draft-raeburn-krb-rijndael-krb-05.txt   [plain text]

Kerberos Working Group                                        K. Raeburn
Document: draft-raeburn-krb-rijndael-krb-05.txt                      MIT
                                                           June 20, 2003
                                               expires December 20, 2003

                     AES Encryption for Kerberos 5

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026 [RFC2026]. Internet-Drafts
   are working documents of the Internet Engineering Task Force (IETF),
   its areas, and its working groups. Note that other groups may also
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   The list of current Internet-Drafts can be accessed at

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   Recently the US National Institute of Standards and Technology chose
   a new Advanced Encryption Standard, which is significantly faster and
   (it is believed) more secure than the old DES algorithm.  This
   document is a specification for the addition of this algorithm to the
   Kerberos cryptosystem suite.

   Comments should be sent to the author, or to the IETF Kerberos
   working group (ietf-krb-wg@anl.gov).

1. Introduction

   This document defines encryption key and checksum types for Kerberos
   5 using the AES algorithm recently chosen by NIST.  These new types
   support 128-bit block encryption, and key sizes of 128 or 256 bits.

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   Using the "simplified profile" of [KCRYPTO], we can define a pair of
   encryption and checksum schemes.  AES is used with cipher text
   stealing to avoid message expansion, and SHA-1 [SHA1] is the
   associated checksum function.

2. Conventions Used in this Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119.

3. Protocol Key Representation

   The profile in [KCRYPTO] treats keys and random octet strings as
   conceptually different.  But since the AES key space is dense, we can
   use any bit string of appropriate length as a key.  We use the byte
   representation for the key described in [AES], where the first bit of
   the bit string is the high bit of the first byte of the byte string
   (octet string) representation.

4. Key Generation From Pass Phrases or Random Data

   Given the above format for keys, we can generate keys from the
   appropriate amounts of random data (128 or 256 bits) by simply
   copying the input string.

   To generate an encryption key from a pass phrase and salt string, we
   use the PBKDF2 function from PKCS #5 v2.0 ([PKCS5]), with parameters
   indicated below, to generate an intermediate key (of the same length
   as the desired final key), which is then passed into the DK function
   with the 8-octet ASCII string "kerberos" as is done for des3-cbc-
   hmac-sha1-kd in [KCRYPTO].  (In [KCRYPTO] terms, the PBKDF2 function
   produces a "random octet string", hence the application of the
   random-to-key function even though it's effectively a simple identity
   operation.)  The resulting key is the user's long-term key for use
   with the encryption algorithm in question.

    tkey = random2key(PBKDF2(passphrase, salt, iter_count, keylength))
    key = DK(tkey, "kerberos")

   The pseudorandom function used by PBKDF2 will be a SHA-1 HMAC of the
   passphrase and salt, as described in Appendix B.1 to PKCS#5.

   The number of iterations is specified by the string-to-key parameters
   supplied.  The parameter string is four octets indicating an unsigned
   number in big-endian order.  This is the number of iterations to be
   performed.  If the value is 00 00 00 00, the number of iterations to
   be performed is 4294967296 (2**32).  (Thus the minimum expressable

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   iteration count is 1.)

   For environments where slower hardware is the norm, implementations
   may wish to limit the number of iterations to prevent a spoofed
   response from consuming lots of client-side CPU time; it is
   recommended that this bound be no less than 50000.  Even for
   environments with fast hardware, 4 billion iterations is likely to
   take a fairly long time; much larger bounds might still be enforced,
   and it might be wise for implementations to permit interruption of
   this operation by the user if the environment allows for it.

   If the string-to-key parameters are not supplied, the value used is
   00 00 10 00 (decimal 4096, indicating 4096 iterations).

   Note that this is NOT a requirement, nor even a recommendation, for
   this value to be used in "optimistic preauthentication" (e.g.,
   attempting timestamp-based preauthentication using the user's long-
   term key, without having first communicated with the KDC) in the
   absence of additional information, nor as a default value for sites
   to use for their principals' long-term keys in their Kerberos
   database.  It is simply the interpretation of the absence of the
   string-to-key parameter field when the KDC has had an opportunity to
   provide it.

