]> ELVIS-PLUS

PO Box 81 Moscow (Zelenograd) 124460 Russian Federation +7 495 276 0211 svan@elvis.ru

AEAD MGM This document defines a set of encryption transforms for use in the Encapsulating Security Payload (ESP) and in the Internet Key Exchange version 2 (IKEv2) protocols, which are parts of the IP Security (IPsec) protocol suite. The transforms are based on the GOST R 34.12-2015 block ciphers (which are named "Magma" and "Kuznyechik") in Multilinear Galois Mode (MGM) and the external rekeying approach. This specification was developed to facilitate implementations that wish to support the GOST algorithms. This document does not imply IETF endorsement of the cryptographic algorithms used in this document.

Introduction The IP Security (IPsec) protocol suite consists of several protocols, of which the Encapsulating Security Payload (ESP) and the Internet Key Exchange version 2 (IKEv2) are most widely used. This document defines four transforms for ESP and IKEv2 based on Russian cryptographic standard algorithms (often referred to as "GOST" algorithms). These definitions are based on the recommendations established by the Federal Agency on Technical Regulating and Metrology (Rosstandart), which describe how Russian cryptographic standard algorithms are used in ESP and IKEv2. The transforms defined in this document are based on two block ciphers from Russian cryptographic standard algorithms -- "Kuznyechik" and "Magma" in Multilinear Galois Mode (MGM) . These transforms provide Authenticated Encryption with Associated Data (AEAD). An external rekeying mechanism, described in , is also used in these transforms to limit the load on session keys. Because the GOST specification includes the definition of both 128-bit ("Kuznyechik") and 64-bit ("Magma") block ciphers, both are included in this document. Implementers should make themselves aware of the relative security and other cost-benefit implications of the two ciphers. See for more details. This specification was developed to facilitate implementations that wish to support the GOST algorithms. This document does not imply IETF endorsement of the cryptographic algorithms used in this document.

Requirements Language 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.

Overview Russian cryptographic standard algorithms, often referred to as "GOST" algorithms, constitute a set of cryptographic algorithms of different types -- ciphers, hash functions, digital signatures, etc. In particular, Russian cryptographic standard defines two block ciphers -- "Kuznyechik" (also defined in ) and "Magma" (also defined in ). Both ciphers use a 256-bit key. "Kuznyechik" has a block size of 128 bits, while "Magma" has a 64-bit block. Multilinear Galois Mode (MGM) is an AEAD mode defined in and . It is claimed to provide defense against some attacks on well-known AEAD modes, like Galois/Counter Mode (GCM). defines mechanisms that can be used to limit the number of times any particular session key is used. One of these mechanisms, called external rekeying with tree-based construction (defined in ), is used in the defined transforms. For the purpose of deriving subordinate keys, the Key Derivation Function (KDF) KDF_GOSTR3411_2012_256, defined in , is used. This KDF is based on a Hashed Message Authentication Code (HMAC) construction with a Russian GOST hash function defined in Russian cryptographic standard (also defined in ).

Description of Transforms This document defines four transforms of Type 1 (Encryption Algorithm) for use in ESP and IKEv2. All of them use MGM as the mode of operation with tree-based external rekeying. The transforms differ in underlying ciphers and in cryptographic services they provide.

• ENCR_KUZNYECHIK_MGM_KTREE (Transform ID 32) is an AEAD transform based on the "Kuznyechik" algorithm; it provides confidentiality and message authentication and thus can be used in both ESP and IKEv2.
• ENCR_MAGMA_MGM_KTREE (Transform ID 33) is an AEAD transform based on the "Magma" algorithm; it provides confidentiality and message authentication and thus can be used in both ESP and IKEv2.
• ENCR_KUZNYECHIK_MGM_MAC_KTREE (Transform ID 34) is a MAC-only transform based on the "Kuznyechik" algorithm; it provides no confidentiality and thus can only be used in ESP, but not in IKEv2.
• ENCR_MAGMA_MGM_MAC_KTREE (Transform ID 35) is a MAC-only transform based on the "Magma" algorithm; it provides no confidentiality and thus can only be used in ESP, but not in IKEv2.

Note that transforms ENCR_KUZNYECHIK_MGM_MAC_KTREE and ENCR_MAGMA_MGM_MAC_KTREE don't provide any confidentiality, but they are defined as Type 1 (Encryption Algorithm) transforms because of the need to include an Initialization Vector (IV), which is impossible for Type 3 (Integrity Algorithm) transforms.

