Kerberos Working Group K. Raeburn
Document: draft-ietf-krb-wg-crypto-00.txt MIT
January 5, 2002
expires July 5, 2002
Encryption and Checksum Specifications
for Kerberos 5
Abstract
The Kerberos protocol [Kerb] uses cryptography to protect messages of
various sizes, using stream encryption ciphers, or more commonly,
block encryption ciphers with block chaining.
This document describes a framework for defining encryption and
checksum mechanisms, defining an abstraction layer between the
Kerberos protocol and related protocols, and the actual mechanism
specifications. This should allow either side to be extended more
cleanly without requiring changes to the other.
Several mechanisms are also defined in this document. Some are taken
from RFC 1510, modified in form to fit this new framework, and
occasionally modified in content when the old specification was
incorrect. Some new mechanisms are presented here as well. No
requirements for implementation of specific mechanisms are made here.
Status
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
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.html.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
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Table of Contents
Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Table of Contents . . . . . . . . . . . . . . . . . . . . . . . . . 2
Work Still Needed . . . . . . . . . . . . . . . . . . . . . . . . . 3
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1. Concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Encryption mechanism attributes . . . . . . . . . . . . . . . . . 5
3. Checksum mechanism attributes . . . . . . . . . . . . . . . . . . 8
4. Simplified profile for CBC-mode ciphers with key derivation . . . 9
4.1. A key derivation function [Horowitz] . . . . . . . . . . . . . 10
4.2. Simplified profile parameters . . . . . . . . . . . . . . . . . 12
4.3. Cryptosystem profile based on simplified profile . . . . . . . 13
4.4. Checksum profiles based on simplified profile . . . . . . . . . 15
5. Profiles for Kerberos encryption systems . . . . . . . . . . . . 15
5.1. null . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.2. DES-based encryption systems . . . . . . . . . . . . . . . . . 17
5.3. Triple-DES Encryption with Key Derivation . . . . . . . . . . . 23
6. Profiles for Kerberos checksums . . . . . . . . . . . . . . . . . 25
6.1. RSA MD4 Cryptographic Checksum Using DES . . . . . . . . . . . 25
6.2. The RSA MD5 Checksum . . . . . . . . . . . . . . . . . . . . . 26
6.3. RSA MD5 Cryptographic Checksum Using DES . . . . . . . . . . . 26
6.4. The CRC-32 Checksum . . . . . . . . . . . . . . . . . . . . . . 27
6.5. The RSA MD4 Checksum . . . . . . . . . . . . . . . . . . . . . 28
6.6. DES CBC checksum . . . . . . . . . . . . . . . . . . . . . . . 28
6.7. RSA MD4 Cryptographic Checksum Using DES alternative . . . . . 29
6.8. DES CBC checksum alternative . . . . . . . . . . . . . . . . . 30
6.9. The HMAC-SHA1-DES3-KD Checksum . . . . . . . . . . . . . . . . 30
7. Use of Kerberos encryption outside this specification . . . . . . 30
8. Assigned Numbers . . . . . . . . . . . . . . . . . . . . . . . . 31
9. Notes to Implementors . . . . . . . . . . . . . . . . . . . . . . 33
10. Security Considerations . . . . . . . . . . . . . . . . . . . . 33
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 33
12. Editor's address . . . . . . . . . . . . . . . . . . . . . . . . 34
13. Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 34
A. Test vectors . . . . . . . . . . . . . . . . . . . . . . . . . . 35
A.1. n-fold . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
A.2. mit_des_string_to_key . . . . . . . . . . . . . . . . . . . . . 35
A.3. DES3 DR and DK . . . . . . . . . . . . . . . . . . . . . . . . 37
A.4. DES3string_to_key . . . . . . . . . . . . . . . . . . . . . . . 38
A.5. DES3 combine-keys . . . . . . . . . . . . . . . . . . . . . . . 39
A.6. Modified CRC-32 . . . . . . . . . . . . . . . . . . . . . . . . 39
Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39
Normative References . . . . . . . . . . . . . . . . . . . . . . . . 40
Informative References . . . . . . . . . . . . . . . . . . . . . . . 41
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Work Still Needed
More talking with cryptographers about the "combine-keys" function in
the simplified profile. I've been talking a little with Uri
Blumenthal, but he hasn't had a lot of time for this.
More detailed list of differences from RFC 1510, to update the
"Significant changes" appendix.
Are sections 2 and 3 what we want to recommend for external use in
section 7, or just a subset?
Look up reference to Bellovin paper on CBC mode use of key as IV
being a bad idea.
Fix anything marked with "@@".
Fix up references section.
Encoding of strings?
Someone remind me, why does get_mic have to produce a fixed-size
output?
Guidelines for mechanism designers for what to document -- something
akin to section 4 of RFC 2411 would make sense.
Introduction
@@ needs update for separation from kerberos-revisions
The Kerberos protocols are designed to encrypt blocks of arbitrary
sizes, using stream encryption ciphers, or more commonly, block
encryption ciphers such as the Data Encryption Standard [DES77] in
conjunction with block chaining and checksum methods [DESM80].
Encryption is used to prove the identities of the network entities
participating in message exchanges. The Key Distribution Center for
each realm is trusted by all principals registered in that realm to
store a secret key in confidence. Proof of knowledge of this secret
key is used to verify the authenticity of a principal.
The Kerberos protocols generally assume that the encryption used is
secure from cryptanalysis; however, in some cases, the order of
fields in the encrypted portions of messages as defined in this
section is chosen to mitigate somewhat the effects of poorly chosen
keys under certain types of cryptographic attacks. It is still
important to choose good keys. If keys are derived from user-typed
passwords, those passwords need to be well chosen to make brute force
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attacks more difficult. Poorly chosen keys still make easy targets
for intruders.
The following sections specify the encryption and checksum mechanisms
currently defined for Kerberos, as well as a framework for defining
future mechanisms. The encodings, chaining, padding and other
requirements for each are described. Test vectors for several
functions are given in appendix A.
1. Concepts
Both encryption and checksum mechanisms are defined in terms of
profiles, detailed in later sections. Each specifies a collection of
operations and attributes that must be defined for a mechanism. A
Kerberos encryption or checksum mechanism specification is not
complete if it does not define all of these operations and
attributes.
An encryption mechanism must provide for confidentiality and
integrity of the original plaintext. (Integrity checking may be
achieved by incorporating a checksum, if the encryption mode does not
provide an integrity check itself.) It must also provide non-
malleability [Bellare98, Dolev91]. Use of a random confounder
prepended to the plaintext is recommended. It should not be possible
to determine if two ciphertexts correspond to the same plaintext,
without knowledge of the key.
A checksum mechanism [1] must provide proof of the integrity of the
associated message, and must preserve the confidentiality of the
message in case it is not sent in the clear. It should be infeasible
to find two plaintexts which have the same checksum. It is NOT
required that an eavesdropper be unable to determine if two checksums
are for the same message; it is assumed that the messages themselves
will be visible to any such eavesdropper.
Due to advances in cryptography, it is considered unwise by some
cryptographers to use the same key for multiple purposes
[@@reference??]. Since keys are used in performing a number of
different functions in Kerberos, it is desirable to use different
keys for each of these purposes, even though we start with a single
long-term or session key.
We do this by enumerating the different uses of keys within Kerberos,
and making the "usage number" an input to the encryption or checksum
mechanisms; this enumeration is outside the scope of this document.
Later sections of this document define simplified profile templates
for encryption and checksum mechanisms that use a key derivation
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function applied to a CBC mode (or similar) cipher and a checksum or
hash algorithm.
