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rfc:rfc3565

Network Working Group J. Schaad Request for Comments: 3565 Soaring Hawk Consulting Category: Standards Track July 2003

     Use of the Advanced Encryption Standard (AES) Encryption
          Algorithm in Cryptographic Message Syntax (CMS)

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 This document specifies the conventions for using the Advanced
 Encryption Standard (AES) algorithm for encryption with the
 Cryptographic Message Syntax (CMS).

Conventions used in this document

 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 BCP 14, RFC 2119
 [MUSTSHOULD].

1. Overview

 This document specifies the conventions for using Advanced Encryption
 Standard (AES) content encryption algorithm with the Cryptographic
 Message Syntax [CMS] enveloped-data and encrypted-data content types.
 CMS values are generated using ASN.1 [X.208-88], using the Basic
 Encoding Rules (BER) [X.209-88] and the Distinguished Encoding Rules
 (DER) [X.509-88].

Schaad Standards Track [Page 1] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

1.1. AES

 The Advanced Encryption Standard (AES) [AES] was developed to replace
 DES [DES].  The AES Federal Information Processing Standard (FIPS)
 Publication specifies a cryptographic algorithm for use by U.S.
 Government organizations.  However, the AES will also be widely used
 by organizations, institutions, and individuals outside of the U.S.
 Government.
 Two researchers who developed and submitted the Rijndael algorithm
 for consideration are both cryptographers from Belgium: Dr. Joan
 Daemen of Proton World International and Dr. Vincent Rijmen, a
 postdoctoral researcher in the Electrical Engineering Department of
 Katholieke Universiteit Leuven.
 The National Institute of Standards and technology (NIST) selected
 the Rijndael algorithm for AES because it offers a combination of
 security, performance, efficiency, ease of implementation, and
 flexibility.  Specifically, Rijndael appears to be consistently a
 very good performer in both hardware and software across a wide range
 of computing environments regardless of its use in feedback or
 non-feedback modes.  Its key setup time is excellent, and its key
 agility is good.  The very low memory requirements of the Rijndael
 algorithm make it very well suited for restricted-space environments,
 in which it also demonstrates excellent performance.  The Rijndael
 algorithm operations are among the easiest to defend against power
 and timing attacks.  Additionally, it appears that some defense can
 be provided against such attacks without significantly impacting the
 algorithm's performance.  Finally, the algorithm's internal round
 structure appears to have good potential to benefit from
 instruction-level parallelism.
 The AES specifies three key sizes: 128, 192 and 256 bits.

2. Enveloped-data Conventions

 The CMS enveloped-data content type consists of encrypted content and
 wrapped content-encryption keys for one or more recipients.  The AES
 algorithm is used to encrypt the content.
 Compliant software MUST meet the requirements for constructing an
 enveloped-data content type stated in [CMS] Section 6,
 "Enveloped-data Content Type".
 The AES content-encryption key MUST be randomly generated for each
 instance of an enveloped-data content type.  The content-encryption
 key (CEK) is used to encrypt the content.

Schaad Standards Track [Page 2] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

 AES can be used with the enveloped-data content type using any of the
 following key management techniques defined in [CMS] Section 6.
 1) Key Transport: The AES CEK is uniquely wrapped for each recipient
 using the recipient's public RSA key and other values.  Section 2.2
 provides additional details.
 2) Key Agreement: The AES CEK is uniquely wrapped for each recipient
 using a pairwise symmetric key-encryption key (KEK) generated using
 an originator's randomly generated private key (ES-DH [DH]) or
 previously generated private key (SS-DH [DH]), the recipient's public
 DH key, and other values.  Section 2.3 provides additional details.
 3) Previously Distributed Symmetric KEK:  The AES CEK is wrapped
 using a previously distributed symmetric KEK (such as a Mail List
 Key).  The methods by which the symmetric KEK is generated and
 distributed are beyond the scope of this document.  Section 2.4
 provides additional details.
 4) Password Encryption:  The AES CEK is wrapped using a KEK derived
 from a password or other shared secret.  Section 2.5 provides
 additional details.
 Documents defining the use of the Other Recipient Info structure will
 need to define the proper use for the AES algorithm if desired.