Tree-Based External Rekeying All four transforms use the same tree-based external rekeying mechanism. The idea is that the key that is provided for the transform is not directly used to protect messages. Instead, a tree of keys is derived using this key as a root. This tree may have several levels. The leaf keys are used for message protection, while intermediate-node keys are used to derive lower-level keys, including leaf keys. See for more details. This construction allows us to protect a large amount of data, at the same time providing a bound on a number of times any particular key in the tree is used, thus defending against some side-channel attacks and also increasing the key lifetime limitations based on combinatorial properties. The transforms defined in this document use a three-level tree. The leaf key that protects a message is computed as follows: where:

KDF (k, l, s)

Key Derivation Function KDF_GOSTR3411_2012_256 (defined in ), which accepts three input parameters -- a key (k), a label (l), and a seed (s) -- and provides a new key as output

K

the root key for the tree (see )

l1, l2, l3

labels defined as 6-octet ASCII strings without null termination:

l1 =

"level1"

l2 =

"level2"

l3 =

"level3"

i1, i2, i3

parameters that determine which keys out of the tree are used on each level. Together, they determine a leaf key that is used for message protection; the length of i1 is one octet, and i2 and i3 are two-octet integers in network byte order

|

indicates concatenation

This construction allows us to generate up to 2⁸ keys on level 1 and up to 2¹⁶ keys on levels 2 and 3. So, the total number of possible leaf keys generated from a single Security Association (SA) key is 2⁴⁰. This specification doesn't impose any requirements on how frequently external rekeying takes place. It is expected that the sending application will follow its own policy dictating how many times the keys on each level must be used.

Initialization Vector Format Each message protected by the defined transforms MUST contain an IV. The IV has a size of 64 bits and consists of four fields. The fields i1, i2, and i3 are parameters that determine the particular leaf key this message was protected with (see ). The fourth field is a counter, representing the message number for this key.

IV Format

where:

i1 (1 octet), i2 (2 octets), i3 (2 octets):

parameters that determine the particular key used to protect this message; 2-octet parameters are integers in network byte order

pnum (3 octets):

message counter in network byte order for the leaf key protecting this message; up to 2²⁴ messages may be protected using a single leaf key

For any given SA, the IV MUST NOT be used more than once, but there is no requirement that IV be unpredictable.

Nonce Format for MGM MGM requires a per-message nonce (called the Initial Counter Nonce, or ICN in ) that MUST be unique in the context of any leaf key. The size of the ICN is n-1 bits, where n is the block size of the underlying cipher. The two ciphers used in the transforms defined in this document have different block sizes, so two different formats for the ICN are defined. MGM specification requires that the nonce be n-1 bits in size, where n is the block size of the underlying cipher. This document defines MGM nonces having n bits (the block size of the underlying cipher) in size. Since n is always a multiple of 8 bits, this makes MGM nonces having a whole number of octets. When used inside MGM, the most significant bit of the first octet of the nonce (represented as an octet string) is dropped, making the effective size of the nonce equal to n-1 bits. Note that the dropped bit is a part of the "zero" field (see Figures  and ), which is always set to 0, so no information is lost when it is dropped.

MGM Nonce Format for Transforms Based on the "Kuznyechik" Cipher For transforms based on the "Kuznyechik" cipher (ENCR_KUZNYECHIK_MGM_KTREE and ENCR_KUZNYECHIK_MGM_MAC_KTREE), the ICN consists of a "zero" octet; a 24-bit message counter; and a 96-bit secret salt, which is fixed for the SA and is not transmitted.

Nonce Format for Transforms Based on the "Kuznyechik" Cipher

where:

zero (1 octet):

set to 0

pnum (3 octets):

the counter for the messages protected by the given leaf key; this field MUST be equal to the pnum field in the IV

salt (12 octets):

secret salt. The salt is a string of bits that are formed when the SA is created (see for details). The salt does not change during the SA's lifetime and is not transmitted on the wire. Every SA will have its own salt.

MGM Nonce Format for Transforms Based on the "Magma" Cipher For transforms based on the "Magma" cipher (ENCR_MAGMA_MGM_KTREE and ENCR_MAGMA_MGM_MAC_KTREE), the ICN consists of a "zero" octet; a 24-bit message counter; and a 32-bit secret salt, which is fixed for the SA and is not transmitted.

Nonce Format for Transforms Based on the "Magma" Cipher

where:

zero (1 octet):

set to 0

pnum (3 octets):

the counter for the messages protected by the given leaf key; this field MUST be equal to the pnum field in the IV

salt (4 octets):

secret salt. The salt is a string of bits that are formed when the SA is created (see for details). The salt does not change during the SA's lifetime and is not transmitted on the wire. Every SA will have its own salt.