We distinguish the "base key" specified by other documents from the
"specific key" to be used for a particular instance of encryption or
checksum operations. It is expected but not required that the
specific key will be one or more separate keys derived from the
original protocol key and the key usage number. The specific key
should not be explicitly referenced outside of this document. The
typical language used in other documents should be something like,
"encrypt this octet string using this key and this usage number";
generation of the specific key and cipher state (described in the
next section) are implicit. (The creation of a new cipher-state
object, or the re-use of one from a previous encryption operation,
may also be explicit.)
New protocols defined in terms of the Kerberos encryption and
checksum types should use their own key usage values. Key usages are
unsigned 32 bit integers; zero is not permitted.
2. Encryption mechanism attributes
An encryption mechanism profile must define the following attributes
and operations. The operations must be defined as functions in the
mathematical sense: no additional or implicit inputs (such as
Kerberos principal names or message sequence numbers) are permitted.
protocol key format
This describes what octet string values represent valid keys. For
encryption mechanisms that don't have perfectly dense key spaces,
this will describe the representation used for encoding keys. It
need not describe specific values that are not valid or desirable
for use; such values should be avoid by all key generation
routines.
specific key structure
This is not a protocol format at all, but a description of the
keying material derived from the chosen key and used to encrypt or
decrypt data or compute or verify a checksum. It may, for
example, be a single key, a set of keys, or a combination of the
original key with additional data. The authors recommend using
one or more keys derived from the original key via one-way
functions.
required checksum mechanism
This indicates a checksum mechanism that must be available when
this encryption mechanism is used. Since Kerberos has no built in
mechanism for negotiating checksum mechanisms, once an encryption
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mechanism has been decided upon, the corresponding checksum
mechanism can simply be used.
key-generation seed length, K
This is the length of the random bitstring needed to generate a
key with the encryption scheme's random-to-key function (described
below). This must be a fixed value so that various techniques for
producing a random bitstring of a given length may be used with
key generation functions.
key generation functions
Keys must be generated in a number of cases, from different types
of inputs. All function specifications must indicate how to
generate keys in the proper wire format, and must avoid generation
of "weak" keys if the cryptosystem has such. Entropy from each
source should be preserved as much as possible. Many of the
inputs, while unknown, may be at least partly predictable (e.g., a
password string is likely to be entirely in the ASCII subset and
of fairly short length in many environments; a semi-random string
may include timestamps); the benefit of such predictability to an
attacker must be minimized.
string-to-key (UTF8String, UTF8String, params)->(protocol-key)
This function generates a key from two UTF-8 strings and an
integer. One of the strings is normally the principal's
password, but is in general merely a secret string. The other
string is a "salt" string intended to produce different keys
from the same password for different users or realms. The
third argument, "params", may be used to pass mechanism-
specific parameters in to this function. Since doing so
implies knowledge of the specific encryption system, it is
intended that this be an uncommon operation done only through
special administrative interfaces, and that normal Kerberos
applications be able to treat this parameter block as an opaque
object.
This should be a one-way function, so that compromising a
user's key in one realm does not compromise the user's key in
another realm, even if the same password (but a different salt
string) is used.
random-to-key (bitstring[K])->(protocol-key)
This function generates a key from a random bit string of a
specific size. It may be assumed that all the bits of the
input string are equally random, even though the entropy
present in the random source may be limited.
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combine-keys (protocol-key, protocol-key)->(protocol-key)
This function takes two input keys and produces a new key.
This function is not used in the basic Kerberos protocol, but
may be used by preauthentication methods or other applications
to be defined later. This operation must be commutative; this
requirement lets us specify "combine keys A and B" in other
documents without worrying about ordering.
key-derivation (protocol-key, integer)->(specific-key)
In this function, the integer input is the key usage value as
described above; the usage values must be assumed to be known
to an attacker. For cryptosystems with dense key spaces, this
function should be something like the key derivation function
outlined in section 1.
default string-to-key parameters (octet string)
This default value for the "params" argument to the string-to-key
function is to be used when the application protocol (Kerberos or
otherwise) does not explicitly set the parameter value. As
indicated above, this parameter block should be treated as an
opaque object in most cases.
cipher state
initial cipher state (specific-key)->(state)
This describes any initial state setup needed before encrypting
arbitrary amounts of data with a given specific key; the specific
key must be the only input needed for this initialization. For
example, a block cipher used in CBC mode must specify an initial
vector. (For security reasons, the key itself should not be used
as the IVEC.) This data may be carried over from one encryption
or decryption operation to another. Unless otherwise specified,
however, each encryption or decryption operation in this RFC uses
a freshly initialized state and is thus independent of all other
encryptions and decryptions.
This state should be treated as opaque in any uses outside of an
encryption algorithm definition.
encrypt (specific-key, state, bytestring)->(state, bytestring)
This function takes the specific key, cipher state, and plaintext
as input, and generates ciphertext and a new cipher state as
outputs. If the basic encryption algorithm itself does not
provide for integrity protection (as DES in CBC mode does not do),
then some form of MAC or checksum must be included that can be
verified by the receiver. Some random factor such as a confounder
should be included so that an observer cannot know if two messages
contain the same plaintext, even if the cipher state and specific
keys are the same. The exact length of the plaintext need not be
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encoded, but if it is not and if padding is required, the padding
must be added at the end of the string so that the decrypted
version may be parsed from the beginning.
The specification of the encryption function must not only
indicate the precise contents of the output bytestring, but also
the output cipher state, if that state is not empty. The
application protocol may carry forward the output cipher state
from one encryption with a given specific key to another; the
effect of this "chaining" must be defined, even if only to say
that it has no effect.
Assuming correctly-produced values for the specific key and cipher
state, no input byte string may result in an error indication.
decrypt (specific-key, state, bytestring)->(state, bytestring)
This function takes the specific key, cipher state, and ciphertext
as inputs, and verifies the integrity of the supplied ciphertext.
If the ciphertext's integrity is intact, this function produces
the plaintext and a new cipher state as outputs; otherwise, an
error indication must be returned, and the data discarded.
The result of the decryption may be longer than the original
plaintext, if the encryption mode requires padding to a multiple
of a block size. If this is the case, any extra padding will be
after the decoded plaintext. An application protocol which needs
to know the exact length of the message must encode a length or
recognizable "end of message" marker within the plaintext. [2]
As with the encryption function, a correct specification for this
function must indicate not only the contents of the output byte
string, but also the resulting cipher state.
These operations and attributes are all that should be required to
support Kerberos and various proposed preauthentication schemes.
3. Checksum mechanism attributes
A checksum mechanism profile must define the following attributes and
operations:
associated encryption algorithm(s)
This essentially indicates the type of encryption key this
checksum mechanism can be used with. A single checksum mechanism
may have more than one associated encryption algorithm if they
share the same wire key format, string-to-key function, and key
derivation function. (This combination means that, for example, a
checksum type and password are adequate to get the specific key
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used to compute a checksum.)
get_mic function
This function generates a MIC token for a given specific key (see
section 2), and message (represented as an octet string), that may
be used to verify the integrity of the associated message. This
function is not required to return the same deterministic result
on every use; it need only generate a token that the verify_mic
routine can check.
The output of this function will also dictate the size of the
checksum. It must be a fixed size.
verify_mic function
Given a specific key, message, and MIC token, this function
ascertains whether the message integrity has been compromised.
For a deterministic get_mic routine, the corresponding verify_mic
may simply generate another checksum and compare them.
The get_mic and verify_mic operations must be able to handle inputs
of arbitrary length; if any padding is needed, the padding scheme
must be specified as part of these functions.
These operations and attributes are all that should be required to
support Kerberos and various proposed preauthentication schemes.
4. Simplified profile for CBC-mode ciphers with key derivation
The profile outlines in sections 2 and 3 describes a large number of
operations that must be defined for encryption and checksum
algorithms to be used with Kerberos. We describe here a simpler
profile from which both encryption and checksum mechanism definitions
can be generated, filling in uses of key derivation in appropriate
places, providing integrity protection, and defining multiple
operations for the cryptosystem profile based on a smaller set of
operations given in the simplified profile. Not all of the existing
cryptosystems for Kerberos fit into this simplified profile, but we
recommend that future cryptosystems use it or something based on it.
[3]
Not all of the operations in the complete profiles are defined
through this mechanism; several must still be defined for each new
algorithm pair.
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4.1. A key derivation function [Horowitz]
Rather than define some scheme by which a "protocol key" is composed
of a large number of encryption keys, we use keys derived from a base
key to perform cryptographic operations. The base key must be used
only for generating the derived keys, and this derivation must be
non-invertible and entropy-preserving. Given these restrictions,
compromise of one derived key does not compromise the other subkeys.
Attack of the base key is limited, since it is only used for
derivation, and is not exposed to any user data.
Since the derived key has as much entropy as the base keys (if the
cryptosystem is good), password-derived keys have the full benefit of
all the entropy in the password.
To generate a derived key from a base key, we generate a pseudorandom
byte string, using an algorithm DR described below, and generate a
key from that byte string using a function dependent on the
encryption algorithm; the input length needed for that function,
which is also dependent on the encryption algorithm, dictates the
length of the string to be generated by the DR algorithm (the value
"k" below).
Derived Key = DK(Base Key, Well-Known Constant)
DK(Key, Constant) = random-to-key(DR(Key, Constant))
DR(Key, Constant) = k-truncate(E(Key, Constant))
Here DR is the random-byte generation function described below, and
DK is the key-derivation function produced from it. In this
construction, E(Key, Plaintext) is a block cipher, Constant is a
well-known constant determined by the specific usage of this
function, and k-truncate truncates its argument by taking the first k
bits. Here, k is the key generation seed length needed for the
encryption system.
The output of the DR function is a string of bits; the actual key is
produced by applying the cryptosystem's random-to-key operation on
this bitstring.
If the output of E is shorter than k bits, then some entropy in the
key will be lost. If the Constant is smaller than the block size of
E, then it must be padded so it may be encrypted.
In either of these situations, a variation of the above construction
is used, where the folded Constant is encrypted, and the resulting
output is fed back into the encryption as necessary (the | indicates
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concatentation):
K1 = E(Key, n-fold(Constant))
K2 = E(Key, K1)
K3 = E(Key, K2)
K4 = ...
DR(Key, Constant) = k-truncate(K1 | K2 | K3 | K4 ...)
n-fold is an algorithm which takes m input bits and ``stretches''
them to form n output bits with equal contribution from each input
bit to the output, as described in [Blumenthal96]:
We first define a primitive called n-folding, which takes a
variable-length input block and produces a fixed-length output
sequence. The intent is to give each input bit approximately
equal weight in determining the value of each output bit. Note
that whenever we need to treat a string of bytes as a number, the
assumed representation is Big-Endian -- Most Significant Byte
first.
To n-fold a number X, replicate the input value to a length that
is the least common multiple of n and the length of X. Before
each repetition, the input is rotated to the right by 13 bit
positions. The successive n-bit chunks are added together using
1's-complement addition (that is, with end-around carry) to yield
a n-bit result....
Test vectors for n-fold are supplied in Appendix A. [4]
In this document, n-fold is always used to produce n bits of output,
where n is the block size of E.
The size of the Constant must not be larger than the block size of E,
because reducing the length of the Constant by n-folding can cause
collisions.
If the size of the Constant is smaller than the block size of E, then
the Constant must be n-folded to the block size of E. This string is
used as input to E. If the block size of E is less than the key
size, then the output from E is taken as input to a second invocation
of E. This process is repeated until the number of bits accumulated
is greater than or equal to the key size of E. When enough bits have
been computed, the first k are taken as the derived key.
Since the derived key is the result of one or more encryptions in the
base key, deriving the base key from the derived key is equivalent to
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determining the key from a very small number of plaintext/ciphertext
pairs. Thus, this construction is as strong as the cryptosystem
itself.
4.2. Simplified profile parameters
These are the operations and attributes that must be defined:
protocol key format
string-to-key function
default string-to-key parameters
key-generation seed length, k
random-to-key function
As above for the normal encryption mechanism profile.
unkeyed hash algorithm, H
This should be a collision-resistant hash algorithm such as SHA-1,
suitable for use in an HMAC. It must support inputs of arbitrary
length.
encryption block size, n
encryption/decryption functions, E and D
These are basic encryption and decryption functions for messages
of sizes that are multiples of the block size. No integrity
checking or confounder should be included here. They take as
input the IV or similar data, a protocol-format key, and a byte
string, returning a new IV and byte string.
The encryption function is not required to use CBC mode, but is
assumed to be using something with similar properties. In
particular, prepending a one-block confounder to the plaintext
should alter the entire ciphertext (comparable to choosing and
including a random initial vector for CBC mode).
While there are still a number of properties to specify, they are
fewer and simpler than in the full profile.
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4.3. Cryptosystem profile based on simplified profile
cryptosystem from simplified profile
----------------------------------------------------------------------
protocol key format As given.
specific key structure Three protocol-format keys: { Kc, Ke, Ki }.
key-generation seed As given.
length
required checksum The checksum mechanism defined by the
mechanism simplified checksum profile given later.
cipher state CBC initial vector (one block), initialized
to all zero.
encryption function The ciphertext output is the concatenation
of the output of the basic encryption
function E and an HMAC using the specified
hash function H, both applied to the padded
plaintext with a confounder:
C1 = E(Ke, conf | plaintext | pad)
H1 = HMAC(Ki, conf | plaintext | pad)
ciphertext = C1 | H1
newstate.ivec = last block of C1
One block of random confounder data is
prepended to the plaintext, and padding
added to the end to bring the length to a
multiple of the encryption algorithm's
block size. The initial vector for
encryption is supplied by the cipher state,
and the last block of the output of E is
the new IVEC for the new cipher state.
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cryptosystem from simplified profile
----------------------------------------------------------------------
decryption function Decryption is performed by extracting the
encrypted portion of the ciphertext,
decrypting using key Ke from the specific
key, and verifying the HMAC. If the HMAC
is incorrect, an error must be reported.
Otherwise, the confounder and padding are
discarded and the remaining plaintext
returned. As with encryption, the cipher
state input indicates the IVEC to use, and
the last block of the encrypted portion of
the ciphertext is put into the new cipher
state to be used as the next IVEC.
default string-to-key As given.
params
key generation functions:
string-to-key function As given.
random-to-key function As given.
combine-keys function @@ Needs to be specified. How about:
combine-keys(K1,K2)
/* First, protect original keys against
exposure with DR. */
R1 = DR(K1, n-fold(K2)) /* length k */
R2 = DR(K2, n-fold(K1)) /* length k */
/* Using k-fold on length 2k means
just add with wrap-around carry. */
rnd = k-fold(R1 | R2)
tkey = random-to-key(rnd)
key = DK(tkey, CombineConstant)
Here CombineConstant is the byte string
{0x63 0x6f 0x6d 0x62 0x69 0x6e 0x65}
corresponding to the ASCII encoding of the
string "combine".
@@ Need a cryptographer to review this.
Asked Uri Blumenthal, he said he'd look it
over when he has time. Have some
suggestions from him, not incorporated yet.
