The Kerberos protocols [Kerb] are designed to encrypt messages of
arbitrary sizes, using block encryption ciphers or, less commonly,
stream encryption ciphers. Encryption is used to prove the
identities of the network entities participating in message
exchanges. However, nothing in the Kerberos protocol requires that
any specific encryption algorithm be used, as long as the algorithm
includes certain operations.
The following sections specify the encryption and checksum mechanisms
currently defined for Kerberos, as well as a framework for defining
future mechanisms. The encoding, chaining, padding, and other
requirements for each are described. Appendix A gives test vectors
for several functions.
Both encryption and checksum mechanisms are profiled in later
sections. Each profile 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. (Incorporating a checksum may
permit integrity checking, if the encryption mode does not provide an
integrity check itself.) It must also provide non-malleability
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RFC 3961 Encryption and Checksum Specifications February 2005
[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 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. Finding two plaintexts
with the same checksum should be infeasible. It is NOT required that
an eavesdropper be unable to determine whether two checksums are for
the same message, as the messages themselves would presumably be
visible to any such eavesdropper.
Due to advances in cryptography, some cryptographers consider using
the same key for multiple purposes unwise. 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 by making the "usage number" an input to the encryption or
checksum mechanisms; such enumeration is outside the scope of this
document. Later sections define simplified profile templates for
encryption and checksum mechanisms that use a key derivation 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" for a specific encryption or checksum operation. It
is expected but not required that the specific key 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.
All data is assumed to be in the form of strings of octets or eight-
bit bytes. Environments with other byte sizes will have to emulate
this behavior in order to get correct results.
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Each algorithm is assigned an encryption type (or "etype") or
checksum type number, for algorithm identification within the
Kerberos protocol. The full list of current type number assignments
is given in section 8.
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 which 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 invalid specific values; all key
generation routines should avoid such values.
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 key
derivation 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
mechanism is decided, the corresponding checksum mechanism can 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 generating
keys that significantly compromise the confidentiality of
encrypted data, if the cryptosystem has such. Entropy from each
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source should be preserved as much as possible. Many of the
inputs, although 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 time stamps). The benefit of such
predictability to an attacker must be minimized.
string-to-key (UTF-8 string, UTF-8 string, opaque)->(protocol-key)
This function generates a key from two UTF-8 strings and an opaque
octet string. One of the strings is usually the principal's pass
phrase, but generally it is 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. Although the
strings provided will use UTF-8 encoding, no specific version of
Unicode should be assumed; all valid UTF-8 strings should be
allowed. Strings provided in other encodings MUST first be
converted to UTF-8 before applying this function.
The third argument, the octet string, may be used to pass
mechanism-specific parameters into this function. Since doing so
implies knowledge of the specific encryption system, generating
non-default parameter values should be an uncommon operation, and
normal Kerberos applications should be able to treat this
parameter block as an opaque object supplied by the Key
Distribution Center or defaulted to some mechanism-specific
constant value.
The string-to-key function should be a one-way function so that
compromising a user's key in one realm does not compromise it in
another, even if the same password (but a different salt) is used.
random-to-key (bitstring[K])->(protocol-key)
This function generates a key from a random bitstring of a
specific size. All the bits of the input string are assumed to be
equally random, even though the entropy present in the random
source may be limited.
key-derivation (protocol-key, integer)->(specific-key)
In this function, the integer input is the key usage value, as
described above. An attacker is assumed to know the usage values.
The specific-key output value was described in section 2.
string-to-key parameter format
This describes the format of the block of data that can be passed
to the string-to-key function above to configure additional
parameters for that function. Along with the mechanism of
encoding parameter values, bounds on the allowed parameters should
also be described to avoid allowing a spoofed KDC to compromise
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the user's password. If practical it may be desirable to
construct the encoding so that values unacceptably weakening the
resulting key cannot be encoded.
Local security policy might permit tighter bounds to avoid excess
resource consumption. If so, the specification should recommended
defaults for these bounds. The description should also outline
possible weaknesses if bounds checks or other validations are not
applied to a parameter string received from the network.
As mentioned above, this should be considered opaque to most
normal applications.
default string-to-key parameters (octet string)
This default value for the "params" argument to the string-to-key
function should be used when the application protocol (Kerberos or
other) does not explicitly set the parameter value. As indicated
above, in most cases this parameter block should be treated as an
opaque object.
cipher state
This describes any information that can be carried over from one
encryption or decryption operation to the next, for use with a
given specific key. For example, a block cipher used in CBC mode
may put an initial vector of one block in the cipher state. Other
encryption modes may track nonces or other data.
This state must be non-empty and must influence encryption so that
messages are decrypted in the same order they were a encrypted, if
the cipher state is carried over from one encryption to the next.
Distinguishing out-of-order or missing messages from corrupted
messages is not required. If desired, this can be done at a
higher level by including sequence numbers and not "chaining" the
cipher state between encryption operations.
The cipher state may not be reused in multiple encryption or
decryption operations. These operations all generate a new cipher
state that may be used for following operations using the same key
and operation.
The contents of the cipher state must be treated as opaque outside
of encryption system specifications.
initial cipher state (specific-key, direction)->(state)
This describes the generation of the initial value for the cipher
state if it is not being carried over from a previous encryption
or decryption operation.
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This describes any initial state setup needed before encrypting
arbitrary amounts of data with a given specific key. The specific
key and the direction of operations to be performed (encrypt
versus decrypt) must be the only input needed for this
initialization.
This state should be treated as opaque in any uses outside of an
encryption algorithm definition.
IMPLEMENTATION NOTE: [Kerb1510] was vague on whether and to what
degree an application protocol could exercise control over the
initial vector used in DES CBC operations. Some existing
implementations permit setting the initial vector. This framework
does not provide for application control of the cipher state
(beyond "initialize" and "carry over from previous encryption"),
as the form and content of the initial cipher state can vary
between encryption systems and may not always be a single block of
random data.