Keying Material We'll call a string of bits that is used to initialize the transforms defined in this specification a "transform key". The transform key is a composite entity consisting of the root key for the tree and the secret salt. The transform key for the ENCR_KUZNYECHIK_MGM_KTREE and ENCR_KUZNYECHIK_MGM_MAC_KTREE transforms consists of 352 bits (44 octets), of which the first 256 bits is a root key for the tree (denoted as K in ) and the remaining 96 bits is a secret salt (see ). The transform key for the ENCR_MAGMA_MGM_KTREE and ENCR_MAGMA_MGM_MAC_KTREE transforms consists of 288 bits (36 octets), of which the first 256 bits is a root key for the tree (denoted as K in ) and the remaining 32 bits is a secret salt (see ). In the case of ESP, the transform keys are extracted from the KEYMAT as defined in . In the case of IKEv2, the transform keys are either SK_ei or SK_er, which are generated as defined in . Note that since these transforms provide authenticated encryption, no additional keys are needed for authentication. This means that, in the case of IKEv2, the keys SK_ai/SK_ar are not used and MUST be treated as having zero length.

Integrity Check Value The length of the authentication tag that MGM can compute is in the range from 32 bits to the block size of the underlying cipher. states that the authentication tag length MUST be fixed for a particular protocol. For transforms based on the "Kuznyechik" cipher (ENCR_KUZNYECHIK_MGM_KTREE and ENCR_KUZNYECHIK_MGM_MAC_KTREE), the resulting Integrity Check Value (ICV) length is set to 96 bits. For transforms based on the "Magma" cipher (ENCR_MAGMA_MGM_KTREE and ENCR_MAGMA_MGM_MAC_KTREE), the full ICV length is set to the block size (64 bits).

Plaintext Padding The transforms defined in this document don't require any plaintext padding, as specified in . This means that only those padding requirements that are imposed by the protocol are applied (4 bytes for ESP, no padding for IKEv2).

AAD Construction

ESP AAD Additional Authenticated Data (AAD) in ESP is constructed differently, depending on the transform being used and whether the Extended Sequence Number (ESN) is in use or not. The ENCR_KUZNYECHIK_MGM_KTREE and ENCR_MAGMA_MGM_KTREE transforms provide confidentiality, so the content of the ESP body is encrypted and the AAD consists of the ESP Security Parameter Index (SPI) and (E)SN. The AAD is constructed similarly to the AAD in . On the other hand, the ENCR_KUZNYECHIK_MGM_MAC_KTREE and ENCR_MAGMA_MGM_MAC_KTREE transforms don't provide confidentiality; they provide only message authentication. For this purpose, the IV and the part of the ESP packet that is normally encrypted are included in the AAD. For these transforms, the encryption capability provided by MGM is not used. The AAD is constructed similarly to the AAD in .

AAD for AEAD Transforms with 32-Bit SN

AAD for AEAD Transforms with 64-Bit ESN

AAD for Authentication-Only Transforms with 32-Bit SN

AAD for Authentication-Only Transforms with 64-Bit ESN

IKEv2 AAD For IKEv2, the AAD consists of the IKEv2 Header, any unencrypted payloads following it (if present), and either the Encrypted payload header () or the Encrypted Fragment payload (), depending on whether IKE fragmentation is used. The AAD is constructed similarly to the AAD in .

AAD for IKEv2 in the Case of the Encrypted Payload

AAD for IKEv2 in the Case of the Encrypted Fragment Payload

Using Transforms When the SA is established, the i1, i2, and i3 parameters are set to 0 by the sender and a leaf key is calculated. The pnum parameter starts from 0 and is incremented with each message protected by the same leaf key. When the sender decides that the leaf should be changed, it increments the i3 parameter and generates a new leaf key. The pnum parameter for the new leaf key is reset to 0, and the process continues. If the sender decides that a third-level key corresponding to i3 is used enough times, it increments i2, resets i3 to 0, and calculates a new leaf key. The pnum is reset to 0 (as with every new leaf key), and the process continues. A similar procedure is used when a second-level key needs to be changed. A combination of i1, i2, i3, and pnum MUST NOT repeat for any particular SA. This means that the wrapping of these counters is not allowed: when i2, i3, or pnum reaches its respective maximum value, a procedure for changing a leaf key, described above, is executed, and if all four parameters reach their maximum values, the IPsec SA becomes unusable. There may be other reasons to recalculate leaf keys besides reaching maximum values for the counters. For example, as described in , it is RECOMMENDED that the sender count the number of octets protected by a particular leaf key and generate a new key when some threshold is reached, and at the latest when reaching the octet limits stated in for each of the ciphers. The receiver always uses i1, i2, and i3 from the received message. If they differ from the values in previously received packets, a new leaf key is calculated. The pnum parameter is always used from the received packet. To improve performance, implementations may cache recently used leaf keys. When a new leaf key is calculated (based on the values from the received message), the old key may be kept for some time to improve performance in the case of possible packet reordering (when packets protected by the old leaf key are delayed and arrive later).

