RFC 989
Network Working Group John Linn (BBNCC)
Request for Comments: 989 IAB Privacy Task Force
February 1987
Privacy Enhancement for Internet Electronic Mail:
Part I: Message Encipherment and Authentication Procedures
STATUS OF THIS MEMO
This RFC suggests a proposed 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, Matt Bishop, Danny Cohen, Tom Daniel, Charles Fox, Morrie
Gasser, Steve Kent (chairman), John Laws, Steve Lipner, Dan Nessett,
Mike Padlipsky, Rob Shirey, Miles Smid, Steve Walker, and Steve
Wilbur.
1 Executive Summary
This RFC defines message encipherment and authentication procedures,
as the initial phase of an effort to provide privacy enhancement
services for electronic mail transfer in the Internet. Detailed key
management mechanisms to support these procedures will be defined in
a subsequent RFC. As a goal of this initial phase, it is intended
that the procedures defined here be compatible with a wide range of
key management approaches, including both conventional (symmetric)
and public-key (asymmetric) approaches for encryption of data
encrypting keys. Use of conventional cryptography for message text
encryption and/or authentication is anticipated.
Privacy enhancement services (confidentiality, authentication, and
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.
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2 Terminology
For descriptive purposes, this RFC uses some terms defined in the OSI
X.400 Message Handling System Model. 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 User
Agent. A User Agent (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's goal is to define 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. Two distinct privacy
enhancement service options are supported:
1. an option providing sender authentication and integrity
verification
2. an option providing sender authentication and integrity
verification in addition to confidentiality service through
encryption
No facility for confidentiality service in the absence of
authentication is provided. Encryption and authentication facilities
may be applied selectively to portions of a message's contents; this
allows less sensitive portions of messages (e.g., descriptive fields)
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to be processed by a recipient's delegate in the absence of the
recipient's personal cryptographic keys.
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 offer compatibility with non-
enhanced Internet components. Privacy enhancements will be
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 offer compatibility 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 offer compatibility with a broad
range of electronic mail user agents (UAs). A large
variety of electronic mail user agent programs, with a
corresponding broad range of user interface paradigms, is
used in the Internet. In order that an electronic mail
privacy enhancement be available to the broadest possible
user community, it is desirable that the selected mechanism
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
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each UA with which enhanced privacy 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.
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).
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:
-- disclosure protection,
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-- sender authenticity, and
-- message integrity measures,
but the following privacy-relevant concerns are not addressed:
-- access control,
-- traffic flow security,
-- address list accuracy,
-- routing control,
-- issues relating to the serial reuse of PCs by multiple users,
-- assurance of message receipt and non-deniability of receipt, and
-- automatic association of acknowledgments with the messages to
which they refer
An important goal is that privacy enhancement mechanisms impose a
minimum of burden on the users they serve. In particular, this goal
suggests eventual automation of the key management mechanisms
supporting message encryption and authentication. In order to
facilitate deployment and testing of pilot privacy enhancement
implementations in the near term, however, compatibility with out-
of-band (e.g., manual) key distribution must also be supported.
A message's sender will determine whether privacy enhancements are to
be performed on a particular message. This will necessitate
mechanisms by which a sender can 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.
A two-level keying hierarchy is used to support privacy-enhanced
message transmission:
1. Data Encrypting Keys (DEKs) are used for encryption of message
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text and for computation of message authentication codes
(MACs). 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. An IK may either be a single symmetric
cryptographic key or, where asymmetric (public-key)
cryptography is used for DEK encryption, the composition of a
public component used by an originator and a secret component
used by a recipient. Ordinarily, the same IK will be used for
all messages sent between a given originator-recipient pair
over a period of time. Each transmitted message includes a
representation of the DEK(s) used for message encryption
and/or authentication, encrypted under an individual IK per
named recipient. This representation is accompanied by an
identifier (IK ID) to enable the recipient to determine which
IK was used, and so to decrypt the representation yielding the
DEK required for message text decryption and/or MAC
verification.
An encoding procedure is employed in order to represent encrypted
message text in a universally transmissible form and to enable
messages encrypted on one type of system to be decrypted on a
different type. Four phases are involved in this process. A
plaintext message is accepted in local form, using the host's native
character set and line representation. The local form is converted
to a canonical message text representation, defined as equivalent to
the inter-SMTP representation of message text. The canonical
representation is padded to an integral multiple of eight octets, as
required by the encryption algorithm. MAC computation is performed,
and (if disclosure protection is required), the padded canonical
representation is encrypted. The output of this step 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 (to be defined in Section 4.8) 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 MAC verification and
decryption on the received message text. First, the printable
encoding is converted to a bitstring. If the transmitted message was
encrypted, it is decrypted into the canonical representation. If the
message was not encrypted, decoding from the printable form produces
the canonical representation directly. The MAC is verified, and the
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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 ISO draft international standard DIS 8227 [1] shall be used for
encryption of message text and for computation of authentication
codes on messages. The DEA-1 is equivalent to the Data Encryption
Standard (DES), as defined in FIPS PUB 46 [2]. When used for these
purposes, the DEA-1 shall be used in the Cipher Block Chaining (CBC)
mode, as defined in ISO DIS 8372 [3]. The CBC mode definition in DIS
8372 is equivalent to that provided in FIPS PUB 81 [4]. A unique
initializing vector (IV) will be generated for and transmitted with
each encrypted electronic mail message.
An algorithm other than DEA-1 may be employed, provided that it
satisfies the following requirements:
1. it must be a 64-bit block cipher, enciphering and deciphering
in 8 octet blocks
2. it is usable in the ECB and CBC modes defined in DIS8372
3. it is able to be keyed using the procedures and parameters
defined in this RFC
4. it is appropriate for MAC computation
5. cryptographic key field lengths are limited to 16 octets
in length
Certain operations require that one key be encrypted under another
key (interchange key) for purposes of transmission. For purposes of
this RFC, such encryption will be performed using DEA-1 in Electronic
Codebook (ECB) mode. An optional facility is available to an
interchange key provider to indicate that an associated key is to be
used for encryption in another mode (e.g., the Encrypt-Decrypt-
Encrypt (EDE) mode used for key encryption and decryption with pairs
of 64-bit keys, as described [5] by ASC X3T1).
Future support of public key algorithms for key encryption is under
consideration, and it is intended that the procedures defined in this
RFC be appropriate to allow such usage. Support of key encryption
modes other than ECB is optional for implementations, however.
Therefore, in support of universal interoperability, interchange key
providers should not specify other modes in the absence of a priori
information indicating that recipients are equipped to perform key
encryption in other modes.
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4.3 Canonical Encoding
Any encryption scheme 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
transport facilities. SMTP, designed primarily for interpersonal
messages and anticipating systems and transport media which may be
restricted to a 7-bit character set, can transmit any 7-bit
characters (but not arbitrary 8-bit binary data) in message text.
SMTP introduces other transparency constraints related to line
lengths and message delimiters. Message text may not contain the
string "." in sequence before the end of a message,
and must contain the string "" at least every 1000
characters. Another important SMTP transparency issue must be noted.
Although SMTP specifies a standard representation for line delimiters
(ASCII ), numerous systems use a different native
representation to delimit lines. For example, the sequences
delimiting lines in mail inbound to UNIX(tm) systems are transformed
to single 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 [6] server.
As a result, if the encryption process generated s or s,
those characters might not be accessible to a recipient UA program at
a destination using 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 MAC 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 the disadvantage that it would
imply internal file (e.g., mailbox) formats which would be
incompatible with the systems on which they reside, an untenable
prospect. 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.
Our approach to this problem selects a canonical character set,
uniformly representable across privacy-enhanced UAs regardless of
their systems' native character sets, to transport encrypted mail
text (but not electronic mail transport headers!) between endpoints.
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In this approach, an outbound privacy-enhanced message is transformed
between four forms, in sequence:
1. (Local_Form) The message text is created (e.g., via an editor)
in the system's native character set, with lines delimited in
accordance with local convention.
2. (Canonicalize) The message text is converted to the universal
canonical form, equivalent to the inter-SMTP representation as
defined in RFC822 [7] (ASCII character set, line
delimiters). (The processing required to perform this
conversion is relatively small, at least on systems whose
native character set is ASCII.)
