Network Working Group J. Pawling
Request for Comments: 2876 WGSI, A Getronics Company
Category: Informational July 2000

Use of the KEA and SKIPJACK Algorithms in CMS

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2000). All Rights Reserved.

Abstract

This document describes the conventions for using the Key Exchange
Algorithm (KEA) and SKIPJACK encryption algorithm in conjunction with
the Cryptographic Message Syntax [CMS] enveloped-data and encrypted-
data content types.

1. IntrodUCtion

Throughout this document, the terms MUST, MUST NOT, SHOULD and MAY
are used in capital letters. This conforms to the definitions in
[MUSTSHOULD]. [MUSTSHOULD] defines the use of these key Words to help
make the intent of standards track documents as clear as possible.
The same key words are used in this document to help implementers
achieve interoperability. Software that claims compliance with this
document MUST provide the capabilities as indicated by the MUST, MUST
NOT, SHOULD and MAY terms. The KEA and SKIPJACK cryptographic
algorithms are described in [SJ-KEA].

2. Content Encryption Process

This section applies to the construction of both the enveloped-data
and encrypted-data content types. Compliant software MUST meet the
requirements stated in [CMS] Section 6.3, "Content-encryption
Process". The input to the encryption process MUST be padded to a
multiple of eight octets using the padding rules described in [CMS]
Section 6.3. The content MUST be encrypted as a single string using
the SKIPJACK algorithm in 64-bit Cipher Block Chaining (CBC) mode
using randomly-generated 8-byte Initialization Vector (IV) and 80-bit
SKIPJACK content-encryption key (CEK) values.

3. Content Decryption Process

This section applies to the processing of both the enveloped-data and
encrypted-data content types. The encryptedContent MUST be decrypted
as a single string using the SKIPJACK algorithm in 64-bit CBC mode.
The 80-bit SKIPJACK CEK and the 8-byte IV MUST be used as inputs to
the SKIPJACK decryption process. Following decryption, the padding
MUST be removed from the decrypted data. The padding rules are
described in [CMS] Section 6.3, "Content-encryption Process".

4. Enveloped-data Conventions

The CMS enveloped-data content type consists of an encrypted content
and wrapped CEKs for one or more recipients. Compliant software MUST
meet the requirements for constructing an enveloped-data content type
stated in [CMS] Section 6, "Enveloped-data Content Type". [CMS]
Section 6 should be studied before reading this section, because this
section does not repeat the [CMS] text.

An 8-byte IV and 80-bit CEK MUST be randomly generated for each
instance of an enveloped-data content type as inputs to the SKIPJACK
algorithm for use to encrypt the content. The SKIPJACK CEK MUST only
be used for encrypting the content of a single instance of an
enveloped-data content type.

KEA and SKIPJACK can be used with the enveloped-data content type
using either of the following key management techniques defined in
[CMS] Section 6:

1) Key Agreement: The SKIPJACK CEK is uniquely wrapped for each
recipient using a pairwise symmetric key-encryption key (KEK)
generated using KEA using the originator's private KEA key,
recipient's public KEA key and other values. Section 4.2 provides
additional details.

2) "Previously Distributed" Symmetric KEK: The SKIPJACK CEK is
wrapped using a "previously distributed" symmetric KEK (such as a
Mail List Key). The methods by which the symmetric KEK is
generated and distributed are beyond the scope of this document.
Section 4.3 provides more details.

[CMS] Section 6 also defines the concept of the key transport key
management technique. The key transport technique MUST NOT be used
with KEA.

4.1. EnvelopedData Fields

The enveloped-data content type is Abstract Syntax Notation.1 (ASN.1)
encoded using the EnvelopedData syntax. The fields of the
EnvelopedData syntax must be populated as follows:

The EnvelopedData version MUST be 2.

If key agreement is being used, then the EnvelopedData originatorInfo
field SHOULD be present and SHOULD include the originator's KEA X.509
v3 certificate containing the KEA public key associated with the KEA
private key used to form each pairwise symmetric KEK used to wrap
each copy of the SKIPJACK CEK. The issuers' X.509 v3 certificates
required to form the complete certification path for the originator's
KEA X.509 v3 certificate MAY be included in the EnvelopedData
originatorInfo field. Self-signed certificates SHOULD NOT be included
in the EnvelopedData originatorInfo field.

