wdiff draft-turner-cmc-serverkeygeneration-00.txt draft-turner-cmc-serverkeygeneration-01.txt

NETWORK WORKING GROUP J. Schaad
Internet-Draft Soaring Hawk Consulting
Intended Status: Proposed Standard S. Turner
Expires: January 31, September 28, 2014 IECA
P. Timmel
National Security Agency
July 30, 2013
March 27, 2014

CMC (Certificate Management over Cryptographic Message Syntax)
Extensions: Server Side Key Generation
draft-turner-cmc-serverkeygeneration-00.txt
draft-turner-cmc-serverkeygeneration-01.txt

Abstract

This document defines a set of extensions to the Certificate
Management over Cryptographic Message Syntax (CMC) protocol that
addresses the desire to support server-side generation of keys. keys for
certificates. This service is provided by the definition of
additional control statements within the CMC architecture.
Additional CMC errors are also defined.

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 http://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
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."

I-D CMC Extensions: Server Key Generation July 30, 2013

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must

I-D CMC Extensions: Server Side Key Generation March 27, 2014

include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.

Table of Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4 3
1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 4
2. Protocol Flows for Supported Scenarios . . . . . . . . . . . . 5
2.1. Shared Secret for Authentication and Key Protection . . . 7 9
2.2 Shared Secret for Authentication and Ephemeral Key for
Protection . . . . . . . . . . . . . . . . . . . . . . . . 9 11
2.3. Certificate for Authentication and Ephemeral Key for
Protection . . . . . . . . . . . . . . . . . . . . . . . . 10 12
2.4. Certificate for Authentication and Key Protection . . . . . 12 13
2.4.1. Same Certificate for Authentication and Key
Protection . . . . . . . . . . . . . . . . . . . . . . 12 14
2.4.2. Different Certificates for Authentication and Key
Protection . . . . . . . . . . . . . . . . . . . . . . 12 14
2.5. RA Scenarios . . . . . . . . . . . . . . . . . . . . . . . 12 14
2.5.1. RA-Generated Key Scenarios . . . . . . . . . . . . . . 14 16
2.5.2. RA-Involved Scenarios . . . . . . . . . . . . . . . . 17 19
3. Generating PKIData and PKIResponse . . . . . . . . . . . . . . 18 20
3.1. Client Requests . . . . . . . . . . . . . . . . . . . . . 19 21
3.2. RA Processing of Client Requests . . . . . . . . . . . . . 19 21
3.3. CA Processing . . . . . . . . . . . . . . . . . . . . . . 21 23
3.4. RA Processing of CA Responses . . . . . . . . . . . . . . 23
3.4. 25
3.5. Client Processing of Responses . . . . . . . . . . . . . . 25 27
4. Shrouding Algorithms . . . . . . . . . . . . . . . . . . . . . 25 27
4.1. Shroud With with a Public Key . . . . . . . . . . . . . . . . . 26 28
4.2. Shroud With with a Shared-Secret Key . . . . . . . . . . . . . . 27 29
5. Returned Key Format . . . . . . . . . . . . . . . . . . . . . . 28 30
6. Server-Side Key Generation . . . . . . . . . . . . . . . . . . 29 31
6.1. Server-Side Key Generation Request Attribute . . . . . . . 29 31
6.2. Server-side Key Generation Response . . . . . . . . . . . . 31 33
7. Additional Error Codes . . . . . . . . . . . . . . . . . . . . 32

I-D CMC Extensions: Server Key Generation July 30, 2013 34
8. Proof-of-Possession . . . . . . . . . . . . . . . . . . . . . 33 35
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 33 35
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 35 37
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 38
11.1 Normative References . . . . . . . . . . . . . . . . . . . 35 38
12.2 Informative References . . . . . . . . . . . . . . . . . . 36 38
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . . 37 39
Appendix B. Additional Message Flows . . . . . . . . . . . . . . . 37 40
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . . 41 43
B.1. Client Requests . . . . . . . . . . . . . . . . . . . . . . 41 43
B.1.1. Shroud with Certificate . . . . . . . . . . . . . . . . 41 43

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B.1.2. Shroud with Public Key . . . . . . . . . . . . . . . . 41 43
B.1.3. Shroud with Shared Secret . . . . . . . . . . . . . . . 41 43
B.2. CA-Generate Key Response . . . . . . . . . . . . . . . . . 41 43
B.3. RA-Generate Key Response . . . . . . . . . . . . . . . . . 41 43
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 41 44

1 Introduction

This document defines a set of extensions to and errors for
Certificate Management over Cryptographic Message Syntax (CMC)
[RFC5272] that allows for server-side generation of key material. material for
certificates [RFC5280]. There are strong reasons for providing this
service:

o Clients may have poor, unknown, or non-existent key generation
capabilities. The creation of private keys relies on the use of
good key generation algorithms and a robust random number
generator. Server key generation can use specialized hardware
that may not always be available on clients.

o Central storage of keys may be desired in some environments to
permit key recovery. This document only addresses a request to
archive server-generated keys; archival of locally generated keys
and the control to retrieve archived keys is out-of-scope.

o Server key generation may be useful for provisioning keys to
disconnected clients (e.g., clients that receive keys from a fill
device [RFC4949] because the clients are not able to connect to
the server due to an air gap).

These extensions to the CMC protocol are designed to provide server
key generation without adding any additional round trips to the
enrollment process; however, additional round trips may be required
based on the mechanism chosen to protect the returned key.

Section 2 describes the enrollment scenarios supported. Section 3
provides CMC requirements. Sections 4 and 5 describe the concepts and

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structures used in transporting private keys between the server and
client applications. Section 6 describes the structure and processes
for server-side key generation. Section 7 describes additional CMC
error codes. Section 8 describes additional exchanges when the
server requires the client provide Proof-of-Possession (POP).
Appendix A provides the ASN.1 module for the CMC controls and errors.
Appendix B provides example encodings.

1.1. Terminology
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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 RFC
2119 [RFC2119].

The terminology in [RFC5272] and in [RFC6402] apply to this profile.
Additionally, familiarity with CMS's (Cryptographic Message Syntax)
[RFC5652] SignedData, AuthenticatedData, and EnvelopedData content
types is
assumed [rfc5652]. assumed.

1.2. Definitions

This section defines some of the terms that are used in this
document:

o Dual-use: Applies to certificates or keys. Certificates that can
be used to verify both digital signatures and to perform key
management, when the Key Usage extension is set to
digitalSignature and either keyAgreement or keyEncipherment, and
keys whose intended use is digitalSignature and either
keyAgreement or keyEncipherment.

o Encryption-capable: Applies to certificates or keys.
Certificates that can be used for key management (i.e., the Key
Usage extension includes keyAgreement or keyEncipherment) and
keys that can be used for key management. This refers to either
dual-use or encryption-only certificates/keys.

o Encryption-only: Applies to certificates or keys. Certificates
that can only be used for key management, when the Key Usage
extension is set to either keyAgreement or keyEncipherment, and
keys whose only intended use is keyAgreement or keyEncipherment.

o Ephemeral key [SP-800-57]: A cryptographic key that is generated
for each execution of a key establishment process and that meets
other requirements of the key type (e.g., unique to each message
or session). Often, ephemeral keys are linked to key agreement
algorithms; however, this document uses the term ephemeral keys

I-D CMC Extensions: Server Key Generation July 30, 2013
to apply to both key transport and key agreement keys. The
ephemeral key has two parts: the private part and the public
part. The client provides the public part to the server to allow
the server to protect the server-generated keys. Note that an
ephemeral key has a security advantage by being unique to the
session; it SHOULD be freshly generated when possible, but MAY be
pre-placed when local key generation is of poor or unknown
quality (see Section 9). An ephemeral key is innately
unauthenticated, and so must be carried in a suitably
authenticated protocol.

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o Identification: A generic term for a process by which a name,
generally assigned by a server, is used to match a request
against a known entity. Identification can be either
authenticated (a subject name in a certificate) or
unauthenticated (a text string).

o Perfect Forward Secrecy (PFS): For this protocol, it is the
property that the compromise of long-term keying material does
not lead to the compromise of the new long-term keying material
carried in the protocol.

o Shared Secret: A value known to two or more entities in advance
of a protocol session in which it will be used, and intended to
be unknown to any others. In this document, the value must be a
suitable basis for derivation of a MAC (Message Authentication
Code) or encryption key. Pass phrases that are used as a shared
secret must be treated as confidential by the holders of the
secret.

o Shrouding: A generic term to cover methods of masking the content
of an object from unauthorized viewers. viewers, taken from RSA's PKCS
specifications. The most common method of shrouding used is
encryption of the data at the application layer. This document
defines two shroud shrouding methods that employ encryption at the
application layer but other shrouding methods can be defined that
don't employ encryption at the application layer.

o Signature-capable: Applies to certificates or keys. Certificates
that can be used to verify signatures and keys that can be used
to generate digital signatures. This refers to either dual-use
or signature-only certificates/keys.

o Signature-only: Applies to certificate or keys. Certificates
that can only be used to verify digital signatures, when the Key
Usage extension is set to digitalSignature, and keys whose only
intended key usage is digital signature.

In this document the server is the entity that generates the key.
This can be either the RA or CA.

2. Protocol Flows for Supported Scenarios

This section describes five the supported scenarios, and specifies the CMC
requests and responses to support them:

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1. In the first scenario (see Section 2.1), clients Clients use a shared secret (e.g., password) (see Section 1.2) to
provide authentication and request that the server use the same
shared secret to encrypt the server-generated keys. Obviously the
security in this scenario, and any scenario in which

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2. Clients use a shared secret to provide authentication and request
that the server use an ephemeral key (see Section 1.2) to encrypt
the server-generated keys.

3. Clients use a key pair previously certified by a CA (i.e., a
private key and a certificate) to support digital signature
authentication and request that the server use an ephemeral key
to encrypt the server-generated keys.

4. Clients use a key pair previously certified by a CA (i.e., a
private key and a certificate) to support digital signature
authentication and request that the server use a certificate to
encrypt the server-generated keys. Some additional details:

* If the client's authentication certificate is used, is predicated on keeping signature-only,
then the shared secret
secret. Here, client also needs an encryption-capable certificate
that the server will use to protect the private key.
* If the client's certificate is dual-use, then the client need only know a shared secret
needs the one certified key pair to generate the SignedData
that encapsulates the certificate request and to decrypt the
EnvelopedData that encapsulates the server-generated key.

