["Ronnie Sahlberg" <ronnie_sahlberg@xxxxxxxxxxxxxx>]

I looked briefly at
http://www.lysator.liu.se/~nisse/nettle/
which seems like a reasonably low-lever library for what we would need and
sopported on many of our platforms.

What you are saying is that I just cant take the shared secret key from the
keytab file and
push it through a des_decrypt(...) function but that I need to do some
manupilation onto the key from the keytab BEFORE i can pass this
key into dissect_des(...) ?
I was under the impression that the salt was only mixed with the password in
order to genreate the key from password+salt => keytab entry
and that the keytab entry was the real key to input into the encrypt/decrypt
engine.
i.e. salt was only required to go from plaintext password to key but then
the key could be used unmodified to pass to the decrypte_/encrypt_
functions.?


that kind of seems complex and suboptimal.
Anyway, do you know for example hat kind of manipulation processing I would
need to do on the key in the keytab in the example I posted
prevoisly before i can feed it to decrypt_xxx() or where i can find the
answer.

remember i am lazy and want it all served on a silver plate :-)



----- Original Message ----- 
From: "Eric Wedel"
Sent: Thursday, February 05, 2004 3:11 PM
Subject: RE: [Ethereal-dev] telnet/authentication Kerberos stuff


> Hi..
>
> Trying to take your questions mostly in order, yes there is an API
> to do the decryption:
>   krb5_c_decrypt(context, key, usage, ivec, input, output)
> in src/lib/crypto/decrypt.c.  Don't know if this is considered "public"
> or not though.
>
> Those formals are roughly:
> context - passed all over the place inside MIT kerb, not used here.
> key - Your key value, from a keytab, ticket or wherever.  _Includes
> encryption type_
> usage - A "salt" value, see below.
> ivec - Might be used by some ciphers, hasn't yet been used in my ADS work.
> input, output - The usual.
>
> That "usage" value is kinda fun.  Don't really know why, but kerberos
> includes
> this structure-specific "salt" value which is factored into the cipher
> calculations.
> This value is a constant that is determined by the higher-level code which
> knows
> what structure it is operating on.  E.g., krb5_mk_req_extended() which
> builds an
> AP-REQ uses value KRB5_KEYUSAGE_AP_REQ_AUTH, which == 11, when encrypting
> its
> authenticator.  This is part of the protocol and cannot be avoided.
>
> A general word on ciphertext:  kerberos always sends these as an
> asn.1-structured
> pair, a "blob" plus an integer encryption type.  The latter helps you to
> determine
> which key to grab from your keytab (along with the principal name of
> course), and
> as shown above is also fed in to the encryption code, which does a table
> lookup based
> on the etype to find the right cipher module.
>
> The master table of known types is in src/lib/crypto/etypes.c.
>
> Once decrypted, the ciphertext is revealed to be yet more asn.1.  You'll
> need to
> browse the source to see what the various fields mean.  As luck would have
> it,
> most of the asn.1 "structure" deserializers may be found in one file:
>   src/lib/krb5/asn1./krb5_decode.c.
> Or this info should be lurking in an RFC somewhere if you prefer.
>
> ---
>
> In answer to your question about decrypting a ticket's payload, yes that
> should
> work, if you use the code as discussed above.  You would then need to grab
> the embedded
> session key and use that to decrypt further traffic in the conversation.
> [iirc, there
> is an optional subkey which may be specified in the AP-REP.  You thus need
> to decrypt
> the AP-REP to see if a subkey is being used and, if so, adopt it as your
> session key.
> Afraid I don't recall under what circumstances it is used.]
>
> regards, Eric
>
>
> -----Original Message-----
> From: Ronnie Sahlberg [mailto:ronnie_sahlberg@xxxxxxxxxxxxxx]
> Sent: Wednesday, February 04, 2004 12:54 AM
> To: Eric Wedel; ethereal-dev@xxxxxxxxxxxx
> Subject: Re: [Ethereal-dev] telnet/authentication Kerberos stuff
>
>
> OK. I will consider to consider your advice. maybe.
> (reluctantly though, i got "advice" on how "sane" it would be to rewrite
> also
> smb, most of the dcerpc interfaces and h.323/voip but look where it got us
> to
> ignore those advices:-) )
>
> anyway, to the point:
> Right now I have a number of captures of Kerberos authenticated services
and
> also the corresponding host/<hostname>
> keytabs containing the principals and the keys. these will be excellent
for
> testing and verification.
>
> My modified version of the kerberos dissector and BER dissection makes it
> very easy to pick up the
> individual fields of the PDU.
> Parsing the keytab files is trivial and should not require any external
> software.
>
>
> I want an easy to use function I can call to decrypt an encrypted array of
> bytes into a new array holding the
> decrypted data. Something like :
> void decrypt_des_cbc_crc(char *encrypted_data, char *decrypted_data, int
> len, char *key, int key_len);
>
> Does MIT or Heimdal kerberos provide shared libraries where I can find a
> function as easy to use as this
> or do i need to do like 1000 lines of setup and stuff first before i can
do
> this easy function?
>
> Do they proivide me with such a simple API?
>
>
>
> I dont really need all of the stuff for things like certificates or so.
> Just a simple function to do  decrypt_des_cbc_crc() and return the
decrypted
> blob which hopefully is asn.1
> encoded which i will then manually dissect.
>
>
> When this works fine I or anyone needing some other method can move on to
> add support for, i dont know
> des_cbc_md5(), whatever.  one by one as people need support for new types.
>
>
> How hard would it be to implement a function as above to just decrypt a
blob
> using des-cbc and not do anything else.
> No asn.1 decoding.  that will be done manually anyway after the blob is
> decrypted.
>
>
> It is a serious question.
>
>
> Another serious question:
> (Assume the encryption type is des-cbc-<something>)
>
> Assume I have a kerberos ticket and the ticket specifies the service
> principal host/foo
> Assume that the encrypted part of the ticket is specified as using the
> encryption type des-cbc-<something>
> Assume i find in the Keytab file the 8 bytes representing the key/secret
for
> this principal   CALL these 8 bytes : KEY.
> IF I then take the content octets from the octet string holding the
> encrypted data and call it  ENC
> IF I would then just try to do
>     decrypt_des_cbc(ENC, CLEAR, KEY)   (assuming i have a function
> decrypt_des_cbc() of course)
> would that transform the ENCrypted byte string into a normal ASN.1/BER
> encoded data structure?
>
> If it would, would decrypt_des_cbc() be very difficult to implement?
>
>
>
>
>
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