This class represent an in-memory or on-disk database of SRP password 
verifiers.
A VerifierDB can be passed to a server handshake to authenticate a 
client based on one of the verifiers.
This class is thread-safe.
  | Method Summary | 
|  | __init__(self,
          filename)Create a new VerifierDB instance.
 | 
|  | __setitem__(self,
          username,
          verifierEntry)Add a verifier entry to the database.
 | 
| tuple | makeVerifier(username,
          password,
          bits)Create a verifier entry which can be stored in a VerifierDB.    (Static method)
 | 
|  | _checkItem(self,
          value,
          username,
          param) | 
|  | _getItem(self,
          username,
          valueStr) | 
|  | _setItem(self,
          username,
          value) | 
  | Inherited from BaseDB | 
| bool | __contains__(self,
          username)Check if the database contains the specified username.
 | 
|  | __delitem__(self,
          username) | 
|  | __getitem__(self,
          username) | 
|  | check(self,
          username,
          param) | 
|  | create(self)Create a new on-disk database.
 | 
| list | keys(self)Return a list of usernames in the database.
 | 
|  | open(self)Open a pre-existing on-disk database.
 |