Internet Engineering Task Force (IETF) P. Wouters
Request for Comments: 8221 Red Hat
Obsoletes: 7321 D. Migault
Category: Standards Track J. Mattsson
ISSN: 2070-1721 Ericsson
T. KivinenOctober 2017 Cryptographic Algorithm Implementation Requirements and Usage Guidance
for Encapsulating Security Payload (ESP) and Authentication Header (AH)
This document replaces RFC 7321, "Cryptographic Algorithm
Implementation Requirements and Usage Guidance for Encapsulating
Security Payload (ESP) and Authentication Header (AH)". The goal of
this document is to enable ESP and AH to benefit from cryptography
that is up to date while making IPsec interoperable.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
The Encapsulating Security Payload (ESP) [RFC4303] and the
Authentication Header (AH) [RFC4302] are the mechanisms for applying
cryptographic protection to data being sent over an IPsec Security
Association (SA) [RFC4301].
This document provides guidance and recommendations so that ESP and
AH can be used with cryptographic algorithms that are up to date.
The challenge of such documents is making sure that, over time, IPsec
implementations can use secure and up-to-date cryptographic
algorithms while keeping IPsec interoperable.
1.1. Updating Algorithm Implementation Requirements and Usage Guidance
The field of cryptography evolves continuously: new, stronger
algorithms appear, and existing algorithms are found to be less
secure than originally thought. Therefore, algorithm implementation
requirements and usage guidance need to be updated from time to time
to reflect the new reality. The choices for algorithms must be
conservative to minimize the risk of algorithm compromise.
Algorithms need to be suitable for a wide variety of CPU
architectures and device deployments ranging from high-end bulk
encryption devices to small, low-power Internet of Things (IoT)
The algorithm implementation requirements and usage guidance may need
to change over time to adapt to the changing world. For this reason,
the selection of mandatory-to-implement algorithms was removed from
the main Internet Key Exchange Protocol Version 2 (IKEv2)
specification [RFC7296] and placed in a separate document.
1.2. Updating Algorithm Requirement Levels
The mandatory-to-implement algorithm of tomorrow should already be
available in most implementations of AH/ESP by the time it is made
mandatory. This document attempts to identify and introduce those
algorithms for future mandatory-to-implement status. There is no
guarantee that the algorithms in use today may become mandatory in
the future. Published algorithms are continuously subjected to
cryptographic attack and may become too weak or could become
completely broken before this document is updated.
This document only provides recommendations for the mandatory-to-
implement algorithms and "too weak" algorithms that are recommended
not to be implemented. As a result, any algorithm listed at the
IPsec IANA registry that is not mentioned in this document MAY be
implemented. It is expected that this document will be updated over
time and future versions will only mention algorithms that have
evolved in status. For clarification, when an algorithm has been
mentioned in [RFC7321], this document states explicitly the update of
Although this document updates the algorithms to keep the AH/ESP
communication secure over time, it also aims at providing
recommendations so that AH/ESP implementations remain interoperable.
AH/ESP interoperability is addressed by an incremental introduction
or deprecation of algorithms. In addition, this document also
considers the new use cases for AH/ESP deployment, such as IoT.
It is expected that deprecation of an algorithm be performed
gradually. This provides time for various implementations to update
their implemented algorithms while remaining interoperable. Unless
there are strong security reasons, an algorithm is expected to be
downgraded from MUST to MUST- or SHOULD, instead of MUST NOT (see
Section 2). Similarly, an algorithm that has not been mentioned as
mandatory-to-implement is expected to be introduced with a SHOULD
instead of a MUST.
The current trend toward IoT and its adoption of AH/ESP requires this
specific use case to be taken into account as well. IoT devices are
resource-constrained devices, and their choice of algorithms is
motivated by minimizing the footprint of the code, the computation
effort, and the size of the messages to send. This document
indicates "(IoT)" when a specified algorithm is specifically listed
for IoT devices. Requirement levels that are marked as "IoT" apply
to IoT devices and to server-side implementations that might
presumably need to interoperate with them, including any general-
purpose VPN gateways.
1.3. Document Audience
The recommendations of this document mostly target AH/ESP
implementers as implementations need to meet both high security
expectations as well as high interoperability between various vendors
and with different versions. Interoperability requires a smooth move
to more secure cipher suites. This may differ from a user point of
view that may deploy and configure AH/ESP with only the safest cipher
This document does not give any recommendations for the use of
algorithms, it only gives recommendations for implementations. The
use of algorithms by a specific user is dictated by their own
security policy requirements, which are outside the scope of this
The algorithms considered here are listed by IANA as part of the
IKEv2 parameters. IKEv1 is out of scope of this document. IKEv1 is
deprecated; the recommendations of this document must not be
considered for IKEv1, nor may IKEv1 parameters be considered by this
The IANA registry for "Internet Key Exchange Version 2 (IKEv2)
Parameters" contains some entries that are not for use with ESP or
AH. This document does not modify the status of those algorithms.
2. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
We define some additional terms here:
SHOULD+ This term means the same as SHOULD. However, it is likely
that an algorithm marked as SHOULD+ will be promoted at
some future time to be a MUST.
SHOULD- This term means the same as SHOULD. However, an algorithm
marked as SHOULD- may be deprecated to a MAY in a future
version of this document.
MUST- This term means the same as MUST. However, we expect at
some point that this algorithm will no longer be a MUST in
a future document. Although its status will be determined
at a later time, it is reasonable to expect that if a
future revision of a document alters the status of a MUST-
algorithm, it will remain at least a SHOULD or a SHOULD-
IoT The Internet of Things.
3. Manual Keying
Manual keying SHOULD NOT be used, as it is inherently dangerous.
Without any secure keying protocol, such as IKE, IPsec does not offer
Perfect Forward Secrecy (PFS) protection; there is no entity to
ensure the refreshing of session keys, the tracking of Security
Parameter Index (SPI) uniqueness, and the single use of nonces,
Initialization Vectors (IVs), and counters. This document was
written for deploying ESP/AH using IKE [RFC7296] and assumes that
keying happens using IKEv2 or higher.
If manual keying is used regardless, Counter Mode algorithms such as
ENCR_AES_CTR, ENCR_AES_CCM, ENCR_AES_GCM, and ENCR_CHACHA20_POLY1305
MUST NOT be used, as it is incompatible with a secure and persistent
handling of the counter (as explained in the Security Considerations
section of [RFC3686]). This particularly applies to IoT devices that
have no state across reboots. At the time of writing, ENCR_AES_CBC
is the only mandatory-to-implement encryption algorithm suitable for
4. Encryption Must Be Authenticated
Encryption without authentication is not effective and MUST NOT be
used. IPsec offers three ways to provide both encryption and
o ESP with an Authenticated Encryption with Associated Data (AEAD)
o ESP with a non-AEAD cipher + authentication
o ESP with a non-AEAD cipher + AH with authentication
The fastest and most modern method is to use ESP with a combined mode
cipher, such as an AEAD cipher, that handles encryption/decryption
and authentication in a single step. In this case, the AEAD cipher
is set as the encryption algorithm, and the authentication algorithm
is set to none. Examples of this are ENCR_AES_GCM_16 and
A more traditional approach is to use ESP with an encryption and an
authentication algorithm. This approach is slower, as the data has
to be processed twice: once for encryption/decryption and once for
authentication. An example of this is ENCR_AES_CBC combined with
The last method that can be used is ESP+AH. This method is NOT
RECOMMENDED. It is the slowest method and also takes up more octets
due to the double header of ESP+AH. This results in a smaller
effective MTU for the encrypted data. With this method, ESP is only
used for confidentiality without an authentication algorithm, and a
second IPsec protocol of type AH is used for authentication. An
example of this is ESP with ENCR_AES_CBC with AH with
5. ESP Encryption Algorithms
| Name | Status | AEAD | Comment |
| ENCR_DES_IV64 | MUST NOT | No | UNSPECIFIED |
| ENCR_DES | MUST NOT | No | [RFC2405] |
| ENCR_3DES | SHOULD NOT | No | [RFC2451] |
| ENCR_BLOWFISH | MUST NOT | No | [RFC2451] |
| ENCR_3IDEA | MUST NOT | No | UNSPECIFIED |
| ENCR_DES_IV32 | MUST NOT | No | UNSPECIFIED |
| ENCR_NULL | MUST | No | [RFC2410] |
| ENCR_AES_CBC | MUST | No | [RFC3602] |
| ENCR_AES_CCM_8 | SHOULD | Yes | [RFC4309](IoT) |
| ENCR_AES_GCM_16 | MUST | Yes | [RFC4106] |
| ENCR_CHACHA20_POLY1305 | SHOULD | Yes | [RFC7634] |
 - This requirement level is for 128-bit and 256-bit keys. 192-bit
keys remain at the MAY level.
(IoT) - This requirement is for interoperability with IoT. Only
128-bit keys are at the given level.
IPsec sessions may have very long lifetime and carry multiple
packets, so there is a need to move to 256-bit keys in the long term.