   Sample test vectors are given in the appendix.

5. Cipher Text Stealing

   Cipher block chaining is used to encrypt messages.  Unlike previous
   Kerberos cryptosystems, we use cipher text stealing to handle the
   possibly partial final block of the message.

   Cipher text stealing is described on pages 195-196 of [AC], and
   section 8 of [RC5]; it has the advantage that no message expansion is
   done during encryption of messages of arbitrary sizes as is typically
   done in CBC mode with padding.

   Cipher text stealing, as defined in [RC5], assumes that more than one
   block of plain text is available.  If exactly one block is to be
   encrypted, that block is simply encrypted with AES (also known as ECB
   mode).  Input of less than one block is padded at the end to one
   block; the values of the padding bits are unspecified.
   (Implementations may use all-zero padding, but protocols should not
   rely on the result being deterministic.  Implementations may use
   random padding, but protocols should not rely on the result not being
   deterministic.  Note that in most cases, the Kerberos encryption
   profile will add a random confounder independent of this padding.)

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   For consistency, cipher text stealing is always used for the last two
   blocks of the data to be encrypted, as in [RC5].  If the data length
   is a multiple of the block size, this is equivalent to plain CBC mode
   with the last two cipher text blocks swapped.

   A test vector is given in the appendix.

6. Kerberos Algorithm Profile Parameters

   This is a summary of the parameters to be used with the simplified
   algorithm profile described in [KCRYPTO]:

   |               protocol key format       128- or 256-bit string     |
   |                                                                    |
   |            string-to-key function       PBKDF2+DK with variable    |
   |                                         iteration count (see       |
   |                                         above)                     |
   |                                                                    |
   |  default string-to-key parameters       00 00 10 00                |
   |                                                                    |
   |        key-generation seed length       key size                   |
   |                                                                    |
   |            random-to-key function       identity function          |
   |                                                                    |
   |                  hash function, H       SHA-1                      |
   |                                                                    |
   |               HMAC output size, h       12 octets (96 bits)        |
   |                                                                    |
   |             message block size, m       1 octet                    |
   |                                                                    |
   |  encryption/decryption functions,       AES in CBC-CTS mode with   |
   |  E and D                                zero ivec (cipher block    |
   |                                         size 16 octets)            |

   Using this profile with each key size gives us two each of encryption
   and checksum algorithm definitions.

7. Assigned Numbers

   The following encryption type numbers are assigned:

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   |                         encryption types                           |
   |         type name                  etype value          key size   |
   |   aes128-cts-hmac-sha1-96              17                 128      |
   |   aes256-cts-hmac-sha1-96              18                 256      |

   The following checksum type numbers are assigned:

   |                          checksum types                            |
   |        type name                 sumtype value           length    |
   |    hmac-sha1-96-aes128                15                   96      |
   |    hmac-sha1-96-aes256                16                   96      |

   These checksum types will be used with the corresponding encryption
   types defined above.

8. Security Considerations

   This new algorithm has not been around long enough to receive the
   decades of intense analysis that DES has received.  It is possible
   that some weakness exists that has not been found by the
   cryptographers analyzing these algorithms before and during the AES
   selection process.

   The use of the HMAC function has drawbacks for certain pass phrase
   lengths.  For example, a pass phrase longer than the hash function
   block size (64 bytes, for SHA-1) is hashed to a smaller size (20
   bytes) before applying the main HMAC algorithm.  However, entropy is
   generally sparse in pass phrases, especially in long ones, so this
   may not be a problem in the rare cases of users with long pass

   Also, generating a 256-bit key from a pass phrase of any length may
   be deceptive, since the effective entropy in pass-phrase-derived key
   cannot be nearly that large.

