Network Working Group J. Linn
Request for Comments: 1113 DEC
Obsoletes RFCs: 989, 1040 IAB Privacy Task Force
August 1989
Privacy Enhancement for Internet Electronic Mail:
Part I -- Message Encipherment and Authentication Procedures
STATUS OF THIS MEMO
This RFC suggests a draft standard elective protocol for the Internet
community, and requests discussion and suggestions for improvements.
Distribution of this memo is unlimited.
ACKNOWLEDGMENT
This RFC is the outgrowth of a series of IAB Privacy Task Force
meetings and of internal working papers distributed for those
meetings. I would like to thank the following Privacy Task Force
members and meeting guests for their comments and contributions at
the meetings which led to the preparation of this RFC: David
Balenson, Curt Barker, Jim Bidzos, Matt Bishop, Danny Cohen, Tom
Daniel, Charles Fox, Morrie Gasser, Russ Housley, Steve Kent
(chairman), John Laws, Steve Lipner, Dan Nessett, Mike Padlipsky, Rob
Shirey, Miles Smid, Steve Walker, and Steve Wilbur.
Table of Contents
1. Executive Summary 2
2. Terminology 3
3. Services, Constraints, and Implications 3
4. Processing of Messages 7
4.1 Message Processing Overview 7
4.1.1 Types of Keys 7
4.1.2 Processing Procedures 8
4.2 Encryption Algorithms and Modes 9
4.3 Privacy Enhancement Message Transformations 10
4.3.1 Constraints 10
4.3.2 Approach 11
4.3.2.1 Step 1: Local Form 12
4.3.2.2 Step 2: Canonical Form 12
4.3.2.3 Step 3: Authentication and Encipherment 12
4.3.2.4 Step 4: Printable Encoding 13
4.3.2.5 Summary of Transformations 15
4.4 Encapsulation Mechanism 15
4.5 Mail for Mailing Lists 17
4.6 Summary of Encapsulated Header Fields 18
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4.6.1 Per-Message Encapsulated Header Fields 20
4.6.1.1 X-Proc-Type Field 20
4.6.1.2 X-DEK-Info Field 21
4.6.2 Encapsulated Header Fields Normally Per-Message 21
4.6.2.1 X-Sender-ID Field 22
4.6.2.2 X-Certificate Field 22
4.6.2.3 X-MIC-Info Field 23
4.6.3 Encapsulated Header Fields with Variable Occurrences 23
4.6.3.1 X-Issuer-Certificate Field 23
4.6.4 Per-Recipient Encapsulated Header Fields 24
4.6.4.1 X-Recipient-ID Field 24
4.6.4.2 X-Key-Info Field 24
4.6.4.2.1 Symmetric Key Management 24
4.6.4.2.2 Asymmetric Key Management 25
5. Key Management 26
5.1 Data Encrypting Keys (DEKs) 26
5.2 Interchange Keys (IKs) 26
5.2.1 Subfield Definitions 28
5.2.1.1 Entity Identifier Subfield 28
5.2.1.2 Issuing Authority Subfield 29
5.2.1.3 Version/Expiration Subfield 29
5.2.2 IK Cryptoperiod Issues 29
6. User Naming 29
6.1 Current Approach 29
6.2 Issues for Consideration 30
7. Example User Interface and Implementation 30
8. Areas For Further Study 31
9. References 32
NOTES 32
1. Executive Summary
This RFC defines message encipherment and authentication procedures,
in order to provide privacy enhancement services for electronic mail
transfer in the Internet. It is one member of a related set of four
RFCs. The procedures defined in the current RFC are intended to be
compatible with a wide range of key management approaches, including
both symmetric (secret-key) and asymmetric (public-key) approaches
for encryption of data encrypting keys. Use of symmetric
cryptography for message text encryption and/or integrity check
computation is anticipated. RFC-1114 specifies supporting key
management mechanisms based on the use of public-key certificates.
RFC-1115 specifies algorithm and related information relevant to the
current RFC and to RFC-1114. A subsequent RFC will provide details
of paper and electronic formats and procedures for the key management
infrastructure being established in support of these services.
Privacy enhancement services (confidentiality, authentication, and
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message integrity assurance) are offered through the use of end-to-
end cryptography between originator and recipient User Agent
processes, with no special processing requirements imposed on the
Message Transfer System at endpoints or at intermediate relay sites.
This approach allows privacy enhancement facilities to be
incorporated on a site-by-site or user-by-user basis without impact
on other Internet entities. Interoperability among heterogeneous
components and mail transport facilities is supported.
2. Terminology
For descriptive purposes, this RFC uses some terms defined in the OSI
X.400 Message Handling System Model per the 1984 CCITT
Recommendations. This section replicates a portion of X.400's
Section 2.2.1, "Description of the MHS Model: Overview" in order to
make the terminology clear to readers who may not be familiar with
the OSI MHS Model.
In the MHS model, a user is a person or a computer application. A
user is referred to as either an originator (when sending a message)
or a recipient (when receiving one). MH Service elements define the
set of message types and the capabilities that enable an originator
to transfer messages of those types to one or more recipients.
An originator prepares messages with the assistance of his or her
User Agent (UA). A UA is an application process that interacts with
the Message Transfer System (MTS) to submit messages. The MTS
delivers to one or more recipient UAs the messages submitted to it.
Functions performed solely by the UA and not standardized as part of
the MH Service elements are called local UA functions.
The MTS is composed of a number of Message Transfer Agents (MTAs).
Operating together, the MTAs relay messages and deliver them to the
intended recipient UAs, which then make the messages available to the
intended recipients.
The collection of UAs and MTAs is called the Message Handling System
(MHS). The MHS and all of its users are collectively referred to as
the Message Handling Environment.
3. Services, Constraints, and Implications
This RFC defines mechanisms to enhance privacy for electronic mail
transferred in the Internet. The facilities discussed in this RFC
provide privacy enhancement services on an end-to-end basis between
sender and recipient UAs. No privacy enhancements are offered for
message fields which are added or transformed by intermediate relay
points.
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Authentication and integrity facilities are always applied to the
entirety of a message's text. No facility for confidentiality
without authentication is provided. Encryption facilities may be
applied selectively to portions of a message's contents; this allows
less sensitive portions of messages (e.g., descriptive fields) to be
processed by a recipient's delegate in the absence of the recipient's
personal cryptographic keys. In the limiting case, where the
entirety of message text is excluded from encryption, this feature
can be used to yield the effective combination of authentication and
integrity services without confidentiality.
In keeping with the Internet's heterogeneous constituencies and usage
modes, the measures defined here are applicable to a broad range of
Internet hosts and usage paradigms. In particular, it is worth
noting the following attributes:
1. The mechanisms defined in this RFC are not restricted to a
particular host or operating system, but rather allow
interoperability among a broad range of systems. All
privacy enhancements are implemented at the application
layer, and are not dependent on any privacy features at
lower protocol layers.
2. The defined mechanisms are compatible with non-enhanced
Internet components. Privacy enhancements are implemented
in an end-to-end fashion which does not impact mail
processing by intermediate relay hosts which do not
incorporate privacy enhancement facilities. It is
necessary, however, for a message's sender to be cognizant
of whether a message's intended recipient implements privacy
enhancements, in order that encoding and possible
encipherment will not be performed on a message whose
destination is not equipped to perform corresponding inverse
transformations.
3. The defined mechanisms are compatible with a range of mail
transport facilities (MTAs). Within the Internet,
electronic mail transport is effected by a variety of SMTP
implementations. Certain sites, accessible via SMTP,
forward mail into other mail processing environments (e.g.,
USENET, CSNET, BITNET). The privacy enhancements must be
able to operate across the SMTP realm; it is desirable that
they also be compatible with protection of electronic mail
sent between the SMTP environment and other connected
environments.
