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RFC 5408

Identity-Based Encryption Architecture and Supporting Data Structures

Pages: 30
Informational
Errata

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Network Working Group                                     G. Appenzeller
Request for Comments: 5408                           Stanford University
Category: Informational                                        L. Martin
                                                        Voltage Security
                                                            M. Schertler
                                                                   Axway
                                                            January 2009


 Identity-Based Encryption Architecture and Supporting Data Structures

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (http://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

Abstract

This document describes the security architecture required to implement identity-based encryption, a public-key encryption technology that uses a user's identity as a public key. It also defines data structures that can be used to implement the technology.
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Table of Contents

1. Introduction ....................................................3 1.1. Terminology ................................................3 2. Identity-Based Encryption .......................................3 2.1. Overview ...................................................3 2.2. Sending a Message That Is IBE-Encrypted ....................5 2.2.1. Sender Obtains Public Parameters ....................5 2.2.2. Construct and Send an IBE-Encrypted Message .........6 2.3. Receiving and Viewing an IBE-Encrypted Message .............6 2.3.1. Recipient Obtains Public Parameters .................7 2.3.2. Recipient Obtains IBE Private Key ...................8 2.3.3. Recipient Decrypts IBE-Encrypted Message ............8 3. Identity Format .................................................9 4. Public Parameter Lookup .........................................9 4.1. Request Method ............................................10 4.2. Parameter and Policy Format ...............................11 4.3. The application/ibe-pp-data MIME Type .....................14 5. Private Key Request Protocol ...................................15 5.1. Overview ..................................................15 5.2. Private Key Request .......................................15 5.3. Request Structure .........................................16 5.4. The application/ibe-key-request+xml MIME type .............17 5.5. Authentication ............................................18 5.6. Server Response Format ....................................18 5.6.1. The IBE100 responseCode ............................19 5.6.2. The IBE101 responseCode ............................20 5.6.3. The IBE201 responseCode ............................20 5.6.4. The IBE300 responseCode ............................21 5.6.5. The IBE301 responseCode ............................21 5.6.6. The IBE303 responseCode ............................21 5.6.7. The IBE304 responseCode ............................22 5.7. The application/ibe-pkg-reply+xml MIME type ...............22 6. ASN.1 Module ...................................................23 7. Security Considerations ........................................25 7.1. Attacks outside the Scope of This Document ................25 7.2. Attacks within the Scope of This Document .................26 7.2.1. Attacks on the Protocols Defined in This Document ..26 8. IANA Considerations ............................................27 8.1. Media Types ...............................................27 8.2. XML Namespace .............................................27 9. References .....................................................28 9.1. Normative References ......................................28 9.2. Informative References ....................................29
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1. Introduction

This document describes the security architecture required to implement identity-based encryption, a public-key encryption technology that uses a user's identity as a public key. It also defines data structures that are required to implement the technology. Objects used in this implementation are defined using ASN.1 [ASN1] and XML [XML].

1.1. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [KEY].

2. Identity-Based Encryption

2.1. Overview

Identity-based encryption (IBE) is a public-key encryption technology that allows a public key to be calculated from an identity and a set of public mathematical parameters and that allows for the corresponding private key to be calculated from an identity, a set of public mathematical parameters, and a domain-wide secret value. An IBE public key can be calculated by anyone who has the necessary public parameters; a cryptographic secret is needed to calculate an IBE private key, and the calculation can only be performed by a trusted server that has this secret. Calculation of both the public and private keys in an IBE system can occur as needed, resulting in just-in-time creation of both public and private keys. This contrasts with other public-key systems [P1363], in which keys are generated randomly and distributed prior to secure communication commencing, and in which private encryption keys need to be securely archived to allow for their recovery if they are lost or destroyed. The ability to calculate a recipient's public key, in particular, eliminates the need for the sender and receiver to interact with each other, either directly or through a proxy such as a directory server, before sending secure messages. A characteristic of IBE systems that differentiates them from other server-based cryptographic systems is that once a set of public parameters is fetched, encryption is possible with no further communication with a server during the validity period of the public parameters. Other server-based systems may require a connection to a server for each encryption operation.
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   This document describes an IBE-based messaging system, how the
   components of such a system work together, and defines data
   structures that support the operation of such a system.  The server
   components required for such a system are the following:

      o  A Public Parameter Server (PPS).  IBE public parameters include
         publicly-sharable cryptographic material, known as IBE public
         parameters, and policy information for an associated PKG.  A
         PPS provides a well-known location for secure distribution of
         IBE public parameters and policy information that describe the
         operation of a PKG.  Section 5 of this document describes the
         protocol that a client uses to communicate with a PPS.

      o  A Private-key Generator (PKG).  The PKG stores and uses
         cryptographic material, known as a master secret, which is used
         for generating a user's IBE private key.  A PKG accepts an IBE
         user's private key request, and after successfully
         authenticating them in some way, returns their IBE private key.
         Section 5 of this document describes the protocol that a client
         uses to communicate with a PKG.

   A logical architecture of such an IBE system would be to have a PKG
   and PPS per name space, such as a DNS zone.  The organization that
   controls the DNS zone would also control the PKG and PPS and thus the
   determination of which PKG or PPS to use when creating public and
   private keys for the organization's members.  In this case, the PPS
   URI/IRI can be uniquely created from a user-friendly name for the
   form of identity that the PPS supports.  This architecture would make
   it clear which set of public parameters to use and where to retrieve
   them for a given identity (for example, an RFC 2821 address [SMTP]).

   IBE-encrypted messages can use standard message formats, such as the
   Cryptographic Message Syntax [CMS].  How to use IBE with the CMS to
   encrypt email messages is defined in [IBECMS].

