add CAST-CERT doc description

Author: Mr-Leshiy

chain-impl-mockchain/doc/format.abnf

@@ -161,13 +161,22 @@ COMMITTE-END      = BLOCK-DATE
 PROPOSALS         = SIZE-ELEMENT-8BIT *PROPOSAL-ELEMENT 
 PROPOSAL-ELEMENT  = TODO
 
-CAST-CERT         = TODO
-
-TALLY-CERT        = VOTE-PLAN-ID PAYLOAD
-PAYLOAD           = %x01 ; Public payload
-                  / %x02 PRIVATE-PAYLOAD ; Private payload       
-PRIVATE-PAYLOAD   = SIZE-ELEMENT-8BIT *PAYLOAD-ELEMENT
-PAYLOAD-ELEMENT   = MEMBERS-SIZE OPTIONS-SIZE *MEMBER-ELEMENT *OPTION-ELEMENT
+CAST-CERT         = VOTE-PLAN-ID PROPOSAL-INDEX CAST-PAYLOAD
+PROPOSAL-INDEX    = U8
+CAST-PAYLOAD      = %x01 CHOICE; Public payload
+                  / %x02 ECRYPTED-VOTE PROOF-VOTE ; Private payload
+CHOICE            = U8
+ECRYPTED-VOTE     = SIZE-ELEMENT-8BIT *CYPHER-TEXT
+CYPHER-TEXT       = 2 * SIZE-ELEMENT-65BIT
+PROOF-VOTE        = SIZE-ELEMENT-8BIT ANNOUNCEMENT CYPHER-TEXT R-RESPONSE
+ANNOUNCEMENT      = 3 * SIZE-ELEMENT-65BIT
+R-RESPONSE        = 2 * SIZE-ELEMENT-65BIT
+
+TALLY-CERT        = VOTE-PLAN-ID TALLY-PAYLOAD
+TALLY-PAYLOAD     = %x01 ; Public payload
+                  / %x02 PRIVATE-TALLY ; Private payload       
+PRIVATE-TALLY     = SIZE-ELEMENT-8BIT *TALLY-ELEMENT
+TALLY-ELEMENT     = MEMBERS-SIZE OPTIONS-SIZE *MEMBER-ELEMENT *OPTION-ELEMENT
 MEMBERS-SIZE      = SIZE-ELEMENT-8BIT
 OPTIONS-SIZE      = SIZE-ELEMENT-8BIT
 MEMBER-ELEMENT    = *SHARE-ELEMENT ; size is equal to the OPTIONS-SIZE value