4. The defined mechanisms are compatible with a broad range of
electronic mail user agents (UAs). A large variety of
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electronic mail user agent programs, with a corresponding
broad range of user interface paradigms, is used in the
Internet. In order that electronic mail privacy
enhancements be available to the broadest possible user
community, selected mechanisms should be usable with the
widest possible variety of existing UA programs. For
purposes of pilot implementation, it is desirable that
privacy enhancement processing be incorporable into a
separate program, applicable to a range of UAs, rather than
requiring internal modifications to each UA with which
privacy-enhanced services are to be provided.
5. The defined mechanisms allow electronic mail privacy
enhancement processing to be performed on personal computers
(PCs) separate from the systems on which UA functions are
implemented. Given the expanding use of PCs and the limited
degree of trust which can be placed in UA implementations on
many multi-user systems, this attribute can allow many users
to process privacy-enhanced mail with a higher assurance
level than a strictly UA-based approach would allow.
6. The defined mechanisms support privacy protection of
electronic mail addressed to mailing lists (distribution
lists, in ISO parlance).
7. The mechanisms defined within this RFC are compatible with a
variety of supporting key management approaches, including
(but not limited to) manual pre-distribution, centralized
key distribution based on symmetric cryptography, and the
use of public-key certificates. Different key management
mechanisms may be used for different recipients of a
multicast message. While support for a particular key
management mechanism is not a minimum essential requirement
for compatibility with this RFC, adoption of the public-key
certificate approach defined in companion RFC-1114 is
strongly recommended.
In order to achieve applicability to the broadest possible range of
Internet hosts and mail systems, and to facilitate pilot
implementation and testing without the need for prior modifications
throughout the Internet, three basic restrictions are imposed on the
set of measures to be considered in this RFC:
1. Measures will be restricted to implementation at endpoints
and will be amenable to integration at the user agent (UA)
level or above, rather than necessitating integration into
the message transport system (e.g., SMTP servers).
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2. The set of supported measures enhances rather than restricts
user capabilities. Trusted implementations, incorporating
integrity features protecting software from subversion by
local users, cannot be assumed in general. In the absence
of such features, it appears more feasible to provide
facilities which enhance user services (e.g., by protecting
and authenticating inter-user traffic) than to enforce
restrictions (e.g., inter-user access control) on user
actions.
3. The set of supported measures focuses on a set of functional
capabilities selected to provide significant and tangible
benefits to a broad user community. By concentrating on the
most critical set of services, we aim to maximize the added
privacy value that can be provided with a modest level of
implementation effort.
As a result of these restrictions, the following facilities can be
provided:
1. disclosure protection,
2. sender authenticity,
3. message integrity measures, and
4. (if asymmetric key management is used) non-repudiation of
origin,
but the following privacy-relevant concerns are not addressed:
1. access control,
2. traffic flow confidentiality,
3. address list accuracy,
4. routing control,
5. issues relating to the casual serial reuse of PCs by
multiple users,
6. assurance of message receipt and non-deniability of receipt,
7. automatic association of acknowledgments with the messages
to which they refer, and
8. message duplicate detection, replay prevention, or other
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stream-oriented services.
A message's sender will determine whether privacy enhancements are to
be performed on a particular message. Therefore, a sender must be
able to determine whether particular recipients are equipped to
process privacy-enhanced mail. In a general architecture, these
mechanisms will be based on server queries; thus, the query function
could be integrated into a UA to avoid imposing burdens or
inconvenience on electronic mail users.
4. Processing of Messages
4.1 Message Processing Overview
This subsection provides a high-level overview of the components and
processing steps involved in electronic mail privacy enhancement
processing. Subsequent subsections will define the procedures in
more detail.
4.1.1 Types of Keys
A two-level keying hierarchy is used to support privacy-enhanced
message transmission:
1. Data Encrypting Keys (DEKs) are used for encryption of
message text and (with certain choices among a set of
alternative algorithms) for computation of message integrity
check (MIC) quantities. DEKs are generated individually for
each transmitted message; no predistribution of DEKs is
needed to support privacy-enhanced message transmission.
2. Interchange Keys (IKs) are used to encrypt DEKs for
transmission within messages. Ordinarily, the same IK will
be used for all messages sent from a given originator to a
given recipient over a period of time. Each transmitted
message includes a representation of the DEK(s) used for
message encryption and/or MIC computation, encrypted under
an individual IK per named recipient. The representation is
associated with "X-Sender-ID:" and "X-Recipient-ID:" fields,
which allow each individual recipient to identify the IK
used to encrypt DEKs and/or MICs for that recipient's use.
Given an appropriate IK, a recipient can decrypt the
corresponding transmitted DEK representation, yielding the
DEK required for message text decryption and/or MIC
verification. The definition of an IK differs depending on
whether symmetric or asymmetric cryptography is used for DEK
encryption:
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2a. When symmetric cryptography is used for DEK
encryption, an IK is a single symmetric key shared
between an originator and a recipient. In this
case, the same IK is used to encrypt MICs as well
as DEKs for transmission. Version/expiration
information and IA identification associated with
the originator and with the recipient must be
concatenated in order to fully qualify a symmetric
IK.
2b. When asymmetric cryptography is used, the IK
component used for DEK encryption is the public
component of the recipient. The IK component used
for MIC encryption is the private component of the
originator, and therefore only one encrypted MIC
representation need be included per message, rather than
one per recipient. Each of these IK
components can be fully qualified in an
"X-Recipient-ID:" or "X-Sender-ID:" field,
respectively.
4.1.2 Processing Procedures
When privacy enhancement processing is to be performed on an outgoing
message, a DEK is generated [1] for use in message encryption and (if
a chosen MIC algorithm requires a key) a variant of the DEK is formed
for use in MIC computation. DEK generation can be omitted for the
case of a message in which all contents are excluded from encryption,
unless a chosen MIC computation algorithm requires a DEK.
An "X-Sender-ID:" field is included in the header to provide one
identification component for the IK(s) used for message processing.
IK components are selected for each individually named recipient; a
corresponding "X-Recipient-ID:" field, interpreted in the context of
a prior "X-Sender-ID:" field, serves to identify each IK. Each "X-
Recipient-ID:" field is followed by an "X-Key-Info:" field, which
transfers a DEK encrypted under the IK appropriate for the specified
recipient. When symmetric key management is used for a given
recipient, the "X-Key-Info:" field also transfers the message's
computed MIC, encrypted under the recipient's IK. When asymmetric
key management is used, a prior "X-MIC-Info:" field carries the
message's MIC encrypted under the private component of the sender.
A four-phase transformation procedure is employed in order to
represent encrypted message text in a universally transmissible form
and to enable messages encrypted on one type of host computer to be
decrypted on a different type of host computer. A plaintext message
is accepted in local form, using the host's native character set and
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line representation. The local form is converted to a canonical
message text representation, defined as equivalent to the inter-SMTP
representation of message text. This canonical representation forms
the input to the MIC computation and encryption processes.
For encryption purposes, the canonical representation is padded as
required by the encryption algorithm. The padded canonical
representation is encrypted (except for any regions which are
explicitly excluded from encryption). The encrypted text (along with
the canonical representation of regions which were excluded from
encryption) is encoded into a printable form. The printable form is
composed of a restricted character set which is chosen to be
universally representable across sites, and which will not be
disrupted by processing within and between MTS entities.
The output of the encoding procedure is combined with a set of header
fields carrying cryptographic control information. The result is
passed to the electronic mail system to be encapsulated as the text
portion of a transmitted message.
When a privacy-enhanced message is received, the cryptographic
control fields within its text portion provide the information
required for the authorized recipient to perform MIC verification and
decryption of the received message text. First, the printable
encoding is converted to a bitstring. Encrypted portions of the
transmitted message are decrypted. The MIC is verified. The
canonical representation is converted to the recipient's local form,
which need not be the same as the sender's local form.
4.2 Encryption Algorithms and Modes
For purposes of this RFC, the Block Cipher Algorithm DEA-1, defined
in ANSI X3.92-1981 [2] shall be used for encryption of message text.
