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signed push: fortify against replay attacks

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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>
This commit is contained in:
Junio C Hamano
2014-08-21 16:45:30 -07:00
parent 9be89160e7
commit b89363e4a5
7 changed files with 187 additions and 29 deletions
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6
Documentation/technical/pack-protocol.txt
View File

@ -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(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

7
Documentation/technical/protocol-capabilities.txt
View File

@ -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.
push-cert
---------
push-cert=<nonce>
-----------------
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.