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+# Exchanging detached signatures
+
+Once an authority has computed and signed a consensus network status, it
+should send its detached signature to each other authority in an HTTP POST
+request to the URL:
+
+`http://<hostname>/tor/post/consensus-signature`
+
+\[XXX Note why we support push-and-then-pull.\]
+
+All of the detached signatures it knows for consensus status should be
+available at:
+
+`http://<hostname>/tor/status-vote/next/consensus-signatures.z`
+
+Assuming full connectivity, every authority should compute and sign the
+same consensus including any flavors in each period.  Therefore, it
+isn't necessary to download the consensus or any flavors of it computed
+by each authority; instead, the authorities only push/fetch each
+others' signatures.  A "detached signature" document contains items as
+follows:
+
+"consensus-digest" SP Digest NL
+
+\[At start, at most once.\]
+
+The digest of the consensus being signed.
+
+"valid-after" SP YYYY-MM-DD SP HH:MM:SS NL
+"fresh-until" SP YYYY-MM-DD SP HH:MM:SS NL
+"valid-until" SP YYYY-MM-DD SP HH:MM:SS NL
+
+\[As in the consensus\]
+
+"additional-digest" SP flavor SP algname SP digest NL
+
+\[Any number.\]
+
+For each supported consensus flavor, every directory authority
+adds one or more "additional-digest" lines.  "flavor" is the name
+of the consensus flavor, "algname" is the name of the hash
+algorithm that is used to generate the digest, and "digest" is the
+hex-encoded digest.
+
+The hash algorithm for the microdescriptor consensus flavor is
+defined as SHA256 with algname "sha256".
+
+```text
+    "additional-signature" SP flavor SP algname SP identity SP
+         signing-key-digest NL signature.
+
+        [Any number.]
+```
+
+For each supported consensus flavor and defined digest algorithm,
+every directory authority adds an "additional-signature" line.
+"flavor" is the name of the consensus flavor.  "algname" is the
+name of the algorithm that was used to hash the identity and
+signing keys, and to compute the signature.  "identity" is the
+hex-encoded digest of the authority identity key of the signing
+authority, and "signing-key-digest" is the hex-encoded digest of
+the current authority signing key of the signing authority.
+
+The "sha256" signature format is defined as the RSA signature of
+the OAEP+-padded SHA256 digest of the item to be signed.  When
+checking signatures, the signature MUST be treated as valid if the
+signature material begins with SHA256(document), so that other
+data can get added later.
+\[To be honest, I didn't fully understand the previous paragraph
+and only copied it from the proposals.  Review carefully. -KL\]
+
+"directory-signature"
+
+\[As in the consensus; the signature object is the same as in the
+consensus document.\]
+
+<a id="dir-spec.txt-3.11"></a>