In Ripple it is often not trivial to change your account. Other users will have extended trust and IOUs will be owed by a particular account.
Because of this Ripple has a Master/Regular key system. The idea is to allow you to change your often used regular key with your emergency Master key that is stored offline.
The public key of an account serves two purposes. First, the hash of the public key is the account ID. Second, the public key is used to validate the transaction signature. Ripple allows you to use two different keys for each of these purposes.
The account ID is the hash of the master account key. The corresponding private key is needed to create the account. But the account also has a field for a public transaction signing key (called the 'regular account key').
Changing the regular account key is useful if you suspected or knew your account had been compromised. It could also be done routinely, if desired, to protect against the risk of leakage due to keyboard loggers, shoulder surfing, and so on.
Accounts have two keys:
To set up an account to using this scheme you do the following steps: