RFC1848 - MIME Object Security Services

Network Working Group S. Crocker
Request For Comments: 1848 CyberCash, Inc.
Category: Standards Track N. Freed
Innosoft International, Inc.
J. Galvin
S. Murphy
Trusted Information Systems
October 1995
MIME Object Security Services
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.
Abstract
This document defines MIME Object Security Services (MOSS), a
protocol that uses the multipart/signed and multipart/encrypted
framework [7] to apply digital signature and encryption services to
MIME objects. The services are offered through the use of end-to-end
cryptography between an originator and a recipient at the application
layer. Asymmetric (public key) cryptography is used in support of
the digital signature service and encryption key management.
Symmetric (secret key) cryptography is used in support of the
encryption service. The procedures are intended to be compatible
with a wide range of public key management approaches, including both
ad hoc and certificate-based schemes. Mechanisms are provided to
support many public key management approaches.
Table of Contents
1. IntrodUCtion ............................................. 3
2. Applying MIME Object Security Services ................... 4
2.1 Digital Signature Service ............................... 4
2.1.1 Canonicalization ...................................... 5
2.1.2 Digital Signature Control Information ................. 7
2.1.2.1 Version: ............................................ 8
2.1.2.2 Originator-ID: ...................................... 8
2.1.2.3 MIC-Info: ........................................... 8
2.1.3 application/moss-signature Content Type Definition .... 9
2.1.4 Use of multipart/signed Content Type .................. 10
2.2 Encryption Service ...................................... 11
2.2.1 Encryption Control Information ........................ 12
2.2.1.1 DEK-Info: ........................................... 13
2.2.1.2 Recipient-ID: ....................................... 14
2.2.1.3 Key-Info: ........................................... 14
2.2.2 application/moss-keys Content Type Definition ......... 15
2.2.3 Use of multipart/encrypted Content Type ............... 16
3. Removing MIME Object Security Services ................... 17
3.1 Digital Signature Service ............................... 18
3.1.1 Preparation ........................................... 18
3.1.2 Verification .......................................... 19
3.1.3 Results ............................................... 19
3.2 Encryption Service ...................................... 20
3.2.1 Preparation ........................................... 20
3.2.2 Decryption ............................................ 20
3.2.3 Results ............................................... 21
4. Identifying Originators, Recipients, and Their Keys ...... 21
4.1 Name Forms .............................................. 23
4.1.1 Email Addresses ....................................... 23
4.1.2 Arbitrary Strings ..................................... 23
4.1.3 Distinguished Names ................................... 23
4.2 Identifiers ............................................. 24
4.2.1 Email Address ......................................... 25
4.2.2 Arbitrary String ...................................... 25
4.2.3 Distinguished Name .................................... 26
4.2.4 Public Key ............................................ 26
4.2.5 Issuer Name and Serial Number ......................... 27
5. Key Management Content Types ............................. 27
5.1 application/mosskey-request Content Type Definition ..... 28
5.2 application/mosskey-data Content Type Definition ........ 29
6. Examples ................................................. 31
6.1 Original Message Prepared for Protection ................ 31
6.2 Sign Text of Original Message ........................... 32
6.3 Sign Headers and Text of Original Message ............... 32
6.4 Encrypt Text of a Message ............................... 33
6.5 Encrypt the Signed Text of a Message .................... 35
6.6 Protecting Audio Content ................................ 37
6.6.1 Sign Audio Content .................................... 37
6.6.2 Encrypt Audio Content ................................. 37
7. Observations ............................................. 38
8. Comparison of MOSS and PEM Protocols ..................... 39
9. Security Considerations .................................. 41
10. Acknowledgements ........................................ 41
11. References .............................................. 41
12. Authors" Addresses ...................................... 43
Appendix A: Collected Grammar .............................. 44
Appendix B: Imported Grammar ............................... 47
1. Introduction
MIME [2], an acronym for "Multipurpose Internet Mail Extensions",
defines the format of the contents of Internet mail messages and
provides for multi-part textual and non-textual message bodies. An
Internet electronic mail message consists of two parts: the headers
and the body. The headers form a collection of field/value pairs
structured according to STD 11, RFC822 [1], whilst the body, if
structured, is defined according to MIME. MIME does not provide for
the application of security services.
PEM [3-6], an acronym for "Privacy Enhanced Mail", defines message
encryption and message authentication procedures for text-based
electronic mail messages using a certificate-based key management
mechanism. The specifications include several features that are
easily and more naturally supported by MIME, for example, the
transfer encoding operation, the Content-Domain header, and the
support services specified by its Part IV [6]. The specification is
limited by specifying the application of security services to text
messages only.
MOSS is based in large part on the PEM protocol as defined by RFC
1421. Many of PEMs features and most of its protocol specification
are included here. A comparison of MOSS and PEM may be found in
Section 8.
In order to make use of the MOSS services, a user (where user is not
limited to being a human, e.g., it could be a process or a role) is
required to have at least one public/private key pair. The public
key must be made available to other users with whom secure
communication is desired. The private key must not be disclosed to
any other user.
An originator"s private key is used to digitally sign MIME objects; a
recipient would use the originator"s public key to verify the digital
signature. A recipient"s public key is used to encrypt the data
encrypting key that is used to encrypt the MIME object; a recipient
would use the corresponding private key to decrypt the data
encrypting key so that the MIME object can be decrypted.
As long as the private keys are protected from disclosure, i.e., the
private keys are Accessible only to the user to whom they have been
assigned, the recipient of a digitally signed message will know from
whom the message was sent and the originator of an encrypted message
will know that only the intended recipient is able to read it. For
assurance, the ownership of the public keys used in verifying digital
signatures and encrypting messages should be verified. A stored
public key should be protected from modification.
The framework defined in [7] provides an embodiment of a MIME object
and its digital signature or encryption keys. When used by MOSS the
framework provides digital signature and encryption services to
single and multi-part textual and non-textual MIME objects.
2. Applying MIME Object Security Services
The application of the MOSS digital signature service requires the
following components.
(1) The data to be signed.
(2) The private key of the originator.