   Note that IBE algorithms are used only for encryption, so if digital
   signatures are required, they will need to be provided by an
   additional mechanism.

   Section 3 of this document describes the identity format that all PPS
   and PKG servers MUST support.
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2.2. Sending a Message That Is IBE-Encrypted

In order to send an encrypted message, an IBE user must perform the following steps: 1. Obtain the recipient's public parameters The public parameters of the recipient's system are needed to perform IBE operations. Once a user obtains these public parameters, he can perform IBE encryption operations. These public parameters may be available at a PPS that is operated by the user's organization, one that is operated by the sender's organization, or by a different organization entirely. 2. Construct and send an IBE-encrypted message In addition to the IBE public parameters, all that is needed to construct an IBE-encrypted message is the recipient's identity, the form of which is defined by the public parameters. When this identity is the same as the identity that a message would be addressed to, then no more information is needed from a user to send them an encrypted message than is needed to send them an unencrypted message. This is one of the major benefits of an IBE-based secure messaging system. Examples of identities are individual, group, or role identifiers.

2.2.1. Sender Obtains Public Parameters

The sender of a message obtains the IBE public parameters that he needs from a PPS that is hosted at a well-known URI or IRI. The IBE public parameters contain all of the information that the sender needs to create an IBE-encrypted message except for the identity of the recipient. Section 4 of this document describes the URI [URI] or IRI [IRI] of a PPS, the format of IBE public parameters, and how to obtain them from a PPS. The URI or IRI from which users obtain IBE public parameters MUST be authenticated in some way. PPS servers MUST support TLS 1.2 [TLS] to satisfy this requirement and SHOULD support its successors. This step is shown below in Figure 1. IBE Public Parameter Request -----------------------------> Sender PPS <----------------------------- IBE Public Parameters Figure 1: Requesting IBE Public Parameters
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   The sender of an IBE-encrypted message selects the PPS and
   corresponding PKG based on his local security policy.  Different PPS
   servers may provide public parameters that specify different IBE
   algorithms or different key strengths, for example.  Or, they may
   require the use of PKG servers that require different levels of
   authentication before granting IBE private keys.

2.2.2. Construct and Send an IBE-Encrypted Message

To IBE-encrypt a message, the sender chooses a content-encryption key (CEK) and uses it to encrypt his message and then encrypts the CEK with the recipient's IBE public key as described in [CMS]. This operation is shown below in Figure 2. The document [IBCS] describes the algorithms needed to implement two forms of IBE, and [IBECMS] describes how to use the Cryptographic Message Syntax (CMS) to encapsulate the encrypted message along with the IBE information that the recipient needs to decrypt an email message. CEK ----> Sender ----> IBE-encrypted CEK ^ | | Recipient's Identity and IBE Public Parameters Figure 2: Using an IBE Public-key Algorithm to Encrypt

2.3. Receiving and Viewing an IBE-Encrypted Message

In order to read an IBE-encrypted message, a recipient of such a message parses it to find the URI or IRI he needs in order to obtain the IBE public parameters that are required to perform IBE calculations as well as to obtain a component of the identity that was used to encrypt the message. Next, the recipient carries out the following steps: 1. Obtain the IBE public parameters An IBE system's public parameters allow it to uniquely create public and private keys. The recipient of an IBE-encrypted message can decrypt an IBE-encrypted message if he has both the IBE public parameters and the necessary IBE private key. The public parameters also provide the URI or IRI of the PKG where the recipient of an IBE-encrypted message can obtain the IBE private keys.
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      2. Obtain the IBE private key from the PKG

         To decrypt an IBE-encrypted message, in addition to the IBE
         public parameters, the recipient needs to obtain the private
         key that corresponds to the public key that the sender used.
         The IBE private key is obtained after successfully
         authenticating to a private key generator (PKG), a trusted
         third party that calculates private keys for users.  The
         recipient then receives the IBE private key over a secure
         connection.

      3. Decrypt the IBE-encrypted Message

         The IBE private key decrypts the CEK (see Section 2.3.3).  The
         CEK is then used to decrypt the encrypted message.

   It may be useful for a PKG to allow users other than the intended
   recipient to receive some IBE private keys.  Giving a mail-filtering
   appliance permission to obtain IBE private keys on behalf of users,
   for example, can allow the appliance to decrypt and scan encrypted
   messages for viruses or other malicious features.

2.3.1. Recipient Obtains Public Parameters

Before he can perform any IBE calculations related to the message that he has received, the recipient of an IBE-encrypted message needs to obtain the IBE public parameters that were used in the encryption operation. This operation is shown below in Figure 3. Because the use of the correct public parameters is vital to the overall security of an IBE system, IBE public parameters MUST be transported to recipients over a secure protocol. PPS servers MUST support TLS 1.2 [TLS] or its successors, using the latest version supported by both parties, for transport of IBE public parameters. In addition, users MUST verify that the subject name in the server certificate matches the URI/IRI of the PPS. The comments in Section 2.2.1 also apply to this operation. IBE Public Parameter Request -----------------------------> Recipient PPS <----------------------------- IBE Public Parameters Figure 3: Requesting IBE Public Parameters
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2.3.2. Recipient Obtains IBE Private Key

To obtain an IBE private key, the recipient of an IBE-encrypted message provides the IBE public key used to encrypt the message and their authentication credentials to a PKG and requests the private key that corresponds to the IBE public key. Section 5 of this document defines the protocol for communicating with a PKG as well as a minimum interoperable way to authenticate to a PKG that all IBE implementations MUST support. Because the security of IBE private keys is vital to the overall security of an IBE system, IBE private keys MUST be transported to recipients over a secure protocol. PKG servers MUST support TLS 1.2 [TLS] or its successors, using the latest version supported by both parties, for transport of IBE private keys. This operation is shown below in Figure 4. IBE Private Key Request ----------------------------> Recipient PKG <---------------------------- IBE Private Key Figure 4: Obtaining an IBE Private Key

