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Versions: 00                                                            
RADIUS Working Group                                             P. Funk
INTERNET-DRAFT                                               O. Tavakoli
<draft-ietf-radius-saltencrypt-00.txt>               Funk Software, Inc.
                                                               D. Mitton
                                                                  D. Fox
                                                      Bay Networks, Inc.
                                                       November 20, 1997

                  Salt-Encryption of RADIUS Attributes

1. Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), ftp.nordu.net (Europe),
   munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
   ftp.isi.edu (US West Coast).

   This document expires May 25, 1998.

2. Abstract

   This document defines a general mechanism for encrypting attributes
   within RADIUS packets. This mechanism permits more than one attribute
   within a RADIUS transaction (request and response) to be encrypted
   without compromising the security of the encryption.

3. Introduction

   For security reasons, it is necessary to encrypt certain attributes
   that are passed between a NAS and a RADIUS server.

   RADIUS [1] defines a password-hiding mechanism for use with the User-
   Password attribute in an Access-Request; namely, that the Value of
   the attribute is XORed with an octet sequence based on a one-way MD5
   digest of the shared secret and the Request Authenticator.

   This mechanism is not extensible to additional attributes in the
   request packet or the RADIUS server’s response packet without
   compromising the encryption. This is because the first 16 octets of
   the XOR value will be identical for each encryption, allowing an
   attacker who knows the clear text value of any of the encrypted

DRAFT             Salt-Encryption of RADIUS Attributes          11/20/97

   attributes to deduce the common XOR value and decipher the other
   encrypted attributes.

   The mechanism defined here -- called "salt-encryption" -- adds a
   unique two-octet Salt value to each attribute to be encrypted. This
   Salt would be concatenated with the shared secret and Request
   Authenticator as input to the MD5 digest to produce an initial 16-
   byte XOR value that is unique for each encrypted attribute in a
   RADIUS transaction. The initial and subsequent XOR values are used to
   encrypt the payload of the attribute. The length of the actual
   information portion of the attribute MAY be obfuscated by encoding
   the payload with the length of the actual data, followed by the data,
   followed by optional padding.

4. Attribute Format

4.1 Standard Form

   A summary of the standard form of the salt-encrypted attribute format
   is shown below. The fields are transmitted from left to right.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     Type      |    Length     |             Salt              |
   | Encrypted Value ...


      The Type field is a single octet as defined in [1].


      The Length field is a single octet as defined in [1].


      The Salt field is two octets, and is used to differentiate
      encryption keys that are based on the same shared secret and
      Request Authenticator.

      The NAS and the RADIUS server are responsible for ensuring that
      each salt within a single packet is unique. To ensure uniqueness
      across a pair of packets constituting a transaction, each Salt in
      an Access-Request packet sent by the NAS must have high-bit clear,
      and each Salt in an Access-Accept, Access-Reject, or Access-
      Challenge packet returned by the RADIUS server must have high-bit

P. Funk et. al.                                                        2

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   Encrypted Value

      The Encrypted Value field is one or more octets, encrypted
      according to the mechanism described below, containing data that
      is length-prefixed and optionally padded.

      The first octet indicates the number of significant data octets to
      follow, excluding any padding.

      The data that follows the first octet contains the information
      specific to the Attribute.

      Following the data, there may be additional octets of padding that
      carry no information but serve to obfuscate the actual length of
      the data. The technique used may be null-padding up to the next
      multiple of 16 octets (as in the password-hiding mechanism defined
      in [1]), padding by a random number of octets, or some other

4.2 Standard Form for Vendor-Specific Attributes

   A Vendor-Specific attribute consists of an attribute of type 26 that
   contains a Vendor-Id and vendor-defined information. According to
   [1], the vendor-defined information SHOULD consist of a sequence of
   one or more sub-attributes, each of which consists of a Vendor type
   and Vendor length.

   A sub-attribute of a Vendor-Specific attribute may be salt-encrypted
   using a format corresponding to an ordinary attribute. A summary of
   the salt-encrypted Vendor-Specific Attribute format is shown below.
   The fields are transmitted from left to right.

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |     Type      |  Length       |            Vendor-Id
        Vendor-Id (cont)           | Vendor type   | Vendor length |
   |             Salt              |  Encrypted Value ...

   All fields are as defined in [1] or as defined above.

4.3 Alternative Forms

   New attributes may be defined that utilize salt-encryption without
   strictly adhering to the standard formats described above. For
   example, it might be desirable to encrypt only part of an attribute,
   keeping the rest in clear text; or to include multiple salts within
   an attribute to encrypt multiple sub-fields. It may also be desirable

P. Funk et. al.                                                        3

DRAFT             Salt-Encryption of RADIUS Attributes          11/20/97

   to eliminate the length prefix and padding from the Encrypted Value,
   particularly for fixed-length data where length obfuscation provides
   no benefit.

   The exact arrangement of the Salt field and the Encrypted Value field
   within an attribute, and whether the Encrypted Value field utilizes
   length-obfuscation, is a matter to be decided for each new attribute
   as it is defined. It is expected that some, but not all, new
   attributes will follow the standard formats as described above.

   All salt-encrypted attributes MUST at least observe the following
   requirements: Each Salt is 2 octets, unique within the packet, with
   high-bit clear in requests and set in responses, and the
   encryption/decryption follows the method outlined below.

5. Method of encryption/decryption

   The salt-encryption method closely corresponds the password-hiding
   method defined in [1]. The differences are:

   (1) The Salt is concatenated to the shared secret and Request
   Authenticator when computing the initial MD5 digest.

   (2) An attribute may be padded to an arbitrary length or not at all.
   However, in order to obfuscate the actual length of the data, a
   padding strategy, such as null-padding to a multiple of 16 octets,
   SHOULD be employed.

   The salt-encryption method proceeds as follows:

   Construct a clear text version of the information to be encrypted;
   call this the Clear Text.

   Call the shared secret S, the pseudo-random 128-bit Request
   Authenticator RA, and the Salt SALT.  Break the Clear Text into
   chunks p1, p2, etc. of up to 16-octets each; the last chunk may
   contain fewer than 16 octets.  Call the ciphertext blocks c(1), c(2),
   etc. We'll need intermediate values b1, b2, etc.

   b1 = MD5(S + RA + SALT)    c(1) = p1 xor b1
   b2 = MD5(S + c(1))         c(2) = p2 xor b2
                   .                       .
                   .                       .
                   .                       .
   bi = MD5(S + c(i-1))       c(i) = pi xor bi

   Note that if the last chunk is fewer than 16 octets only the first
   part of the final MD5 digest bi is used in the XOR operation.

   The resulting Encrypted Value will contain c(1)+c(2)+...+c(i) where +
   denotes concatenation.

P. Funk et. al.                                                        4

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   On receipt, the process is reversed to yield the Clear Text.

6. Security Considerations

   Security is the subject of this document.

7. Author’s Addresses

   Authors may be contacted by email as follows:

   Paul Funk        paul@funk.com
   Oliver Tavakoli  oliver@funk.com
   Dave Mitton      dmitton@baynetworks.com
   Daniel Fox       dfox@baynetworks.com

8. Expiration Date

   This document expires May 25, 1998.

9. References

   [1]  Rigney, C., et. al., "Remote Access Dial In User Service", RFC
   2138, April 1997

P. Funk et. al.                                                        5

| Paul Funk                    | Tel: +1 617 497 6339         |
| Funk Software, Inc.          | Fax: +1 617 547 1031         |
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