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Re: keytab - MIT Keytab Binary File Format Encoder / Decoder

Michael B Allen <mba2000@ioplex.com> writes:

> I have written a small keytab encoder / decoder module in ANSI C. The
> package can be downloaded here:

You should look at the heimdal code for the for the diffrent versions of
the keytab format. esp lib/krb5/keytab_file.c:storage_set_flags().

>   http://www.ioplex.com/utilities/
> Also, inlined below is a brief description of the keytab binary file
> format. There are a few parts missing that I wasn't certain about. Perhaps
> someone knows the answer to these questions?
> 1) What fields are in host byte order? It seems that everything is big
>    endian except the 16 bit vno and and 16 bit keyblock key type?

Depends on version.

> 2) What codeset are strings? Are they UTF-8 or locale dependant?

"kerberos codeset", today, basicly ascii.

> 3) Are my assumptions about num_components correct: 1 for no service,
>    2 with service?

See code in Heimdal, depends on version. Hint, there is no such thing as
"name" and "service" in kerberos5, its just a list of
components. foo/bar/baz/kaka/cookie@EXAMPLE.ORG is a valid name.

> 4) Have I missed anything?
> If people would like to try this on their keytab files the test1.c test
> decodes, encodes a copy, and then decodes the copy. Please let me know
> what you find.

If you clean up the code, i'll happily include the text in the info
documentation in the info documentation togheter with the Heimdal dump
format that today lives in a textfile in kadmin/. Don't worry about the
texinfo markup, I can deal with that.

The "vno32" as you name it, is a heimdal extention, and now adays java
doesn't truncate your keytab when it finds it. In current heimdal there is
a JAVA14 keytab formats that avoids write the "vno32" field.


> Thanks,
> Mike
> --8<--
> The Kerberos Keytab Binary File Format
> Michael B Allen <mba2000 ioplex.com>
> Last updated: Tue May 2 20:51:41 EDT 2006
> The MIT keytab binary format is not a standard format, nor is it
> documentated anywhere in detail. It is however understood by several
> Kerberos implementations including Heimdal and of course MIT and keytab
> files are created by the ktpass.exe utility from Windows. So it has
> established itself as the defacto format for storing Kerberos keys.
> The following C-like structure definitions illustrate the MIT keytab
> format.
>   keytab {
>       uint16_t file_format_version; /* 0x502 */
>       keytab_entry entries[*];
>   };
>   keytab_entry {
>       int32_t size;
>       uint16_t num_components;
>       counted_string realm;
>       counted_string service; /* optional */
>       counted_string name;
>       uint32_t name_type;
>       uint32_t timestamp;
>       uint16_t vno; /* little endian */
>       keyblock key;
>       uint32_t vno32; /* big endian */
>   };
>   counted_string {
>       uint16_t length;
>       uint8_t data[length];
>   };
>   keyblock {
>       uint16_t type;
>       uint8_t length;
>       uint8_t data[length];
>   };
> The keytab file format begins with the 16 bit file_format_version which
> at the time this document was authored is 0x502.
> The file_format_version is immediately followed by an array of
> keytab_entry structures which are prefixed with a 32 bit size indicating
> the number of bytes that follow in the entry. Note that the size should be
> evaluated as signed. This is because a negative value indicates that the
> entry is in fact empty (e.g. it has been deleted) and that the negative
> value of that negative value (which is of course a positive value) is
> the offset to the next keytab_entry. Based on these size values alone
> the entire keytab file can be traversed.
> The size is followed by a 16 bit num_components field indicating the
> number of string components that follow minus one. In practice this value
> will be either 1 to indicate the realm and name follow or 2 to indicate
> the realm, service, and name follow. Each component is a counted_string.
> The realm, optional service, and name components are counted_strings. A
> counted string is simply an array of bytes prefixed with a 16 bit
> length. With a service principal of HTTP/quark.foo.net@FOO.NET the
> service, name and realm would be "HTTP" "quark.foo.net" and "FOO.NET"
> respectively.
> Following the string components is the name_type. This value is usually
> 1 unless ... ?
> The 32 bit timestamp indicates the time the key was established for that
> principal. The value represents the number of seconds since Jan 1, 1970.
> The 16 bit vno field is the version number of the key. This value may
> be overridden by the vno32 field described below.
> The keyblock structure consists of a 16 bit value indicating the keytype
> (e.g. 3 is des-cbc-md5, 23 is arcfour-hmac-md5, 16 is des3-cbc-sha1,
> etc). This is followed by an 8 bit value indicating the length of the
> key data that follows.
> The last field of the keytab_entry struction is optional. If the size of
> the keytab_entry indicates that there are bytes remaining a 32 bit value
> representing the key version number follows. This value superceeds the
> 16 bit value preceeding the keyblock.
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