LAMPS M. Ounsworth
Internet-Draft J. Gray
Intended status: Standards Track S. Mister
Expires: January 13, 2022 Entrust
July 12, 2021
Composite Encryption For Use In Internet PKI
draft-ounsworth-pq-composite-encryption-00
Abstract
With the widespread adoption of post-quantum cryptography will come
the need for an entity to possess multiple public keys on different
cryptographic algorithms. Since the trustworthiness of individual
post-quantum algorithms is at question, a multi-key cryptographic
operation will need to be performed in such a way that breaking it
requires breaking each of the component algorithms individually.
This requires defining new structures for holding composite
encryption data.
This document defines a content encryption process following the
hybrid model as described in the NIST Post-Quantum Crypto FAQ. This
draft defines three composite encryption modes. First, Composite Key
Transport using Encryption primitives which encrypts a message
(typically a content encryption key) for a recipient with a composite
public key composed entirely of encryption keys by encrypting it with
multiple one-time-pad keys, each encrypted under a different
recipient public key. Second, Composite Key Transport using
Encryption and KEM primitives is the generalization of the previous
mode to support a mixture of encryption and KEM algorithms. Third,
Composite Key Exchange is the most general and supports establishing
a shared secret using any combination of encryption, KEM, and key
exchange primitives where a master shared secret is generated using
NIST SP 800-56Cr2.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 13, 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Composite Key Transport using Encryption primitives . . . . . 5
2.1. Algorithm Identifier . . . . . . . . . . . . . . . . . . 6
2.2. Public key and key usage . . . . . . . . . . . . . . . . 6
2.2.1. Composite-OR . . . . . . . . . . . . . . . . . . . . 6
2.3. Algorithm parameters . . . . . . . . . . . . . . . . . . 7
2.4. Encryption process . . . . . . . . . . . . . . . . . . . 7
2.5. Decryption process . . . . . . . . . . . . . . . . . . . 9
3. Composite Key Transport using Encryption and KEM primitives . 11
3.1. Algorithm Identifier . . . . . . . . . . . . . . . . . . 11
3.2. Public key and key usage . . . . . . . . . . . . . . . . 11
3.2.1. Composite-OR . . . . . . . . . . . . . . . . . . . . 12
3.3. Algorithm parameters . . . . . . . . . . . . . . . . . . 12
3.4. Encryption process . . . . . . . . . . . . . . . . . . . 12
3.5. Decryption process . . . . . . . . . . . . . . . . . . . 14
4. Composite Key Exchange . . . . . . . . . . . . . . . . . . . 14
4.1. Algorithm Identifier . . . . . . . . . . . . . . . . . . 14
4.2. Public key and key usage . . . . . . . . . . . . . . . . 15
4.2.1. Composite-OR . . . . . . . . . . . . . . . . . . . . 15
4.3. Algorithm parameters . . . . . . . . . . . . . . . . . . 15
4.4. Encapsulation Process . . . . . . . . . . . . . . . . . . 16
4.5. Decapsulation Process . . . . . . . . . . . . . . . . . . 18
5. In Practice . . . . . . . . . . . . . . . . . . . . . . . . . 20
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. Security Considerations . . . . . . . . . . . . . . . . . . . 20
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7.1. IID property of KEM primitives . . . . . . . . . . . . . 20
7.2. Composite-OR modes . . . . . . . . . . . . . . . . . . . 20
7.3. Policy for Deprecated or Unacceptable Algorithms . . . . 21
8. Appendices . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. ASN.1 Module . . . . . . . . . . . . . . . . . . . . . . 21
8.2. Intellectual Property Considerations . . . . . . . . . . 21
8.3. Making contributions . . . . . . . . . . . . . . . . . . 22
9. Normative References . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
During the transition to post-quantum cryptography, there will be
uncertainty as to the strength of cryptographic algorithms; we will
no longer fully trust traditional cryptography such as RSA, Diffie-
Hellman, DSA and their elliptic curve variants, but we will also not
fully trust their post-quantum replacements until they have had
sufficient scrutiny. Unlike previous cryptographic algorithm
migrations, the choice of when to migrate and which algorithms to
migrate to, is not so clear. Even after the migration period, it may
be advantageous for an entity's cryptographic identity to be composed
of key pairs associated with different public-key algorithms.
