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Valid OpenPGP keys without self-signature?




During OpenPGP interoperability tests, I have recently come across an
unusual situation:

The German company "Robert Bosch GmbH" introduced a PKI on the basis
of a product called "Secure e-mail iT_SEC_outlook". This product uses
old-style V3 RSA keys that are created by the "trust center" of
the company for every user and are signed on creation by the trust
center key. The unusual aspect now is that only the "trust center key"
has a self-signature. All normal user keys have no self-signature but
only the trust center signature on them.

(I have added the trust center key and one user key at the end of
this message to show what I mean.)

My questions now are:

(1) Are such keys a security problem?

    (A key without any signature on it would be open to manipulation,
    of course, but the company claims that their trust center
    signature protects the exact same key contents that a
    self-signature would protect. And according to RFC 2440, this
    really seems to be the case.)

(2) Is such a key conforming to the OpenPGP spec (or at least
    interoperable with a conforming OpenPGP product)?

    (I re-read the spec yesterday and could not find any obvious
    violation although such a key loses lots of features because
    aspects like "Key expiration time", "Preferred symmectric
    algorithms", "Preferred hash algorithms", "Preferred compression
    algorithms", "Key server preferences" and the "Primary user id"
    flag all reside in the self-signature that is missing here.)

(3) Which OpenPGP products support such unusual public keys?

    (In my tests, PGP 5.x imported the user key without any error
    message, whereas GnuPG 1.2.x refused to import the key even in
    "--expert" mode. And a PGP 8.02 user reported to me that PGP 8.02
    also refuses to import any keys without self-signature.)

As I either have to find a solution to this interoperability problem
or have to prove to the company that their setup is insecure, I would
be very happy about any answers to the above questions.

- Wolfgang Redtenbacher

---------------------------------------------------------------------
Redtenbacher Software                Tel.:   +49 7159 17046
Roemerstr. 11/1                      Fax:    +49 7159 17047
D-71272 Renningen                    e-mail: wolfgang@xxxxxxxxxxxxxxx
---------------------------------------------------------------------

The following is the trust center key of the company:
  Rb.Trustcenter@xxxxxxxxx
  Rb.Trustcenter@xxxxxxxxxxxx
  Rb.Trustcenter@xxxxxxxxxxxx

-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: Secure e-mail iT_SEC_outlook V2.0.2
Comment: <http://www.it-sec.com>
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==
=J70u
-----END PGP PUBLIC KEY BLOCK-----

The following is a user key without self-signature, but with the
trust center signature on it:
   Uwe.Wetzel@xxxxxxxxx
   Uwe.Wetzel@xxxxxxxxxxxx

-----BEGIN PGP PUBLIC KEY BLOCK-----
Version: Secure e-mail iT_SEC_outlook V2.0.2
Comment: <http://www.it-sec.com>
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=+NiC
-----END PGP PUBLIC KEY BLOCK-----