3. (Encipher/Authenticate) A padded version of the canonical
plaintext representation is created by appending zero-valued
octets to the end of the representation until the length is an
integral multiple of 8 octets, as is required to perform
encryption in the DEA-1 CBC mode. No padding is applied if
the canonical plaintext representation's length is already a
multiple of 8 octets. This padded representation is used as
the input to the encryption function and to the MAC
computation function.
4. (Encode to Printable Form) The bits resulting from the
encryption operation are 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). Use
of a 64-character subset of International Alphabet IA5 is
proposed, 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 encoding function's output is delimited into text lines
(using local conventions), with each line containing 64
printable characters. The encoding process is performed as
follows, transforming strings of 3 arbitrary (8-bit)
characters to strings of 4 encoded characters:
4a. Proceeding from left to right across the input characters
(considered as a contiguous bitstring), each group of 6
bits 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 1, are selected so as to be
universally representable, and the set excludes
characters with particular significance to SMTP e.g.,
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".", "", "").
4b. If fewer than 3 input characters are available in a final
quantum, 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 a 65th reserved, universally
representable character ("="). Use of a reserved
character for padding allows compensatory processing to
be performed by a recipient, allowing the decoded message
text's length to be precisely the same as the input
message's length. A final 3-octet input quantum will be
represented as a 4 octet encoding with no terminal "=", a
2-octet input quantum will be represented as 3 octets
followed by one terminal "=", and a 1-octet input quantum
will be represented as 2 octets followed by two
occurrences of "=".
A sender may exclude one or more portions of a message from
encryption/authentication processing. Explicit action is required to
exclude a portion of a message from such processing; by default,
encryption/authentication 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/authentication processing. An
excluded area is represented in the inter-SMTP transmission form
(universal across communicating systems) by bracketing with the
reserved delimiter "*". Cryptographic state is preserved
transparently across an excluded area and continued after the end of
the excluded area. A printable encoding quantum (per step 4b) is
completed before the delimiter "*" is output to initiate or terminate
the representation of an excluded block. Note that the
canonicalizing transformation (step 2 above) and the encoding to
printable form (step 4 above) are applied to all portions of message
text, even those excluded from encryption and authentication.
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)))
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.
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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 delimit portions of an
encoded message to which encryption/authentication
processing has not been applied.
Printable Encoding Characters
Table 1
4.4 Encapsulation Mechanism
Encapsulation of privacy-enhanced messages within an enclosing layer
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. 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 RFC934 [8] 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. Note that, while encryption and/or authentication
processing of transmitted mail may depend on information contained in
the enclosing header (e.g., "To:"), 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
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processing can be applied recursively.
Sensitive data should be 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. 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. 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 the "Subject:" field, it is recommended that the
enclosing header contain a "Subject:" field indicating that
"Encrypted Mail Follows".
A specific point regarding the integration of privacy-enhanced mail
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.
4.5 Processing for Authentication Without Confidentiality
When a message is to be authenticated without confidentiality
service, a DEK is generated [9] for use in MAC computation, and a MAC
is computed using that DEK. For each individually identified
recipient, an IK is selected and identified with an "X-IK-ID:" field.
Each "X-IK-ID:" field is followed by an "X-Key-Info:" field which
transfers the key under which MAC computation was performed,
encrypted under the IK identified by the preceding "X-IK-ID:" field,
along with a representation of the MAC encrypted under the same IK.
The encapsulated text portion following the encapsulated header is
canonically encoded, and coded into printable characters for
transmission, but not encrypted.
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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-IV:", "X-IK-ID:", "X-Key-Info:",
and "X-Pad-Count:". 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 "Subject:", etc.)
Post-Encapsulation Boundary (Post-EB)
-----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
Message Encapsulation
Figure 1
4.6 Processing for Authentication and Confidentiality
When a message is to be authenticated with confidentiality service, a
DEK is generated for use in MAC computation and a variant of the DEK
is formed for use in message encryption. For each individually
identified recipient, an IK is selected and identified with an "X-
IK-ID:" field. Each "X-IK-ID:" field is followed by an "X-Key-Info:"
field, which transfers the DEK and computed MAC, each encrypted under
the IK identified in the preceding "X-IK-ID:" field. The
encapsulated text portion following the encapsulated header is
canonically encoded, encrypted, and coded into printable characters
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for transmission.