The deployment of composite public keys and composite encryption
using post-quantum algorithms will face two challenges:
o Algorithm strength uncertainty: During the transition period, some
post-quantum signature and encryption algorithms will not be fully
trusted, while the trust in legacy public key algorithms will
start to erode. A relying party may learn some time after
deployment that a public key algorithm has become untrustworthy,
but in the interim, they may not know which algorithm an adversary
has compromised.
o Backwards compatibility: During the transition period, post-
quantum algorithms will not be supported by all clients.
This document provides mechanisms to address algorithm strength
uncertainty by building on ~~ reference draft-ounsworth-pq-composite-
pubkeys ~~ by providing formats for both wrapping a content
encryption key using multiple public key encryption mechanisms, or
performing key exchange using a combination of encryption, key
encapsulation, and key exchange primitives. The issue of backwards
compatibility is addressed with support for Composite Or recipient
keys in each mode.
This document is intended for general applicability anywhere that
content encryption or key exchange is used.
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1.1. Terminology
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.
The following terms are used in this document:
ALGORITHM: An information object class for identifying the type of
cryptographic operation to be performed. This document is primarily
concerned with algorithms for producing encryption keys.
BER: Basic Encoding Rules (BER) as defined in [X.690].
COMPONENT ALGORITHM: A single basic algorithm which is contained
within a composite algorithm.
COMPOSITE ALGORITHM: An algorithm which is a sequence of one or more
component algorithms..
DER: Distinguished Encoding Rules as defined in [X.690].
PUBLIC / PRIVATE KEY: The public and private portion of an asymmetric
cryptographic key, making no assumptions about which algorithm.
PRIMITIVE PUBLIC KEY / SIGNATURE: A public key or signature object of
a non-composite algorithm type.
SIGNATURE: A digital cryptographic signature, making no assumptions
about which algorithm.
SECRET or SHARED SECRET: Cryptographic material established between
two parties. May be generated by one party and send encrypted to the
other, or may be the output of an exchange of public information
between two or more parties that generates a unique shared value for
all involved parties.
KEY DERIVATION FUNCTION: A function used to derive secure secret keys
using shared secrets, hashing and other cryptographic primitives.
COMPOSITE ENCRYPTION KEY: A structure that contains a sequence of
content encryption keys, or secrets used to derive a content
encryption keys.
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2. Composite Key Transport using Encryption primitives
In this composite encryption mode, a message to be encrypted is
provided by the calling application. This message to be encrypted is
assumed to have length less than the maximum message size of the
chosen encryption algorithms, as is the case when a suitably-sized
symmetric key is encrypted.
This mode is compatible with protocols requiring a key transport
primitive, such as CMS' KeyTransRecipientInfo [RFC5652].
Composite Key Transport using Encryption primitives uses a trivial
XOR one-time-pad scheme, as defined in Section 2.4. It transports n
one-time-pad secret keys of the same length as the content to be
encryption, where n is the number of recipient component public keys,
and each one-time-pad secret key is encrypted under a different
recipient component public key. The trivial XOR key-sharing scheme
requires the recipient to use all component private keys in order to
recover the content encryption key. Note that it would be possible
to use an "n of m" or "threshold" secret sharing scheme if it was
desired for the recipient to be able to complete the key transport
using a subset of their private keys, but that mechanism is not
defined in this document.
EDNOTE: we have not been able to find a reference and security
analysis for the trivial XOR key-sharing scheme. This may need
review by CFRG. We could re-frame this process as "a one-time pad
with n-1 one-time pad keys, which we transport using the recipients
public keys", then this could leverage one-time pad security
analysis.
Composite encryption uses the following structure:
EDNOTE: Should a different composite OID be used to determine the
type of composite encryption (Key Transport or Key Agreement?).
Probably not because the desired key usage will be handled in the
protocols that uses this privitive.
CompositeEncryptedKey ::= EncryptedKey{ SEQUENCE SIZE (2..Max) OF OCTET STRING}
EDNOTE: This ASN.1 probably does not compile. The intent is that
this fits into any EncryptedKey field, but defines some structure
within the existing EncryptedKey ::= OCTET STRING, but I'm not sure
exactly how to specify that.
Where each OCTET STRING within the SEQUENCE contains an encrypted
one-time-pad secret key encrypted under one of the recipient
component public keys. The CompositeEncryptedKey MUST list encrypted
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values in the same order as the recipient public key's component
keys.