   The iteration count in PBKDF2 appears to be useful primarily as a
   constant multiplier for the amount of work required for an attacker
   using brute-force methods.  Unfortunately, it also multiplies, by the
   same amount, the work needed by a legitimate user with a valid
   password.  Thus the work factor imposed on an attacker (who may have

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   many powerful workstations at his disposal) must be balanced against
   the work factor imposed on the legitimate user (who may have a PDA or
   cell phone); the available computing power on either side increases
   as time goes on, as well.  A better way to deal with the brute-force
   attack is through preauthentication mechanisms that provide better
   protection of the user's long-term key.  Use of such mechanisms is
   out of scope for this document.

   If a site does wish to use this means of protection against a brute-
   force attack, the iteration count should be chosen based on the
   facilities expected to be available to an attacker, and the amount of
   work the attacker should be required to perform to acquire the key or

   As an example:

      The author's tests on a 2GHz Pentium 4 system indicated that in
      one second, nearly 90000 iterations could be done, producing a
      256-bit key.  This was using the SHA-1 assembly implementation
      from OpenSSL, and a pre-release version of the PBKDF2 code for
      MIT's Kerberos package, on a single system.  No attempt was made
      to do multiple hashes in parallel, so we assume an attacker doing
      so can probably do at least 100000 iterations per second --
      rounded up to 2**17, for ease of calculation.  For simplicity, we
      also assume the final AES encryption step costs nothing.

      Paul Leach estimates [LEACH] that a password-cracking dictionary
      may have on the order of 2**21 entries, with capitalization,
      punctuation, and other variations contributing perhaps a factor of
      2**11, giving a ballpark estimate of 2**32.

      Thus, for a known iteration count N and a known salt string, an
      attacker with some number of computers comparable to the author's
      would need roughly N*2**15 CPU seconds to convert the entire
      dictionary plus variations into keys.

      An attacker using a dozen such computers for a month would have
      roughly 2**25 CPU seconds available.  So using 2**12 (4096)
      iterations would mean an attacker with a dozen such computers
      dedicated to a brute-force attack against a single key (actually,
      any password-derived keys sharing the same salt and iteration
      count) would process all the variations of the dictionary entries
      in four months, and on average, would likely find the user's
      password in two months.

      Thus, if this form of attack is of concern, an iteration count a
      few orders of magnitude higher should be chosen, and users should
      be required to change their passwords every few months.  Perhaps

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      several orders of magnitude, since many users will tend to use the
      shorter and simpler passwords (as much as they can get away with,
      given a site's password quality checks) that the attacker would
      likely try first.

      Since this estimate is based on currently available CPU power, the
      iteration counts used for this mode of defense should be increased
      over time, at perhaps 40%-60% each year or so.

      Note that if the attacker has a large amount of storage available,
      intermediate results could be cached, saving a lot of work for the
      next attack with the same salt and a greater number of iterations
      than had been run at the point where the intermediate results were
      saved.  Thus, it would be wise to generate a new random salt
      string when passwords are changed.  The default salt string,
      derived from the principal name, only protects against the use of
      one dictionary of keys against multiple users.

   If the PBKDF2 iteration count can be spoofed by an intruder on the
   network, and the limit on the accepted iteration count is very high,
   the intruder may be able to introduce a form of denial of service
   attack against the client by sending a very high iteration count,
   causing the client to spend a great deal of CPU time computing an
   incorrect key.

   An intruder spoofing the KDC reply, providing a low iteration count,
   and reading the client's reply from the network may be able to reduce
   the work needed in the brute-force attack outlined above.  Thus,
   implementations may wish to enforce lower bounds on the number of
   iterations that will be used.

   Since threat models and typical end-user equipment will vary widely
   from site to site, allowing site-specific configuration of such
   bounds is recommended.

