path: root/Documentation
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authorJunio C Hamano <>2014-08-21 23:45:30 (GMT)
committerJunio C Hamano <>2014-09-17 21:27:40 (GMT)
commitb89363e4a5277038629491f8765c0598f366326c (patch)
tree254c02373450167a86f33afdeac9cd851859ca00 /Documentation
parent9be89160e7382a88e56a02bcf38f4694dd6542d6 (diff)
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 <>
Diffstat (limited to 'Documentation')
4 files changed, 35 insertions, 9 deletions
diff --git a/Documentation/config.txt b/Documentation/config.txt
index 0d01e32..dd6fd65 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -2038,17 +2038,17 @@ rebase.autostash::
successful rebase might result in non-trivial conflicts.
Defaults to false.
- By default, `git receive-pack` will advertise that it
- accepts `git push --signed`. Setting this variable to
- false disables it (this is a tentative variable that
- will go away at the end of this series).
By default, git-receive-pack will run "git-gc --auto" after
receiving data from git-push and updating refs. You can stop
it by setting this variable to false.
+ By setting this variable to a string, `git receive-pack`
+ will accept a `git push --signed` and verifies it by using
+ a "nonce" protected by HMAC using this string as a secret
+ key.
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 e6df234..2d4b452 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]).
+ 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 --signed" sent a nonce when we did not ask it to
+ send one.
+ "git push --signed" did not send any nonce header.
+ "git push --signed" sent a bogus nonce.
+ "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
+ 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
diff --git a/Documentation/technical/pack-protocol.txt b/Documentation/technical/pack-protocol.txt
index 7b543dc..dda1206 100644
--- a/Documentation/technical/pack-protocol.txt
+++ b/Documentation/technical/pack-protocol.txt
@@ -485,6 +485,7 @@ references.
PKT-LINE("certificate version 0.1" LF)
PKT-LINE("pusher" SP ident LF)
PKT-LINE("pushee" SP url LF)
+ PKT-LINE("nonce" SP nonce LF)
*PKT-LINE(command LF)
*PKT-LINE(gpg-signature-lines LF)
@@ -533,6 +534,11 @@ Currently, the following header fields are defined:
authentication material) the user who ran `git push`
intended to push into.
+`nonce` nonce::
+ The 'nonce' string the receiving repository asked the
+ pushing user to include in the certificate, to prevent
+ replay attacks.
The GPG signature lines are a detached signature for the contents
recorded in the push certificate before the signature block begins.
The detached signature is used to certify that the commands were
diff --git a/Documentation/technical/protocol-capabilities.txt b/Documentation/technical/protocol-capabilities.txt
index a478cc4..0c92dee 100644
--- a/Documentation/technical/protocol-capabilities.txt
+++ b/Documentation/technical/protocol-capabilities.txt
@@ -251,10 +251,11 @@ If the upload-pack server advertises this capability, fetch-pack may
send "want" lines with SHA-1s that exist at the server but are not
advertised by upload-pack.
The receive-pack server that advertises this capability is willing
-to accept a signed push certificate. A send-pack client MUST NOT
+to accept a signed push certificate, and asks the <nonce> to be
+included in the push certificate. A send-pack client MUST NOT
send a push-cert packet unless the receive-pack server advertises
this capability.