2.1. Algorithm Identifier
The id-alg-composite-encryption object identifier MUST be used to
identify the usage of this mode
id-alg-composite-encryption OBJECT IDENTIFIER ::= {
id-alg-composite-encryption OBJECT IDENTIFIER ::= {
joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027)
Algorithm(80) Composite(4) id-alg-composite-encryption(4) }
EDNOTE: this is a temporary OID for the purposes of prototyping.
Permanent OIDs should be requested from IANA, see Section 6.
2.2. Public key and key usage
The recipient MUST have a composite public key which supports key
transport operations. Where the recipient public key has an
associated keyUsage as specified in [RFC5280], it MUST have keyUsage:
keyEncipherment. In other words, the mechanism specified in this
section applies only if all of the recipient's public keys are
associated with encryption algorithms.
2.2.1. Composite-OR
The design intent of this mode is to support migration scenarios
where a recipient has been provisioned with a composite key
containing algorithms that its peers may not yet support. This mode
allows the sender to encrypt for a subset of the recipient's public
keys. Support for Composite OR subset encryption is indicated by the
recipient at key generation time by marking its composite key with
the id-composite-or-key algorithm identifier as defined in ~~~cite
properly draft-ounsworth-pq-composite-keys~~~. To maximize security
strength of the ciphertext, clients SHOULD encrypt for as many keys
as they support and as the migration and compatibility situation
allow.
Policy mechanisms defining allowed subets of algorithms could be
applied here, but are out of scope of this document. As defined in
this document, a recipient marking their public key as id-composite-
or-key must accept the risk that a sender may encrypt sensitive data
for it using any one of its component keys in isolation. Composite
Or is a direct tradeoff of lower security for increased migration
flexibility.
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2.3. Algorithm parameters
The composite key transport using encryption mode does not require
additional parameters, and therefore any associated Params are
ABSENT.
2.4. Encryption process
The process for performing Composite Key Transport using Encryption
primitives is as follows:
The first n-1 one-time-pad keys are random bit strings of the same
length as the content encryption key. The final one-time-pad key is
computed by XOR'ing the content encryption key with each of n-1
previous keys.
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Input:
n The number of recipient component public keys
P1, P2, .., Pn Recipient component public keys
A1, A2, .., An Cryptographic algorithms to be used with
public keys P1, P2, .., Pn
CEK The Content Encryption Key
SIZE The size of the Content Encryption Key in bits
Output:
E1, E2, .., En EncryptedKey values corresponding to each recipient
component public key
Intermediate values:
S1, S2, .., Sn-1 One-time-pad secret keys to be encapsulated by each
component algorithm
C One-time-pad ciphertext of the CEK under S1, S2, .., Sn-1
Generation Procedure:
1. If recipient public key is of type id-composite-or-key, determine the
index of the last recipient public key to be encrypted for
i_last := index of last Pi to be encrypted for
Else,
i_last = n
2. To generate secret keys Sn, compute the following
C = CEK
for i := 1 to n
a. If id-composite-or-key and Pi is to be skipped
Ei := emptyOctetString
continue to next i
b. If i == i_last
Ei = encrypt(C, Pi, Ai)
break
Else,
Si := random_bits(SIZE)
C := C XOR Si
Ei = encrypt(Si, Pi, Ai)
3. Output E1, E2, .., En
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Where "random_bits(SIZE)" is a cryptographically-secure random bit
generator outputting SIZE bits, and where "emptyOctetString" is the
octet string of length 0.
EDNOTE: we currently do not define a composite algorithmID type to
carry A1, A2, .., An. We may need to add one analogously to the
CompositeParams ::= SEQUENCE SIZE (2..MAX) OF AlgorithmIdentifier
that we have in the composite signutares draft.
If the sender does not support Composite Or encryption, this
algorithm may be simplified by omitting step 1, 2a, and the if i ==
i_last statement in 2b.
The design intent is that Composite Or encryption with a single
recipient key collapses to being equivalent to direct encryption of
the CEK.
2.5. Decryption process
To obtain the content-encryption key from a CompositeEncryptedKey,
each component algorithm MUST be used to decrypt the set of one-time-
pad keys. The keys are then XOR'ed together to recover the content
encryption key.