2.3.3. Recipient Decrypts IBE-Encrypted Message

After obtaining the necessary IBE private key, the recipient uses that IBE private key and the corresponding IBE public parameters to decrypt the CEK. This operation is shown below in Figure 5. He then uses the CEK to decrypt the encrypted message content. An example of how to do this with email messages is given in [IBECMS]. IBE-encrypted CEK ----> Recipient ----> CEK ^ | | IBE Private Key and IBE Public Parameters Figure 5: Using an IBE Public-Key Algorithm to Decrypt
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3. Identity Format

An IBEIdentityInfo type MUST be used to represent the identity of a recipient. This is defined to be the following: IBEIdentityInfo ::= SEQUENCE { district IA5String, serial INTEGER, identityType OBJECT IDENTIFIER, identityData OCTET STRING } An IBEIdentityInfo type is used to calculate public and private IBE keys. Because of this, such a structure is typically DER-encoded [DER]. The fields of an IBEIdentityInfo structure have the following meanings. The district is an IA5String that represents the URI [URI] or IRI [IRI] where the recipient of an IBE-encrypted message can retrieve the IBE public parameters needed to encrypt or decrypt a message encrypted for this identity. Applications MUST support the method described in Section 4 for doing this and MAY support other methods. IRIs MUST be handled according to the procedures specified in Section 7.4 of [PKIX]. The serial is an INTEGER that defines a unique set of IBE public parameters in the event that more than one set of parameters is used by a single district. identityType is an OBJECT IDENTIFIER that defines the format that the identityData field is encoded with. An example of a useful IBEIdentityInfo type is given in [IBECMS]. This example is tailored to the use of IBE in encrypting email. Because the information that comprises an identity is very dependent on the application, this document does not define an identityType that all applications MUST support.

4. Public Parameter Lookup

This section specifies how a component of an IBE system can retrieve the public parameters. A sending or receiving client MUST allow configuration of these parameters manually, for example, through editing a configuration file. However, for simplified configuration, a client SHOULD also implement the public parameter URI/IRI request method described in this document to fetch the public parameters
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   based on a configured URI/IRI.  This is especially useful for
   federating between IBE systems.  By specifying a single URI/IRI, a
   client can be configured to fetch all the relevant parameters for a
   remote PKG.  These public parameters can then be used to encrypt
   messages to recipients who authenticate to and retrieve private keys
   from that PKG.

   The following section outlines the URI/IRI request method to retrieve
   a parameter block and describes the structure of the parameter block
   itself.  The technique for fetching IBE public parameters using the
   process defined in this section is indicated by the OID uriPPSOID,
   which is defined to be the following:

   uriPPSOID OBJECT IDENTIFIER ::= {
     joint-iso-itu-t(2) country(16) us(840)
     organization(1) identicrypt(114334)
     pps-schemas(3) ic-schemas(1) pps-uri(1) version(1)
   }

4.1. Request Method

The configuration URI/IRI SHOULD be an HTTPS URI [HTTP] and MAY additionally support IRIs [IRI] for this purpose. To retrieve the IBE public parameters, the client SHOULD use the HTTP GET method as defined in [HTTP]. The request MUST happen over a secure protocol. The requesting client MUST support TLS 1.2 [TLS] or its successors and SHOULD use the latest version supported by both parties. When requesting the URI/IRI, the client MUST only accept the system parameter block if the server identity was verified successfully by TLS 1.2 [TLS] or its successors. A successful GET request returns in its body the base64 [B64] encoding of the DER-encoded [DER] IBESysParams structure that is described in the next section. This structure MUST be encoded as an application/ibe-pp-data MIME type.
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4.2. Parameter and Policy Format

The IBE public parameters are a structure of the form IBESysParams ::= SEQUENCE { version INTEGER { v2(2) }, districtName IA5String, districtSerial INTEGER, validity ValidityPeriod, ibePublicParameters IBEPublicParameters, ibeIdentityType OBJECT IDENTIFIER, ibeParamExtensions IBEParamExtensions OPTIONAL } The fields of an IBESysParams structure have the following meanings. The version field specifies the version of the IBESysParams format. For the format described in this document, this MUST be set to 2. The districtName field is an IA5String that MUST be an encoding of an URI [URI] or IRI [IRI]. IRIs MUST be handled according to the procedures specified in Section 7.4 of [PKIX]. The districtSerial field is an integer that represents a unique set of IBE public parameters that are available at the URI or IRI defined by the districtName. If new parameters are published for a districtName, the districtSerial MUST be increased to a number greater than the previously-used districtSerial. The validity field defines the lifetime of a specific instance of the IBESysParams and is defined to be the following: ValidityPeriod ::= SEQUENCE { notBefore GeneralizedTime, notAfter GeneralizedTime } The values of notBefore and notAfter MUST be expressed in Greenwich Mean Time (Zulu), MUST include seconds (i.e., times are always YYYYMMDDHHMMSSZ), even where the number of seconds is equal to zero, and MUST be expressed to the nearest second. A client MUST verify that the date on which it uses the IBE public parameters falls between the notBefore time and the notAfter time of the IBE public parameters, and it MUST NOT use the parameters for IBE encryption operations if they do not.
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   IBE public parameters MUST be regenerated and republished whenever
   the values of ibePublicParameters, ibeIdentityType, or
   ibeParamExtensions change for a district.  A client SHOULD refetch
   the IBE public parameters at an application-configurable interval to
   ensure that it has the most current version of the IBE public
   parameters.