The data to be signed is prepared according to the description below.
The digital signature is created by generating a hash of the data and
encrypting the hash value with the private key of the originator.
The digital signature, some additional ancillary information
described below, and the data are then embodied in a multipart/signed
body part. Finally, the multipart/signed body part may be
transferred to a recipient or processed further, for example, it may
be encrypted.
The application of the MOSS encryption service requires the following
components.
(1) The data to be encrypted.
(2) A data encrypting key to encrypt the data.
(3) The public key of the recipient.
The data to be encrypted is prepared according to the description
below. The originator creates a data encrypting key and encrypts the
data. The recipient"s public key is used to encrypt the data
encrypting key. The encrypted data, the encrypted data encrypting
key, and some additional ancillary information described below are
then embodied in a multipart/encrypted body part, ready to be
transferred to a recipient or processed further, for example, it may
be signed.
The next two sections describe the digital signature and encryption
services, respectively, in detail.
2.1. Digital Signature Service
The MOSS digital signature service is applied to MIME objects,
specifically a MIME body part. The MIME body part is created
according to a local convention and then made available to the
digital signature service.
The following sequence of steps comprises the application of the
digital signature service.
(1) The body part to be signed must be canonicalized.
(2) The digital signature and other control information must be gen-
erated.
(3) The control information must be embodied in an appropriate MIME
content type.
(4) The control information body part and the data body part must be
embodied in a multipart/signed content type.
Each of these steps is described below.
2.1.1. Canonicalization
The body part must be converted to a canonical form that is uniquely
and unambiguously representable in at least the environment where the
digital signature is created and the environment where the digital
signature will be verified, i.e., the originator and recipient"s
environment, respectively. This is required in order to ensure that
both the originator and recipient have the same data with which to
calculate the digital signature; the originator needs to be able to
create the digital signature value while the recipient needs to be
able to compare a re-computed value with the received value. If the
canonical form is representable on many different host computers, the
signed data may be forwarded by recipients to additional recipients,
who will also be able to verify the original signature. This service
is called forwardable authentication.
The canonicalization transformation is a two step process. First,
the body part must be converted to a form that is unambiguously
representable on as many different host computers as possible.
Second, the body part must have its line delimiters converted to a
unique and unambiguous representation.
The representation chosen to satisfy the first step is 7bit, as
defined by MIME; the high order bit of each octet of the data to be
signed must be zero. A MIME body part is comprised of two parts:
headers and content. Since the headers of body parts are already
required to be represented in 7bit, this step does not require
changes to the headers. This step requires that if the content is
not already 7bit then it must be encoded with an appropriate MIME
content transfer encoding and a Content-Transfer-Encoding: header
must be added to the headers. For example, if the content to be
signed contains 8bit or binary data, the content must be encoded with
either the quoted-printable or base64 encoding as defined by MIME.
IMPLEMENTORS NOTE: Since the MIME standard eXPlicitly disallows
nested content transfer encodings, i.e., the content types
multipart and message may not themselves be encoded, the 7bit
transformation requires each nested body part to be individually
encoded in a 7bit representation. Any valid MIME encoding, e.g.,
quoted-printable or base64, may be used and, in fact, a different
encoding may be used on each of the non-7bit body parts.
Representing all content types in a 7bit format transforms them into
text-based content types. However, text-based content types present
a unique problem. In particular, the line delimiter used for a
text-based content type is specific to a local environment; different
environments use the single character carriage-return (<CR>), the
single character line-feed (<LF>), or the two character sequence
"carriage-return line-feed (<CR><LF>)".
The application of the digital signature service requires that the
same line delimiter be used by both the originator and the recipient.
This document specifies that the two character sequence "<CR><LF>"
must be used as the line delimiter. Thus, the second step of the
canonicalization transformation includes the conversion of the local
line delimiter to the two character sequence "<CR><LF>".
The conversion to the canonical line delimiter is only required for
the purposes of computing the digital signature. Thus, originators
must apply the line delimiter conversion before computing the digital
signature but must transfer the data without the line delimiter
conversion. Similarly, recipients must apply the line delimiter
conversion before computing the digital signature.
NOTE: An originator can not transfer the content with the line
delimiter conversion intact because the conversion process is not
idempotent. In particular, SMTP servers may themselves convert
the line delimiter to a local line delimiter, prior to the message
being delivered to the recipient. Thus, a recipient has no way of
knowing if the conversion is present or not. If the recipient
applies the conversion to a content in which it is already
present, the resulting content may have two line delimiters
present, which would cause the verification of the signature to
fail.
IMPLEMENTORS NOTE: Implementors should be aware that the
conversion to a 7bit representation is a function that is required
in a minimally compliant MIME user agent. Further, the line
delimiter conversion required here is distinct from the same
conversion included in that function. Specifically, the line
delimiter conversion applied when a body part is converted to a
7bit representation (transfer encoded) is performed prior to the
application of the transfer encoding. The line delimiter
conversion applied when a body part is signed is performed after
the body part is converted to 7bit (transfer encoded). Both line
delimiter conversions are required.
2.1.2. Digital Signature Control Information
The application of the digital signature service generates control
information which includes the digital signature itself. The syntax
of the control information is that of a set of RFC822 headers,
except that the folding of header values onto continuation lines is
explicitly forbidden. Each header and value pair generated by the
digital signature service must be output on exactly one line.
The complete set of headers generated by the digital signature
service is as follows.
Version:
indicates which version of the MOSS protocol the remaining headers
represent.
Originator-ID:
indicates the private key used to create the digital signature and
the corresponding public key to be used to verify it.
MIC-Info:
contains the digital signature value.
Each invocation of the digital signature service must emit exactly
one Version: header and at least one pair of Originator-ID: and MIC-
Info: headers. The Version: header must always be emitted first.
The Originator-ID: and MIC-Info: headers are always emitted in pairs
in the order indicated. This specification allows an originator to
generate multiple signatures of the data, presumably with different
signature algorithms, and to include them all in the control
information. The interpretation of the presence of multiple
signatures is outside the scope of this specification except that a
MIC-Info: header is always interpreted in the context of the
immediately preceding Originator-ID: header.