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Input:
n The number of recipient component public keys
SK1, SK2, .., SKn Recipient component secret keys
A1, A2, .., An Cryptographic algorithms to be used with
public keys P1, P2, .., Pn
E1, E2, .., En EncryptedKey values corresponding to each recipient
component public key
Intermediate values:
S1, S2, .., Sn One-time-pad keys and ciphertext to be decapsulated
by each component algorithm
Output:
CEK The Content Encryption Key
Generation Procedure:
1. Recover each one-time-pad key
for i := 1 to n
if Ei == emptyOctetString
Si := emptyOctetString
Else,
Si := decrypt(Ei, SKi)
2. Recover the CEK;
For each one-time-pad key
CEK = S1
for i := 2 to n
if Si != emptyOctetString
CEK = CEK XOR Si
3. Output CEK
The if statement in step 1 (and ensuring its proper bit length for
the XOR in step 2) is the only modification required to support
Composite Or encryption. The designers have intentionally omitted a
check that the recipient key is of type id-composite-or-key because
even if the sender erroneously used composite or subset encryption
for a recipient key which is not of type id-composite-or-key, the
damage has already been done by encrypting and transmitting the data,
no further harm can be done by decrypting it. However, where
appropriate, clients SHOULD indicate a warning to users that this
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data was transmitted with weaker encrypting than their public key
allows.
EDNOTE: investigate whether this is actually a special case of the
next mechanism, and therefore both sections can be folded together.
3. Composite Key Transport using Encryption and KEM primitives
This composite encryption mode is the generalization of the mode
defined in Section 2 to support a composite recipient public key
which may contain a mixture of one or more encryption component
algorithms with zero or more key encapsulation mechanism (KEM)
component algorithms.
This mode is compatible with protocols requiring a key transport
primitive, such as CMS' KeyTransRecipientInfo [RFC5652].
Security consideration: for a recipient composite public key to be
applicable to this mode, all component KEMs MUST produce a shared
secret whose bits are independent and uniformly distributed (aka
"uniformly IID" or "uniformly random" or "full entropy") and
therefore the shared secret is safe to use direcly as a symmetric
key. If a recipient public key contains component KEMs which are not
know to have this property, then implementors SHOULD use the more
general mode described in Section 4 which incorporates the use of a
key derivation function. See Section 7.1 for a further discussion of
this security consideration.
EDNOTE: also put this in the Security considerations section.
3.1. Algorithm Identifier
The id-alg-composite-kem object identifier MUST be used to identify
the usage of this mode
id-alg-composite-kem OBJECT IDENTIFIER ::= {
id-alg-composite-encryption OBJECT IDENTIFIER ::= {
joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027)
Algorithm(80) Composite(4) id-alg-composite-kem(5)}
EDNOTE: this is a temporary OID for the purposes of prototyping.
Permanent OIDs should be requested from IANA, see Section 6.
3.2. Public key and key usage
The recipient MUST have a composite public key which supports key
transport or key encapsulation operations. Where the recipient
public key has an associated keyUsage as specified in [RFC5280], it
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MUST have keyUsage: keyEncipherment. In other words, the mechanism
specified in this section applies only if all of the recipient's
public keys are encryption or KEM algorithms.
In addition, for a recipient composite public key to be applicable to
this mode, all component KEMs MUST be capable of producing a shared
secret of SIZE bits, where SIZE is the length in bits of the content
encryption key (CEK) to be transported. This is assumed for the
remainder of this section.
3.2.1. Composite-OR
The design intent of this mode is to support migration scenarios
where a recipient has been provisioned with a composite key
containing algorithms that its peers may not yet support. This mode
allows the sender to encrypt for a subset of the recipient's public
keys. Support for Composite OR subset encryption is indicated by the
recipient at key generation time by marking its composite key with
the id-composite-or-key algorithm identifier as defined in ~~~cite
properly draft-ounsworth-pq-composite-keys~~~.
Policy mechanisms defining allowed subets of algorithms could be
applied here, but are out of scope of this document. As defined in
this document, a recipient marking their public key as id-composite-
or-key must accept the risk that a sender may encrypt sensitive data
for it using any one of its component keys in isolation. Composite
Or is a direct tradeoff of lower security for increased migration
flexibility.
3.3. Algorithm parameters
The composite key transport using encryption and KEM mode does not
require additional parameters, and therefore any associated Params
are ABSENT.