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cryptosystem from simplified profile
----------------------------------------------------------------------
key-derivation function Three keys are generated, using the DK
function described above, and the key usage
number, represented as a 32-bit integer in
big-endian byte order. One is used for
generating checksums only; the other two
are used for encrypting and integrity
protection for ciphertext. These keys are
generated as follows:
Kc = DK(base-key, usage | 0x99);
Ke = DK(base-key, usage | 0xAA);
Ki = DK(base-key, usage | 0x55);
4.4. Checksum profiles based on simplified profile
When an encryption system is defined using the simplified profile
given in section 4.2, a checksum algorithm may be defined for it as
follows:
checksum mechanism from simplified profile
----------------------------------------------
associated cryptosystem as defined above
get_mic HMAC(Kc, message)
verify_mic get_mic and compare
The HMAC function and key Kc are as described in section 4.3.
5. Profiles for Kerberos encryption systems
These are the currently defined encryption systems for Kerberos. The
astute reader will notice that some of them do not fulfill all of the
requirements outlined above. These weaker encryption systems are
defined for backwards compatibility; newer implementations should
attempt to make use of the stronger encryption systems when possible.
The full list of current encryption type number assignments is given
in section 8.
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5.1. null
If no encryption is in use, the encryption system is said to be the
NULL encryption system. In the NULL encryption system there is no
checksum, confounder or padding. The ciphertext is simply the
plaintext. The null Key is used by the null encryption system and is
zero octets in length.
This encryption system should not be used for protection of data. It
exists primarily to associate with the rsa-md5 checksum type, but may
also be useful for testing protocol implementations.
null
------------------------------------------------
protocol key format zero-length bit string
specific key structure empty
required checksum rsa-md5
mechanism
key-generation seed 0
length
cipher state none
initial cipher state none
encryption function identity
decryption function identity, no integrity
check
default string-to-key none
params
key generation functions:
string-to-key empty string
random-to-key empty string
combine-keys empty string
key-derivation empty string
The null encryption algorithm is assigned the etype value zero (0).
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5.2. DES-based encryption systems
These encryption systems encrypt information under the Data
Encryption Standard [DES77] using the cipher block chaining mode
[DESM80]. A checksum is computed as described below and placed in
the cksum field. DES blocks are 8 bytes. As a result, the data to
be encrypted (the concatenation of confounder, checksum, and message)
must be padded to an 8 byte boundary before encryption. The values
of the padding bytes are unspecified.
Plaintext and DES ciphtertext are encoded as blocks of 8 octets which
are concatenated to make the 64-bit inputs for the DES algorithms.
The first octet supplies the 8 most significant bits (with the
octet's MSbit used as the DES input block's MSbit, etc.), the second
octet the next 8 bits, ..., and the eighth octet supplies the 8 least
significant bits.
Encryption under DES using cipher block chaining requires an
additional input in the form of an initialization vector; this vector
is specified for each encryption system, below.
The DES specifications identify some 'weak' and 'semi-weak' keys;
those keys shall not be used for encrypting messages for use in
Kerberos. Additionally, because of the way that keys are derived for
the encryption of checksums, keys shall not be used that yield 'weak'
or 'semi-weak' keys when eXclusive-ORed with the hexadecimal constant
0xF0F0F0F0F0F0F0F0.
A DES key is 8 octets of data. This consists of 56 bits of actual
key data, and 8 parity bits, one per octet. The key is encoded as a
series of 8 octets written in MSB-first order. The bits within the
key are also encoded in MSB order. For example, if the encryption
key is (B1,B2,...,B7,P1,B8,...,B14,P2,B15,...,B49,P7,B50,...,B56,P8)
where B1,B2,...,B56 are the key bits in MSB order, and P1,P2,...,P8
are the parity bits, the first octet of the key would be
B1,B2,...,B7,P1 (with B1 as the MSbit). See the [DESM80]
introduction for reference.
Encryption data format
The format for the data to be encrypted includes a one-block
confounder, a checksum, the encoded plaintext, and any necessary
padding, as described in the following diagram. The msg-seq field
contains the part of the protocol message described in section 5
which is to be encrypted.
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+-----------+----------+---------+-----+
|confounder | checksum | msg-seq | pad |
+-----------+----------+---------+-----+
One generates a random confounder of one block, placing it in
confounder; zeroes out the checksum field (of length appropriate to
exactly hold the checksum to be computed); calculates the appropriate
checksum over confounder, check, and msg-seq, placing the result in
check; adds the necessary padding; then encrypts using the specified
encryption type and the appropriate key.
String to key transformation
To generate a DES key from a UTF-8 text string (password), a "salt"
is concatenated to the text string, and then padded with ASCII nulls
to an 8 byte boundary.
This string is then fan-folded and eXclusive-ORed with itself to form
an 8 byte DES key. Before eXclusive-ORing a block, every byte is
shifted one bit to the left to leave the lowest bit zero. The key is
the "corrected" by correcting the parity on the key, and if the key
matches a 'weak' or 'semi-weak' key as described in the DES
specification, it is eXclusive-ORed with the constant
0x00000000000000F0. This key is then used to generate a DES CBC
checksum on the initial string (with the salt appended). The result
of the CBC checksum is the "corrected" as described above to form the
result which is return as the key.
Pseudocode follows:
key_correction(key) {
fixparity(key);
if (is_weak_key_key(key))
key = key XOR 0xF0;
return(key);
}
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mit_des_string_to_key(string,salt) {
odd = 1;
s = string | salt;
tempkey = NULL;
pad(s); /* with nulls to 8 byte boundary */
for (8byteblock in s) {
if (odd == 0) {
odd = 1;
reverse(8byteblock)
}
else odd = 0;
left shift every byte in 8byteblock one bit;
tempkey = tempkey XOR 8byteblock;
}
tempkey = key_correction(tempkey);
key = key_correction(DES-CBC-check(s,tempkey));
return(key);
}
des_string_to_key(string,salt,params) {
if (length(params) == 0)
type = 0;
else if (length(params) == 1)
type = params[0];
else
error("invalid params");
if (type == 0)
mit_des_string_to_key(string,salt);
else if (type == 1)
afs_des_string_to_key(string,salt);
else
error("invalid params");
}
The AFS string-to-key algorithm is not defined here, but a parameter
block containing a byte value of one (1) is reserved for its use.
5.2.1. DES with MD5
The des-cbc-md5 encryption mode encrypts information under DES in CBC
mode with an all-zero initial vector, with an MD5 checksum (described
in [MD5-92]) computed and placed in the checksum field.
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The encryption system parameters for des-cbc-md5 are:
des-cbc-md5
--------------------------------------------------------------------
protocol key format 8 bytes, parity in low bit of each
specific key structure copy of original key
required checksum rsa-md5-des
mechanism
key-generation seed 8 bytes
length
cipher state 8 bytes (CBC initial vector)
initial cipher state all-zero
encryption function des-cbc(confounder | checksum | msg | pad,
ivec=oldstate)
where
checksum = md5(confounder | 0000... | msg)
newstate = last block of des-cbc output
decryption function decrypt encrypted text and verify checksum
newstate = last block of ciphertext
default string-to-key empty string
params
key generation functions:
string-to-key des_string_to_key
random-to-key copy input, then fix parity bits (discards
low bit of each input byte)
combine-keys bitwise XOR, then fix parity bits
key-derivation identity
The des-cbc-md5 encryption type is assigned the etype value three
(3).
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5.2.2. DES with MD4
The des-cbc-md4 encryption mode also encrypts information under DES
in CBC mode, with an all-zero initial vector. An MD4 checksum
(described in [MD4-92]) is computed and placed in the checksum field.
des-cbc-md4
--------------------------------------------------------------------
protocol key format 8 bytes, parity in low bit of each
specific key structure copy of original key
required checksum rsa-md4-des
mechanism
key-generation seed 8 bytes
length
cipher state 8 bytes (CBC initial vector)
initial cipher state all-zero
encryption function des-cbc(confounder | checksum | msg | pad,
ivec=oldstate)
where
checksum = md4(confounder | 0000... | msg)
newstate = last block of des-cbc output
decryption function decrypt encrypted text and verify checksum
newstate = last block of ciphertext
default string-to-key empty string
params
key generation functions:
string-to-key des_string_to_key
random-to-key copy input, then fix parity bits
combine-keys bitwise XOR, then fix parity bits
key-derivation identity
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The des-cbc-md4 encryption algorithm is assigned the etype value two
(2).