4.7 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
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. 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 MAC 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 MAC 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.
4.8 Summary of Added Header and Control Fields
This section summarizes the syntax and semantics of the new header
and control fields to be added to messages in the course of privacy
enhancement processing, indicating whether a particular field occurs
in a message's encapsulated header portion or its encapsulated text
portion. Figure 2 shows the appearance of a small example
encapsulated message using these fields. In all cases, hexadecimal
quantities are represented as contiguous strings of digits, where
each digit is represented by a character from the ranges "0"-"9" or
upper case "A"-"F". Unless otherwise specified, all arguments are to
be processed in a case-sensitive fashion.
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
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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.
The "X-IK-ID" and "X-Key-Info" fields are the only encapsulated
header fields with lengths which can vary beyond a size conveniently
printable on a line. Whitespace may be used between the subfields of
these fields to fold them in the manner of RFC-822; such whitespace
is not to be interpreted as a part of a subfield.
-----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
X-Proc-Type: 1,E
X-Pad-Count: 1
X-IV: F8143EDE5960C597
X-IK-ID: JL:3:ECB
X-Key-Info: 9FD3AAD2F2691B9A,B70665BB9BF7CBCD
X-IK-ID: JL:1:ECB
X-Key-Info: 161A3F75DC82EF26,E2EF532C65CBCFF7
LLrHB0eJzyhP+/fSStdW8okeEnv47jxe7SJ/iN72ohNcUk2jHEUSoH1nvNSIWL9M
8tEjmF/zxB+bATMtPjCUWbz8Lr9wloXIkjHUlBLpvXR0UrUzYbkNpk0agV2IzUpk
J6UiRRGcDSvzrsoK+oNvqu6z7Xs5Xfz5rDqUcMlK1Z6720dcBWGGsDLpTpSCnpot
dXd/H5LMDWnonNvPCwQUHt==
-----PRIVACY-ENHANCED MESSAGE BOUNDARY-----
Example Encapsulated Message
Figure 2
X-IK-ID: This field is placed in the encapsulated header
portion of a message to identify the Interchange Key
used for encryption of an associated Data Encrypting
Key or keys (used for message text encryption and/or
MAC computation). This field is used for messages
authenticated without confidentiality service and for
messages authenticated with confidentiality service.
The field contains (in order) an Issuing Authority
subfield and an IK Qualifier subfield, and may also
contain an optional IK Use Indicator subfield. The
subfields are delimited by the colon character (":"),
optionally followed by whitespace. Section 5.1.2,
Interchange Keys, discusses the semantics of these
subfields and specifies the alphabet from which they
are chosen. Note that multiple X-IK-ID fields may
occur within a single encapsulated header. Each X-
IK-ID field is associated with an immediately
subsequent X-Key-Info field.
X-IV: This field is placed in the encapsulated header
portion of a message to carry the Initializing Vector
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RFC 989 February 1987
used for message encryption. It is used only for
messages where confidentiality service is applied.
Following the field name, and one or more delimiting
whitespace characters, a 64-bit Initializing Vector
is represented as a contiguous string of 16
hexadecimal digits.
X-Key-Info: This field is placed in a message's encapsulated
header portion to transfer two items: a DEK and a
MAC. Both items are encrypted under the IK
identified by a preceding X-IK-ID field; they are
represented as two strings of contiguous hexadecimal
digits, separated by a comma. For DEA-1, 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.
The MAC is a 64-bit quantity, represented as 16
hexadecimal digits. The MAC is computed under an
unmodified version of the DEK. Message encryption is
performed using a variant of the DEK, formed by
modulo-2 addition of the hexadecimal quantity
F0F0F0F0F0F0F0F0 to the DEK.
X-Pad-Count: This field is placed in the encapsulated header
portion of a message to indicate the number of zero-
valued octets which were added to pad the input
stream to the encryption function to an integral
multiple of eight octets, as required by the DEA-1
CBC encryption mode. A decimal number in the range
0-7 follows the field name, and one or more
delimiting whitespace characters. Inclusion of this
field allows disambiguation between terminal zero-
valued octets in message text (admittedly, a
relatively unlikely prospect) and zero-valued octets
inserted for padding purposes.