   Any benefit against other attacks specific to the HMAC or SHA-1
   algorithms is probably achieved with a fairly small number of

   In the "optimistic preauthentication" case mentioned in section 3,
   the client may attempt to produce a key without first communicating
   with the KDC.  If the client has no additional information, it can
   only guess as to the iteration count to be used.  Even such
   heuristics as "iteration count X was used to acquire tickets for the
   same principal only N hours ago" can be wrong.  Given the
   recommendation above for increasing the iteration counts used over
   time, it is impossible to recommend any specific default value for
   this case; allowing site-local configuration is recommended.  (If the

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   lower and upper bound checks described above are implemented, the
   default count for optimistic preauthentication should be between
   those bounds.)

   Cipher text stealing mode, since it requires no additional padding in
   most cases, will reveal the exact length of each message being
   encrypted, rather than merely bounding it to a small range of
   possible lengths as in CBC mode.  Such obfuscation should not be
   relied upon at higher levels in any case; if the length must be
   obscured from an outside observer, it should be done by intentionally
   varying the length of the message to be encrypted.

   The author is not a cryptographer.  Caveat emptor.

9. IANA Considerations


10. Acknowledgements

   Thanks to John Brezak, Gerardo Diaz Cuellar, Ken Hornstein, Paul
   Leach, Marcus Watts and others for feedback on earlier versions of
   this document.

A. Sample test vectors

   Sample values for the PBKDF2 HMAC-SHA1 string-to-key function are
   included below.

   Iteration count = 1
   Pass phrase = "password"
   Salt = "ATHENA.MIT.EDUraeburn"
   128-bit PBKDF2 output:
       cd ed b5 28 1b b2 f8 01 56 5a 11 22 b2 56 35 15
   128-bit AES key:
       42 26 3c 6e 89 f4 fc 28 b8 df 68 ee 09 79 9f 15
   256-bit PBKDF2 output:
       cd ed b5 28 1b b2 f8 01 56 5a 11 22 b2 56 35 15
       0a d1 f7 a0 4b b9 f3 a3 33 ec c0 e2 e1 f7 08 37
   256-bit AES key:
       fe 69 7b 52 bc 0d 3c e1 44 32 ba 03 6a 92 e6 5b
       bb 52 28 09 90 a2 fa 27 88 39 98 d7 2a f3 01 61

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   Iteration count = 2
   Pass phrase = "password"
   128-bit PBKDF2 output:
       01 db ee 7f 4a 9e 24 3e 98 8b 62 c7 3c da 93 5d
   128-bit AES key:
       c6 51 bf 29 e2 30 0a c2 7f a4 69 d6 93 bd da 13
   256-bit PBKDF2 output:
       01 db ee 7f 4a 9e 24 3e 98 8b 62 c7 3c da 93 5d
       a0 53 78 b9 32 44 ec 8f 48 a9 9e 61 ad 79 9d 86
   256-bit AES key:
       a2 e1 6d 16 b3 60 69 c1 35 d5 e9 d2 e2 5f 89 61
       02 68 56 18 b9 59 14 b4 67 c6 76 22 22 58 24 ff

   Iteration count = 1200
   Pass phrase = "password"
   Salt = "ATHENA.MIT.EDUraeburn"
   128-bit PBKDF2 output:
       5c 08 eb 61 fd f7 1e 4e 4e c3 cf 6b a1 f5 51 2b
   128-bit AES key:
       4c 01 cd 46 d6 32 d0 1e 6d be 23 0a 01 ed 64 2a
   256-bit PBKDF2 output:
       5c 08 eb 61 fd f7 1e 4e 4e c3 cf 6b a1 f5 51 2b
       a7 e5 2d db c5 e5 14 2f 70 8a 31 e2 e6 2b 1e 13
   256-bit AES key:
       55 a6 ac 74 0a d1 7b 48 46 94 10 51 e1 e8 b0 a7
       54 8d 93 b0 ab 30 a8 bc 3f f1 62 80 38 2b 8c 2a