The DEA-1 is equivalent to the Data Encryption Standard (DES), as
defined in FIPS PUB 46 [3]. When used for encryption of text, the
DEA-1 shall be used in the Cipher Block Chaining (CBC) mode, as
defined in ISO IS 8372 [4]. The identifier string "DES-CBC", defined
in RFC-1115, signifies this algorithm/mode combination. The CBC mode
definition in IS 8372 is equivalent to that provided in FIPS PUB 81
[5] and in ANSI X3.106-1983 [16]. Use of other algorithms and/or
modes for message text processing will require case-by-case study to
determine applicability and constraints. Additional algorithms and
modes approved for use in this context will be specified in
successors to RFC-1115.
It is an originator's responsibility to generate a new pseudorandom
initializing vector (IV) for each privacy-enhanced electronic mail
message unless the entirety of the message is excluded from
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encryption. Section 4.3.1 of [17] provides rationale for this
requirement, even in a context where individual DEKs are generated
for individual messages. The IV will be transmitted with the
message.
Certain operations require that one key be encrypted under an
interchange key (IK) for purposes of transmission. A header facility
indicates the mode in which the IK is used for encryption. RFC-1115
specifies encryption algorithm/mode identifiers, including DES-ECB,
DES-EDE, and RSA. All implementations using symmetric key management
should support DES-ECB IK use, and all implementations using
asymmetric key management should support RSA IK use.
RFC-1114, released concurrently with this RFC, specifies asymmetric,
certificate-based key management procedures to support the message
processing procedures defined in this document. The message
processing procedures can also be used with symmetric key management,
given prior distribution of suitable symmetric IKs through out-of-
band means. Support for the asymmetric approach defined in RFC-1114
is strongly recommended.
4.3 Privacy Enhancement Message Transformations
4.3.1 Constraints
An electronic mail encryption mechanism must be compatible with the
transparency constraints of its underlying electronic mail
facilities. These constraints are generally established based on
expected user requirements and on the characteristics of anticipated
endpoint and transport facilities. An encryption mechanism must also
be compatible with the local conventions of the computer systems
which it interconnects. In our approach, a canonicalization step is
performed to abstract out local conventions and a subsequent encoding
step is performed to conform to the characteristics of the underlying
mail transport medium (SMTP). The encoding conforms to SMTP
constraints, established to support interpersonal messaging. SMTP's
rules are also used independently in the canonicalization process.
RFC-821's [7] Section 4.5 details SMTP's transparency constraints.
To prepare a message for SMTP transmission, the following
requirements must be met:
1. All characters must be members of the 7-bit ASCII character
set.
2. Text lines, delimited by the character pair <CR><LF>, must
be no more than 1000 characters long.
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3. Since the string <CR><LF>.<CR><LF> indicates the end of a
message, it must not occur in text prior to the end of a
message.
Although SMTP specifies a standard representation for line delimiters
(ASCII <CR><LF>), numerous systems use a different native
representation to delimit lines. For example, the <CR><LF> sequences
delimiting lines in mail inbound to UNIX systems are transformed to
single <LF>s as mail is written into local mailbox files. Lines in
mail incoming to record-oriented systems (such as VAX VMS) may be
converted to appropriate records by the destination SMTP [8] server.
As a result, if the encryption process generated <CR>s or <LF>s,
those characters might not be accessible to a recipient UA program at
a destination which uses different line delimiting conventions. It
is also possible that conversion between tabs and spaces may be
performed in the course of mapping between inter-SMTP and local
format; this is a matter of local option. If such transformations
changed the form of transmitted ciphertext, decryption would fail to
regenerate the transmitted plaintext, and a transmitted MIC would
fail to compare with that computed at the destination.
The conversion performed by an SMTP server at a system with EBCDIC as
a native character set has even more severe impact, since the
conversion from EBCDIC into ASCII is an information-losing
transformation. In principle, the transformation function mapping
between inter-SMTP canonical ASCII message representation and local
format could be moved from the SMTP server up to the UA, given a
means to direct that the SMTP server should no longer perform that
transformation. This approach has a major disadvantage: internal
file (e.g., mailbox) formats would be incompatible with the native
forms used on the systems where they reside. Further, it would
require modification to SMTP servers, as mail would be passed to SMTP
in a different representation than it is passed at present.
4.3.2 Approach
Our approach to supporting privacy-enhanced mail across an
environment in which intermediate conversions may occur encodes mail
in a fashion which is uniformly representable across the set of
privacy-enhanced UAs regardless of their systems' native character
sets. This encoded form is used to represent mail text from sender
to recipient, but the encoding is not applied to enclosing mail
transport headers or to encapsulated headers inserted to carry
control information between privacy-enhanced UAs. The encoding's
characteristics are such that the transformations anticipated between
sender and recipient UAs will not prevent an encoded message from
being decoded properly at its destination.
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A sender may exclude one or more portions of a message from
encryption processing, but authentication processing is always
applied to the entirety of message text. Explicit action is required
to exclude a portion of a message from encryption processing; by
default, encryption is applied to the entirety of message text. The
user-level delimiter which specifies such exclusion is a local
matter, and hence may vary between sender and recipient, but all
systems should provide a means for unambiguous identification of
areas excluded from encryption processing.
An outbound privacy-enhanced message undergoes four transformation
steps, described in the following four subsections.
4.3.2.1 Step 1: Local Form
The message text is created in the system's native character set,
with lines delimited in accordance with local convention.
4.3.2.2 Step 2: Canonical Form
The entire message text, including both those portions subject to
encipherment processing and those portions excluded from such
processing, is converted to a universal canonical form, analogous to
the inter-SMTP representation [9] as defined in RFC-821 and RFC-822
[10] (ASCII character set, <CR><LF> line delimiters). The processing
required to perform this conversion is minimal on systems whose
native character set is ASCII. (Note: Since the output of the
canonical encoding process will never be submitted directly to SMTP,
but only to subsequent steps of the privacy enhancement encoding
process, the dot-stuffing transformation discussed in RFC-821,
section 4.5.2, is not required.) Since a message is converted to a
standard character set and representation before encryption, it can
be decrypted and its MIC can be verified at any type of destination
host computer. The decryption and MIC verification is performed
before any conversions which may be necessary to transform the
message into a destination-specific local form.
4.3.2.3 Step 3: Authentication and Encipherment
The canonical form is input to the selected MIC computation algorithm
in order to compute an integrity check quantity for the message. No
padding is added to the canonical form before submission to the MIC
computation algorithm, although certain MIC algorithms will apply
their own padding in the course of computing a MIC.
Padding is applied to the canonical form as needed to perform
encryption in the DEA-1 CBC mode, as follows: The number of octets to
be encrypted is determined by subtracting the number of octets
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excluded from encryption from the total length of the canonically
encoded text. Octets with the hexadecimal value FF (all ones) are
appended to the canonical form as needed so that the text octets to
be encrypted, along with the added padding octets, fill an integral
number of 8-octet encryption quanta. No padding is applied if the
number of octets to be encrypted is already an integral multiple of
8. The use of hexadecimal FF (a value outside the 7-bit ASCII set)
as a padding value allows padding octets to be distinguished from
valid data without inclusion of an explicit padding count indicator.
The regions of the message which have not been excluded from
encryption are encrypted. To support selective encipherment
processing, an implementation must retain internal indications of the
positions of excluded areas excluded from encryption with relation to
non-excluded areas, so that those areas can be properly delimited in
the encoding procedure defined in step 4. If a region excluded from
encryption intervenes between encrypted regions, cryptographic state
(e.g., IVs and accumulation of octets into encryption quanta) is
preserved and continued after the excluded region.
4.3.2.4 Step 4: Printable Encoding
Proceeding from left to right, the bit string resulting from step 3
is encoded into characters which are universally representable at all
sites, though not necessarily with the same bit patterns (e.g.,
although the character "E" is represented in an ASCII-based system as
hexadecimal 45 and as hexadecimal C5 in an EBCDIC-based system, the
local significance of the two representations is equivalent). This
encoding step is performed for all privacy-enhanced messages, even if
an entire message is excluded from encryption.
