Network Working Group P. Gutmann
Request for Comments: 3211 University of Auckland
Category: Standards Track December 2001
Password-based Encryption for CMS
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document provides a method of encrypting data using user-
supplied passwords and, by extension, any form of variable-length
keying material which is not necessarily an algorithm-specific
fixed-format key. The Cryptographic Message Syntax data format does
not currently contain any provisions for password-based data
encryption.
1. Introduction
This document describes a password-based content encryption mechanism
for CMS. This is implemented as a new RecipientInfo type and is an
extension to the RecipientInfo types currently defined in RFC 2630.
The format of the messages are described in ASN.1 [ASN1].
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC 2119.
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1.1 Password-based Content Encryption
CMS currently defined three recipient information types for public-
key key wrapping (KeyTransRecipientInfo), conventional key wrapping
(KEKRecipientInfo), and key agreement (KeyAgreeRecipientInfo). The
recipient information described here adds a fourth type,
PasswordRecipientInfo, which provides for password-based key
wrapping.
1.2 RecipientInfo Types
The new recipient information type is an extension to the
RecipientInfo type defined in section 6.2 of CMS, extending the types
to:
RecipientInfo ::= CHOICE {
ktri KeyTransRecipientInfo,
kari [1] KeyAgreeRecipientInfo,
kekri [2] KEKRecipientInfo,
pwri [3] PasswordRecipientinfo -- New RecipientInfo type
}
Although the recipient information generation process is described in
terms of a password-based operation (since this will be its most
common use), the transformation employed is a general-purpose key
derivation one which allows any type of keying material to be
converted into a key specific to a particular content-encryption
algorithm. Since the most common use for password-based encryption
is to encrypt files which are stored locally (rather than being
transmitted across a network), the term "recipient" is somewhat
misleading, but is used here because the other key transport
mechanisms have always been described in similar terms.
1.2.1 PasswordRecipientInfo Type
Recipient information using a user-supplied password or previously
agreed-upon key is represented in the type PasswordRecipientInfo.
Each instance of PasswordRecipientInfo will transfer the content-
encryption key (CEK) to one or more recipients who have the
previously agreed-upon password or key-encryption key (KEK).
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- Always set to 0
keyDerivationAlgorithm
[0] KeyDerivationAlgorithmIdentifier OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
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The fields of type PasswordRecipientInfo have the following meanings:
version is the syntax version number. It MUST be 0. Details of
the CMSVersion type are discussed in CMS [RFC2630], section
10.2.5.
keyDerivationAlgorithm identifies the key-derivation algorithm,
and any associated parameters, used to derive the KEK from the
user-supplied password. If this field is absent, the KEK is
supplied from an external source, for example a crypto token such
as a smart card.
keyEncryptionAlgorithm identifies the key-encryption algorithm,
and any associated parameters, used to encrypt the CEK with the
KEK.
encryptedKey is the result of encrypting the content-encryption
key with the KEK.
1.2.2 Rationale
Password-based key wrapping is a two-stage process, a first stage in
which a user-supplied password is converted into a KEK if required,
and a second stage in which the KEK is used to encrypt a CEK. These
two stages are identified by the two algorithm identifiers. Although
the PKCS #5v2 standard [RFC2898] goes one step further to wrap these
up into a single algorithm identifier, this design is particular to
that standard and may not be applicable for other key wrapping
mechanisms. For this reason the two steps are specified separately.
The current format doesn't provide any means of differentiating
between multiple password recipient infos, which would occur for
example if two passwords are used to encrypt the same data.
Unfortunately there is a lack of existing practice in this area,
since typical applications follow the model of encrypting data such
as a file with a single password obtained from the user. Without any
clear requirements, an appropriate multiple password mechanism would
be difficult (perhaps impossible) to define at this time. If
sufficient demand emerges then this may be addressed in a future
version of this document, for example by adding an optional
identification field of an appropriate form.
2 Supported Algorithms
This section lists the algorithms that must be implemented.
Additional algorithms that should be implemented are also included.