2.1. EnvelopedData Fields

 The enveloped-data content type is ASN.1 encoded using the
 EnvelopedData syntax.  The fields of the EnvelopedData syntax MUST be
 populated as follows:
 The EnvelopedData version is determined based on a number of factors.
 See [CMS] section 6.1 for the algorithm to determine this value.
 The EnvelopedData recipientInfos CHOICE is dependent on the key
 management technique used.  Section 2.2, 2.3, 2.4 and 2.5 provide
 additional information.
 The EnvelopedData encryptedContentInfo contentEncryptionAlgorithm
 field MUST specify a symmetric encryption algorithm.  Implementations
 MUST support content encryption with AES, but implementations MAY
 support other algorithms as well.
 The EnvelopedData unprotectedAttrs MAY be present.

Schaad Standards Track [Page 3] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

2.2. KeyTransRecipientInfo Fields

 The enveloped-data content type is ASN.1 encoded using the
 EnvelopedData syntax.  The fields of the EnvelopedData syntax MUST be
 populated as follows:
 The KeyTransRecipientInfo version MUST be either 0 or 2.  If the
 RecipientIdentifier is the CHOICE issuerAndSerialNumber, then the
 version MUST be 0.  If the RecipientIdentifier is
 subjectKeyIdentifier, then the version MUST be 2.
 The KeyTransRecipientInfo RecipientIdentifier provides two
 alternatives for specifying the recipient's certificate, and thereby
 the recipient's public key.  The recipient's certificate MUST contain
 a RSA public key.  The CEK is encrypted with the recipient's RSA
 public key.  The issuerAndSerialNumber alternative identifies the
 recipient's certificate by the issuer's distinguished name and the
 certificate serial number; the subjectKeyIdentifier identifies the
 recipient's certificate by the X.509 subjectKeyIdentifier extension
 value.
 The KeyTransRecipientInfo keyEncryptionAlgorithm field specifies the
 key transport algorithm (i.e., RSAES-OAEP [RSA-OAEP]), and the
 associated parameters used to encrypt the CEK for the recipient.
 The KeyTransRecipientInfo encryptedKey is the result of encrypting
 the CEK with the recipient's RSA public key.

2.3. KeyAgreeRecipientInfo Fields

 This section describes the conventions for using ES-DH or SS-DH and
 AES with the CMS enveloped-data content type to support key
 agreement.  When key agreement is used, then the RecipientInfo
 keyAgreeRecipientInfo CHOICE MUST be used.
 The KeyAgreeRecipient version MUST be 3.
 The EnvelopedData originatorInfo field MUST be the originatorKey
 alternative.  The originatorKey algorithm fields MUST contain the
 dh-public-number object identifier with absent parameters.  The
 originatorKey publicKey MUST contain the originator's ephemeral
 public key.
 The EnvelopedData ukm MAY be present.
 The EnvelopedData keyEncrytionAlgorithm MUST be the id-alg-ESDH
 algorithm identifier [CMSALG].

Schaad Standards Track [Page 4] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

2.3.1. ES-DH/AES Key Derivation

 Generation of the AES KEK to be used with the AES-key wrap algorithm
 is done using the method described in [DH].

2.3.1.1. Example 1

 ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09
                    0a 0b 0c 0d 0e 0f 10 11 12 13
 The key wrap algorithm is AES-128 wrap, so we need 128 bits (16
 bytes) of keying material.
 No partyAInfo is used.
 Consequently, the input to SHA-1 is:
 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
 30 1b
    30 11
       06 09 60 86 48 01 65 03 04 01 05           ; AES-128 wrap OID
       04 04
          00 00 00 01                             ; Counter
    a2 06
       04 04
       00 00 00 80                                ; key length
 And the output is the 32 bytes:
 d6 d6 b0 94 c1 02 7a 7d e6 e3 11 72 94 a3 53 64 49 08 50 f9
 Consequently,
 K= d6 d6 b0 94 c1 02 7a 7d e6 e3 11 72 94 a3 53 64

Schaad Standards Track [Page 5] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

2.3.1.2. Example 2

 ZZ is the 20 bytes 00 01 02 03 04 05 06 07 08 09
                    0a 0b 0c 0d 0e 0f 10 11 12 13
 The key wrap algorithm is AES-256 key wrap, so we need 256 bits (32
 bytes) of keying material.
 The partyAInfo used is the 64 bytes
 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
 01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
 Consequently, the input to first invocation of SHA-1 is:
 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
 30 5f
    30 11
       06 09 60 86 48 01 65 03 04 01 2d            ; AES-256 wrap OID
       04 04
          00 00 00 01                              ; Counter
    a0 42
       04 40
          01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo
          01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
          01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
          01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
    a2 06
       04 04
          00 00 01 00                              ; key length
 And the output is the 20 bytes:
 88 90 58 5C 4E 28 1A 5C 11 67 CA A5 30 BE D5 9B 32 30 D8 93