A 64-character subset of International Alphabet IA5 is used, enabling
6 bits to be represented per printable character. (The proposed
subset of characters is represented identically in IA5 and ASCII.)
Two additional characters, "=" and "*", are used to signify special
processing functions. The character "=" is used for padding within
the printable encoding procedure. The character "*" is used to
delimit the beginning and end of a region which has been excluded
from encipherment processing. The encoding function's output is
delimited into text lines (using local conventions), with each line
except the last containing exactly 64 printable characters and the
final line containing 64 or fewer printable characters. (This line
length is easily printable and is guaranteed to satisfy SMTP's 1000-
character transmitted line length limit.)
The encoding process represents 24-bit groups of input bits as output
strings of 4 encoded characters. Proceeding from left to right across
a 24-bit input group extracted from the output of step 3, each 6-bit
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group is used as an index into an array of 64 printable characters.
The character referenced by the index is placed in the output string.
These characters, identified in Table 0, are selected so as to be
universally representable, and the set excludes characters with
particular significance to SMTP (e.g., ".", "<CR>", "<LF>").
Special processing is performed if fewer than 24 bits are available
in an input group, either at the end of a message or (when the
selective encryption facility is invoked) at the end of an encrypted
region or an excluded region. A full encoding quantum is always
completed at the end of a message and before the delimiter "*" is
output to initiate or terminate the representation of a block
excluded from encryption. When fewer than 24 input bits are
available in an input group, zero bits are added (on the right) to
form an integral number of 6-bit groups. Output character positions
which are not required to represent actual input data are set to the
character "=". Since all canonically encoded output is an integral
number of octets, only the following cases can arise: (1) the final
quantum of encoding input is an integral multiple of 24 bits; here,
the final unit of encoded output will be an integral multiple of 4
characters with no "=" padding, (2) the final quantum of encoding
input is exactly 8 bits; here, the final unit of encoded output will
be two characters followed by two "=" padding characters, or (3) the
final quantum of encoding input is exactly 16 bits; here, the final
unit of encoded output will be three characters followed by one "="
padding character.
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4.3.2.5 Summary of Transformations
In summary, the outbound message is subjected to the following
composition of transformations:
Transmit_Form = Encode(Encipher(Canonicalize(Local_Form)))
The inverse transformations are performed, in reverse order, to
process inbound privacy-enhanced mail:
Local_Form = DeCanonicalize(Decipher(Decode(Transmit_Form)))
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y (1) *
(1) The character "*" is used to enclose portions of an
encoded message to which encryption processing has not
been applied.
Printable Encoding Characters
Table 1
Note that the local form and the functions to transform messages to
and from canonical form may vary between the sender and recipient
systems without loss of information.
4.4 Encapsulation Mechanism
Encapsulation of privacy-enhanced messages within an enclosing layer
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of headers interpreted by the electronic mail transport system offers
a number of advantages in comparison to a flat approach in which
certain fields within a single header are encrypted and/or carry
cryptographic control information. Encapsulation provides generality
and segregates fields with user-to-user significance from those
transformed in transit. All fields inserted in the course of
encryption/authentication processing are placed in the encapsulated
header. This facilitates compatibility with mail handling programs
which accept only text, not header fields, from input files or from
other programs. Further, privacy enhancement processing can be
applied recursively. As far as the MTS is concerned, information
incorporated into cryptographic authentication or encryption
processing will reside in a message's text portion, not its header
portion.
The encapsulation mechanism to be used for privacy-enhanced mail is
derived from that described in RFC-934 [11] which is, in turn, based
on precedents in the processing of message digests in the Internet
community. To prepare a user message for encrypted or authenticated
transmission, it will be transformed into the representation shown in
Figure 1.
As a general design principle, sensitive data is protected by
incorporating the data within the encapsulated text rather than by
applying measures selectively to fields in the enclosing header.
Examples of potentially sensitive header information may include
fields such as "Subject:", with contents which are significant on an
end-to-end, inter-user basis. The (possibly empty) set of headers to
which protection is to be applied is a user option. It is strongly
recommended, however, that all implementations should replicate
copies of "X-Sender-ID:" and "X-Recipient-ID:" fields within the
encapsulated text.
If a user wishes disclosure protection for header fields, they must
occur only in the encapsulated text and not in the enclosing or
encapsulated header. If disclosure protection is desired for a
message's subject indication, it is recommended that the enclosing
header contain a "Subject:" field indicating that "Encrypted Mail
Follows".
If an authenticated version of header information is desired, that
data can be replicated within the encapsulated text portion in
addition to its inclusion in the enclosing header. For example, a
sender wishing to provide recipients with a protected indication of a
message's position in a series of messages could include a copy of a
timestamp or message counter field within the encapsulated text.
A specific point regarding the integration of privacy-enhanced mail
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facilities with the message encapsulation mechanism is worthy of
note. The subset of IA5 selected for transmission encoding
intentionally excludes the character "-", so encapsulated text can be
distinguished unambiguously from a message's closing encapsulation
boundary (Post-EB) without recourse to character stuffing.
Enclosing Header Portion
(Contains header fields per RFC-822)
Blank Line
(Separates Enclosing Header from Encapsulated Message)
Encapsulated Message
Pre-Encapsulation Boundary (Pre-EB)
-----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
Encapsulated Header Portion
(Contains encryption control fields inserted in plaintext.
Examples include "X-DEK-Info:", "X-Sender-ID:", and
"X-Key-Info:".
Note that, although these control fields have line-oriented
representations similar to RFC-822 header fields, the set
of fields valid in this context is disjoint from those used
in RFC-822 processing.)
Blank Line
(Separates Encapsulated Header from subsequent encoded
Encapsulated Text Portion)
Encapsulated Text Portion
(Contains message data encoded as specified in Section 4.3;
may incorporate protected copies of enclosing and
encapsulated header fields such as "Subject:", etc.)
Post-Encapsulation Boundary (Post-EB)
-----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
Message Encapsulation
Figure 1
4.5 Mail for Mailing Lists
When mail is addressed to mailing lists, two different methods of
processing can be applicable: the IK-per-list method and the IK-per-
recipient method. The choice depends on the information available to
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the sender and on the sender's preference.
If a message's sender addresses a message to a list name or alias,
use of an IK associated with that name or alias as a entity (IK-per-
list), rather than resolution of the name or alias to its constituent
destinations, is implied. Such an IK must, therefore, be available
to all list members. For the case of asymmetric key management, the
list's private component must be available to all list members. This
alternative will be the normal case for messages sent via remote
exploder sites, as a sender to such lists may not be cognizant of the
set of individual recipients. Unfortunately, it implies an
undesirable level of exposure for the shared IK, and makes its
revocation difficult. Moreover, use of the IK-per-list method allows
any holder of the list's IK to masquerade as another sender to the
list for authentication purposes.
If, in contrast, a message's sender is equipped to expand the
destination mailing list into its individual constituents and elects
to do so (IK-per-recipient), the message's DEK (and, in the symmetric
key management case, MIC) will be encrypted under each per-recipient
IK and all such encrypted representations will be incorporated into
the transmitted message. Note that per-recipient encryption is
required only for the relatively small DEK and MIC quantities carried
in the "X-Key-Info:" field, not for the message text which is, in
general, much larger. Although more IKs are involved in processing
under the IK-per-recipient method, the pairwise IKs can be
individually revoked and possession of one IK does not enable a
successful masquerade of another user on the list.
4.6 Summary of Encapsulated Header Fields
This section summarizes the syntax and semantics of the encapsulated
header fields to be added to messages in the course of privacy
enhancement processing. The fields are presented in three groups.
Normally, the groups will appear in encapsulated headers in the order
in which they are shown, though not all fields in each group will
appear in all messages. In certain indicated cases, it is recommended
that the fields be replicated within the encapsulated text portion as
well as being included within the encapsulated header. Figures 2 and
3 show the appearance of small example encapsulated messages. Figure
2 assumes the use of symmetric cryptography for key management.
