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krb-aead-id.lyx
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AEAD Encryption Types for Kerberos 5
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draft-howard-krb-aead-01
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AEAD Encryption Types for Kerberos 5
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Security Area
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Internet-Draft
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\begin_layout Author
Luke Howard
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PADL Software
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PADL
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L.
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Howard
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PO Box 59
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Australia
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3145
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\begin_layout Abstract
This document updates
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to support encryption mechanisms that can authenticate associated data,
such as Counter with CBC-MAC (CCM) and Galois/Counter Mode (GCM).
These mechanisms are often more performant and need not expand the message
as much as conventional modes.
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Introduction
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This document updates
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for encryption mechanisms that support Authenticated Encryption with Associated
Data (AEAD), such as Counter with CBC-MAC (CCM) and Galois/Counter Mode
(GCM).
These mechanisms provide the ability to authenticate additional data associated
with a plaintext.
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In addition, these mechanisms often have performance advantage over conventional
encryption modes such as Cipher Block Chaining (CBC) and Ciphertext Stealing
(CTS) as they can be efficiently parallelized and the absence of a confounder
allows for shorter messages.
For example, the ciphertext output by the AEAD encryption mechanisms described
in this document is 28 bytes shorter than those specified in
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.
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Requirements notation
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The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD",
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Authenticated Encryption with Associated Data (AEAD) Overview
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provides for authenticated encryption of plaintext in Kerberos 5, that
is, it provides both for confidentiality and a way to check the for integrity
and authenticity of a message.
Some applications can benefit from protecting the integrity and authenticity
of unencrypted data accompanying a ciphertext: this is termed Authenticated
Encryption with Associated Data (AEAD).
(A general description of AEAD is given in
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.)
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Existing encryption and checksum mechanisms can provide AEAD through generic
composition, where the checksum is made over both the associated data and
plaintext.
Alternatively, modern encryption mechanisms such as those profiled in
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explicitly provide for AEAD.
Both approaches are discussed in this document.
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Algorithms that are specifically designed for AEAD may have additional constrain
ts to be imposed on cryptosystems that would otherwise be definable in terms
of
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.
We term these algorithms native AEAD, to distinguish them from the generic
composition of non-AEAD encryption and checksum types.
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Updates to RFC 3961
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The changes described below amend
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for use with native AEAD encryption types.
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\begin_layout Subsection
Additional Requirements
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The native AEAD algorithms profiled in
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and
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have the property where the reuse of a particular combination of secret
key and initialization vector destroys all security guarantees of the underlyin
g mechanism.
This contrasts with existing Kerberos ciphers such as those defined in
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where the random confounder (used instead of an initialization vector)
need not be non-repeating.
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To accommodate the use of native AEAD cryptosystems with Kerberos, we allow
an encryption mechanism profile to adjust its behavior depending on whether
a long-term is being used.
It is valid for a profile to not support long-term keys.
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Where an ephemeral key is used, applications MUST guarantee that each invocation
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Native AEAD encryption types share the same namespace as existing Kerberos
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facilities such as
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RFC4537
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.
Encryption types that only support ephemeral keys SHALL NOT be advertised
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Removal of Mandatory Requirements
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requires that an encryption mechanism profile define both an associated
checksum type and a string-to-key function.
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Native AEAD algorithms do not provide for integrity protection outside of
authenticated encryption, so we remove the requirement for an associated
checksum type.
(An application that only requires integrity protection may invoke the
authenticated encryption function with a zero length plaintext.)
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These ciphers are not required to support long-term keys, so we remove the
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Application-managed Cipherstate
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Whilst
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requires that the cipherstate be opaque to the application, we relax this
so that the application may manage some or all of the cipherstate explicitly.
Native AEAD ciphers SHALL nominate how many bits of their cipherstate may
be deterministic and, by extension, may be managed by the application.
(The encryption types defined in this document allow all bits of the cipherstat
e to be managed by the application.)