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2.1 Key Derivation Algorithms
These algorithms are used to convert the password into a KEK. The
key derivation algorithms are:
KeyDerivationAlgorithmIdentifer ::= AlgorithmIdentifier
Conforming implementations MUST include PBKDF2 [RFC2898]. Appendix B
contains a more precise definition of the allowed algorithm type than
is possible using 1988 ASN.1.
2.2 Key Encryption Algorithms
These algorithms are used to encrypt the CEK using the derived KEK.
The key encryption algorithms are:
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
The PasswordRecipientInfo key encryption algorithm identifier is:
id-alg-PWRI-KEK OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 9 }
The AlgorithmIdentifier parameters field for this algorithm contains
the KEK encryption algorithm used with the the key wrap algorithm
specified in section 2.3.
There is no requirement that the CEK algorithm match the KEK
encryption algorithm, although care should be taken to ensure that,
if different algorithms are used, they offer an equivalent level of
security (for example wrapping a Triple-DES key with an RC2/40 key
leads to a severe impedance mismatch in encryption strength).
Conforming implementations MUST implement the id-alg-PWRI-KEK key
wrap algorithm. For the KEK encryption algorithms used by id-alg-
PWRI-KEK, conforming implementations MUST include Triple-DES in CBC
mode and MAY include other algorithms such as AES, CAST-128, RC5,
IDEA, Skipjack, Blowfish, and encryption modes as required.
Implementations SHOULD NOT include any KSG (keystream generator)
ciphers such as RC4 or a block cipher in OFB mode, and SHOULD NOT
include a block cipher in ECB mode.
2.2.1 Rationale
The use of a level of indirection in specifying the
KeyEncryptionAlgorithmIdentifier allows alternative wrapping
algorithms to be used in the future. If the KEK algorithm were
specified directly in this field then any use of an alternative
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wrapping algorithm would require a change to the
PasswordRecipientInfo structure rather than simply a change to the
key encryption algorithm identifier.
The parameter field for this algorithm identifier could be specified
to default to triple-DES, however due to the confusion over NULL vs
absent parameters in algorithm identifiers it's left explicit with no
default value.
2.3.1 Key Wrap
The key wrap algorithm encrypts a CEK with a KEK in a manner which
ensures that every bit of plaintext effects every bit of ciphertext.
This makes it equivalent in function to the package transform
[PACKAGE] without requiring additional mechanisms or resources such
as hash functions or cryptographically strong random numbers. The
key wrap algorithm is performed in two phases, a first phase which
formats the CEK into a form suitable for encryption by the KEK, and a
second phase which wraps the formatted CEK using the KEK.
Key formatting: Create a formatted CEK block consisting of the
following:
1. A one-byte count of the number of bytes in the CEK.
2. A check value containing the bitwise complement of the first
three bytes of the CEK.
3. The CEK.
4. Enough random padding data to make the CEK data block a
multiple of the KEK block length and at least two KEK cipher
blocks long (the fact that 32 bits of count+check value are
used means that even with a 40-bit CEK, the resulting data size
will always be at least two (64-bit) cipher blocks long). The
padding data does not have to be cryptographically strong,
although unpredictability helps. Note that PKCS #5 padding is
not used, since the length of the data is already known.
The formatted CEK block then looks as follows:
CEK byte count || check value || CEK || padding (if required)
Key wrapping:
1. Encrypt the padded key using the KEK.
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2. Without resetting the IV (that is, using the last ciphertext
block as the IV), encrypt the encrypted padded key a second
time.
The resulting double-encrypted data is the EncryptedKey.
2.3.2 Key Unwrap
Key unwrapping:
1. Using the n-1'th ciphertext block as the IV, decrypt the n'th
ciphertext block.
2. Using the decrypted n'th ciphertext block as the IV, decrypt
the 1st ... n-1'th ciphertext blocks. This strips the outer
layer of encryption.