Figure 3 illustrates an example encapsulated message in which
asymmetric key management is used.
Unless otherwise specified, all field arguments are processed in a
case-sensitive fashion. In most cases, numeric quantities are
represented in header fields as contiguous strings of hexadecimal
digits, where each digit is represented by a character from the
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ranges "0"-"9" or upper case "A"-"F". Since public-key certificates
and quantities encrypted using asymmetric algorithms are large in
size, use of a more space-efficient encoding technique is appropriate
for such quantities, and the encoding mechanism defined in Section
4.3.2.4 of this RFC, representing 6 bits per printed character, is
adopted. The example shown in Figure 3 shows asymmetrically
encrypted quantities (e.g., "X-MIC-Info:", "X-Key-Info:") with 64-
character printed representations, corresponding to 384 bits. The
fields carrying asymmetrically encrypted quantities also illustrate
the use of folding as defined in RFC-822, section 3.1.1.
-----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
X-Proc-Type: 3,ENCRYPTED
X-DEK-Info: DES-CBC,F8143EDE5960C597
X-Sender-ID: linn@ccy.bbn.com::
X-Recipient-ID: linn@ccy.bbn.com:ptf-kmc:3
X-Key-Info: DES-ECB,RSA-MD2,9FD3AAD2F2691B9A,B70665BB9BF7CBCD,
A60195DB94F727D3
X-Recipient-ID: privacy-tf@venera.isi.edu:ptf-kmc:4
X-Key-Info: DES-ECB,RSA-MD2,161A3F75DC82EF26,E2EF532C65CBCFF7,
9F83A2658132DB47
LLrHB0eJzyhP+/fSStdW8okeEnv47jxe7SJ/iN72ohNcUk2jHEUSoH1nvNSIWL9M
8tEjmF/zxB+bATMtPjCUWbz8Lr9wloXIkjHUlBLpvXR0UrUzYbkNpk0agV2IzUpk
J6UiRRGcDSvzrsoK+oNvqu6z7Xs5Xfz5rDqUcMlK1Z6720dcBWGGsDLpTpSCnpot
dXd/H5LMDWnonNvPCwQUHt==
-----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
Example Encapsulated Message (Symmetric Case)
Figure 2
-----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
X-Proc-Type: 3,ENCRYPTED
X-DEK-Info: DES-CBC,F8143EDE5960C597
X-Sender-ID: linn@ccy.bbn.com::
X-Certificate:
jHUlBLpvXR0UrUzYbkNpk0agV2IzUpk8tEjmF/zxB+bATMtPjCUWbz8Lr9wloXIk
YbkNpk0agV2IzUpk8tEjmF/zxB+bATMtPjCUWbz8Lr9wloXIkjHUlBLpvXR0UrUz
agV2IzUpk8tEjmFjHUlBLpvXR0UrUz/zxB+bATMtPjCUWbz8Lr9wloXIkYbkNpk0
X-Issuer-Certificate:
TMtPjCUWbz8Lr9wloXIkYbkNpk0agV2IzUpk8tEjmFjHUlBLpvXR0UrUz/zxB+bA
IkjHUlBLpvXR0UrUzYbkNpk0agV2IzUpk8tEjmF/zxB+bATMtPjCUWbz8Lr9wloX
vXR0UrUzYbkNpk0agV2IzUpk8tEjmF/zxB+bATMtPjCUWbz8Lr9wloXIkjHUlBLp
X-MIC-Info: RSA-MD2,RSA,
5rDqUcMlK1Z6720dcBWGGsDLpTpSCnpotJ6UiRRGcDSvzrsoK+oNvqu6z7Xs5Xfz
X-Recipient-ID: linn@ccy.bbn.com:RSADSI:3
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X-Key-Info: RSA,
lBLpvXR0UrUzYbkNpk0agV2IzUpk8tEjmF/zxB+bATMtPjCUWbz8Lr9wloXIkjHU
X-Recipient-ID: privacy-tf@venera.isi.edu:RSADSI:4
X-Key-Info: RSA,
NcUk2jHEUSoH1nvNSIWL9MLLrHB0eJzyhP+/fSStdW8okeEnv47jxe7SJ/iN72oh
LLrHB0eJzyhP+/fSStdW8okeEnv47jxe7SJ/iN72ohNcUk2jHEUSoH1nvNSIWL9M
8tEjmF/zxB+bATMtPjCUWbz8Lr9wloXIkjHUlBLpvXR0UrUzYbkNpk0agV2IzUpk
J6UiRRGcDSvzrsoK+oNvqu6z7Xs5Xfz5rDqUcMlK1Z6720dcBWGGsDLpTpSCnpot
dXd/H5LMDWnonNvPCwQUHt==
-----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
Example Encapsulated Message (Asymmetric Case)
Figure 3
Although the encapsulated header fields resemble RFC-822 header
fields, they are a disjoint set and will not in general be processed
by the same parser which operates on enclosing header fields. The
complexity of lexical analysis needed and appropriate for
encapsulated header field processing is significantly less than that
appropriate to RFC-822 header processing. For example, many
characters with special significance to RFC-822 at the syntactic
level have no such significance within encapsulated header fields.
When the length of an encapsulated header field is longer than the
size conveniently printable on a line, whitespace may be used to fold
the field in the manner of RFC-822, section 3.1.1. Any such inserted
whitespace is not to be interpreted as a part of a subfield. As a
particular example, due to the length of public-key certificates and
of quantities encrypted using asymmetric algorithms, such quantities
may often need to be folded across multiple printed lines. In order
to facilitate such folding in a uniform manner, the bits representing
such a quantity are to be divided into an ordered set (with leftmost
bits first) of zero or more 384-bit groups (corresponding to 64-
character printed representations), followed by a final group of bits
which may be any length up to 384 bits.
4.6.1 Per-Message Encapsulated Header Fields
This group of encapsulated header fields contains fields which occur
no more than once in a privacy-enhanced message, generally preceding
all other encapsulated header fields.
4.6.1.1 X-Proc-Type Field
The "X-Proc-Type:" encapsulated header field, required for all
privacy-enhanced messages, identifies the type of processing
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performed on the transmitted message. Only one "X-Proc-Type:" field
occurs in a message; the "X-Proc-Type:" field must be the first
encapsulated header field in the message.
The "X-Proc-Type:" field has two subfields, separated by a comma.
The first subfield is a decimal number which is used to distinguish
among incompatible encapsulated header field interpretations which
may arise as changes are made to this standard. Messages processed
according to this RFC will carry the subfield value "3" to
distinguish them from messages processed in accordance with prior
RFCs 989 and 1040.
The second subfield may assume one of two string values: "ENCRYPTED"
or "MIC-ONLY". Unless all of a message's encapsulated text is
excluded from encryption, the "X-Proc-Type:" field's second subfield
must specify "ENCRYPTED". Specification of "MIC-ONLY", when applied
in conjunction with certain combinations of key management and MIC
algorithm options, permits certain fields which are superfluous in
the absence of encryption to be omitted from the encapsulated header.
In particular, "X-Recipient-ID:" and "X-Key-Info:" fields can be
omitted for recipients for whom asymmetric cryptography is used,
assuming concurrent use of a keyless MIC computation algorithm. The
"X-DEK-Info:" field can be omitted for all "MIC-ONLY" messages.
4.6.1.2 X-DEK-Info Field
The "X-DEK-Info:" encapsulated header field identifies the message
text encryption algorithm and mode, and also carries the Initializing
Vector used for message encryption. No more than one "X-DEK-Info:"
field occurs in a message; the field is required except for messages
specified as "MIC-ONLY" in the "X-Proc-Type:" field.
The "X-DEK-Info:" field carries two arguments, separated by a comma.
For purposes of this RFC, the first argument must be the string
"DES-CBC", signifying (as defined in RFC-1115) use of the DES
algorithm in the CBC mode. The second argument represents a 64-bit
Initializing Vector (IV) as a contiguous string of 16 hexadecimal
digits. Subsequent revisions of RFC-1115 will specify any additional
values which may appear as the first argument of this field.