5.2.3. DES with CRC
The des-cbc-crc encryption type uses DES in CBC mode, with a 4-octet
CRC-based checksum computed as described in section 6.4. (Note that
this is not a standard CRC-32 checksum, but a slightly modified one.)
Unless otherwise specified, the key should be used as the
initialization vector, unlike for the other Kerberos DES encryption
schemes. The other details of the encryption of this data are
identical to those for the des-cbc-md5 encryption mode.
Note that, since the CRC-32 checksum is not collision-proof, an
attacker could use a probabilistic chosen-plaintext attack to
generate a valid message even if a confounder is used [SG92]. The
use of collision-proof checksums is recommended for environments
where such attacks represent a significant threat.
des-cbc-crc
--------------------------------------------------------------------
protocol key format 8 bytes, parity in low bit of each
specific key structure copy of original key
required checksum rsa-md5-des
mechanism
key-generation seed 8 bytes
length
cipher state 8 bytes (CBC initial vector)
initial cipher state copy of original key
encryption function des-cbc(confounder | checksum | msg | pad,
ivec=oldstate)
where
checksum = crc(confounder | 00000000
| msg)
newstate = last block of des-cbc output
decryption function decrypt encrypted text and verify checksum
newstate = last block of ciphertext
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des-cbc-crc
--------------------------------------------------------------------
default string-to-key empty string
params
key generation functions:
string-to-key des_string_to_key
random-to-key copy input, then fix parity bits
combine-keys bitwise XOR, then fix parity bits
key-derivation identity
The des-cbc-crc encryption algorithm is assigned the etype value one
(1).
5.3. Triple-DES Encryption with Key Derivation
This encryption type is based on the Triple DES cryptosystem in
Outer-CBC mode, and the HMAC-SHA1 [Krawczyk96] message authentication
algorithm.
A Triple DES key is the concatenation of three DES keys as described
above for des-cbc-md5. A Triple DES key is generated from random
data by creating three DES keys from separate sequences of random
data.
EncryptedData using this type must be generated as described in
section 4.3. If the length of the input data is not a multiple of
the block size, zero octets must be used to pad the plaintext to the
next eight-octet boundary. The counfounder must be eight random
octets (one block).
The simplified profile for Triple DES, with key derivation as defined
in section 4, is as follows:
des3-cbc-hmac-sha1-kd
------------------------------------------------
protocol key format 24 bytes, parity in low
bit of each
key-generation seed 21 bytes
length
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des3-cbc-hmac-sha1-kd
------------------------------------------------
hash function SHA-1
block size 8 bytes
default string-to-key none
params
encryption decryption functions
key generation functions:
random-to-key see below
string-to-key DES3string-to-key (see
below)
The des3-cbc-hmac-sha1-kd encryption type is assigned the value
sixteen (16).
5.3.1. Triple DES Key Production (random-to-key, string-to-key)
The 168 bits of random key data are converted to a protocol key value
as follows. First, the 168 bits are divided into three groups of 56
bits, which are expanded individually into 64 bits as follows:
1 2 3 4 5 6 7 p
9 10 11 12 13 14 15 p
17 18 19 20 21 22 23 p
25 26 27 28 29 30 31 p
33 34 35 36 37 38 39 p
41 42 43 44 45 46 47 p
49 50 51 52 53 54 55 p
56 48 40 32 24 16 8 p
The "p" bits are parity bits computed over the data bits. The output
of the three expansions are concatenated to form the protocol key
value.
When the HMAC-SHA1 of a string is computed, the key is used in the
protocol key form.
The string-to-key function is used to tranform UTF-8 passwords into
DES3 keys. The DES3 string-to-key function relies on the "N-fold"
algorithm and DK function, described in section 4.
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The n-fold algorithm is applied to the password string concatenated
with a salt value. For 3-key triple DES, the operation will involve
a 168-fold of the input password string, to generate an intermediate
key, from which the user's long-term key will be derived with the DK
function. The DES3 string-to-key function is shown here in
pseudocode:
DES3string-to-key(passwordString, salt, params)
if (params != emptyString)
error("invalid params");
s = passwordString + salt
tmpKey = random-to-key(168-fold(s))
key = DK (tmpKey, KerberosConstant)
No weak-key checking is performed. The KerberosConstant value is the
byte string {0x6b 0x65 0x72 0x62 0x65 0x72 0x6f 0x73}. These values
correspond to the ASCII encoding for the string "kerberos".
6. Profiles for Kerberos checksums
These are the checksum types currently defined for Kerberos. The
full list of current checksum type number assignments is given in
section 8.
6.1. RSA MD4 Cryptographic Checksum Using DES
The RSA-MD4-DES checksum calculates a keyed collision-proof checksum
by prepending an 8 octet confounder before the text, applying the RSA
MD4 checksum algorithm [MD4-92], and encrypting the confounder and
the checksum using DES in cipher-block-chaining (CBC) mode using a
variant of the key, where the variant is computed by eXclusive-ORing
the key with the constant 0xF0F0F0F0F0F0F0F0 [@@REF 39]. The
initialization vector should be zero. The resulting checksum is 24
octets long. This checksum is tamper-proof and believed to be
collision-proof.
The DES specifications identify some weak keys' and 'semi-weak keys';
those keys shall not be used for generating RSA-MD4 checksums for use
in Kerberos.
rsa-md4-des
----------------------------------------------------------------
associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
get_mic des-cbc(key XOR 0xF0F0F0F0F0F0F0F0,
conf | rsa-md4(conf | msg),
ivec=0)
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rsa-md4-des
----------------------------------------------------------------
verify_mic decrypt and verify rsa-md4 checksum
The rsa-md4-des checksum algorithm is assigned a checksum type number
of three (3).
6.2. The RSA MD5 Checksum
The RSA-MD5 checksum calculates a checksum using the RSA MD5
algorithm [MD5-92]. The algorithm takes as input an input message of
arbitrary length and produces as output a 128-bit (16 octet)
checksum. RSA-MD5 is believed to be collision-proof. However, since
it is unkeyed, it must be used with caution. Currently it is used by
some implementations in places where the checksum itself is part of a
larger message that will be encrypted. Its use is not recommended.
rsa-md5
----------------------------------------------
associated cryptosystem null
get_mic rsa-md5(msg)
verify_mic get_mic and compare
The rsa-md5 checksum algorithm is assigned a checksum type number of
seven (7).
6.3. RSA MD5 Cryptographic Checksum Using DES
The RSA-MD5-DES checksum calculates a keyed collision-proof checksum
by prepending an 8 octet confounder before the text, applying the RSA
MD5 checksum algorithm, and encrypting the confounder and the
checksum using DES in cipher-block-chaining (CBC) mode using a
variant of the key, where the variant is computed by eXclusive-ORing
the key with the hexadecimal constant 0xF0F0F0F0F0F0F0F0. The
initialization vector should be zero. The resulting checksum is 24
octets long. This checksum is tamper-proof and believed to be
collision-proof.
The DES specifications identify some 'weak keys' and 'semi-weak
keys'; those keys shall not be used for encrypting RSA-MD5 checksums
for use in Kerberos.