   It is possible to create identities for use in IBE that have a time
   component, as described in [IBECMS], for example.  If such an
   identity is used, the time component of the identity MUST fall
   between the notBefore time and the notAfter times of the IBE public
   parameters.

   IBEPublicParameters is a structure containing public parameters that
   correspond to IBE algorithms that the PKG supports.  This structure
   is defined to be following:

   IBEPublicParameters ::= SEQUENCE (1..MAX) OF
     IBEPublicParameter

   IBEPublicParameter  ::= SEQUENCE {
     ibeAlgorithm          OBJECT IDENTIFIER,
     publicParameterData   OCTET STRING
   }

   The ibeAlgorithm OID specifies an IBE algorithm.  The OIDs for two
   IBE algorithms (the Boneh-Franklin and Boneh-Boyen algorithms) and
   their publicParameterData structures are defined in [IBCS].

   The publicParameterData is a DER-encoded [DER] structure that
   contains the actual cryptographic parameters.  Its specific structure
   depends on the algorithm.

   The IBESysParams of a district MUST contain an OID that identifies at
   least one algorithm and MAY contain OIDs that identify more than one
   algorithm.  It MUST NOT contain two or more IBEPublicParameter
   entries with the same algorithm.  A client that wants to use
   IBESysParams can chose any of the algorithms specified in the
   publicParameterData structure.  A client MUST implement at least the
   Boneh-Franklin algorithm [IBCS] and MAY implement the Boneh-Boyen
   [IBCS] and other algorithms.  If a client does not support any of the
   supported algorithms, it MUST generate an error message and fail.

   ibeIdentityType is an OID that defines the type of identities that
   are used with this district.  The OIDs and the required and optional
   fields for each OID are application dependent.  An example of this is
   given in [IBECMS], which defines an identity format suitable for use
   in encrypting email.
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   IBEParamExtensions is a set of extensions that can be used to define
   additional parameters that particular implementations may require.
   This structure is defined as follows:

   IBEParamExtensions ::= SEQUENCE OF IBEParamExtension

   IBEParamExtension ::= SEQUENCE {
     ibeParamExtensionOID     OBJECT IDENTIFIER,
     ibeParamExtensionValue   OCTET STRING
   }

   The contents of the octet string ibeParamExtensionValue are defined
   by the specific ibeParamExtensionOID.  The IBEParamExtensions of a
   district MAY have any number of extensions, including zero.  One
   example of extensions that have been used in practice is to provide a
   URI where an encrypted message can be decrypted and viewed by users
   of webmail systems.  Another example is to provide commercial
   branding information, so that a bank can provide a different user
   interface for customers of different lines of business.

   If a client receives public parameters that contain an extension that
   it is unable to process, it MUST NOT use the values in the
   IBESysParams structure for any cryptographic operations.  Clients
   MUST be able to process an IBESysParams structure that contains no
   IBEParamExtensions.

   The pkgURI OID that is defined below defines an IBEParamExtensions
   structure that contains the URI or IRI of a Private Key Generator.
   The presence of this OID in an IBEParamExtension indicates that
   clients MUST use the protocol defined in Section 5 of this document
   to obtain IBE private keys from the PKG and MUST do so using the
   URI/IRI that is defined by the ibeParamExtensionValue in the
   IBEParamExtension.

   If the PKG is publicly-accessible, this extension SHOULD be present
   to allow the automatic retrieval of private keys for recipients of
   encrypted messages.  For this extension, the ibeParamExtensionValue
   OCTET STRING is an IA5String containing the URI [URI] or IRI [IRI] of
   the PKG where IBE private keys can be obtained.  IRIs MUST be handled
   according to the procedures specified in Section 7.4 of [PKIX].

   ibeParamExt OBJECT IDENTIFIER ::= {
     ibcs ibcs3(3) parameter-extensions(2)
   }

   pkgURI OBJECT IDENTIFIER ::= { ibeParamExt pkgURI(1) }
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4.3. The application/ibe-pp-data MIME Type

The following summarizes the properties of the application/ibe-pp-data MIME type. MIME media type name: application MIME subtype name: ibe-pp-data Mandatory parameters: none Optional parameters: none Encoding considerations: This media type MUST be encoded as 7-bit (US-ASCII text [ASCII]). Security considerations: The data conveyed as this media type are the public parameters needed for the operation of a cryptographic system. To ensure that the parameters can be trusted, the request for these parameters must take place over a secure protocol, such as TLS 1.2 or its successors. To ensure the validity of the server, the client MUST verify the server certificate and MUST abort the parameter request if the verification of the server certificate of the TLS connection fails. This media type contains no active content and does not use compression. Interoperability considerations: There are no known interoperability considerations for this media type. Applications that use this media type: Applications that implement IBE in compliance with this specification will use this media type. The most commonly used of these applications are encrypted email and file encryption. Additional information: none Person and email address for further information: Luther Martin, martin@voltage.com. Intended usage: COMMON Author/Change controller: Luther Martin, martin@voltage.com.
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5. Private Key Request Protocol

