[pkg-gnupg-maint] Bug#978630: gnupg: --check-sigs trusts weak digest alg if weak digest was trusted when importing key

[Ansgar]

On Tue, 2020-12-29 at 13:15 +0100, Ansgar wrote:
> gpg --check-sigs seems to trust digest algs depending on what digest
> algs were trusted when the key was imported:
>
> I have `weak-digest SHA1` and `weak-digest RIPEMD160` in my gpg.conf
> and observed this behavior:

As a further observeration, this doesn't happen when the key was
retrieved by `--recv-keys`:

+---
| $ gpg --keyserver-options no-self-sigs-only --keyserver keyserver.ubuntu.com --recv-keys
| B1AEA6F29103A00A4D5212A15B3C275D60BF72BE
| gpg: Note: signatures using the SHA1 algorithm are rejected
| gpg: key 0x5B3C275D60BF72BE: 2 signatures not checked due to missing keys
| gpg: key 0x5B3C275D60BF72BE: 4 bad signatures
| gpg: key 0x5B3C275D60BF72BE: public key "[...]" imported
| gpg: marginals needed: 3  completes needed: 1  trust model: pgp
| gpg: depth: 0  valid:   1  signed:   5  trust: 0-, 0q, 0n, 0m, 0f, 1u
| gpg: depth: 1  valid:   5  signed:   7  trust: 0-, 1q, 2n, 0m, 2f, 0u
| gpg: depth: 2  valid:   7  signed:   2  trust: 0-, 0q, 4n, 2m, 1f, 0u
| gpg: depth: 3  valid:   1  signed:   2  trust: 0-, 0q, 0n, 1m, 0f, 0u
| gpg: next trustdb check due at 2021-03-20
| gpg: Total number processed: 1
| gpg:               imported: 1
| $ gpg --keyring /usr/share/keyrings/debian-keyring.gpg  --check-sigs 0x5B3C275D60BF72BE
| gpg: Note: signatures using the SHA1 algorithm are rejected
| pub   rsa4096/0x5B3C275D60BF72BE 2013-02-24 [SC] [expires: 2025-02-23]
|       B1AEA6F29103A00A4D5212A15B3C275D60BF72BE
| uid                   [ unknown] [...]
| sig!3        0x5B3C275D60BF72BE 2020-07-16  [...]
| sig!2        0x69F2FC516EA71993 2020-08-05  [...]
| uid                   [ unknown] [...]
| sig!3        0x5B3C275D60BF72BE 2020-02-21  [...]
| sig%3        0x5B3C275D60BF72BE 2018-02-23  [Invalid digest algorithm]
| sig%3        0x5B3C275D60BF72BE 2013-02-24  [Invalid digest algorithm]
| sig!2        0x69F2FC516EA71993 2020-08-05  [...]
| sub   rsa4096/0xD1660B54B5E3F109 2013-02-24 [E] [expires: 2025-02-23]
| sig!         0x5B3C275D60BF72BE 2020-02-21  [...]
|
| gpg: 5 good signatures
| gpg: 2 signatures not checked due to errors
+---

Signature rejected as expected.

+---
| $ gpg --allow-weak-digest-algos --keyring /usr/share/keyrings/debian-keyring.gpg --check-sigs 0x5B3C275D60BF72BE
| pub   rsa4096/0x5B3C275D60BF72BE 2013-02-24 [SC] [expires: 2025-02-23]
|       B1AEA6F29103A00A4D5212A15B3C275D60BF72BE
| uid                   [ unknown] [...]
| sig!3        0x5B3C275D60BF72BE 2020-07-16  [...]
| sig!2        0x69F2FC516EA71993 2020-08-05  [...]
| uid                   [ unknown] [...]
| sig!3        0x5B3C275D60BF72BE 2020-02-21  [...]
| sig!3        0x5B3C275D60BF72BE 2018-02-23  [...]
| sig!3        0x5B3C275D60BF72BE 2013-02-24  [...]
| sig!2        0x69F2FC516EA71993 2020-08-05  [...]
| sub   rsa4096/0xD1660B54B5E3F109 2013-02-24 [E] [expires: 2025-02-23]
| sig!         0x5B3C275D60BF72BE 2020-02-21  [...]
|
| gpg: 7 good signatures
+---

And `--allow-weak-digest-algos` passed to the `--check-sigs` call
shows the signatures as valid as I would expect and what didn't happen
when the key was imported using `--import`.

Ansgar