Schaad Standards Track [Page 6] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

 The input to second invocation of SHA-1 is:
 00 01 02 03 04 05 06 07 08 09 0a 0b 0c 0d 0e 0f 10 11 12 13 ; ZZ
 30 5f
    30 11
       06 09 60 86 48 01 65 03 04 01 2d            ; AES-256 wrap OID
       04 04
          00 00 00 02                              ; Counter
    a0 42
       04 40
          01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01 ; partyAInfo
          01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
          01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
          01 23 45 67 89 ab cd ef fe dc ba 98 76 54 32 01
    a2 06
       04 04
          00 00 01 00                              ; key length
 And the output is the 20 bytes:
 CB A8 F9 22 BD 1B 56 A0 71 C9 6F 90 36 C6 04 2C AA 20 94 37
 Consequently,
 K = 88 90 58 5C 4E 28 1A 5C 11 67 CA A5 30 BE D5 9B
     32 30 D8 93 CB A8 F9 22 BD 1B 56 A0

2.3.2. AES CEK Wrap Process

 The AES key wrap algorithm encrypts one AES key in another AES key.
 The algorithm produces an output 64-bits longer than the input AES
 CEK, the additional bits are a checksum.  The algorithm uses 6*n AES
 encryption/decryption operations where n is number of 64-bit blocks
 in the AES CEK.  Full details of the AES key wrap algorithm are
 available at [AES-WRAP].
 NIST has assigned the following OIDs to define the AES key wrap
 algorithm.
      id-aes128-wrap OBJECT IDENTIFIER ::= { aes 5 }
      id-aes192-wrap OBJECT IDENTIFIER ::= { aes 25 }
      id-aes256-wrap OBJECT IDENTIFIER ::= { aes 45 }
 In all cases the parameters field MUST be absent.  The OID gives the
 KEK key size, but does not make any statements as to the size of the
 wrapped AES CEK.  Implementations MAY use different KEK and CEK

Schaad Standards Track [Page 7] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

 sizes.  Implements MUST support the CEK and the KEK having the same
 length.  If different lengths are supported, the KEK MUST be of equal
 or greater length than the CEK.

2.4. KEKRecipientInfo Fields

 This section describes the conventions for using AES with the CMS
 enveloped-data content type to support previously distributed
 symmetric KEKs.  When a previously distributed symmetric KEK is used
 to wrap the AES CEK, then the RecipientInfo KEKRecipientInfo CHOICE
 MUST be used.  The methods used to generate and distribute the
 symmetric KEK are beyond the scope of this document.  One possible
 method of distributing keys is documented in [SYMKEYDIST].
 The KEKRecipientInfo fields MUST be populated as specified in [CMS]
 Section 6.2.3, KEKRecipientInfo Type.
 The KEKRecipientInfo keyEncryptionAlgorithm algorithm field MUST be
 one of the OIDs defined in section 2.3.2 indicating that the AES wrap
 function is used to wrap the AES CEK.  The KEKRecipientInfo
 keyEncryptionAlgorithm parameters field MUST be absent.
 The KEKRecipientInfo encryptedKey field MUST include the AES CEK
 wrapped using the previously distributed symmetric KEK as input to
 the AES wrap function.

2.5. PasswordRecipientInfo Fields

 This section describes the conventions for using AES with the CMS
 enveloped-data content type to support password-based key management.
 When a password derived KEK is used to wrap the AES CEK, then the
 RecipientInfo PasswordRecipientInfo CHOICE MUST be used.
 The keyEncryptionAlgorithm algorithm field MUST be one of the OIDs
 defined in section 2.3.2 indicating the AES wrap function is used to
 wrap the AES CEK.  The keyEncryptionAlgorithm parameters field MUST
 be absent.
 The encryptedKey field MUST be the result of the AES key wrap
 algorithm applied to the AES CEK value.

3. Encrypted-data Conventions

 The CMS encrypted-data content type consists of encrypted content
 with implicit key management.  The AES algorithm is used to encrypt
 the content.