3. Decrypt the inner layer of encryption using the KEK.
Key format verification:
1a. If the CEK byte count is less than the minimum allowed key
size (usually 5 bytes for 40-bit keys) or greater than the
wrapped CEK length or not valid for the CEK algorithm (eg not
16 or 24 bytes for triple DES), the KEK was invalid.
1b. If the bitwise complement of the key check value doesn't match
the first three bytes of the key, the KEK was invalid.
2.3.3 Example
Given a content-encryption algorithm of Skipjack and a KEK algorithm
of Triple-DES, the wrap steps are as follows:
1. Set the first 4 bytes of the CEK block to the Skipjack key size
(10 bytes) and the bitwise complement of the first three bytes
of the CEK.
2. Append the 80-bit (10-byte) Skipjack CEK and pad the total to
16 bytes (two triple-DES blocks) using 2 bytes of random data.
2. Using the IV given in the KeyEncryptionAlgorithmIdentifer,
encrypted the padded Skipjack key.
3. Without resetting the IV, encrypt the encrypted padded key a
second time.
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The unwrap steps are as follows:
1. Using the first 8 bytes of the double-encrypted key as the IV,
decrypt the second 8 bytes.
2. Without resetting the IV, decrypt the first 8 bytes.
3. Decrypt the inner layer of encryption using the the IV given in
the KeyEncryptionAlgorithmIdentifer to recover the padded
Skipjack key.
4. If the length byte isn't equal to the Skipjack key size (80
bits or 10 bytes) or the bitwise complement of the check bytes
doesn't match the first three bytes of the CEK, the KEK was
invalid.
2.3.4 Rationale for the Double Wrapping
If many CEKs are encrypted in a standard way with the same KEK and
the KEK has a 64-bit block size then after about 2^32 encryptions
there is a high probability of a collision between different blocks
of encrypted CEKs. If an opponent manages to obtain a CEK, they may
be able to solve for other CEKs. The double-encryption wrapping
process, which makes every bit of ciphertext dependent on every bit
of the CEK, eliminates this collision problem (as well as preventing
other potential problems such as bit-flipping attacks). Since the IV
is applied to the inner layer of encryption, even wrapping the same
CEK with the same KEK will result in a completely different wrapped
key each time.
An additional feature of the double wrapping is that it doesn't
require the use of any extra algorithms such as hash algorithms in
addition to the wrapping algorithm itself, allowing it to be
implemented in devices which only support one type of encryption
algorithm. A typical example of such a device is a crypto token such
as a smart card which often only supports a single block cipher and a
single public-key algorithm, making it impossible to wrap keys if the
use of an additional algorithm were required.
3. Test Vectors
This section contains two sets of test vectors, a very basic set for
DES which can be used to verify correctness and which uses an
algorithm which is freely exportable from the US, and a stress-test
version which uses very long passphrase and key sizes and a mixture
of algorithms which can be used to verify the behaviour in extreme
cases.
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The basic test contains two subtests, a known-answer test for the key
derivation stage and a full test of the key wrapping. Both tests use
a DES-CBC key derived from the password "password" with salt { 12 34
56 78 78 56 34 12 } using 5 iterations of PBKDF2. In the known
answer test the IV is set to all zeroes (equivalent to using ECB) and
used to encrypt an all-zero data block.
