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rfc:rfc2541

Network Working Group D. Eastlake Request for Comments: 2541 IBM Category: Informational March 1999

              DNS Security Operational Considerations

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) The Internet Society (1999).  All Rights Reserved.

Abstract

 Secure DNS is based on cryptographic techniques.  A necessary part of
 the strength of these techniques is careful attention to the
 operational aspects of key and signature generation, lifetime, size,
 and storage.  In addition, special attention must be paid to the
 security of the high level zones, particularly the root zone.  This
 document discusses these operational aspects for keys and signatures
 used in connection with the KEY and SIG DNS resource records.

Acknowledgments

 The contributions and suggestions of the following persons (in
 alphabetic order) are gratefully acknowledged:
       John Gilmore
       Olafur Gudmundsson
       Charlie Kaufman

Eastlake Informational [Page 1] RFC 2541 DNS Security Operational Considerations March 1999

Table of Contents

 Abstract...................................................1
 Acknowledgments............................................1
 1. Introduction............................................2
 2. Public/Private Key Generation...........................2
 3. Public/Private Key Lifetimes............................2
 4. Public/Private Key Size Considerations..................3
 4.1 RSA Key Sizes..........................................3
 4.2 DSS Key Sizes..........................................4
 5. Private Key Storage.....................................4
 6. High Level Zones, The Root Zone, and The Meta-Root Key..5
 7. Security Considerations.................................5
 References.................................................6
 Author's Address...........................................6
 Full Copyright Statement...................................7

1. Introduction

 This document describes operational considerations for the
 generation, lifetime, size, and storage of DNS cryptographic keys and
 signatures for use in the KEY and SIG resource records [RFC 2535].
 Particular attention is paid to high level zones and the root zone.

2. Public/Private Key Generation

 Careful generation of all keys is a sometimes overlooked but
 absolutely essential element in any cryptographically secure system.
 The strongest algorithms used with the longest keys are still of no
 use if an adversary can guess enough to lower the size of the likely
 key space so that it can be exhaustively searched.  Technical
 suggestions for the generation of random keys will be found in [RFC
 1750].
 Long term keys are particularly sensitive as they will represent a
 more valuable target and be subject to attack for a longer time than
 short period keys.  It is strongly recommended that long term key
 generation occur off-line in a manner isolated from the network via
 an air gap or, at a minimum, high level secure hardware.

3. Public/Private Key Lifetimes

 No key should be used forever.  The longer a key is in use, the
 greater the probability that it will have been compromised through
 carelessness, accident, espionage, or cryptanalysis.  Furthermore, if

Eastlake Informational [Page 2] RFC 2541 DNS Security Operational Considerations March 1999

 key rollover is a rare event, there is an increased risk that, when
 the time does come to change the key, no one at the site will
 remember how to do it or operational problems will have developed in
 the key rollover procedures.
 While public key lifetime is a matter of local policy, these
 considerations imply that, unless there are extraordinary
 circumstances, no long term key should have a lifetime significantly
 over four years.  In fact, a reasonable guideline for long term keys
 that are kept off-line and carefully guarded is a 13 month lifetime
 with the intent that they be replaced every year.  A reasonable
 maximum lifetime for keys that are used for transaction security or
 the like and are kept on line is 36 days with the intent that they be
 replaced monthly or more often.  In many cases, a key lifetime of
 somewhat over a day may be reasonable.
 On the other hand, public keys with too short a lifetime can lead to
 excessive resource consumption in re-signing data and retrieving
 fresh information because cached information becomes stale.  In the
 Internet environment, almost all public keys should have lifetimes no
 shorter than three minutes, which is a reasonable estimate of maximum
 packet delay even in unusual circumstances.

4. Public/Private Key Size Considerations

 There are a number of factors that effect public key size choice for
 use in the DNS security extension.  Unfortunately, these factors
 usually do not all point in the same direction.  Choice of zone key
 size should generally be made by the zone administrator depending on
 their local conditions.
 For most schemes, larger keys are more secure but slower.  In
 addition, larger keys increase the size of the KEY and SIG RRs.  This
 increases the chance of DNS UDP packet overflow and the possible
 necessity for using higher overhead TCP in responses.

4.1 RSA Key Sizes

 Given a small public exponent, verification (the most common
 operation) for the MD5/RSA algorithm will vary roughly with the
 square of the modulus length, signing will vary with the cube of the
 modulus length, and key generation (the least common operation) will
 vary with the fourth power of the modulus length.  The current best
 algorithms for factoring a modulus and breaking RSA security vary
 roughly with the 1.6 power of the modulus itself.  Thus going from a
 640 bit modulus to a 1280 bit modulus only increases the verification
 time by a factor of 4 but may increase the work factor of breaking
 the key by over 2^900.

Eastlake Informational [Page 3] RFC 2541 DNS Security Operational Considerations March 1999

 The recommended minimum RSA algorithm modulus size is 704 bits which
 is believed by the author to be secure at this time.  But high level
 zones in the DNS tree may wish to set a higher minimum, perhaps 1000
 bits, for security reasons.  (Since the United States National
 Security Agency generally permits export of encryption systems using
 an RSA modulus of up to 512 bits, use of that small a modulus, i.e.
 n, must be considered weak.)
 For an RSA key used only to secure data and not to secure other keys,
 704 bits should be adequate at this time.

