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CAT working group M. Swift
Internet Draft J. Brezak
Document: draft-brezak-win2k-krb-rc4-hmac-00.txt Microsoft
Category: Informational September, 1999
The Windows 2000 RC4-HMAC Kerberos encryption type
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026 [1]. 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."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
1. Abstract
The Windows 2000 implementation of Kerberos introduces a new
encryption type based on the RC4 encryption algorithm and using an
MD5 HMAC for checksum. This is offered as an alternative to using
the existing DES based encryption types.
The RC4-HMAC encryption types are used to ease upgrade of existing
Windows NT environments, provide strong crypto (128-bit key
lengths), and provide exportable (meet United States government
export restriction requirements) encryption.
The Windows 2000 implementation of Kerberos contains new encryption
and checksum types for two reasons: for export reasons early in the
development process, 56 bit DES encryption could not be exported,
and because upon upgrade from Windows NT 4.0 to Windows 2000,
accounts will not have the appropriate DES keying material to do the
standard DES encryption. Furthermore, 3DES is not available for
export, and there was a desired to use a single flavor of encryption
in the product for both US and international products.
As a result, there are two new encryption types and one new checksum
type introduced in Windows 2000.
2. Conventions used in this document
Swift Category - Informational 1
Windows 2000 RC4-HMAC Kerberos E-Type July 1999
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 RFC-2119 [2].
3. Key Generation
On upgrade from existing Windows NT domains, the user accounts would
not have a DES based key available to enable the use of DES base
encryption types specified in RFC 1510. The key used for RC4-HMAC is
the same as the existing Windows NT key for compatibility reasons.
Once the account password is changed, the DES based keys are created
and maintained. Once the DES keys are available DES based encryption
types can be used with Kerberos.
The RC4-HMAC String to key function is defined as follow:
String2Key(password)
K = MD4(UNICODE(password))
The RC4-HMAC keys are generated by using the Windows UNICODE version
of the password. Each Windows UNICODE character is encoded in
little-endian format of 2 octets each. Then performing an MD4 [6]
hash operation on just the UNICODE characters of the password (not
including the terminating zero octets).
4. Basic Operations
The MD5 HMAC function is defined in [3]. It is used in this
encryption type for checksum operations. Refer to [3] for details on
its operation. In this document this function is referred to as
HMAC(Key, Data) returning the checksum using the specified key on
the data.
The basic MD5 hash operation is used in this encryption type and
defined in [7]. In this document this function is referred to as
MD5(Key, Data) returning the checksum using the specified key on the
data.
The basic RC4 encryption operation is used in this encryption type
and defined in [8]. In this document the function is referred to as
RC4(Key, Data) returning the encrypted data using the specified key
on the data.
These encryption types use key derivation as defined in [9] (RFC-
1510BIS) in Section titled "Key Derivation". With each message, the
message type (T) is used as a component of the keying material.
The lengths of ASCII encoded character strings include the trailing
terminator character (0).
The concat(a,b,c,...) function will return the logical concatenation
(left to right) of the values of the arguments.
Swift Category - Informational 2
Windows 2000 RC4-HMAC Kerberos E-Type July 1999
The nonce(n) function returns a pseudo-random number of "n" octets.
5. Checksum Types
There is one checksum type used in this encryption type. The
Kerberos constant for this type is:
#define KERB_CHECKSUM_HMAC_MD5 (-138)
The function is defined as follows:
K - is the Key
T - the message type, encoded as a little-endian four byte integer
CHKSUM(K, T, data)
Ksign = HMAC(K, "signature key") //includes zero octet at end
tmp = MD5(Ksign, concat(T, data))
CHKSUM = HMAC(K, tmp)
6. Encryption Types
There are two encryption types used in these encryption types. The
Kerberos constants for these types are:
#define KERB_ETYPE_RC4_HMAC 23
#define KERB_ETYPE_RC4_HMAC_EXP 24
The basic encryption function is defined as follow:
T = the message type, encoded as a little-endian four byte integer.
ENCRYPT(K, T, data)
if (K.enctype == KERB_ETYPE_RC4_HMAC_EXP)
L = "fiftysixbits" //includes zero octet at end
Else
L = "" // one octet of zero
Ksign = HMAC(K, concat(L, T))
Confounder = nonce(8) // get an 8 octet nonce for a confounder
Checksum = HMAC(Ksign, concat(Confounder, data))
Ke = Ksign
if (L == "fiftysixbits") memset(&Ke[7], 0x0ab, 9)
Ke2 = HMAC(Ke, Checksum)
data = RC4(Ke2, data)
The header field on the encrypted data in KDC messages is:
typedef struct _RC4_MDx_HEADER {
UCHAR Checksum[16];
UCHAR Confounder[8];
} RC4_MDx_HEADER, *PRC4_MDx_HEADER;
Swift Category - Informational 3
Windows 2000 RC4-HMAC Kerberos E-Type July 1999
The character constant "fiftysixbits" evolved from the time when a
56-bit key length was all that was exportable from the United
States. It is now used to recognize that the key length is of
"exportable" length.