The following values are obtained for the known-answer test:
PKCS #5v2 values:
input 70 61 73 73 77 6f 72 64
passphrase: "password"
input salt: 12 34 56 78 78 56 34 12
iterations: 5
output key: D1 DA A7 86 15 F2 87 E6
known answer: 9B BD 78 FC 11 A3 A9 08
The following values are obtained when wrapping a 64-bit (parity-
adjusted) DES-EBC key:
PKCS #5v2 values:
input 70 61 73 73 77 6f 72 64
passphrase: "password"
input salt: 12 34 56 78 78 56 34 12
iterations: 5
output key: D1 DA A7 86 15 F2 87 E6
CEK formatting phase:
length byte: 08
key check: 73 9D 83
CEK: 8C 62 7C 89 73 23 A2 F8
padding: C4 36 F5 41
complete 08 73 9D 83 8C 62 7C 89 73 23 A2 F8 C4 36 F5 41
CEK block:
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Key wrap phase (wrap CEK block using DES key):
IV: EF E5 98 EF 21 B3 3D 6D
first encr. 06 A0 43 86 1E 82 88 E4 8B 59 9E B9 76 10 00 D4
pass output:
second encr. B8 1B 25 65 EE 37 3C A6 DE DC A2 6A 17 8B 0C 10
pass output:
ASN.1 encoded PasswordRecipientInfo:
0 A3 68: [3] {
2 02 1: INTEGER 0
5 A0 26: [0] {
7 06 9: OBJECT IDENTIFIER id-PBKDF2 (1 2 840 113549 1 5 12)
18 30 13: SEQUENCE {
20 04 8: OCTET STRING
: 12 34 56 78 78 56 34 12
30 02 1: INTEGER 5
: }
: }
34 30 32: SEQUENCE {
36 06 11: OBJECT IDENTIFIER id-alg-PWRI-KEK
: (1 2 840 113549 1 9 16 3 9)
33 30 17: SEQUENCE {
35 06 5: OBJECT IDENTIFIER des-CBC (1 3 14 3 2 7)
42 04 8: OCTET STRING
: EF E5 98 EF 21 B3 3D 6D
: }
: }
68 04 16: OCTET STRING
: B8 1B 25 65 EE 37 3C A6 DE DC A2 6A 17 8B 0C 10
: }
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The following values are obtained when wrapping a 256-bit key (for
example one for AES or Blowfish) using a triple DES-CBC key derived
from the passphrase "All n-entities must communicate with other
n-entities via n-1 entiteeheehees" with salt
{ 12 34 56 78 78 56 34 12 } using 500 iterations of PBKDF2.
PKCS #5v2 values:
input 41 6C 6C 20 6E 2D 65 6E 74 69 74 69 65 73 20 6D
passphrase: 75 73 74 20 63 6F 6D 6D 75 6E 69 63 61 74 65 20
77 69 74 68 20 6F 74 68 65 72 20 6E 2d 65 6E 74
69 74 69 65 73 20 76 69 61 20 6E 2D 31 20 65 6E
74 69 74 65 65 68 65 65 68 65 65 73
"All n-entities must communicate with other "
"n-entities via n-1 entiteeheehees"
input
salt: 12 34 56 78 78 56 34 12
iterations: 500
output 6A 89 70 BF 68 C9 2C AE A8 4A 8D F2 85 10 85 86
3DES key: 07 12 63 80 CC 47 AB 2D
CEK formatting phase:
length byte: 20
key check: 73 9C 82
CEK: 8C 63 7D 88 72 23 A2 F9 65 B5 66 EB 01 4B 0F A5
D5 23 00 A3 F7 EA 40 FF FC 57 72 03 C7 1B AF 3B
padding: FA 06 0A 45
complete 20 73 9C 82 8C 63 7D 88 72 23 A2 F9 65 B5 66 EB
CEK block: 01 4B 0F A5 D5 23 00 A3 F7 EA 40 FF FC 57 72 03
C7 1B AF 3B FA 06 0A 45
Key wrap phase (wrap CEK block using 3DES key):
IV: BA F1 CA 79 31 21 3C 4E
first encr. F8 3F 9E 16 78 51 41 10 64 27 65 A9 F5 D8 71 CD
pass output: 27 DB AA 41 E7 BD 80 48 A9 08 20 FF 40 82 A2 80
96 9E 65 27 9E 12 6A EB
second encr. C0 3C 51 4A BD B9 E2 C5 AA C0 38 57 2B 5E 24 55
pass output: 38 76 B3 77 AA FB 82 EC A5 A9 D7 3F 8A B1 43 D9
EC 74 E6 CA D7 DB 26 0C
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ASN.1 encoded PasswordRecipientInfo:
0 A3 96: [3] {
2 02 1: INTEGER 0
5 A0 27: [0] {
7 06 9: OBJECT IDENTIFIER id-PBKDF2 (1 2 840 113549 1 5 12)
18 30 14: SEQUENCE {
20 04 8: OCTET STRING
: 12 34 56 78 78 56 34 12
30 02 2: INTEGER 500
: }
: }
34 30 35: SEQUENCE {
36 06 11: OBJECT IDENTIFIER id-alg-PWRI-KEK
: (1 2 840 113549 1 9 16 3 9)
34 30 20: SEQUENCE {
36 06 8: OBJECT IDENTIFIER des-EDE3-CBC (1 2 840 113549 3 7)