3.4. Encryption process
Given these conditions are met, the encryption process defined in
Section 2.4 is modified as follows:
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Input:
n The number of recipient component public keys
P1, P2, .., Pn Recipient component public keys
CEK The Content Encryption Key
SIZE The size of the Content Encryption Key in bits
Output:
E1, E2, .., En EncryptedKey values corresponding to each recipient
component public key
Intermediate values:
S1, S2, .., Sn One-time-pad secret keys to be encapsulated by each
component algorithm
Generation Procedure:
1. If recipient public key is of type id-composite-or-key, determine the
index of the last recipient public key to be encrypted for
i_last := index of last Pi to be encrypted for
Else,
i_last = n
2.
for i := 1 to n
a. if id-composite-or-key and Pi is to be skipped
Ei := emptyOctetString
continue to next i
b. If i == i_last
continue to next i
Else, if Pi is of type KEM:
Si,Ei := encaps(Pi)
CEK := CEK XOR Si
Else:
Si := random_bits(SIZE)
CEK := CEK XOR Si
Ei := encrypt(Si, Pi)
3. Encrypt the final CEK value
Ei_last = encrypt(CEK, Pi_last)
4. Output E1, E2, .., En
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Where "random_bits(SIZE)" is a cryptographically-secure random bit
generator outputting SIZE bits, and where "emptyOctetString" is the
octet string of length 0.
If the sender does not support Composite Or encryption, this
algorithm may be simplified by omitting step 1, 2a, and the if i ==
i_last statement in 2b.
The design intent is that Composite Or encryption with a single
recipient key collapses to being equivalent to direct encryption of
the CEK.
3.5. Decryption process
The decryption process defined in Section 2.5 applies directly where
"decrypt()" is substituted for "decaps()" when the underlying
primitive is a KEM.
4. Composite Key Exchange
This mode is the most general in that it supports a composite
recipient public key which MAY contain an arbitrary mixture of
encryption, key encapsulation mechanism (KEM), and key agreement
component algorithms. Due to the nature of key agreement algorithms,
this mode cannot take a content encryption key as input, but instead
generates a master shared secret as an output. As such, the
nomenclature in this mode differs from the modes above.
This mode is compatible with protocols requiring a key agreement
primitive, such as CMS' KeyAgreeRecipientInfo [RFC5652].
Composite key exchange uses the underlying primitive to either
encrypt for, encapsulate, or interactively do key agreement with each
of the recipient's public keys, then all shared secrets are
concatenated together and a KDF is applies as prescribed by NIST SP
800-56Cr2 [SP80056cr2].
4.1. Algorithm Identifier
The id-alg-composite-keyex object identifier MUST be used to identify
the usage of this mode
id-alg-composite-keyex OBJECT IDENTIFIER ::= {
id-alg-composite-encryption OBJECT IDENTIFIER ::= {
joint-iso-itu-t(2) country(16) us(840) organization(1) entrust(114027)
Algorithm(80) Composite(4) id-alg-composite-encryption(6) }
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EDNOTE: this is a temporary OID for the purposes of prototyping.
Permanent OIDs should be requested from IANA, see Section 6.
4.2. Public key and key usage
The recipient MUST have a composite public key which supports key
transport, key encapsulation, or key exchange operations. Where the
recipient public key has an associated keyUsage as specified in
[RFC5280], it MUST have keyUsage: keyEncipherment, keyAgreement.
This mode is the most general and places the fewest restrictions on
the recipient public key.
EDNOTE: I think this violates our public key draft where we say that
the public key's KU MUST apply to all components. ... we did not want
mixing of signatures and encryption keys, but I think in this case we
do want to allow mixing of keyEncipherment and keyExchange keys. Not
sure how to fix that.
4.2.1. Composite-OR
The design intent of this mode is to support migration scenarios
where a recipient has been provisioned with a composite key
containing algorithms that its peers may not yet support. This mode
allows the sender to encrypt for a subset of the recipient's public
keys. Support for Composite OR subset encryption is indicated by the
recipient at key generation time by marking its composite key with
the id-composite-or-key algorithm identifier as defined in ~~~cite
properly draft-ounsworth-pq-composite-keys~~~.
Policy mechanisms defining allowed subets of algorithms could be
applied here, but are out of scope of this document. As defined in
this document, a recipient marking their public key as id-composite-
or-key must accept the risk that a sender may encrypt sensitive data
for it using any one of its component keys in isolation. Composite
Or is a direct tradeoff of lower security for increased migration
flexibility.
4.3. Algorithm parameters
The composite key exchange mode requires additional parameters to
specify the KDF used to combine shared secrets into a master shared
secret.