The EnvelopedData RecipientInfo CHOICE is dependent on the key
management technique used. Sections 4.2 and 4.3 provide more
information.

The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
algorithm field MUST be the id-fortezzaConfidentialityAlgorithm
object identifier (OID). The EnvelopedData encryptedContentInfo
contentEncryptionAlgorithm parameters field MUST include the random
8-byte IV used as the input to the content encryption process.

The EnvelopedData unprotectedAttrs MAY be present.

4.2. Key Agreement

This section describes the conventions for using KEA and SKIPJACK
with the CMS enveloped-data content type to support key agreement.
When key agreement is used, then the RecipientInfo
keyAgreeRecipientInfo CHOICE MUST be used.

If the EnvelopedData originatorInfo field does not include the
originator's KEA X.509 v3 certificate, then each recipientInfos
KeyAgreementRecipientInfo originator field MUST include the
issuerAndSerialNumber CHOICE identifying the originator's KEA X.509
v3 certificate. If the EnvelopedData originatorInfo field includes
the originator's KEA X.509 v3 certificate, then each recipientInfos
KeyAgreementRecipientInfo originator field MUST include either the
subjectKeyIdentifier CHOICE containing the value from the
subjectKeyIdentifier extension of the originator's KEA X.509 v3
certificate or the issuerAndSerialNumber CHOICE identifying the

originator's KEA X.509 v3 certificate. To minimize the size of the
EnvelopedData, it is recommended that the subjectKeyIdentifier CHOICE
be used.

In some environments, the KeyAgreementRecipientInfo originator field
MAY include the originatorKey CHOICE. The originatorKey CHOICE
SHOULD NOT be used with KEA for e-mail transactions. Within a
controlled security architecture, a module may produce KEA key pairs
for use in conjunction with internal/local storage of encrypted data.
In this case, there may not be an X.509 certificate associated with a
(possibly) short term or one time use public KEA key. When
originatorKey is used, then the KEA public key MUST be conveyed in
the publicKey BIT STRING as specified in [KEA] Section 3.1.2. The
originatorKey algorithm identifier MUST be the id-
keyExchangeAlgorithm OID. The originatorKey algorithm parameters
field MUST contain the KEA "domain identifier" (ASN.1 encoded as an
OCTET STRING) identifying the KEA algorithm parameters (i.e., p/q/g
values) associated with the KEA public key. [KEA] Section 3.1.1
describes the method for computing the KEA domain identifier value.

4.2.1. SKIPJACK CEK Wrap Process

The SKIPJACK CEK is uniquely wrapped for each recipient of the
EnvelopedData using a pairwise KEK generated using the KEA material
of the originator and the recipient along with the originator's User
Keying Material (UKM) (i.e. Ra). The CMS EnvelopedData syntax
provides two options for wrapping the SKIPJACK CEK for each recipient
using a KEA-generated KEK. The "shared Originator UKM" option SHOULD
be used when constructing EnvelopedData objects. The "unique
originator UKM" option MAY be used when constructing EnvelopedData
objects. Compliant software MUST be capable of processing
EnvelopedData objects constructed using both options.

1) Shared Originator UKM Option: CMS provides the ability for a
single, shared originator's UKM to be used to generate each pairwise
KEK used to wrap the SKIPJACK CEK for each recipient. When using the
shared originator UKM option, a single RecipientInfo
KeyAgreeRecipientInfo structure MUST be constructed to contain the
wrapped SKIPJACK CEKs for all of the KEA recipients sharing the same
KEA parameters. The KeyAgreeRecipientInfo structure includes
multiple RecipientEncryptedKey fields that each contain the SKIPJACK
CEK wrapped for a specific recipient.