4.6.2 Encapsulated Header Fields Normally Per-Message
This group of encapsulated header fields contains fields which
ordinarily occur no more than once per message. Depending on the key
management option(s) employed, some of these fields may be absent
from some messages. The "X-Sender-ID" field may occur more than once
in a message if different sender-oriented IK components (perhaps
corresponding to different versions) must be used for different
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recipients. In this case later occurrences override prior
occurrences. If a mixture of symmetric and asymmetric key
distribution is used within a single message, the recipients for each
type of key distribution technology should be grouped together to
simplify parsing.
4.6.2.1 X-Sender-ID Field
The "X-Sender-ID:" encapsulated header field, required for all
privacy-enhanced messages, identifies a message's sender and provides
the sender's IK identification component. It should be replicated
within the encapsulated text. The IK identification component
carried in an "X-Sender-ID:" field is used in conjunction with all
subsequent "X-Recipient-ID:" fields until another "X-Sender-ID:"
field occurs; the ordinary case will be that only a single "X-
Sender-ID:" field will occur, prior to any "X-Recipient-ID:" fields.
The "X-Sender-ID:" field contains (in order) an Entity Identifier
subfield, an (optional) Issuing Authority subfield, and an (optional)
Version/Expiration subfield. The optional subfields are omitted if
their use is rendered redundant by information carried in subsequent
"X-Recipient-ID:" fields; this will ordinarily be the case where
symmetric cryptography is used for key management. The subfields are
delimited by the colon character (":"), optionally followed by
whitespace.
Section 5.2, Interchange Keys, discusses the semantics of these
subfields and specifies the alphabet from which they are chosen.
Note that multiple "X-Sender-ID:" fields may occur within a single
encapsulated header. All "X-Recipient-ID:" fields are interpreted in
the context of the most recent preceding "X-Sender-ID:" field; it is
illegal for an "X-Recipient-ID:" field to occur in a header before an
"X-Sender-ID:" has been provided.
4.6.2.2 X-Certificate Field
The "X-Certificate:" encapsulated header field is used only when
asymmetric key management is employed for one or more of a message's
recipients. To facilitate processing by recipients (at least in
advance of general directory server availability), inclusion of this
field in all messages is strongly recommended. The field transfers a
sender's certificate as a numeric quantity, represented with the
encoding mechanism defined in Section 4.3.2.4 of this RFC. The
semantics of a certificate are discussed in RFC-1114. The
certificate carried in an "X-Certificate:" field is used in
conjunction with "X-Sender-ID:" and "X-Recipient-ID:" fields for
which asymmetric key management is employed.
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4.6.2.3 X-MIC-Info Field
The "X-MIC-Info:" encapsulated header field, used only when
asymmetric key management is employed for at least one recipient of a
message, carries three arguments, separated by commas. The first
argument identifies the algorithm under which the accompanying MIC is
computed; RFC-1115 specifies the acceptable set of MIC algorithm
identifiers. The second argument identifies the algorithm under
which the accompanying MIC is encrypted; for purposes of this RFC,
the string "RSA" as described in RFC-1115 must occur, identifying
use of the RSA algorithm. The third argument is a MIC,
asymmetrically encrypted using the originator's private component.
As discussed earlier in this section, the asymmetrically encrypted
MIC is represented using the technique described in Section 4.3.2.4
of this RFC.
The "X-MIC-Info:" field will occur immediately following the
message's "X-Sender-ID:" field and any "X-Certificate:" or "X-
Issuer-Certificate:" fields. Analogous to the "X-Sender-ID:" field,
an "X-MIC-Info:" field applies to all subsequent recipients for whom
asymmetric key management is used.
4.6.3 Encapsulated Header Fields with Variable Occurrences
This group of encapsulated header fields contains fields which will
normally occur variable numbers of times within a message, with
numbers of occurrences ranging from zero to non-zero values which are
independent of the number of recipients.
4.6.3.1 X-Issuer-Certificate Field
The "X-Issuer-Certificate:" encapsulated header field is meaningful
only when asymmetric key management is used for at least one of a
message's recipients. A typical "X-Issuer-Certificate:" field would
contain the certificate containing the public component used to sign
the certificate carried in the message's "X-Certificate:" field, for
recipients' use in chaining through that certificate's certification
path. Other "X-Issuer-Certificate:" fields, typically representing
higher points in a certification path, also may be included by a
sender. The order in which "X-Issuer-Certificate:" fields are
included need not correspond to the order of the certification path;
the order of that path may in general differ from the viewpoint of
different recipients. More information on certification paths can be
found in RFC-1114.
The certificate is represented in the same manner as defined for the
"X-Certificate:" field, and any "X-Issuer-Certificate:" fields will
ordinarily follow the "X-Certificate:" field directly. Use of the
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"X-Issuer-Certificate:" field is optional even when asymmetric key
management is employed, although its incorporation is strongly
recommended in the absence of alternate directory server facilities
from which recipients can access issuers' certificates.
4.6.4 Per-Recipient Encapsulated Header Fields
This group of encapsulated header fields normally appears once for
each of a message's named recipients. As a special case, these
fields may be omitted in the case of a "MIC-ONLY" message to
recipients for whom asymmetric key management is employed, given that
the chosen MIC algorithm is keyless.
4.6.4.1 X-Recipient-ID Field
The "X-Recipient-ID:" encapsulated header field identifies a
recipient and provides the recipient's IK identification component.
One "X-Recipient-ID:" field is included for each of a message's named
recipients. It should be replicated within the encapsulated text.
The field contains (in order) an Entity Identifier subfield, an
Issuing Authority subfield, and a Version/Expiration subfield. The
subfields are delimited by the colon character (":"), optionally
followed by whitespace.
Section 5.2, Interchange Keys, discusses the semantics of the
subfields and specifies the alphabet from which they are chosen. All
"X-Recipient-ID:" fields are interpreted in the context of the most
recent preceding "X-Sender-ID:" field; it is illegal for an "X-
Recipient-ID:" field to occur in a header before an "X-Sender-ID:"
has been provided.
4.6.4.2 X-Key-Info Field
One "X-Key-Info:" field is included for each of a message's named
recipients. Each "X-Key-Info:" field is interpreted in the context
of the most recent preceding "X-Recipient-ID:" field; normally, an
"X-Key-Info:" field will immediately follow its associated "X-
Recipient-ID:" field. The field's argument(s) differ depending on
whether symmetric or asymmetric key management is used for a
particular recipient.
4.6.4.2.1 Symmetric Key Management
When symmetric key management is employed for a given recipient, the
"X-Key-Info:" encapsulated header field transfers four items,
separated by commas: an IK Use Indicator, a MIC Algorithm Indicator,
a DEK and a MIC. The IK Use Indicator identifies the algorithm and
mode in which the identified IK was used for DEK encryption for a
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particular recipient. For recipients for whom symmetric key
management is used, it may assume the reserved string values "DES-
ECB" or "DES-EDE", as defined in RFC-1115.
The MIC Algorithm Indicator identifies the MIC computation algorithm
used for a particular recipient; values for this subfield are defined
in RFC-1115. The DEK and MIC are encrypted under the IK identified
by a preceding "X-Recipient-ID:" field and prior "X-Sender-ID:"
field; they are represented as two strings of contiguous hexadecimal
digits, separated by a comma.
When DEA-1 is used for message text encryption, the DEK
representation will be 16 hexadecimal digits (corresponding to a 64-
bit key); this subfield can be extended to 32 hexadecimal digits
(corresponding to a 128-bit key) if required to support other
algorithms.
Symmetric encryption of MICs is always performed in the same
encryption mode used to encrypt the message's DEK. Encrypted MICs,
like encrypted DEKs, are represented as contiguous strings of
hexadecimal digits. The size of a MIC is dependent on the choice of
MIC algorithm as specified in the MIC Algorithm Indicator subfield.
