Questions on code signing

[muredanta]

Wow. Thanks very much for such a detailed reply. GPG can be counter-intuitive at times, but it seems there is always a way. Shalom!


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‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Tuesday, August 27, 2019 6:30 PM, Werner Koch <wk at gnupg.org> wrote:

> On Tue, 27 Aug 2019 00:18, gnupg-users at gnupg.org said:
>
> > (1) If a file is signed but the signature is incorrect, 'gpg2 -d'
> > returns a non-zero status code, so the remote script knows not to
>
> Right but as stated somewhere in the docs, you should never ever rely on
> the status code fomr the binary. Well, except for the gpgv tool which
> we developed just for the purpose to verify signatures. You can't use
> it however with encrypted+signed data.
>
> > like gpg2 will not catch that case. Is there an option I can give in
> > addition to -d that REQUIRES the file to be signed? I realize you can
>
> No, you need to check that there is a valid signature by checking the
> --status-fd messages. It is a bit tricky to get this right because
> there is a variety of things which can be found in the input to gpg.
> Using detached signatures makes things much easier because you really
> known what has been signed. You still need to check the status-fd
> messages.
>
> You may also use the GPGME library (or the gpgme-json wrapper) to make
> things easier. GPGME takes care of also nitty-gritty you would need to
> do.
>
> > (2) If a file is signed and the remote script has the signer's public
> > key, then a --verify operation will succeed. The trouble is, what if
> > the file is signed by some signature in the remote script's keyring
> > other than the expected code signing key? Is there an option that can
> > be given to --verify (or along with -d, if there's a solution to (1))
> > that can specify the exact signing key (or keys) required? I tried
>
> Your script or progrtam should check which key was used for signing. Or
> you make sure that your local keyring only has that very key. Watch out
> for this --status-fd token:
>
> VALIDSIG <args>
>
> The args are:
>
> - <fingerprint_in_hex>
>
>     - <sig_creation_date>
>
>     - <sig-timestamp>
>
>     - <expire-timestamp>
>
>     - <sig-version>
>
>     - <reserved>
>
>     - <pubkey-algo>
>
>     - <hash-algo>
>
>     - <sig-class>
>
>     - [ <primary-key-fpr> ]
>
>
> This status indicates that the signature is cryptographically
> valid. This is similar to GOODSIG, EXPSIG, EXPKEYSIG, or REVKEYSIG
> (depending on the date and the state of the signature and signing
> key) but has the fingerprint as the argument. Multiple status
> lines (VALIDSIG and the other appropriate *SIG status) are emitted
> for a valid signature. All arguments here are on one long line.
> sig-timestamp is the signature creation time in seconds after the
> epoch. expire-timestamp is the signature expiration time in
> seconds after the epoch (zero means "does not
> expire"). sig-version, pubkey-algo, hash-algo, and sig-class (a
> 2-byte hex value) are all straight from the signature packet.
> PRIMARY-KEY-FPR is the fingerprint of the primary key or identical
> to the first argument. This is useful to get back to the primary
> key without running gpg again for this purpose.
>
> The primary-key-fpr parameter is used for OpenPGP and not
> available for CMS signatures. The sig-version as well as the sig
> class is not defined for CMS and currently set to 0 and 00.
>
> Note, that *-TIMESTAMP may either be a number of seconds since
> Epoch or an ISO 8601 string which can be detected by the presence
> of the letter 'T'.
>
> This can be easily parsed with awk(1) or other tools.
>
> > (3) If a file is both encrypted and signed, is there a way to merely
> > verify the signature other than to include a detached signature? Right
> > now, using --verify on a single file that was encrypted/signed with
> > -es just gives "gpg: verify signatures failed: Unexpected error." Is
> > there a way around that, so I don't need to use a detached signature?
>
> You can extra the signature from the encrypted+signed data:
>
> gpg --unwrap -d -o SIG <ENC+SIG
>
> and then run
>
> gpgv -o SIGNEDFILE SIG && echo verified!
>
> --unwrap is not documented and has the minor problem that it also keeps the
> compression layer. However, gpgv groks that compression layer and works
> as with a standard signature. The signature is on SIGNEDFILE which gpgv
> outputs for you.
>
> Shalom-Salam,
>
> Werner
>
> ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
> Die Gedanken sind frei. Ausnahmen regelt ein Bundesgesetz.