7. Key Strength Negotiation
A Kerberos client and server can negotiate over key length if they
are using mutual authentication. If the client is unable to perform
full strength encryption, it may propose a key in the "subkey" field
of the authenticator, using a weaker encryption type. The server
must then either return the same key or suggest its own key in the
subkey field of the AP reply message. The key used to encrypt data
is derived from the key returned by the server. If the client is
able to perform strong encryption but the server is not, it may
propose a subkey in the AP reply without first being sent a subkey
in the authenticator.
8. GSSAPI Kerberos V5 Mechanism Type
8.1 Mechanism Specific Changes
The GSSAPI per-message tokens also require new checksum and
encryption types. The GSS-API per-message tokens must be changed to
support these new encryption types (See [5] Section 1.2.2). The
sealing algorithm identifier (SEAL_ALG) for an RC4 based encryption
is:
Byte 4..5 SEAL_ALG 0x10 0x00 - RC4
The signing algorithm identifier (SGN_ALG) for MD5 HMAC is:
Byte 2..3 SGN ALG 0x11 0x00 - HMAC
The only support quality of protection is:
#define GSS_KRB5_INTEG_C_QOP_DEFAULT 0x0
In addition, when using an RC4 based encryption type, the sequence
number is sent in big-endian rather than little-endian order.
8.2 GSSAPI Checksum Type
The GSSAPI checksum type and algorithm is defined in Section 5. Only
the first 8 octets of the checksum are used. The resulting checksum
is stored in the SGN_CKSUM field (See [5] Section 1.2) for
GSS_GetMIC() and GSS_Wrap(conf_flag=FALSE).
8.3 GSSAPI Encryption Types
There are two encryption types for GSSAPI message tokens, one that
is 128 bits in strength, and one that is 56 bits in strength as
defined in Section 6.
Swift Category - Informational 4
Windows 2000 RC4-HMAC Kerberos E-Type July 1999
All padding is rounded up to 1 byte. One byte is needed to say that
there is 1 byte of padding. The DES based mechanism type uses 8 byte
padding. See [5] Section 1.2.2.3.
The encryption mechanism used for GSS based messages is as follow:
GSS-ENCRYPT(K, T, data)
IV = SND_SEQ
K = XOR(K, 0xf0f0f0f0f0f0f0f0f0f0f0f0f0f0f0)
if (K.enctype == KERB_ETYPE_RC4_HMAC_EXP)
L = "fortybits" //includes zero octet at end
else
L = "" // one octet of zero
Ksign = HMAC(K, concat(L, T))
Ke = Ksign
if (L == "fortybits") memset(&Ke[7], 0x0ab, 9)
Ke2 = HMAC(Ke, IV)
Data = RC4(Ke2, data)
SND_SEQ = RC4(Ke, seq#)
The sequence number (SND_SEQ) and IV are used as defined in [5]
Section 1.2.2.
The character constant "fortybits" evolved from the time when a 40-
bit key length was all that was exportable from the United States.
It is now used to recognize that the key length is of "exportable"
length.
8. Security Considerations
Care must be taken in implementing this encryption type because it
uses a stream cipher. If a different IV isn<73>t used in each direction
when using a session key, the encryption is weak. By using the
sequence number as an IV, this is avoided.
9. References
1 Bradner, S., "The Internet Standards Process -- Revision 3", BCP
9, RFC 2026, October 1996.
2 Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
3 Krawczyk, H., Bellare, M., Canetti, R.,"HMAC: Keyed-Hashing for
Message Authentication", RFC 2104, February 1997
4 Kohl, J., Neuman, C., "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993
5 Linn, J., "The Kerberos Version 5 GSS-API Mechanism", RFC-1964,
June 1996
Swift Category - Informational 5
Windows 2000 RC4-HMAC Kerberos E-Type July 1999
6 R. Rivest, "The MD4 Message-Digest Algorithm", RFC-1320, April
1992
7 R. Rivest, "The MD5 Message-Digest Algorithm", RFC-1321, April
1992
8 RC4 is a proprietary encryption algorithm available under license
from RSA Data Security Inc. For licensing information,
contact:
RSA Data Security, Inc.
100 Marine Parkway
Redwood City, CA 94065-1031
9 Neuman, C., Kohl, J., Ts'o, T., "The Kerberos Network
Authentication Service (V5)", draft-ietf-cat-kerberos-revisions-
04.txt, June 25, 1999
10. Author's Addresses
Mike Swift
Microsoft
One Microsoft Way
Redmond, Washington
Email: mikesw@microsoft.com
John Brezak
Microsoft
One Microsoft Way
Redmond, Washington
Email: jbrezak@microsoft.com
Swift Category - Informational 6
Windows 2000 RC4-HMAC Kerberos E-Type July 1999
11. 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."
Swift Category - Informational 7