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The format for the checksum is described in the following diagram:
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| des-cbc(confounder+rsa-md5(confounder+msg), key=var(key), iv=0) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
rsa-md5-des
----------------------------------------------------------------
associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
get_mic des-cbc(key XOR 0xF0F0F0F0F0F0F0F0,
conf | rsa-md5(conf | msg))
verify_mic decrypt and verify rsa-md5 checksum
The rsa-md5-des checksum algorithm is assigned a checksum type number
of eight (8).
6.4. The CRC-32 Checksum
This CRC-32 checksum calculates a checksum based on a cyclic
redundancy check as described in ISO 3309 [ISO3309], modified as
described below. The resulting checksum is four (4) octets in
length. The CRC-32 is neither keyed nor collision-proof; thus, the
use of this checksum is not recommended. An attacker using a
probabilistic chosen-plaintext attack as described in [@@REF 13??]
might be able to generate an alternative message that satisfies the
checksum. The use of collision-proof checksums is recommended for
environments where such attacks represent a significant threat.
The CRC-32 checksum used in the des-cbc-crc encryption mode is
identical to the 32-bit FCS described in ISO 3309 with two
exceptions: the sum with the all-ones polynomial times x**k is
omitted, and the final remainder is not ones-complemented. ISO 3309
describes the FCS in terms of bits, while this document describes the
Kerberos protocol in terms of octets. To disambiguate the ISO 3309
definition for the purpose of computing the CRC-32 in the des-cbc-crc
encryption mode, the ordering of bits in each octet shall be assumed
to be LSB-first. Given this assumed ordering of bits within an
octet, the mapping of bits to polynomial coefficients shall be
identical to that specified in ISO 3309.
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crc32
------------------------------------------------
associated cryptosystem des-cbc-md5, des-cbc-
md4, des-cbc-crc
get_mic crc32(msg)
verify_mic compute checksum and
compare
The crc32 checksum algorithm is assigned a checksum type number of
one (1).
6.5. The RSA MD4 Checksum
The RSA-MD4 checksum calculates a checksum using the RSA MD4
algorithm [MD4-92]. The algorithm takes as input an input message of
arbitrary length and produces as output a 128-bit (16 octet)
checksum. RSA-MD4 is believed to be collision-proof.
rsa-md4
------------------------------------------------
associated cryptosystem des-cbc-md5, des-cbc-
md4, des-cbc-crc
get_mic md4(msg)
verify_mic compute checksum and
compare
The rsa-md4 checksum algorithm is assigned a checksum type number of
two (2).
6.6. DES CBC checksum
The DES-MAC checksum is computed by prepending an 8 octet confounder
to the plaintext, performing a DES CBC-mode encryption on the result
using the key and an initialization vector of zero, taking the last
block of the ciphertext, prepending the same confounder and
encrypting the pair using DES in cipher-block-chaining (CBC) mode
using a a variant of the key, where the variant is computed by
eXclusive-ORing the key with the constant 0xF0F0F0F0F0F0F0F0. The
initialization vector should be zero. The resulting checksum is 128
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bits (16 octets) long, 64 bits of which are redundant. This checksum
is tamper-proof and collision-proof.
The DES specifications identify some "weak" and "semiweak" keys;
those keys shall not be used for generating DES-MAC checksums for use
in Kerberos, nor shall a key be used whose variant is "weak" or
"semi-weak".
des-mac
----------------------------------------------------------------
associated des-cbc-md5, des-cbc-md4, des-cbc-crc
cryptosystem
get_mic des-cbc(key XOR 0xF0F0F0F0F0F0F0F0,
conf | des-mac(key, conf | msg, ivec=0),
ivec=0)
verify_mic decrypt, compute DES MAC using confounder,
compare
The des-mac checksum algorithm is assigned a checksum type number of
four (4).
6.7. RSA MD4 Cryptographic Checksum Using DES alternative
The RSA-MD4-DES-K checksum calculates a keyed collision-proof
checksum by applying the RSA MD4 checksum algorithm and encrypting
the results using DES in cipherblock-chaining (CBC) mode using a DES
key as both key and initialization vector. The resulting checksum is
16 octets long. This checksum is tamper-proof and believed to be
collision-proof. Note that this checksum type is the old method for
encoding the RSA-MD4-DES checksum and it is no longer recommended.
rsa-md4-des-k
----------------------------------------------------------------
associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
get_mic des-cbc(key, md4(msg), ivec=key)
verify_mic compute CRC-32 and compare
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The rsa-md4-des-k checksum algorithm is assigned a checksum type
number of six (6).
6.8. DES CBC checksum alternative
The DES-MAC-K checksum is computed by performing a DES CBC-mode
encryption of the plaintext, and using the last block of the
ciphertext as the checksum value. It is keyed with an encryption key
and an initialization vector; any uses which do not specify an
additional initialization vector will use the key as both key and
initialization vector. The resulting checksum is 64 bits (8 octets)
long. This checksum is tamper-proof and collision-proof. Note that
this checksum type is the old method for encoding the DESMAC checksum
and it is no longer recommended.
The DES specifications identify some "weak keys"; those keys shall
not be used for generating DES-MAC checksums for use in Kerberos.
des-mac-k
----------------------------------------------------------------
associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
get_mic des-mac(key, msg, ivec=key or given)
verify_mic compute MAC and compare
The des-mac-k checksum algorithm is assigned a checksum type number
of five (5).
6.9. The HMAC-SHA1-DES3-KD Checksum
This checksum type is defined as outlined in section 3 above, using
the des3-hmac-sha1-kd encryption algorithm parameters from section
5.3. The checksum is thus a SHA-1 HMAC using the computed key Kc
over the message to be protected.
The hmac-sha1-des3-kd checksum algorithm is assigned a checksum type
number of twelve (12).
7. Use of Kerberos encryption outside this specification
Several Kerberos-based application protocols and preauthentication
systems have been designed and deployed that perform encryption and
message integrity checks in various ways. While in some cases there
may be good reason for specifying these protocols in terms of
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specific encryption or checksum algorithms, we anticipate that in
many cases this will not be true, and more generic approaches
independent of particular algorithms will be desirable. Rather than
having each protocol designer reinvent schemes for protecting data,
using multiple keys, etc, we have attempted to present in this
section a general framework that should be sufficient not only for
the Kerberos protocol itself but also for many preauthentication
systems and application protocols, while trying to avoid some of the
assumptions that can work their way into such protocol designs.
Some problematic assumptions we've seen, and sometimes made, include:
that a random bitstring is always valid as a key (not true for DES
keys with parity); that the basic block encryption chaining mode
provides no integrity checking, or can easily be separated from such
checking (not true for many modes in development that do both
simultaneously); that a checksum for a message always results in the
same value (not true if a confounder is incorporated); that an
initial vector is used (may not be true if a block cipher in CBC mode
is not in use); that the key is a clever thing to use as the initial
vector for CBC mode encryption (not true @@REF Bellovin paper).
Such assumptions, while they may hold for any given set of encryption
and checksum algorithms, may not be true of the next algorithms to be
defined, leaving the application protocol unable to make use of those
algorithms without updates to its specification.
The Kerberos protocol uses only the attributes and operations
described in sections 2 and 3. Preauthentication systems and
application protocols making use of Kerberos are encouraged to use
them as well. The specific key and string-to-key parameters should
generally be treated as opaque. While the string-to-key parameters
are manipulated as an octet string, the representation for the
specific key structure is implementation-defined; it may not even be
a single object.
While we don't recommend it, some application protocols will
undoubtedly continue to use the key data directly, even if only in
some of the currently existing protocol specifications. An
implementation intended to support general Kerberos applications may
therefore need to make the key data available, as well as the
attributes and operations described in sections 2 and 3. [5]
8. Assigned Numbers
The following encryption type numbers are already assigned or
reserved for use in Kerberos and related protocols.
