S/MIME Version 2 Message SpecificationRSA Data Security, Inc.100 Marine Parkway, #500Redwood CityCA94065(415) 595-8782spock@rsa.comInternet Mail Consortium127 Segre PlaceSanta CruzCA95060(408) 426-9827phoffman@imc.orgWorldtalk13122 NE 20th St., Suite CBellevueWA98005(425) 882-8861blaker@deming.comQUALCOMM IncorporatedEudora Division6455 Lusk BoulevardSan DiegoCalifornia92121-2779(800) 238-3672lgl@qualcomm.comNetscape Communications Corporation501 East Middlefield RoadMountain ViewCA94043(415) 254-1900repka@netscape.com
Security
Applications
authenticationcryptographicdigital signatureencryptionmultipurpose internet mail extensionssecurity
S/MIME (Secure/Multipurpose Internet Mail Extensions) provides a
consistent way to send and receive secure MIME data. Based on the
popular Internet MIME standard, S/MIME provides the following
cryptographic security services for electronic messaging
applications: authentication, message integrity and non-repudiation
of origin (using digital signatures) and privacy and data security
(using encryption).
S/MIME can be used by traditional mail user agents (MUAs) to add
cryptographic security services to mail that is sent, and to
interpret cryptographic security services in mail that is received.
However, S/MIME is not restricted to mail; it can be used with any
transport mechanism that transports MIME data, such as HTTP. As such,
S/MIME takes advantage of the object-based features of MIME and
allows secure messages to be exchanged in mixed-transport systems.
Further, S/MIME can be used in automated message transfer agents that
use cryptographic security services that do not require any human
intervention, such as the signing of software-generated documents and
the encryption of FAX messages sent over the Internet.
Please note: The information in this document is historical material
being published for the public record. It is not an IETF standard.
The use of the word "standard" in this document indicates a standard
for adopters of S/MIME version 2, not an IETF standard.
This document describes a protocol for adding cryptographic signature
and encryption services to MIME data. The MIME standard [MIME-SPEC]
provides a general structure for the content type of Internet
messages and allows extensions for new content type applications.
This memo defines how to create a MIME body part that has been
cryptographically enhanced according to PKCS #7 [PKCS-7]. This memo
also defines the application/pkcs7-mime MIME type that can be used to
transport those body parts. This memo also defines how to create
certification requests that conform to PKCS #10 [PKCS-10], and the
application/pkcs10 MIME type for transporting those requests.
This memo also discusses how to use the multipart/signed MIME type
defined in [MIME-SECURE] to transport S/MIME signed messages. This
memo also defines the application/pkcs7-signature MIME type, which is
also used to transport S/MIME signed messages. This specification is
compatible with PKCS #7 in that it uses the data types defined by
PKCS #7.
In order to create S/MIME messages, an agent has to follow
specifications in this memo, as well as some of the specifications
listed in the following documents:
- "PKCS #1: RSA Encryption", [PKCS-1]
- "PKCS #7: Cryptographic Message Syntax", [PKCS-7]
- "PKCS #10: Certification Request Syntax", [PKCS-10]
Throughout this memo, there are requirements and recommendations made
for how receiving agents handle incoming messages. There are separate
requirements and recommendations for how sending agents create
outgoing messages. In general, the best strategy is to "be liberal in
what you receive and conservative in what you send". Most of the
requirements are placed on the handling of incoming messages while
the recommendations are mostly on the creation of outgoing messages.
The separation for requirements on receiving agents and sending
agents also derives from the likelihood that there will be S/MIME
systems that involve software other than traditional Internet mail
clients. S/MIME can be used with any system that transports MIME
data. An automated process that sends an encrypted message might not
be able to receive an encrypted message at all, for example. Thus,
the requirements and recommendations for the two types of agents are
listed separately when appropriate.
Throughout this memo, the terms MUST, MUST NOT, SHOULD, and SHOULD
NOT are used in capital letters. This conforms to the definitions in
[MUSTSHOULD]. [MUSTSHOULD] defines the use of these key words to
help make the intent of standards track documents as clear as
possible. The same key words are used in this document to help
implementors achieve interoperability.
For the purposes of this memo, the following definitions apply.
ASN.1: Abstract Syntax Notation One, as defined in CCITT X.208.
BER: Basic Encoding Rules for ASN.1, as defined in CCITT X.209.
Certificate: A type that binds an entity's distinguished name to a
public key with a digital signature.
DER: Distinguished Encoding Rules for ASN.1, as defined in CCITT
X.509.
7-bit data: Text data with lines less than 998 characters long, where
none of the characters have the 8th bit set, and there are no NULL
characters. <CR> and <LF> occur only as part of a <CR><LF> end of
line delimiter.
8-bit data: Text data with lines less than 998 characters, and where
none of the characters are NULL characters. <CR> and <LF> occur only
as part of a <CR><LF> end of line delimiter.
Binary data: Arbitrary data.
Transfer Encoding: A reversible transformation made on data so 8-bit
or binary data may be sent via a channel that only transmits 7-bit
data.
Appendix C contains important information about how S/MIME agents
following this specification should act in order to have the greatest
interoperability with earlier implementations of S/MIME.
The PKCS #7 message format allows for a wide variety of options in
content and algorithm support. This section puts forth a number of
support requirements and recommendations in order to achieve a base
level of interoperability among all S/MIME implementations.
Receiving agents MUST support SHA-1 [SHA1] and MD5 [MD5].
Sending agents SHOULD use SHA-1.
Receiving agents MUST support rsaEncryption, defined in [PKCS-1].
Receiving agents MUST support verification of signatures using RSA
public key sizes from 512 bits to 1024 bits.
Sending agents MUST support rsaEncryption. Outgoing messages are
signed with a user's private key. The size of the private key is
determined during key generation.
Receiving agents MUST support rsaEncryption. Incoming encrypted
messages contain symmetric keys which are to be decrypted with a
user's private key. The size of the private key is determined during
key generation.