2) Unique Originator UKM Option: CMS also provides the ability for a
unique originator UKM to be used to generate each pairwise KEK used
to wrap the SKIPJACK CEK for each recipient. When using the unique
originator UKM option, a separate RecipientInfo KeyAgreeRecipientInfo
structure MUST be constructed for each recipient. Each
KeyAgreeRecipientInfo structure includes a single
RecipientEncryptedKey field containing the SKIPJACK CEK wrapped for
the recipient. This option requires more overhead than the shared
UKM option because the KeyAgreeRecipientInfo fields (i.e. version,
originator, ukm, keyEncryptionAlgorithm) must be repeated for each
recipient.

The next two paragraphs apply to both options.

The KeyAgreeRecipientInfo keyEncryptionAlgorithm algorithm field MUST
include the id-kEAKeyEncryptionAlgorithm OID. The
KeyAgreeRecipientInfo keyEncryptionAlgorithm parameters field MUST
contain a KeyWrapAlgorithm as specified in [CMS] Appendix A, "ASN.1
Module". The algorithm field of KeyWrapAlgorithm MUST be the id-
fortezzaWrap80 OID indicating that the FORTEZZA 80-bit wrap function
is used to wrap the 80-bit SKIPJACK CEK. Since the FORTEZZA 80-bit
wrap function includes an integrity check value, the wrapped SKIPJACK
key is 96 bits long. The parameters field of KeyWrapAlgorithm MUST
be absent.

If the originator is not already an eXPlicit recipient, then a copy
of the SKIPJACK CEK SHOULD be wrapped for the originator and included
in the EnvelopedData. This allows the originator to decrypt the
contents of the EnvelopedData.

4.2.1.1. SKIPJACK CEK Wrap Process Using A Shared Originator UKM Value

This section describes how a shared originator UKM value is used as
an input to KEA to generate each pairwise KEK used to wrap the
SKIPJACK CEK for each recipient.

When using the shared originator UKM option, a single RecipientInfo
KeyAgreeRecipientInfo structure MUST be constructed to contain the
wrapped SKIPJACK CEKs for all of the KEA recipients using the same
set of KEA parameters. If all recipients' KEA public keys were
generated using the same set of KEA parameters, then there MUST only
be a single RecipientInfo KeyAgreeRecipientInfo structure for all of
the KEA recipients. If the recipients' KEA public keys were
generated using different sets of KEA parameters, then multiple
RecipientInfo KeyAgreeRecipientInfo fields MUST be constructed
because the originatorIdentifierOrKey will be different for each
distinct set of recipients' KEA parameters.

A unique 128-byte originator's UKM MUST be generated for each
distinct set of recipients' KEA parameters. The originator's UKM
MUST be placed in each KeyAgreeRecipientInfo ukm OCTET STRING.

The originator's and recipient's KEA parameters MUST be identical to
use KEA to successfully generate a pairwise KEK. [KEA] describes how
a KEA public key is conveyed in an X.509 v3 certificate. [KEA]
states that the KEA parameters are not included in KEA certificates;
instead, a "domain identifier" is supplied in the
subjectPublicKeyInfo algorithm parameters field of every KEA
certificate. The values of the KEA domain identifiers in the
originator's and recipient's KEA X.509 v3 certificates can be
compared to determine if the originator's and recipient's KEA
parameters are identical.

The following steps MUST be repeated for each recipient:

1) KEA MUST be used to generate the pairwise KEK based on the
originator's UKM, originator's private KEA key, recipient's 128
byte public KEA key (oBTained from the recipient's KEA X.509 v3
certificate) and the recipient's 128-byte public KEA key used as
the Rb value.

2) The SKIPJACK CEK MUST be wrapped using the KEA-generated pairwise
KEK as input to the FORTEZZA 80-bit wrap function. The FORTEZZA
80-bit wrap function takes the 80-bit SKIPJACK CEK along with a
16-bit integrity checkvalue and produces a 96-bit result using the
KEA-generated pairwise KEK.

3) A new RecipientEncryptedKey SEQUENCE MUST be constructed for the
recipient.