Params ::= KeyDerivationAlgorithmIdentifier
The KeyDerivationAlgorithmIdentifier type is specified in [RFC5652].
The KeyDerivationAlgorithmIdentifier definition is repeated here for
completeness.
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KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier
4.4. Encapsulation Process
Composite key exchange uses the underlying primitive to either
encrypt for, encapsulate, or interactively do key agreement with each
of the recipient's public keys, then all shared secrets are
concatenated together and a KDF is applies as prescribed by NIST SP
800-56Cr2 [SP80056cr2].
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Input:
P1, P2, .., Pn Public keys for the n component encryption
algorithms, a CompositePublicKey
SIZE The size, in bits, for shared secrets to be combined by both
parties into a content encryption key. This value SHOULD correspond
to the size of the content encryption key.
KDF A key derivation function
Output:
E1, E2, .., En EncryptedKey values corresponding to each recipient
component public key
M Master shared secret
Ciphertext and master secret Generation Procedure:
1. Generate a set of one-time-pad secret keys of
the same length as the content encryption key
for i := 1 to n
a. if id-composite-or-key and Pi is to be skipped
Si = emptyOctetString
Ei := emptyOctetString
continue to next i
b. if P1 is of type KEM or keyExchange:
Si,Ei := encaps(Pi)
else:
Si := random_bits(SIZE)
Ei := encrypt(Si, Pi)
2. Generate Z via concatenation
Z = S1 || S2 || .. || Sn
3. Generate the master shared secret via a KDF
M = KDF(Z)
4. Output M
Output E1, E2, .., En
Where "emptyOctetString" is the octet string of length 0 that serves
as a no-op or identity element for the concatenation in step 2.
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In cases where KDF is extensible output function, the length of M
must be carried in the KeyDerivationAlgorithmIdentifier defined in
Section 4.3.
EDNOTE: It isn't clear to us how one uses the defined HKDF
algorithmid (RFC 8619) here. Those OIDs specify a hash, but no
output length or seed or info parameter either implicitely or
explicitely. But we also don't see how it would be used with CMS
either, for the same reason. ..?
If the sender does not support Composite Or encryption, this
algorithm may be simplified by omitting step 2a.
EDNOTE: investigate whether step 3 really belongs here, or whether
the surrounding protocol (ex. CMS EnvelopedData) will perform a
final KDF anyways. We believe that outputting an IID master secret
is consistent with modern KEM behaviour.
4.5. Decapsulation Process
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Input:
n The number of recipient component public keys
SK1, SK2, .., SKn Recipient component secret keys
E1, E2, .., En EncryptedKey values corresponding to each recipient
component public key
KDF A key derivation function
Intermediate values:
S1, S2, .., Sn Shared secrets to be encapsulated by
each component algorithm
Output:
M Master shared secret
Master Secret Recovery Procedure:
1. Recover each shared secret
for i := 1 to n
if Ei == null
Si = EMPTY_STRING
Si := decrypt_or_decaps(Ei, SKi)
2. Generate Z via concatenation
Z = S1 || S2 || .. || Sn
3. Generate the master shared secret via a KDF
M = KDF(Z)
4. Output M
"EMPTY_STRING" indicates a string or byte array of length zero so
that that value as essentially omitted from the concatenation in step
2.
The if statement in step 1 is the only modification required to
support Composite Or encryption. The designers have intentionally
omitted a check that the recipient key is of type id-composite-or-key
because even if the sender erroneously used composite or subset for a
recipient key which is not of type id-composite-or-key, the damage
has already been done by generating a master secret and potentially
transmitting data encrypted with it, no further harm can be done by
decrypting it. However, where appropriate, clients SHOULD indicate a
warning to users that this data was transmitted with weaker
encrypting than their public key allows.
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5. In Practice
This section addresses practical issues of how this draft affects
other protocols and standards.
6. IANA Considerations
The following OIDs are to be assigned by IANA. The authors suggest
that IANA assign OIDs for composite encryption on the id-pkix arc:
id-alg-composite OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) algorithms(6) composite(??) id-alg-composite-encryption(??)}
id-alg-composite-kem OBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) algorithms(6) composite(??) id-alg-composite-kem(??)}
id-alg-composite-keyex OBJECT IDENTIFIER ::= {
iOBJECT IDENTIFIER ::= {
iso(1) identified-organization(3) dod(6) internet(1) security(5)
mechanisms(5) pkix(7) algorithms(6) composite(??) id-alg-composite-encryption(??)}
7. Security Considerations
7.1. IID property of KEM primitives
Composite Key Transport using Encryption and KEM primitives defined
in Section 3 directly uses the shared secret output from the
underlying KEM primitevas as a one-time-pad key to encrypt the CEK.