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Encryption type etype block minimum confounder section
value size pad size size
----------------------------------------------------------------------
NULL 0 1 0 0 5.1
des-cbc-crc 1 8 4 8 5.2.3
des-cbc-md4 2 8 0 8 5.2.2
des-cbc-md5 3 8 0 8 5.2.1
des3-cbc-sha1-kd 16 8 0 8 5.3
Other numbers have been reserved for use in encryption systems not
defined here. Encryption type numbers are unfortunately overloaded
on occasion in Kerberos-related protocols, so some of the reserved
numbers do not and will not correspond to encryption systems fitting
the profile presented here.
Encryption type etype value comment
----------------------------------------------------------------------
[reserved] 4
des3-cbc-md5 5
[reserved] 6
des3-cbc-sha1 7
dsaWithSHA1-CmsOID 9 (pkinit)
md5WithRSAEncryption-CmsOID 10 (pkinit)
sha1WithRSAEncryption-CmsOID 11 (pkinit)
rc2CBC-EnvOID 12 (pkinit)
rsaEncryption-EnvOID 13 (pkinit from PKCS#1 v1.5)
rsaES-OAEP-ENV-OID 14 (pkinit from PKCS#1 v2.0)
des-ede3-cbc-Env-OID 15 (pkinit)
rc4-hmac 23 (swift)
rc4-hmac-exp 24 (swift)
subkey-keynaterial 65 (opaque mhur)
The following checksum type numbers are assigned or reserved. As
with encryption type numbers, some overloading of checksum numbers
has occurred.
Checksum type sumtype checksum section
value size
----------------------------------------------------------------------
CRC32 1 4 6.4
rsa-md4 2 16 6.5
rsa-md4-des 3 24 6.1
des-mac 4 16 6.6
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des-mac-k 5 8 6.8
rsa-md4-des-k 6 16 6.7
rsa-md5 7 16 6.2
rsa-md5-des 8 24 6.3
rsa-md5-des3 9 24
hmac-sha1-des3-kd 12 20 6.9
hmac-sha1-des3 13 20
sha1 (unkeyed) 14 20
[reserved] 0x8003 ? [GSS-KRB5]
9. Notes to Implementors
The "interface" described here is the minimal information that must
be defined to make a cryptosystem useful within Kerberos in an
interoperable fashion. It is not an attempt to define a complete API
for cryptographic functionality within Kerberos. Actual
implementations providing clean APIs will probably find it useful to
make additional information available, which should be possible to
derive from a specification written to the framework given here. For
example, an application designer may wish to determine the largest
number of bytes that can be encrypted without overflowing a certain
size output buffer, or conversely, the maximum number of bytes that
might be obtained by decrypting a given ciphertext message.
The presence of a mechanism in this document should not be taken as
an indication that it must be implemented for compliance with any
specification; required mechanisms will be specified elsewhere.
Indeed, some of the mechanisms described here for backwards
compatibility are now considered rather weak for protecting critical
data.
10. Security Considerations
Well, sure... weak encryption or checksum algorithms. Warnings made
in the various sections. Reference EFF book on DES cracking, RFC on
DES for IPsec.
11. Acknowledgements
This document is an extension of the encryption specification
included in RFC 1510 by B. Clifford Neuman and John Kohl, and much of
the text of the background, concepts, and DES specifications are
drawn directly from that document.
Marc Horowitz wrote the original specification of triple-DES and key
derivation in a pair of Internet Drafts (under the names draft-
horowitz-key-derivation and draft-horowitz-kerb-key-derivation) which
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were later folded into a draft revision of RFC 1510, from which this
document was later split off.
The abstract framework presented in this document was put together by
Jeff Altman, Sam Hartman, Jeff Hutzelman, Cliff Neuman, Ken Raeburn,
and Tom Yu, and the details were refined several times based on
comments from John Brezak and others.
Miroslav Jurisic provided one of the UTF-8 test cases for the string-
to-key functions.
Uri Blumenthal provided comments on the "combine-keys" function
proposed for use with triple-DES.
12. Editor's address
Kenneth Raeburn
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139
raeburn@mit.edu
13. Full Copyright Statement
Copyright (C) The Internet Society (2002). 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
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE."
A. Test vectors
This section provides test vectors for various functions defined or
described in section 6. For convenience, most inputs are ASCII
strings, though some UTF-8 samples should be provided for string-to-
key functions. Keys and other binary data are specified as
hexadecimal strings.
A.1. n-fold
The n-fold function is defined in section 6.4. As noted there, the
sample vector in the original paper defining the algorithm appears to
be incorrect. Here are values provided by Marc Horowitz:
64-fold("012345") =
64-fold(303132333435) = be072631276b1955
56-fold("password") =
56-fold(70617373776f7264) = 78a07b6caf85fa
64-fold("Rough Consensus, and Running Code") =
64-fold(526f75676820436f6e73656e7375732c20616e642052756e
6e696e6720436f6465) = bb6ed30870b7f0e0
168-fold("password") =
168-fold(70617373776f7264) =
59e4a8ca7c0385c3c37b3f6d2000247cb6e6bd5b3e
192-fold("MASSACHVSETTS INSTITVTE OF TECHNOLOGY"
192-fold(4d41535341434856534554545320494e5354495456544520
4f4620544543484e4f4c4f4759) =
db3b0d8f0b061e603282b308a50841229ad798fab9540c1b
A.2. mit_des_string_to_key
The function mit_des_string_to_key is defined in section 6.5.2. We
present here several test values, with some of the intermediate
results. The fourth test demonstrates the use of UTF-8 with three
characters. The last two tests are specifically constructed so as to
trigger the weak-key fixups for the intermediate key produced by fan-
folding; we have no test cases that cause such fixups for the final
key.
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UTF-8 encodings used in test vector:
eszett C3 9F s-caron C5 A1 c-acute C4 87
Test vector:
salt: "ATHENA.MIT.EDUraeburn"
415448454e412e4d49542e4544557261656275726e
password: "password" 70617373776f7264
fan-fold result: c01e38688ac86c2e
intermediate key: c11f38688ac86d2f
DES key: cbc22fae235298e3
salt: "WHITEHOUSE.GOVdanny" 5748495445484f5553452e474f5664616e6e79
password: "potatoe" 706f7461746f65
fan-fold result: a028944ee63c0416
intermediate key: a129944fe63d0416
DES key: df3d32a74fd92a01
salt: "EXAMPLE.COMbuckaroo" 4558414d504c452e434f4d6275636b61726f6f
password: "penny" 70656e6e79
fan-fold result: 96d2d87e925c64ee
intermediate key: 97d3d97f925d64ef
DES key: 9443a2e532fdc4f1
salt: "ATHENA.MIT.EDUJuri" + s-caron + "i" + c-acute
415448454e412e4d49542e4544554a757269c5a169c487
password: eszett c39f
fan-fold result: b8f6c40e305afc9e
intermediate key: b9f7c40e315bfd9e
DES key: 62c81a5232b5e69d
salt: "AAAAAAAA" 4141414141414141
password: "11119999" 3131313139393939
fan-fold result: e0e0e0e0f0f0f0f0
intermediate key: e0e0e0e0f1f1f101
DES key: 984054d0f1a73e31
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salt: "FFFFAAAA" 4646464641414141
password: "NNNN6666" 4e4e4e4e36363636
fan-fold result: 1e1e1e1e0e0e0e0e
intermediate key: 1f1f1f1f0e0e0efe
DES key: c4bf6b25adf7a4f8
A.3. DES3 DR and DK
These tests show the derived-random and derived-key values for the
des3-hmac-sha1-kd encryption scheme, using the DR and DK functions
defined in section 6.5.5. The input keys were randomly generated;
the usage values are from this specification.