For that purpose, the requirement level for 128-bit keys and 256-bit
keys is MUST (when applicable). In that sense, the status for
256-bit keys has been raised from MAY in [RFC7321] to MUST.
IANA has allocated codes for cryptographic algorithms that have not
been specified by the IETF. Such algorithms are noted as
UNSPECIFIED. Usually, the use of these algorithms is limited to
specific cases, and the absence of specification makes
interoperability difficult for IPsec communications. These
algorithms were not mentioned in [RFC7321], and this document
clarifies that such algorithms MUST NOT be implemented for IPsec
Similarly, IANA also allocated code points for algorithms that are
not expected to be used to secure IPsec communications. Such
algorithms are noted as non-IPsec. As a result, these algorithms
MUST NOT be implemented.
Various ciphers that are older, not well tested, and never widely
implemented have been changed to MUST NOT.
ENCR_3DES status has been downgraded from MAY in [RFC7321] to SHOULD
NOT. ENCR_CHACHA20_POLY1305 is a more modern approach and
alternative for ENCR_3DES than ENCR_AES_CBC, and so it is expected to
be favored to replace ENCR_3DES.
ENCR_BLOWFISH has been downgraded to MUST NOT as it has been
deprecated for years by TWOFISH, which is not standardized for ESP
and therefore not listed in this document. Some implementations
support TWOFISH using a private range number.
ENCR_NULL status was set to MUST in [RFC7321] and remains a MUST to
enable the use of ESP with only authentication, which is preferred
over AH due to NAT traversal. ENCR_NULL is expected to remain MUST
by protocol requirements.
ENCR_AES_CBC status remains at MUST. ENCR_AES_CBC MUST be
implemented in order to enable interoperability between
implementations that followed [RFC7321]. However, there is a trend
for the industry to move to AEAD encryption, and the overhead of
ENCR_AES_CBC remains quite large, so it is expected to be replaced by
AEAD algorithms in the long term.
ENCR_AES_CCM_8 status was set to MAY in [RFC7321] and has been raised
from MAY to SHOULD in order to interact with IoT devices. As this
case is not a general use case for VPNs, its status is expected to
remain as SHOULD.
ENCR_AES_GCM_16 status has been updated from SHOULD+ to MUST in order
to favor the use of authenticated encryption and AEAD algorithms.
ENCR_AES_GCM_16 has been widely implemented for ESP due to its
increased performance and key longevity compared to ENCR_AES_CBC.
ENCR_CHACHA20_POLY1305 was not ready to be considered at the time of
[RFC7321]. It has been recommended by the Crypto Forum Research
Group (CFRG) and others as an alternative to AES-CBC and AES-GCM. At
the time of writing, there are not enough ESP implementations of
ENCR_CHACHA20_POLY1305 to be able to introduce it at the SHOULD+
level. Its status has been set to SHOULD and is expected to become
MUST in the long term.
6. ESP and AH Authentication Algorithms
Authentication algorithm recommendations in this section are
targeting two types of communications:
o Authenticated-only communications without encryption, such as ESP
with NULL encryption or AH communications.
o Communications that are encrypted with a non-AEAD algorithm that
MUST be combined with an authentication algorithm.
| Name | Status | Comment |
| AUTH_NONE | MUST / | [RFC7296][RFC5282] |
| | MUST NOT | AEAD-only |
| AUTH_HMAC_MD5_96 | MUST NOT | [RFC2403][RFC7296] |
| AUTH_HMAC_SHA1_96 | MUST- | [RFC2404][RFC7296] |
| AUTH_DES_MAC | MUST NOT | UNSPECIFIED |
| AUTH_KPDK_MD5 | MUST NOT | UNSPECIFIED |
| AUTH_AES_XCBC_96 | SHOULD / MAY | [RFC3566][RFC7296] |
| | | (IoT) |
| AUTH_AES_128_GMAC | MAY | [RFC4543] |
| AUTH_AES_256_GMAC | MAY | [RFC4543] |
| AUTH_HMAC_SHA2_256_128 | MUST | [RFC4868] |
| AUTH_HMAC_SHA2_512_256 | SHOULD | [RFC4868] |
(IoT) - This requirement is for interoperability with IoT.
AUTH_NONE has been downgraded from MAY in [RFC7321] to MUST NOT. The
only case where AUTH_NONE is acceptable is when authenticated
encryption algorithms are selected from Section 5. In all other
cases, AUTH_NONE MUST NOT be selected. As ESP and AH both provide
authentication, one may be tempted to combine these protocols to
provide authentication. As mentioned by [RFC7321], it is NOT
RECOMMENDED to use ESP with NULL authentication (with non-
authenticated encryption) in conjunction with AH; some configurations
of this combination of services have been shown to be insecure
[PD10]. In addition, AUTH_NONE authentication cannot be combined
with ESP NULL encryption.