Sending agents MUST support rsaEncryption. Sending agents MUST
support encryption of symmetric keys with RSA public keys at key
sizes from 512 bits to 1024 bits.
The PKCS #7 defines six distinct content types: "data", "signedData",
"envelopedData", "signedAndEnvelopedData", "digestedData", and
"encryptedData". Receiving agents MUST support the "data",
"signedData" and "envelopedData" content types. Sending agents may or
may not send out any of the content types, depending on the services
that the agent supports.
Sending agents MUST use the "data" content type as the content within
other content types to indicate the message content which has had
security services applied to it.
Sending agents MUST use the signedData content type to apply a
digital signature to a message or, in a degenerate case where there
is no signature information, to convey certificates.
This content type is used to apply privacy protection to a message. A
sender needs to have access to a public key for each intended message
recipient to use this service. This content type does not provide
authentication.
The SignerInfo type allows the inclusion of unauthenticated and
authenticated attributes to be included along with a signature.
Receiving agents MUST be able to handle zero or one instance of each
of the signed attributes described in this section.
Sending agents SHOULD be able to generate one instance of each of the
signed attributes described in this section, and SHOULD include these
attributes in each signed message sent.
Additional attributes and values for these attributes may be defined
in the future. Receiving agents SHOULD handle attributes or values
that it does not recognize in a graceful manner.
The signing-time attribute is used to convey the time that a message
was signed. Until there are trusted timestamping services, the time
of signing will most likely be created by a message originator and
therefore is only as trustworthy as the originator.
Sending agents MUST encode signing time through the year 2049 as
UTCTime; signing times in 2050 or later MUST be encoded as
GeneralizedTime. Agents MUST interpret the year field (YY) as
follows: if YY is greater than or equal to 50, the year is
interpreted as 19YY; if YY is less than 50, the year is interpreted
as 20YY.
The S/MIME capabilities attribute includes signature algorithms (such
as "md5WithRSAEncryption"), symmetric algorithms (such as "DES-CBC"),
and key encipherment algorithms (such as "rsaEncryption"). It also
includes a non-algorithm capability which is the preference for
signedData. SMIMECapabilities was designed to be flexible and
extensible so that, in the future, a means of identifying other
capabilities and preferences such as certificates can be added in a
way that will not cause current clients to break.
The semantics of the S/MIME capabilites attribute specify a partial
list as to what the client announcing the SMIMECapabilites can
support. A client does not have to list every capability it supports,
and probably should not list all its capabilities so that the
capabilities list doesn't get too long. In an SMIMECapabilities
encoding, the OIDs are listed in order of their preference, but
SHOULD be logically separated along the lines of their categories
(signature algorithms, symmetric algorithms, key encipherment
algorithms, etc.)
The structure of SMIMECapabilities was designed to facilitate simple
table lookups and binary comparisons in order to determine matches.
For instance, the DER-encoding for the SMIMECapability for DES EDE3
CBC MUST be identically encoded regardless of the implementation.
In the case of symmetric algorithms, the associated parameters for
the OID MUST specify all of the parameters necessary to differentiate
between two instances of the same algorithm. For instance, the number
of rounds and block size for RC5 must be specified in addition to the
key length.
There is a list of OIDs (the registered SMIMECapability list) that is
centrally maintained and is separate from this memo. The list of OIDs
is maintained by the Internet Mail Consortium at
<http://www.imc.org/ietf-smime/oids.html>.
The OIDs that correspond to algorithms SHOULD use the same OID as the
actual algorithm, except in the case where the algorithm usage is
ambiguous from the OID. For instance, in an earlier memo,
rsaEncryption was ambiguous because it could refer to either a
signature algorithm or a key encipherment algorithm. In the event
that an OID is ambiguous, it needs to be arbitrated by the maintainer
of the registered S/MIME capabilities list as to which type of
algorithm will use the OID, and a new OID MUST be allocated under the
smimeCapabilities OID to satisfy the other use of the OID.
The registered S/MIME capabilities list specifies the parameters for
OIDs that need them, most notably key lengths in the case of
variable-length symmetric ciphers. In the event that there are no
differentiating parameters for a particular OID, the parameters MUST
be omitted, and MUST NOT be encoded as NULL.
Additional values for SMIMECapability may be defined in the future.
Receiving agents MUST handle a SMIMECapabilities object that has
values that it does not recognize in a graceful manner.
Receiving agents MUST support decryption using the RC2 [RC2] or a
compatible algorithm at a key size of 40 bits, hereinafter called
"RC2/40". Receiving agents SHOULD support decryption using DES EDE3
CBC, hereinafter called "tripleDES" [3DES] [DES].
Sending agents SHOULD support encryption with RC2/40 and tripleDES.
When a sending agent creates an encrypted message, it has to decide
which type of encryption to use. The decision process involves using
information garnered from the capabilities lists included in messages
received from the recipient, as well as out-of-band information such
as private agreements, user preferences, legal restrictions, and so
on.
Section 2.5 defines a method by which a sending agent can optionally
announce, among other things, its decrypting capabilities in its
order of preference. The following method for processing and
remembering the encryption capabilities attribute in incoming signed
messages SHOULD be used.
- If the receiving agent has not yet created a list of capabilities
for the sender's public key, then, after verifying the signature
on the incoming message and checking the timestamp, the receiving
agent SHOULD create a new list containing at least the signing
time and the symmetric capabilities.
- If such a list already exists, the receiving agent SHOULD verify
that the signing time in the incoming message is greater than the
signing time stored in the list and that the signature is valid.
If so, the receiving agent SHOULD update both the signing time and
capabilities in the list. Values of the signing time that lie far
in the future (that is, a greater discrepancy than any reasonable
clock skew), or a capabilitie lists in messages whose signature
could not be verified, MUST NOT be accepted.
The list of capabilities SHOULD be stored for future use in creating
messages.