The characteristics of the four scenarios are as follows:

o Scenarios that employ shared secrets (Scenarios 1 and 2) are
consider by some to be more human friendly than scenarios that
employ certificates because there is less initial overhead
because a certificate, CA is not initially and clients do not need to find
their smartcard, remember their PIN, etc. Also, shared secrets
can be used for rekey but this requires distributing a new shared
secret with the newly created private key; this behavior is not
described in this document.

o For scenarios that use a shared secret for key protection (i.e.,
Scenario 1), the client needs to maintain secrecy of the shared
secret, which is often difficult for clients, for the lifetime of
the key (see Section 9). Further, using the same
secret for authentication and encryption potentially increases
exposure of 9) to ensure that if the server-generated
and encrypted private key among the entities
comprising the RA is intercepted and CA. Note that the use of split-shared shared secret (i.e., one for authentication and one for encryption)
would alleviate the last concern, but this is beyond
later disclosed/discovered that the scope of
this document.

2. encrypted private key private
key can not be decrypted. In other words, no PFS is provided if
a shared secret is used to protect the second scenario (see Section 2.2), clients returned encrypted private
key.

o For scenarios that use a shared secret to provide for client authentication
(i.e., Scenarios 1 and request that 2), the server use
an ephemeral key (see Section 1.2) client needs to encrypt the server-
generated keys. Here, maintain secrecy
of the client can still use their shared secret to request the certificate but now once encrypted the
private key for as long as it is needed for
authentication. Servers can only become known to the client. Also, the key
that protects mitigate this issue by limiting the response is ephemeral even if
lifetime of the
authentication shared secret is not.

3. In the third scenario (see Section 2.3), clients to one-time-use.

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o For scenarios that use a
certificate ephemeral keys to support digital signature authentication protect the returned
private key (i.e., Scenarios 2 and
request that 3):

* PFS is provided presuming the server use an ephemeral key is forgotten as
well as any other information necessary to encrypt generate the
server-generated keys. Here,
ephemeral key after the primary motivation certificate request is that successfully
processed by the client.

* Client use of the ephemeral key mitigates the risk of
compromise of a pre-
existing pre-existing certificate and key while in the
supply chain (see Section 9).

4. In the fourth scenario (see Section 2.4), clients use a
certificate to support digital signature authentication and
request

o For scenarios that the server only use certificates (i.e., Scenario 4), PFS
is not provided because the the client uses a certificate to encrypt long-term private
key for private key protection and if compromised the server-
generated keys. Here, clients next key
can request a certificate with a
certificate or certificates also be compromised.

Note that the client already has. There there are still security existing CP-based (Certificate Policy)
requirements for protecting the private key(s) associated with
the certificate(s) (see Section 9). If two
certificates are used (i.e., one

In the scenarios above, the entity generating the key pair can be the
Certification Authority (CA) or the Registration Authority (RA), in
fact the RA or CA can use a separate key generator device (e.g. a
hardware security module) but interactions with that device is out of
scope. Sections 2.1-2.4 depict protocol flows for encryption scenarios with a
client and one CA only where the CA generates the keys. Section 2.5
depicts protocol flows for
digital signatures), scenarios that involve the separation between signing and key
exchange functions is preserved, client, RA, and supports
CA where the RA generates the keys. Section 2.5 also depicts
protocol flows for so-called "RA-involved" scenarios where the CA
generates the keys but the RA performs identity checks.

All scenarios described herein require the server to have some a broader range
priori knowledge of
algorithms. Some additional details: the client. How this knowledge is obtained is
out of scope, but the scenarios offer different
opportunities/constraints for obtaining the information:

o If In the shared secret cases, the method for prior knowledge has to
cope with securely delivering and storing the shared secret.
o In the certificate cases, there are two alternatives:
* The key pair was generated by the server, so the client's authentication certificate is signature-only,
then
evidence of that prior knowledge.
* The certificate was issued from another CA. Even when there
are no grounds for cross-certification, the client also needs certificate can
still be used as an encryption-capable artifact for registration/enrollment prior
to the client making a certificate request, which is
advantageous because the key pair bound to the identifier in
the certificate enables the server to authenticate the source

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(i.e. having March 27, 2014

of the Key Usage extension set eventual certificate request and positively link it to either keyAgreement
or keyEncipherment)
the registration information.

Note: The initial certificate key pairs could be considered a
special case of the shared secret scenario that improves on the
security of the shared secret mechanism and mitigates some of the
management burden and cost. For example, the certificates could be
special-purpose--issued by the manufacturer and recognized by the
server will use solely for authentication against a registration list (i.e.,
not usable for anything else). If a manufacturer initializes the
device with a shared secret, then that shared secret has to be
distributed securely to protect the
private key.

o eventual enrolling CA via the device
owner, but independently of the device. If the client's manufacturer
instead installs a key pair and generic certificate, the
certificate is dual-use, then can take the client only
needs place of the one certified shared secret that would
otherwise have to be independently provided to the central key pair
generation CA. The management process is roughly the same, but the
information that has to be handled now longer has to be kept
secret. That means there are many more options for how that
information can be managed and distributed.

Normally the client's identifier and the shared secret are generated
by the server and then securely transported to the client, there is
no practical reason why this cannot be done in the opposite
direction. In this case, the client will generate an identifier,
shared secret and an ephemeral key. When the SignedData server receives the CMC
message, it recognizes that encapsulates it does not correspond to an existing
identifier/secret pair and puts the certificate request on hold. The end-entity
then communicates the identifier and secret to decrypt the
EnvelopedData that encapsulates the server-generated key.

5. server via an out-
of-band means. The first four scenarios assume server then performs the necessary user
management dealing with identity validation and certificate setup.
If that passes and the keys are generated at ephemeral passes applicable public key
validation tests, then the Certification Authority (CA) certificate will be issued and that the interactions do
response returned protected with ephemeral key. If it does not
involve a Registration Authority (RA) [RFC5280]. But, key
generation pass
checking then the certificate will fail to issue. An ephemeral is
generated by an RA the client, to ensure PFS is also supported as well provided as scenarios when the RA verifies client may
not get the client's identity (see Section 2.5). same degree of confidentiality because the client is
unaware how the private key has been provided.

Note that there is no scenario where the response is protected with a
key/certificate that only supports digital signatures. This is
because the "protection" afforded by digital signatures alone does
not include confidentiality, which is required to ensure that the
server-generated private key is only disclosed to the client.

Note also that there is also no scenario where the client uses an
encryption-only certificate and is unable to generate a digital
signature to provide authentication. This is because the

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"protection" afforded by encryption-only certificates does not
include authentication. Technically, there are dual-service Authenticated
Encryption with Associated Data (AEAD) algorithms (i.e., dual-service
algorithms) that support both authentication and encryption but their
use is beyond the scope of this document.

In all of the scenarios, the client can validate that the response
came from the CA or RA by validating the digital signature on the
SignedData to a Trust Anchor (TA). After the EnvelopedData is
decrypted, the client can 1) verify that the private key is
associated with the public key in the returned certificate and/or 2)
verify that the certificate validates back to an authorized TA.

The scenarios in the subsections assume that the transaction
identifier and nonce controls are used for transaction processing and
replay protection, respectively, but they are optional, as specified
in [RFC5272]. Also, the scenarios assume the CMC Status Information
v2 control is not included when the response is a success, as allowed
by [RFC5272]. See Appendix B for additional example scenarios.

A client requesting a certificate for a different name than one
already issued, either based on the certificate being used in
Scenarios 3 and 4 or the name associated with the shared secret in
Scenarios 1 and 2, includes the Change Subject Name attribute to
ensure the server will not reject the request because the name in the
certificate used to sign the request does not match the name in the
request. Note this is not depicted in the diagrams that follow
because the attribute is included within the ServerKeyGenRequest.

2.1. Shared Secret for Authentication and Key Protection

The shared secret allows the server to authenticate the client and
allows the server to encrypt the server-side generated key for the
client. The shared secret is distributed via an out-of-band

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mechanism that is out-of-scope of this document. Also note the
server and client need to share a non-secret identification string
that the client can assert in a request so that the server will know
which shared secret is being used.

When the client generates their its request, the client includes the
following control attributes in a PKIData content type [RFC5272]:
Server Key Generation Request (see Section 6.1), Transaction
Identifier [RFC5272], Sender Nonce [RFC5272], and Identification
[RFC5272]. The Server Key Generation Request control indicates that
the shroudMethod is shroud with shared-secret key (see Section 4.2).
The PKIData is encapsulated in a CMS AuthenticatedData content type
and the password RecipientInfo (i.e., pwri CHOICE) is used [RFC5652].
Note that reqSequence, cmsSequence, and otherMsgSequence are not

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included in the PKIData for the server-side key generation request.
The following depicts this:

+----------------------------------------------------------------+
|AuthenticatedData: ReceipientInfo: RecipientInfo: pwri |
|+--------------------------------------------------------------||
||PKIData: control: ServerKeyGenRequest (ShroudWithSharedSecret)||
|| control: TransationID ||
|| control: SenderNonce ||
|| control: Identification ||
|+--------------------------------------------------------------+|
+----------------------------------------------------------------+

After the server authenticates the client, client and verified the request,
the server generates a response that includes the server-generated
key and any associated parameters in an Asymmetric Key Package
content type [RFC5958]. The Asymmetric Key Package is then
encapsulated within a SignedData SignedData, which is signed by the server, and
that is further encapsulated within an EnvelopedData using the
password RecipientInfo (i.e., pwri CHOICE). The EnvelopedData EnvelopedData, which
is encrypted for the client, is then placed in a PKIResponse
cmsSequence [RFC5272] and the following controls are included in the
PKIResponse: Transaction Identifier, Sender Nonce, Recipient Nonce
[RFC5272], and Server Key Generation Response (see Section 4.2). The
PKIResponse is then encapsulated in a SignedData SignedData, which is signed by
the server, and the client's certificate associated with the server-generated server-
generated key is placed in the outer-most SignedData's certificate certificates
field [RFC5652]. The following depicts this:

+---------------------------------------------------------+
|SignedData: Signed by the CA |
| Client's certificate found here in certificates field |
|+-------------------------------------------------------+|
||PKIResponse: control: TransactionID ||
|| control: SenderNonce ||
|| control: RecipientNonce ||

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|| control: ServerKeyGenResponse ||
|| ||
|| cmsSequence: ||
|| +----------------------------------------+||
|| |EnvelopedData: RecipientInfo: pwri |||
|| |+----------------------+ |+-----------------------------+ |||
|| ||SignedData ||SignedData: Signed by the CA | |||
|| ||+--------------------+| ||+---------------------------+| |||
|| |||AsymmetricKeyPackage|| |||AsymmetricKeyPackage || |||
|| ||+--------------------+| ||+---------------------------+| |||
|| |+----------------------+ |+-----------------------------+ |||
|| +----------------------------------------+||

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|+-------------------------------------------------------+|
+---------------------------------------------------------+

2.2 Shared Secret for Authentication and Ephemeral Key for Protection

The shared secret allows the server to authenticate the client and
the ephemeral key allows the server to use a different key to encrypt
the server-side generated key for the client. The shared secret is
distributed via an out-of-band mechanism that is out-of-scope of this
document. Also note that the client needs an identification string
to allow the server to determine which shared secret is being used.