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In order to meet the security requirements of the underlying cryptosystems,
applications using ephemeral keys MUST NOT invoke the authenticated encryption
function with a previously used (key, cipherstate) combination.
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Applications that support native AEAD ciphers MAY impose a minimum deterministic
octet length requirement on the cipherstate.
Such applications MUST NOT be used with ciphers with a shorter cipherstate
length.
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Applications MAY set the deterministic cipherstate component directly on
each invocation of the authenticated encryption function.
This allows a non-repeating counter such as a sequence number to be embedded
in the cipherstate, without preventing the application from processing
out-of-order messages.
Alternatively, an application MAY set the initial cipherstate to a random
value and pass the cipherstate output from the authenticated encryption
and decryption functions to subsequent invocations.
Using a randomly generated cipherstate on each invocation may place a limit
on the number of invocations of the authenticated encryption function when
compared with a deterministic construction.
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\begin_layout Subsection
Encrypt/decrypt with Associated Data
\end_layout
\begin_layout Standard
We define the following functions for encrypting and decrypting with associated
data:
\end_layout
\begin_layout Itemize
encrypt-with-ad (specific-key, state, associated data, octet string, is-longterm
)->(state, octet string)
\end_layout
\begin_layout Itemize
decrypt-with-ad (specific-key, state, associated data, octet string, is-longterm
)->(state, octet string)
\end_layout
\begin_layout Standard
The associated data parameter is input only and contains data that is to
be authenticated, but not encrypted.
If the associated data or plaintext are zero length strings, then these
functions are equivalent respectively to the checksum and encryption functions
described in
\begin_inset Flex EntityXRef
status open
\begin_layout Plain Layout
RFC3961
\end_layout
\end_inset
.
\end_layout
\begin_layout RevisionRemark
NB: these functions only allow a single associated data and plaintext buffer
to be supplied, which is not compatible with applications such as DCE RPC
that interleave the associated data and plaintext buffers.
Support for such applications is implementation dependent and not guaranteed
to work with native AEAD ciphers that only take a single buffer of each.
\end_layout
\begin_layout Standard
The boolean is-longterm parameter indicates whether specific-key is a long-term
key or a session key.
\end_layout
\begin_layout Standard
Correspondingly, the encrypt and decrypt functions are updated to take the
is-longterm parameter:
\end_layout
\begin_layout Itemize
encrypt (specific-key, state, octet string, is-longterm)->(state, octet
string)
\end_layout
\begin_layout Itemize
decrypt (specific-key, state, octet string, is-longterm)->(state, octet
string)
\end_layout
\begin_layout Standard
A profile that specifices encrypt-with-ad and decrypt-with-ad need not explicitl
y specify encrypt and decrypt functions.
Profiles that adhere to
\begin_inset Flex EntityXRef
status open
\begin_layout Plain Layout
RFC3961
\end_layout
\end_inset
take an implicit value of TRUE for is-longterm.
\end_layout
\begin_layout Subsection
AEAD with Simplified Profile
\end_layout
\begin_layout Standard
We allow the authentication of associated data with existing cryptosystems
that follow the Simplified Profile defined in
\begin_inset Flex EntityXRef
status open
\begin_layout Plain Layout
RFC3961
\end_layout
\end_inset
through the generic composition of the encryption and checksum functions.
The is-longterm parameter is ignored.
For consistency with
\begin_inset Flex EntityXRef
status open
\begin_layout Plain Layout
RFC3961
\end_layout
\end_inset
the following definition uses 1-based indexing.
\end_layout
\begin_layout Standard
\begin_inset listings
inline false
status open
\begin_layout Plain Layout
encrypt-with-ad conf = Random string of length c
\end_layout
\begin_layout Plain Layout
pad = Shortest string to bring confounder
\end_layout
\begin_layout Plain Layout
and plaintext to a length that's a
\end_layout
\begin_layout Plain Layout
multiple of M
\end_layout
\begin_layout Plain Layout