46 04 8: OCTET STRING
: BA F1 CA 79 31 21 3C 4E
: }
: }
71 04 40: OCTET STRING
: C0 3C 51 4A BD B9 E2 C5 AA C0 38 57 2B 5E 24 55
: 38 76 B3 77 AA FB 82 EC A5 A9 D7 3F 8A B1 43 D9
: EC 74 E6 CA D7 DB 26 0C
: }
4. Security Considerations
The security of this recipient information type rests on the security
of the underlying mechanisms employed, for which further information
can be found in RFC 2630 and PKCS5v2. More importantly, however,
when used with a password the security of this information type rests
on the entropy of the user-selected password, which is typically
quite low. Pass phrases (as opposed to simple passwords) are
STRONGLY RECOMMENDED, although it should be recognized that even with
pass phrases it will be difficult to use this recipient information
type to derive a KEK with sufficient entropy to properly protect a
128-bit (or higher) CEK.
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5. IANA Considerations
The PasswordRecipientInfo key encryption algorithms are identified by
object identifiers (OIDs). OIDs were assigned from an arc
contributed to the S/MIME Working Group by the RSA Security. Should
additional encryption algorithms be introduced, the advocates for
such algorithms are expected to assign the necessary OIDs from their
own arcs. No action by the IANA is necessary for this document or
any anticipated updates.
Acknowledgments
The author would like to thank Jim Schaad, Phil Griffin, and the
members of the S/MIME Working Group for their comments and feedback
on this document.
Author Address
Peter Gutmann
University of Auckland
Private Bag 92019
Auckland, New Zealand
EMail:
[email protected]
References
[ASN1] CCITT Recommendation X.208: Specification of Abstract
Syntax Notation One (ASN.1), 1988.
[RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2630] Housley, R., "Cryptographic Message Syntax", RFC 2630, June
1999.
[RFC2898] Kaliski, B., "PKCS #5: Password-Based Cryptography
Specification, Version 2.0", RFC 2898, September 2000.
[PACKAGE] All-or-Nothing Encryption and the Package Transform, R.
Rivest, Proceedings of Fast Software Encryption '97, Haifa,
Israel, January 1997.
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Appendix A: ASN.1:1988 Module
PasswordRecipientInfo-88
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) pwri(17) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
AlgorithmIdentifier
FROM AuthenticationFramework { joint-iso-itu-t ds(5) module(1)
authenticationFramework(7) 3 }
CMSVersion, EncryptedKey
FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) cms(1) };
-- The following PDU is defined in PKCS5 { iso(1) member-body(2)
-- us(840) rsadsi(113549) pkcs(1) pkcs-5(5) modules(16)
-- pkcs5v2-0(1) }, however it can't be imported because because
-- it's specified in 1994/1997 ASN.1. Because of this it's copied
-- here from the source but rephrased as 1988 ASN.1. Further
-- details are given in [RFC 2898].
PBKDF2-params ::= SEQUENCE {
salt OCTET STRING,
iterationCount INTEGER (1..MAX),
keyLength INTEGER (1..MAX) OPTIONAL,
prf AlgorithmIdentifier
DEFAULT { algorithm id-hmacWithSHA1, parameters NULL } }
-- The PRF algorithm is also defined in PKCS5 and can neither be
-- imported nor expressed in 1988 ASN.1, however it is encoded as
-- an AlgorithmIdentifier with the OID:
id-hmacWithSHA1 OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) digestAlgorithm(2) 7 }
-- and NULL parameters. Further details are given in [RFC 2898].