When the client provides an ephemeral key to protect the response,
the client includes the following control attributes in a PKIData
content type [RFC5272]: Server Key Generation Request control (see
Section 6.1), Transaction Identifier [RFC5272], Sender Nonce
[RFC5272], and Identification [RFC5272]. The Server Key Generation
Request control indicates that the shroudMethod is shroud with public
key and that the bareKey CHOICE is used (see Section 4.1). The
PKIData is encapsulated in an AuthenticatedData content type and the
password RecipientInfo (i.e., pwri CHOICE) is used [RFC5652]. Note
that reqSequence, cmsSequence, or otherMsgSequence are not included.
The following depicts this:

+-------------------------------------------------------------+
|AuthenticatedData: ReceipientInfo: RecipientInfo: pwri |
|+-----------------------------------------------------------+|
||PKIData: control: ServerKeyGenRequest (ShroudWithPublicKey)||
|| control: TransationID ||
|| control: SenderNonce ||
|| control: Identification ||
|+-----------------------------------------------------------+|
+-------------------------------------------------------------+

After the server has authenticated the client, client and verified the
request, the server returns the server-generated key and any
associated parameters in an Asymmetric

I-D CMC Extensions: Server Key Generation July 30, 2013 Key Package content type
[RFC5958]. The Asymmetric Key Package is then encapsulated within a SignedData
SignedData, which is signed by the server, and that is further
encapsulated within an EnvelopedData using the key agreement or key
transport RecipientInfo (i.e., kari or ktri CHOICE). The EnvelopedData
EnvelopedData, which is encrypted for the client, is then placed in a
PKIResponse cmsSequence [RFC5272] and the following controls are
included: Transaction Identifier, Sender Nonce, Recipient Nonce
[RFC5272], and Server Key Generation Response (see Section 6.2). The
PKIResponse is then encapsulated in a SignedData SignedData, which is signed by
the server, and the certificate associated with the server-generated
key is placed in the outer-most SignedData's certificate certificates field

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[RFC5652]. The following depicts this:

+-----------------------------------------------------------+
|SignedData: Signed by the CA |
| Client's certificate found here in certificates field |
|+---------------------------------------------------------+|
||PKIResponse: control: TransactionID ||
|| control: SenderNonce ||
|| control: RecipientNonce ||
|| control: ServerKeyGenResponse ||
|| ||
|| cmsSequence: ||
|| +------------------------------------------+||
|| |EncryptedData: |EnvelopedData: RecipientInfo: kari or ktri|||
|| |+----------------------+ |+------------------------------+ |||
|| ||SignedData ||SignedData: Signed by the CA | |||
|| ||+--------------------+| ||+----------------------------+| |||
|| |||AsymmetricKeyPackage|| |||AsymmetricKeyPackage || |||
|| ||+--------------------+| ||+----------------------------+| |||
|| |+----------------------+ |+------------------------------+ |||
|| +------------------------------------------+||
|+---------------------------------------------------------+|
+-----------------------------------------------------------+

2.3. Certificate for Authentication and Ephemeral Key for Protection

This scenario differs from the scenarios in Sections 2.1 and 2.2 in
that the client encapsulates the PKIData in a SignedData instead of
an AuthenticatedData (i.e., the client uses its private key
associated with its signature-capable certificate to sign the
PKIData) but is similar to Section 2.2 for the response. As implied
in [RFC5272], clients omit the Identification and Identity Proof
controls when using certificates to support digital signature
authentication. A client requesting a certificate for a different
name includes the Change Subject Name attribute to ensure the server
will not reject the request because the name in the certificate used
to sign the request does not match the name in the request.

I-D CMC Extensions: Server Key Generation July 30, 2013

When the client generates their its request, the client includes the
following control attributes in a PKIData content type [RFC5272]:
Server Key Generation Request (see Section 6.1), Transaction
Identifier [RFC5272], and Sender Nonce [RFC5272]. The Server Key
Generation Request control indicates the shroudMethod is shroud with
public key and the bareKey CHOICE is used (see Section 4.1). The
PKIData is encapsulated in a SignedData content type [RFC5652]. Note
that reqSequence, cmsSequence, and otherMsgSequence are not included
in the PKIData for the server-side key generation request. The
following depicts this:

+--------------------------------------------------------------+
|SignedData: Signed by the Client |

I-D CMC Extensions: Server Side Key Generation March 27, 2014

|+------------------------------------------------------------||
||PKIData: control: ServerKeyGenRequest (ShroudWithPublicKey) ||
|| control: TransationID ||
|| control: SenderNonce ||
|+------------------------------------------------------------+|
+--------------------------------------------------------------+

After the server has authenticated the client, client and verified the
request, the server returns the server-generated key and any
associated parameters in an AsymmetricKeyPackage content type
[RFC5958]. The AsymmetricKeyPackage is then encapsulated within a SignedData
SignedData, which is signed by the server, and that is encapsulated
within an EnvelopedData using the key agreement or key transport
RecipientInfo (i.e., kari or ktri CHOICE). The
EnvelopedData EnvelopedData, which
is encrypted for the client, is then placed in a PKIResponse
cmsSequence [RFC5272] and the following controls are included:
Transaction Identifier, Sender Nonce, Recipient Nonce [RFC5272], and
Server Key Generation Response (see Section 6.2). The PKIResponse is
then encapsulated in a SignedData SignedData, which is signed by the server, and
the certificate associated with the server-generated key is placed in
the outer-most SignedData's certificate certificates field [RFC5652]. The
following depicts this:

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|| ||+--------------------+| ||+---------------------------+| |||
|| |||AsymmetricKeyPackage|| |||AsymmetricKeyPackage || |||
|| ||+--------------------+| ||+---------------------------+| |||
|| |+----------------------+ |+-----------------------------+ |||
|| +------------------------------------------+||
|+---------------------------------------------------------+|
+-----------------------------------------------------------+

2.4. Certificate for Authentication and Key Protection

If a client already has been issued a signature-capable certificate,

I-D CMC Extensions: Server Side Key Generation March 27, 2014

then it can use this certificate to authenticate the requests. If
the certificate also indicates support for encryption (i.e., the key
usage extension is set to keyEncipherment or keyAgreement), then the
client can request that the server use the same certificate to
protect the server-generated key (see Section 2.4.1). If the
certificate does not indicate support for encryption, then the client
can provide the server with another certificate to use to encrypt the
server-generated key (see Section 2.4.2). The certificate that
protects the server-generated key MUST be encryption-capable.

These scenarios differ from the scenarios in Section 2.3 in that the
response is protected with a previously certified key instead of an
ephemeral key. As specified in [RFC5272], clients omit the
Identification and Identity Proof controls when using certificates to
support digital signature authentication. A client requesting a
certificate for a different name includes the Change Subject Name
attribute to ensure the server will not reject the request because
the name in the certificate used to sign the request does not match
the name in the request.

2.4.1. Same Certificate for Authentication and Key Protection

This scenario is the same as in Section 2.3. except the Server Key
Generation Request control includes the certificate or certIdentifier
CHOICE instead of the bareKey CHOICE. The certidentifier is
sufficient information for the server since the same certificate will
have already been used for verifying the encapsulating SignedData.

2.4.2. Different Certificates for Authentication and Key Protection

When the client's certificate that

This scenario is used to sign the PKIData does
not support encryption but the client has another certificate that
does, the client can include this other certificate same as in Section 2.3. except the Server Key
Generation Request control's control includes the certificate field. CHOICE instead of
the bareKey CHOICE. When using two certificates, the names in the
two certificates MUST match to ensure the CA will not reject the
request due to name mismatches.

2.5. RA Scenarios
I-D CMC Extensions: Server Key Generation July 30, 2013

In Sections 2.1-2.4, client-to-CA protocol flows were illustrated
where the CA generates the client's key and no RA was involved. This
section illustrates client-to-RA-to-CA protocol flows. The scenarios
divide into two basic categories, according to whether the RA or the
CA generates the key pair.

Regardless of whether the RA or the CA generates the request, the
intent of the RA here is to be transparent. That is, client's
initial the same request regardless of the entity that ultimately
generates the keys.

When the RA generates the key on behalf of the client, the RA
effectively fleshes
out the taggedRequst from the client with the RA-generated public key
and applies POP with the corresponding private key. This becomes the

I-D CMC Extensions: Server Side Key Generation March 27, 2014

requestSequences in a proxy for new PKIData that the RA will send to the CA,
and that repeats the controls received from the client. Then the PA
signs the new PKIData with its own signing key. In this way, the RA
effectively becomes the client from the CA's perspective. However,
the RA and CA already have an established trust relationship (i.e.,
the RA has been issued a certificate from the CA), which might not be
true for the client. The protocol exchange between the RA and CA is
identical to other
situations in which a client requests enrolling with a certificate. This means that CMC Full PKI Request;
therefore, the CA need not know about or support the Server Key
Generation Request and Server Key Generation Response controls. The
PKIResponse to the client now has to accommodate the fact that the
asymmetric key package is generated by the RA, whereas the
certificate is generated by the CA. This necessitates that the RA
intercepts whatever response the CA returns to get the client's
certificate and that the RA generates a signed response that includes
the asymmetric key package as well as the client's certificate. See
section 9 for security considerations.

When the RA participates in the process but does not generate the
key, there are two possibilities. If the RA does not contribute to
the protocol (its effects may be procedural or out-of-band), then it
can simply pass the messages it receives to the other party when
warranted. If that is not warranted, the RA would generate the usual
response for the associated failure. No message flow for this
possibility is included. Alternatively, the RA may be responsible
for processing certain aspects of the request, and needs to vouch for
that when forwarding the client request to the CA. The RA does this
per [RFC5272] by embedding the client request in a Full PKI Request
that it signs, containing controls for the processing that it
performs. Here Here, as in Sections 2.1-2.4, the CA needs to fully
understand and support the Server Key Generation Request and Server
Key Generation Response controls, since the CA has to generate the
key and construct the asymmetric key package.

In the figures that follow, SKGReq is the Server Key Generation
Request control (see Section 6.1), the SKGRes denotes the Server Key
Generation Response control (see Section 6.2), TransactionId denotes
the Transaction Identifier control [RFC5272], SenderNonce denotes the
Sender Nonce control [RFC5272], RecipientNonce denotes the Recipient
Nonce control [RFC57272]. [RFC5272]. AKP denotes an Asymmetric Key Package
[RFC5958] (i.e., the private key). Also, {} denotes encryption
(i.e., EnvelopedData), <> denotes signatures (i.e., SignedData), with
() identifying some of the information in is carried in unsignedAttrs
for clarity, and [] denotes authentication (i.e., SignedData or
AuthenticatedData). AuthenticatedData is used when clients use a
shared secret for authentication. control denotes controlSequence,
reqSeq denotes requestSequence, and cmsSeq denotes cmsSequence.
PKCS#10 [RFC2986] [RFC2986], CRMF (Certificate Request Message Format)

I-D CMC Extensions: Server Side Key Generation July 30, 2013 March 27, 2014

[RFC4211], and CRMF (Certificate Request Message Format) [RFC4211] other request message [RFC5272] are the certification requests.
request formats supported.