Before sending a message, the sending agent MUST decide whether it is
willing to use weak encryption for the particular data in the
message. If the sending agent decides that weak encryption is
unacceptable for this data, then the sending agent MUST NOT use a
weak algorithm such as RC2/40. The decision to use or not use weak
encryption overrides any other decision in this section about which
encryption algorithm to use.
Sections 2.6.2.1 through 2.6.2.4 describe the decisions a sending
agent SHOULD use in deciding which type of encryption should be
applied to a message. These rules are ordered, so the sending agent
SHOULD make its decision in the order given.
If the sending agent has received a set of capabilities from the
recipient for the message the agent is about to encrypt, then the
sending agent SHOULD use that information by selecting the first
capability in the list (that is, the capability most preferred by the
intended recipient) for which the sending agent knows how to encrypt.
The sending agent SHOULD use one of the capabilities in the list if
the agent reasonably expects the recipient to be able to decrypt the
message.
If:
- the sending agent has no knowledge of the encryption capabilities
of the recipient,
- and the sending agent has received at least one message from the
recipient,
- and the last encrypted message received from the recipient had a
trusted signature on it,
then the outgoing message SHOULD use the same encryption algorithm as
was used on the last signed and encrypted message received from the
recipient.
If:
- the sending agent has no knowledge of the encryption capabilities
of the recipient,
- and the sending agent is willing to risk that the recipient may
not be able to decrypt the message,
then the sending agent SHOULD use tripleDES.
If:
- the sending agent has no knowledge of the encryption capabilities
of the recipient,
- and the sending agent is not willing to risk that the recipient
may not be able to decrypt the message,
then the sending agent MUST use RC2/40.
Like all algorithms that use 40 bit keys, RC2/40 is considered by
many to be weak encryption. A sending agent that is controlled by a
human SHOULD allow a human sender to determine the risks of sending
data using RC2/40 or a similarly weak encryption algorithm before
sending the data, and possibly allow the human to use a stronger
encryption method such as tripleDES.
If a sending agent is composing an encrypted message to a group of
recipients where the encryption capabilities of some of the
recipients do not overlap, the sending agent is forced to send more
than one message. It should be noted that if the sending agent
chooses to send a message encrypted with a strong algorithm, and then
send the same message encrypted with a weak algorithm, someone
watching the communications channel can decipher the contents of the
strongly-encrypted message simply by decrypting the weakly-encrypted
message.
This section describes the S/MIME message formats and how they are
created. S/MIME messages are a combination of MIME bodies and PKCS
objects. Several MIME types as well as several PKCS objects are used.
The data to be secured is always a canonical MIME entity. The MIME
entity and other data, such as certificates and algorithm
identifiers, are given to PKCS processing facilities which produces a
PKCS object. The PKCS object is then finally wrapped in MIME.
S/MIME provides one format for enveloped-only data, several formats
for signed-only data, and several formats for signed and enveloped
data. Several formats are required to accommodate several
environments, in particular for signed messages. The criteria for
choosing among these formats are also described.
The reader of this section is expected to understand MIME as
described in [MIME-SPEC] and [MIME-SECURE].
S/MIME is used to secure MIME entities. A MIME entity may be a sub-
part, sub-parts of a message, or the whole message with all its sub-
parts. A MIME entity that is the whole message includes only the MIME
headers and MIME body, and does not include the RFC-822 headers. Note
that S/MIME can also be used to secure MIME entities used in
applications other than Internet mail.
The MIME entity that is secured and described in this section can be
thought of as the "inside" MIME entity. That is, it is the
"innermost" object in what is possibly a larger MIME message.
Processing "outside" MIME entities into PKCS #7 objects is described
in Section 3.2, 3.4 and elsewhere.
The procedure for preparing a MIME entity is given in [MIME-SPEC].
The same procedure is used here with some additional restrictions
when signing. Description of the procedures from [MIME-SPEC] are
repeated here, but the reader should refer to that document for the
exact procedure. This section also describes additional requirements.
A single procedure is used for creating MIME entities that are to be
signed, enveloped, or both signed and enveloped. Some additional
steps are recommended to defend against known corruptions that can
occur during mail transport that are of particular importance for
clear-signing using the multipart/signed format. It is recommended
that these additional steps be performed on enveloped messages, or
signed and enveloped messages in order that the message can be
forwarded to any environment without modification.
These steps are descriptive rather than prescriptive. The implementor
is free to use any procedure as long as the result is the same.
Step 1. The MIME entity is prepared according to the local
conventions
Step 2. The leaf parts of the MIME entity are converted to
canonical form
Step 3. Appropriate transfer encoding is applied to the leaves of
the MIME entity
When an S/MIME message is received, the security services on the
message are removed, and the result is the MIME entity. That MIME
entity is typically passed to a MIME-capable user agent where, it is
further decoded and presented to the user or receiving application.
Each MIME entity MUST be converted to a canonical form that is
uniquely and unambiguously representable in the environment where the
signature is created and the environment where the signature will be
verified. MIME entities MUST be canonicalized for enveloping as well
as signing.
The exact details of canonicalization depend on the actual MIME type
and subtype of an entity, and are not described here. Instead, the
standard for the particular MIME type should be consulted. For
example, canonicalization of type text/plain is different from
canonicalization of audio/basic. Other than text types, most types
have only one representation regardless of computing platform or
environment which can be considered their canonical representation.
In general, canonicalization will be performed by the sending agent
rather than the S/MIME implementation.
The most common and important canonicalization is for text, which is
often represented differently in different environments. MIME
entities of major type "text" must have both their line endings and
character set canonicalized. The line ending must be the pair of
characters <CR><LF>, and the charset should be a registered charset
[CHARSETS]. The details of the canonicalization are specified in
[MIME-SPEC]. The chosen charset SHOULD be named in the charset
parameter so that the receiving agent can unambiguously determine the
charset used.
Note that some charsets such as ISO-2022 have multiple
representations for the same characters. When preparing such text for
signing, the canonical representation specified for the charset MUST
be used.