Schaad Standards Track [Page 8] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

 Compliant software MUST meet the requirements for constructing an
 enveloped-data content type stated in [CMS] Section 8,
 "Encrypted-data Content Type".
 The encrypted-data content type is ASN.1 encoded using the
 EncryptededData syntax.  The fields of the EncryptedData syntax MUST
 be populated as follows:
 The EncryptedData version is determined based on a number of factors.
 See [CMS] section 9.1 for the algorithm to determine this value.
 The EncryptedData encryptedContentInfo contentEncryptionAlgorithm
 field MUST specify a symmetric encryption algorithm.  Implementations
 MUST support encryption using AES, but implementations MAY support
 other algorithms as well.
 The EncryptedData unprotectedAttrs MAY be present.

4. Algorithm Identifiers and Parameters

 This section specified algorithm identifiers for the AES encryption
 algorithm.

4.1. AES Algorithm Identifiers and Parameters

 The AES algorithm is defined in [AES].  RSAES-OAEP [RSA-OAEP] MAY be
 used to transport AES keys.
 AES is added to the set of symmetric content encryption algorithms
 defined in [CMSALG].  The AES content-encryption algorithm, in Cipher
 Block Chaining (CBC) mode, for the three different key sizes are
 identified by the following object identifiers:
     id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 }
     id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 }
     id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }
 The AlgorithmIdentifier parameters field MUST be present, and the
 parameters field MUST contain a AES-IV:
     AES-IV ::= OCTET STRING (SIZE(16))
 Content encryption algorithm identifiers are located in the
 EnvelopedData EncryptedContentInfo contentEncryptionAlgorithm and the
 EncryptedData EncryptedContentInfo contentEncryptionAlgorithm fields.

Schaad Standards Track [Page 9] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

 Content encryption algorithms are used to encrypt the content located
 in the EnvelopedData EncryptedContentInfo encryptedContent and the
 EncryptedData EncryptedContentInfo encryptedContent fields.

5. SMIMECapabilities Attribute Conventions

 An S/MIME client SHOULD announce the set of cryptographic functions
 it supports by using the S/MIME capabilities attribute.  This
 attribute provides a partial list of object identifiers of
 cryptographic functions and MUST be signed by the client.  The
 algorithm OIDs SHOULD be logically separated in functional categories
 and MUST be ordered with respect to their preference.
 RFC 2633 [MSG], Section 2.5.2 defines the SMIMECapabilities signed
 attribute (defined as a SEQUENCE of SMIMECapability SEQUENCEs) to be
 used to specify a partial list of algorithms that the software
 announcing the SMIMECapabilities can support.

5.1. AES S/MIME Capability Attributes

 If an S/MIME client is required to support symmetric encryption with
 AES, the capabilities attribute MUST contain the AES object
 identifier specified above in the category of symmetric algorithms.
 The parameter with this encoding MUST be absent.
 The encodings for the mandatory key sizes are:
       Key Size                   Capability
        128          30 0B 06 09 60 86 48 01 65 03 04 01 02
        196          30 0B 06 09 60 86 48 01 65 03 04 01 16
        256          30 0B 06 09 60 86 48 01 65 03 04 01 2A
 When a sending agent creates an encrypted message, it has to decide
 which type of encryption algorithm to use.  In general the decision
 process involves information obtained from the capabilities lists
 included in messages received from the recipient, as well as other
 information such as private agreements, user preferences, legal
 restrictions, and so on.  If users require AES for symmetric
 encryption, the S/MIME clients on both the sending and receiving side
 MUST support it, and it MUST be set in the user preferences.

Schaad Standards Track [Page 10] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

6. Security Considerations

 If RSA-OAEP [PKCS#1v2.0] and RSA PKCS #1 v1.5 [PKCS#1v1.5] are both
 used to transport the same CEK, then an attacker can still use the
 Bleichenbacher attack against the RSA PKCS #1 v1.5 encrypted key.  It
 is generally unadvisable to mix both RSA-OAEP and RSA PKCS#1 v1.5 in
 the same set of recipients.
 Implementations must protect the RSA private key and the CEK.
 Compromise of the RSA private key may result in the disclosure of all
 messages protected with that key.  Compromise of the CEK may result
 in disclosure of the associated encrypted content.
 The generation of AES CEKs relies on random numbers.  The use of
 inadequate pseudo-random number generators (PRNGs) to generate these
 values can result in little or no security.  An attacker may find it
 much easier to reproduce the PRNG environment that produced the keys,
 searching the resulting small set of possibilities, rather than brute
 force searching the whole key space.  The generation of quality
 random numbers is difficult.  RFC 1750 [RANDOM] offers important
 guidance in this area.
 When wrapping a CEK with a KEK, the KEK MUST always be at least the
 same length as the CEK.  An attacker will generally work at the
 weakest point in an encryption system.  This would be the smaller of
 the two key sizes for a brute force attack.