2.1.2.1. Version:
The version header is defined by the grammar token <version> as
follows.
<version> ::= "Version:" "5" CRLF
Its value is constant and MOSS implementations compliant with this
specification must recognize only this value and generate an error if
any other value is found.
2.1.2.2. Originator-ID:
The purpose of the originator header is two-fold: to directly
identify the public key to be used to verify the digital signature
and to indirectly identify the user who owns both it and its
corresponding private key. Typically, a recipient is less interested
in the actual public key value, although obviously the recipient
needs the value to verify the signature, and more interested in
identifying its owner. Thus, the originator header may convey either
or both pieces of information:
the public key to be used to verify the signature
the name of the owner and which of the owner"s public keys to use
to verify the signature
The decision as to what information to place in the value rests
entirely with the originator. The suggested value is to include
both. Recipients with whom the originator has previously
communicated will have to verify that the information presented is
consistent with what is already known. New recipients will want all
of the information, which they will need to verify prior to storing
in their local database.
The originator header is defined by the grammar token <origid> as
follows.
<origid> ::= "Originator-ID:" <id> CRLF
The grammar token <id> is defined in Section 4.
2.1.2.3. MIC-Info:
The purpose of the Message Integrity Check (MIC) header is to convey
the digital signature value. Its value is a comma separated list of
three arguments: the hash (or MIC) algorithm identifier, the
signature algorithm identifier, and the digital signature.
The MIC header is defined by the grammar token <micinfo> as follows.
<micinfo> ::= "MIC-Info:" <micalgid> "," <ikalgid> ","
<asymsignmic> CRLF
The grammar tokens for the MIC algorithms and identifiers
(<micalgid>), signature algorithms and identifiers (<ikalgid>), and
signed MIC formats (<asymsignmic>) are defined by RFC1423. They are
also reprinted in Appendix B.
IMPLEMENTORS NOTE: RFC1423 is referenced by the PEM protocol,
which includes support for symmetric signatures and key
management. As a result, some of the grammar tokens defined
there, for example, <ikalgid>, will include options that are not
legal for this protocol. These options must be ignored and have
not been included in the appendix.
2.1.3. application/moss-signature Content Type Definition
(1) MIME type name: application
(2) MIME suBType name: moss-signature
(3) Required parameters: none
(4) Optional parameters: none
(5) Encoding considerations: quoted-printable is always sufficient
(6) Security considerations: none
The "application/moss-signature" content type is used on the second
body part of an enclosing multipart/signed. Its content is comprised
of the digital signature of the data in the first body part of the
enclosing multipart/signed and other control information required to
verify that signature, as defined by Section 2.1.2. The label
"application/moss-signature" must be used as the value of the
protocol parameter of the enclosing multipart/signed; the protocol
parameter must be present.
Part of the signature verification information will be the Message
Integrity Check (MIC) algorithm(s) used during the signature creation
process. The MIC algorithm(s) identified in this body part must
match the MIC algorithm(s) identified in the micalg parameter of the
enclosing multipart/signed. If it does (they do) not, a user agent
should identify the discrepancy to a user and it may choose to either
halt or continue processing, giving precedence to the algorithm(s)
identified in this body part.
An application/moss-signature body part is constructed as follows:
Content-Type: application/moss-signature
<mosssig>
where the grammar token <mosssig> is defined as follows.
<mosssig> ::= <version> ( 1*<origasymflds> )
<version> ::= "Version:" "5" CRLF
<origasymflds> ::= <origid> <micinfo>
<origid> ::= "Originator-ID:" <id> CRLF
<micinfo> ::= "MIC-Info:" <micalgid> "," <ikalgid> ","
<asymsignmic> CRLF
The token <id> is defined in Section 4. All other tokens are defined
in Section 2.1.2.3.
2.1.4. Use of multipart/signed Content Type
The definition of the multipart/signed content type in [7] specifies
three steps for creating the body part.
(1) The body part to be digitally signed is created according to a
local convention, for example, with a text editor or a mail user
agent.
(2) The body part is prepared for the digital signature service
according to the protocol parameter, in this case according to
Section 2.1.1.
(3) The prepared body part is digitally signed according to the
protocol parameter, in this case according to Section 2.1.2.
The multipart/signed content type is constructed as follows.
(1) The value of its required parameter "protocol" is set to
"application/moss-signature".
(2) The signed body part becomes its first body part.
(3) Its second body part is labeled "application/moss-signature" and
is filled with the control information generated by the digital
signature service.
(4) The value of its required parameter "micalg" is set to the same
value used in the MIC-Info: header in the control information.
If there is more than one MIC-Info: header present the value is
set to a comma separated list of values from the MIC-Info
headers. The interpretation of the order of the list of values
is outside the scope of this specification.
A multipart/signed content type with the MOSS protocol might look as
follows:
Content-Type: multipart/signed;
protocol="application/moss-signature";
micalg="rsa-md5"; boundary="Signed Message"
--Signed Message
Content-Type: text/plain
This is some example text.
--Signed Message
Content-Type: application/moss-signature
Version: 5
Originator-ID: ID-INFORMATION
MIC-Info: RSA-MD5,RSA,SIGNATURE-INFORMATION
--Signed Message--
where ID-INFORMATION and SIGNATURE-INFORMATION are descriptive of the
content that would appear in a real body part.
2.2. Encryption Service
The MOSS encryption service is applied to MIME objects, specifically
a MIME body part. The MIME body part is created according to a local
convention and then made available to the encryption service.
The following sequence of steps comprises the application of the
encryption service.
(1) The body part to be encrypted must be in MIME canonical form.
(2) The data encrypting key and other control information must be
generated.
(3) The control information must be embodied in an appropriate MIME
content type.
(4) The control information body part and the encrypted data body
part must be embodied in a multipart/encrypted content type.
The first step is defined by MIME. The latter three steps are
described below.