Security Considerations The most important security consideration for MGM is that the nonce MUST NOT repeat for a given key. For this reason, the transforms defined in this document MUST NOT be used with manual keying. Excessive use of the same key can give an attacker advantages in breaking security properties of the transforms defined in this document. For this reason, the amount of data that any particular key is used to protect should be limited. This is especially important for algorithms with a 64-bit block size (like "Magma"), which currently are generally considered insecure after protecting a relatively small amount of data. For example, Section 3.4 of limits the number of blocks that are allowed to be encrypted with the Triple DES cipher to 2²⁰ (8 MB of data). This document defines a rekeying mechanism that allows the mitigation of weak security of a 64-bit block cipher by frequently changing the encryption key. For transforms defined in this document, recommends limiting the number of octets protected with a single K_msg key by the following values:

• 2⁴¹ octets for transforms based on the "Kuznyechik" cipher (ENCR_KUZNYECHIK_MGM_KTREE and ENCR_KUZNYECHIK_MGM_MAC_KTREE)
• 2²⁸ octets for transforms based on the "Magma" cipher (ENCR_MAGMA_MGM_KTREE and ENCR_MAGMA_MGM_MAC_KTREE)

These values are based on combinatorial properties and may be further restricted if side-channel attacks are taken into consideration. Note that the limit for transforms based on the "Kuznyechik" cipher is unreachable because, due to the construction of the transforms, the number of protected messages is limited to 2²⁴ and each message (either IKEv2 messages or ESP datagrams) is limited to 2¹⁶ octets in size, giving 2⁴⁰ octets as the maximum amount of data that can be protected with a single K_msg. discusses the possibility of truncating authentication tags in MGM as a trade-off between message expansion and the probability of forgery. This specification truncates an authentication tag length for transforms based on the "Kuznyechik" cipher to 96 bits. This decreases message expansion while still providing a very low probability of forgery: 2^-96. An attacker can send a lot of packets with arbitrarily chosen i1, i2, and i3 parameters. This will 1) force a recipient to recalculate the leaf key for every received packet if i1, i2, and i3 are different from these values in previously received packets, thus consuming CPU resources and 2) force a recipient to make verification attempts (that would fail) on a large amount of data, thus allowing the attacker a deeper analysis of the underlying cryptographic primitive (see ). Implementations MAY initiate rekeying if they deem that they receive too many packets with an invalid ICV. Security properties of MGM are discussed in .

IANA Considerations IANA maintains a registry called "Internet Key Exchange Version 2 (IKEv2) Parameters" with a subregistry called "Transform Type Values". IANA has added the following four Transform IDs to the "Transform Type 1 - Encryption Algorithm Transform IDs" subregistry. Transform IDs

Number	Name	ESP Reference	IKEv2 Reference
32	ENCR_KUZNYECHIK_MGM_KTREE	RFC 9227	RFC 9227
33	ENCR_MAGMA_MGM_KTREE	RFC 9227	RFC 9227
34	ENCR_KUZNYECHIK_MGM_MAC_KTREE	RFC 9227	Not allowed
35	ENCR_MAGMA_MGM_MAC_KTREE	RFC 9227	Not allowed

References Normative References Informative References Federal Agency on Technical Regulating and Metrology (In Russian) Federal Agency on Technical Regulating and Metrology (In Russian) Federal Agency on Technical Regulating and Metrology (In Russian) Federal Agency on Technical Regulating and Metrology (In Russian) National Institute of Standards and Technology ETH Zurich Mozilla Cloudflare

Test Vectors In the following test vectors, binary data is represented in hexadecimal format. The numbers in square brackets indicate the size of the corresponding data in decimal format.

1. ENCR_KUZNYECHIK_MGM_KTREE (Example 1):
2. ENCR_KUZNYECHIK_MGM_KTREE (Example 2):
3. ENCR_MAGMA_MGM_KTREE (Example 1):
4. ENCR_MAGMA_MGM_KTREE (Example 2):
5. ENCR_KUZNYECHIK_MGM_MAC_KTREE (Example 1):
6. ENCR_KUZNYECHIK_MGM_MAC_KTREE (Example 2):
7. ENCR_MAGMA_MGM_MAC_KTREE (Example 1):
8. ENCR_MAGMA_MGM_MAC_KTREE (Example 2):

Acknowledgments The author wants to thank , , , and for valuable input during the publication process for this document.