signed push: allow stale nonce in stateless mode

When operating with the stateless RPC mode, we will receive a nonce
issued by another instance of us that advertised our capability and
refs some time ago.  Update the logic to check received nonce to
detect this case, compute how much time has passed since the nonce
was issued and report the status with a new environment variable
GIT_PUSH_CERT_NONCE_SLOP to the hooks.

GIT_PUSH_CERT_NONCE_STATUS will report "SLOP" in such a case.  The
hooks are free to decide how large a slop it is willing to accept.

Strictly speaking, the "nonce" is not really a "nonce" anymore in
the stateless RPC mode, as it will happily take any "nonce" issued
by it (which is protected by HMAC and its secret key) as long as it
is fresh enough.  The degree of this security degradation, relative
to the native protocol, is about the same as the "we make sure that
the 'git push' decided to update our refs with new objects based on
the freshest observation of our refs by making sure the values they
claim the original value of the refs they ask us to update exactly
match the current state" security is loosened to accomodate the
stateless RPC mode in the existing code without this series, so
there is no need for those who are already using smart HTTP to push
to their repositories to be alarmed any more than they already are.

In addition, the server operator can set receive.certnonceslop
configuration variable to specify how stale a nonce can be (in
seconds).  When this variable is set, and if the nonce received in
the certificate that passes the HMAC check was less than that many
seconds old, hooks are given "OK" in GIT_PUSH_CERT_NONCE_STATUS
(instead of "SLOP") and the received nonce value is given in
GIT_PUSH_CERT_NONCE, which makes it easier for a simple-minded
hook to check if the certificate we received is recent enough.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 26 insertions, 0 deletions

diff --git a/Documentation/config.txt b/Documentation/config.txt
index dd6fd65e9f..d73366f6b8 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -2049,6 +2049,19 @@ receive.certnonceseed::
 	a "nonce" protected by HMAC using this string as a secret
 	key.
 
+receive.certnonceslop::
+	When a `git push --signed` sent a push certificate with a
+	"nonce" that was issued by a receive-pack serving the same
+	repository within this many seconds, export the "nonce"
+	found in the certificate to `GIT_PUSH_CERT_NONCE` to the
+	hooks (instead of what the receive-pack asked the sending
+	side to include).  This may allow writing checks in
+	`pre-receive` and `post-receive` a bit easier.  Instead of
+	checking `GIT_PUSH_CERT_NONCE_SLOP` environment variable
+	that records by how many seconds the nonce is stale to
+	decide if they want to accept the certificate, they only
+	can check `GIT_PUSH_CERT_NONCE_STATUS` is `OK`.
+
 receive.fsckObjects::
 	If it is set to true, git-receive-pack will check all received
 	objects. It will abort in the case of a malformed object or a
diff --git a/Documentation/git-receive-pack.txt b/Documentation/git-receive-pack.txt
index 2d4b45242c..9016960e27 100644
--- a/Documentation/git-receive-pack.txt
+++ b/Documentation/git-receive-pack.txt
@@ -89,6 +89,19 @@ the following environment variables:
 	"git push --signed" sent a bogus nonce.
 `OK`;;
 	"git push --signed" sent the nonce we asked it to send.
+`SLOP`;;
+	"git push --signed" sent a nonce different from what we
+	asked it to send now, but in a previous session.  See
+	`GIT_PUSH_CERT_NONCE_SLOP` environment variable.
+
+`GIT_PUSH_CERT_NONCE_SLOP`::
+	"git push --signed" sent a nonce different from what we
+	asked it to send now, but in a different session whose
+	starting time is different by this many seconds from the
+	current session.  Only meaningful when
+	`GIT_PUSH_CERT_NONCE_STATUS` says `SLOP`.
+	Also read about `receive.certnonceslop` variable in
+	linkgit:git-config[1].
 
 This hook is called before any refname is updated and before any
 fast-forward checks are performed.