2^s} -> V_s
the transformation inverse to the mapping Int_s (the interpretation of an integer as a binary string);
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_{s-i+1}) in V_i (most significant bits);
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 (least significant bits);
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
(incrementing the least significant c bits of the bit string, regarded as the binary representation of an integer);
a^s
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 smallest integer that is greater than or equal to x;
floor(x)
the biggest integer that is less than or equal to x;
k
the bit length of the K; k is assumed to be divisible by 8;
n
the block size of the block cipher (in bits); n is assumed to be divisible by 8;
b
the number of data blocks in the plaintext P (b = ceil(|P|/n));
N
the section size (the number of bits that are processed with one section key before this key is transformed).
A plaintext message P and the corresponding 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, respectively. The first b-1 blocks
P_i and C_i are in V_n for i = 1, 2, ... , b-1. The b-th
blocks P_b and C_b may be incomplete blocks, i.e., in V_r, where r <= n if not otherwise specified.
Choosing Constructions and Security Parameters
External re-keying is an approach assuming that a key is transformed after encrypting a limited number of entire messages.
The external re-keying method is chosen at the protocol level, regardless of the underlying block cipher or the encryption mode.
External re-keying is recommended for protocols that process relatively short messages or
protocols that have a way to divide a long message into manageable pieces.
Through external re-keying, the number of messages that can be
securely processed with a single initial key K is
substantially increased without a loss of message length.
External re-keying has the following advantages
It increases the lifetime of an initial key by increasing the number of messages processed with this key.
It has minimal impact on performance when the number of messages processed under one initial key is sufficiently large.
It provides forward and backward security of data-processing keys.
However, the use of external re-keying has the following disadvantage:
in case of restrictive key lifetime limitations, the message sizes can become
inconvenient due to the impossibility of processing sufficiently large messages, so it could
be necessary to perform additional fragmentation at the
protocol level. For example, if the key lifetime L is 1 GB and the message length m = 3 GB,
then this message cannot be processed as a whole, and it should be divided into three fragments that will be processed separately.
Internal re-keying is an approach assuming that a key is transformed during each separate message processing.
Such procedures are integrated into the base modes of operations, so every internal re-keying mechanism
is defined for the particular operation mode and the block size of the used cipher.
Internal re-keying is recommended for protocols that process long messages:
the size of each single message can be substantially increased
without loss in the number of messages that can be securely processed with a single initial key.
Internal re-keying has the following advantages:
It increases the lifetime of an initial key by increasing the size of the messages processed with one initial key.
It has minimal impact on performance.
Internal re-keying mechanisms without a master key do not affect short-message transformation at all.
It is transparent (works like any mode of operation):
it does not require changes of initialization vectors
(IVs) and a restart of MACing.
However, the use of internal re-keying has the following disadvantages:
a specific method must not be chosen independently of a mode of operation.
internal re-keying mechanisms without a master key do not provide backward security of data-processing keys.
Any block cipher modes of operations with internal re-keying can be jointly used with any external
re-keying mechanisms. Such joint usage increases both the number of messages processed with one initial key
and their maximum possible size.
If the adversary has access to the data-processing interface, the use of the same cryptographic primitives both for data-processing and re-keying transformation
decreases the code size but can lead to some possible vulnerabilities (the possibility of mounting a chosen-plaintext attack may lead to the compromise of the following keys).
This vulnerability can be eliminated by using different primitives for
data processing and re-keying, e.g., block cipher for data processing and hash for re-keying (see and ).
However, in this case, the security of the whole scheme cannot be reduced
to standard notions like pseudorandom function
(PRF) or pseudorandom permutation (PRP), so security estimations become more difficult and unclear.
Summing up the abovementioned issues briefly:
If a protocol assumes processing of long records (e.g., ), internal re-keying should be used.
If a protocol assumes processing of a significant number of ordered records, which can be
considered as a single data stream (e.g., , ), internal re-keying may also be used.
For protocols that allow out-of-order delivery and lost records (e.g., , ), external
re-keying should be used as, in this case, records
cannot be considered as a single data stream. If the records are also long enough, internal re-keying
should also be used during each separate message processing.
