{rend,tor}-spec: clarify linkspec ID multiplicity issues

We were previously a bit unclear on how to handle multiple linkspecs
of type ed25519, and our spec didn't actually permit Tor's current
behavior.

Now we say that both Ed25519 ID and Legacy ID linkspecs MUST appear
at most once in a list of linkspecs, and that parties SHOULD
enforce this.

This is "problem 1" on torspec#193.

2 files changed, 10 insertions, 7 deletions

diff --git a/rend-spec-v3.txt b/rend-spec-v3.txt
index 3f76824..757fc1a 100644
--- a/rend-spec-v3.txt
+++ b/rend-spec-v3.txt
@@ -238,7 +238,7 @@ Table of contents:
            LSPEC  (Link specifier)                [LSLEN bytes]
 
    Link specifier types are as described in tor-spec.txt. Every set of
-   link specifiers MUST include at minimum specifiers of type [00]
+   link specifiers SHOULD include at minimum specifiers of type [00]
    (TLS-over-TCP, IPv4), [02] (legacy node identity) and [03] (ed25519
    identity key).
 
@@ -1380,7 +1380,7 @@ Table of contents:
             point section]
 
           The link-specifiers is a base64 encoding of a link specifier
-          block in the format described in BUILDING-BLOCKS.
+          block in the format described in [BUILDING-BLOCKS] above.
 
           As of 0.4.1.1-alpha, services include both IPv4 and IPv6 link
           specifiers in descriptors. All available addresses SHOULD be
@@ -1392,8 +1392,9 @@ Table of contents:
           recognize; instead, it should use them verbatim in its EXTEND
           request to the introduction point.
 
-          The client MAY perform basic validity checks on the link
-          specifiers in the descriptor. These checks SHOULD NOT leak
+          The client SHOULD perform the basic validity checks on the link
+          specifiers in the descriptor, described in `tor-spec.txt`
+          section 5.1.2. These checks SHOULD NOT leak
           detailed information about the client's version, configuration,
           or consensus. (See 3.3 for service link specifier handling.)
 
@@ -1903,7 +1904,7 @@ Table of contents:
 
    The hidden service should handle invalid or unrecognised link specifiers
    the same way as clients do in section 2.5.2.2. In particular, services
-   MAY perform basic validity checks on link specifiers, and SHOULD NOT
+   SHOULD perform basic validity checks on link specifiers, and SHOULD NOT
    reject unrecognised link specifiers, to avoid information leaks.
 
    The ONION_KEY_TYPE field is:
diff --git a/tor-spec.txt b/tor-spec.txt
index 34a3b44..8f30624 100644
--- a/tor-spec.txt
+++ b/tor-spec.txt
@@ -1128,7 +1128,9 @@ see tor-design.pdf.
            be listed.
 
    Nodes MUST ignore unrecognized specifiers, and MUST accept multiple
-   instances of specifiers other than 'legacy identity'.
+   instances of specifiers other than 'legacy identity' and
+   'Ed25519 identity'.  (Nodes SHOULD reject link specifier lists
+   that include multiple instances of either one of those specifiers.)
 
    For purposes of indistinguishability, implementations SHOULD send
    these link specifiers, if using them, in this order: [00], [02], [03],
@@ -1154,7 +1156,7 @@ see tor-design.pdf.
    target OR did not prove its ownership of any such identity key.
    If only one identity key is provided, but the extending OR knows
    the other (from directory information), then the OR SHOULD also
-   enforce that key.
+   enforce the key in the directory.
 
    If an extending OR has a channel with a given Ed25519 ID and RSA
    identity, and receives a request for that Ed25519 ID and a