New Kerberos application protocols should not assume control over
the initial vector, or that one even exists. However, a general-
purpose implementation may wish to provide the capability, in case
applications explicitly setting it are encountered.
encrypt (specific-key, state, octet string)->(state, octet string)
This function takes the specific key, cipher state, and a non-
empty plaintext string as input and generates ciphertext and a new
cipher state as outputs. If the basic encryption algorithm itself
does not provide for integrity protection (e.g., DES in CBC mode),
then some form of verifiable MAC or checksum must be included.
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 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 indicate not
only the precise contents of the output octet string, but also the
output cipher state. The application protocol may carry the
output cipher state forward from one encryption with a given
specific key to another; the effect of this "chaining" must be
defined [2].
Assuming that values for the specific key and cipher state are
correctly-produced, no input octet string may result in an error
indication.
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decrypt (specific-key, state, octet string)->(state, octet string)
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, as, for example, when the encryption mode adds padding
to reach a multiple of a block size. If this is the case, any
extra octets must come after the decoded plaintext. An
application protocol that needs to know the exact length of the
message must encode a length or recognizable "end of message"
marker within the plaintext [3].
As with the encryption function, a correct specification for this
function must indicate not only the contents of the output octet
string, but also the resulting cipher state.
pseudo-random (protocol-key, octet-string)->(octet-string)
This pseudo-random function should generate an octet string of
some size that is independent of the octet string input. The PRF
output string should be suitable for use in key generation, even
if the octet string input is public. It should not reveal the
input key, even if the output is made public.
These operations and attributes are all that is required to support
Kerberos and various proposed preauthentication schemes.
For convenience of certain application protocols that may wish to use
the encryption profile, we add the constraint that, for any given
plaintext input size, a message size must exist between that given
size and that size plus 65,535 such that the length of the decrypted
version of the ciphertext will never have extra octets at the end.
Expressed mathematically, for every message length L1, there exists a
message size L2 such that
L2 >= L1
L2 < L1 + 65,536
for every message M with |M| = L2, decrypt(encrypt(M)) = M
A document defining a new encryption type should also describe known
weaknesses or attacks, so that its security may be fairly assessed,
and should include test vectors or other validation procedures for
the operations defined. Specific references to information that is
readily available elsewhere are sufficient.
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A checksum mechanism profile must define the following attributes and
operations:
associated encryption algorithm(s)
This indicates the types of encryption keys this checksum
mechanism can be used with.
A keyed checksum mechanism may have more than one associated
encryption algorithm if they share the same wire-key format,
string-to-key function, default string-to-key-parameters, and key
derivation function. (This combination means that, for example, a
checksum type, key usage value, and password are adequate to get
the specific key used to compute a checksum.)
An unkeyed checksum mechanism can be used with any encryption
type, as the key is ignored, but its use must be limited to cases
where the checksum itself is protected, to avoid trivial attacks.
get_mic function
This function generates a MIC token for a given specific key (see
section 3) 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
for each 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 no larger than 65,535 octets.
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 the two.
The get_mic and verify_mic operations must allow 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.
As with encryption mechanism definition documents, documents defining
new checksum mechanisms should indicate validation processes and
known weaknesses.
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The profile outlined in sections 3 and 4 describes a large number of
operations that must be defined for encryption and checksum
algorithms to be used with Kerberos. Here we describe a simpler
profile that can generate both encryption and checksum mechanism
definitions, 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. 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 [4].
Not all the operations in the complete profiles are defined through
this mechanism; several must still be defined for each new algorithm
pair.
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 others. Attack of
the base key is limited, as it is only used for derivation and is not
exposed to any user data.
To generate a derived key from a base key, we generate a pseudorandom
octet string by using an algorithm DR, described below, and generate
a key from that octet string by 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). These procedures are based on the key derivation in
[Blumenthal96].
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,
initial-cipher-state))
Here DR is the random-octet generation function described below, and
DK is the key-derivation function produced from it. In this
construction, E(Key, Plaintext, CipherState) is a cipher, Constant is
a well-known constant determined by the specific usage of this
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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 Constant is smaller than the cipher block size of E, then it
must be expanded with n-fold() so it can be encrypted. If the output
of E is shorter than k bits, it is fed back into the encryption as
many times as necessary. The construct is as follows (where |
indicates concatentation):
K1 = E(Key, n-fold(Constant), initial-cipher-state)
K2 = E(Key, K1, initial-cipher-state)
K3 = E(Key, K2, initial-cipher-state)
K4 = ...
DR(Key, Constant) = k-truncate(K1 | K2 | K3 | K4 ...)
n-fold is an algorithm that 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 octets 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 [5].
In this section, n-fold is always used to produce c bits of output,
where c is the cipher block size of E.
The size of the Constant must not be larger than c, because reducing
the length of the Constant by n-folding can cause collisions.
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If the size of the Constant is smaller than c, then the Constant must
be n-folded to length c. This string is used as input to E. If the
block size of E is less than the random-to-key input 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 random-to-key input size. When enough bits have
been computed, the first k are taken as the random data used to
create the key with the algorithm-dependent random-to-key function.
As 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
determining the key from a very small number of plaintext/ciphertext
pairs. Thus, this construction is as strong as the cryptosystem
itself.
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 with fixed-
size output, suitable for use in an HMAC [HMAC]. It must support
inputs of arbitrary length. Its output must be at least the
message block size (below).
HMAC output size, h
This indicates the size of the leading substring output by the
HMAC function that should be used in transmitted messages. It
should be at least half the output size of the hash function H,
and at least 80 bits; it need not match the output size.
message block size, m
This is the size of the smallest units the cipher can handle in
the mode in which it is being used. Messages will be padded to a
multiple of this size. If a block cipher is used in a mode that
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can handle messages that are not multiples of the cipher block
size, such as CBC mode with cipher text stealing (CTS, see [RC5]),
this value would be one octet. For traditional CBC mode with
padding, it would be the underlying cipher's block size.
This value must be a multiple of eight bits (one octet).
encryption/decryption functions, E and D
These are basic encryption and decryption functions for messages
of sizes that are multiples of the message block size. No
integrity checking or confounder should be included here. For
inputs these functions take the IV or similar data, a protocol-
format key, and an octet string, returning a new IV and octet
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 cipher block-size confounder to the
plaintext should alter the entire ciphertext (comparable to
choosing and including a random initial vector for CBC mode).