For external re-keying:
If it is desirable to separate transformations used for data processing and key updates,
hash function-based re-keying should be used.
If parallel data processing is required, then parallel external re-keying should be used.
If restrictive key lifetime limitations are present, external tree-based re-keying should be used.
For internal re-keying:
If the property of forward and backward security is desirable for data-processing keys and
if additional key material can be easily obtained for the data-processing stage, internal re-keying with a master key should be used.
External Re-keying Mechanisms
This section presents an approach to increasing the initial key lifetime by using
a transformation of a data-processing key (frame key) after
processing a limited number of entire messages (frame).
It provides external parallel and serial re-keying mechanisms (see ).
These mechanisms use initial key K only for frame key generation and never use it directly for data processing.
Such mechanisms operate outside of the base modes of operations and do not change them at all; therefore,
they are called "external re-keying" mechanisms in this document.
External re-keying mechanisms are recommended for usage in protocols
that process quite small messages, since the maximum gain in increasing the initial key lifetime is achieved by increasing the number of messages.
External re-keying increases the initial 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 side-channel analysis methods (side-channel limitation),
let L2 be a key lifetime limitation induced by methods based on the combinatorial properties
of a 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 an initial key K according to these limitations.
Let L = min(L1, L2), q = min (q1, q2), and q * m <= L. As
the L1 limitation is usually much stronger than the L2 limitation (L1 < L2), the final key lifetime restriction
is equal to the most restrictive limitation L1. Thus, as
displayed in , without re-keying, only q1 (q1 * m <= L1) messages can be safely processed.
Suppose that the safety margin for the protocol P is fixed and the external re-keying approach is applied to the initial key K to generate the sequence of frame keys.
The frame keys are generated in such a way that the leakage of
a previous frame 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 initial key K can be calculated
on the basis of a new combinatorial limitation L2'.
It is proven (see ) that the security of the mode of operation that uses external re-keying leads to an increase when compared to base mode
without re-keying (thus, L2 < L2'). Hence, as displayed in
, the resulting key lifetime limitation if using external re-keying can be increased up to L2'.
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 a 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 initial key K must be
renegotiated after processing 128 MB / 1 KB = 131072 messages.
If an external re-keying mechanism is used,
the key lifetime limitation L1 goes off. Hence, the resulting key lifetime limitation L2' can be set to more than 1 TB.
Thus, if an external re-keying mechanism is used, more than 1 TB / 1 KB = 2^30 messages can be processed before the initial key K is renegotiated.
This is 8192 times greater than the number of messages that
can be processed when an external re-keying mechanism is not used.
Methods of Key Lifetime Control
Suppose L is an amount of data that can be safely processed with one frame key.
For i in {1, 2, ... , t}, the frame key K^i (see Figures and ) should be transformed after processing q_i messages,
where q_i can be calculated in accordance with one of the following approaches:
Explicit approach:
q_i is such that |M^{i,1}| + ... + |M^{i,q_i}| <= L, |M^{i,1}| + ... + |M^{i,q_i+1}| > L.
This approach allows use of the frame key K^i in
an almost optimal way,
but it can be applied only when messages cannot be lost or reordered (e.g., TLS records).
Implicit approach:
q_i = L / m_max, i = 1, ... , t.
The amount of data processed with one frame 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 when messages may be lost or reordered (e.g., DTLS records).
Dynamic key changes:
We can organize the key change using the Protected Point to Point () solution by building a protected tunnel between the endpoints in which the
information about frame key updating can be safely passed across.
This can be useful, for example, when we want the
adversary to not detect the key change during the protocol evaluation.
Parallel Constructions
External parallel re-keying mechanisms generate frame keys K^1, K^2, ... directly from the initial key K independently of each other.
The main idea behind external re-keying with a parallel
construction is presented in :
The frame key K^i, i = 1, ... , t - 1 is updated after processing a certain number of messages (see ).