2.5.1. RA-Generated Key Scenarios

There are some differences in the protocol flows when an RA generates
the key:

o The RA MUST be issued a certificate from the CA. This means all
of the RA-generated PKIData are encapsulated in a SignedData.
Further, the RA's certificate can be used for identification and
linking identity and POP information.

o The RA generates the certification request for the client by:

1. Using the name from the certificate for the key the RA will
use to sign the PKIData.

2. Using the information from the client's request.

3. The RA includes the Change Subject Name control [RFC6402]
either in the PKCS #10 or CRMF TaggedRequest because the POP
and identity linking CMC requirements are that the name
in the certificate used to sign for verifying the PKIData matches signature
must match the name in the
certification certificate request. The Change
Subject Name attribute allows the names to be different.

4. Modifying the information taggedRequest as necessary. Notably adding the
public key but also adding certificate extensions, etc. Note
that the Modify Certificate Template control is not needed as
the RA is generating a new PKIData.

5. Generating the POP information for the certificateRequest
containing the RA-generated keys. For keys that support
digital signatures, the RA includes the POPSigningKey in the
CRMF or the signature in the PKCS #10. For encryption-only
keys, the RA can indicate that it performed POP by including
the RA POP Witness control. Note the CA could force the RA to
prove it has possession of the key with the
encrypted/decrypted POP mechanism for [RFC57272], [RFC5272], but this adds
additional round trips and is discussed later in this section.

6. Signing the certification request (i.e., the PKIData) with the
RA's private key.

The RA can also generate bulk requests (i.e., include more than one
request in cmsSequence) with the Bulk Request control [RFC5272] but
these controls are not depicted in the following sections to simplify
the protocol flows. When the RA is requesting more than one key for

I-D CMC Extensions: Server Side Key Generation July 30, 2013 March 27, 2014

a given client, the RA includes each request in the reqSequence.

The following message flow applies when the RA generates the key. It
supports all of the previously defined choices for authentication and
shrouding. The diagram below depicts use of a shared secret for
authentication by including the Identification control and
encapsulating the client's PKIData in an AuthenticatedData. If a
digital signature is used for authentication, the Identification
control is omitted and the client encapsulates its PKIData in a
SignedData.

Client RA CA
| | |
|----------------->| |
| [PKIData | |
| control: SKGReq, | |
| TransactionID, | |
| SenderNonce, | |
| Identification] | |
| |-------------------------------------->|
| | <PKIData |
| | control: TransactionId, SenderNonce, |
| | reqSeq:* PKCS #10 or CRMF> |
| | (RA signed) |
| |<--------------------------------------|
| | <PKIResponse
| | control: TransactionId, SenderNonce,
| | RecipientNonce>
|<-----------------|
| | (CA signed with includes issued
|<-----------------| client certificate)
| <PKIResponse
| control: TransactionId, SenderNonce, RecipientNonce, SKGRes
| cmsSeq: {<AKP>}>
| (RA signed PKIResponse with CA-issued client certificate)

* Includes ChangeSubjectName attribute in PKCS #10 or CRMF.

NOTE: There's no need for the RA to provide the SKGReq or the {<AKP>}
to the CA. The CA won't be able to access the contents of the
{<AKP>} because it's encrypted for the client and the CA's response
is always returned to the RA because the RA needs to provide the
generated {<AKP>} back to the client.

Whether the client's PKIData is encapsulated in a SignedData or
AuthenticatedData depends on whether the client used a certificate or
shared secret for authentication, respectively. Additionally, the
RecipientInfo for the EnvelopedData encapsulating the <AKP> depends
on whether the key was protected with a shared secret (pwri),

I-D CMC Extensions: Server Side Key Generation March 27, 2014

ephemeral key (ktri or kari), or certificate (ktri or kari).

I-D CMC Extensions: Server Key Generation July 30, 2013

The RA intercepts the response from the CA; it strips the CA's
signature and creates a new PKIResponse for the client. The
controlSequence is comprised of the Transaction Identifier and
Recipient Nonce fields from the client's request, the RA's Sender
Nonce, and the Server Key Generation Response (see Section 6.2); the
cmsSequence includes the RA-generated {<AKP>}; and the RA signs the
PKIRepsonse and includes the client's certificate, which was returned
in the CA's SignedData.

When the RA is generating an encryption-only key pair for the client,
and the CA wishes to force the RA to prove it has possession of the
private key, but the RA cannot use it to generate a one-time
signature, then the flow is as follows:

Client RA CA
| | |
|----------------->| |
| [PKIData | |
| control: SKGReq, | |
| TransactionID, | |
| SenderNonce, | |
| Identification] | |
| |--------------------------------------->|
| | <PKIData |
| | control: TransactionId, SenderNonce, |
| | RAPOPWitness |
| | reqSeq:* PKCS #10 or CRMF> |
| |<---------------------------------------|
| | <PKIResponse |
| | control: CMCStatusInfoV2 (popRequired),|
| | TransactionId, SenderNonce, |
| | RecipientNonce, EncryptedPOP> |
| |--------------------------------------->|
| | <PKIData |
| | control: TransactionId, SenderNonce, |
| | RecipientNonce, DecryptedPOP> |
| |<---------------------------------------|
| | <PKIResponse
| | control: TransactionId, SenderNonce,
|<-----------------| RecipientNonce>
| <PKIResponse
| control: TransactionId, SenderNonce, RecipientNonce, SKGRes
| cmsSeq: <{<AKP>}>>

* Includes ChangeSubjectName attribute in PKCS #10 or CRMF.

I-D CMC Extensions: Server Side Key Generation March 27, 2014

NOTE: The number of round trips between the RA and CA in the above
figure is twice as many as the first figure in this Section and in

I-D CMC Extensions: Server Key Generation July 30, 2013
Section 2.5.1.1; however, the additional round trip is specified in
[RFC5272] (i.e., this document does not introduce the additional
round trip). The additional round trip is necessary when the CA
forces the RA to perform POP with the CA. While the additional round
trip might be problematic between the client and server, the quality
of communication connectivity between RA and CA should not make the
additional round trips as problematic as between clients and RAs or
CAs.

2.5.2. RA-Involved Scenarios

This section illustrates a message flow for when the CA generates the
client's key. Here too the RA MUST be issued a certificate from the
CA, which means that all of the RA-generated PKIData are encapsulated
in a SignedData. Further, the RA's certificate can be used for
identification and linking identity and POP information and the RA
can include the RA Identity Witness control to tell the CA that it
performed the client identity checks; the RA will omit the control if
it does not perform these checks.

The RA can include a Modify Certification Request control [RFC5272]
in the PKIData that encapsulates the client's request but these
controls are not shown below. The RA does this when it wishes to
modify the request present in the Server Key Generation Request
control. The RA MUST NOT use the RA POP Witness control if the CA is
to generate the key. This control indicates that the RA performed
POP, but the key for which POP is claimed has not yet been generated.

The diagram below depicts the client's use of a shared secret for
authentication by including the Identification control and
encapsulating the client's PKIData in an AuthenticatedData. If a
digital signature is used for authentication, the Identification
control is omitted and the client's PKIData is encapsulated in a
SignedData. The RA encapsulates the client's request in its PKIData
by placing the client request in the cmsSequence, and includes
controls such as Transaction Identifier and Sender Nonce controls as
well as RA Identity Witness control if the RA checks the client's
identity.

I-D CMC Extensions: Server Side Key Generation July 30, 2013 March 27, 2014

| Identification] | |
| |--------------------------------------->|
| | <PKIData |
| | control: TransactionId, SenderNonce, |
| | RAIdentityWitness |
| | cmsSeq: [PKIData |
| | control: SKGReq, TransactionID, |
| | SenderNonce, Identification]>|
| |<---------------------------------------|
| | <PKIResponse
| | control: TransactionId, SenderNonce,
| | RecipientNonce
| | cmsSeq: <PKIResponse
| | control: TransactionID, SenderNonce,
| | RecipientNonce, SKGRes
| | cmsSeq: {<AKP>} >
| | (CA signed with includes issued
| | client certificate)
|<-----------------| > (CA signed)
| <PKIResponse
| control: SKGRes, TransactionId, SenderNonce, RecipientNonce RecipientNonce, SKGRes
| cmsSeq: {<AKP>}>
| (CA signed with issued client certificate)

When the RA receives the request from the CA, it strips the CA's
response for the RA off and passes the inner response to the client
unchanged. The difference between this scenario and the scenarios in
Section 2.5.1 is that the signature on the PKIResponse is generated
by the CA not the RA.

Note that the additional round trips to prove possession of an
encryption-only key depicted in Section 2.5.1 are unnecessary here
because the CA generates the asymmetric key pair and it does not need
to prove to itself that it has the keys.

3. Generating PKIData and PKIResponse

[RFC5272] defines PKIData as follows (included here for convenience):

PKIData ::= SEQUENCE {
controlSequence SEQUENCE SIZE(0..MAX) OF TaggedAttribute,
reqSequence SEQUENCE SIZE(0..MAX) OF TaggedRequest,
cmsSequence SEQUENCE SIZE(0..MAX) OF TaggedContentInfo,
otherMsgSequence SEQUENCE SIZE(0..MAX) OF OtherMsg
}

[RFC5272] defines PKIResponse as follows (included here for
convenience):

I-D CMC Extensions: Server Side Key Generation March 27, 2014

PKIResponse ::= SEQUENCE {
controlSequence SEQUENCE SIZE(0..MAX) OF TaggedAttribute,

I-D CMC Extensions: Server Key Generation July 30, 2013
cmsSequence SEQUENCE SIZE(0..MAX) OF TaggedContentInfo,
otherMsgSequence SEQUENCE SIZE(0..MAX) OF OtherMsg
}

3.1. Client Requests

When the client generates their its request, the Server Key Generation
Request control (see Section 6.1) is included in controlSequence; the
other sequences (i.e., reqSequence, cmsSequence, and
otherMsgSequence) are omitted. If a shared secret is used for
authentication, the Identification control [RFC5272] is included in
the controlSequence to ensure that the server can locate the shared-
secret needed to authenticate the request. Additional controls can
be included in controlSequence such as Sender Nonce and Transaction
Identifier [RFC5272]. The reqSequence, which is included for client-
generated key certification requests, is not needed as the Server Key
Generation Request control includes the certification request. The
client's request is either encapsulated in an AuthenticatedData or a
SignedData depending on whether the client is using a shared secret
or a digital signature key to authenticate the request. If the
client wishes to request a certificate with a different name than the
one that is present in the certificate that authenticates the
request, request
or associated with the shared secret, the client includes must nevertheless
populate the certificateRequest with the Subject Name used in the
existing certificate, and then include the Change Subject Name
control
[RFC6402] to ensure identify the Subject Name that is desired instead.
Otherwise the server will not reject the request because
the name in the certificate used to sign the request does not match
the name as required in the request.
[RFC6402].