5.1. Overview

When requesting a private key, a client has to transmit three parameters: 1. The IBE algorithm for which the key is being requested 2. The identity for which it is requesting a key 3. Authentication credentials for the individual requesting the key The identity for which a client requests a key may not necessarily be the same as the identity that the authentication credentials validate. This may happen, for example, when a single user has access to multiple aliases. For example, an email user may have access to the keys that correspond to two different email addresses, e.g., bob@example.com and bob.smith@example.com. This section defines the protocol to request private keys, a minimum user authentication method for interoperability, and how to pass authentication credentials to the server. It assumes that a client has already determined the URI/IRI of the PKG. This can be done from information included in the IBE message format and the public parameters of the IBE system. The technique for fetching an IBE private key using the process defined in this section is indicated by the OBJECT IDENTIFIER pkgURI, which is defined to be the following: ibcs OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840) organization(1) identicrypt(114334) ibcs(1) } ibeParamExt OBJECT IDENTIFIER ::= { ibcs ibcs3(3) parameter-extensions(2) } pkgURI OBJECT IDENTIFIER ::= { ibeParamExt pkgURI(1) }

5.2. Private Key Request

To request a private key, a client MUST perform a HTTP POST method as defined in [HTTP]. The request MUST take place over a secure protocol. The requesting client MUST support TLS 1.2 [TLS] or its
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   successors, using the latest version supported by both the client and
   the PKG.  When requesting the URI/IRI, the client MUST verify the
   server certificate [HTTPTLS], and it MUST abort the key request if
   the server certificate verification of the TLS connection fails.
   Doing so is critical to protect the authentication credentials and
   the private key against man-in-the-middle attacks when it is
   transmitted from the key server to the client.

5.3. Request Structure

The POST method contains in its body the following XML structure that MUST be encoded as an application/ibe-key-request+xml MIME type: <ibe:request xmlns:ibe="urn:ietf:params:xml:ns:ibe"> <ibe:header> <ibe:client version="clientID"/> </ibe:header> <ibe:body> <ibe:keyRequest> <ibe:algorithm> algorithmOID </ibe:algorithm> <ibe:id> ibeIdentityInfo </ibe:id> </ibe:keyRequest> <ibe:authData> ibeAuthData </ibe:authData> </ibe:body> </ibe:request> A <ibe:request> SHOULD include an <ibe:client> element in the <ibe:header> part of a key request that contains an ASCII string that identifies the client type and client version. A key request MUST contain an <ibe:oid> element that contains the base64 [B64] encoding of a DER-encoded [DER] object identifier that identifies the algorithm for which a key is requested. OIDs for the Boneh-Boyen (BB1) and Boneh-Franklin (BF) algorithms are listed in [IBCS]. A key request MUST contain an <ibe:id> element that contains the identity that the private key is being requested for. This identity is the base64 [B64] encoding of a DER-encoded [DER] ASN.1 structure. This structure defines a user's identity in a way appropriate for the application. An example of such a structure that is appropriate for use in encrypting email is defined in [IBECMS].
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   A key request MAY contain an <ibe:authData> element.  This element
   contains authentication information that the PKG can use to determine
   whether or not a request for a particular IBE private key should be
   granted.

   A client MAY include optional additional XML elements in the
   <ibe:body> part of the key request.  A PKG MUST be able to process
   key requests that contain no such optional elements.

5.4. The application/ibe-key-request+xml MIME type

The following summarizes the properties of the application/ibe-key-request+xml MIME type. MIME media type name: application MIME subtype name: ibe-key-request+xml Mandatory parameters: none Optional parameters: none Encoding considerations: This media type MUST be encoded as US-ASCII [ASCII]. Security considerations: The data conveyed in this media type may contain authentication credentials. Because of this, its confidentiality and integrity is extremely important. To ensure this, the request for an IBE private key must take place over a secure protocol, such as TLS 1.2 or its successors. To ensure the validity of the server, the client MUST verify the server certificate and MUST abort the key request if the verification of the server certificate of the TLS connection fails. This media type contains no active content and does not use compression. Interoperability considerations: There are no known interoperability considerations for this media type. Applications that use this media type: Applications that implement IBE in compliance with this specification will use this media type. The most commonly used of these applications are encrypted email and file encryption. Additional information: none Person and email address for further information: Luther Martin, martin@voltage.com.
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   Intended usage: COMMON

   Author/Change controller: Luther Martin, martin@voltage.com.

5.5. Authentication

When a client requests a key from a PKG, the PKG MUST authenticate the client before issuing the key. Authentication may either be done through the key request structure or as part of the secure transport protocol. A client or server implementing the request protocol MUST support HTTP Basic Auth [AUTH] and SHOULD also support HTTP Digest Auth [AUTH]. Applications MAY also use other means of authentication that are appropriate for the application. An email application, for example, might rely on deployed email infrastructure for this. For authentication methods that are not done by the transport protocol, a client MAY include additional authentication information in XML elements in the <ibe:authData> element of a key request. If a client does not know how to authenticate to a server, the client MAY send a key request without authentication information. If the key server requires the client to authenticate externally, it MAY reply with an IBE201 responseCode (as defined below) to redirect the client to the correct authentication mechanism. After receiving an authentication credential from this external mechanism, a client can then use the credential to form a key request that contains the additional authentication data.