Parallel Construction Based on a KDF on a Block Cipher
The ExtParallelC re-keying mechanism is based on the key derivation function on a block cipher and is used to generate t frame keys as follows:
where R = ceil(t * k/n).
Parallel Construction Based on a KDF on a Hash Function
The ExtParallelH re-keying mechanism is based on the key derivation function HKDF-Expand, described
in , and is used to generate t frame keys as follows:
K^1 | K^2 | ... | K^t = ExtParallelH(K, t * k) = HKDF-Expand(K, label, t * k),
where label is a string (may be a zero-length string) that is defined by a specific protocol.
Tree-Based Construction
The application of an external tree-based mechanism leads to the construction
of the key tree with the initial key K (root key) at
the 0 level and the frame keys K^1, K^2, ... at the
last level, as described in .
The tree height h and the number of keys Wj, j in {1,
... , h}, which can be partitioned from the "parent" key, are defined in accordance
with a specific protocol and key lifetime limitations for the used derivation functions.
Each j-level key K{j,w}, where j in {1, ... , h}, w in
{1, ... , W1 * ... * Wj}, is derived from the
(j-1)-level "parent" key K{j-1, ceil(w/Wi)}
(and other appropriate input data) using the j-th
level derivation function. This function can be based on the block cipher function
or on the hash function and is defined in accordance with a specific protocol.
The i-th frame K^i, i in {1, 2, ... , W1*...*Wh}, can be calculated as follows:
where KDF_j is the j-th level derivation function that takes two arguments
(the parent key value and the integer in a range from 1 to W1 * ... * Wj) and outputs the j-th level key value.
The frame key K^i is updated after processing a certain number of messages (see ).
In order to create an efficient implementation, during frame key K^i generation, the derivation functions KDF_j, j in {1, ... , h-1} should be used only when ceil(i / (W{j+1} * ... * Wh)) != ceil((i - 1) / (W{j+1} * ... * Wh));
otherwise, it is necessary to use a previously generated value. This approach also makes it possible to take countermeasures against side-channel attacks.
Consider an example. Suppose h = 3, W1 = W2 = W3 = W, and KDF_1, KDF_2, KDF_3 are key derivation functions based on the KDF_GOSTR3411_2012_256 (hereafter simply KDF)
function described in . The resulting ExtKeyTree function can be defined as follows:
where i in {1, 2, ... , W^3}.
A structure similar to the external tree-based mechanism can be found in Section 6 of .
Serial Constructions
External serial re-keying mechanisms generate frame keys,
each of which depends on the secret state (K*_1, K*_2,
...) that is updated after the generation of each new
frame key; see .
Similar approaches are used in the protocol and the updating traffic key mechanism and
were proposed for use in the protocol.
External serial re-keying mechanisms have the obvious disadvantage of the impossibility to be implemented in parallel,
but they can be preferred if additional forward secrecy is desirable: in case all keys are securely deleted after usage,
the compromise of a current secret state at some time does not lead to a compromise of all previous secret states and frame keys.
In terms of , compromise of application_traffic_secret_N does not compromise all previous application_traffic_secret_i, i < N.
The main idea behind external re-keying with a serial
construction is presented in :
The frame key K^i, i = 1, ... , t - 1, is updated after processing a certain number of messages (see ).
Serial Construction Based on a KDF on a Block Cipher
The frame key K^i is calculated using the ExtSerialC transformation as follows:
where j = 1, ... , t - 1.
Serial Construction Based on a KDF on a Hash Function
The frame key K^i is calculated using the ExtSerialH transformation as follows:
where i = 1, ... , t; HKDF-Expand is the HMAC-based
key derivation function, as described in ; and K*_i is calculated as follows:
K*_1 = K,
K*_{j+1} = HKDF-Expand(K*_j, label2, k), where j = 1, ... , t - 1,
where label1 and label2 are different strings from V*
that are defined by a specific protocol (see, for
example, TLS 1.3 updating traffic keys algorithm
).
Using Additional Entropy during Re-keying
In many cases, using additional entropy during re-keying
won't increase security but may give a false sense of
that. Therefore, one can rely on additional entropy only after conducting a deep security analysis.