2.2.1. Encryption Control Information
The application of the encryption service generates control
information which includes the data encrypting key used to encrypt
the data itself. The syntax of the control information is that of a
set of RFC822 headers, except that the folding of header values onto
continuation lines is explicitly forbidden. Each header and value
pair generated by the encryption service must be output on exactly
one line.
First, the originator must retrieve the public key of the recipient.
The retrieval may be from a local database or from a remote service.
The acquisition of the recipient"s public key is outside the scope of
the specification, although Section 5 defines one possible mechanism.
With the public key, the originator encrypts the data encrypting key
according to the Key-Info: header defined below. The complete set of
headers generated by the encryption service is as follows.
Version:
indicates which version of the MOSS protocol the remaining headers
represent and is defined in Section 2.1.2.1.
DEK-Info:
indicates the algorithm and mode used to encrypt the data.
Recipient-ID:
indicates the public key used to encrypt the data encrypting key
that was used to encrypt the data.
Key-Info:
contains data encrypting key encrypted with the recipient"s public
key.
Each invocation of the encryption service must emit exactly one
Version: header, exactly one DEK-Info: header, and at least one pair
of Recipient-ID: and Key-Info: headers. Headers are always emitted
in the order indicated. The Recipient-ID: and Key-Info: headers are
always emitted in pairs in the order indicated, one pair for each
recipient of the encrypted data. A Key-Info: header is always
interpreted in the context of the immediately preceding Recipient-ID:
header.
IMPLEMENTORS NOTE: Implementors should always generate a
Recipient-ID: and Key-Info header pair representing the originator
of the encrypted data. By doing so, if an originator sends a
message to a recipient that is returned undelivered, the
originator will be able to decrypt the message and determine an
appropriate course of action based on its content. If not, an
originator will not be able to review the message that was sent.
2.2.1.1. DEK-Info:
The purpose of the data encrypting key information header is to
indicate the algorithm and mode used to encrypt the data, along with
any cryptographic parameters that may be required, e.g.,
initialization vectors. Its value is either a single argument
indicating the algorithm and mode or a comma separated pair of
arguments where the second argument carries any cryptographic
parameters required by the algorithm and mode indicated in the first
argument.
The data encrypting key information header is defined by the grammar
token <dekinfo> as follows.
<dekinfo> ::= "DEK-Info" ":" <dekalgid>
[ "," <dekparameters> ] CRLF
The grammar tokens for the encryption algorithm and mode identifier
(<dekalgid>) and the optional cryptographic parameters
(<dekparameters>) are defined by RFC1423. They are also reprinted
in Appendix B.
2.2.1.2. Recipient-ID:
The purpose of the recipient header is to identify the private key
that must be used to decrypt the data encrypting key that will be
used to decrypt the data. Presumably the recipient owns the private
key and thus is less interested in identifying the owner of the key
and more interested in the private key value itself. Nonetheless,
the recipient header may convey either or both pieces of information:
the public key corresponding to the private key to be used to
decrypt the data encrypting key
the name of the owner and which of the owner"s private keys to use
to decrypt the data encrypting key
The decision as to what information to place in the value rests
entirely with the originator. The suggested choice is to include
just the public key. However, some recipients may prefer that
originators not include their public key. How this preference is
conveyed to and managed by the originator is outside the scope of
this specification.
The recipient header is defined by the grammar token <recipid> as
follows.
<recipid> ::= "Recipient-ID:" <id> CRLF
The grammar token <id> is defined in Section 4.
2.2.1.3. Key-Info:
The purpose of the key information header is to convey the encrypted
data encrypting key. Its value is a comma separated list of two
arguments: the algorithm and mode identifier in which the data
encrypting key is encrypted and the encrypted data encrypting key.
The key information header is defined by the grammar token
<asymkeyinfo> as follows.
<asymkeyinfo> ::= "Key-Info" ":" <ikalgid> "," <asymencdek> CRLF
The grammar tokens for the encryption algorithm and mode identifier
(<ikalgid>) and the encrypted data encrypting key format
(<asymsignmic>) are defined by RFC1423. They are also reprinted in
Appendix B.
IMPLEMENTORS NOTE: RFC1423 is referenced by the PEM protocol,
which includes support for symmetric signatures and key
management. As a result, some of the grammar tokens defined
there, for example, <ikalgid>, will include options that are not
legal for this protocol. These options must be ignored and have
not been included in the appendix.
2.2.2. application/moss-keys Content Type Definition
(1) MIME type name: application
(2) MIME subtype name: moss-keys
(3) Required parameters: none
(4) Optional parameters: none
(5) Encoding considerations: quoted-printable is always sufficient
(6) Security considerations: none
The "application/moss-keys" content type is used on the first body
part of an enclosing multipart/encrypted. Its content is comprised
of the data encryption key used to encrypt the data in the second
body part and other control information required to decrypt the data,
as defined by Section 2.2.1. The label "application/moss-keys" must
be used as the value of the protocol parameter of the enclosing
multipart/encrypted; the protocol parameter must be present.
An application/moss-keys body part is constructed as follows:
Content-Type: application/moss-keys
<mosskeys>
where the <mosskeys> token is defined as follows.
<mosskeys> ::= <version> <dekinfo> 1*<recipasymflds>
<version> ::= "Version:" "5" CRLF
<dekinfo> ::= "DEK-Info" ":" <dekalgid>
[ "," <dekparameters> ] CRLF
<recipasymflds> ::= <recipid> <asymkeyinfo>
<recipid> ::= "Recipient-ID:" <id> CRLF
<asymkeyinfo> ::= "Key-Info" ":" <ikalgid> "," <asymencdek> CRLF
The token <id> is defined in Section 4. The token <version> is
defined in Section 2.1.2.1. All other tokens are defined in Section
2.2.1.3.
2.2.3. Use of multipart/encrypted Content Type
The definition of the multipart/encrypted body part in [7] specifies
three steps for creating the body part.
(1) The body part to be encrypted is created according to a local
convention, for example, with a text editor or a mail user
agent.
(2) The body part is prepared for encryption according to the
protocol parameter, in this case the body part must be in MIME
canonical form.
(3) The prepared body part is encrypted according to the protocol
parameter, in this case according to Section 2.2.1.