3.2. RA Processing of Client Requests

If an RA is involved, then it can do the following:

o Forward the request as-is to the CA. This happens when the CA
authenticates the request, performs the Identity identity checks, and
generates the keys.

o Not authenticate Authenticate or not the request and place one or more client client-
authenticated PKIData in cmsSequence; reqSequence and
otherMsgSequence are omitted. Here the RA does not have the
shared secret necessary to authenticate the request. The RA can
also include additional controls in controlSequence such as the
Modify Certification Request control if the RA needs to modify
the client's request and the Sender Nonce and Transaction
Identifier controls for replay protection and transaction processing. If the RA performs
the Identity checks it can include the RA Identity Witness
control. After generation of the PKIData, the RA encapsulates it
in a SignedData as part of the digital signature process.

I-D CMC Extensions: Server Side Key Generation July 30, 2013

o Authenticate the request and place one or more client PKIData in
cmsSequence; reqSequence and otherMsgSequence are omitted. The
RA can also include additional controls in controlSequence such
as the Modify Certification Request control if the RA needs to
modify the client's request in the Server Key Generation Request
control and Sender Nonce and Transaction Identifier for replay
protection and transaction control. March 27, 2014

processing. If the RA also performs the Identity check, checks it includes can
include the RA Identity Proof Witness control [RFC6402] otherwise it is
omitted. After generation of the its PKIData, the RA encapsulates it
in a SignedData as part of the digital signature process.

o Authenticate the request and generate the client's keys. When
the RA generates the client's key, the RA generates a new PKIData
with a reqSequence; cmsSequence and otherMsgSequence are omitted.
The RA includes must assert its own name as the Subject Name in the
certificateRequest, and include the Change Subject Name attribute, attribute
carrying the intended client name, as specified in [RFC6402], in
the PKCS#10 or CRMF because [RFC6402] requires that the name in
the request will not match the name in the certificate used to
authenticate the request. If the RA-generated key is signature-
capable, POP is provided in the typical fashion (i.e., the
embedded CRMF or PKCS#10 request includes the POP). The RA can
also include additional controls in controlSequence such as
Sender Nonce and Transaction Identifier. After generation of the its
PKIData, the RA encapsulates it in a SignedData as part of the
digital signature process.

o Reject the client's request and return a PKIResponse with an
appropriate reason in the CMC Status Information V2 control.
Additionally, the RA includes Transaction Identifier, Sender
Nonce, and Recipient Nonce if the request included Transaction
Identifier and Sender Nonce controls. The PKIResponse is
encapsulated in a SignedData as part of the digital signature
process. This document defines three additional error conditions
(see Section 7):

* For a Server Key Generation Request control using the
ShroudWithPublicKey choice of certificate or certIdentifier,
the RA can check that the certificate provided to protect the
returned private key validates back to an authorized TA. If
the certificate does not validate back to their its TA, then the RA
returns a PKIResponse with a CMC Status Information v2 control
indicating the request failed with an extendedFailInfo
indicating badCertificate (see Section 7) encapsulated in a
SignedData. Note that the RA performing this check will lessen
the load on the CA, but it need only be done by the RA when the
RA is generating the client's keys; when the CA is generating
the keys technically it's up to the CA to perform this check if
it receives a CA-generated key request from a client.

I-D CMC Extensions: Server Key Generation July 30, 2013

* For a Server Key Generation Request control using the
ShroudWithSharedSecret choice and where the RA knows the shared
secret, the RA will reject the request if the shared secret
does not match the one on the RA by returning a PKIResponse

I-D CMC Extensions: Server Side Key Generation March 27, 2014

with a CMC Status Information control indicating the request
failed with an extendedFailInfo indicating badSharedSecret (see
Section 7) encapsulated in a SignedData. This is done because
client authentication failed. failed or the HMAC output was corrupted.

* For a Server Key Generation Request control that has archiveKey
set to TRUE, the RA is generating the client's keys, and the RA
does not support archive, the RA will reject the request by
returning a PKIResponse with a CMC Status Information v2
control indicating the request failed with an extendedFailInfo
indicating archiveNotSupported (see Section 7) encapsulated in
a SignedData. If the RA knows the CA also does not support
archival of keys, the RA, if it wishes, can reject the
certificate request
in the same fashion; when the CA is generating the keys,
technically it is up to the CA to perform this check if it
receives a CA-generated key request from a client. Note that
the RA performing this check will lessen the load on the CA,
but it need only be done by the RA when the RA is generating
the client's keys; when the CA is generating the
keys technically it is up to the CA to perform this check if it
receives a CA-generated key request from a client. keys.

RA's can also batch more than one request together, by including each
client request in a separate cmsSequence or reqSequence (for Simple
PKI requests) along with a Batch Request control in the RA's
PKIRequest control field. After generation of the PKIData, the RA
encapsulates it in a SignedData as part of the digital signature
process.

When verifying SignedData signatures, the RA verifies it back to an
authorized TA.

3.3. CA Processing

Receipt of a PKIResponse with a CMC Status Information control that
indicates failed results requires an out-of-band mechanism to
adjudicate the error in order to avoid a loop between the client/RA
and CA.

CA processing of requests depends on the number of layers of
encapsulation:

o Requests with a single layer of encapsulation will be validated
back to an authorized TA if they are encapsulated in a SignedData

I-D CMC Extensions: Server Key Generation July 30, 2013
or authenticated with the shared secret if they are encapsulated
in an AuthenticatedData. For AuthenticatedData encapsulated
requests the server locates the necessary shared secret with the
information found in the Identification control. For a PKIRequst
PKIRequest with a reqSequence, the server verifies the POP.
Regardless of the encapsulation technique, the server performs
the Identity checks and processes other controls such as
Transaction Identifier and Sender Nonce. If any of these checks
fail or processing of a control fails, the CA rejects the
certification request with the appropriate error code, as
specified in [RFC5272].

I-D CMC Extensions: Server Side Key Generation March 27, 2014

o Requests with multiple layers of encapsulation (i.e., those
requests that are RA-involved) will first validate the signature
on the outer SignedData back to an authorized TA and process any
controls present such as RA Identity Witness, Modify Certificate
Template, Sender Nonce, and Transaction Identifier, as per
[RFC5272][RFC6402]. Inner requests are also processed as
specified in the previous bullet. Failure to validate back to an
authorized TA or control processing failures result in rejected
requests with the appropriate error code, as specified in
[RFC5752].
[RFC5272].

CAs may require that the RA prove that it has possession of
encryption-only keys that do not support one-time signature use, by
returning a PKIResponse indicating the request failed because POP is
required and including the Encrypted POP control along with other
appropriate controls. The response is signed by the CA. See Section
2.5.1.

After successfully authenticating the request and verifying the
client's identity, the CA generates:

o Responses for single layer encapsulated requests for RA-generated
keys by issuing the certificate. If no controls were present in
the request (see Appendix B), the PKIResponse is a Simple PKI
Response [RFC5751], which includes no content and therefore no
signature. With controls (e.g., Transaction Identifier, Sender
Nonce, and Recipient Nonce), the PKIResponse includes the
appropriate controls and is signed by the CA. The CA places the
certificate in the SignedData certificates field.

o Responses for multi-layered encapsulation requests for RA-
generated keys (See Appendix B) beginning as with the previous
bullet followed by placing to form the inner response response. This is placed in a cmsSequence
entry. The
cmsSequences of the outer PKIResponse PKIResponse, which also includes the
Batch Response control as well as any other necessary controls in controlSequence and
the
controlSequence. The CA generates a signature for the
encapsulating SignedData.

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o Responses for single layer encapsulated requests for CA-generated
keys by generating the asymmetric key pair and issuing the
certificate. The signed CA-generated PKIResponse includes the
Server Key Generation Response control (see Section 6.2) along
with other controls based on whether they were present in the
controlSequence as well as the signed and then encrypted
Asymmetric Key Package in cmsSequence. The CA places the
certificate in the SignedData certificates field.

o Responses for multi-layered encapsulation requests for CA-

I-D CMC Extensions: Server Side Key Generation March 27, 2014

generated keys beginning with the previous bullet followed by
encapsulating the inner response in cmsSequence. cmsSequence for the outer
PKIResponse. The outer response also includes controls as
necessary in controlSequence and the CA generates a signature. signature for
an encapsulating SignedData.

In all cases, the certificate issued is an X.509 certificate
[RFC5280].

If the CA is unable to perform the request at this time or the entire
request cannot be processed, it can return a signed PKIResponse
indicating with a CMC Status Information control with a status of
pending or partial along with pendInfo, which the client or RA uses
to know when to ask the CA next about the request.

When the CA fails to or refuses to process the request, it returns a
PKIResponse with a CMC Status Information control with the
appropriate error code from [RFC5272] or from Section 7 of this
document. Additionally, it includes Transaction Identifier, Sender
Nonce, and Recipient Nonce in the response if the request included
Transaction Identifier and Sender Nonce controls.

3.4. RA Processing of CA Responses

If the CA rejected the RA's request as indicated by a PKIResponse
with CMC Status Information control that is failed, indicates "failed", then an out-of-
band
out-of-band mechanism may be necessary to determine the cause of
failure in order to avoid a loop of the RA returning the same request
at a later time only to also have it rejected.

If the CA returned a pending or partial response, the RA will use the
information in the CMC Status Information control's pendInfo to poll
the CA with a signed PKIRequest with a Query Pending control. CA
processing continues as in Section 3.3.

RA's that are challenged by the CA to prove possession of an
encryption-only RA-generated key validate the CA's signature back to
an authorized TA, decrypt the POP, and process any other controls
that are present. If any of these fail, then the RA terminate terminates the
request and inform informs the operator of the fault. Assuming the checks
pass, the RA generates a PKIData that includes a Decrypted POP
control and any other controls with no cmsSequence, reqSequence, or

I-D CMC Extensions: Server Key Generation July 30, 2013
otherMsgSequence. The RA encapsulates the PKIData or the PKIResponse in a SignedData as
part of the digital signature process and sends it to the CA. CA
processing resumes as in Section 3.3.