The result of encrypting one cipher block (of size c, above) must
be deterministic for the random octet generation function DR in
the previous section to work. For best security, it should also
be no larger than c.
cipher block size, c
This is the block size of the block cipher underlying the
encryption and decryption functions indicated above, used for key
derivation and for the size of the message confounder and initial
vector. (If a block cipher is not in use, some comparable
parameter should be determined.) It must be at least 5 octets.
This is not actually an independent parameter; rather, it is a
property of the functions E and D. It is listed here to clarify
the distinction between it and the message block size, m.
Although there are still a number of properties to specify, they are
fewer and simpler than in the full profile.
The above key derivation function is used to produce three
intermediate keys. One is used for computing checksums of
unencrypted data. The other two are used for encrypting and
checksumming plaintext to be sent encrypted.
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The ciphertext output is the concatenation of the output of the basic
encryption function E and a (possibly truncated) HMAC using the
specified hash function H, both applied to the plaintext with a
random confounder prefix and sufficient padding to bring it to a
multiple of the message block size. When the HMAC is computed, the
key is used in the protocol key form.
Decryption is performed by removing the (partial) HMAC, decrypting
the remainder, and verifying the HMAC. The cipher state is an
initial vector, initialized to zero.
The substring notation "[1..h]" in the following table should be read
as using 1-based indexing; leading substrings are used.
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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 As defined below in section 5.4.
mechanism
cipher state Initial vector (usually of length c)
initial cipher state All bits zero
encryption function conf = Random string of length c
pad = Shortest string to bring confounder
and plaintext to a length that's a
multiple of m.
(C1, newIV) = E(Ke, conf | plaintext | pad,
oldstate.ivec)
H1 = HMAC(Ki, conf | plaintext | pad)
ciphertext = C1 | H1[1..h]
newstate.ivec = newIV
decryption function (C1,H1) = ciphertext
(P1, newIV) = D(Ke, C1, oldstate.ivec)
if (H1 != HMAC(Ki, P1)[1..h])
report error
newstate.ivec = newIV
default string-to-key As given.
params
pseudo-random function tmp1 = H(octet-string)
tmp2 = truncate tmp1 to multiple of m
PRF = E(DK(protocol-key, prfconstant),
tmp2, initial-cipher-state)
The "prfconstant" used in the PRF operation is the three-octet string
"prf".
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Cryptosystem from Simplified Profile
------------------------------------------------------------------------
key generation functions:
string-to-key function As given.
random-to-key function As given.
key-derivation function The "well-known constant" used for the DK
function is the key usage number, expressed as
four octets in big-endian order, followed by
one octet indicated below.
Kc = DK(base-key, usage | 0x99);
Ke = DK(base-key, usage | 0xAA);
Ki = DK(base-key, usage | 0x55);
When an encryption system is defined with the simplified profile
given in section 5.2, 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)[1..h]
verify_mic get_mic and compare
The HMAC function and key Kc are as described in section 5.3.
These profiles describe the encryption and checksum systems defined
for Kerberos. The astute reader will notice that some of them do not
fulfill all the requirements outlined in previous sections. These
systems are defined for backward compatibility; newer implementations
should (whenever possible) attempt to utilize encryption systems that
satisfy all the profile requirements.
The full list of current encryption and checksum type number
assignments, including values currently reserved but not defined in
this document, is given in section 8.
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These checksum types use no encryption keys and thus can be used in
combination with any encryption type, but they may only be used with
caution, in limited circumstances where the lack of a key does not
provide a window for an attack, preferably as part of an encrypted
message [6]. Keyed checksum algorithms are recommended.
The RSA-MD5 checksum calculates a checksum by 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 (sixteen octet)
checksum.
rsa-md5
----------------------------------------------
associated cryptosystem any
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).
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 (sixteen octet)
checksum.
rsa-md4
----------------------------------------------
associated cryptosystem any
get_mic md4(msg)
verify_mic get_mic and compare
The rsa-md4 checksum algorithm is assigned a checksum type number of
two (2).
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This CRC-32 checksum calculates a checksum based on a cyclic
redundancy check as described in ISO 3309 [CRC] but 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 [SG92] might be
able to generate an alternative message that satisfies the checksum.
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, whereas this document describes
the Kerberos protocol in terms of octets. To clarify 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.
Test values for this modified CRC function are included in appendix
A.5.
crc32
----------------------------------------------
associated cryptosystem any
get_mic crc32(msg)
verify_mic get_mic and compare
The crc32 checksum algorithm is assigned a checksum type number of
one (1).
These encryption systems encrypt information under the Data
Encryption Standard [DES77] by using the cipher block chaining mode
[DESM80]. A checksum is computed as described below and placed in
the cksum field. DES blocks are eight bytes. As a result, the data
to be encrypted (the concatenation of confounder, checksum, and
message) must be padded to an eight byte boundary before encryption.
The values of the padding bytes are unspecified.
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Plaintext and DES ciphertext are encoded as blocks of eight octets,
which are concatenated to make the 64-bit inputs for the DES
algorithms. The first octet supplies the eight most significant bits
(with the octet's MSB used as the DES input block's MSB, etc.), the
second octet the next eight bits, and so on. 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 below for each encryption system.
The DES specifications [DESI81] identify four 'weak' and twelve
'semi-weak' keys; these keys SHALL NOT be used for encrypting
messages for use in Kerberos. The "variant keys" generated for the
RSA-MD5-DES, RSA-MD4-DES, and DES-MAC checksum types by an
eXclusive-OR of a DES key with a constant are not checked for this
property.
A DES key is eight octets of data. This consists of 56 bits of
actual key data, and eight parity bits, one per octet. The key is
encoded as a series of eight 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 most significant bit). 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 to be encrypted.
+-----------+----------+---------+-----+
|confounder | checksum | msg-seq | pad |
+-----------+----------+---------+-----+
One generates a random confounder of one block, placing it in
'confounder'; zeros out the 'checksum' field (of length appropriate
to exactly hold the checksum to be computed); adds the necessary
padding; calculates the appropriate checksum over the whole sequence,
placing the result in 'checksum'; and then encrypts using the
specified encryption type and the appropriate key.
