signed push: fortify against replay attacks

In order to prevent a valid push certificate for pushing into an
repository from getting replayed in a different push operation, send
a nonce string from the receive-pack process and have the signer
include it in the push certificate.  The receiving end uses an HMAC
hash of the path to the repository it serves and the current time
stamp, hashed with a secret seed (the secret seed does not have to
be per-repository but can be defined in /etc/gitconfig) to generate
the nonce, in order to ensure that a random third party cannot forge
a nonce that looks like it originated from it.

The original nonce is exported as GIT_PUSH_CERT_NONCE for the hooks
to examine and match against the value on the "nonce" header in the
certificate to notice a replay, but returned "nonce" header in the
push certificate is examined by receive-pack and the result is
exported as GIT_PUSH_CERT_NONCE_STATUS, whose value would be "OK"
if the nonce recorded in the certificate matches what we expect, so
that the hooks can more easily check.

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

1 files changed, 19 insertions, 0 deletions

diff --git a/Documentation/git-receive-pack.txt b/Documentation/git-receive-pack.txt
index e6df234926..2d4b45242c 100644
--- a/Documentation/git-receive-pack.txt
+++ b/Documentation/git-receive-pack.txt
@@ -72,6 +72,24 @@ the following environment variables:
 	using the same mnemonic as used in `%G?` format of `git log`
 	family of commands (see linkgit:git-log[1]).
 
+`GIT_PUSH_CERT_NONCE`::
+	The nonce string the process asked the signer to include
+	in the push certificate.  If this does not match the value
+	recorded on the "nonce" header in the push certificate, it
+	may indicate that the certificate is a valid one that is
+	being replayed from a separate "git push" session.
+
+`GIT_PUSH_CERT_NONCE_STATUS`::
+`UNSOLICITED`;;
+	"git push --signed" sent a nonce when we did not ask it to
+	send one.
+`MISSING`;;
+	"git push --signed" did not send any nonce header.
+`BAD`;;
+	"git push --signed" sent a bogus nonce.
+`OK`;;
+	"git push --signed" sent the nonce we asked it to send.
+
 This hook is called before any refname is updated and before any
 fast-forward checks are performed.
 
@@ -147,6 +165,7 @@ service:
 	if test -n "${GIT_PUSH_CERT-}" && test ${GIT_PUSH_CERT_STATUS} = G
 	then
 		(
+			echo expected nonce is ${GIT_PUSH_NONCE}
 			git cat-file blob ${GIT_PUSH_CERT}
 		) | mail -s "push certificate from $GIT_PUSH_CERT_SIGNER" push-log@mydomain
 	fi