Network Working Group Jim Galvin INTERNET DRAFT Sandy Murphy draft-ietf-pem-sigenc-02.txt Steve Crocker Ned Freed November 1994 Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted Status of this Memo This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas, and its Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet Drafts as reference material or to cite them other than as ``work in progress''. To learn the current status of any Internet Draft, please check the 1id-abstracts.txt listing contained in one of the Internet Drafts Shadow Directories on ds.internic.net (US East Coast), venera.isi.edu (US West Coast), munnari.oz.au (Pacific Rim), or nic.nordu.net (Europe). Abstract This document defines two new content types for specifying the application of security services to MIME message bodies. MIME, an acronym for "Multipurpose Internet Mail Extensions", defines the format of the contents of Internet mail messages and provides for multi-part textual and non-textual message bodies. The new content types are subtypes of multipart: signed and encrypted. Each will contain two body parts: one for the protected data and one for the control information necessary to remove the protection. The type and contents of the control information body parts are determined by the value of the protocol parameter of the enclosing multipart/signed or multipart/encrypted content type, which is required to be present. Galvin/Murphy/Crocker/Freed Expires: May 1995 [Page 1]
INTERNET DRAFT Security Multiparts November 1994 1. Introduction An Internet electronic mail message consists of two parts: the headers and the body. The headers form a collection of field/value pairs structured according to RFC822 [1], whilst the body, if structured, is defined according to MIME [2]. The basic MIME specification does not provide specific security protection. This document defines a framework whereby security protection provided by other protocols may be used with MIME in a complementary fashion. The resulting combined service provides security for single-part and multi-part textual and non-textual messages. The framework is provided by defining two new security subtypes of the MIME multipart content type: signed and encrypted. In each of the security subtypes, there are exactly two related body parts: one for the protected data and one for the control information. This should allow the framework to be used by all protocols providing signature and encryption services. A three-step process is specified for the origination and reception of the multipart contents. The details of the processing performed during each step is specified by the protocol being used. By registering new values for the required protocol parameter, the framework is easily extended to accommodate a variety of protocols. A MIME agent that includes support for this framework will be able to parse protected body parts, even when the value of the protocol parameter is unrecognized and the body part can not be displayed to the user. 2. Definition of Security Subtypes of Multipart The multipart/signed content type specifies how to support authentication and integrity services via digital signature. The control information is carried in the second of the two required body parts. The multipart/encrypted content type specifies how to support confidentiality via encryption. The control information is carried in the first of the two required body parts. Galvin/Murphy/Crocker/Freed Expires: May 1995 [Page 2]
INTERNET DRAFT Security Multiparts November 1994 2.1. Definition of Multipart/Signed (1) MIME type name: multipart (2) MIME subtype name: signed (3) Required parameters: boundary, protocol, and micalg (4) Optional parameters: none (5) Encoding considerations: 7bit, 8bit, or binary depending on transport requirements and encoding of the digitally signed data (6) Security considerations: Must be treated as opaque while in transit The multipart/signed content type contains exactly two body parts. The first body part is the body part over which the digital signature was created, including its content type label. The second body part contains the control information necessary to verify the digital signature. The first body part may contain any valid MIME content type, labelled accordingly. The second body part is labelled according to the value of the protocol parameter. The attribute token for the protocol parameter is "protocol", i.e., parameter := "protocol" "=" value The value token is comprised of the type and sub-type tokens of the Content-Type: header of the second body part, i.e., value := type "/" subtype where the type and subtype tokens are defined by the MIME [2] specification. The semantics of the protocol parameter are defined according to its value. The attribute token for the micalg parameter is "micalg", i.e., parameter := "micalg" "=" value The Message Integrity Check (MIC) is the name given to the quantity computed over the body part with a message digest or hash function, in support of the digital signature service. Valid value tokens are Galvin/Murphy/Crocker/Freed Expires: May 1995 [Page 3]