The multipart/encrypted content type is constructed as follows.
(1) The value of its required parameter "protocol" is set to
"application/moss-keys".
(2) The first body part is labeled "application/moss-keys" and is
filled with the control information generated by the encryption
service.
(3) The encrypted body part becomes the content of its second body
part, which is labeled "application/octet-stream".
A multipart/encrypted content type with the MOSS protocol might look
as follows:
Content-Type: multipart/encrypted;
protocol="application/moss-keys";
boundary="Encrypted Message"
--Encrypted Message
Content-Type: application/moss-keys
Version: 5
DEK-Info: DES-CBC,DEK-INFORMATION
Recipient-ID: ID-INFORMATION
Key-Info: RSA,KEY-INFORMATION
--Encrypted Message
Content-Type: application/octet-stream
ENCRYPTED-DATA
--Encrypted Message--
where DEK-INFORMATION, ID-INFORMATION, and KEY-INFORMATION are
descriptive of the content that would appear in a real body part.
3. Removing MIME Object Security Services
The verification of the MOSS digital signature service requires the
following components.
(1) A recipient to verify the digital signature.
(2) A multipart/signed body part with two body parts: the signed
data and the control information.
(3) The public key of the originator.
The signed data and control information of the enclosing
multipart/signed are prepared according to the description below.
The digital signature is verified by re-computing the hash of the
data, decrypting the hash value in the control information with the
originator"s public key, and comparing the two hash values. If the
two hash values are equal, the signature is valid.
The decryption of the MOSS encryption service requires the following
components.
(1) A recipient to decrypt the data.
(2) A multipart/encrypted body part with two body parts: the
encrypted data and the control information.
(3) The private key of the recipient.
The encrypted data and control information of the enclosing
multipart/encrypted are prepared according to the description below.
The data encrypting key is decrypted with the recipient"s private key
and used to decrypt the data.
The next two sections describe the digital signature and encryption
services in detail, respectively.
3.1. Digital Signature Service
This section describes the processing steps necessary to verify the
MOSS digital signature service. The definition of the
multipart/signed body part in [7] specifies three steps for receiving
it.
(1) The digitally signed body part and the control information body
part are prepared for processing.
(2) The prepared body parts are made available to the digital
signature verification process.
(3) The results of the digital signature verification process are
made available to the user and processing continues with the
digitally signed body part, as returned by the digital signature
verification process.
Each of these steps is described below.
3.1.1. Preparation
The digitally signed body part (the data) and the control information
body part are separated from the enclosing multipart/signed body
part.
The control information is prepared by removing any content transfer
encodings that may be present.
The digitally signed body part is prepared by leaving the content
transfer encodings intact and canonicalizing the line delimiters
according to Step 2 of Section 2.1.1.
3.1.2. Verification
First, the recipient must obtain the public key of the originator.
The public key may be contained in the control information or it may
be necessary for the recipient to retrieve the public key based on
information present in the control information. The retrieval may be
from a local database or from a remote service. The acquisition of
the originator"s public key is outside the scope of the
specification, although Section 5 defines one possible mechanism.
With the public key, the recipient decrypts the hash value contained
in the control information. Then, a new hash value is computed over
the body part purported to have been digitally signed.
Finally, the two hash values are compared to determine the accuracy
of the digital signature.
3.1.3. Results
There are two required components of the results of the verification
process. The first is an indication as to whether a public key could
be found that allows the hash values in the previous step to compare
equal. Such an indication verifies only that the data received is
the same data that was digitally signed.
The second indication identifies the owner of the public key who is
presumably the holder of the private key that created the digital
signature. The indication must include a testament as to the
accuracy of the owner identification.
At issue is a recipient knowing who created the digital signature.
In order for the recipient to know with certainty who digitally
signed the message, the binding between the owner"s name and the
public key must have been verified by the recipient prior to the
verification of the digital signature. The verification of the
binding may have been completed offline and stored in a trusted,
local database or, if the owner"s name and public key are embodied in
a certificate, it may be possible to complete it in realtime. See
Section 5 for more information.
3.2. Encryption Service
This section describes the processing steps necessary to decrypt the
MOSS encryption service. The definition of the multipart/encrypted
body part in [7] specifies three steps for receiving it.
(1) The encrypted body part and the control information body part
are prepared for processing.
(2) The prepared body parts are made available to the decryption
process.
(3) The results of the decryption process are made available to the
user and processing continues with the decrypted body part, as
returned by the decryption process.
Each of these steps is described below.
3.2.1. Preparation
The encrypted body part (the data) and the control information body
part are separated from the enclosing multipart/encrypted body part.
The body parts are prepared for the decryption process by removing
any content transfer encodings that may be present.
3.2.2. Decryption
First, the recipient must locate the encrypted data encrypting key in
the control information. Each Recipient-ID: header is checked in
order to see if it identifies the recipient or a public key of the
recipient.
If it does, the immediately following Key-Info: header will contain
the data encrypting key encrypted with the public key of the
recipient. The recipient must use the corresponding private key to
decrypt the data encrypting key.
The data is decrypted with the data encrypting key. The decrypted
data will be a MIME object, a body part, ready to be processed by a
MIME agent.
3.2.3. Results
If the recipient is able to locate and decrypt a data encrypting key,
from the point of view of MOSS the decryption should be considered
successful. An indication of the owner of the private key used to
decrypt the data encrypting key must be made available to the user.
Ultimately, the success of the decryption is dependent on the ability
of a MIME agent to continue processing with the decrypted body part.
4. Identifying Originators, Recipients, and Their Keys
In the PEM specifications, public keys are required to be embodied in
certificates, an object that binds each public key with a
distinguished name. A distinguished name is a name form that
identifies the owner of the public key. The embodiment is issued by
a certification authority, a role that is expected to be trustworthy
insofar as the certification authority would have procedures to
verify the identity of the owner prior to issuing the certificate.