For example, good PRF constructions do not require additional entropy for the quality of keys, so, in
most cases, there is no need to use additional entropy with external re-keying mechanisms based on secure KDFs.
However, in some situations, mixed-in entropy can still increase security in the case of a time-limited but
complete breach of the system when an adversary can access the frame-key generation interface
but cannot reveal the master keys (e.g., when the master
keys are stored in a Hardware Security Module (HSM)).
For example, an external parallel construction based on a KDF on a hash function with a mixed-in entropy can be described as follows:
K^i = HKDF-Expand(K, label_i, k),
where label_i is additional entropy that must be sent to
the recipient (e.g., sent jointly with an encrypted message).
The entropy label_i and the corresponding key K^i must be generated directly before message processing.
Internal Re-keying Mechanisms
This section presents an approach to increasing the key lifetime by using a transformation of a data-processing key (section key)
during each separate message processing. Each message is processed starting with the same key (the first section key),
and each section key is updated after processing N bits of the
message (section).
This section provides internal re-keying mechanisms called
ACPKM (Advanced Cryptographic Prolongation of Key Material)
and ACPKM-Master that do not use a master key and use a master key, respectively.
Such mechanisms are integrated into the base modes of operation
and actually form new modes of operation. Therefore,
they are called "internal re-keying" mechanisms in this document.
Internal re-keying mechanisms are 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 provides almost no increase in the number of messages that can be processed with one initial key.
Internal re-keying increases the key lifetime through the following approach.
Suppose protocol P uses some base mode of operation.
Let L1 and L2 be a side channel and combinatorial limitations, respectively, and
for some fixed number of messages q, let m1, m2 be the lengths of messages
that can be safely processed with a single initial key K according to these limitations.
Thus, by analogy with the without
re-keying the final key lifetime restriction, as displayed in
,
is equal to L1, and only q messages of the length m1 can be safely processed.
Suppose that the safety margin for the protocol P is fixed and
the internal re-keying approach is applied to the base mode of operation.
Suppose further that every message is processed with a section key, which 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 initial key K can be calculated
on the basis of a new combinatorial limitation L2'. The
security of the mode of operation that uses internal re-keying
increases when compared to the base mode of operation
without re-keying (thus, L2 < L2'). Hence, as displayed in
, the resulting key lifetime limitation if using internal re-keying can be increased up to L2'.
Note: The key transformation process is depicted in a simplified form.
A specific approach (ACPKM and ACPKM-Master re-keying 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 considered security models.
Consider an example. Suppose L1 = 128 MB and L2 = 10 TB.
Let the message size in the protocol be large/unlimited (which
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 initial key K would be required after processing only four messages.
If an internal re-keying mechanism with section size N = 1 MB is used, more than L1 / N = 128 MB / 1 MB = 128
messages can be processed before the renegotiation of initial key K (instead of four messages when an internal re-keying mechanism is not used).
Note that only one section of each message is processed with the section key K^i, and, consequently, the key lifetime limitation L1 goes off.
Hence, the resulting key lifetime limitation L2' can be set to more than 10 TB (in cases when a single large message is processed using the initial key K).
Methods of Key Lifetime Control
Suppose L is an amount of data that can be safely
processed with one section key and N is a section size (fixed parameter).
Suppose M^{i}_1 is the first section of message M^{i}, i =
1, ... , q (see Figures and
);
the parameter q can then be calculated in accordance with one of the following two approaches:
Explicit approach:
q_i is such that |M^{1}_1| + ... + |M^{q}_1| <= L, |M^{1}_1| + ... + |M^{q+1}_1| > L
This approach allows use of the section key K^i in an almost optimal way,
but it can be applied only when messages cannot be lost or reordered (e.g., TLS records).
Implicit approach:
q = L / N.
The amount of data processed with one section key
K^i is calculated under the assumption that the
length of every message is equal to or greater than section size N
and thus can be considerably less than the key lifetime limitation L. On the other hand, this approach
can be applied when messages may be lost or reordered (e.g., DTLS records).
