Documentation/technical: describe signature formats

We use different types of signature formats in different places.
Set up the infrastructure and overview to describe them systematically
in our technical documentation.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/Makefile

@@ -76,6 +76,7 @@ TECH_DOCS += technical/protocol-common
 TECH_DOCS += technical/racy-git
 TECH_DOCS += technical/send-pack-pipeline
 TECH_DOCS += technical/shallow
+TECH_DOCS += technical/signature-format
 TECH_DOCS += technical/trivial-merge
 SP_ARTICLES += $(TECH_DOCS)
 SP_ARTICLES += technical/api-index

Documentation/technical/signature-format.txt

@@ -0,0 +1,17 @@
+Git signature format
+====================
+
+== Overview
+
+Git uses cryptographic signatures in various places, currently objects (tags,
+commits, mergetags) and transactions (pushes). In every case, the command which
+is about to create an object or transaction determines a payload from that,
+calls gpg to obtain a detached signature for the payload (`gpg -bsa`) and
+embeds the signature into the object or transaction.
+
+Signatures always begin with `-----BEGIN PGP SIGNATURE-----`
+and end with `-----END PGP SIGNATURE-----`, unless gpg is told to
+produce RFC1991 signatures which use `MESSAGE` instead of `SIGNATURE`.
+
+The signed payload and the way the signature is embedded depends
+on the type of the object resp. transaction.