In MOSS, a user is not required to have a certificate. The MOSS
services require that the user have at least one public/private key
pair. The MOSS protocol requires the digital signature and
encryption services to emit Originator-ID: and Recipient-ID: headers,
as appropriate. In the discussion above the actual value of these
headers was omitted, having been relegated to this section. Although
the value of each of these headers serves a distinct purpose, for
simplicity the single grammar token <id> represents the value that
may be assigned to either header.
One possible value for the Originator-ID: and Recipient-ID: headers
is the public key values themselves. However, while it is true that
the public keys alone could be exchanged and used by users to
communicate, the values are, in fact, large and cumbersome. In
addition, public keys would appear as a random sequence of characters
and, as a result, would not be immediately consumable by human users.
NOTE: It should be pointed out that a feature of being able to
specify the public key explicitly is that it allows users to
exchange encrypted, anonymous mail. In particular, receiving
users will always know a message comes from the same originating
user even if the real identity of the originating user is unknown.
Recognizing that the use of public keys is, in general, unsuitable
for use by humans, MOSS allows other identifiers in Originator-ID:
and Recipient-ID: headers. These other identifiers are comprised of
two parts: a name form and a key selector.
The name form is chosen and asserted by the user who owns the
public/private key pair. Three name forms are specified by this
document. The use of a distinguished name is retained for
compatibility with PEM (and compatibility with the X.500 Directory
should it become a ubiquitous service). However, the Internet
community has a great deal of experience with the use of electronic
mail addresses as a name form. Also, arbitrary strings are useful to
identify the owners of public keys when private name forms are used.
Hence, email addresses and arbitrary strings are included as name
forms to increase flexibility.
Since a user may have more than one public key and may wish to use
the same name form for each public key, a name form is insufficient
for uniquely identifying a public key. A unique "key selector" must
be assigned to each public key. The combination of a name form and
the key selector uniquely identifies a public key. Throughout this
document, this combination is called an identifier. There are 5
identifiers specified by this document.
NOTE: In the simplest case, key selectors will be assigned by the
owners of the public/private key pairs. This works best when
users generate their own key pairs for personal use, from which
they distribute their public key to others asserting by
declaration that the public key belongs to them. When the
assertion that the public key belongs to them is made by a third
party, for example when a certification authority issues a
certificate to a user according to [4], the key selector may be
assigned by that third party.
The value of the key selector must be unique with respect to the name
form with which it forms an identifier. Although the same key
selector value may be used by more than one name form it must not be
used for two different keys with the same name form. When considered
separately, neither a name form nor a key selector is sufficient for
identifying the public key to be used. Either could be used to
determine a set of public keys that may be tried in turn until the
desired public key is identified.
With a public/private key pair for one"s self and software that is
MOSS aware, an originating user may digitally sign arbitrary data and
send it to one or more recipients. With the public keys of the
recipients, a user may encrypt the data so that only the intended
recipients can decrypt and read it. With the name forms assigned to
the public keys, originators and recipients can easily recognize
their peers in a communication.
In the next section the 3 name forms are described in detail.
Following that is the specification of the 5 identifiers.
4.1. Name Forms
There are 3 name forms specified by this document: email addresses,
distinguished names, and arbitrary strings.
4.1.1. Email Addresses
The email address (grammar token <emailstr>) used must be a valid
RFC822 address, which is defined in terms of one of the two grammar
tokens <addr-spec> or <route-addr>. The grammar for these two tokens
is included in the Appendix as a convenience; the definitive source
for these tokens is necessarily RFC822 [1].
<emailstr> ::= <addr-spec> / <route-addr>
; an electronic mail address as defined by
; one of these two tokens from RFC822
For example, the strings "crocker@tis.com", "galvin@tis.com",
"murphy@tis.com", and "ned@innosoft.com" are all email addresses.
4.1.2. Arbitrary Strings
The arbitrary string (grammar token <string>) must have a length of
at least 1. There are no other restrictions on the value chosen.
<string> ::= ; a non-null sequence of characters
For example, the string
the SAAG mailing list maintainer
is an arbitrary string.
4.1.3. Distinguished Names
The distinguished name (grammar token <dnamestr>) must be constructed
according to the guidelines of the X.500 Directory. The actual
syntax of the distinguished name is outside the scope of this
specification. However, RFC1422, for example, specifies syntactic
restrictions based on its choice of a certification hierarchy for
certificates.
For the purposes of conveying a distinguished name from an originator
to a recipient, it must be ASN.1 encoded and then printably encoded
according to the base64 encoding defined by MIME.
<dnamestr> ::= <encbin>
; a printably encoded, ASN.1 encoded
; distinguished name (as defined by the "Name"
; production specified in X.501 [8])
For example,
/Country Name=US
/State or Province Name=MD
/Organization Name=Trusted Information Systems
/Organizational Unit Name=Glenwood
/Common Name=James M. Galvin/
is a distinguished name in a user friendly format (line breaks and
leading spaces present only to improve readability). When encoded,
it would appear as follows (line breaks present only to improve
readability):
MG0xCzAJBgNVBAYTAlVTMQswCQYDVQQIEwJNRDEkMCIGA1UEChMbVHJ1c3RlZCBJ
bmZvcm1hdGlvbiBTeXN0ZW1zMREwDwYDVQQLEwhHbGVud29vZDEYMBYGA1UEAxMP
SmFtZXMgTS4gR2Fsdmlu
4.2. Identifiers
There are 5 types of identifiers specified by this document:
email address identifiers
arbitrary string identifiers
distinguished name identifiers
the public keys themselves
issuer name serial number pairs from a certificate
All of these have approximately the same structure (except issuer
name and serial number which has "TYPE, STRING, KEYSEL" for
historical reasons):
TYPE, KEYSEL, STRING
The TYPE field is a literal string chosen from the set "EN", "STR",
"DN", "PK", and "IS", one for each of the possible identifiers.
The KEYSEL field is used to distinguish between the multiple public
keys that may be associated with the name form in the STRING field.
Its value must be unique with respect to all other key selectors used
with the same name form. An example would be to use a portion (low-
order 16 or 32 bits) or all of the actual public key used.
The STRING field is the name form and has a different syntax
according to the value of the TYPE field.
