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Versions: 00 01 02 03 04 05 RFC 3211

Internet Draft                                      Editor: Peter Gutmann
draft-ietf-smime-password-03.txt                    University of Auckland
October 21, 2000
Expires April 2001

                   Password-based Encryption for S/MIME

Status of this memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.

Internet-Drafts are working documents of the Internet Engineering Task
Force (IETF), its areas, and its working groups.  Note that other
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The list of current Internet-Drafts can be accessed at
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The list of Internet-Draft Shadow Directories can be accessed at
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Abstract

The Cryptographic Message Syntax data format doesn't currently
contain any provisions for password-based data encryption.  This
document provides a method of encrypting data using user-supplied
passwords and, by extension, any form of variable-length keying
material which isn't necessarily an algorithm-specific fixed-format
key.

1. Introduction

This document describes a password-based content encryption mechanism
for S/MIME.  This is implemented as a new RecipientInfo type and is
an extension to the RecipientInfo types currently defined in RFC 2640
[RFC2640].

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 [RFC2119].

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 }

The fields of type PasswordRecipientInfo have the following meanings:

  version is the syntax version number.  It shall always be 0.

  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 content-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 [RFC 2898] 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.

2 Supported Algorithms

This section lists the algorithms that must be implemented.
Additional algorithms that should be implemented are also included.

2.1 Key Derivation Algorithms

These algorithms are used to convert the password into a KEK.  The
key derivation algorithms are:

    KeyDerivationAlgorithmIdentifer ::= AlgorithmIdentifier

CMS 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 content (the key) using the
derived KEK.  The content encryption algorithms are PBES2-Encs [RFC
2898].  Appendix B contains a more precise definition of the allowed
algorithm types than is possible using 1988 ASN.1.

CMS implementations MUST include Triple-DES in CBC mode, SHOULD
include RC2 in CBC mode, and MAY include other algorithms such as
AES, CAST-128, RC5, IDEA, Skipjack, Blowfish, and encryption modes as
required.  CMS implementations SHOULD NOT include any KSG ciphers
such as RC4 or a block cipher in OFB mode, and SHOULD NOT include a
block cipher in ECB mode.  The use of RC2 has special requirements,
see section 2.4 for details.

2.3 Symmetric Key Encryption Algorithms

The key wrap algorithm is used to wrap the CEK with the KEK.  There
is no requirement that the content-encryption algorithm match the KEK
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).

The key wrap algorithm specified below is independent of the content-
encryption or wrapping algorithms, relying only on the use of a block
cipher to perform the wrapping.

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 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.

  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.

  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.

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 CEK's 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 CEK's.  If an opponent manages to obtain a CEK, they may
be able to solve for other CEK's.  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 doens'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.

2.4 Special Handling for RC2 Keys

For a variety of historical, political, and software-peculiarity
reasons which are beyond the scope of this document, the handling of
keys for the RC2 algorithm [RFC2268] by different implementations is
somewhat arbitrary.  In particular, the choice of actual vs effective
key bits used in the algorithm is often unclear.  The standard RC2
AlgorithmIdentifier only allows the effective key bits to be
specified, leaving the actual key bits to be communicated via out-of-
band means, which in some cases means hardcoding them into
applications.  Solving this problem requires two things, a precise
definition of how keys represented with the standard RC2
AlgorithmIdentifier are handled, and a new RC2 AlgorithmIdentifier
which allows keys currently in use by different applications to be
handled.

2.4.1 Handling of RC2 with RFC 2268 AlgorithmIdentifier

RFC 2268 defines the following AlgorithmIdentifier for RC2:

    rc2CBC OBJECT IDENTIFIER ::= {iso(1) member-body(2) US(840)
                        rsadsi(113549) encryptionAlgorithm(3) 2}

    RC2-CBCParameter ::= CHOICE {
      iv IV,
      params SEQUENCE {
        version INTEGER,
        iv OCTET STRING
        }
      }

where the version field encodes the effective key size in a complex
manner specified in the RFC.  Where this algorithm identifier is
used, the actual key size shall be the same size as the effective key
size as given by the version field.  When RC2 is to be used,
implementations should use this AlgorithmIdentifier and parameters,
and when this AlgorithmIdentifier is used the actual key size MUST
NOT be a value other than the effective key size (to use a different
size, see section 2.4.2).