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RFC 3961 Encryption and Checksum Specifications February 2005
String or Random-Data to Key Transformation
To generate a DES key from two UTF-8 text strings (password and
salt), the two strings are concatenated, password first, and the
result is then padded with zero-valued octets to a multiple of eight
octets.
The top bit of each octet (always zero if the password is plain
ASCII, as was assumed when the original specification was written) is
discarded, and the remaining seven bits of each octet form a
bitstring. This is then fan-folded and eXclusive-ORed with itself to
produce a 56-bit string. An eight-octet key is formed from this
string, each octet using seven bits from the bitstring, leaving the
least significant bit unassigned. The key is then "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 then
"corrected" as described above to form the result, which is returned
as the key.
For purposes of the string-to-key function, the DES CBC checksum is
calculated by CBC encrypting a string using the key as IV and the
final eight byte block as the checksum.
Pseudocode follows:
removeMSBits(8byteblock) {
/* Treats a 64 bit block as 8 octets and removes the MSB in
each octet (in big endian mode) and concatenates the
result. E.g., the input octet string:
01110000 01100001 11110011 01110011 11110111 01101111
11110010 01100100
results in the output bitstring:
1110000 1100001 1110011 1110011 1110111 1101111
1110010 1100100 */
}
reverse(56bitblock) {
/* Treats a 56-bit block as a binary string and reverses it.
E.g., the input string:
1000001 1010100 1001000 1000101 1001110 1000001
0101110 1001101
results in the output string:
1011001 0111010 1000001 0111001 1010001 0001001
0010101 1000001 */
}
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RFC 3961 Encryption and Checksum Specifications February 2005
add_parity_bits(56bitblock) {
/* Copies a 56-bit block into a 64-bit block, left shifts
content in each octet, and add DES parity bit.
E.g., the input string:
1100000 0001111 0011100 0110100 1000101 1100100
0110110 0010111
results in the output string:
11000001 00011111 00111000 01101000 10001010 11001000
01101101 00101111 */
}
key_correction(key) {
fixparity(key);
if (is_weak_key(key))
key = key XOR 0xF0;
return(key);
}
mit_des_string_to_key(string,salt) {
odd = 1;
s = string | salt;
tempstring = 0; /* 56-bit string */
pad(s); /* with nulls to 8 byte boundary */
for (8byteblock in s) {
56bitstring = removeMSBits(8byteblock);
if (odd == 0) reverse(56bitstring);
odd = ! odd;
tempstring = tempstring XOR 56bitstring;
}
tempkey = key_correction(add_parity_bits(tempstring));
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
error("invalid params");
}
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RFC 3961 Encryption and Checksum Specifications February 2005
One common extension is to support the "AFS string-to-key" algorithm,
which is not defined here, if the type value above is one (1).
For generation of a key from a random bitstring, we start with a 56-
bit string and, as with the string-to-key operation above, insert
parity bits. If the result is a weak or semi-weak key, we modify it
by eXclusive-OR with the constant 0x00000000000000F0:
des_random_to_key(bitstring) {
return key_correction(add_parity_bits(bitstring));
}
The des-cbc-md5 encryption mode encrypts information under DES in CBC
mode with an all-zero initial vector and with an MD5 checksum
(described in [MD5-92]) computed and placed in the checksum field.
The encryption system parameters for des-cbc-md5 are as follows:
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 | pad)
newstate = last block of des-cbc output
decryption function decrypt encrypted text and verify checksum
newstate = last block of ciphertext
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RFC 3961 Encryption and Checksum Specifications February 2005
des-cbc-md5
--------------------------------------------------------------------
default string-to-key empty string
params
pseudo-random function des-cbc(md5(input-string), ivec=0)
key generation functions:
string-to-key des_string_to_key
random-to-key des_random_to_key
key-derivation identity
The des-cbc-md5 encryption type is assigned the etype value three
(3).
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 | pad)
newstate = last block of des-cbc output
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RFC 3961 Encryption and Checksum Specifications February 2005
des-cbc-md4
--------------------------------------------------------------------
decryption function decrypt encrypted text and verify checksum
newstate = last block of ciphertext
default string-to-key empty string
params
pseudo-random function des-cbc(md5(input-string), ivec=0)
key generation functions:
string-to-key des_string_to_key
random-to-key copy input, then fix parity bits
key-derivation identity
Note that des-cbc-md4 uses md5, not md4, in the PRF definition.
The des-cbc-md4 encryption algorithm is assigned the etype value two
(2).
The des-cbc-crc encryption type uses DES in CBC mode with the key
used as the initialization vector, with a four-octet CRC-based
checksum computed as described in section 6.1.3. Note that this is
not a standard CRC-32 checksum, but a slightly modified one.
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)
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RFC 3961 Encryption and Checksum Specifications February 2005
des-cbc-crc
--------------------------------------------------------------------
initial cipher state copy of original key
encryption function des-cbc(confounder | checksum | msg | pad,
ivec=oldstate)
where
checksum = crc(confounder | 00000000
| msg | pad)
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
pseudo-random function des-cbc(md5(input-string), ivec=0)
key generation functions:
string-to-key des_string_to_key
random-to-key copy input, then fix parity bits
key-derivation identity
The des-cbc-crc encryption algorithm is assigned the etype value one
(1).
The RSA-MD5-DES checksum calculates a keyed collision-proof checksum
by prepending an eight octet confounder before the text, applying the
RSA MD5 checksum algorithm, and encrypting the confounder and the
checksum by using DES in cipher-block-chaining (CBC) mode with 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.
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RFC 3961 Encryption and Checksum Specifications February 2005
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).
The RSA-MD4-DES checksum calculates a keyed collision-proof checksum
by prepending an eight 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 with a
variant of the key, where the variant is computed by eXclusive-ORing
the key with the constant 0xF0F0F0F0F0F0F0F0 [7]. The initialization
vector should be zero. The resulting checksum is 24 octets long.
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)
verify_mic decrypt and verify rsa-md4 checksum
The rsa-md4-des checksum algorithm is assigned a checksum type number
of three (3).