The identifier used in each of the originator and recipient fields is
described by the following grammar. The definition of the key
selector token is included here since it used by several of the
identifiers below.
<id> ::= <id-email> / <id-string> / <id-dname>
/ <id-publickey> / <id-issuer>
<keysel> ::= 1*<hexchar>
; hex dump of a non-null sequence of octets
Each of the identifier name forms is described below.
4.2.1. Email Address
The email address identifier has the following syntax.
<id-email> ::= "EN" "," <keysel> "," <emailstr> CRLF
The syntax of the token <emailstr> is defined in Section 4.1.1.
For example:
EN,1,galvin@tis.com
is an email address identifier.
4.2.2. Arbitrary String
The arbitrary string identifier has the following syntax.
<id-string> ::= "STR" "," <keysel> "," <string> CRLF
The syntax of the token <string> is defined in Section 4.1.2.
For example:
STR,1,The SAAG mailing list maintainer
is an arbitrary string identifier.
4.2.3. Distinguished Name
The distinguished name identifier has the following syntax.
<id-dname> ::= "DN" "," <keysel> "," <dnamestr> CRLF
The syntax of the token <dnamestr> is defined in Section 4.1.3.
For example (line breaks present only to improve readability):
DN,1,MG0xCzAJBgNVBAYTAlVTMQswCQYDVQQIEwJNRDEkMCIGA1UEChMbVHJ1c3R
lZCBJbmZvcm1hdGlvbiBTeXN0ZW1zMREwDwYDVQQLEwhHbGVud29vZDEYMBYGA1U
EAxMPSmFtZXMgTS4gR2Fsdmlu
is a distinguished name identifier.
4.2.4. Public Key
The public key identifier has the following syntax.
<id-publickey> ::= "PK" "," <publickey> [ "," <id-subset> ] CRLF
<publickey> ::= <encbin>
; a printably encoded, ASN.1 encoded public
; key (as defined by the
; "SubjectPublicKeyInfo" production specified
; in X.509 [9])
<id-subset> ::= <id-email> / <id-string> / <id-dname>
The production SubjectPublicKeyInfo is imported from the X.500
Directory from the certificate object. It is currently the best
choice for a general purpose public key encoding.
For example, (line breaks present only to improve readability):
PK,MHkwCgYEVQgBAQICAwADawAwaAJhAMAHQ45ywA357G4fqQ61aoC1fO6BekJmG
4475mJkwGIUxvDkwuxe/EFdPkXDGBxzdGrW1iuh5K8kl8KRGJ9wh1HU4TrghGdhn
0Lw8gG67Dmb5cBhY9DGwq0CDnrpKZV3cQIDAQAB
is a public key identifier without the optional <id-subset>.
In normal usage, the token <id-subset> is expected to be present. It
represents a mechanism by which an identifier (name form and key
selector) can be associated with a public key. Recipients of a
public key identifier must take care to verify the accuracy of the
purported association. If they do not, it may be possible for a
malicious originator to assert an identifier that accords the
originator unauthorized privileges. See Section 5.2 for more
details.
For example, (line breaks present only to improve readability):
PK,MHkwCgYEVQgBAQICAwADawAwaAJhAMAHQ45ywA357G4fqQ61aoC1fO6BekJmG
4475mJkwGIUxvDkwuxe/EFdPkXDGBxzdGrW1iuh5K8kl8KRGJ9wh1HU4TrghGdhn
0Lw8gG67Dmb5cBhY9DGwq0CDnrpKZV3cQIDAQAB,EN,2,galvin@tis.com
is a public key identifier with the optional <id-subset>.
4.2.5. Issuer Name and Serial Number
The issuer name and serial number identifier has the following
syntax.
<id-issuer> ::= "IS" "," <dnamestr> "," <serial> CRLF
<serial> ::= 1*<hexchar>
; hex dump of a certificate serial number
The <id-issuer> identifier is included for compatibility with the
ID-ASymmetric fields defined in [3] (and compatibility with X.500
Directory certificates should they become ubiquitously available).
Its syntax was chosen such that the older fields are easily converted
to this new form by prefixing the old value with "IS" (and replacing
the field name of [3] with an appropriate new ID field name). For
example, (line breaks present only to improve readability):
IS,MFMxCzAJBgNVBAYTAlVTMQswCQYDVQQIEwJNRDEkMCIGA1UEChMbVHJ1c3
RlZCBJbmZvcm1hdGlvbiBTeXN0ZW1zMREwDwYDVQQLEwhHbGVud29vZA==,02
is an issuer name and serial number identifier according to MOSS,
while
MFMxCzAJBgNVBAYTAlVTMQswCQYDVQQIEwJNRDEkMCIGA1UEChMbVHJ1c3
RlZCBJbmZvcm1hdGlvbiBTeXN0ZW1zMREwDwYDVQQLEwhHbGVud29vZA==,02
is an issuer name and serial number identifier according to PEM.
5. Key Management Content Types
This document defines two key management content types: one for
requesting cryptographic key material and one for sending
cryptographic key material. Since MOSS depends only on the existence
of public/private key pairs, these content types provide a means for
conveying public keys and an assertion as to the identity of the
owner. In addition, in order to be compatible with the certificate-
base key management system proposed by RFC1422, the content types
may also be used to convey certificate and certificate revocation
list material.
The functions defined here are based on the exchange of body parts.
In particular, a user would send a message containing at least one
application/mosskey-request content, as defined below. In response,
a user would expect to receive a message containing at least one
application/mosskey-data content, as defined below. MIME provides a
convenient framework for a user to send several request body parts
and to receive several data (response) body parts in one message.
5.1. application/mosskey-request Content Type Definition
(1) MIME type name: application
(2) MIME subtype name: mosskey-request
(3) Required parameters: none
(4) Optional parameters: none
(5) Encoding considerations: quoted-printable is always sufficient
(6) Security Considerations: none
The content of this body part corresponds to the following
production.