4.6.4.2.2 Asymmetric Key Management
When asymmetric key management is employed for a given recipient, the
"X-Key-Info:" field transfers two quantities, separated by commas.
The first argument is an IK Use Indicator identifying the algorithm
(and mode, if applicable) in which the DEK is encrypted; for purposes
of this RFC, the IK Use Indicator subfield will always assume the
reserved string value "RSA" (as defined in RFC-1115) for recipients
for whom asymmetric key management is employed, signifying use of the
RSA algorithm. The second argument is a DEK, encrypted (using
asymmetric encryption) under the recipient's public component.
Throughout this RFC we have adopted the terms "private component" and
"public component" to refer to the quantities which are,
respectively, kept secret and made publically available in asymmetric
cryptosystems. This convention is adopted to avoid possible
confusion arising from use of the term "secret key" to refer to
either the former quantity or to a key in a symmetric cryptosystem.
As discussed earlier in this section, the asymmetrically encrypted
DEK is represented using the technique described in Section 4.3.2.4
of this RFC.
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5. Key Management
Several cryptographic constructs are involved in supporting the
privacy-enhanced message processing procedure. A set of fundamental
elements is assumed. Data Encrypting Keys (DEKs) are used to encrypt
message text and (for some MIC computation algorithms) in the message
integrity check (MIC) computation process. Interchange Keys (IKs)
are used to encrypt DEKs and MICs for transmission with messages. In
a certificate-based asymmetric key management architecture,
certificates are used as a means to provide entities' public
components and other information in a fashion which is securely bound
by a central authority. The remainder of this section provides more
information about these constructs.
5.1 Data Encrypting Keys (DEKs)
Data Encrypting Keys (DEKs) are used for encryption of message text
and (with some MIC computation algorithms) for computation of message
integrity check quantities (MICs). It is strongly recommended that
DEKs be generated and used on a one-time, per-message, basis. A
transmitted message will incorporate a representation of the DEK
encrypted under an appropriate interchange key (IK) for each of the
named recipients.
DEK generation can be performed either centrally by key distribution
centers (KDCs) or by endpoint systems. Dedicated KDC systems may be
able to implement stronger algorithms for random DEK generation than
can be supported in endpoint systems. On the other hand,
decentralization allows endpoints to be relatively self-sufficient,
reducing the level of trust which must be placed in components other
than a message's originator and recipient. Moreover, decentralized
DEK generation at endpoints reduces the frequency with which senders
must make real-time queries of (potentially unique) servers in order
to send mail, enhancing communications availability.
When symmetric cryptography is used, one advantage of centralized
KDC-based generation is that DEKs can be returned to endpoints
already encrypted under the IKs of message recipients rather than
providing the IKs to the senders. This reduces IK exposure and
simplifies endpoint key management requirements. This approach has
less value if asymmetric cryptography is used for key management,
since per-recipient public IK components are assumed to be generally
available and per-sender private IK components need not necessarily
be shared with a KDC.
5.2 Interchange Keys (IKs)
Interchange Key (IK) components are used to encrypt DEKs and MICs.
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In general, IK granularity is at the pairwise per-user level except
for mail sent to address lists comprising multiple users. In order
for two principals to engage in a useful exchange of privacy-enhanced
electronic mail using conventional cryptography, they must first
possess common IK components (when symmetric key management is used)
or complementary IK components (when asymmetric key management is
used). When symmetric cryptography is used, the IK consists of a
single component, used to encrypt both DEKs and MICs. When
asymmetric cryptography is used, a recipient's public component is
used as an IK to encrypt DEKs (a transformation invertible only by a
recipient possessing the corresponding private component), and the
originator's private component is used to encrypt MICs (a
transformation invertible by all recipients, since the originator's
certificate provides the necessary public component of the
originator).
While this RFC does not prescribe the means by which interchange keys
are provided to appropriate parties, it is useful to note that such
means may be centralized (e.g., via key management servers) or
decentralized (e.g., via pairwise agreement and direct distribution
among users). In any case, any given IK component is associated with
a responsible Issuing Authority (IA). When certificate-based
asymmetric key management, as discussed in RFC-1114, is employed, the
IA function is performed by a Certification Authority (CA).
When an IA generates and distributes an IK component, associated
control information is provided to direct how it is to be used. In
order to select the appropriate IK(s) to use in message encryption, a
sender must retain a correspondence between IK components and the
recipients with which they are associated. Expiration date
information must also be retained, in order that cached entries may
be invalidated and replaced as appropriate.
Since a message may be sent with multiple IK components identified,
corresponding to multiple intended recipients, each recipient's UA
must be able to determine that recipient's intended IK component.
Moreover, if no corresponding IK component is available in the
recipient's database when a message arrives, the recipient must be
able to identify the required IK component and identify that IK
component's associated IA. Note that different IKs may be used for
different messages between a pair of communicants. Consider, for
example, one message sent from A to B and another message sent (using
the IK-per-list method) from A to a mailing list of which B is a
member. The first message would use IK components associated
individually with A and B, but the second would use an IK component
shared among list members.
When a privacy-enhanced message is transmitted, an indication of the
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IK components used for DEK and MIC encryption must be included. To
this end, the "X-Sender-ID:" and "X-Recipient-ID:" encapsulated
header fields provide the following data:
1. Identification of the relevant Issuing Authority (IA subfield)
2. Identification of an entity with which a particular IK
component is associated (Entity Identifier or EI subfield)
3. Version/Expiration subfield
The colon character (":") is used to delimit the subfields within an
"X-Sender-ID:" or "X-Recipient-ID:". The IA, EI, and
version/expiration subfields are generated from a restricted
character set, as prescribed by the following BNF (using notation as
defined in RFC-822, sections 2 and 3.3):
IKsubfld := 1*ia-char
ia-char := DIGIT / ALPHA / "'" / "+" / "(" / ")" /
"," / "." / "/" / "=" / "?" / "-" / "@" /
"%" / "!" / '"' / "_" / "<" / ">"
An example "X-Recipient-ID:" field is as follows:
X-Recipient-ID: linn@ccy.bbn.com:ptf-kmc:2
This example field indicates that IA "ptf-kmc" has issued an IK
component for use on messages sent to "linn@ccy.bbn.com", and that
the IA has provided the number 2 as a version indicator for that IK
component.
5.2.1 Subfield Definitions
The following subsections define the subfields of "X-Sender-ID:" and
"X-Recipient-ID:" fields.
5.2.1.1 Entity Identifier Subfield
An entity identifier is constructed as an IKsubfld. More
restrictively, an entity identifier subfield assumes the following
form:
<user>@<domain-qualified-host>
In order to support universal interoperability, it is necessary to
assume a universal form for the naming information. For the case of
installations which transform local host names before transmission
into the broader Internet, it is strongly recommended that the host
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name as presented to the Internet be employed.
5.2.1.2 Issuing Authority Subfield
An IA identifier subfield is constructed as an IKsubfld. IA
identifiers must be assigned in a manner which assures uniqueness.
This can be done on a centralized or hierarchic basis.
5.2.1.3 Version/Expiration Subfield
A version/expiration subfield is constructed as an IKsubfld. The
version/expiration subfield format may vary among different IAs, but
must satisfy certain functional constraints. An IA's
version/expiration subfields must be sufficient to distinguish among
the set of IK components issued by that IA for a given identified
entity. Use of a monotonically increasing number is sufficient to
distinguish among the IK components provided for an entity by an IA;
use of a timestamp additionally allows an expiration time or date to
be prescribed for an IK component.
5.2.2 IK Cryptoperiod Issues
An IK component's cryptoperiod is dictated in part by a tradeoff
between key management overhead and revocation responsiveness. It
would be undesirable to delete an IK component permanently before
receipt of a message encrypted using that IK component, as this would
render the message permanently undecipherable. Access to an expired
IK component would be needed, for example, to process mail received
by a user (or system) which had been inactive for an extended period
of time. In order to enable very old IK components to be deleted, a
message's recipient desiring encrypted local long term storage should
transform the DEK used for message text encryption via re-encryption
under a locally maintained IK, rather than relying on IA maintenance
of old IK components for indefinite periods.