key: dce06b1f64c857a11c3db57c51899b2cc1791008ce973b92
usage: 0000000155
DR: 935079d14490a75c3093c4a6e8c3b049c71e6ee705
DK: 925179d04591a79b5d3192c4a7e9c289b049c71f6ee604cd
key: 5e13d31c70ef765746578531cb51c15bf11ca82c97cee9f2
usage: 00000001aa
DR: 9f58e5a047d894101c469845d67ae3c5249ed812f2
DK: 9e58e5a146d9942a101c469845d67a20e3c4259ed913f207
key: 98e6fd8a04a4b6859b75a176540b9752bad3ecd610a252bc
usage: 0000000155
DR: 12fff90c773f956d13fc2ca0d0840349dbd39908eb
DK: 13fef80d763e94ec6d13fd2ca1d085070249dad39808eabf
key: 622aec25a2fe2cad7094680b7c64940280084c1a7cec92b5
usage: 00000001aa
DR: f8debf05b097e7dc0603686aca35d91fd9a5516a70
DK: f8dfbf04b097e6d9dc0702686bcb3489d91fd9a4516b703e
key: d3f8298ccb166438dcb9b93ee5a7629286a491f838f802fb
usage: 6b65726265726f73
DR: 2270db565d2a3d64cfbfdc5305d4f778a6de42d9da
DK: 2370da575d2a3da864cebfdc5204d56df779a7df43d9da43
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key: b55e983467e551b3e5d0e5b6c80d45769423a873dc62b30e
usage: 636f6d62696e65
DR: 0127398bacc81a2a62bc45f8d4c151bbcdd5cb788a
DK: 0126388aadc81a1f2a62bc45f8d5c19151bacdd5cb798a3e
key: c1081649ada74362e6a1459d01dfd30d67c2234c940704da
usage: 0000000155
DR: 348056ec98fcc517171d2b4d7a9493af482d999175
DK: 348057ec98fdc48016161c2a4c7a943e92ae492c989175f7
key: 5d154af238f46713155719d55e2f1f790dd661f279a7917c
usage: 00000001aa
DR: a8818bc367dadacbe9a6c84627fb60c294b01215e5
DK: a8808ac267dada3dcbe9a7c84626fbc761c294b01315e5c1
key: 798562e049852f57dc8c343ba17f2ca1d97394efc8adc443
usage: 0000000155
DR: c813f88b3be2b2f75424ce9175fbc8483b88c8713a
DK: c813f88a3be3b334f75425ce9175fbe3c8493b89c8703b49
key: 26dce334b545292f2feab9a8701a89a4b99eb9942cecd016
usage: 00000001aa
DR: f58efc6f83f93e55e695fd252cf8fe59f7d5ba37ec
DK: f48ffd6e83f83e7354e694fd252cf83bfe58f7d5ba37ec5d
A.4. DES3string_to_key
These are the keys generated for some of the above input strings for
triple-DES with key derivation as defined in section 5.3.1.
salt: "ATHENA.MIT.EDUraeburn"
passwd: "password"
key: 850bb51358548cd05e86768c313e3bfef7511937dcf72c3e
salt: "WHITEHOUSE.GOVdanny"
passwd: "potatoe"
key: dfcd233dd0a43204ea6dc437fb15e061b02979c1f74f377a
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salt: "EXAMPLE.COMbuckaroo"
passwd: "penny"
key: 6d2fcdf2d6fbbc3ddcadb5da5710a23489b0d3b69d5d9d4a
salt: "ATHENA.MIT.EDUJuri" + s-caron + "i" + c-acute
passwd: eszett
key: 16d5a40e1ce3bacb61b9dce00470324c831973a7b952feb0
A.5. DES3 combine-keys
PLACEHOLDER, FILL IN BEFORE PUBLICATION
A.6. Modified CRC-32
PLACEHOLDER, GET DATA FROM TOM
Notes
[1] While Message Authentication Code (MAC) or Message Integrity
Check (MIC) would be more appropriate terms for many of the
uses in this section, we continue to use the term "checksum"
for historical reasons.
[2] In the case of Kerberos, the encrypted objects will generally
be ASN.1 DER encodings, which contain indications of their
length in the first few octets.
[3] As of the time of this writing, some new modes of operation
have been proposed, some of which may permit encryption and
integrity protection simultaneously. After some of these
proposals have been subjected to adequate analysis, we may
wish to formulate a new simplified profile based on one of
them.
[4] It should be noted that the sample vector in Appendix B.2 of
the original paper appears to be incorrect. Two independent
implementations from the specification (one in C by Marc
Horowitz, and another in Scheme by Bill Sommerfeld) agree on
a value different from that in [Blumenthal96].
[5] Perhaps one of the more common reasons for directly
performing encryption is direct control over the negotiation
and to select a "sufficiently strong" encryption algorithm
(whatever that means in the context of a given application).
While Kerberos directly provides no facility for negotiating
encryption types between the application client and server,
there are other means for accomplishing similar goals. For
example, requesting only "strong" session key types from the
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KDC, and assuming that the type actually returned by the KDC
will be understood and supported by the application server.
Normative References
This section copied from kerberos-revisions draft. Drop the ones we
don't need, add anything new that we do need. Move informational-
only references to the next section. Update old I-D references to
RFCs, or find other sources.
[Blumenthal96]
Blumenthal, U., "A Better Key Schedule for DES-Like Ciphers",
Proceedings of PRAGOCRYPT '96, 1996.
[Bellare98]
Bellare, M., Desai, A., Pointcheval, D., Rogaway, P., "Relations
Among Notions of Security for Public-Key Encryption Schemes".
Extended abstract published in Advances in Cryptology- Crypto 98
Proceedings, Lecture Notes in Computer Science Vol. 1462, H.
Krawcyzk ed., Springer-Verlag, 1998.
[DES77]
National Bureau of Standards, U.S. Department of Commerce, "Data
Encryption Standard," Federal Information Processing Standards
Publication 46, Washington, DC (1977).
[DESM80]
National Bureau of Standards, U.S. Department of Commerce, "DES
Modes of Operation," Federal Information Processing Standards
Publication 81, Springfield, VA (December 1980).
[Dolev91]
Dolev, D., Dwork, C., Naor, M., "Non-malleable cryptography",
Proceedings of the 23rd Annual Symposium on Theory of Computing,
ACM, 1991.
[ISO3309]
International Organization for Standardization, "ISO Information
Processing Systems - Data Communication - High-Level Data Link
Control Procedure - Frame Structure," IS 3309 (October 1984). 3rd
Edition.
[Krawczyk96]
Krawczyk, H., Bellare, and M., Canetti, R., "HMAC: Keyed-Hashing
for Message Authentication", draft-ietf-ipsec-hmac-md5-01.txt,
August, 1996. @@ Now RFC 2202.
[MD4-92]
R. Rivest, "The MD4 Message Digest Algorithm," RFC 1320, MIT
Laboratory for Computer Science (April 1992).
[MD5-92]
R. Rivest, "The MD5 Message Digest Algorithm," RFC 1321, MIT
Laboratory for Computer Science (April 1992).
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[MNSS87]
S. P. Miller, B. C. Neuman, J. I. Schiller, and J. H. Saltzer,
Section E.2.1: Kerberos Authentication and Authorization System,
M.I.T. Project Athena, Cambridge, Massachusetts (December 21,
1987).
[SG92]
Stuart G. Stubblebine and Virgil D. Gligor, "On Message Integrity
in Cryptographic Protocols," in Proceedings of the IEEE Symposium
on Research in Security and Privacy, Oakland, California (May
1992).
Informative References
[GSS-KRB5]
@@ blah blah blah, RFC 1964
... EFF DES-cracking book ...
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