docs/mutable.txt: put fingerprint in read-write URI too, it makes everything easier

docs/mutable.txt

@@ -99,13 +99,12 @@ encrypted child names to rw-URI/ro-URI pairs.
 
 Each SDMF slot is created with a public/private key pair. The public key is
 known as the "verification key", while the private key is called the
-"signature key". The private key and public key are concatenated and the
-result is hashed to form the "write key" (an AES symmetric key). The write
-key is then hashed to form the "read key". The read key is hashed to form the
-"storage index" (a unique string used as an index to locate stored data).
+"signature key". The private key is hashed to form the "write key" (an AES
+symmetric key). The write key is then hashed to form the "read key". The read
+key is hashed to form the "storage index" (a unique string used as an index
+to locate stored data).
 
-The public key is hashed by itself to form the "verification key hash". The
-private key is encrypted
+The public key is hashed by itself to form the "verification key hash".
 
 The write key is hashed a different way to form the "write enabler master".
 For each storage server on which a share is kept, the write enabler master is
@@ -126,8 +125,19 @@ by concatenating the readkey with the IV, the hashing the results and
 truncating to 16 bytes. The IV is randomly generated each time the slot is
 updated, and stored next to the encrypted data.
 
-The read-write URI consists of just the write key. The read-only URI contains
-the read key and the verification key hash.
+The read-write URI consists of the write key and the verification key hash.
+The read-only URI contains the read key and the verification key hash. The
+verify-only URI contains the storage index and the verification key hash.
+
+ URI:SSK-RW:b2a(writekey):b2a(verification_key_hash)
+ URI:SSK-RO:b2a(readkey):b2a(verification_key_hash)
+ URI:SSK-Verify:b2a(storage_index):b2a(verification_key_hash)
+
+Note that this allows the read-only and verify-only URIs to be derived from
+the read-write URI without actually retrieving the public keys. Also note
+that it means the read-write agent must validate both the private key and the
+public key when they are first fetched. All users validate the public key in
+exactly the same way.
 
 The SDMF slot is allocated by sending a request to the storage server with a
 desired size, the storage index, and the write enabler for that server's
@@ -150,6 +160,7 @@ pieces are:
   * the signature key, encrypted with the write key
 
 The access pattern for read is:
+ * hash read-key to get storage index
  * use storage index to locate 'k' shares with identical 'R' values
    * either get one share, read 'k' from it, then read k-1 shares
    * or read, say, 5 shares, discover k, either get more or be finished
@@ -166,12 +177,15 @@ The access pattern for read is:
  * submit plaintext to application
 
 The access pattern for write is:
+ * hash write-key to get read-key, hash read-key to get storage index
  * use the storage index to locate at least one share
  * read verification key and encrypted signature key
  * decrypt signature key using write-key
- * concatenate signature and verification keys, compare against write-key
- * hash verification key to form read-key
+ * hash signature key, compare against write-key
+ * hash verification key, compare against verification key hash
  * encrypt plaintext from application with read-key
+   * application can encrypt some data with the write-key to make it only
+     available to writers (use this for transitive read-onlyness of dirnodes)
  * erasure-code crypttext to form shares
  * split shares into blocks
  * compute Merkle tree of blocks, giving root "r" for each share