4.2 DSS Key Sizes

 DSS keys are probably roughly as strong as an RSA key of the same
 length but DSS signatures are significantly smaller.

5. Private Key Storage

 It is recommended that, where possible, zone private keys and the
 zone file master copy be kept and used in off-line, non-network
 connected, physically secure machines only.  Periodically an
 application can be run to add authentication to a zone by adding SIG
 and NXT RRs and adding no-key type KEY RRs for subzones/algorithms
 where a real KEY RR for the subzone with that algorithm is not
 provided. Then the augmented file can be transferred, perhaps by
 sneaker-net, to the networked zone primary server machine.
 The idea is to have a one way information flow to the network to
 avoid the possibility of tampering from the network.  Keeping the
 zone master file on-line on the network and simply cycling it through
 an off-line signer does not do this.  The on-line version could still
 be tampered with if the host it resides on is compromised.  For
 maximum security, the master copy of the zone file should be off net
 and should not be updated based on an unsecured network mediated
 communication.
 This is not possible if the zone is to be dynamically updated
 securely [RFC 2137]. At least a private key capable of updating the
 SOA and NXT chain must be on line in that case.
 Secure resolvers must be configured with some trusted on-line public
 key information (or a secure path to such a resolver) or they will be
 unable to authenticate.  Although on line, this public key
 information must be protected or it could be altered so that spoofed
 DNS data would appear authentic.

Eastlake Informational [Page 4] RFC 2541 DNS Security Operational Considerations March 1999

 Non-zone private keys, such as host or user keys, generally have to
 be kept on line to be used for real-time purposes such as DNS
 transaction security.

6. High Level Zones, The Root Zone, and The Meta-Root Key

 Higher level zones are generally more sensitive than lower level
 zones.  Anyone controlling or breaking the security of a zone thereby
 obtains authority over all of its subdomains (except in the case of
 resolvers that have locally configured the public key of a
 subdomain).  Therefore, extra care should be taken with high level
 zones and strong keys used.
 The root zone is the most critical of all zones.  Someone controlling
 or compromising the security of the root zone would control the
 entire DNS name space of all resolvers using that root zone (except
 in the case of resolvers that have locally configured the public key
 of a subdomain). Therefore, the utmost care must be taken in the
 securing of the root zone. The strongest and most carefully handled
 keys should be used.  The root zone private key should always be kept
 off line.
 Many resolvers will start at a root server for their access to and
 authentication of DNS data.  Securely updating an enormous population
 of resolvers around the world will be extremely difficult.  Yet the
 guidelines in section 3 above would imply that the root zone private
 key be changed annually or more often and if it were staticly
 configured at all these resolvers, it would have to be updated when
 changed.
 To permit relatively frequent change to the root zone key yet
 minimize exposure of the ultimate key of the DNS tree, there will be
 a "meta-root" key used very rarely and then only to sign a sequence
 of regular root key RRsets with overlapping time validity periods
 that are to be rolled out. The root zone contains the meta-root and
 current regular root KEY RR(s) signed by SIG RRs under both the
 meta-root and other root private key(s) themselves.
 The utmost security in the storage and use of the meta-root key is
 essential.  The exact techniques are precautions to be used are
 beyond the scope of this document.  Because of its special position,
 it may be best to continue with the same meta-root key for an
 extended period of time such as ten to fifteen years.

7. Security Considerations

 The entirety of this document is concerned with operational
 considerations of public/private key pair DNS Security.

Eastlake Informational [Page 5] RFC 2541 DNS Security Operational Considerations March 1999

References

 [RFC 1034]   Mockapetris, P., "Domain Names - Concepts and
              Facilities", STD 13, RFC 1034, November 1987.
 [RFC 1035]   Mockapetris, P., "Domain Names - Implementation and
              Specifications", STD 13, RFC 1035, November 1987.
 [RFC 1750]   Eastlake, D., Crocker, S. and J. Schiller, "Randomness
              Requirements for Security", RFC 1750, December 1994.
 [RFC 2065]   Eastlake, D. and C. Kaufman, "Domain Name System
              Security Extensions", RFC 2065, January 1997.
 [RFC 2137]   Eastlake, D., "Secure Domain Name System Dynamic
              Update", RFC 2137, April 1997.
 [RFC 2535]   Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.
 [RSA FAQ]    RSADSI Frequently Asked Questions periodic posting.

Author's Address

 Donald E. Eastlake 3rd
 IBM
 65 Shindegan Hill Road, RR #1
 Carmel, NY 10512
 Phone:   +1-914-276-2668(h)
          +1-914-784-7913(w)
 Fax:     +1-914-784-3833(w)
 EMail:   dee3@us.ibm.com

Eastlake Informational [Page 6] RFC 2541 DNS Security Operational Considerations March 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  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
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Eastlake Informational [Page 7]

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