When generating any of the secured MIME entities below, except the
signing using the multipart/signed format, no transfer encoding at
all is required. S/MIME implementations MUST be able to deal with
binary MIME objects. If no Content-Transfer-Encoding header is
present, the transfer encoding should be considered 7BIT.
S/MIME implementations SHOULD however use transfer encoding described
in section 3.1.3 for all MIME entities they secure. The reason for
securing only 7-bit MIME entities, even for enveloped data that are
not exposed to the transport, is that it allows the MIME entity to be
handled in any environment without changing it. For example, a
trusted gateway might remove the envelope, but not the signature, of
a message, and then forward the signed message on to the end
recipient so that they can verify the signatures directly. If the
transport internal to the site is not 8-bit clean, such as on a
wide-area network with a single mail gateway, verifying the signature
will not be possible unless the original MIME entity was only 7-bit
data.
If a multipart/signed entity is EVER to be transmitted over the
standard Internet SMTP infrastructure or other transport that is
constrained to 7-bit text, it MUST have transfer encoding applied so
that it is represented as 7-bit text. MIME entities that are 7-bit
data already need no transfer encoding. Entities such as 8-bit text
and binary data can be encoded with quoted-printable or base-64
transfer encoding.
The primary reason for the 7-bit requirement is that the Internet
mail transport infrastructure cannot guarantee transport of 8-bit or
binary data. Even though many segments of the transport
infrastructure now handle 8-bit and even binary data, it is sometimes
not possible to know whether the transport path is 8-bit clear. If a
mail message with 8-bit data were to encounter a message transfer
agent that can not transmit 8-bit or binary data, the agent has three
options, none of which are acceptable for a clear-signed message:
- The agent could change the transfer encoding; this would
invalidate the signature.
- The agent could transmit the data anyway, which would most likely
result in the 8th bit being corrupted; this too would invalidate
the signature.
- The agent could return the message to the sender.
[MIME-SECURE] prohibits an agent from changing the transfer encoding
of the first part of a multipart/signed message. If a compliant agent
that can not transmit 8-bit or binary data encounters a
multipart/signed message with 8-bit or binary data in the first part,
it would have to return the message to the sender as undeliverable.
This example shows a multipart/mixed message with full transfer
encoding. This message contains a text part and an attachment. The
sample message text includes characters that are not US-ASCII and
thus must be transfer encoded. Though not shown here, the end of each
line is <CR><LF>. The line ending of the MIME headers, the text, and
transfer encoded parts, all must be <CR><LF>.
Note that this example is not of an S/MIME message.
Content-Type: multipart/mixed; boundary=bar
--bar
Content-Type: text/plain; charset=iso-8859-1
Content-Transfer-Encoding: quoted-printable
=A1Hola Michael!
How do you like the new S/MIME specification?
I agree. It's generally a good idea to encode lines that begin with
From=20because some mail transport agents will insert a greater-
than (>) sign, thus invalidating the signature.
Also, in some cases it might be desirable to encode any =20
trailing whitespace that occurs on lines in order to ensure =20
that the message signature is not invalidated when passing =20
a gateway that modifies such whitespace (like BITNET). =20
--bar
Content-Type: image/jpeg
Content-Transfer-Encoding: base64
iQCVAwUBMJrRF2N9oWBghPDJAQE9UQQAtl7LuRVndBjrk4EqYBIb3h5QXIX/LC//
jJV5bNvkZIGPIcEmI5iFd9boEgvpirHtIREEqLQRkYNoBActFBZmh9GC3C041WGq
uMbrbxc+nIs1TIKlA08rVi9ig/2Yh7LFrK5Ein57U/W72vgSxLhe/zhdfolT9Brn
HOxEa44b+EI=
--bar--
The application/pkcs7-mime type is used to carry PKCS #7 objects of
several types including envelopedData and signedData. The details of
constructing these entities is described in subsequent sections. This
section describes the general characteristics of the
application/pkcs7-mime type.
This MIME type always carries a single PKCS #7 object. The PKCS #7
object must always be BER encoding of the ASN.1 syntax describing the
object. The contentInfo field of the carried PKCS #7 object always
contains a MIME entity that is prepared as described in section 3.1.
The contentInfo field must never be empty.
Since PKCS #7 objects are binary data, in most cases base-64 transfer
encoding is appropriate, in particular when used with SMTP transport.
The transfer encoding used depends on the transport through which the
object is to be sent, and is not a characteristic of the MIME type.
Note that this discussion refers to the transfer encoding of the PKCS
#7 object or "outside" MIME entity. It is completely distinct from,
and unrelated to, the transfer encoding of the MIME entity secured by
the PKCS #7 object, the "inside" object, which is described in
section 3.1.
Because there are several types of application/pkcs7-mime objects, a
sending agent SHOULD do as much as possible to help a receiving agent
know about the contents of the object without forcing the receiving
agent to decode the ASN.1 for the object. The MIME headers of all
application/pkcs7-mime objects SHOULD include the optional "smime-
type" parameter, as described in the following sections.
For the application/pkcs7-mime, sending agents SHOULD emit the
optional "name" parameter to the Content-Type field for compatibility
with older systems. Sending agents SHOULD also emit the optional
Content-Disposition field [CONTDISP] with the "filename" parameter.
If a sending agent emits the above parameters, the value of the
parameters SHOULD be a file name with the appropriate extension:
MIME Type File Extension
application/pkcs7-mime .p7m
(signedData, envelopedData)
application/pkcs7-mime .p7c
(degenerate signedData
"certs-only" message)
application/pkcs7-signature .p7s
application/pkcs10 .p10
In addition, the file name SHOULD be limited to eight characters
followed by a three letter extension. The eight character filename
base can be any distinct name; the use of the filename base "smime"
SHOULD be used to indicate that the MIME entity is associated with
S/MIME.