Constructions that Do Not Require a Master Key
This section describes the block cipher modes that use the ACPKM
re-keying mechanism, which
does not use a master key; an initial key is used directly for the data encryption.
ACPKM Re-keying Mechanisms
This section defines a periodical key transformation without a master key, which is
called the ACPKM re-keying mechanism. This mechanism can be applied to one of the
base encryption modes (CTR and GCM block cipher modes)
to get an extension of this encryption mode that uses periodical key
transformation without a 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 re-keying 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 the key lifetime.
The section size N MUST be divisible by the block size n.
The main idea behind internal re-keying without a
master key is presented in :
During the processing of the input message M with the length m in some encryption mode that
uses the ACPKM key transformation of the initial 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 in {1, 2, ... , l - 1} and M_l is in V_r, r <= N).
The first section of each message is processed with the section key K^1 = K. To process the (i + 1)-th section of each message, the section key K^{i+1}
is calculated using the ACPKM transformation as follows:
where J = ceil(k/n) 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}:
Note: The constant D is such that D_1, ... , D_J are pairwise different for any allowed n and k values.
Note: The highest bit of each octet of the constant D is equal to 1.
This condition is important as, in conjunction with a
certain mode message length limitation, it allows
prevention of collisions of block cipher permutation inputs
in cases with key transformation and message processing (for more details, see Section 4.4 of ).
CTR-ACPKM Encryption Mode
This section defines a CTR-ACPKM encryption mode that uses the ACPKM internal re-keying
mechanism for the periodical key transformation.
The CTR-ACPKM mode can be considered as the base encryption mode CTR (see ) extended by the ACPKM re-keying mechanism.
The CTR-ACPKM encryption mode can be used with the following parameters:
64 <= n <= 512.
128 <= k <= 512.
The number c of bits in a specific part of the block to be incremented
is such that 32 <= c <= 3 / 4 n, c is a multiple of 8.
The maximum message size m_max = n * 2^{c-1}.
The CTR-ACPKM mode encryption and decryption procedures are defined as follows:
The initial counter nonce (ICN) value for each message that is encrypted under the given initial key K must be
chosen in a unique manner.
GCM-ACPKM Authenticated Encryption Mode
This section defines the GCM-ACPKM authenticated encryption mode that uses the ACPKM internal re-keying mechanism for the periodical key transformation.
The GCM-ACPKM mode can be considered as the base authenticated encryption mode GCM (see ) extended by the ACPKM re-keying mechanism.
The GCM-ACPKM authenticated encryption mode can be used with the following parameters:
n in {128, 256}.
128 <= k <= 512.
The number c of bits in a specific part of the block to be incremented
is such that 1 / 4 n <= c <= 1 / 2 n, c is a multiple of 8.
Authentication tag length t.
The maximum message size m_max = min{n * (2^{c-1} - 2), 2^{n/2} - 1}.
The GCM-ACPKM mode encryption and decryption procedures are defined as follows:
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 ):
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 counter nonce ICN value for each message that is encrypted under the given initial key K must be
chosen in a unique manner.
The key for computing values E_{K}(ICB_0) and H is not updated and is
equal to the initial key K.
Constructions that Require a Master Key
This section describes the block cipher modes that use the ACPKM-Master
re-keying mechanism, which
use the initial key K as a master key, so K is never used directly for data processing but is
used for key derivation.
ACPKM-Master Key Derivation from the Master Key
This section defines periodical key transformation with a master key, which is
called the ACPKM-Master re-keying mechanism. This mechanism can be applied to one of the base modes of operation (CTR, GCM, CBC, CFB, OMAC modes)
for getting an extension that uses periodical key transformation with a master key. This extension can be considered as a new mode of operation.
Additional parameters that define the functioning of modes of operation
that use the ACPKM-Master re-keying mechanism are the
section size N, the change frequency T* of the master
keys K*_1, K*_2, ... (see ), and the size d of the section key material.
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 the key lifetime.