5.6. Server Response Format

The key server replies to the HTTP request with an HTTP response. If the response contains a client error or server error status code, the client MUST abort the key request and fail. If the PKG replies with an HTTP response that has a status code indicating success, the body of the reply MUST contain the following XML structure that MUST be encoded as an application/ibe-pkg-reply+xml MIME type: <ibe:response xmlns:ibe="urn:ietf:params:xml:ns:ibe"> <ibe:responseType value="responseCode"/> <ibe:body> bodyTags </ibe:body> </ibe:response>
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   The responseCode attribute contains an ASCII string that describes
   the type of response from the key server.  The list of currently-
   defined responseCodes and their associated meanings is:

      IBE100 KEY_FOLLOWS
      IBE101 RESERVED
      IBE201 FOLLOW_ENROLL_URI
      IBE300 SYSTEM_ERROR
      IBE301 INVALID_REQUEST
      IBE303 CLIENT_OBSOLETE
      IBE304 AUTHORIZATION DENIED

5.6.1. The IBE100 responseCode

If the key request was successful, the key server responds with a responseCode of IBE100, and the <ibe:body> MUST contain a <ibe:privateKey> element that contains a valid private key. An example of this is shown below. <ibe:response xmlns:ibe="urn:ietf:params:xml:ns:ibe"> <ibe:responseType value="IBE100"/> <ibe:body> <ibe:privateKey> privateKey </ibe:privateKey> </ibe:body> </ibe:response> The privateKey is the base64 [B64] encoding of the DER encoding [DER] of the following structure: IBEPrivateKeyReply ::= SEQUENCE { pkgIdentity IBEIdentityInfo, pgkAlgorithm OBJECT IDENTIFIER, pkgKeyData OCTET STRING, pkgOptions SEQUENCE SIZE (1..MAX) OF PKGOption } PKGOption ::= SEQUENCE { optionID OBJECT IDENTIFIER, optionValue OCTET STRING } The pkgIdentity is an IBEIdentityInfo structure that MUST be identical to the IBEIdentityInfo structure that was sent in the key request.
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   The pkgAlgorithm is an OID that identifies the algorithm of the
   returned private key.  The OIDs for the BB1 and BF algorithms are
   defined in [IBCS].

   The pkgKeyData is a structure that contains the actual private key.
   Private-key formats for the BB1 and BF algorithms are defined in
   [IBCS].

   A server MAY pass back additional information to a client in the
   pkgOptions structure.  A client that receives a IBEPrivateKeyReply
   from a PKG that contains information in a pkgOptions structure that
   it is unable process MUST NOT use the IBE private key in the
   IBEPrivateKeyReply structure for any cryptographic operations.  A
   client MUST be able to process an IBEPrivateKeyReply that contains no
   PKGOptions structure.

5.6.2. The IBE101 responseCode

The responseCode IBE101 is reserved to ensure interoperability with earlier versions of the protocol described in this document. An example of such a response is shown below. A response with the IBE101 responseCode SHOULD contain no body. If information is contained in the body of such a response, the client receiving the response MUST discard any data that is contained in the body. <ibe:response xmlns:ibe="urn:ietf:params:xml:ns:ibe"> <ibe:responseType value="IBE101"/> <ibe:body> This message must be discarded by the recipient </ibe:body </ibe:response>

5.6.3. The IBE201 responseCode

A PKG MAY support authenticating users to external authentication mechanisms. If this is the case, the server replies to the client with responseCode IBE201 and the body of the response MUST contain a <ibe:location> element that specifies the URI of the authentication mechanism. An example of such a response is shown below. <ibe:response xmlns:ibe="urn:ietf:params:xml:ns:ibe"> <ibe:responseType value="IBE201"/> <ibe:body> <ibe:location URI="http://www.example.com/enroll.asp"/> </ibe:body> </ibe:response>
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   The client can now contact the URI returned in such a response using
   the same mechanisms as defined in Section 5.2 to obtain an
   authentication credential.  Once the client has obtained the
   credential from the authentication mechanism at this URI, it sends a
   new key request to the PKG with the correct authentication
   credentials contained in the request, placing the authentication
   credential in the <ibe:authData> element of a key request as
   described in Section 5.5.

5.6.4. The IBE300 responseCode

The IBE300 responseCode indicates that an internal server error has occurred. Information that may help diagnose the error MAY be included in the body of such a response. An example of such a response is shown below. Upon receiving a IBE300 responseCode, the client MUST abort the key request and discard any data that was included in the body of the response. <ibe:response xmlns:ibe="urn:ietf:params:xml:ns:ibe"> <ibe:responseType value="IBE300"/> <ibe:body> Widget phlebotomy failure </ibe:body> </ibe:response>

5.6.5. The IBE301 responseCode

The IBE303 responseCode indicates that an invalid key request has been received by the server. Information that may help diagnose the error MAY be included in the body of such a response. An example of such a response is shown below. Upon receiving an IBE301 responseCode, the client MUST abort the key request and discard any data that was included in the body of the response. <ibe:response xmlns:ibe="urn:ietf:params:xml:ns:ibe"> <ibe:responseType value="IBE301"/> <ibe:body> Some additional stuff </ibe:body> </ibe:response>

5.6.6. The IBE303 responseCode

The IBE303 responseCode indicates that the server is unable to correctly process the request because the version of the request is no longer supported by the server. Information that may help diagnose the error MAY be included in the body of such a response.
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   An example of such a response is shown below.  Upon receiving an
   IBE303 responseCode, the client MUST abort the key request and
   discard any data that was included in the body of the response.

   <ibe:response xmlns:ibe="urn:ietf:params:xml:ns:ibe">
      <ibe:responseType value="IBE303"/>
      <ibe:body>
         Version 3.3 or later needed
      </ibe:body>
   </ibe:response>

5.6.7. The IBE304 responseCode

The IBE304 responseCode indicates that a valid key request has been received by the server, but the authentication credentials provided were invalid. Information that may help diagnose the error MAY be included in the body of such a response. An example of such a response is shown below. Upon receiving an IBE304 responseCode, the client MUST abort the key request and discard any data that was included in the body of the response. <ibe:response xmlns:ibe="urn:ietf:params:xml:ns:ibe"> <ibe:responseType value="IBE304"/> <ibe:body> Helpful error message </ibe:body> </ibe:response>

5.7. The application/ibe-pkg-reply+xml MIME type

The following summarizes the properties of the application/ibe-pkg-reply+xml MIME type. MIME media type name: application MIME subtype name: ibe-pkg-reply+xml Mandatory parameters: none Optional parameters: none Encoding considerations: This media type MUST be encoded as US-ASCII [ASCII]. Security considerations: The data conveyed as this media type is an IBE private key, so its confidentiality and integrity are extremely important. To ensure this, the response from the server that contains an IBE private key must take place over a secure
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      protocol, such as TLS 1.2 or its successors.  To ensure the
      validity of the server, the client MUST verify the server
      certificate and MUST abort the key request if the verification of
      the server certificate of the TLS connection fails.  This media
      type contains no active content and does not use compression.