Including a file name serves two purposes. It facilitates easier use
of S/MIME objects as files on disk. It also can convey type
information across gateways. When a MIME entity of type
application/pkcs7-mime (for example) arrives at a gateway that has no
special knowledge of S/MIME, it will default the entity's MIME type
to application/octet-stream and treat it as a generic attachment,
thus losing the type information. However, the suggested filename for
an attachment is often carried across a gateway. This often allows
the receiving systems to determine the appropriate application to
hand the attachment off to, in this case a stand-alone S/MIME
processing application. Note that this mechanism is provided as a
convenience for implementations in certain environments. A proper
S/MIME implementation MUST use the MIME types and MUST NOT rely on
the file extensions.
This section describes the format for enveloping a MIME entity
without signing it.
Step 1. The MIME entity to be enveloped is prepared according to
section 3.1.
Step 2. The MIME entity and other required data is processed into a
PKCS #7 object of type envelopedData.
Step 3. The PKCS #7 object is inserted into an application/pkcs7-
mime MIME entity.
The smime-type parameter for enveloped-only messages is "enveloped-
data". The file extension for this type of message is ".p7m".
A sample message would be:
Content-Type: application/pkcs7-mime; smime-type=enveloped-data;
name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m
rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
0GhIGfHfQbnj756YT64V
There are two formats for signed messages defined for S/MIME:
application/pkcs7-mime and SignedData, and multipart/signed. In
general, the multipart/signed form is preferred for sending, and
receiving agents SHOULD be able to handle both.
There are no hard-and-fast rules when a particular signed-only format
should be chosen because it depends on the capabilities of all the
receivers and the relative importance of receivers with S/MIME
facilities being able to verify the signature versus the importance
of receivers without S/MIME software being able to view the message.
Messages signed using the multipart/signed format can always be
viewed by the receiver whether they have S/MIME software or not. They
can also be viewed whether they are using a MIME-native user agent or
they have messages translated by a gateway. In this context, "be
viewed" means the ability to process the message essentially as if it
were not a signed message, including any other MIME structure the
message might have.
Messages signed using the signedData format cannot be viewed by a
recipient unless they have S/MIME facilities. However, if they have
S/MIME facilities, these messages can always be verified if they were
not changed in transit.
This signing format uses the application/pkcs7-mime MIME type. The
steps to create this format are:
Step 1. The MIME entity is prepared according to section 3.1
Step 2. The MIME entity and other required data is processed into a
PKCS #7 object of type signedData
Step 3. The PKCS #7 object is inserted into an
application/pkcs7-mime MIME entity
The smime-type parameter for messages using application/pkcs7-mime
and SignedData is "signed-data". The file extension for this type of
message is ".p7m".
A sample message would be:
Content-Type: application/pkcs7-mime; smime-type=signed-data;
name=smime.p7m
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7m
567GhIGfHfYT6ghyHhHUujpfyF4f8HHGTrfvhJhjH776tbB9HG4VQbnj7
77n8HHGT9HG4VQpfyF467GhIGfHfYT6rfvbnj756tbBghyHhHUujhJhjH
HUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H7n8HHGghyHh
6YT64V0GhIGfHfQbnj75
This format is a clear-signing format. Recipients without any S/MIME
or PKCS processing facilities are able to view the message. It makes
use of the multipart/signed MIME type described in [MIME-SECURE]. The
multipart/signed MIME type has two parts. The first part contains the
MIME entity that is to be signed; the second part contains the
signature, which is a PKCS #7 detached signature.
This MIME type always contains a single PKCS #7 object of type
signedData. The contentInfo field of the PKCS #7 object must be
empty. The signerInfos field contains the signatures for the MIME
entity. The details of the registered type are given in Appendix D.
The file extension for signed-only messages using application/pkcs7-
signature is ".p7s".
Step 1. The MIME entity to be signed is prepared according to
section 3.1, taking special care for clear-signing.
Step 2. The MIME entity is presented to PKCS #7 processing in order
to obtain an object of type signedData with an empty
contentInfo field.
Step 3. The MIME entity is inserted into the first part of a
multipart/signed message with no processing other than that
described in section 3.1.
Step 4. Transfer encoding is applied to the detached signature and
it is inserted into a MIME entity of type
application/pkcs7-signature
Step 5. The MIME entity of the application/pkcs7-signature is
inserted into the second part of the multipart/signed
entity
The multipart/signed Content type has two required parameters: the
protocol parameter and the micalg parameter.
The protocol parameter MUST be "application/pkcs7-signature". Note
that quotation marks are required around the protocol parameter
because MIME requires that the "/" character in the parameter value
MUST be quoted.
The micalg parameter allows for one-pass processing when the
signature is being verified. The value of the micalg parameter is
dependent on the message digest algorithm used in the calculation of
the Message Integrity Check. The value of the micalg parameter SHOULD
be one of the following:
Algorithm used Value
-------------- ---------
MD5 md5
SHA-1 sha1
any other unknown
(Historical note: some early implementations of S/MIME emitted and
expected "rsa-md5" and "rsa-sha1" for the micalg parameter.)
Receiving agents SHOULD be able to recover gracefully from a micalg
parameter value that they do not recognize.
Content-Type: multipart/signed;
protocol="application/pkcs7-signature";
micalg=sha1; boundary=boundary42
--boundary42
Content-Type: text/plain
This is a clear-signed message.
--boundary42
Content-Type: application/pkcs7-signature; name=smime.p7s
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p7s
ghyHhHUujhJhjH77n8HHGTrfvbnj756tbB9HG4VQpfyF467GhIGfHfYT6
4VQpfyF467GhIGfHfYT6jH77n8HHGghyHhHUujhJh756tbB9HGTrfvbnj
n8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
7GhIGfHfYT64VQbnj756
--boundary42--
To achieve signing and enveloping, any of the signed-only and
encrypted-only formats may be nested. This is allowed because the
above formats are all MIME entities, and because they all secure MIME
entities.
An S/MIME implementation MUST be able to receive and process
arbitrarily nested S/MIME within reasonable resource limits of the
recipient computer.
It is possible to either sign a message first, or to envelope the
message first. It is up to the implementor and the user to choose.