2.4.2 Handling of RC2 with Other Key Sizes

If the use of an actual key size of other than the effective key size
is required, implementations MUST use the following
AlgorithmIdentifier:

    id-alg-pwri-rc2CBC OBJECT IDENTIFIER ::= {1 3 6 1 4 1 3029 666 13}
    pwri-rc2CBCParameter ::= SEQUENCE {
      actualKeySize INTEGER,        -- Actual key size in bits
      effectiveKeySize INTEGER,     -- Effective key size in bits
      iv OCTET STRING
      }

This allows arbitrary actual and effective key sizes to be specified
for compatibility with existing usage.  Although implementations
SHOULD NOT use this alternative (using instead the one in section
2.4.1) experience has shown that implementors will continue to use
oddball RC2 parameters anyway, so new implementations should be
prepared to encounter and handle actual and effective key sizes
ranging from 40 up to around 200 bits.

2.4.3 Rationale

The reason for providing for the handling of oddball key sizes is
compatibility with existing applications, for example a mailing-list
exploder or mail gateway may take an RSA-wrapped CEK generated by a
current application and repackage it with a KEK, so we need a
mechanism for handling strange key lengths in a manner which is
compatible with existing usage.  The alternative RC2
AlgorithmIdentifier, although not recommended, provides a means of
ensuring this compatibility.

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.

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:

Key wrap phase (wrap CEK block using 3DES 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
            :       }
            :     }
  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
            :     }
  52 04   16:   OCTET STRING
            :     B8 1B 25 65 EE 37 3C A6 DE DC A2 6A 17 8B 0C 10
            :   }

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

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   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
            :     }
  56 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 2640 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.

Author Address

Peter Gutmann
University of Auckland
Private Bag 92019
Auckland, New Zealand
pgut001@cs.auckland.ac.nz

References

  ASN1  Recommendation X.680: Specification of Abstract Syntax
        Notation One (ASN.1), 1994.

  RFC2119 Key Words for Use in RFCs to Indicate Requirement Levels,
        S.Bradner, March 1997.

  RFC2268 A Description of the RC2(r) Encryption Algorithm, R.Rivest,
        March 1998.

  RFC2630 Cryptographic Message Syntax, R.Housley, June 1999.

  RFC2898 PKCS #5: Password-Based Cryptography Specification, Version
        2.0, B.Kaliski, September 2000.

  PACKAGE All-or-Nothing Encryption and the Package Transform,
        R.Rivest, Proceedings of Fast Software Encryption '97, Haifa,
        Israel, January 1997.


Appendix A: ASN.1:1988 Module

PasswordRecipientInfo
    { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
      smime(16) modules(0) pwri(12) }

DEFINITIONS IMPLICIT TAGS ::=
BEGIN

IMPORTS

  FROM PKCS5 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
               pkcs-5(5) }
    PBKDF2-params, PBES2-Encs;

PasswordRecipientInfo ::= SEQUENCE {
  version CMSVersion,       -- Always set to 0
  keyDerivationAlgorithm
                    [0] KeyDerivationAlgorithmIdentifier OPTIONAL,
  keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
  encryptedKey EncryptedKey }

KeyDerivationAlgorithmIdentifer ::= AlgorithmIdentifier

KeyEncryptionAlgorithmIdentifer ::= AlgorithmIdentifier

END

Appendix B: ASN.1:1994 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
    { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9)
      smime(16) modules(0) pwri(12) }

DEFINITIONS IMPLICIT TAGS ::=
BEGIN

IMPORTS

  FROM PKCS5 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1)
               pkcs-5(5) }
    PBKDF2-params, PBES2-Encs;

PasswordRecipientInfo ::= SEQUENCE {
  version CMSVersion,       -- Always set to 0
  keyDerivationAlgorithm
                    [0] KeyDerivationAlgorithmIdentifier OPTIONAL,
  keyEncryptionAlgorithm KeyEncryptionAlgorithmIdentifier,
  encryptedKey EncryptedKey }

KeyDerivationAlgorithmIdentifer ALGORITHM-IDENTIFIER ::= {
  { SYNTAX PBKDF2-params IDENTIFIED BY id-PBKDF2 },
  ...
  }

KeyEncryptionAlgorithmIdentifer ALGORITHM-IDENTIFIER ::= PBES2-Encs

END

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