<request> ::= <version>
( <subject> / <issuer> / <certification> )
<version> ::= "Version:" "5" CRLF
<subject> ::= "Subject:" <id> CRLF
<issuer> ::= "Issuer:" <id> CRLF
<certification> ::= "Certification:" <encbin> CRLF
A user would use this content type to specify needed cryptographic
key information. The message containing this content type might be
directed towards an automatic or manual responder, which may be
mail-based, depending on the local implementation and environment.
The application/mosskey-request content type is an independent body
part because it is entirely independent of any other body part.
If the application/mosskey-request content contains a Certification:
field it requests certification of the self-signed certificate in the
field value. If the content contains an Issuer: field it requests
the Certificate Revocation List (CRL) chain beginning with the CRL of
the issuer identified in the field value. If the content contains a
Subject: field it requests either the public key of the subject or a
certificate chain beginning with the subject identified in the field
value, or both if both exist.
The Subject: and Issuer: fields each contain a value of type <id>,
which is defined in Section 4.
One possible response to receiving an application/mosskey-request
body part is to construct and return an application/mosskey-data body
part. When returning public keys, certificate chains, and
certificate revocation list chains, if there exists more than one,
several application/mosskey-data body parts are to be returned in the
reply message, one for each.
5.2. application/mosskey-data Content Type Definition
The principal objective of this content type is to convey
cryptographic keying material from a source to a destination. This
might be in response to the receipt of an application/mosskey-request
content type or it might be in anticipation of receiving an
application/mosskey-request if it is not sent, e.g., it may be
combined with a multipart/signed object by an originator to ensure
that a recipient has the cryptographic keying material necessary to
verify the signature. When combined with other content types, the
processing by a recipient is enhanced if the application/mosskey-data
content type is positioned in its enclosing content type prior to the
content types that will make use of its cryptographic keying
material.
However, no explicit provision is made in this document for
determining the authenticity or accuracy of the data being conveyed.
In particular, when a public key and its identifier is conveyed,
there is nothing to prevent the source or an interloper along the
path from the source to the destination from substituting alternate
values for either the public key or the identifier.
It is incumbent upon a recipient to verify the authenticity and
accuracy of the data received in this way prior to its use. This
problem can be addressed by the use of certificates, since a
certification hierarchy is a well-defined mechanism that conveniently
supports the automatic verification of the data. Alternatively, the
source of the application/mosskey-data body part could digitally sign
it. In this way, if the destination believes that a correct source"s
public key is available locally and if the destination believes the
source would convey accurate data, then the contents of the
application/mosskey-data from the source could be believed to be
accurate.
NOTE: Insofar as a certificate represents a mechanism by which a
third party vouches for the binding between a name and a public
key, the signing of an application/mosskey-data body part is a
similar mechanism.
(1) MIME type name: application
(2) MIME subtype name: mosskey-data
(3) Required parameters: none
(4) Optional parameters: none
(5) Encoding considerations: quoted-printable is always sufficient.
(6) Security Considerations: none
The content of this body part corresponds to the following
production.
<mosskeydata> ::= <version>
( <publickeydata> / <certchain> / <crlchain> )
<version> ::= "Version:" "5" CRLF
<publickeydata> ::= "Key:" "PK" "," <publickey> ","
<id-subset> CRLF
<certchain> ::= <cert> *( [ <crl> ] <cert> )
<crlchain> ::= 1*( <crl> [ <cert> ] )
<cert> ::= "Certificate:" <encbin> CRLF
<crl> ::= "CRL:" <encbin> CRLF
This content type is used to transfer public keys, certificate
chains, or Certificate Revocation List (CRL) chains. The information
in the body part is entirely independent of any other body part.
(Note that the converse is not true: the validity of a protected body
part cannot be determined without the proper public keys,
certificates, or current CRL information.) As such, the
application/mosskey-data content type is an independent body part.
The <publickeydata> production contains exactly one public key. It
is used to bind a public key with its corresponding name form and key
selector. It is recommended that when responders are returning this
information that the enclosing body part be digitally signed by the
responder in order to protect the information. The <id-subset> token
is defined in Section 4.2.4.
The <certchain> production contains one certificate chain. A
certificate chain starts with the requested certificate and continues
with the certificates of subsequent issuers. Each issuer certificate
included must have issued the preceding certificate. For each
issuer, a CRL may be supplied. A CRL in the chain belongs to the
immediately following issuer. Therefore, it potentially contains the
immediately preceding certificate.
The <crlchain> production contains one certificate revocation list
chain. The CRLs in the chain begin with the requested CRL and
continue with the CRLs of subsequent issuers. The issuer of each CRL
is presumed to have issued a certificate for the issuer of the
preceding CRL. For each CRL, the issuer"s certificate may be
supplied. A certificate in the chain must belong to the issuer of
the immediately preceding CRL.
The relationship between a certificate and an immediately preceding
CRL is the same in both <certchain> and <crlchain>. In a <certchain>
the CRLs are optional. In a <crlchain> the certificates are
optional.
6. Examples
Each example is included as a separate section for ease of reference.
6.1. Original Message Prepared for Protection
Except as explicitly indicated, the following message is used as the
message to be protected.
To: Ned Freed <ned@innosoft.com>
Subject: Hi Ned!
How do you like the new MOSS?
Jim
6.2. Sign Text of Original Message
When the text of the original message is signed, it will look like
this, where lines with an ampersand "&" are digitally signed (note
the use of the public key identifier with the included email name
identifier, on the lines marked with an asterisk "*"):
To: Ned Freed <ned@innosoft.com>
Subject: Hi Ned!
MIME-Version: 1.0
Content-Type: multipart/signed;
protocol="application/moss-signature";
micalg="rsa-md5"; boundary="Signed Boundary"
--Signed Boundary
& Content-Type: text/plain; charset="us-ascii"
& Content-ID: <21436.785186814.2@tis.com>
&
& How do you like the new MOSS?
&
& Jim
--Signed Boundary
Content-Type: application/moss-signature
Content-ID: <21436.785186814.1@tis.com>
Content-Transfer-Encoding: quoted-printable
Version: 5
* Originator-ID: PK,MHkwCgYEVQgBAQICAwADawAwaAJhAMAHQ45ywA357