When signing first, the signatories are then securely obscured by the
enveloping. When enveloping first the signatories are exposed, but it
is possible to verify signatures without removing the enveloping.
This may be useful in an environment were automatic signature
verification is desired, as no private key material is required to
verify a signature.
The certificates only message or MIME entity is used to transport
certificates, such as in response to a registration request. This
format can also be used to convey CRLs.
Step 1. The certificates are made available to the PKCS #7
generating process which creates a PKCS #7 object of type
signedData. The contentInfo and signerInfos fields must be
empty.
Step 2. The PKCS #7 signedData object is enclosed in an
application/pkcs7-mime MIME entity
The smime-type parameter for a certs-only message is "certs-only".
The file extension for this type of message is ".p7c".
A typical application which allows a user to generate cryptographic
information has to submit that information to a certification
authority, who transforms it into a certificate. PKCS #10 describes a
syntax for certification requests. The application/pkcs10 body type
MUST be used to transfer a PKCS #10 certification request.
The details of certification requests and the process of obtaining a
certificate are beyond the scope of this memo. Instead, only the
format of data used in application/pkcs10 is defined.
PKCS #10 defines the ASN.1 type CertificationRequest for use in
submitting a certification request. Therefore, when the MIME content
type application/pkcs10 is used, the body MUST be a
CertificationRequest, encoded using the Basic Encoding Rules (BER).
Although BER is specified, instead of the more restrictive DER, a
typical application will use DER since the CertificationRequest's
CertificationRequestInfo has to be DER-encoded in order to be signed.
A robust application SHOULD output DER, but allow BER or DER on
input.
Data produced by BER or DER is 8-bit, but many transports are limited
to 7-bit data. Therefore, a suitable 7-bit Content-Transfer-Encoding
SHOULD be applied. The base64 Content-Transfer-Encoding SHOULD be
used with application/pkcs10, although any 7-bit transfer encoding
may work.
For sending a certificate-signing request, the application/pkcs10
message format MUST be used to convey a PKCS #10 certificate-signing
request. Note that for sending certificates and CRLs messages without
any signed content, the application/pkcs7-mime message format MUST be
used to convey a degenerate PKCS #7 signedData "certs-only" message.
To send an application/pkcs10 body, the application generates the
cryptographic information for the user. The details of the
cryptographic information are beyond the scope of this memo.
Step 1. The cryptographic information is placed within a PKCS #10
CertificationRequest.
Step 2. The CertificationRequest is encoded according to BER or DER
(typically, DER).
Step 3. As a typical step, the DER-encoded CertificationRequest is
also base64 encoded so that it is 7-bit data suitable for
transfer in SMTP. This then becomes the body of an
application/pkcs10 body part.
The result might look like this:
Content-Type: application/pkcs10; name=smime.p10
Content-Transfer-Encoding: base64
Content-Disposition: attachment; filename=smime.p10
rfvbnj756tbBghyHhHUujhJhjH77n8HHGT9HG4VQpfyF467GhIGfHfYT6
7n8HHGghyHhHUujhJh4VQpfyF467GhIGfHfYGTrfvbnjT6jH7756tbB9H
f8HHGTrfvhJhjH776tbB9HG4VQbnj7567GhIGfHfYT6ghyHhHUujpfyF4
0GhIGfHfQbnj756YT64V
A typical application only needs to send a certification request. It
is a certification authority that has to receive and process the
request. The steps for recovering the CertificationRequest from the
message are straightforward but are not presented here. The
procedures for processing the certification request are beyond the
scope of this document.
Because S/MIME takes into account interoperation in non-MIME
environments, several different mechanisms are employed to carry the
type information, and it becomes a bit difficult to identify S/MIME
messages. The following table lists criteria for determining whether
or not a message is an S/MIME message. A message is considered an
S/MIME message if it matches any below.
The file suffix in the table below comes from the "name" parameter in
the content-type header, or the "filename" parameter on the content-
disposition header. These parameters that give the file suffix are
not listed below as part of the parameter section.
MIME type: application/pkcs7-mime
parameters: any
file suffix: any
MIME type: application/pkcs10
parameters: any
file suffix: any
MIME type: multipart/signed
parameters: protocol="application/pkcs7-signature"
file suffix: any
MIME type: application/octet-stream
parameters: any
file suffix: p7m, p7s, aps, p7c, p10
A receiving agent MUST provide some certificate retrieval mechanism
in order to gain access to certificates for recipients of digital
envelopes. This memo does not cover how S/MIME agents handle
certificates, only what they do after a certificate has been
validated or rejected. S/MIME certification issues are covered in a
different document.
At a minimum, for initial S/MIME deployment, a user agent could
automatically generate a message to an intended recipient requesting
that recipient's certificate in a signed return message. Receiving
and sending agents SHOULD also provide a mechanism to allow a user to
"store and protect" certificates for correspondents in such a way so
as to guarantee their later retrieval.
An S/MIME agent or some related administrative utility or function
MUST be capable of generating RSA key pairs on behalf of the user.
Each key pair MUST be generated from a good source of non-
deterministic random input and protected in a secure fashion.
A user agent SHOULD generate RSA key pairs at a minimum key size of
768 bits and a maximum key size of 1024 bits. A user agent MUST NOT
generate RSA key pairs less than 512 bits long. Some agents created
in the United States have chosen to create 512 bit keys in order to
get more advantageous export licenses. However, 512 bit keys are
considered by many to be cryptographically insecure.
Implementors should be aware that multiple (active) key pairs may be
associated with a single individual. For example, one key pair may be
used to support confidentiality, while a different key pair may be
used for authentication.
This entire memo discusses security. Security issues not covered in
other parts of the memo include:
40-bit encryption is considered weak by most cryptographers. Using
weak cryptography in S/MIME offers little actual security over
sending plaintext. However, other features of S/MIME, such as the
specification of tripleDES and the ability to announce stronger
cryptographic capabilities to parties with whom you communicate,
allow senders to create messages that use strong encryption. Using
weak cryptography is never recommended unless the only alternative is
no cryptography. When feasible, sending and receiving agents should
inform senders and recipients the relative cryptographic strength of
messages.