The section size N MUST be divisible by the block size n. The master key frequency T* MUST be divisible by d and by n.
The main idea behind internal re-keying with a master
key is presented in :
During the processing of the input message M with the length m in some mode of operation that
uses ACPKM-Master key transformation with the initial key K and the master 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, ... , l - 1} and M_l is in V_r, r <= N). The j-th section of each message is processed
with the key material K[j], j in {1, ... , l}, |K[j]| = d, which is calculated with the ACPKM-Master algorithm as follows:
Note: The parameters d and l MUST be such that d * l <= n * 2^{n/2-1}.
CTR-ACPKM-Master Encryption Mode
This section defines a CTR-ACPKM-Master encryption mode that uses the ACPKM-Master internal re-keying
mechanism for the periodical key transformation.
The CTR-ACPKM-Master encryption mode can be considered as the base encryption mode CTR (see ) extended by the ACPKM-Master re-keying mechanism.
The CTR-ACPKM-Master encryption mode can be used with the following parameters:
64 <= n <= 512.
128 <= k <= 512.
The number c of bits in a specific part of the block to be incremented
is such that 32 <= c <= 3 / 4 n, c is a multiple of 8.
The maximum message size m_max = min{N * (n * 2^{n/2-1} / k), n * 2^c}.
The key material K[j] that is used for one section processing is equal to K^j, |K^j| = k bits.
The CTR-ACPKM-Master mode encryption and decryption procedures are defined as follows:
The initial counter nonce ICN value for each message that is encrypted under the given initial key must be
chosen in a unique manner.
GCM-ACPKM-Master Authenticated Encryption Mode
This section defines a GCM-ACPKM-Master authenticated encryption mode that uses the ACPKM-Master internal re-keying
mechanism for the periodical key transformation.
The GCM-ACPKM-Master authenticated encryption mode can be considered as the base authenticated encryption mode GCM (see ) extended by the ACPKM-Master re-keying mechanism.
The GCM-ACPKM-Master authenticated encryption mode can be used with the following parameters:
n in {128, 256}.
128 <= k <= 512.
The number c of bits in a specific part of the block to be incremented
is such that 1 / 4 n <= c <= 1 / 2 n, c is a multiple of 8.
authentication tag length t.
the maximum message size m_max = min{N * ( n * 2^{n/2-1} / k), n * (2^c - 2), 2^{n/2} - 1}.
The key material K[j] that is used for the j-th section processing is equal to K^j, |K^j| = k bits.
The GCM-ACPKM-Master mode encryption and decryption procedures are defined as follows:
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 ):
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 counter nonce ICN value for each message that is encrypted under the given initial key must be
chosen in a unique manner.
CBC-ACPKM-Master Encryption Mode
This section defines a CBC-ACPKM-Master encryption mode that uses the ACPKM-Master internal re-keying
mechanism for the periodical key transformation.
The CBC-ACPKM-Master encryption mode can be considered as the base encryption mode CBC (see ) extended by the ACPKM-Master re-keying mechanism.
The CBC-ACPKM-Master encryption mode can be used with the following parameters:
64 <= n <= 512.
128 <= k <= 512.
The maximum message size m_max = N * (n * 2^{n/2-1} / k).
In the specification of the CBC-ACPKM-Master 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 out of the scope of this document. An example of a padding method can be found in Appendix A of .
The key material K[j] that is used for the j-th 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 E_{K}.
The CBC-ACPKM-Master mode encryption and decryption procedures are defined as follows:
The initialization vector IV for any particular execution of the encryption process must be unpredictable.
CFB-ACPKM-Master Encryption Mode
This section defines a CFB-ACPKM-Master encryption mode that uses the ACPKM-Master internal re-keying
mechanism for the periodical key transformation.
The CFB-ACPKM-Master encryption mode can be considered as the base encryption mode CFB (see ) extended by the ACPKM-Master re-keying mechanism.
The CFB-ACPKM-Master encryption mode can be used with the following parameters:
64 <= n <= 512.
128 <= k <= 512.
The maximum message size m_max = N * (n * 2^{n/2-1} / k).