4) The value of the subjectKeyIdentifier extension from the
recipient's KEA X.509 v3 certificate MUST be placed in the
recipient's RecipientEncryptedKey rid rKeyId subjectKeyIdentifier
field. The KeyAgreeRecipientIdentifier CHOICE MUST be rKeyId.
The date and other fields MUST be absent from the
recipientEncryptedKey rid rKeyId SEQUENCE.

5) The wrapped SKIPJACK CEK MUST be placed in the recipient's
RecipientEncryptedKey encryptedKey OCTET STRING.

6) The recipient's RecipientEncryptedKey MUST be included in the
KeyAgreeRecipientInfo recipientEncryptedKeys SEQUENCE OF
RecipientEncryptedKey.

4.2.1.2. SKIPJACK CEK Wrap Process Using Unique Originator UKM Values

This section describes how a unique originator UKM value is generated
for each recipient to be used as an input to KEA to generate that
recipient's pairwise KEK.

The following steps MUST be repeated for each recipient:

1) A new RecipientInfo KeyAgreeRecipientInfo structure MUST be
constructed.

2) A unique 128-byte originator's UKM MUST be generated. The
originator's UKM MUST be placed in the KeyAgreeRecipientInfo ukm
OCTET STRING.

3) KEA MUST be used to generate the pairwise KEK based on the
originator's UKM, originator's private KEA key, recipient's 128-
byte public KEA key and recipient's 128-byte public KEA key used
as the Rb value.

4) The SKIPJACK CEK MUST be wrapped using the KEA-generated pairwise
KEK as input to the FORTEZZA 80-bit wrap function. The FORTEZZA
80-bit wrap function takes the 80-bit SKIPJACK CEK along with a
16-bit integrity check value and produces a 96-bit result using
the KEA-generated pairwise KEK.

5) A new RecipientEncryptedKey SEQUENCE MUST be constructed.

6) The value of the subjectKeyIdentifier extension from the
recipient's KEA X.509 v3 certificate MUST be placed in the
RecipientEncryptedKey rid rKeyId subjectKeyIdentifier field. The
KeyAgreeRecipientIdentifier CHOICE MUST be rKeyId. The date and
other fields MUST be absent from the RecipientEncryptedKey rid
rKeyId SEQUENCE.

7) The wrapped SKIPJACK CEK MUST be placed in the
RecipientEncryptedKey encryptedKey OCTET STRING.

8) The recipient's RecipientEncryptedKey MUST be the only
RecipientEncryptedKey present in the KeyAgreeRecipientInfo
recipientEncryptedKeys SEQUENCE OF RecipientEncryptedKey.

9) The RecipientInfo containing the recipient's KeyAgreeRecipientInfo
MUST be included in the EnvelopedData RecipientInfos SET OF
RecipientInfo.

4.2.2. SKIPJACK CEK Unwrap Process

This section describes the recipient processing using KEA to generate
the SKIPJACK KEK and the subsequent decryption of the SKIPJACK CEK.

1) Compliant software MUST be capable of processing EnvelopedData
objects constructed using both the shared and the unique
originator UKM options. To support the shared UKM option, the
receiving software MUST be capable of searching for the
recipient's RecipientEncryptedKey in a KeyAgreeRecipientInfo
recipientEncryptedKeys SEQUENCE OF RecipientEncryptedKey. To
support the unique UKM option, the receiving software MUST be
capable of searching for the recipient's RecipientEncryptedKey in
the EnvelopedData recipientInfos SET OF RecipientInfo, with each
RecipientInfo containing exactly one RecipientEncryptedKey. For
each RecipientEncryptedKey, if the rid rkeyId CHOICE is present,
then the receiving software MUST attempt to match the value of the
subjectKeyIdentifier extension from the recipient's KEA X.509 v3
certificate with the RecipientEncryptedKey rid rKeyId
subjectKeyIdentifier field. If the rid issuerAndSerialNumber
CHOICE is present, then the receiving software MUST attempt to
match the values of the issuer name and serial number from the
recipient's KEA X.509 v3 certificate with the
RecipientEncryptedKey rid issuerAndSerialNumber field.

2) The receiving software MUST extract the originator's UKM from the
ukm OCTET STRING contained in the same KeyAgreeRecipientInfo that
includes the recipient's RecipientEncryptedKey.