It is impossible for most software or people to estimate the value of
a message. Further, it is impossible for most software or people to
estimate the actual cost of decrypting a message that is encrypted
with a key of a particular size. Further, it is quite difficult to
determine the cost of a failed decryption if a recipient cannot
decode a message. Thus, choosing between different key sizes (or
choosing whether to just use plaintext) is also impossible. However,
decisions based on these criteria are made all the time, and
therefore this memo gives a framework for using those estimates in
choosing algorithms.
If a sending agent is sending the same message using different
strengths of cryptography, an attacker watching the communications
channel can determine the contents of the strongly-encrypted message
by decrypting the weakly-encrypted version. In other words, a sender
should not send a copy of a message using weaker cryptography than
they would use for the original of the message.
The syntax for SMIMECapability is:
SMIMECapability ::= SEQUENCE {
capabilityID OBJECT IDENTIFIER,
parameters OPTIONAL ANY DEFINED BY capabilityID }
SMIMECapabilities ::= SEQUENCE OF SMIMECapability
RC2-CBC OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) encryptionAlgorithm(3) 2}
For the effective-key-bits (key size) greater than 32 and less than
256, the RC2-CBC algorithm parameters are encoded as:
RC2-CBC parameter ::= SEQUENCE {
rc2ParameterVersion INTEGER,
iv OCTET STRING (8)}
For the effective-key-bits of 40, 64, and 128, the
rc2ParameterVersion values are 160, 120, 58 respectively.
DES-EDE3-CBC OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) encryptionAlgorithm(3) 7}
For DES-CBC and DES-EDE3-CBC, the parameter should be encoded as:
CBCParameter :: IV
where IV ::= OCTET STRING -- 8 octets.
md5 OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) digestAlgorithm(2) 5}
sha-1 OBJECT IDENTIFIER ::=
{iso(1) identified-organization(3) oiw(14) secsig(3) algorithm(2) 26}
rsaEncryption OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 1}
rsa OBJECT IDENTIFIER ::=
{joint-iso-ccitt(2) ds(5) algorithm(8) encryptionAlgorithm(1) 1}
md2WithRSAEncryption OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 2}
md5WithRSAEncryption OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 4}
sha-1WithRSAEncryption OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-1(1) 5}
signingTime OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 5}
smimeCapabilities OBJECT IDENTIFIER ::=
{iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) 15}
[3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"
IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.
[CHARSETS] Character sets assigned by IANA. See
<ftp://ftp.isi.edu/in-notes/iana/assignments/character-sets>.
[CONTDISP] Troost, R., Dorner, S and K. Moore, "Communicating
Presentation Information in Internet Messages: The Content-
Disposition Header Field", RFC 2183, August 1997.
[DES] ANSI X3.106, "American National Standard for Information
Systems-Data Link Encryption," American National Standards Institute,
1983.
[MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321, April
1992.
[MIME-SPEC] The primary definition of MIME.
Freed, N., and N. Borenstein, "MIME Part 1: Format of Internet
Message Bodies", RFC 2045, November 1996.
Freed, N., and N. Borenstein, "MIME Part 2: Media Types", RFC 2046,
November 1996.
Moore, K., "MIME Part 3: Message Header Extensions for Non-ASCII
Text", RFC 2047, November 1996.
Freed, N., Klensin, J., and J. Postel, "MIME Part 4: Registration
Procedures", RFC 2048, November 1996.
Freed, N., and N. Borenstein, "MIME Part 5: Conformance Criteria and
Examples", RFC 2049, November 1996.
[MIME-SECURE] Galvin, J., Murphy, S., Crocker, S., and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC 1847, October 1995.
[MUSTSHOULD] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[PKCS-1] Kaliski, B., "PKCS #1: RSA Encryption Version 1.5", RFC
2313, March 1998.
[PKCS-7] Kaliski, B., "PKCS #7: Cryptographic Message Syntax Version
1.5", RFC 2315, March 1998.
[PKCS-10] Kaliski, B., "PKCS #10: Certification Request Syntax
Version 1.5", RFC 2314, March 1998.
[RC2] Rivest, R., "Description of the RC2(r) Encryption Algorithm",
RFC 2268, January 1998.
[SHA1] NIST FIPS PUB 180-1, "Secure Hash Standard," National
Institute of Standards and Technology, U.S. Department of Commerce,
DRAFT, 31 May 1994.
S/MIME was originally developed by RSA Data Security, Inc. Many
developers implemented S/MIME agents before this document was
published. All S/MIME receiving agents SHOULD make every attempt to
interoperate with these earlier implementations of S/MIME.
Some early implementations of S/MIME agents used the following MIME
types:
application/x-pkcs7-mime
application/x-pkcs7-signature
application/x-pkcs10
In each case, the "x-" subtypes correspond to the subtypes described
in this document without the "x-".
Early S/MIME documentation had two profiles for encryption:
"restricted" and "unrestricted". The difference between these
profiles historically came about due to US Government export
regulations, as described at the end of this section. It is expected
that in the future, there will be few agents that only use the
restricted profile.
Briefly, the restricted profile required the ability to encrypt and
decrypt using RSA's trade-secret RC2 algorithm in CBC mode with 40-
bit keys. The unrestricted profile required the ability to encrypt
and decrypt using RSA's trade-secret RC2 algorithm in CBC mode with
40-bit keys, and to encrypt and decrypt using tripleDES. The
restricted profile also had non-mandatory suggestions for other
algorithms, but these were not widely implemented.
It is important to note that many current implementations of S/MIME
use the restricted profile.
