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Re: IETF mtg discussion comments

From: Paul C. Kocher <pck@best.com>
Date: Sun, 30 Jun 1996 14:02:33 -0700
Message-Id: <199606302102.OAA11590@shellx.best.com>
To: ietf-tls@w3.org, Bennet Yee <bsy@cs.ucsd.edu>
Cc: pck@cryptography.com
Bennet Yee writes:
1.  Does pre-encryption prevent MACs from being encrypted?
> First, Paul Kocher said that in order to support pre-encrypted data,
> the MAC (independent of whether it is an on-the-fly computed MAC or a
> pre-computed MAC) must be left outside of the encryption.  That is,
> the record format will include a header, pre-encrypted data, and an
> un-encrypted MAC.
> This is not quite correct.
> What -is- required is that the MAC must go to the end of the record
> (true for SSLv3.x / PCT / now-dropped TLS-draft) and that per-record
> IVs be permitted for block ciphers -- or have the IV for the next
> record be the last data block, excluding the MAC block(s).
> Consider first the case of a stream cipher.  To have encrypted MACs,
> we simply run the stream cipher to generate enough output to encrypt
> (xor with) the MAC and store that with the "compiled" or pre-encrypted
> version of the data.  When we've sent the encrypted data and it's time
> to send the MAC, we can grab the saved stream cipher output and xor it
> with the MAC prior to transmission.
>  [Long description of how to do this for a block cipher deleted.]

I was well aware of this, but assumed that since it was such a grotesque
layering violation it was not an acceptable option.  Changing
the MAC around is a relatively minor change anyway (and happened to
be virtually the only change which *doesn't* really cause layering 

> 2.  Constancy of Communication Channel Security
>  [ Deleted]
> 3.  Symmetry Breaking
>  [ Deleted ]
> 4.  Channel Security Constancy and Pre-encryption
> It may seem that the Channel Security Constancy principle would argue
> against the use of pre-encryption.  (This was what Eric Rescorla
> referred to during the meeting as Bennet's Law, but I bet the idea
> predates me.)
> [...]  Furthermore, the
> point of pre-encryption is actually two fold: first, the more widely
> recognized desire to increase (web/ftp/etc) server performance, and
> second, the security of multiply retransmitted data.
> The first is pretty obvious.  If we can pre-encrypt data (via a sort
> of compilation process) on servers, the servers can save on cycles
> that would otherwise have to be used to do on-the-fly encryption.
> The second is slightly subtler.  For oft retransmitted data,
> re-encrypting them under different keys, especially weak, 40-bit keys,
> for many transmissions provides attackers with more partial
> information about the plaintext.  If there is only one encrypted
> version of the data that is always re-sent, less partial information
> about the data is leaked.

TLS/SSL and other cryptosystems using 40-bit keys always include
a nonce or salt with the key to prevent birthday-paradox style 
attacks against messages encrypted with more than one key.

While having multiple copies of the plaintext encrypted under
different keys could theoretically help for some kinds of 
cryptanalytic attack, this sort of data could be collected
using a chosen plaintext attack as well (which the protocol
must be able to resist).

> [...]
> 5.  Modularization and Layering, and Pre-encryption
> As a general purpose cryptographic protocol, TLS's embodiment in an
> implementation should make it easy to use but difficult to misuse.
> Furthermore, the implementation should be cleanly engineered, so that
> security reviews are feasible.  Does pre-encryption violate
> "layering", a sound engineering principle?
> As I've mentioned in other email to this list, transforming a
> plaintext file into a pre-encrypted file may be thought of as a
> compilation process.  This translator is intimately bound with the
> record layer, since it must know the record format as well as the
> ciphers to use, and the key management layer simply passes the
> compiled file down to the record layer uninterpreted except for
> extracting (and sending) the pre-encryption key and verifying that the
> cipher spec matches.
> I don't view this as a terrible layering violation -- it's nothing
> more than viewing the data transmission process as incorporating a
> just-in-time data compiler that caches, with the key management layer
> specifying a fixed encryption key for the file.

In the server it sounds as though you're merging all the layers 
into one to avoid the problem, which isn't pretty though I suppose
that's more of an implementation issue.  The protocol-level layering
issues revolve around the question of how to send the pre-encrypted 
data/MAC keys over the wire, how the keys get injected, how to
avoid killing pipelined implementations, etc.

> 6.  Pre-MAC-ing Data

[ Longish but interesting description of the two pre-MACed data 
constructions omitted since pre-MACing currently seems to be
off the agenda. ]

> 7.  Cryptographic "Linking" of the Password with the Master Secret

If password authentication is supported, there are three general

1)  Send the password (hashed under the Randoms) encrypted under
    the standard encryption key.  Advantage:  Cleaner, simpler,
    can be done by clients without altering the protocol.
    Disadvantage:  Someone can break the encryption key then
    attack the password using brute force.

2)  Send the password hashed under the master secret or some
    value derived from it.  (I don't necessarily recommend the MAC 
    secret; its job is to protect data from being altered, and 
    probably shouldn't be used for other things.  One possible
    way to implement it would be to add a field in the data fed
    into the handshake hashes to for the password, if password
    authentication is supported.  Disadvantage:  Protocol
    changes required.  Advantage: Breaking encryption does not
    allow brute force attack against the password.

3)  Applications could get the master secret and hash the password 
    with it, then send the result across the wire.  Disadvantage: 
    major layering violations in applications.  Advantage:  No
    protocol changes required.

Two issues we need to decide:

  - Whether to support password authentication at the TLS level.
     (Type 2 can be done today by applications using SSL 3.0)
     I think everyone here wants to see a certificate-based
     infrastructure get developed as quickly as possible, but it
     isn't clear whether giving people an alternative to certs
     is good (because it helps them make the transision to
     certs more smooth) or bad (because it gives them a way to
     avoid switching to certs).

  - Whether passwords have to be protected from brute force

> 8.  Features are Optional
> Pre-encryption, pre-MAC-ing, ans password authentication are all
> independent options that are (typically) server configuration choices.
> If you don't want them, clients should be able to leave these out of
> the crypto function negotiation.  (Though pre-encryption, in my
> opinion, should always be used since it -helps- security when the
> cipher strength matches [pre-encryption uses the same cipher as normal
> encryption].)

Pre-encryption selection will require significant changes to how the
ciphersuites are negotiated, since some can support pre-encrypted data
and others probably won't be able to (i.e., Fortezza).

I don't really agree that pre-encryption helps cryptographic security --
it actually introduces new potential weaknesses.  For example, there
is the risk that the encryption mechanism (probably based on a hash 
function) used to send the key over the wire could be broken.  Its
only real advantage is improved performance.  (Which I do think could be
useful for some applications as secure FTP servers.)

> 9.  Misc


Paul Kocher (pck@cryptography.com) |     Voicemail: +1-(415)-354-8004
Crypto consultant                  |           FAX: +1-(415)-321-1483
Received on Sunday, 30 June 1996 17:02:52 UTC

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