The main requirement for a `zcrypt`ed zephyr class is to distribute a key to all the intended users of the class. Typically, this is done by storing the key in AFS. You can set that up like this:
- mkdir -p ~/Public/zcrypt/label/ # Pick an arbitrary label for your class.
- fs sa ~/Public/zcrypt/label/ system:anyuser none # Keep randoms from reading your key
- fs sa ~/Public/zcrypt/label/ system:groupname read # Allow an appropriate user/group to read the key
+ # Pick an arbitrary label for your class.
+ mkdir -p ~/Public/zcrypt/label/
+ # Keep randoms from reading your key
+ fs setacl -dir ~/Public/zcrypt/label/ -acl system:anyuser none
+ # Allow an appropriate user/group to read the key
+ fs setacl -dir ~/Public/zcrypt/label/ -acl system:groupname read
+ # Generate a 126-byte key with no NUL or newline bytes
tr -d '\000\n' < /dev/urandom | head -c 126 > ~/Public/zcrypt/label/key.zcrypt
-The first three lines create a directory to store the key in, and set the permissions properly. You should replace `label` with an appropriate name; you may want to use something besides the class name in order to help keep the class name private. Subbing to the class will disclose traffic patterns and instances used, so you may want to use the traditional "secret class" (keeping the name secret) as a first line of defense, in addition to `zcrypt`.
+The first three lines create a directory to store the key, and set the permissions properly. You should replace `label` with an appropriate name; you may want to use something besides the class name in order to help keep the class name private. Subbing to the class will disclose traffic patterns and instances used, so you may want to use the traditional "secret class" (keeping the name secret) as a first line of defense, in addition to `zcrypt`.
-The last line creates the key, which should be a random byte string of at least 126 characters, none of which should be null or newlines.
+The last line creates the key, which should be a random byte string of at least 126 characters, none of which should be null or newlines. (Nulls and newlines terminate the key, so if `/dev/urandom` happens to give a null or newline early (which is certainly plausible) you could end up with a much weaker key than you expected.)
## Subbing to a `zcrypt`ed zephyr class
-This may vary between clients. For traditional zephyr clients, you should sub as usual (in Barnowl, run `:sub classname`).
+This may vary between clients. For traditional zephyr clients, you should sub as usual (in BarnOwl, run `:sub classname`, or edit your `.zephyr.subs` file, then run `:loadsubs` in BarnOwl).
You should also have been told a key path. In the example above, that would be `/mit/user/Public/zcrypt/label/key.zcrypt` (where `user` is your Athena username).
Replace `classname` and the path appropriately. (You can also leave off "AES:" to use the key with 1DES, but that is very weak, and should not be used in new classes.)
-<small>* This is only mostly true. Zephyr supports limiting who can sub to zephyr classes, and Moira has support to control zephyr's support. However, there's no self-service support for setting up ACL'd classes, and I don't believe anybody with the relevant bits in interested in setting up classes for student groups or similar uses, so the technical ability isn't practically useful.</small>
+<small>* This is only mostly true. Zephyr supports limiting who can sub to zephyr classes, and Moira has support to control zephyr's support. However, there's no self-service support for setting up ACL'd classes, and I don't believe anybody with the relevant bits is interested in setting up classes for student groups or similar uses, so the technical ability isn't practically useful.</small>
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