Therefore the output of the KEM primitive of needs to meet the
security properties of a one-time-pad key, namely that its bits are
independent and identically distributed (IID). In particular, key
agreement schemes such as ECDH or SIKE do not produce shared secrets
that meet this requirement and therefore MUST use the fully general
mechanism Composite Key Exchange defined in Section 4.
EDNOTE: Should this be brough to CFRG to decide which KEMs are
appropriate to use with this mechanism? It may be possible that we
need to run the KEM output through a KDF; but we're trying to avoid
needing to carry a KDF AlgID here.
7.2. Composite-OR modes
Composite-OR eases migration at the expense of security. For
composite encryption and key encapsulation, the weakening of security
is entirely at the discretion of the sender, since once data has been
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encrypted and transmitted with weak ciphers, there is nothing the
recipient can do to protect the data against record & decrypt
attacks. Clients performing Composite Or encryption operations MUST
ensure that the recipient's public key is of type id-composite-or-key
before producing a ciphertext with a subset of the recipient's public
keys.
For some cases of composite key exchange, notably when the underlying
key exchange primitive is used in a fully interactive (aka
"ephemeral-ephemeral") mode, the sender cannot begin encrypting data
until the recipient has completed the key exchange. The recipient
SHOULD reject the connection if one or more null ciphertexts are
encountered when the recipient's public key is not of type id-
composite-or-key.
7.3. Policy for Deprecated or Unacceptable Algorithms
Within the context of composite encryption, the sender holds the
responsibility to ensure that chosen algorithms are of sufficient
strength prior to encrypting and transmitting sensitive data under
them. Composite is designed to provide security redundancy and to
remain strong as long as at least one of the component algorithms
remains strong.
When encrypting for a Composite-OR public key and using a subset of
the recipient's public key, then these redundancy guarantees no
longer apply. The sender SHOULD employ a policy mechanism to ensure
that they are using a combination of algorithms of sufficient
strength. Even though this document does not define such a policy
mechanism, but implementors making use of Composite-OR encryption are
strongly encouraged to implement a policy mechanism.
8. Appendices
8.1. ASN.1 Module
~~ TODO ~~
8.2. Intellectual Property Considerations
The following IPR Disclosure relates to this draft:
https://datatracker.ietf.org/ipr/3588/
We are grateful to all, including any contributors who may have been
inadvertently omitted from this list.
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This document borrows text from similar documents, including those
referenced below. Thanks go to the authors of those documents.
"Copying always makes things easier and less error prone" -
[RFC8411].
8.3. Making contributions
Additional contributions to this draft are weclome. Please see the
working copy of this draft at, as well as open issues at:
https://github.com/EntrustCorporation/draft-ounsworth-pq-composite-
encryption
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
DOI 10.17487/RFC2986, November 2000,
.
[RFC3211] Gutmann, P., "Password-based Encryption for CMS",
RFC 3211, DOI 10.17487/RFC3211, December 2001,
.
[RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210,
DOI 10.17487/RFC4210, September 2005,
.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, .
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[RFC8411] Schaad, J. and R. Andrews, "IANA Registration for the
Cryptographic Algorithm Object Identifier Range",
RFC 8411, DOI 10.17487/RFC8411, August 2018,
.
[SP80056cr2]
NIST, "SP 800-56c Rev. 2: Recommendation for Key-
Derivation Methods in Key-Establishment Schemes", August
2020.
[X.690] ITU-T, "Information technology - ASN.1 encoding Rules:
Specification of Basic Encoding Rules (BER), Canonical
Encoding Rules (CER) and Distinguished Encoding Rules
(DER)", ISO/IEC 8825-1:2015, November 2015.
Authors' Addresses
Mike Ounsworth
Entrust Limited
2500 Solandt Road -- Suite 100
Ottawa, Ontario K2K 3G5
Canada
Email: mike.ounsworth@entrust.com
John Gray
Entrust Limited
Email: john.gray@entrust.com
Serge Mister
Entrust Limited
Email: serge.mister@entrust.com
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