X-Proc-Type: This field is placed in the encapsulated header
portion of a message to identify the type of
processing performed on the transmitted message. The
first subfield is a decimal version number, which
will be used if future developments make it necessary
to redefine the interpretation of encapsulated header
fields. At present, this field may assume only the
value "1". The second subfield, delimited by a
comma, assumes one of two single-character alphabetic
values: "A" and "E", to signify, respectively, (1)
authentication processing only and (2) the
combination of authentication and confidentiality
service through encryption.
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5 Key Management
5.1 Types of Keys
5.1.1 Data Encrypting Keys (DEKs)
Data Encrypting Keys (DEKs) are used for encryption of message text
and for computation of message authentication codes (MACs). It is
strongly recommended that DEKs be generated and used on a one-time
basis. A transmitted message will incorporate a representation of
the DEK encrypted under an interchange key (IK) known to the
authorized recipient.
DEK generation can be performed either centrally by key distribution
centers (KDCs) or by endpoint systems. One advantage of centralized
KDC-based generation is that DEKs can be returned to endpoints
already encrypted under the IKs of message recipients. This reduces
IK exposure and simplifies endpoint key management requirements.
Further, dedicated KDC systems may be able to implement better
algorithms for random key 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 by
endpoints reduces the frequency with which senders must make real-
time queries of (potentially unique) servers in order to send mail,
enhancing communications availability.
5.1.2 Interchange Keys (IKs)
Interchange Keys (IKs) are used to encrypt Data Encrypting Keys. In
general, the granularity of IK usage 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 share a common interchange key. When asymmetric
cryptography is used, an originator and recipient must possess
appropriate public and secret components which, in composition,
constitute an interchange key.
The means by which interchange keys are provided to appropriate
parties are outside the scope of this RFC, but may be centralized
(e.g., via key management servers) or decentralized (e.g., via direct
distribution among users). In any case, a given IK is associated
with a responsible Issuing Authority (IA). When an IA generates and
distributes an IK, associated control information must be provided to
direct how that IK is to be used. In order to select the appropriate
IK to use in message encryption, a sender must retain a
correspondence between IKs 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
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appropriate.
When a privacy-enhanced message is transmitted, an indication of the
IK (or IKs, in the case of a message sent to multiple recipients)
used for DEK encryption must be included. To this end, the IK ID
construct is defined to provide the following data:
1. Identification of the relevant Issuing Authority (IA
subfield)
2. Qualifier string to distinguish the particular IK within
the set of IKs distributed by the IA (IK qualifier
subfield)
3. (Optional) Indicator of IK usage mode (IK use indicator
subfield)
The subfields of an IK ID are delimited with the colon character
(":"). The IA and IK qualifier 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):
IAorIKQual := 1*ia-char
ia-char := DIGIT / ALPHA / "'" / "+" / "(" / ")" /
"," / "." / "/" / "=" / "?" / "-" / "@" /
"%" / "!" / '"' / "_" / "<" / ">"
The IK use indicator subfield assumes a value from a small set of
reserved strings, described later in this section.
IA identifiers must be assigned in a manner which assures uniqueness.
This can be done on a centralized or hierarchic basis.
The IK qualifier string format may vary among different IAs, but must
satisfy certain functional constraints. An IA's IK qualifiers must
be sufficient to distinguish among the set of IKs issued by that IA.
Since a message may be sent with multiple IK IDs, corresponding to
multiple intended recipients, each recipient must be able to
determine which IK is intended for it. Moreover, if no corresponding
IK is available in the recipient's database when a message arrives,
the recipient must be able to determine which IK to request and to
identify that IK'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 an IK shared
between A and B, but the second would use an IK shared among list
members.
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While use of a monotonically increasing number as an IK qualifier is
sufficient to distinguish among the set of IKs distributed by an IA,
it offers no facility for a recipient lacking a matching IK to
determine the appropriate IK to request. This suggests that sender
and recipient name information should be incorporated into an IK
qualifier, along with a number to distinguish among multiple IKs used
between a sender/recipient pair. In order to support universal
interoperability, it is necessary to assume a universal form for the
naming information. General definition of such a form requires
further study; issues and possible approaches will be noted in
Section 6. As an interim measure, the following IK qualifier format
is suggested:
//
where and are in the following form:
@
For the case of installations which transform local host names before
transmission into the broader Internet, it is strongly recommended
that the host name as presented to the Internet be employed. The
is a contiguous string of decimal digits.