3) The receiving software MUST locate the originator's KEA X.509 v3
certificate identified by the originator field contained in the
same KeyAgreeRecipientInfo that includes the recipient's
RecipientEncryptedKey.

4) KEA MUST be used to generate the pairwise KEK based on the
originator's UKM, originator's 128-byte public KEA key (extracted
from originator's KEA X.509 v3 certificate), recipient's private
KEA key (associated with recipient's KEA X.509 v3 certificate
identified by the RecipientEncryptedKey rid field) and the
originator's 128-byte public KEA key used as the Rb value.

5) The SKIPJACK CEK MUST be unwrapped using the KEA-generated
pairwise KEK as input to the FORTEZZA 80-bit unwrap function.

6) The unwrapped 80-bit SKIPJACK CEK resulting from the SKIPJACK CEK
unwrap process and the 8-byte IV obtained from the EnvelopedData
encryptedContentInfo contentEncryptionAlgorithm parameters field
are used as inputs to the SKIPJACK content decryption process to
decrypt the EnvelopedData encryptedContent.

4.3. "Previously Distributed" Symmetric KEK

This section describes the conventions for using SKIPJACK with the
CMS enveloped-data content type to support "previously distributed"
symmetric KEKs. When a "previously distributed" symmetric KEK is
used to wrap the SKIPJACK CEK, then the RecipientInfo
KEKRecipientInfo CHOICE MUST be used. The methods used to generate
and distribute the symmetric KEK are beyond the scope of this
document.

The KEKRecipientInfo fields MUST be populated as specified in [CMS]
Section 6.2.3, "KEKRecipientInfo Type". The KEKRecipientInfo
keyEncryptionAlgorithm algorithm field MUST be the id-fortezzaWrap80
OID indicating that the FORTEZZA 80-bit wrap function is used to wrap
the 80-bit SKIPJACK CEK. The KEKRecipientInfo keyEncryptionAlgorithm
parameters field MUST be absent. The KEKRecipientInfo encryptedKey
field MUST include the SKIPJACK CEK wrapped using the "previously
distributed" symmetric KEK as input to the FORTEZZA 80-bit wrap
function.

5. Encrypted-data Conventions

The CMS encrypted-data content type consists of an encrypted content,
but no recipient information. The method for conveying the SKIPJACK
CEK required to decrypt the encrypted-data encrypted content is
beyond the scope of this document. Compliant software MUST meet the
requirements for constructing an encrypted-data content type stated
[CMS] Section 8, "Encrypted-data Content Type". [CMS] Section 8
should be studied before reading this section, because this section
does not repeat the [CMS] text.

The encrypted-data content type is ASN.1 encoded using the
EncryptedData syntax. The fields of the EncryptedData syntax must be
populated as follows:

The EncryptedData version MUST be set according to [CMS] Section 8.

The EncryptedData encryptedContentInfo contentEncryptionAlgorithm
algorithm field MUST be the id-fortezzaConfidentialityAlgorithm OID.
The EncryptedData encryptedContentInfo contentEncryptionAlgorithm
parameters field MUST include the random 8-byte IV used as the input
to the content encryption process.

The EncryptedData unprotectedAttrs MAY be present.

6. FORTEZZA 80-bit Wrap Function

The United States Government has not published the description of the
FORTEZZA 80-bit wrap function.

7. SMIMECapabilities Attribute Conventions

RFC2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed
attribute (defined as a SEQUENCE of SMIMECapability SEQUNCEs) to be
used to specify a partial list of algorithms that the software
announcing the SMIMECapabilities can support. When constructing a
signedData object, compliant software MAY include the
SMIMECapabilities signed attribute announcing that it supports the
KEA and SKIPJACK algorithms.

