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yamltab

yamltab converts Kerberos keytabs to YAML/JSON and the other way around.

Keytab input, YAML/JSON output

Given a keytab file, yamltab will output a YAML representation of its contents, suitable for human edition

$ yamltab example.keytab
version: 2
entries:
- spn: HTTP/[email protected]
  principal:
    name_type: KRB5_NT_PRINCIPAL
    components:
    - HTTP
    - application.example.com
    realm: EXAMPLE.COM
  kvno: 42
  date: '2020-04-28T14:04:12+0000'
  enctype: AES256_CTS_HMAC_SHA1_96
  key: 0123456789abcdeffedcba9876543210abcdeffedcba9876543210abcdef1234

JSON output is also supported:

$ yamltab -f json example.keytab | jq '.entries[0].principal.realm'
"EXAMPLE.COM"

Data layouts

yamltab offers several "data layouts" for its YAML/JSON output:

  • raw simply reflects the raw, complete binary structure: it reflects records and holes, tails (i.e. data in a record beyond its entry length) and exposes raw values (name types, timestamps, encryption types, etc.), without any kind of interpretation.
  • full is the same structure as raw with additional properties for the sake of readability.
  • simple, shown above, is the default format; it aims at exposing everything that matters in the keytab in a user-friendly way.
$ yamltab -l raw -f json example.keytab | jq '.records[0].entry.timestamp'
1588082652
$ yamltab -l full -f json example.keytab | jq '.records[0].entry.date'
"2020-04-28T14:04:12+0000"
$ yamltab -l simple -f json example.keytab | jq '.entries[0].date'
"2020-04-28T14:04:12+0000"

YAML/JSON input, keytab output

yamltab also offers the ability to convert YAML/JSON input into a binary keytab:

$ yamltab example.keytab.yaml | file -
/dev/stdin: Kerberos Keytab file, realm=EXAMPLE.COM, principal=HTTP/application.example.com, type=1, date=Tue Apr 28 14:04:12 2020, kvno=42

It supports all three data layouts: raw is useful to control every detail of the keytab; full is processed the same way. Internally, simple input is converted to the full data layout; the resulting structure can be inspected using --simple-to-full.

Other options

--names-types and --enc-types list known name and encryption types, respectively. They can be combined with --format:

$ ./yamltab -f json --enc-types | jq '.AES256_CTS_HMAC_SHA1_96'
18

--v1-endianness exists as a vague attempt to support keytab v1 format. However, keytab v2 has been around since 1992 and real-life keytab v1 files are nowhere to be found. Consequently, keytab v1 support is completely untested and very likely to result in a stacktrace or absurd output.

Keytab edition

Since yamltab can convert a keytab to YAML and this YAML representation back into a keytab, then it should be easy to assemble a wrapper that allows interactive edition of a keytab file as YAML by invoking your favourite editor. This is what the keytab-editor script does.

Subtleties

The simple format is liable to expose two overlapping properties: spn and principal. (the former is convenient, the latter is comprehensive). When processing its YAML input, yamltab takes principal into account and ignores spn... unless principal happens to be missing, in which case yamltab attempts to convert spn into a KRB5_NT_PRINCIPAL principal.

name_type_raw, enc_type_raw and timestamp override name_type, enc_type and date respectively.

Date handling is NOT clever. Stick to the YYYY-mm-DDTHHMMSS+ZZZZ format. That said, it is possible to write date: now in simple mode.

In simple mode,kvno_in_tail: True can be used to force storage of the key version number as a 32-bit unsigned integer in the tail (i.e. after the entry). extra_tail can be used to inject arbitrary data in the tail. In full mode, the following keys strive to reflect kvno:

  • kvno: 8-bit key version number found in the entry, always present;
  • tail_kvno: 32-bit key version number found right after the entry, present only if actually found;
  • actual_kvno: kvno value that matters, always present.

Limitations: yamltab spares you the pain of hex-editing keytab files; however, it offers no encryption-related features. Keys are always handled as hexadecimal blobs, and password hashing is simply not supported.