Normative References

 [AES]         National Institute of Standards.  FIPS Pub 197:
               Advanced Encryption Standard (AES).  26 November 2001.
 [CMS]         Housley, R., "Cryptographic Message Syntax (CMS)", RFC
               3369, August 2002.
 [AES-WRAP]    Schaad, J. and R. Housley, "Advanced Encryption
               Standard (AES) Key Wrap Algorithm", RFC 3394, September
               2002.
 [CMSALG]      Housley, R., "Cryptographic Message Syntax (CMS)
               Algorithms, RFC 3370, August 2002.
 [DES]         National Institute of Standards and Technology. FIPS
               Pub 46: Data Encryption Standard.  15 January 1977.
 [DH]          Rescorla, E., "Diffie-Hellman Key Agreement Method",
               RFC 2631, June 1999.

Schaad Standards Track [Page 11] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

 [MUSTSHOULD]  Bradner, S., "Key Words for Use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RSA-OAEP]    Housley, R. "Use of the RSAES-OAEP Key Transport
               Algorithm in the Cryptographic Message Syntax (CMS)",
               RFC 3560, July 2003.
 [X.208-88]    CCITT.  Recommendation X.208: Specification of Abstract
               Syntax Notation One (ASN.1).  1988.
 [X.209-88]    CCITT.  Recommendation X.209: Specification of Basic
               Encoding Rules for Abstract Syntax Notation One
               (ASN.1). 1988.
 [X.509-88]    CCITT.  Recommendation X.509: The Directory -
               Authentication Framework.  1988.

Informational References

 [MSG]         Ramsdell, B., Editor, "S/MIME Version 3 Message
               Specification", RFC 2633, June 1999.
 [PKCS#1v1.5]  Kaliski, B., "PKCS #1: RSA Encryption, Version 1.5",
               RFC 2313, March 1998.
 [PKCS#1v2.0]  Kaliski, B., "PKCS #1: RSA Encryption, Version 2.0",
               RFC 2437, October 1998.
 [RANDOM]      Eastlake, D., Crocker, S. and J. Schiller, "Randomness
               Recommendations for Security", RFC 1750, December 1994.
 [SYMKEYDIST]  Turner, S., "CMS Symmetric Key Management and
               Distribution", Work in Progress, January 2003.

Acknowledgements

 This document is the result of contributions from many professionals.
 We appreciate the hard work of all members of the IETF S/MIME Working
 Group.

Schaad Standards Track [Page 12] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

Appendix A ASN.1 Module

CMSAesRsaesOaep {iso(1) member-body(2) us(840) rsadsi(113549)

    pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-aes(19) }

DEFINITIONS IMPLICIT TAGS ::= BEGIN

– EXPORTS ALL – IMPORTS

  1. - PKIX

AlgorithmIdentifier

        FROM PKIXExplicit88 {iso(1) identified-organization(3) dod(6)
            internet(1) security(5) mechanisms(5) pkix(7) id-mod(0)
            id-pkix1-explicit(18)};

– AES information object identifiers –

aes OBJECT IDENTIFIER ::= { joint-iso-itu-t(2) country(16) us(840)

             organization(1) gov(101) csor(3)_ nistAlgorithms(4)  1 }

– AES using CBC-chaining mode for key sizes of 128, 192, 256

id-aes128-CBC OBJECT IDENTIFIER ::= { aes 2 } id-aes192-CBC OBJECT IDENTIFIER ::= { aes 22 } id-aes256-CBC OBJECT IDENTIFIER ::= { aes 42 }

– AES-IV is a the parameter for all the above object identifiers.

AES-IV ::= OCTET STRING (SIZE(16))

– AES Key Wrap Algorithm Identifiers - Parameter is absent

id-aes128-wrap OBJECT IDENTIFIER ::= { aes 5 } id-aes192-wrap OBJECT IDENTIFIER ::= { aes 25 } id-aes256-wrap OBJECT IDENTIFIER ::= { aes 45 }

END

Author's Address

 Jim Schaad
 Soaring Hawk Consulting
 EMail: jimsch@exmsft.com

Schaad Standards Track [Page 13] RFC 3565 Use of the AES Encryption Algorithm in CMS July 2003

Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Schaad Standards Track [Page 14]

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