The RSA-MD4-DES-K checksum calculates a keyed collision-proof
checksum by applying the RSA MD4 checksum algorithm and encrypting
the results by using DES in cipher block chaining (CBC) mode with 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; it is no longer
recommended.
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RFC 3961 Encryption and Checksum Specifications February 2005
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 decrypt, compute checksum and compare
The rsa-md4-des-k checksum algorithm is assigned a checksum type
number of six (6).
The DES-MAC checksum is computed by prepending an eight octet
confounder to the plaintext, padding with zero-valued octets if
necessary to bring the length to a multiple of eight octets,
performing a DES CBC-mode encryption on the result by using the key
and an initialization vector of zero, taking the last block of the
ciphertext, prepending the same confounder, and encrypting the pair
by using DES in cipher-block-chaining (CBC) mode with 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 bits (sixteen octets)
long, 64 bits of which are redundant. This checksum is tamper-proof
and collision-proof.
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 | pad, 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).
The DES-MAC-K checksum is computed by performing a DES CBC-mode
encryption of the plaintext, with zero-valued padding bytes if
necessary to bring the length to a multiple of eight octets, and by
using the last block of the ciphertext as the checksum value. It is
keyed with an encryption key that is also used as the initialization
vector. The resulting checksum is 64 bits (eight octets) long. This
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RFC 3961 Encryption and Checksum Specifications February 2005
checksum is tamper-proof and collision-proof. Note that this
checksum type is the old method for encoding the DESMAC checksum; it
is no longer recommended.
des-mac-k
----------------------------------------------------------------
associated cryptosystem des-cbc-md5, des-cbc-md4, des-cbc-crc
get_mic des-mac(key, msg | pad, ivec=key)
verify_mic compute MAC and compare
The des-mac-k checksum algorithm is assigned a checksum type number
of five (5).
This encryption and checksum type pair is based on the Triple DES
cryptosystem in Outer-CBC mode and on the HMAC-SHA1 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.
Encrypted data using this type must be generated as described in
section 5.3. If the length of the input data is not a multiple of
the block size, zero-valued octets must be used to pad the plaintext
to the next eight-octet boundary. The confounder must be eight
random octets (one block).
The simplified profile for Triple DES, with key derivation as defined
in section 5, is as follows:
des3-cbc-hmac-sha1-kd, hmac-sha1-des3-kd
------------------------------------------------
protocol key format 24 bytes, parity in low
bit of each
key-generation seed 21 bytes
length
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RFC 3961 Encryption and Checksum Specifications February 2005
des3-cbc-hmac-sha1-kd, hmac-sha1-des3-kd
------------------------------------------------
hash function SHA-1
HMAC output size 160 bits
message block size 8 bytes
default string-to-key empty string
params
encryption and triple-DES encrypt and
decryption functions decrypt, in outer-CBC
mode (cipher block size
8 octets)
key generation functions:
random-to-key DES3random-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). The hmac-sha1-des3-kd checksum algorithm is assigned a
checksum type number of twelve (12).
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:
DES3random-to-key:
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, each corrected to avoid "weak" and "semi-
weak" keys as in section 6.2, are concatenated to form the protocol
key value.
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The string-to-key function is used to transform UTF-8 passwords into
DES3 keys. The DES3 string-to-key function relies on the "N-fold"
algorithm and DK function, described in section 5.
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)
Weak key checking is performed in the random-to-key and DK
operations. 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".
Several Kerberos-based application protocols and preauthentication
systems have been designed and deployed that perform encryption and
message integrity checks in various ways. Although in some cases
there may be good reason for specifying these protocols in terms of
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
have 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
the following: a random bitstring is always valid as a key (not true
for DES keys with parity); 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); a checksum for a message always results in the same
value (not true if a confounder is incorporated); an initial vector
is used (may not be true if a block cipher in CBC mode is not in
use).
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RFC 3961 Encryption and Checksum Specifications February 2005
Although such assumptions the may hold for any given set of
encryption and checksum algorithms, they 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 3 and 4. 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. Although 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.
We don't recommend doing so, but 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 3 and 4 [8].
The following encryption-type numbers are already assigned or
reserved for use in Kerberos and related protocols.
encryption type etype section or comment
-----------------------------------------------------------------
des-cbc-crc 1 6.2.3
des-cbc-md4 2 6.2.2
des-cbc-md5 3 6.2.1
[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)
des3-cbc-sha1-kd 16 6.3
aes128-cts-hmac-sha1-96 17 [KRB5-AES]
aes256-cts-hmac-sha1-96 18 [KRB5-AES]
rc4-hmac 23 (Microsoft)
rc4-hmac-exp 24 (Microsoft)
subkey-keymaterial 65 (opaque; PacketCable)
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RFC 3961 Encryption and Checksum Specifications February 2005
(The "des3-cbc-sha1" assignment is a deprecated version using no key
derivation. It should not be confused with des3-cbc-sha1-kd.)
Several numbers have been reserved for use in encryption systems not
defined here. Encryption-type numbers have unfortunately been
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.
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 or
value size reference
---------------------------------------------------------------------
CRC32 1 4 6.1.3
rsa-md4 2 16 6.1.2
rsa-md4-des 3 24 6.2.5
des-mac 4 16 6.2.7
des-mac-k 5 8 6.2.8
rsa-md4-des-k 6 16 6.2.6
rsa-md5 7 16 6.1.1
rsa-md5-des 8 24 6.2.4
rsa-md5-des3 9 24 ??
sha1 (unkeyed) 10 20 ??
hmac-sha1-des3-kd 12 20 6.3
hmac-sha1-des3 13 20 ??
sha1 (unkeyed) 14 20 ??
hmac-sha1-96-aes128 15 20 [KRB5-AES]
hmac-sha1-96-aes256 16 20 [KRB5-AES]
[reserved] 0x8003 ? [GSS-KRB5]
Encryption and checksum-type numbers are signed 32-bit values. Zero
is invalid, and negative numbers are reserved for local use. All
standardized values must be positive.