The key material K[j] that is used for the j-th section processing is equal to K^j, |K^j| = k bits.
The CFB-ACPKM-Master mode encryption and decryption procedures are defined as follows:
The initialization vector IV for any particular execution of the encryption process must be unpredictable.
OMAC-ACPKM-Master Authentication Mode
This section defines an OMAC-ACPKM-Master message authentication code calculation mode that uses the ACPKM-Master internal re-keying
mechanism for the periodical key transformation.
The OMAC-ACPKM-Master mode can be considered as the base message authentication code calculation mode OMAC, which is also known as CMAC (see ), extended by the ACPKM-Master re-keying mechanism.
The OMAC-ACPKM-Master message authentication code calculation mode can be used with the following parameters:
n in {64, 128, 256}.
128 <= k <= 512.
The maximum message size m_max = N * (n * 2^{n/2-1} / (k + n)).
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:
Here, R_n takes the following values:
n = 64: R_{64} = 0^{59} | 11011.
n = 128: R_{128} = 0^{120} | 10000111.
n = 256: R_{256} = 0^{145} | 10000100101.
The OMAC-ACPKM-Master message authentication code calculation mode is defined as follows:
Joint Usage of External and Internal Re-keying
Both external re-keying and internal re-keying have their own
advantages and disadvantages, which are discussed in .
For instance, using external re-keying can essentially limit the message length, while in the case of internal re-keying,
the section size, which can be chosen as the maximal possible for operational properties, limits the number of separate messages.
Therefore, the choice of re-keying mechanism (either external or internal) depends on particular protocol features.
However, some protocols may have features that require to take advantages provided by both external and internal re-keying mechanisms:
for example, the protocol mainly transmits messages of small length, but it must additionally support very long message processing.
In such situations, it is necessary to use external and internal re-keying jointly, since these techniques negate each other's disadvantages.
For composition of external and internal re-keying techniques, any mechanism described in can be used with any mechanism described in .
For example, consider the GCM-ACPKM mode with external serial re-keying
based on a KDF on a hash function. Denote by a frame size the number of messages in each frame
(in the case of implicit approach to the key lifetime control) for external re-keying.
Let L be a key lifetime limitation. The section size N for internal re-keying and
the frame size q for external re-keying must be chosen in such a way
that q * N must not exceed L.
Suppose that t messages (ICN_i, P_i, A_i), with initial counter nonce ICN_i, plaintext P_i, and
additional authenticated data A_i will be processed before renegotiation.
For authenticated encryption of each message (ICN_i, P_i, A_i), i = 1, ..., t, the following
algorithm can be applied:
Note that nonces ICN_i, that are used under the same frame key, must be unique for each message.
Security Considerations
Re-keying should be used to increase a priori security properties of ciphers in hostile environments (e.g., with side-channel adversaries).
If some efficient attacks are known for a cipher, it must not be used. Thus, re-keying cannot be used as a patch for vulnerable ciphers.
Base cipher properties must be well analyzed because the security of re-keying mechanisms is based on the security of a block cipher as a pseudorandom function.
Re-keying is not intended to solve any postquantum security issues for symmetric cryptography,
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 re-keying is to limit the size of plaintext transformed under one initial key.
Re-keying can provide backward security only if previous key material is securely deleted after usage by all parties.
IANA Considerations
This document has no IANA actions.