The SMIMECapability SEQUENCE representing KEA MUST include the id-
kEAKeyEncryptionAlgorithm OID in the capabilityID field and MUST
include a KeyWrapAlgorithm SEQUENCE in the parameters field. The
algorithm field of KeyWrapAlgorithm MUST be the id-fortezzaWrap80
OID. The parameters field of KeyWrapAlgorithm MUST be absent. The
SMIMECapability SEQUENCE for KEA SHOULD be included in the key
management algorithms portion of the SMIMECapabilities list. The
SMIMECapability SEQUENCE representing KEA MUST be DER-encoded as the
following hexadecimal string:

3018 0609 6086 4801 6502 0101 1830 0b06 0960 8648 0165 0201 0117

The SMIMECapability SEQUENCE representing SKIPJACK MUST include the
id-fortezzaConfidentialityAlgorithm OID in the capabilityID field and
the parameters field MUST be absent. The SMIMECapability SEQUENCE
for SKIPJACK SHOULD be included in the symmetric encryption
algorithms portion of the SMIMECapabilities list. The
SMIMECapability SEQUENCE representing SKIPJACK MUST be DER-encoded as
the following hexadecimal string:

300b 0609 6086 4801 6502 0101 0400

8. Object Identifier Definitions

The following OIDs are specified in [INFO], but are repeated here for
the reader's convenience:

id-keyExchangeAlgorithm OBJECT IDENTIFIER ::= {joint-iso-ccitt(2)
country(16) us(840) organization(1) gov(101) dod(2) infosec(1)
algorithms(1) keyExchangeAlgorithm (22)}

id-fortezzaWrap80 OBJECT IDENTIFIER ::= {joint-iso-ccitt(2)
country(16) us(840) organization(1) gov(101) dod(2) infosec(1)
algorithms(1) fortezzaWrap80Algorithm (23)}

id-kEAKeyEncryptionAlgorithm OBJECT IDENTIFIER ::= {joint-iso-
ccitt(2) country(16) us(840) organization(1) gov(101) dod(2)
infosec(1) algorithms(1) kEAKeyEncryptionAlgorithm (24)}

id-fortezzaConfidentialityAlgorithm OBJECT IDENTIFIER ::= {joint-
iso-ccitt(2) country(16) us(840) organization(1) gov(101) dod(2)
infosec(1) algorithms(1) fortezzaConfidentialityAlgorithm (4)}

As specified in [USSUP1], when the id-
fortezzaConfidentialityAlgorithm OID is present in the
AlgorithmIdentifier algorithm field, then the AlgorithmIdentifier
parameters field MUST be present and MUST include the SKIPJACK IV
ASN.1 encoded using the following syntax:

Skipjack-Parm ::= SEQUENCE { initialization-vector OCTET STRING }

Note: [CMS] Section 2, "General Overview" describes the ASN.1
encoding conventions for the CMS content types including the
enveloped-data and encrypted-data content types in which the id-
fortezzaConfidentialityAlgorithm OID and parameters will be present.

References

[CMS] Housley, R., "Cryptographic Message Syntax", RFC2630,
June 1999.

[KEA] Housley, R. and W. Polk, "Representation of Key Exchange
Algorithm (KEA) Keys in Internet X.509 Public Key
Infrastructure Certificates", RFC2528, March 1999.

[INFO] Registry of INFOSEC Technical Objects, 22 July 1999.

[MSG] Ramsdell, B., "S/MIME Version 3 Message Specification",
RFC2633, June 1999.

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

[SJ-KEA] SKIPJACK and KEA Algorithm Specifications, Version 2.0,
http://csrc.nist.gov/encryption/skipjack-kea.htm.

[USSUP1] Allied Communication Publication 120 (ACP120) Common
Security Protocol (CSP) United States (US) Supplement
No. 1, June 1998;
http://www.armadillo.huntsville.al.us/Fortezza_docs/missi2.Html#specs.

Acknowledgments

The following people have made significant contributions to this
memo: David Dalkowski, Phillip Griffin, Russ Housley, Pierce
Leonberger, Rich Nicholas, Bob Relyea and Jim Schaad.

Author's Address

John Pawling
Wang Government Services, Inc. (WGSI),
A Getronics Company
141 National Business Pkwy, Suite 210
Annapolis Junction, MD 20701

Phone: (301) 939-2739
(410) 880-6095
EMail: john.pawling@wang.com

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