6. User Naming
6.1 Current Approach
Unique naming of electronic mail users, as is needed in order to
select corresponding keys correctly, is an important topic and one
which has received significant study. Our current architecture
associates IK components with user names represented in a universal
form ("user@domain-qualified-host"), relying on the following
properties:
1. The universal form must be specifiable by an IA as it
distributes IK components and known to a UA as it processes
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received IK components and IK component identifiers. If a
UA or IA uses addresses in a local form which is different
from the universal form, it must be able to perform an
unambiguous mapping from the universal form into the local
representation.
2. The universal form, when processed by a sender UA, must have
a recognizable correspondence with the form of a recipient
address as specified by a user (perhaps following local
transformation from an alias into a universal form).
It is difficult to ensure these properties throughout the Internet.
For example, an MTS which transforms address representations between
the local form used within an organization and the universal form as
used for Internet mail transmission may cause property 2 to be
violated.
6.2 Issues for Consideration
The use of flat (non-hierarchic) electronic mail user identifiers,
which are unrelated to the hosts on which the users reside, may offer
value. As directory servers become more widespread, it may become
appropriate for would-be senders to search for desired recipients
based on such attributes. Personal characteristics, like social
security numbers, might be considered. Individually-selected
identifiers could be registered with a central authority, but a means
to resolve name conflicts would be necessary.
A point of particular note is the desire to accommodate multiple
names for a single individual, in order to represent and allow
delegation of various roles in which that individual may act. A
naming mechanism that binds user roles to keys is needed. Bindings
cannot be immutable since roles sometimes change (e.g., the
comptroller of a corporation is fired).
It may be appropriate to examine the prospect of extending the
DARPA/DoD domain system and its associated name servers to resolve
user names to unique user IDs. An additional issue arises with
regard to mailing list support: name servers do not currently perform
(potentially recursive) expansion of lists into users. ISO and CSNet
are working on user-level directory service mechanisms, which may
also bear consideration.
7. Example User Interface and Implementation
In order to place the mechanisms and approaches discussed in this RFC
into context, this section presents an overview of a prototype
implementation. This implementation is a standalone program which is
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invoked by a user, and lies above the existing UA sublayer. In the
UNIX system, and possibly in other environments as well, such a
program can be invoked as a "filter" within an electronic mail UA or
a text editor, simplifying the sequence of operations which must be
performed by the user. This form of integration offers the advantage
that the program can be used in conjunction with a range of UA
programs, rather than being compatible only with a particular UA.
When a user wishes to apply privacy enhancements to an outgoing
message, the user prepares the message's text and invokes the
standalone program (interacting with the program in order to provide
address information and other data required to perform privacy
enhancement processing), which in turn generates output suitable for
transmission via the UA. When a user receives a privacy-enhanced
message, the UA delivers the message in encrypted form, suitable for
decryption and associated processing by the standalone program.
In this prototype implementation (based on symmetric key management),
a cache of IK components is maintained in a local file, with entries
managed manually based on information provided by originators and
recipients. This cache is, effectively, a simple database. IK
components are selected for transmitted messages based on the
sender's identity and on recipient names, and corresponding "X-
Sender-ID:" and "X-Recipient-ID:" fields are placed into the
message's encapsulated header. When a message is received, these
fields are used as a basis for a lookup in the database, yielding the
appropriate IK component entries. DEKs and IVs are generated
dynamically within the program.
Options and destination addresses are selected by command line
arguments to the standalone program. The function of specifying
destination addresses to the privacy enhancement program is logically
distinct from the function of specifying the corresponding addresses
to the UA for use by the MTS. This separation results from the fact
that, in many cases, the local form of an address as specified to a
UA differs from the Internet global form as used in "X-Sender-ID:"
and "X-Recipient-ID:" fields.
8. Areas For Further Study
The procedures defined in this RFC are sufficient to support
implementation of privacy-enhanced electronic mail transmission among
cooperating parties in the Internet. Further effort will be needed,
however, to enhance robustness, generality, and interoperability. In
particular, further work is needed in the following areas:
1. User naming techniques, and their relationship to the domain
system, name servers, directory services, and key management
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functions.
2. Detailed standardization of Issuing Authority and directory
service functions and interactions.
3. Privacy-enhanced interoperability with X.400 mail.
We anticipate generation of subsequent RFCs which will address these
topics.
9. References
This section identifies background references which may be useful to
those contemplating use of the mechanisms defined in this RFC.
ISO 7498/Part 2 - Security Architecture, prepared by ISO/TC97/SC
21/WG 1 Ad hoc group on Security, extends the OSI Basic Reference
Model to cover security aspects which are general architectural
elements of communications protocols, and provides an annex with
tutorial and background information.
US Federal Information Processing Standards Publication (FIPS
PUB) 46, Data Encryption Standard, 15 January 1977, defines the
encipherment algorithm used for message text encryption and
Message Authentication Code (MAC) computation.
FIPS PUB 81, DES Modes of Operation, 2 December 1980, defines
specific modes in which the Data Encryption Standard algorithm
may to be used to perform encryption.
FIPS PUB 113, Computer Data Authentication, May 1985, defines a
specific procedure for use of the Data Encryption Standard
algorithm to compute a MAC.
NOTES:
[1] Key generation for MIC computation and message text encryption
may either be performed by the sending host or by a centralized
server. This RFC does not constrain this design alternative.
Section 5.1 identifies possible advantages of a centralized
server approach if symmetric key management is employed.
[2] American National Standard Data Encryption Algorithm (ANSI
X3.92-1981), American National Standards Institute, Approved 30
December 1980.
[3] Federal Information Processing Standards Publication 46, Data
Encryption Standard, 15 January 1977.
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[4] Information Processing Systems: Data Encipherment: Modes of
Operation of a 64-bit Block Cipher.
[5] Federal Information Processing Standards Publication 81, DES
Modes of Operation, 2 December 1980.
[6] ANSI X9.17-1985, American National Standard, Financial
Institution Key Management (Wholesale), American Bankers
Association, April 4, 1985, Section 7.2.
[7] Postel, J., "Simple Mail Transfer Protocol" RFC-821,
USC/Information Sciences Institute, August 1982.
[8] This transformation should occur only at an SMTP endpoint, not at
an intervening relay, but may take place at a gateway system
linking the SMTP realm with other environments.
[9] Use of the SMTP canonicalization procedure at this stage was
selected since it is widely used and implemented in the Internet
community, not because SMTP interoperability with this
intermediate result is required; no privacy-enhanced message will
be passed to SMTP for transmission directly from this step in the
four-phase transformation procedure.
[10] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", RFC-822, August 1982.
[11] Rose, M. and E. Stefferud, "Proposed Standard for Message
Encapsulation", RFC-934, January 1985.
[12] CCITT Recommendation X.411 (1988), "Message Handling Systems:
Message Transfer System: Abstract Service Definition and
Procedures".
[13] CCITT Recommendation X.509 (1988), "The Directory -
Authentication Framework".
[14] Kille, S., "Mapping between X.400 and RFC-822", RFC-987, June
1986.
[15] Federal Information Processing Standards Publication 113,
Computer Data Authentication, May 1985.
[16] American National Standard for Information Systems - Data
Encryption Algorithm - Modes of Operation (ANSI X3.106-1983),
American National Standards Institute - Approved 16 May 1983.
[17] Voydock, V. and S. Kent, "Security Mechanisms in High-Level
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Network Protocols", ACM Computing Surveys, Vol. 15, No. 2, Pages
135-171, June 1983.
Author's Address
John Linn
Secure Systems
Digital Equipment Corporation
85 Swanson Road, BXB1-2/D04
Boxborough, MA 01719-1326
Phone: 508-264-5491
EMail: Linn@ultra.enet.dec.com
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