The IK use indicator subfield is an optional facility, provided to
identify the encryption mode in which the IK is to be used.
Currently, this subfield may assume the following reserved string
values: "ECB" and "EDE"; the default value is ECB.
An example IK ID adhering to this recommendation is as follows:
ptf-kmc:linn@CCY.BBN.COM/privacy-tf@C.ISI.EDU/2:ECB
This IK ID would indicate that IA "ptf-kmc" has issued an IK for use
on messages sent from "linn@CCY.BBN.COM" to "privacy-tf@C.ISI.EDU",
that the IA has associated number 2 with that IK, and that the IK is
to be used in ECB mode.
IKs will remain valid for a period which will be longer than a single
message and will be identified by an expiration time distributed
along with the IK; IK cryptoperiod is dictated in part by a tradeoff
between key management overhead and revocation responsiveness. It
would be undesirable to delete an IK permanently before receipt of a
message encrypted using that IK, as this would render the message
permanently undecipherable. Access to an expired IK 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 IKs 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 IKs for indefinite
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periods.
6 User Naming
Unique naming of electronic mail users, as is needed in order to
select corresponding keys correctly, is an important topic and one
requiring significant study. A logical association exists between
key distribution and name/directory server functions; their
relationship is a topic deserving further consideration. These
issues have not been fully resolved at this writing. The interim
architecture relies on association of IKs with user names represented
in a universal form, which has the following properties:
1. The universal form must be specifiable by an IA as it
distributes IKs and known to a UA as it processes
received IKs and IK IDs. 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 global form used
for Internet mail transmission may cause property 2 to be violated.
The use of flat (non-hierarchic) electronic mail user identifiers,
which are unrelated to the hosts on which the users reside, appears
useful. 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 Domain
Name 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
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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 [10]
which is invoked by a user, and lies above the existing UA sublayer.
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, a cache of IKs is maintained in a
local file, with entries managed manually based on pairwise
agreements between originators and recipients. This cache is,
effectively, a simple database. IKs are selected for transmitted
messages based on recipient names, and corresponding IK IDs are
placed into the message's encapsulated header. When a message is
received, the IK ID is used as a basis for a lookup in the database,
yielding the appropriate IK entry. DEKs and IVs are generated
dynamically within the program.
Options (e.g., authentication only vs. authentication with
confidentiality service) are selected by command line arguments to
the standalone program. Destination addresses are specified in the
same fashion. 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 for IK ID fields.
8 Areas For Further Study
The procedures defined in this RFC are sufficient to support pilot
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. Standardization of Issuing Authority functions, including
protocols for communications among IAs and between User Agents
and IAs
3. Use of public key encryption algorithms to encrypt data
encrypting keys
4. 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
MAC computation.
FIPS PUB 81, DES Modes of Operation, 2 December 1980, defines
specific modes in which the Data Encryption Standard algorithm
is to be used to perform encryption and MAC computation.
NOTES:
[1] Information Processing Systems: Data Encipherment: Block
Cipher Algorithm DEA 1.
[2] Federal Information Processing Standards Publication 46, Data
Encryption Standard, 15 January 1977.
[3] Information Processing Systems: Data Encipherment: Modes of
Operation of a 64-bit Block Cipher
[4] Federal Information Processing Standards Publication 81, DES
Modes of Operation, 2 December 1980.
Linn, Privacy Task Force [Page 22]
RFC 989 February 1987
[5] Addendum to the Transport Layer Protocol Definition for
Providing Connection Oriented End to End Cryptographic Data
Protection Using a 64-Bit Block Cipher, X3T1-85-50.3, draft of
19 December 1985, Gaithersburg, MD, p. 15.
[6] 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.
[7] Crocker, D. Standard for the Format of ARPA Internet Text
Messages (RFC822), August 1982.
[8] Rose, M. T., and Stefferud, E. A., Proposed Standard for
Message Encapsulation, January 1985.
[9] Key generation for authentication 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 5.1.1 identifies possible advantages of
a centralized server approach.
[10] Note that in the UNIX(tm) 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.
Linn, Privacy Task Force [Page 23]