gpg-interface: prefer check_signature() for GPG verification

This commit refactors the use of verify_signed_buffer() outside of
gpg-interface.c to use check_signature() instead.  It also turns
verify_signed_buffer() into a file-local function since it's now only
invoked internally by check_signature().

There were previously two globally scoped functions used in different
parts of Git to perform GPG signature verification:
verify_signed_buffer() and check_signature().  Now only
check_signature() is used.

The verify_signed_buffer() function doesn't guard against duplicate
signatures as described by Michał Górny [1].  Instead it only ensures a
non-erroneous exit code from GPG and the presence of at least one
GOODSIG status field.  This stands in contrast with check_signature()
that returns an error if more than one signature is encountered.

The lower degree of verification makes the use of verify_signed_buffer()
problematic if callers don't parse and validate the various parts of the
GPG status message themselves.  And processing these messages seems like
a task that should be reserved to gpg-interface.c with the function
check_signature().

Furthermore, the use of verify_signed_buffer() makes it difficult to
introduce new functionality that relies on the content of the GPG status
lines.

Now all operations that does signature verification share a single entry
point to gpg-interface.c.  This makes it easier to propagate changed or
additional functionality in GPG signature verification to all parts of
Git, without having odd edge-cases that don't perform the same degree of
verification.

[1] https://dev.gentoo.org/~mgorny/articles/attack-on-git-signature-verification.html

Signed-off-by: Hans Jerry Illikainen <hji@dyntopia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 0 insertions, 9 deletions

diff --git a/gpg-interface.h b/gpg-interface.h
index 3e624ec289..93cc3aff5c 100644
--- a/gpg-interface.h
+++ b/gpg-interface.h
@@ -46,15 +46,6 @@ size_t parse_signature(const char *buf, size_t size);
 int sign_buffer(struct strbuf *buffer, struct strbuf *signature,
 		const char *signing_key);
 
-/*
- * Run "gpg" to see if the payload matches the detached signature.
- * gpg_output, when set, receives the diagnostic output from GPG.
- * gpg_status, when set, receives the status output from GPG.
- */
-int verify_signed_buffer(const char *payload, size_t payload_size,
-			 const char *signature, size_t signature_size,
-			 struct strbuf *gpg_output, struct strbuf *gpg_status);
-
 int git_gpg_config(const char *, const char *, void *);
 void set_signing_key(const char *);
 const char *get_signing_key(void);