   Iteration count = 5
   Pass phrase = "password"
   128-bit PBKDF2 output:
       d1 da a7 86 15 f2 87 e6 a1 c8 b1 20 d7 06 2a 49
   128-bit AES key:
       e9 b2 3d 52 27 37 47 dd 5c 35 cb 55 be 61 9d 8e
   256-bit PBKDF2 output:
       d1 da a7 86 15 f2 87 e6 a1 c8 b1 20 d7 06 2a 49
       3f 98 d2 03 e6 be 49 a6 ad f4 fa 57 4b 6e 64 ee
   256-bit AES key:
       97 a4 e7 86 be 20 d8 1a 38 2d 5e bc 96 d5 90 9c
       ab cd ad c8 7c a4 8f 57 45 04 15 9f 16 c3 6e 31
   (This test is based on values given in [PECMS].)

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   Iteration count = 1200
   Pass phrase = (64 characters)
   Salt="pass phrase equals block size"
   128-bit PBKDF2 output:
       13 9c 30 c0 96 6b c3 2b a5 5f db f2 12 53 0a c9
   128-bit AES key:
       59 d1 bb 78 9a 82 8b 1a a5 4e f9 c2 88 3f 69 ed
   256-bit PBKDF2 output:
       13 9c 30 c0 96 6b c3 2b a5 5f db f2 12 53 0a c9
       c5 ec 59 f1 a4 52 f5 cc 9a d9 40 fe a0 59 8e d1
   256-bit AES key:
       89 ad ee 36 08 db 8b c7 1f 1b fb fe 45 94 86 b0
       56 18 b7 0c ba e2 20 92 53 4e 56 c5 53 ba 4b 34

   Iteration count = 1200
   Pass phrase = (65 characters)
   Salt = "pass phrase exceeds block size"
   128-bit PBKDF2 output:
       9c ca d6 d4 68 77 0c d5 1b 10 e6 a6 87 21 be 61
   128-bit AES key:
       cb 80 05 dc 5f 90 17 9a 7f 02 10 4c 00 18 75 1d
   256-bit PBKDF2 output:
       9c ca d6 d4 68 77 0c d5 1b 10 e6 a6 87 21 be 61
       1a 8b 4d 28 26 01 db 3b 36 be 92 46 91 5e c8 2a
   256-bit AES key:
       d7 8c 5c 9c b8 72 a8 c9 da d4 69 7f 0b b5 b2 d2
       14 96 c8 2b eb 2c ae da 21 12 fc ee a0 57 40 1b

   Iteration count = 50
   Pass phrase = g-clef (0xf09d849e)
   Salt = "EXAMPLE.COMpianist"
   128-bit PBKDF2 output:
       6b 9c f2 6d 45 45 5a 43 a5 b8 bb 27 6a 40 3b 39
   128-bit AES key:
       f1 49 c1 f2 e1 54 a7 34 52 d4 3e 7f e6 2a 56 e5
   256-bit PBKDF2 output:
       6b 9c f2 6d 45 45 5a 43 a5 b8 bb 27 6a 40 3b 39
       e7 fe 37 a0 c4 1e 02 c2 81 ff 30 69 e1 e9 4f 52
   256-bit AES key:
       4b 6d 98 39 f8 44 06 df 1f 09 cc 16 6d b4 b8 3c
       57 18 48 b7 84 a3 d6 bd c3 46 58 9a 3e 39 3f 9e

   Some test vectors for CBC with cipher text stealing, using an initial
   vector of all-zero.