The "interface" described here is the minimal information that must
be defined to make a cryptosystem useful within Kerberos in an
interoperable fashion. The use of functional notation used in some
places is not an attempt to define an API for cryptographic
functionality within Kerberos. Actual implementations providing
clean APIs will probably make additional information available, that
could be derived 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
Raeburn Standards Track [Page 32]
RFC 3961 Encryption and Checksum Specifications February 2005
certain size output buffer or conversely, the maximum number of bytes
that might be obtained by decrypting a ciphertext message of a given
size. (In fact, an implementation of the GSS-API Kerberos mechanism
[GSS-KRB5] will require some of these.)
The presence of a mechanism in this document should not be taken to
indicate that it must be implemented for compliance with any
specification; required mechanisms will be specified elsewhere.
Indeed, some of the mechanisms described here for backward
compatibility are now considered rather weak for protecting critical
data.
Recent years have brought so many advancements in large-scale attacks
capability against DES that it is no longer considered a strong
encryption mechanism. Triple-DES is generally preferred in its
place, despite its poorer performance. See [ESP-DES] for a summary
of some of the potential attacks and [EFF-DES] for a detailed
discussion of the implementation of particular attacks. However,
most Kerberos implementations still have DES as their primary
interoperable encryption type.
DES has four 'weak' keys and twelve 'semi-weak' keys, and the use of
single-DES here avoids them. However, DES also has 48 'possibly-
weak' keys [Schneier96] (note that the tables in many editions of the
reference contains errors) that are not avoided.
DES weak keys have the property that E1(E1(P)) = P (where E1 denotes
encryption of a single block with key 1). DES semi-weak keys, or
"dual" keys, are pairs of keys with the property that E1(P) = D2(P),
and thus E2(E1(P)) = P. Because of the use of CBC mode and the
leading random confounder, however, these properties are unlikely to
present a security problem.
Many of the choices concerning when to perform weak-key corrections
relate more to compatibility with existing implementations than to
any risk analysis.
Although checks are also done for the component DES keys in a
triple-DES key, the nature of the weak keys make it extremely
unlikely that they will weaken the triple-DES encryption. It is only
slightly more likely than having the middle of the three sub-keys
match one of the other two, which effectively converts the encryption
to single-DES - a case we make no effort to avoid.
Raeburn Standards Track [Page 33]
RFC 3961 Encryption and Checksum Specifications February 2005
The true 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 of course recommended for environments where such
attacks represent a significant threat. The "simplifications" (read:
bugs) introduced when CRC-32 was implemented for Kerberos cause
leading zeros effectively to be ignored, so messages differing only
in leading zero bits will have the same checksum.
[HMAC] and [IPSEC-HMAC] discuss weaknesses of the HMAC algorithm.
Unlike [IPSEC-HMAC], the triple-DES specification here does not use
the suggested truncation of the HMAC output. As pointed out in
[IPSEC-HMAC], SHA-1 was not developed for use as a keyed hash
function, which is a criterion of HMAC. [HMAC-TEST] contains test
vectors for HMAC-SHA-1.
The mit_des_string_to_key function was originally constructed with
the assumption that all input would be ASCII; it ignores the top bit
of each input byte. Folding with XOR is also not an especially good
mixing mechanism for preserving randomness.
The n-fold function used in the string-to-key operation for des3-
cbc-hmac-sha1-kd was designed to cause each bit of input to
contribute equally to the output. It was not designed to maximize or
equally distribute randomness in the input, and conceivably
randomness may be lost in cases of partially structured input. This
should only be an issue for highly structured passwords, however.
[RFC1851] discusses the relative strength of triple-DES encryption.
The relatively slow speed of triple-DES encryption may also be an
issue for some applications.
[Bellovin91] suggests that analyses of encryption schemes include a
model of an attacker capable of submitting known plaintexts to be
encrypted with an unknown key, as well as be able to perform many
types of operations on known protocol messages. Recent experiences
with the chosen-plaintext attacks on Kerberos version 4 bear out the
value of this suggestion.
The use of unkeyed encrypted checksums, such as those used in the
single-DES cryptosystems specified in [Kerb1510], allows for cut-
and-paste attacks, especially if a confounder is not used. In
addition, unkeyed encrypted checksums are vulnerable to chosen-
plaintext attacks: An attacker with access to an encryption oracle
can easily encrypt the required unkeyed checksum along with the
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RFC 3961 Encryption and Checksum Specifications February 2005
chosen plaintext. [Bellovin99] These weaknesses, combined with a
common implementation design choice described below, allow for a
cross-protocol attack from version 4 to version 5.
The use of a random confounder is an important means to prevent an
attacker from making effective use of protocol exchanges as an
encryption oracle. In Kerberos version 4, the encryption of constant
plaintext to constant ciphertext makes an effective encryption oracle
for an attacker. The use of random confounders in [Kerb1510]
frustrates this sort of chosen-plaintext attack.
Using the same key for multiple purposes can enable or increase the
scope of chosen-plaintext attacks. Some software that implements
both versions 4 and 5 of the Kerberos protocol uses the same keys for
both versions. This enables the encryption oracle of version 4 to be
used to attack version 5. Vulnerabilities to attacks such as this
cross-protocol attack make it unwise to use a key for multiple
purposes.
This document, like the Kerberos protocol, does not address limiting
the amount of data a key may be used with to a quantity based on the
robustness of the algorithm or size of the key. It is assumed that
any defined algorithms and key sizes will be strong enough to support
very large amounts of data, or they will be deprecated once
significant attacks are known.
This document also places no bounds on the amount of data that can be
handled in various operations. To avoid denial of service attacks,
implementations will probably seek to restrict message sizes at some
higher level.
Two registries for numeric values have been created: Kerberos
Encryption Type Numbers and Kerberos Checksum Type Numbers. These
are signed values ranging from -2147483648 to 2147483647. Positive
values should be assigned only for algorithms specified in accordance
with this specification for use with Kerberos or related protocols.
Negative values are for private use; local and experimental
algorithms should use these values. Zero is reserved and may not be
assigned.
Positive encryption- and checksum-type numbers may be assigned
following either of two policies described in [BCP26].
Standards-track specifications may be assigned values under the
Standards Action policy.
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RFC 3961 Encryption and Checksum Specifications February 2005
Specifications in non-standards track RFCs may be assigned values
after Expert Review. A non-IETF specification may be assigned values
by publishing an Informational or standards-track RFC referencing the
external specification; that specification must be public and
published in some permanent record, much like the IETF RFCs. It is
highly desirable, though not required, that the full specification be
published as an IETF RFC.