   Interoperability considerations: There are no known interoperability
      considerations for this media type.

   Applications that use this media type: Applications that implement
      IBE in compliance with this specification will use this media
      type.  The most commonly used of these applications are encrypted
      email and file encryption.

   Additional information: none

   Person and email address for further information: Luther Martin,
      martin@voltage.com.

   Intended usage: COMMON

   Author/Change controller: Luther Martin, martin@voltage.com.

6. ASN.1 Module

The following ASN.1 module summarizes the ASN.1 definitions discussed in this document. IBEARCH-module { joint-iso-itu-t(2) country(16) us(840) organization(1) identicrypt(114334) ibcs(1) ibearch(5) module(5) version(1) } DEFINITIONS IMPLICIT TAGS ::= BEGIN IBESysParams ::= SEQUENCE { version INTEGER { v2(2) }, districtName IA5String, districtSerial INTEGER, validity ValidityPeriod, ibePublicParameters IBEPublicParameters, ibeIdentityType OBJECT IDENTIFIER, ibeParamExtensions IBEParamExtensions OPTIONAL }
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   ValidityPeriod ::= SEQUENCE {
     notBefore     GeneralizedTime,
     notAfter      GeneralizedTime
   }

   IBEPublicParameters ::= SEQUENCE (1..MAX) OF
     IBEPublicParameter

   IBEPublicParameter  ::= SEQUENCE {
     ibeAlgorithm          OBJECT IDENTIFIER,
     publicParameterData   OCTET STRING
   }

   IBEParamExtensions ::= SEQUENCE OF IBEParamExtension

   IBEParamExtension ::= SEQUENCE {
     ibeParamExtensionOID     OBJECT IDENTIFIER,
     ibeParamExtensionValue   OCTET STRING
   }

   ibcs OBJECT IDENTIFIER ::= {
     joint-iso-itu-t(2) country(16) us(840)
     organization(1) identicrypt(114334) ibcs(1)
   }

   ibeParamExt OBJECT IDENTIFIER ::= {
     ibcs ibcs3(3) parameter-extensions(2)
   }

   pkgURI OBJECT IDENTIFIER ::= { ibeParamExt pkgURI(1) }

   IBEPrivateKeyReply ::= SEQUENCE {
     pkgIdentity  IBEIdentityInfo,
     pgkAlgorithm OBJECT IDENTIFIER,
     pkgKeyData   OCTET STRING,
     pkgOptions   SEQUENCE SIZE (1..MAX) OF PKGOption
   }

   PKGOption  ::=  SEQUENCE  {
     optionID    OBJECT IDENTIFIER,
     optionValue OCTET STRING
   }

   uriPPSOID OBJECT IDENTIFIER ::= {
     joint-iso-itu-t(2) country(16) us(840)
     organization(1) identicrypt(114334)
     pps-schemas(3) ic-schemas(1) pps-uri(1) version(1)
   }
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   IBEIdentityInfo ::= SEQUENCE {
     district        IA5String,
     serial          INTEGER,
     identityType    OBJECT IDENTIFIER,
     identityData    OCTET STRING
   }

   END

7. Security Considerations

7.1. Attacks outside the Scope of This Document

Attacks on the cryptographic algorithms that are used to implement IBE are outside the scope of this document. Such attacks are detailed in [IBCS], which defines parameters that give 80-bit, 112-bit, and 128-bit encryption strength. We assume that capable administrators of an IBE system will select parameters that provide a sufficient resistance to cryptanalytic attacks by adversaries. Attacks that give an adversary the ability to access or change the information on a PPS or PKG, especially the cryptographic material (referred to in this document as the master secret), will defeat the security of an IBE system. In particular, if the cryptographic material is compromised, the adversary will have the ability to recreate any user's private key and therefore decrypt all messages protected with the corresponding public key. To address this concern, it is highly RECOMMENDED that best practices for physical and operational security for PPS and PKG servers be followed and that these servers be configured (sometimes known as hardened) in accordance with best current practices [NIST]. An IBE system SHOULD be operated in an environment where illicit access is infeasible for attackers to obtain. Attacks that require administrative access or IBE-user-equivalent access to machines used by either the client or the server components defined in this document are also outside the scope of this document. We also assume that all administrators of a system implementing the protocols that are defined in this document are trustworthy and will not abuse their authority to bypass the security provided by an IBE system. Similarly, we assume that users of an IBE system will behave responsibly, not sharing their authentication credentials with others. Thus, attacks that require such assumptions are outside the scope of this document.
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7.2. Attacks within the Scope of This Document

Attacks within the scope of this document are those that allow an adversary to: o passively monitor information transmitted between users of an IBE system and the PPS and PKG o masquerade as a PPS or PKG o perform a denial-of-service (DoS) attack on a PPS or PKG o easily guess an IBE users authentication credential