INTERNET DRAFT Security Multiparts November 1994 defined by the specification for the value of the protocol parameter. The value may be a comma (",") separated list of tokens, indicating the presence of multiple MIC algorithms. As a result, the comma (",") character is explicitly excluded from the list of characters that may be included in a token used as a value of the micalg parameter. If multiple MIC algorithms are specified, the purpose and use of the multiple algorithms is defined by the protocol. If the MIC algorithm is also specified in the control information and the value there does not agree with the value in this parameter, it must be treated as an error. NOTE: The presence of the micalg parameter on the multipart/signed content type header is explicitly intended to support one-pass processing. MIME implementations may locate the second body part by inputting the first body part and computing the specified MIC values until the boundary identifying the second body part is found. The entire multipart/signed body part must be treated as opaque while it is in transit from an originator to a recipient. Intermediate message transfer agents must not alter the content of a multipart/signed in any way, including, but not limited to, changing the content transfer encoding of the body part or any of its encapsulated body parts. When creating a multipart/signed body part, the following sequence of steps describes the processing necessary. It must be emphasized that these steps are descriptive, not prescriptive, and in no way impose restrictions or requirements on implementations of this specification. (1) The content of the body part to be protected is prepared according to a local convention. The content is then transformed into a MIME body part, including an appropriate set of headers. (2) The body part (content and headers) to be digitally signed is prepared for signature according to the value of the protocol parameter. The MIME headers of the signed body part are included in the signature to protect the integrity of the MIME labelling of the data that is signed. (3) The prepared body part is made available to the signature creation process according to a local convention. The signature creation process must make available to a MIME implementation two data streams: the control information necessary to verify the signature, which the MIME implementation will place in the second body part, and the digitally signed body part, which the MIME implementation Galvin/Murphy/Crocker/Freed Expires: May 1995 [Page 4]
INTERNET DRAFT Security Multiparts November 1994 will use as the first body part. When receiving a multipart/signed body part, the following sequence of steps describes the processing necessary. It must be emphasized that these steps are descriptive, not prescriptive, and in no way impose restrictions or requirements on implementations of this specification. (1) The first body part and the control information in the second body part must be prepared for the signature verification process according to the value of the protocol parameter. (2) The prepared body parts must be made available to the signature verification process according to a local convention. The signature verification process must make available to the MIME implementation the result of the signature verification and the body part that was digitally signed. NOTE: The result of the signature verification is likely to include a testament of the success or failure of the verification. Also, in the usual case, the body part returned after signature verification will be the same as the body part that was received. We do not insist that this be the case to allow for protocols that may modify the body part during the signature processing. (3) The result of the signature verification process is made available to the user and the MIME implementation continues processing with the verified body part, i.e., the body part returned by the signature verification process. 2.2. Definition of Multipart/Encrypted (1) MIME type name: multipart (2) MIME subtype name: encrypted (3) Required parameters: boundary, protocol (4) Optional parameters: none (5) Encoding considerations: 7bit, 8bit, or binary depending on transport requirements and encoding of the encrypted data Galvin/Murphy/Crocker/Freed Expires: May 1995 [Page 5]
INTERNET DRAFT Security Multiparts November 1994 (6) Security considerations: none The multipart/encrypted content type contains exactly two body parts. The first body part contains the control information necessary to decrypt the data in the second body part and is labelled according to the value of the protocol parameter. The second body part contains the data which was encrypted and is always labelled application/octet- stream. The attribute token for the protocol parameter is "protocol", i.e., parameter := "protocol" "=" value The value token is comprised of the type and sub-type tokens of the Content-Type: header of the first body part, i.e., value := type "/" subtype where the type and subtype tokens are defined by the MIME [2] specification. The semantics of the protocol parameter are defined according to its value. When creating a multipart/encrypted body part, the following sequence of steps describes the processing necessary. It must be emphasized that these steps are descriptive, not prescriptive, and in no way impose restrictions or requirements on implementations of this specification. (1) The contents of the body part to be protected is prepared according to a local convention. The contents are then transformed into a MIME body part, including an appropriate set of headers. (2) The body part (content and headers) to be encrypted is prepared for encryption according to the value of the protocol parameter. The MIME headers of the encrypted body part are included in the encryption to protect from disclosure the MIME labelling of the data that is encrypted. (3) The prepared body part is made available to the encryption process according to a local convention. The encryption process must make available to a MIME implementation two data streams: the control information necessary to decrypt the body part, which the MIME implementation will place in the first body part, and the encrypted body part, which the MIME implementation will place in the second body part and label application/octet-stream. Thus, when used in a Galvin/Murphy/Crocker/Freed Expires: May 1995 [Page 6]