Smaller encryption type values should be used for IETF standards-
track mechanisms, and much higher values (16777216 and above) for
other mechanisms. (Rationale: In the Kerberos ASN.1 encoding,
smaller numbers encode to smaller octet sequences, so this favors
standards-track mechanisms with slightly smaller messages.) Aside
from that guideline, IANA may choose numbers as it sees fit.
Internet-Draft specifications should not include values for
encryption- and checksum-type numbers. Instead, they should indicate
that values would be assigned by IANA when the document is approved
as an RFC. For development and interoperability testing, values in
the private-use range (negative values) may be used but should not be
included in the draft specification.
Each registered value should have an associated unique reference
name. The lists given in section 8 were used to create the initial
registry; they include reservations for specifications in progress in
parallel with this document, and certain other values believed to
already be in use.
This document is an extension of the encryption specification
included in [Kerb1510] by B. Clifford Neuman and John Kohl, and much
of the text of the background, concepts, and DES specifications is
drawn directly from that document.
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.
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) that
were later folded into a draft revision of [Kerb1510], from which
this document was later split off.
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RFC 3961 Encryption and Checksum Specifications February 2005
Tom Yu provided the text describing the modifications to the standard
CRC algorithm as Kerberos implementations actually use it, and some
of the text in the Security Considerations section.
Miroslav Jurisic provided information for one of the UTF-8 test cases
for the string-to-key functions.
Marcus Watts noticed some errors in earlier versions and pointed out
that the simplified profile could easily be modified to support
cipher text stealing modes.
Simon Josefsson contributed some clarifications to the DES "CBC
checksum" and string-to-key and weak key descriptions, and some test
vectors.
Simon Josefsson, Louis LeVay, and others also caught some errors in
earlier versions of this document.
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RFC 3961 Encryption and Checksum Specifications February 2005
This section provides test vectors for various functions defined or
described in this document. For convenience, most inputs are ASCII
strings, though some UTF-8 samples are provided for string-to-key
functions. Keys and other binary data are specified as hexadecimal
strings.
The n-fold function is defined in section 5.1. As noted there, the
sample vector in the original paper defining the algorithm appears to
be incorrect. Here are some test cases provided by Marc Horowitz and
Simon Josefsson:
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
168-fold("Q") =
168-fold(51) =
518a54a2 15a8452a 518a54a2 15a8452a
518a54a2 15
168-fold("ba") =
168-fold(6261) =
fb25d531 ae897449 9f52fd92 ea9857c4
ba24cf29 7e
Here are some additional values corresponding to folded values of the
string "kerberos"; the 64-bit form is used in the des3 string-to-key
(section 6.3.1).
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RFC 3961 Encryption and Checksum Specifications February 2005
64-fold("kerberos") =
6b657262 65726f73
128-fold("kerberos") =
6b657262 65726f73 7b9b5b2b 93132b93
168-fold("kerberos") =
8372c236 344e5f15 50cd0747 e15d62ca
7a5a3bce a4
256-fold("kerberos") =
6b657262 65726f73 7b9b5b2b 93132b93
5c9bdcda d95c9899 c4cae4de e6d6cae4
Note that the initial octets exactly match the input string when the
output length is a multiple of the input length.
These are the keys generated for some of the above input strings for
triple-DES with key derivation as defined in section 6.3.1.
salt: "ATHENA.MIT.EDUraeburn"
passwd: "password"
key: 850bb51358548cd05e86768c313e3bfef7511937dcf72c3e
salt: "WHITEHOUSE.GOVdanny"
passwd: "potatoe"
key: dfcd233dd0a43204ea6dc437fb15e061b02979c1f74f377a
salt: "EXAMPLE.COMbuckaroo"
passwd: "penny"
key: 6d2fcdf2d6fbbc3ddcadb5da5710a23489b0d3b69d5d9d4a
salt: "ATHENA.MIT.EDUJuri" + s-caron(U+0161) + "i"
+ c-acute(U+0107)
passwd: eszett(U+00DF)
key: 16d5a40e1ce3bacb61b9dce00470324c831973a7b952feb0
salt: "EXAMPLE.COMpianist"
passwd: g-clef(U+1011E)
key: 85763726585dbc1cce6ec43e1f751f07f1c4cbb098f40b19
Below are modified-CRC32 values for various ASCII and octet strings.
Only the printable ASCII characters are checksummed, without a C-
style trailing zero-valued octet. The 32-bit modified CRC and the
sequence of output bytes as used in Kerberos are shown. (The octet
values are separated here to emphasize that they are octet values and
not 32-bit numbers, which will be the most convenient form for
manipulation in some implementations. The bit and byte order used
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RFC 3961 Encryption and Checksum Specifications February 2005
internally for such a number is irrelevant; the octet sequence
generated is what is important.)
mod-crc-32("foo") = 33 bc 32 73
mod-crc-32("test0123456789") = d6 88 3e b8
mod-crc-32("MASSACHVSETTS INSTITVTE OF TECHNOLOGY") = f7 80 41 e3
mod-crc-32(8000) = 4b 98 83 3b
mod-crc-32(0008) = 32 88 db 0e
mod-crc-32(0080) = 20 83 b8 ed
mod-crc-32(80) = 20 83 b8 ed
mod-crc-32(80000000) = 3b b6 59 ed
mod-crc-32(00000001) = 96 30 07 77
The encryption and checksum mechanism profiles are new. The old
specification defined a few operations for various mechanisms but
didn't outline what abstract properties should be required of new
mechanisms, or how to ensure that a mechanism specification is
complete enough for interoperability between implementations. The
new profiles differ from the old specification in a few ways:
Some message definitions in [Kerb1510] could be read as permitting
the initial vector to be specified by the application; the text
was too vague. It is explicitly not permitted in this
specification. Some encryption algorithms may not use
initialization vectors, so relying on chosen, secret
initialization vectors for security is unwise. Also, the
prepended confounder in the existing algorithms is roughly
equivalent to a per-message initialization vector that is revealed
in encrypted form. However, carrying state across from one
encryption to another is explicitly permitted through the opaque
"cipher state" object.