Assuming the response is a success:

I-D CMC Extensions: Server Side Key Generation March 27, 2014

o If there is no signature, either the keys are RA-generated or are
client-generated.
CA-generated. If the keys are client-generated, CA-generated, then the RA returns
the message to the client unmodified. If the key is RA-
generated, the RA generates a PKIResponse for the client that
includes the signed and then encrypted Asymmetric Key Package in
cmsSequence (see Section 5), the Server Key Generation Response
control (see Section 6.2), and any other controls. Finally, the
RA encapsulates the PKIResponse for the client in a SignedData as
part of the digital signature process and includes the client's
certificate (from the CA) in the SignedData's certificate field.

o If there is a signature:

* The response is for an RA-generated key request that contained
controls and the RA knows this by the presence of the
Transaction Identifier, Sender Nonce, or Recipient Nonce
controls (and possibly other controls) but there is no
cmsSequence or CMC Status Information control indicating the
request failed or is pending. The RA generates a PKIResponse
for the client that includes the signed and then encrypted
Asymmetric Key Package in cmsSequence (see Section 5), the
Server Key Generation Response control (see Section 6.2), and
any other controls as appropriate. Finally, the RA
encapsulates the PKIResponse for the client in a SignedData as
part of the digital signature process and includes the client's
certificate from the CA's response in the RA's SignedData
certificate
certificates field.

* The response is for a request where the CA generated the key
and the RA knows this by the presence of a cmsSequence element.
The RA processes any controls and assuming the processing
passes the RA strips off the outer SignedData and forwards the
cmsSequence element (i.e., the SignedData) to the client.

* The response is for a batch request and the RA knows this by
the presence of a Batch Response control. The RA process any
controls and assuming the processing passes the RA strips off
the outer SignedData and forwards the inner SignedData(s) from
the cmsSequence to the appropriate clients. If any of the
requests are marked as failed or pending, then the processing
earlier in this section applies.

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The RA, if it wishes, can also check the returned certificate to make
sure it validates back to an authorized TA and that the returned
certificate is consistent with the certificate request found in the
Server Key Generation Request control. These checks cut down on
errors at the client. If the RA detects that the certificate is not
consistent, the RA SHOULD NOT return the certificate to the client

I-D CMC Extensions: Server Side Key Generation March 27, 2014

and the RA SHOULD request that the certificate be revoked.

RA-generated keys for which a PKIResponse with a CMC Status
Information control that is not success SHOULD NOT return the Server
Key Generation Response or the encapsulated Asymmetric Key Package to
the client because the the CA didn't certify the public key.

3.4.

3.5. Client Processing of Responses

Clients validate the signature on all responses back to an authorized
TA.

Responses signed by an RA with a client certificate signed by a CA
whose certificate includes a id-kp-cmcCA EKU (Extended Key Usage)
[RFC6402] will violate the should "SHOULD" requirement found in [RFC6042] [RFC6402]
that the PKIResponse be signed with by an entity with the same name as
found in the certificate. Because the RA has generated the keys
there are many more bad things an RA can do so this seemed like a
tradeoff worth making.

4. Shrouding Algorithms

For the server-side key generation control attribute described in
this document to function, clients need to tell the server in advance
what encryption algorithm and what key value is to be used for
encrypting the returned private key. The encrypted data returned is
returned as an EnvelopedData object as defined by [RFC5652] and
placed in the cmsSequence field of a PKIResponse [RFC5272]. Clients
also need to tell the server in advance what digital signature and hash
algorithms it supports to ensure the certification response and
certificate can be verified.

Each request control for which the response includes encrypted data
contains two fields to define type of encryption used:
algCapabilities and shroudMethod.

The algCapabilities field, see Section 6.1, contains the advertised
capabilities of the client-side entity. This field uses the S/MIME
Capabilities type defined in section 2.5.2 of [RFC5751]. The
capabilities to be listed are digital signature algorithms, message
digest algorithms, content encryption algorithms, key agreement
algorithms, key encipherment algorithms, key-wrap algorithms and key

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derivation algorithms. Encodings for SMIME Capability values for
Elliptic Curve Key Agreement, Key Derivation Function, and Key Wrap
algorithms can be found in [RFC5753], Message Digest and Signature
algorithms can be found in [RFC5754], and AES Key Wrap with Padding
can be found in [RFC5959].

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The shroudMethod field, see field (see Section 6.1, 6.1) defines the method by which
the server will do the key management of the content encryption key
(CEK) value in EnvelopedData. The shroudMethod field uses the type
ShroudMethod. This type is defined as:

ShroudMethod ::= AlgorithmIdentifier {
SHROUD-ALGORITHM, { ShroudAlgorithmSet }
}

When a new shroud method is defined it includes (a) the source of the
key material, (b) the public or salting information, and (c) the
method of protecting the Content Encryption Key (CEK) using the
requested data, source key material and salt. This document defines
two shroud methods: id-cmc-shroudWithPublicKey and id-cmc-
shroudWithSharedSecret. Clients and servers MUST support id-cmc-
shroudWithPublicKey. Client and servers SHOULD support id-cmc-
shroudWithSharedSecret.

Other shrouding methods could be defined in the future that would not
involve the use of EnvelopedData. For example, one could conceive of
a shrouding method that required the use of Transport Layer Security
(TLS) [RFC5246] to provide the necessary security between the server
and the client. This document does not define any such mechanism.

4.1. Shroud With with a Public Key

Clients can indicate that the server use a public key, either wrapped
in a certificate or as a bare public key, to protect the server-
generated key. For this option, the key material is either included
or referenced by a key identifier. The following object identifier
identifies the shroudWithPublicKey shroud method:

id-cmc-shroudWithPublicKey OBJECT IDENTIFER ::= { id-cmc XX }

shroudWithPublicKey has the ASN.1 type ShroudWithPublicKey:

srda-shroudWithPublicKey SHROUD-ALGORITHM ::= {
IDENTIFIED BY id-cmc-shroudWithPublicKey,
PARAMS TYPE ShroudWithPublicKey ARE required,
SMIME-CAPS { IDENTIFIED BY id-cmc-shroudWithPublicKey }
}

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ShroudWithPublicKey ::= CHOICE {
certificate Certificate,
certIdentifier [1] SignerIdentifier,
bareKey [2] SEQUENCE {
publicKey SubjectPublicKeyInfo,
ski SubjectKeyIdentifier

I-D CMC Extensions: Server Side Key Generation March 27, 2014

}
}

The fields of type ShroudWithPublicKey have the following meanings:

o certificate provides a public key certificate containing the
public key to be used for encrypting the server-generated private
key from the server to the client.

o certIdentifier provides a pointer to a public key certificate
located in the SignedData that encapsulates the client's PKIData.

For the above two fields, servers SHOULD check that the subject
and
and, if included, subject alternative names match in some way
with the entity that the private key is destined for. Servers do
this to ensure the key they have made for the client is intended
for the correct client. This mechanism is beyond the scope of
this document.

o bareKey allows for an arbitrary public key to be used to return
the encrypted private key.

- publicKey contains the public key that is to be used for
encrypting the private key returned from the server to the
client.

- ski contains the SubjectKeyIdentifier that will be used in CMS
EnvelopedData to identify the public key when encrypting the
private key from the server to the client.

When this method is used with the certificate option, the server
validates the certificate back to a trust anchor. Further, the
server checks that the client provided certificate belongs to the
same client that authenticated the certification request (e.g. the
certificate subjects match, or the client provided certificate
belongs to the same entity as the authentication shared secret). If
either of these checks fails, then the server returns a CMCFailInfo
with the value of badCertificate, which is defined in Section 7.

4.2. Shroud With with a Shared-Secret Key

Clients can indicate that servers use a shared secret value to
protect the server-generated private key. For this option, the key
material is identified by the identifier, identifier; the key derivation
algorithms supported by the client are included in the

I-D CMC Extensions: Server Key Generation July 30, 2013
algCapabilities field. No salting material is provided by the
client. The derived key is then used as a key encryption key in the
EnvelopedData recipient info structure. The following object

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identifier identifies the shroudWithSharedSecret control attribute:

id-cmc-shroudWithSharedSecret OBJECT IDENTIFER ::= {id-cmc XX}

shroudWithSharedSecret attribute values have the ASN.1 type
ShroudWithSharedSecret:

shrda-shroudWithSharedSecret SHROUD-ALGORITHM ::= {
IDENTIFIED BY id-cmc-shroudWithSharedSecret
PARAMS TYPE ShroudWithSharedSecret ARE required
SMIME-CAPS { IDENTIFIED BY id-cmc-shroudWithSharedSecret }
}

ShroudWithSharedSecret ::= UTF8String

The common identification string for the client and the server is
placed includes an identifier in the ShroudWithSharedSecret
field, which is a an UTFString
[RFC5280]. [RFC5280], that the server uses to
locate the shared secret to be used to protect the returned server-
generated private key. The secret identified by the
ShroudWithSharedSecret field may be different than the secret
referred to by the identification control, which is used to identify
the shared secret used to authenticate the request. In addition addition, the
client needs to place both a key derivation function and a key wrap
function in the set of capabilities advertised by the client in the
algCapabilities field.
The identification string is used to identify the pass phrase or
shared key.

When this method is used, the server checks that the chosen shared
secret belongs to the authenticated identity of the entity that
generated the certification request. If this check fails, then the
server returns a CMCFailInfo with the value of badSharedSecret, which
is defined in Section 7. In general, while it is expected that the
same identity token and shared secret used to do the identity
authentication are used to derive the key encryption key this is not
required.

5. Returned Key Format

The server returns the private key and optionally the corresponding
public key to the client with the AsymmetricKeyPackage content type
[RFC5958]. The AsymmetricKeyPackage is first encapsulated in a
Cryptographic Message Syntax (CMS) SignedData content type [RFC5652],
then the inner SignedData is further encapsulated in an EnvelopedData
content type [RFC5652], and finally the EnvelopedData is encapsulated
in an outer SignedData. The Content Hints attribute [RFC2634] can be
used in the outer SignedData to provide a hint as to the inner most
content type (i.e., the AsymmetricKeyPacakge). There MUST be only
one OneAsymmetricKey present in the AsymmetricKeyPackage sequence.
When more than one private key is to be returned, multiple

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AsymmetricKeyPackage content types are needed. For example, when
returning more than one private key the separate AsymmetricKeyPackage
content types can be individually encapsulated in SignedData,
packaged together with the ContentCollection content type [RFC4073],
protected with an EnvelopedData around the ContentCollection, and
finally the EnvelopedData is encapsulated in a SignedData. The
public key SHOULD be included in the AsymmetricKeyPackage.

6. Server-Side Key Generation

This section provides the control attributes necessary for doing
server-side generation of keys for clients. The client places the
request for the key generation in a request message and sends it to
the server. The server will generate the key pair, create a
certificate for the public key and return the data in a response
message, or the server will return a failure indication.