INTERNET DRAFT Security Multiparts November 1994 multipart/encrypted, the application/octet-stream data is comprised of a nested MIME body part. When receiving a multipart/encrypted body part, the following sequence of steps describes the processing necessary. It must be emphasized that these steps are descriptive, not prescriptive, and in no way impose restrictions or requirements on implementations of this specification. (1) The second body part and the control information in the first body part must be prepared for the decryption process according to the value of the protocol parameter. (2) The prepared body parts must be made available to the decryption process according to a local convention. The decryption process must make available to the MIME implementation the result of the decryption and the decrypted form of the encrypted body part. NOTE: The result of the decryption process is likely to include a testament of the success or failure of the decryption. Failure may be due to an inability to locate the proper decryption key or the proper recipient field, etc. (3) The result of the decryption process is made available to the user and the MIME implementation continues processing with the decrypted body part, i.e., the body part returned by the decryption process. NOTE: A MIME implementation will not be able to display the received form of the second body part because the application of encryption will transform the body part. This transformation will not be described in the MIME headers (Content-Type: and Content-Transfer-Encoding:) but, rather, will be described in the content of the first body part. Therefore, an implementation should wait until the encryption has been removed before attempting to display the content. 3. Definition of Control Information Content Types This document requires the definition of two additional content types, one each for when a digital signature is applied to a body part and for when encryption is applied to a body part. These content types are to be used in multipart/signed and multipart/encrypted body parts, Galvin/Murphy/Crocker/Freed Expires: May 1995 [Page 7]
INTERNET DRAFT Security Multiparts November 1994 respectively, as indicated above. Their definition must be included in the specification defining the use of the security protocol indicated in the protocol parameter. 4. Definition of Key Management Content Types It is recognized that many cryptographic techniques require a mechanism for distributing the cryptographic keys used in support of their services. As a result, many security protocols will define two additional body parts: one for the purpose of requesting cryptographic keys and one for the purpose of distributing cryptographic keys. 5. Security Considerations This specification describes an enhancement to MIME to support signed and encrypted body parts. In that context this entire document is about security. 6. Acknowledgements David H. Crocker suggested the use of a multipart structure for MIME-PEM interaction. 7. References [1] David H. Crocker. Standard for the Format of ARPA Internet Text Messages. RFC 822, University of Delaware, August 1982. [2] Nathaniel Borenstein and Ned Freed. MIME (Multipurpose Internet Mail Extension) Part One: Mechanisms for Specifying and Describing the Format of Internet Message Bodies. RFC 1521, Bellcore and Innosoft, September 1993. Obsoletes RFC 1341. 8. Authors' Addresses Jim Galvin email: galvin@tis.com Sandy Murphy email: sandy@tis.com Steve Crocker email: crocker@tis.com Trusted Information Systems Galvin/Murphy/Crocker/Freed Expires: May 1995 [Page 8]
INTERNET DRAFT Security Multiparts November 1994 3060 Washington Road Glenwood, MD 21738 Tel: +1 301 854 6889 FAX: +1 301 854 5363 Ned Freed Innosoft International, Inc. 1050 East Garvey Avenue South West Covina, CA 91790 Tel: +1 818 919 3600 FAX: +1 818 919 3614 email: ned@innosoft.com Galvin/Murphy/Crocker/Freed Expires: May 1995 [Page 9]
INTERNET DRAFT Security Multiparts November 1994 Table of Contents Status of this Memo ............................................. 1 Abstract ........................................................ 1 1 Introduction ................................................... 2 2 Definition of Security Subtypes of Multipart ................... 2 2.1 Definition of Multipart/Signed ............................... 3 2.2 Definition of Multipart/Encrypted ............................ 5 3 Definition of Control Information Content Types ................ 7 4 Definition of Key Management Content Types ..................... 8 5 Security Considerations ........................................ 8 6 Acknowledgements ............................................... 8 7 References ..................................................... 8 8 Authors' Addresses ............................................. 8 Galvin/Murphy/Crocker/Freed Expires: May 1995 [Page 10]