7.2.1. Attacks on the Protocols Defined in This Document

All communications between users of an IBE system and the PPS or PKG are protected using TLS 1.2 [TLS]. The IBE system defined in this document provides no additional security protections for the communications between IBE users and the PPS or PKG. Therefore, the described IBE system is completely dependent on the TLS security mechanisms for authentication of the PKG or PPS server and for confidentiality and integrity of the communications. Should there be a compromise of the TLS security mechanisms, the integrity of all communications between an IBE user and the PPS or PKG will be suspect. The protocols defined in this document do not explicitly defend against an attacker masquerading as a legitimate IBE PPS or PKG. The protocols rely on the server authentication mechanism of TLS [TLS]. In addition to the TLS server authentication mechanism, IBE client software can provide protection against this possibility by providing user interface capabilities that allow users to visually determine that a connection to PPS and PKG servers is legitimate. This additional capability can help ensure that users cannot easily be tricked into providing valid authorization credentials to an attacker. The protocols defined in this document are also vulnerable to attacks against an IBE PPS or PKG. Denial-of-service attacks against either component can result in users' being unable to encrypt or decrypt using IBE, and users of an IBE system SHOULD take the appropriate countermeasures [DOS, BGPDOS] that their use of IBE requires. The IBE user authentication method selected by an IBE PKG SHOULD be of sufficient strength to prevent attackers from easily guessing the IBE user's authentication credentials through trial and error.
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8. IANA Considerations

8.1. Media Types

With this specification, IANA has registered three media types in the standard registration tree. These are application/ibe-pp-data, application/ibe-key-request+xml, and application/ibe-pkg-reply+xml. The media type application/ibe-pp-data is defined in Section 4.3 of this document. The media type application/ibe-key-request+xml is defined in Section 5.4 of this document. The media type application/ibe-pkg-reply+xml is defined in Section 5.7 of this document.

8.2. XML Namespace

The IANA is requested to register the following namespace identifier: urn:ietf:params:xml:ns:ibe Registrant Contact: Luther Martin Voltage Security 1070 Arastradero Rd Suite 100 Palo Alto, CA 94304 Phone: +1 650 543 1280 Email: martin@voltage.com XML: <?xml version="1.0"?> <!DOCTYPE html PUBLIC "-//W3C/DTD XHTML Basic 1.0//EN" "http://www.w3.org/TR/xhtml-basic/xhtml-basic10.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="content-type" content="text/html;charset=iso-8859-1"/> <title>Identity-Based Encryption</title> </head> <body> <h1>Namespace for Identity-Based Encryption</h1> <h2>urn:ietf:params:xml:ns:ibe</h2> <p> <a href="http://www.rfc-editor.org/rfc/rfc5408.txt">RFC5408</a>. </p> </body> </html>
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9. References

9.1. Normative References

[ASCII] ISO/IEC 646:1991 - Information Technology - ISO 7-bit Coded Character Set for Information Exchange. [ASN1] ITU-T Recommendation X.680: Information Technology - Abstract Syntax Notation One, 1997. [AUTH] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S., Leach, P., Luotonen, A., and L. Stewart, "HTTP Authentication: Basic and Digest Access Authentication", RFC 2617, June 1999. [B64] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies", RFC 2045, November 1996. [CMS] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3852, July 2004. [DER] ITU-T Recommendation X.690: OSI Networking and System Aspects: Abstract Syntax Notation One (ASN.1), July 2002. [DOS] Ferguson, P. and D. Senie, "Network Ingress Filtering: Defeating Denial of Service Attacks which employ IP Source Address Spoofing", BCP 38, RFC 2827, May 2000. [HTTP] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. [HTTPTLS] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000. [IBCS] Boyen, X. and L. Martin, "Identity-Based Cryptography Standard (IBCS) #1: Supersingular Curve Implementations of the BF and BB1 Cryptosystems", RFC 5091, December 2007. [IRI] Duerst, M. and M. Suignard, "Internationalized Resource Identifiers (IRIs)", RFC 3987, January 2005. [KEY] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
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   [PKIX]    Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
             Housley, R., and W. Polk, "Internet X.509 Public Key
             Infrastructure Certificate and Certificate Revocation List
             (CRL) Profile", RFC 5280, May 2008.

   [SMTP]    Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
             October 2008.

   [TLS]     Dierks, T. and E. Rescorla, "The Transport Layer Security
             (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [URI]     Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
             Resource Identifier (URI): Generic Syntax", STD 66, RFC
             3986, January 2005.

   [XML]     W3C, Extensible Markup Language (XML) 1.0 (Fourth Edition),
             September 2006.

9.2. Informative References

[BGPDOS] Turk, D., "Configuring BGP to Block Denial-of-Service Attacks", RFC 3882, September 2004. [IBECMS] Martin, L. and M. Schertler, "Using the Boneh-Franklin Identity-Based Encryption Algorithm with the Cryptographic Message Syntax (CMS)", RFC 5409, January 2009. [NIST] M. Souppaya, J. Wack and K. Kent, "Security Configuration Checklist Program for IT Products - Guidance for Checklist Users and Developers", NIST Special Publication SP 800-70, May 2005. [P1363] IEEE P1363, "Standard Specifications for Public-Key Cryptography", 2001.
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Authors' Addresses

Guido Appenzeller Stanford University Gates Building 3A Stanford, CA 94305 Phone: +1 650 732 2273 EMail: appenz@cs.stanford.edu Luther Martin Voltage Security 1070 Arastradero Rd, Suite 100 Palo Alto, CA 94304 USA Phone: +1 650 543 1280 EMail: martin@voltage.com Mark Schertler Axway 1600 Seaport Blvd, Suite 400 Redwood City, CA 94063 USA Phone: +1 650 216 2039 EMail: mschertler@us.axway.com