6.1. Server-Side Key Generation Request Attribute

The client initiates a request for server-side key generation by
including the server-side key generation request attribute in the
control attributes section of a PKIData object. The request
attribute includes information about how to return the generated key
as well as any client suggested items for the certificate. The
control attribute for doing server-side key generation is identified
by the following OID:

id-cmc-serverKeyGenRequest OBJECT IDENTIFIER ::= { id-cmc XX }

The Server-Side Key Generation Request control attribute has the
following ASN.1 definition:

cmc-serverKeyGenRequest CMC-CONTROL ::= {
ServerKeyGenRequest IDENTIFIED BY id-cmc-serverKeyGenRequest
}

ServerKeyGenRequest ::= SEQUENCE {
certificateRequest CertTemplate, TaggedRequest,
shroudMethod ShroudMethod,
algCapabilities SMimeCapabilties OPTIONAL,
archiveKey BOOLEAN DEFAULT TRUE
}

The fields in ServerKeyGenRequest have the following meaning:

o certificateRequest contains the data fields that the client
suggests for the certificate being requested for the server
generated key pair. The format is TaggedRequest from [RFC5272],

I-D CMC Extensions: Server Side Key Generation July 30, 2013 March 27, 2014

which supports both PKCS#10 and CRMF requests. In all instances,
the bodyPartID is set to zero.

o shroudMethod contains the identifier of the type of algorithm to
be used in deriving the key used to encrypt the private key.

o algCapabilities contains the set of algorithm capabilities being
advertised by the client. The server uses algorithms from this
set in the ServerKeyGenResponse object to encrypt the private key
of the server-generated key pair. This field is optional because
this information might be carried in a signed attribute, included
within a certificate or be part of the local configuration.

o archiveKey is set to TRUE if the client wishes the key to be
archived as well as generated on the server. Further processing
by the server when this is set to TRUE is out-of-scope.

The client can request that the generated key be for a specific
algorithm by placing data in the publicKey publicKey.attribute field of the
CRMF request or in the subjectPKInfo subjectPKInfo.attribute field of the PKCS#10
request. If the publicKey or subjectPKInfo field is populated, then
the public key subjectPublicKey is a zero length zero-length bit string. If the client
requests a specific algorithm, the server either generates a key for
that algorithm (with the parameters if defined) or fails to process
the request. If the request fails for this reason, the server
returns a CMCFailInfo with a value of badAlg [RFC5272].

As specified in [RFC5272]:

"A server is not required to use all of the values suggested by
the client in the certificate template. Servers MUST be able to
process all extensions defined in [RFC5280]. Servers are not
required to be able to process other V3 X.509 extensions
transmitted using this protocol, nor are they required to be able
to process other, private extensions. Servers are permitted to
modify client-requested extensions. Servers MUST NOT alter an
extension so as to invalidate the original intent of a client-
requested extension. (For example change key usage from key
exchange to digital signature.) If a certification request is
denied due to the inability to handle a requested extension, the
server MUST respond with a CMCFailInfo with a value of
unsupportedExt."

A server that does not recognize the algorithm identified in
shroudMethod will reject the request. The server returns a
CMCFailInfo with a value of badAlg [RFC5272].

A server that does not support at least one of the algCapabilities

I-D CMC Extensions: Server Side Key Generation March 27, 2014

will reject the request. The server returns a CMCFailInfo with a

I-D CMC Extensions: Server Key Generation July 30, 2013
value of badAlg [RFC5272].

If archiveKey is set to true and the server does not support
archiving of private keys, the request will be rejected by the
server. The server returns a CMCFailInfo with a value of
archiveNotSupported, see Section 7.

6.2. Server-side Key Generation Response

The server creates a server-side key generation response attribute
for every key generation request made and successfully completed.
The response message has a pointer to both the original request
attribute and to the body part in the current message that holds the
encrypted private keys. The response message also can contain a
pointer to the certificate issued. The key generation response
control attribute is identified by the OID:

id-cmc-serverKeyGenResponse OBJECT IDENTIFIER ::= { id-cmc XX }

The Server-Side Key Generation Response control attribute has the
following ASN.1 definition:

cmc-serverKeyGenResponse CMC-CONTROL ::= {
ServerKeyGenResponse IDENTIFIED BY id-cmc-serverKeyGenResponse
}

ServerKeyGenResponse ::= SEQUENCE {
cmsBodyPartId BodyPartID,
requestBodyPartId BodyPartID,
issuerAndSerialNumber IssuerAndSerialNumber OPTIONAL
}

The fields in ServerKeyGenResponse have the following meaning:

o cmsBodyPartId identifies a TaggedContentInfo contained within the
enclosing PKIData. The ContentInfo object is of type
EnvelopedData and has an encapsulated content of id-ct-KP-
aKeyPackage (see Section 4).

o requestBodyPartId contains the body part identifier for the
server-side key generation request control attribute. This
allows for clients to associate the resulting key and certificate
with the original request.

o issuerAndSerialNumber if present contains the identity of the
certificate issued to satisfy the request. The certificate is
placed in the certificate bag of the immediately encapsulating
signedData object.

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SignedData object.

As specified in [RFC5272]:

"Clients MUST NOT assume the certificates are in any order.
Servers SHOULD include all intermediate certificates needed to
form complete chains to one or more self-signed certificates, not
just the newly issued certificate(s) in the certificate bag. The
server MAY additionally return CRLs in the CRL bag. Servers MAY
include self-signed certificates. Clients MUST NOT implicitly
trust included self-signed certificate(s) merely due to its
presence in the certificate bag."

7. Additional Error Codes

This section defines ExtendedFailInfo errors from this document:

cmc-err-keyGeneration EXTENDED-FAILURE-INFO ::= {
TYPE ErrorList IDENTIFIED BY id-tbd
}

ErrorList ::= INTEGER {
archiveNotSupported (1),
badCertificate (2),
badSharedSecret (3)
}

o archiveNotSupported indicates that the server does not support
archiving of private keys. The syntax for the ExtendedFailInfo
is as follows:

cmc-err-archiveNotSupport EXTENDED-FAILURE-INFO ::={
IDENTIFIED BY id-tbd
}

o badCertificate indicates that the certificate to be used to
encrypt the response did not validate back to a RA/CA trust
anchor or the certificate does not belong to the client. The
syntax for the ExtendedFailInfo is as follows:

cmc-err-badCertificate EXTENDED-FAILURE-INFO ::={
IDENTIFIED BY id-tbd
}

o badSharedSecret indicates that the shared secret provided used by the
client does not match that stored by the server.

cmc-err-badSharedSecret EXTENDED-FAILURE-INFO ::={
IDENTIFIED BY id-tbd
}

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IDENTIFIED BY id-tbd
}

8. Proof-of-Possession

Some servers may require that the client prove that it has taken
possession of the server-generated key. This proof requires an
additional roundtrip beyond those previously discussed.

For certificates returned that support digital signatures the process
is as described in [RFC5272]: the server indicates in CMCStatus that
status is
confirmRequired, confirmRequired; the client returns the Confirm Certificate
Acceptance control in PKIData signed with the server-generated
private key, key; and the server responds with a CMCStatus of success.

For certificates returned that only support encryption, the server
indicates the CMCStatus is popRequired and includes the Encrypted POP
control,
control; the client returns the Decrypted POP control. Whether the
PKIRequest from the client is encapsulated in an Authenticated Data AuthenticatedData or Signed Data
SignedData depends on which mechanism was used during the server key
generation request.

9. Security Considerations

Central generation of digital signature keys contains risks and is
not always appropriate. Organization-specific CPs (Certificate
Policies) [RFC3647] define whether or not server-side generation of
digital signature keys is permitted.

There

For the choice of mechanisms to protect the server-generated key,
there is a balance that needs to be maintained between the use of a
potentially poorly generated one time one-time key (i.e., the shared secret)
and the use of a key externally provided. For externally provided
keys, the external provider of the key will be able to decrypt the
key delivery message as long as it was captured. For poorly generated one time
one-time keys, any external party might be able to guess the key and
thus decrypt the key delivery message. Different types of keys will
have different requirements for what a poorly generated key means.
Generators of RSA keys need to be able to do good prime checking, checking;
generators of Diffie-
Hellman Diffie-Hellman (DH) or Elliptic Curve Diffie-Hellman
(ECDH) keys only need a moderate quality random number generator if
the group parameters are externally provided. provided and of good quality.

This specification requires implementations to generate key pairs and
other random values [RFC4086]. The use of inadequate pseudo-random
number generators (PRNGs) can result in little or no security. The
generation of quality random numbers is difficult. NIST Special
Publication 800-90 [SP-800-90] and FIPS 186 [FIPS-186] offer

I-D CMC Extensions: Server Side Key Generation March 27, 2014

guidance.

Private keys, regardless of where they are generated, must be
appropriately protected from disclosure or modification on the

I-D CMC Extensions: Server Key Generation July 30, 2013
server, in transit, and on the client. Cryptographic algorithms and
keys used to protect the private key should be at least as strong as
the private key's intended strength.

This document describes the CA signing certificates and messages as
well as encrypting messages. It should not be assumed that the CA
uses the same key for all of these operations. In fact, CAs may wish
to limit the exposure of their private keys by using different keys
or different purposes.

Key agreement algorithms (i.e., Diffie-Hellam or Elliptic Curve
Diffie-Hellman) can be used to protect the returned server-generated
key. These algorithms support a number of different schemes [SP-800-
56]. Normally, an Ephemeral-Static (E-S) scheme is used (more
formally known as "(Cofactor) One-Pass Diffie-Hellman,
C(1e,1s,ECCCDH) Scheme") see [RFC5753], but here the client provides
an ephemeral key to the server so an S-E scheme is used when the key
is encrypted for the client. Regardless, the client needs to
generate an ephemeral key and provide it to the server and this key
needs to use the same parameters (i.e., p, q, g for DH and elliptic
curve for ECDH) as the server. The client's parameters MUST be
present in the publicKey or certificate field of the Server Key
Generation Request control or MUST be in the certificate referred to
by the ski in the same control. The client can find the server's
parameters in the server's certificate; to make it easy on clients,
server certificates MUST include parameters. How the client obtains
the server's certificate is out of scope.

Servers that support the features specified herein need to document
their procedures in a CPS (Certificate Practice Statement) [RFC3647].
CAs that certify server-generated private keys are certifying that
they've taken due diligence to ensure that the private key is only
known to and used by the subject. Depending on the Certification
Policy (CP) [RFC3647], the keys have been allocated to the subject,
but the keys may not be strictly owned by the subject. The CA (and
the enterprise it supports) has a reason for issuing the keys (e.g.,
employer to employee; school to student) and because the enterprise
CA generated the private keys it is accountable for the
trustworthiness of the private key. But, the subject should beware of
using it for other purposes.