AUTH_HMAC_MD5_96 and AUTH_KPDK_MD5 were not mentioned in [RFC7321].
As MD5 is known to be vulnerable to collisions, these algorithms MUST
NOT be used.
AUTH_HMAC_SHA1_96 has been downgraded from MUST in [RFC7321] to MUST-
as there is an industry-wide trend to deprecate its usage.
AUTH_DES_MAC was not mentioned in [RFC7321]. As DES is known to be
vulnerable, it MUST NOT be used.
AUTH_AES_XCBC_96 is set as SHOULD only in the scope of IoT, as IoT
deployments tend to prefer AES-based Hashed Message Authentication
Code (HMAC) functions in order to avoid implementing SHA2. For the
wide VPN deployment, as it has not been widely adopted, it has been
downgraded from SHOULD to MAY.
AUTH_AES_128_GMAC status has been downgraded from SHOULD+ to MAY.
Along with AUTH_AES_192_GMAC and AUTH_AES_256_GMAC, these algorithms
should only be used for AH and not for ESP. If using ENCR_NULL,
AUTH_HMAC_SHA2_256_128 is recommended for integrity. If using AES-
GMAC in ESP without authentication, ENCR_NULL_AUTH_AES_GMAC is
recommended. Therefore, these algorithms are kept at MAY.
AUTH_HMAC_SHA2_256_128 was not mentioned in [RFC7321], as no
SHA2-based authentication was mentioned. AUTH_HMAC_SHA2_256_128 MUST
be implemented in order to replace AUTH_HMAC_SHA1_96. Note that due
to a long standing common implementation bug of this algorithm that
truncates the hash at 96 bits instead of 128 bits, it is recommended
that implementations prefer AUTH_HMAC_SHA2_512_256 over
AUTH_HMAC_SHA2_256_128 if they implement AUTH_HMAC_SHA2_512_256.
AUTH_HMAC_SHA2_512_256 SHOULD be implemented as a future replacement
of AUTH_HMAC_SHA2_256_128 or when stronger security is required.
This value has been preferred to AUTH_HMAC_SHA2_384, as the
additional overhead of AUTH_HMAC_SHA2_512 is negligible.
7. ESP and AH Compression Algorithms
| Name | Status | Comment |
| IPCOMP_OUI | MUST NOT | UNSPECIFIED |
| IPCOMP_DEFLATE | MAY | [RFC3173] |
| IPCOMP_LZS | MAY | [RFC2395] |
| IPCOMP_LZJH | MAY | [RFC3051] |
Compression was not mentioned in [RFC7321]. As it is not widely
deployed, it remains optional and at the MAY level.
8. Summary of Changes from RFC 7321
The following table summarizes the changes from RFC 7321.
| Algorithm | RFC 7321 | RFC 8221 |
| ENCR_AES_GCM_16 | SHOULD+ | MUST |
| ENCR_AES_CCM_8 | MAY | SHOULD |
| ENCR_AES_CTR | MAY | MAY(*) |
| ENCR_3DES | MAY | SHOULD NOT |
| AUTH_HMAC_SHA1_96 | MUST | MUST- |
| AUTH_AES_128_GMAC | SHOULD+ | MAY |
| AUTH_NONE | MAY | MUST / MUST NOT |
(*) This algorithm is not mentioned in the above sections, so it
defaults to MAY.
9. IANA Considerations
This document does not require any IANA actions.
10. Security Considerations
The security of a system that uses cryptography depends on both the
strength of the cryptographic algorithms chosen and the strength of
the keys used with those algorithms. The security also depends on
the engineering and administration of the protocol used by the system
to ensure that there are no non-cryptographic ways to bypass the
security of the overall system.
This document concerns itself with the selection of cryptographic
algorithms for the use of ESP and AH, specifically with the selection
of mandatory-to-implement algorithms. The algorithms identified in
this document as "MUST implement" or "SHOULD implement" are not known
to be broken at the current time, and cryptographic research to date
leads us to believe that they will likely remain secure into the
foreseeable future. However, this is not necessarily forever.
Therefore, we expect that revisions of that document will be issued
from time to time to reflect the current best practice in this area.
Some of the wording in this document was adapted from [RFC7321], the
document that this one obsoletes, which was written by D. McGrew and
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