ReferencesNormative ReferencesKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.The AES-CMAC AlgorithmThe National Institute of Standards and Technology (NIST) has recently specified the Cipher-based Message Authentication Code (CMAC), which is equivalent to the One-Key CBC MAC1 (OMAC1) submitted by Iwata and Kurosawa. This memo specifies an authentication algorithm based on CMAC with the 128-bit Advanced Encryption Standard (AES). This new authentication algorithm is named AES-CMAC. The purpose of this document is to make the AES-CMAC algorithm conveniently available to the Internet Community. This memo provides information for the Internet community.HMAC-based Extract-and-Expand Key Derivation Function (HKDF)This document specifies a simple Hashed Message Authentication Code (HMAC)-based key derivation function (HKDF), which can be used as a building block in various protocols and applications. The key derivation function (KDF) is intended to support a wide range of applications and requirements, and is conservative in its use of cryptographic hash functions. This document is not an Internet Standards Track specification; it is published for informational purposes.Guidelines on the Cryptographic Algorithms to Accompany the Usage of Standards GOST R 34.10-2012 and GOST R 34.11-2012The purpose of this document is to make the specifications of the cryptographic algorithms defined by the Russian national standards GOST R 34.10-2012 and GOST R 34.11-2012 available to the Internet community for their implementation in the cryptographic protocols based on the accompanying algorithms.These specifications define the pseudorandom functions, the key agreement algorithm based on the Diffie-Hellman algorithm and a hash function, the parameters of elliptic curves, the key derivation functions, and the key export functions.Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.
Recommendation for Key Derivation Using Pseudorandom Functions
National Institute of Standards and Technology
Datagram Transport Layer Security Version 1.2Cryptographic Message Syntax (CMS)The Secure Shell (SSH) Transport Layer ProtocolIP Encapsulating Security Payload (ESP)The Transport Layer Security (TLS) Protocol Version 1.3
Recommendation for Block Cipher Modes of Operation: Galois/Counter Mode (GCM) and GMAC
Recommendation for Block Cipher Modes of Operation: Methods and Techniques
Informative References
Retail Financial Services Symmetric Key Management - Part 3: Derived Unique Key Per Transaction
American National Standards Institute
Increasing the Lifetime of a Key: A Comparative Analysis of the Security of Re-keying Techniques
On the Practical (In-)Security of 64-bit Block Ciphers: Collision Attacks on HTTP over TLS and OpenVPN
TEMPEST attacks against AES. Covertly stealing keys for 200 euro
A Tutorial on Linear and Differential Cryptanalysis
The Double Ratchet Algorithm
Subject: [Cfrg] Dynamic Key Changes on Encrypted Sessions. - Draft I-D Attached
On Making U2F Protocol Leakage-Resilient via Re-keying
Key Updating for Leakage Resiliency With Application to AES Modes of Operation
A Leakage-Resilient Mode of Operation
Practical Leakage-Resilient Symmetric Cryptography
How to Construct Random Functions
Key Regression: Enabling Efficient Key Distribution for Secure Distributed Storage
Secure Group Services for Storage Area Networks
One-Way Cross-Trees and Their Applications
Towards Sound Fresh Re-Keying with Hard (Physical) Learning Problems
Increasing the Lifetime of Symmetric Keys for the GCM Mode by Internal Re-keying
Test ExamplesTest Examples for External Re-keyingExternal Re-keying with a Parallel ConstructionExternal Re-keying with a Serial ConstructionTest Examples for Internal Re-keyingInternal Re-keying Mechanisms that Do Not Require a Master KeyInternal Re-keying Mechanisms with a Master Key K2 = K1 << 1
K1:
00000: 78 21 C7 C7 6C BD 79 63 56 AC F8 8E 69 6A 00 07
K2:
00000: F0 43 8F 8E D9 7A F2 C6 AD 59 F1 1C D2 D4 00 0E
Plaintext M_5:
00000: 33 44 55 66 77 88 99 AA BB CC EE FF 0A 00 11 22
Using K1, padding is not required
Input block M_5 (xor) C_4:
00000: FD E6 71 37 E6 05 2D 8F 94 A1 9D 55 60 E8 0C A4
Output block C_5:
00000: B3 AD B8 92 18 32 05 4C 09 21 E7 B8 08 CF A0 B8
Message authentication code T:
00000: B3 AD B8 92 18 32 05 4C 09 21 E7 B8 08 CF A0 B8
]]>Acknowledgments
We thank Mihir Bellare, Scott Fluhrer, Dorothy Cooley, Yoav Nir, Jim Schaad, Paul Hoffman, Dmitry Belyavsky, Yaron Sheffer, Alexey Melnikov, and Spencer Dawkins
for their useful comments.
Contributors