The use of key derivation is new.
Several new methods are introduced, including generation of a key
in wire-protocol format from random input data.
The means for influencing the string-to-key algorithm are laid out
more clearly.
Triple-DES support is new.
The pseudo-random function is new.
The des-cbc-crc, DES string-to-key and CRC descriptions have been
updated to align them with existing implementations.
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RFC 3961 Encryption and Checksum Specifications February 2005
[Kerb1510] did not indicate what character set or encoding might be
used for pass phrases and salts.
In [Kerb1510], key types, encryption algorithms, and checksum
algorithms were only loosely associated, and the association was not
well described. In this specification, key types and encryption
algorithms have a one-to-one correspondence, and associations between
encryption and checksum algorithms are described so that checksums
can be computed given negotiated keys, without requiring further
negotiation for checksum types.
Notes
[1] Although Message Authentication Code (MAC) or Message Integrity
Check (MIC) would be more appropriate terms for many of the uses
in this document, we continue to use the term checksum for
historical reasons.
[2] Extending CBC mode across messages would be one obvious example
of this chaining. Another might be the use of counter mode, with
a counter randomly initialized and attached to the ciphertext; a
second message could continue incrementing the counter when
chaining the cipher state, thus avoiding having to transmit
another counter value. However, this chaining is only useful for
uninterrupted, ordered sequences of messages.
[3] 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.
[4] As of the time of this writing, 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.
[5] 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].
[6] For example, in MIT's implementation of [Kerb1510], the rsa-md5
unkeyed checksum of application data may be included in an
authenticator encrypted in a service's key.
[7] Using a variant of the key limits the use of a key to a
particular function, separating the functions of generating a
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RFC 3961 Encryption and Checksum Specifications February 2005
checksum from other encryption performed using the session key.
The constant 0xF0F0F0F0F0F0F0F0 was chosen because it maintains
key parity. The properties of DES precluded the use of the
complement. The same constant is used for similar purpose in the
Message Integrity Check in the Privacy Enhanced Mail standard.
[8] 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). Although Kerberos
directly provides no direct facility for negotiating encryption
types between the application client and server, there are other
means to accomplish similar goals (for example, requesting only
"strong" session key types from the KDC, and assuming that the
type actually returned by the KDC will be understood and
supported by the application server).
Normative References
[BCP26] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[Bellare98] Bellare, M., Desai, A., Pointcheval, D., and P.
Rogaway, "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.
[Blumenthal96] Blumenthal, U. and S. Bellovin, "A Better Key Schedule
for DES-Like Ciphers", Proceedings of PRAGOCRYPT '96,
1996.
[CRC] International Organization for Standardization, "ISO
Information Processing Systems - Data Communication -
High-Level Data Link Control Procedure - Frame
Structure," IS 3309, 3rd Edition, October 1984.
[DES77] National Bureau of Standards, U.S. Department of
Commerce, "Data Encryption Standard," Federal
Information Processing Standards Publication 46,
Washington, DC, 1977.
Raeburn Standards Track [Page 47]
RFC 3961 Encryption and Checksum Specifications February 2005
[DESI81] National Bureau of Standards, U.S. Department of
Commerce, "Guidelines for implementing and using NBS
Data Encryption Standard," Federal Information
Processing Standards Publication 74, Washington, DC,
1981.
[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., and M. Naor, "Non-malleable
cryptography", Proceedings of the 23rd Annual
Symposium on Theory of Computing, ACM, 1991.
[HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
Keyed-Hashing for Message Authentication", RFC 2104,
February 1997.
[KRB5-AES] Raeburn, K., "Advanced Encryption Standard (AES)
Encryption for Kerberos 5", RFC 3962, February 2005.
[MD4-92] Rivest, R., "The MD4 Message-Digest Algorithm", RFC
1320, April 1992.
[MD5-92] Rivest, R., "The MD5 Message-Digest Algorithm ", RFC
1321, April 1992.
[SG92] Stubblebine, S. and V. 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
[Bellovin91] Bellovin, S. M. and M. Merrit, "Limitations of the
Kerberos Authentication System", in Proceedings of the
Winter 1991 Usenix Security Conference, January, 1991.
[Bellovin99] Bellovin, S. M. and D. Atkins, private communications,
1999.
[EFF-DES] Electronic Frontier Foundation, "Cracking DES: Secrets
of Encryption Research, Wiretap Politics, and Chip
Design", O'Reilly & Associates, Inc., May 1998.
[ESP-DES] Madson, C. and N. Doraswamy, "The ESP DES-CBC Cipher
Algorithm With Explicit IV", RFC 2405, November 1998.
Raeburn Standards Track [Page 48]
RFC 3961 Encryption and Checksum Specifications February 2005
[GSS-KRB5] Linn, J., "The Kerberos Version 5 GSS-API Mechanism",
RFC 1964, June 1996.
[HMAC-TEST] Cheng, P. and R. Glenn, "Test Cases for HMAC-MD5 and
HMAC-SHA-1", RFC 2202, September 1997.
[IPSEC-HMAC] Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96
within ESP and AH", RFC 2404, November 1998.
[Kerb] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", Work in
Progress, September 2004.
[Kerb1510] Kohl, J. and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September
1993.
[RC5] Baldwin, R. and R. Rivest, "The RC5, RC5-CBC, RC5-
CBC-Pad, and RC5-CTS Algorithms", RFC 2040, October
1996.
[RFC1851] Karn, P., Metzger, P., and W. Simpson, "The ESP Triple
DES Transform", RFC 1851, September 1995.
[Schneier96] Schneier, B., "Applied Cryptography Second Edition",
John Wiley & Sons, New York, NY, 1996. ISBN 0-471-
12845-7.
Editor's Address
Kenneth Raeburn
Massachusetts Institute of Technology
77 Massachusetts Avenue
Cambridge, MA 02139
EMail: raeburn@mit.edu
Raeburn Standards Track [Page 49]
RFC 3961 Encryption and Checksum Specifications February 2005
Full Copyright Statement
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
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Acknowledgement
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Internet Society.
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