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+<a id="dir-spec.txt-3.3"></a>
+
+# Computing microdescriptors
+
+Microdescriptors are a stripped-down version of server descriptors
+generated by the directory authorities which may additionally contain
+authority-generated information.  Microdescriptors contain only the
+most relevant parts that clients care about.  Microdescriptors are
+expected to be relatively static and only change about once per week.
+Microdescriptors do not contain any information that clients need to
+use to decide which servers to fetch information about, or which
+servers to fetch information from.
+
+Microdescriptors are a straight transform from the server descriptor
+and the consensus method.  Microdescriptors have no header or footer.
+Microdescriptors are identified by the hash of its concatenated
+elements without a signature by the router.  Microdescriptors do not
+contain any version information, because their version is determined
+by the consensus method.
+
+Starting with consensus method 8, microdescriptors contain the
+following elements taken from or based on the server descriptor.  Order
+matters here, because different directory authorities must be able to
+transform a given server descriptor and consensus method into the exact
+same microdescriptor.
+
+"onion-key" NL a public key in PEM format
+
+\[Exactly once, at start\]
+\[No extra arguments\]
+
+The "onion-key" element as specified in section 2.1.1.
+
+When generating microdescriptors for consensus method 30 or later,
+the trailing = sign must be absent. For consensus method 29 or
+earlier, the trailing = sign must be present.
+
+"ntor-onion-key" SP base-64-encoded-key NL
+
+\[Exactly once\]
+
+The "ntor-onion-key" element as specified in section 2.1.1.
+
+(Only included when generating microdescriptors for
+consensus-method 16 or later.)
+
+\[Before Tor 0.4.5.1-alpha, this field was optional.\]
+
+"a" SP address ":" port NL
+
+\[Any number\]
+
+Additional advertised addresses for the OR.
+
+Present currently only if the OR advertises at least one IPv6
+address; currently, the first address is included and all others are
+omitted. Any other IPv4 or IPv6 addresses should be ignored.
+
+Address and port are as for "or-address" as specified in
+section 2.1.1.
+
+(Only included when generating microdescriptors for
+consensus-methods 14 to 27.)
+
+"family" names NL
+
+\[At most once\]
+
+The "family" element as specified in section 2.1.1.
+
+When generating microdescriptors for consensus method 29 or later,
+the following canonicalization algorithm is applied to improve
+compression:
+
+```text
+           For all entries of the form $hexid=name or $hexid~name,
+           remove the =name or ~name portion.
+
+           Remove all entries of the form $hexid, where hexid is not
+           40 hexadecimal characters long.
+
+           If an entry is a valid nickname, put it into lower case.
+
+           If an entry is a valid $hexid, put it into upper case.
+
+           If there are any entries, add a single $hexid entry for
+           the relay in question, so that it is a member of its own
+           family.
+
+           Sort all entries in lexical order.
+
+           Remove duplicate entries.
+```
+
+(Note that if an entry is not of the form "nickname", "$hexid",
+"$hexid=nickname" or "$hexid~nickname", then it will be unchanged:
+this is what makes the algorithm forward-compatible.)
+
+"p" SP ("accept" / "reject") SP PortList NL
+
+\[At most once.\]
+
+The exit-policy summary as specified in sections 3.4.1 and 3.8.2.
+
+\[With microdescriptors, clients don't learn exact exit policies:
+clients can only guess whether a relay accepts their request, try the
+BEGIN request, and might get end-reason-exit-policy if they guessed
+wrong, in which case they'll have to try elsewhere.\]
+
+\[In consensus methods before 5, this line was omitted.\]
+
+"p6" SP ("accept" / "reject") SP PortList NL
+
+\[At most once\]
+
+The IPv6 exit policy summary as specified in sections 3.4.1 and
+3.8.2. A missing "p6" line is equivalent to "p6 reject 1-65535".
+
+(Only included when generating microdescriptors for
+consensus-method 15 or later.)
+
+"id" SP "rsa1024" SP base64-encoded-identity-digest NL
+
+\[At most once\]
+
+The node identity digest (as described in tor-spec.txt), base64
+encoded, without trailing =s.  This line is included to prevent
+collisions between microdescriptors.
+
+Implementations SHOULD ignore these lines: they are
+added to microdescriptors only to prevent collisions.
+
+(Only included when generating microdescriptors for
+consensus-method 18 or later.)
+
+"id" SP "ed25519" SP base64-encoded-ed25519-identity NL
+
+\[At most once\]
+
+The node's master Ed25519 identity key, base64 encoded,
+without trailing =s.
+
+All implementations MUST ignore this key for any microdescriptor
+whose corresponding entry in the consensus includes the
+'NoEdConsensus' flag.
+
+(Only included when generating microdescriptors for
+consensus-method 21 or later.)
+
+"id" SP keytype ... NL
+
+\[At most once per distinct keytype.\]
+
+Implementations MUST ignore "id" lines with unrecognized
+key-types in place of "rsa1024" or "ed25519"
+
+(Note that with microdescriptors, clients do not learn the RSA identity of
+their routers: they only learn a hash of the RSA identity key.  This is
+all they need to confirm the actual identity key when doing a TLS
+handshake, and all they need to put the identity key digest in their
+CREATE cells.)