Due to US Government export regulations, an S/MIME agent which
supports a strong content encryption algorithm such as DES would not
be freely exportable outside of North America. US software
manufacturers have been compelled to incorporate an exportable or
"restricted" content encryption algorithm in order to create a widely
exportable version of their product. S/MIME agents created in the US
and intended for US domestic use (or use under special State
Department export licenses) can utilize stronger, "unrestricted"
content encryption. However, in order to achieve interoperability,
such agents need to support whatever exportable algorithm is
incorporated in restricted S/MIME agents.
The RC2 symmetric encryption algorithm has been approved by the US
Government for "expedited" export licensing at certain key sizes.
Consequently, support for the RC2 algorithm in CBC mode is required
for baseline interoperability in all S/MIME implementations. Support
for other strong symmetric encryption algorithms such as RC5 CBC, DES
CBC and DES EDE3-CBC for content encryption is strongly encouraged
where possible.
To: ietf-types@iana.org
Subject: Registration of MIME media type application/pkcs7-mime
MIME media type name: application
MIME subtype name: pkcs7-mime
Required parameters: none
Optional parameters: name, filename, smime-type
Encoding considerations: Will be binary data, therefore should use
base64 encoding
Security considerations: Described in [PKCS-7]
Interoperability considerations: Designed to carry data formatted
with PKCS-7, as described in [PKCS-7]
Published specification: RFC 2311
Applications which use this media type: Secure Internet mail and
other secure data transports.
Additional information:
File extension(s): .p7m and .p7c
Macintosh File Type Code(s):
Person & email address to contact for further information:
Steve Dusse, spock@rsa.com
Intended usage: COMMON
To: ietf-types@iana.org
Subject: Registration of MIME media type application/pkcs7-signature
MIME media type name: application
MIME subtype name: pkcs7-signature
Required parameters: none
Optional parameters: name, filename
Encoding considerations: Will be binary data, therefore should use
base64 encoding
Security considerations: Described in [PKCS-7]
Interoperability considerations: Designed to carry digital
signatures with PKCS-7, as described in [PKCS-7]
Published specification: RFC 2311
Applications which use this media type: Secure Internet mail and
other secure data transports.
Additional information:
File extension(s): .p7s
Macintosh File Type Code(s):
Person & email address to contact for further information:
Steve Dusse, spock@rsa.com
Intended usage: COMMON
To: ietf-types@iana.org
Subject: Registration of MIME media type application/pkcs10
MIME media type name: application
MIME subtype name: pkcs10
Required parameters: none
Optional parameters: name, filename
Encoding considerations: Will be binary data, therefore should use
base64 encoding
Security considerations: Described in [PKCS-10]
Interoperability considerations: Designed to carry digital
certificates formatted with PKCS-10, as described in [PKCS-10]
Published specification: RFC 2311
Applications which use this media type: Secure Internet mail and
other transports where certificates are required.
Additional information:
File extension(s): .p10
Macintosh File Type Code(s):
Person & email address to contact for further information:
Steve Dusse, spock@rsa.com
Intended usage: COMMON
The rationale behind the multiple formats for signing has to do with
the MIME subtype defaulting rules of the application and multipart
top-level types, and the behavior of currently deployed gateways and
mail user agents.
Ideally, the multipart/signed format would be the only format used
because it provides a truly backwards compatible way to sign MIME
entities. In a pure MIME environment with very capable user agents,
this would be possible. The world, however, is more complex than
this.
One problem with the multipart/signed format occurs with gateways to
non-MIME environments. In these environments, the gateway will
generally not be S/MIME aware, will not recognize the
multipart/signed type, and will default its treatment to
multipart/mixed as is prescribed by the MIME standard. The real
problem occurs when the gateway also applies conversions to the MIME
structure of the original message that is being signed and is
contained in the first part of the multipart/signed structure, such
as the gateway converting text and attachments to the local format.
Because the signature is over the MIME structure of the original
message, but the original message is now decomposed and transformed,
the signature cannot be verified. Because MIME encoding of a
particular set of body parts can be done in many different ways,
there is no way to reconstruct the original MIME entity over which
the signature was computed.
A similar problem occurs when an attempt is made to combine an
existing user agent with a stand-alone S/MIME facility. Typical user
agents do not have the ability to make a multipart sub-entity
available to a stand-alone application in the same way they make leaf
MIME entities available to "viewer" applications. This user agent
behavior is not required by the MIME standard and thus not widely
implemented. The result is that it is impossible for most user agents
to hand off the entire multipart/signed entity to a stand-alone
application.
To work around these two problems, the application/pkcs7-mime type
can be used. When going through a gateway, it will be defaulted to
the MIME type of application/octet-stream and treated as a single
opaque entity. That is, the message will be treated as an attachment
of unknown type, converted into the local representation for an
attachment and thus can be made available to an S/MIME facility
completely intact. A similar result is achieved when a user agent
similarly treats the application/pkcs7-mime MIME entity as a simple
leaf node of the MIME structure and makes it available to viewer
applications.
Another way to work around these problems is to encapsulate the
multipart/signed MIME entity in a MIME entity that will not be
damaged by the gateway. At the time that this memo is being written,
there is a proposal for a MIME entity "application/mime" for this
purpose. However, no implementations of S/MIME use this type of
wrapping.
While this document primarily addresses the Internet, it is useful to
compose and receive S/MIME secured messages in non-MIME environments.
This is particularly the case when it is desired that security be
implemented end-to-end. Other discussion here addresses the receipt
of S/MIME messages in non-MIME environments. Here the composition of
multipart/signed entities is addressed.
When a message is to be sent in such an environment, the
multipart/signed entity is created as described above. That entity is
then treated as an opaque stream of bits and added to the message as
an attachment. It must have a file name that ends with ".aps", as
this is the sole mechanism for recognizing it as an S/MIME message by
the receiving agent.
When this message arrives in a MIME environment, it is likely to have
a MIME type of application/octet-stream, with MIME parameters giving
the filename for the attachment. If the intervening gateway has
carried the file type, it will end in ".aps" and be recognized as an
S/MIME message.
Significant contributions to the content of this memo were made by
many people, including Jim Schaad, Jeff Thompson, and Jeff Weinstein.