When using an ephemeral key for protecting the server-generated key,
A
a compromised signature key, when used by the intended party, will
not automatically jeopardize the security of the server-generated

I-D CMC Extensions: Server Side Key Generation March 27, 2014

keys. Procedural controls can help to ensure a one-to-one mapping
between verified requests and intended parties (i.e. mitigate the
risk of masquerade using a compromised authentication key and
certificate), but that is outside the scope of this document.

POP is important, but important; [RFC4211] provides some information about POP; for
server-generated keys it can only be provided after the server-generated server-
generated key has been returned by the client (see Section 8).
Whether a server requires POP is a CP dependent [RFC3647], but highly
recommended.

When the shared secret is used to provide client authentication and

I-D CMC Extensions: Server Key Generation July 30, 2013
protect the server-generated private key, the shared secret must be
kept secret for the lifetime of the key. This is because disclosure
could allow attackers to determine the server-generated private key.
This is different than certification requests with client-generated
keys because the shared secret is no longer needed after the
authentication.

If the key generator and the server are not collocated, then the
exchange between these two entities must be protected from
unauthorized disclosure and modification and both entities must have
a trust relationship. However, these exchanges are beyond the scope
of this document. Note that the CA needs access to the public key to
generate the certificate. If the key generator encrypts the
generated key for the client, then the key generator needs to provide
the public key to the CA (possibly through the RA).

Returning the key to the wrong client can be bad. If an encrypted
key is returned to the wrong client, then it is only bad if the key
was encrypted for the wrong client and then something much worse is
afoot. If the encrypted key is returned to the wrong client and it
is encrypted for the right client (i.e., it was misdirected), then it
is bad but the unencrypted key hasn't been disclosed to an
unauthorized client. The protection afforded by the confidentiality
algorithm is what protects the misdirected key from unauthorized
disclosure.

10. IANA Considerations

This document makes use of object identifiers to register CMC control
attributes and CMC error codes. Additionally, an object identifier
is used to identify the ASN.1 module found in Appendix A. All are
defined in an arc delegated by IANA to the PKIX Working Group. The
current contents of the arc are located here:

http://www.imc.org/ietf-pkix/pkix-oid.asn

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They were obtained by sending a request to ietf-pkix-oid-reg@imc.org.
When the PKIX WG closes, this arc and registration procedures will
be transferred to IANA. No further action by IANA is necessary for
this document or any anticipated updates.

11. References

11.1 Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

I-D CMC Extensions: Server Key Generation July 30, 2013

[RFC2986] Nystrom, M. and B. Kaliski, "PKCS #10: Certification
Request Syntax Specification Version 1.7", RFC 2986,
November 2000.

[RFC4211] Schaad, J., "Internet X.509 Public Key Infrastructure
Certificate Request Message Format (CRMF)", RFC 4211,
September 2005.

[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, June 2008.

[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, May 2008.

[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, September 2009.

[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, January 2010.

[RFC5958] Turner, S., "Asymmetric Key Packages", RFC 5958, August
2010.

[RFC6402] Schaad, J., "Certificate Management over CMS (CMC)
Updates", RFC 6402, November 2011.

12.2 Informative References

[FIPS-186] National Institute of Standards and Technology (NIST),
FIPS 186-3 DRAFT: Digital Signature Standard (DSS),
November 2008.

I-D CMC Extensions: Server Side Key Generation March 27, 2014

[RFC2634] Hoffman, P., Ed., "Enhanced Security Services for S/MIME",
RFC 2634, June 1999.

[RFC3647] Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S.
Wu, "Internet X.509 Public Key Infrastructure Certificate
Policy and Certification Practices Framework", RFC 3647,
November 2003.

[RFC4073] Housley, R., "Protecting Multiple Contents with the
Cryptographic Message Syntax (CMS)", RFC 4073, May 2005.

[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,

I-D CMC Extensions: Server Key Generation July 30, 2013
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.

[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI
36, RFC 4949, August 2007.

[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.

[RFC5753] Turner, S. and D. Brown, "Use of Elliptic Curve
Cryptography (ECC) Algorithms in Cryptographic Message
Syntax (CMS)", RFC 5753, January 2010.

[RFC5754] Turner, S., "Using SHA2 Algorithms with Cryptographic
Message Syntax", RFC 5754, January 2010.

[RFC5959] Turner, S., "Algorithms for Asymmetric Key Package Content
Type", RFC 5959, August 2010.

[SP-800-56] Barker, E., Johnson, D., and M. Smid, "Recommendation for
Pair-Wise Key Establishment Schemes Using Discrete
Logarithm Cryptography", NIST Special Publication 800-56A
Revision 1, March 2007.

[SP-800-57] National Institute of Standards and Technology (NIST),
Special Publication 800-57: Recommendation for Key
Management - Part 1 (Revised), March 2007.

[SP-800-90] National Institute of Standards and Technology (NIST),
Special Publication 800-90: Recommendation for Random
Number Generation Using Deterministic Random Number Bit
Generators (Revised), March 2007.

Appendix A. ASN.1 Module

To be supplied later.

I-D CMC Extensions: Server Side Key Generation March 27, 2014

Appendix B. Additional Message Flows

This appendix forms a non-normative part of this specification.

The main body of this document has portrayed protocol flows with
optional controls. This was done to explain the more complicated
scenarios. This appendix depicts the flows without those optional
controls.

For example the figure in Section 2.5.1 without the TransactionId,
SenderNonce, and RecipientNonce, appear as follows:

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Client RA CA
| | |
|----------------->| |
| [PKIData | |
| control: SKGReq, | |
| Identification] | |
| |-------------------------------------->|
| | <PKIData |
| | reqSeq:* PKCS #10 or CRMF> |
| |<--------------------------------------|
|<-----------------| PKIResponse
| <PKIResponse
| control: SKGRes
| cmsSeq: {<AKP>}>

* Includes ChangeSubjectName attribute in PKCS #10 or CRMF.

The PKIResponse from the CA is a certs-only message, which does not
include a signature.

Likewise for the figure in Section 2.5.2:

Client RA CA
| | |
|----------------->| |
| [PKIData | |
| control: SKGReq, | |
| Identification] | |
| |--------------------------------------->|
| | <PKIData |
| | control: RAIdentityWitness |
| | cmsSeq: [PKIData |
| | control: SKGReq, Identification]> |
| |<---------------------------------------|
| | <PKIResponse
| | cmsSeq: <PKIResponse

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| | control: SKGRes
|<-----------------| cmsSeq: {<AKP>} > >
| <PKIResponse
| control: SKGRes
| cmsSeq: {<AKP>}>

If the RA doesn't perform the Identity checks, then it can forward
the client's request without the additional layers of encapsulation.

Client RA CA
| | |
|----------------->| |

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| [PKIData | |
| control: SKGReq, | |
| Identification] | |
| |--------------------------------------->|
| | [PKIData |
| | control: SKGReq, Identification] |
| |<---------------------------------------|
| | <PKIResponse
| | control: SKGRes
| | cmsSeq: {<AKP>}>
|<-----------------|
| <PKIResponse
| control: SKGRes
| cmsSeq: {<AKP>}>

Not to be outdone, the scenarios in Section 2.5.1 can be more
complicated by an RA that batch requests together. The following
depicts a number of clients sending requests together that an RA then
batches together. The unsigned PKIResponse (a certs-only message)
includes all of the certificates issued. The CA can also return
individual responses as opposed to batching them all together or it
can batch them together in some other combination.

Client(1-n) RA CA
| | |
|----------------->| |
| [PKIData | |
| control: SKGReq, | |
| Identification] | |
| |-------------------------------------->|
| | <PKIData |
| | cmsSeq (1-n):* PKCS #10 or CRMF> |
| |<--------------------------------------|
| | <PKIResponse
| | control: BatchResponse
|<-----------------| cmsSeq (1-n): certs-only>

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| <PKIResponse(1-n)
| control: SKGRes
| cmsSeq: {<AKP>}>

* Includes ChangeSubjectName attribute in PKCS #10 or CRMF.

In another scenario, all but one of the requests were successfully
processed. The RA returns those that were successful back to the
clients but later polls the CA, based on the value CMCStatusInfoV2
pendInfo, for the one that was not successful. The CA returns the
one successful request.

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Client(1-n) RA CA
| | |
|----------------->| |
| [PKIData | |
| control: SKGReq, | |
| Identification] | |
| |-------------------------------------->|
| | <PKIData |
| | control: BatchRequest |
| | cmsSeq (1-n):* PKCS #10 or CRMF> |
| |<--------------------------------------|
| | <PKIResponse
| | control:CMCStatusInfoV2 (partial),
| | BatchResponse
| | cmsSeq (1-n): certs-only>
|<-----------------|
| <PKIResponse(1-*)|
| control: SKGRes |
| cmsSeq: {<AKP>}> |
| |-------------------------------------->|
| | <PKIData |
| | control: QueryPending> |
| |<--------------------------------------|
| | PKIResponse
|<-----------------|
| <PKIResponse
| control: SKGRes
| cmsSeq: {<AKP>}>

* Includes ChangeSubjectName attribute in PKCS #10 or CRMF.

Batching the requests can also be performed for CA-generated keys as
shown below. The RA Identity Witness controls indicates all those
client requests that it performed Identity checks on.

Client RA CA

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| | control: SKGReq, TransactionID, |
| | SenderNonce, Identification]>|
| |<---------------------------------------|
| | <PKIResponse
| | control: TransactionId, SenderNonce,
| | RecipientNonce, BatchResponse
| | cmsSeq (1-n): <PKIResponse
| | control: TransactionID, SenderNonce,
| | RecipientNonce, SKGRes
|<-----------------| cmsSeq: {<AKP>} >>
| <PKIResponse(1-n)
| control: SKGRes, TransactionId, SenderNonce, RecipientNonce
| cmsSeq: {<AKP>}>

Appendix B. Examples

To be supplied later.

B.1. Client Requests

B.1.1. Shroud with Certificate

B.1.2. Shroud with Public Key

B.1.3. Shroud with Shared Secret

B.2. CA-Generate Key Response

B.3. RA-Generate Key Response
I-D CMC Extensions: Server Side Key Generation March 27, 2014

Authors' Addresses

Jim Schaad
Soaring Hawk Consulting

Email: jimsch@exmsft.com

Sean Turner
IECA, Inc.
3057 Nutley Street, Suite 106
Fairfax, VA 22031
USA

Email: turners@ieca.com

I-D CMC Extensions: Server Key Generation July 30, 2013

Paul Timmel
National Information Assurance Research Laboratory
National Security Agency

Email: pstimme@tycho.ncsc.mil