-- Implementation note: Because of the inability to precisely
-- specify the PBKDF2 PDU or its parameters in 1988 ASN.1, it is
-- likely that implementors will also encounter alternative
-- interpretations of these parameters, usually using an alternate
-- OID from the IPsec arc which is generally used for HMAC-SHA1:
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RFC 3211 Password-based Encryption for CMS December 2001
--
-- hMAC-SHA1 OBJECT IDENTIFIER ::= { iso(1)
-- identified-organization(3) dod(6) internet(1) security(5)
-- mechanisms(5) 8 1 2 }
--
-- with absent (rather than NULL) parameters.
-- The PasswordRecipientInfo
id-alg-PWRI-KEK OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 9 }
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- Always set to 0
keyDerivationAlgorithm
[0] KeyDerivationAlgorithmIdentifier OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
KeyDerivationAlgorithmIdentifier ::= AlgorithmIdentifier
KeyEncryptionAlgorithmIdentifier ::= AlgorithmIdentifier
END -- PasswordRecipientInfo-88 --
Appendix B: ASN.1:1997 Module
This appendix contains the same information as Appendix A in a more
recent (and precise) ASN.1 notation, however Appendix A takes
precedence in case of conflict.
PasswordRecipientInfo-97
{ iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) pwri(18) }
DEFINITIONS IMPLICIT TAGS ::=
BEGIN
IMPORTS
id-PBKDF2, PBKDF2-params,
FROM PKCS5 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
pkcs-5(5) }
CMSVersion, EncryptedKey, des-ede3-cbc, CBCParameter
FROM CryptographicMessageSyntax { iso(1) member-body(2) us(840)
rsadsi(113549) pkcs(1) pkcs-9(9)
smime(16) modules(0) cms(1) };
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RFC 3211 Password-based Encryption for CMS December 2001
id-alg-PWRI-KEK OBJECT IDENTIFIER ::= { iso(1) member-body(2)
us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) alg(3) 9 }
PasswordRecipientInfo ::= SEQUENCE {
version CMSVersion, -- Always set to 0
keyDerivationAlgorithm
[0] KeyDerivationAlgorithmIdentifier OPTIONAL,
keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
encryptedKey EncryptedKey }
KeyDerivationAlgorithmIdentifier ::=
AlgorithmIdentifier {{ KeyDerivationAlgorithms }}
KeyDerivationAlgorithms ALGORITHM ::= {
{ OID id-PBKDF2 PARMS PBKDF2-params },
...
}
KeyEncryptionAlgorithmIdentifier ::=
AlgorithmIdentifier {{ KeyEncryptionAlgorithms }}
KeyEncryptionAlgorithms ALGORITHM ::= {
{ OID id-alg-PWRI-KEK PARMS
AlgorithmIdentifier {{ PWRIAlgorithms }} },
...
}
-- Algorithm identifiers for algorithms used with the
-- id-alg-PWRI-KEK key wrap algorithm. Currently only 3DES is a
-- MUST, all others are optional
PWRIAlgorithms ALGORITHM ::= {
{ OID des-ede3-cbc PARMS CBCParameter },
...
}
-- Supporting definitions. We could also pull in the
-- AlgorithmIdentifier from an appropriately recent X.500 module (or
-- wherever) but it's just as easy (and more convenient for readers)
-- to provide a definition here
AlgorithmIdentifier { ALGORITHM:IOSet } ::= SEQUENCE {
algorithm ALGORITHM.&id({IOSet}),
parameters ALGORITHM.&Type({IOSet}{@algorithm}) OPTIONAL
}
ALGORITHM ::= CLASS {
&id OBJECT IDENTIFIER UNIQUE,
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&Type OPTIONAL
}
WITH SYNTAX { OID &id [PARMS &Type] }
END -- PasswordRecipientInfo-97 --
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Gutmann Standards Track [Page 17]