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   AES 128-bit key:
       63 68 69 63 6b 65 6e 20 74 65 72 69 79 61 6b 69

       49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
       c6 35 35 68 f2 bf 8c b4 d8 a5 80 36 2d a7 ff 7f

       49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
       20 47 65 6e 65 72 61 6c 20 47 61 75 27 73 20
       fc 00 78 3e 0e fd b2 c1 d4 45 d4 c8 ef f7 ed 22
       97 68 72 68 d6 ec cc c0 c0 7b 25 e2 5e cf e5

       49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
       20 47 65 6e 65 72 61 6c 20 47 61 75 27 73 20 43
       39 31 25 23 a7 86 62 d5 be 7f cb cc 98 eb f5 a8
       97 68 72 68 d6 ec cc c0 c0 7b 25 e2 5e cf e5 84

       49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
       20 47 65 6e 65 72 61 6c 20 47 61 75 27 73 20 43
       68 69 63 6b 65 6e 2c 20 70 6c 65 61 73 65 2c
       97 68 72 68 d6 ec cc c0 c0 7b 25 e2 5e cf e5 84
       b3 ff fd 94 0c 16 a1 8c 1b 55 49 d2 f8 38 02 9e
       39 31 25 23 a7 86 62 d5 be 7f cb cc 98 eb f5

       49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
       20 47 65 6e 65 72 61 6c 20 47 61 75 27 73 20 43
       68 69 63 6b 65 6e 2c 20 70 6c 65 61 73 65 2c 20
       97 68 72 68 d6 ec cc c0 c0 7b 25 e2 5e cf e5 84
       9d ad 8b bb 96 c4 cd c0 3b c1 03 e1 a1 94 bb d8
       39 31 25 23 a7 86 62 d5 be 7f cb cc 98 eb f5 a8

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       49 20 77 6f 75 6c 64 20 6c 69 6b 65 20 74 68 65
       20 47 65 6e 65 72 61 6c 20 47 61 75 27 73 20 43
       68 69 63 6b 65 6e 2c 20 70 6c 65 61 73 65 2c 20
       61 6e 64 20 77 6f 6e 74 6f 6e 20 73 6f 75 70 2e
       97 68 72 68 d6 ec cc c0 c0 7b 25 e2 5e cf e5 84
       39 31 25 23 a7 86 62 d5 be 7f cb cc 98 eb f5 a8
       48 07 ef e8 36 ee 89 a5 26 73 0d bc 2f 7b c8 40
       9d ad 8b bb 96 c4 cd c0 3b c1 03 e1 a1 94 bb d8

Normative References

   [AC] Schneier, B., "Applied Cryptography", second edition, John Wiley
   and Sons, New York, 1996.

   [AES] National Institute of Standards and Technology, U.S. Department
   of Commerce, "Advanced Encryption Standard", Federal Information
   Processing Standards Publication 197, Washington, DC, November 2001.

   [KCRYPTO] Raeburn, K., "Encryption and Checksum Specifications for
   Kerberos 5", draft-ietf-krb-wg-crypto-01.txt, May, 2002.  Work in

   [PKCS5] Kaliski, B., "PKCS #5: Password-Based Cryptography
   Specification Version 2.0", RFC 2898, September 2000.

   [RC5] Baldwin, R, and R. Rivest, "The RC5, RC5-CBC, RC5-CBC-Pad, and
   RC5-CTS Algorithms", RFC 2040, October 1996.

   [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
   3", RFC 2026, October 1996.

   [SHA1] National Institute of Standards and Technology, U.S.
   Department of Commerce, "Secure Hash Standard", Federal Information
   Processing Standards Publication 180-1, Washington, DC, April 1995.

Informative References

   [LEACH] Leach, P., email to IETF Kerberos working group mailing list,
   5 May 2003, ftp://ftp.ietf.org/ietf-mail-archive/krb-wg/2003-05.mail.

   [PECMS] Gutmann, P., "Password-based Encryption for CMS", RFC 3211,
   December 2001.

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Author's Address

   Kenneth Raeburn
   Massachusetts Institute of Technology
   77 Massachusetts Avenue
   Cambridge, MA 02139

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Notes to RFC Editor

   Assuming this document goes through Last Call along with the Kerberos
   crypto framework draft, the reference entry for [KCRYPTO] will list
   the draft name, not the RFC number.  This should be replaced with the
   RFC info.

   Remove Kerberos working group contact info from the Abstract; it's
   right for the draft, but not the final RFC.

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