`class CompositeKey : PublicKey`

A tree data structure that enables the representation of composite public keys, which are used to represent the signing requirements for multi-signature scenarios such as RAFT notary services. A composite key is a list of leaf keys and their contributing weight, and each leaf can be a conventional single key or a composite key. Keys contribute their weight to the total if they are matched by the signature.

For complex scenarios, such as *"Both Alice and Bob need to sign to consume a state S"*, we can represent
the requirement by creating a tree with a root CompositeKey, and Alice and Bob as children.
The root node would specify *weights* for each of its children and a *threshold* – the minimum total weight required
(e.g. the minimum number of child signatures required) to satisfy the tree signature requirement.

Using these constructs we can express e.g. 1 of N (OR) or N of N (AND) signature requirements. By nesting we can
create multi-level requirements such as *"either the CEO or 3 of 5 of his assistants need to sign"*.

## Builder |
A helper class for building a CompositeKey. `class Builder` |

## NodeAndWeight |
Holds node - weight pairs for a CompositeKey. Ordered first by weight, then by node's hashCode. Each node should be assigned with a positive weight to avoid certain types of weight underflow attacks. `data class NodeAndWeight : Comparable<NodeAndWeight>, ASN1Object` |

## children |
Τhe order of the children may not be the same to what was provided in the builder. `val children: List<NodeAndWeight>` |

## leafKeys |
Set of all leaf keys of that CompositeKey. `val leafKeys: Set<PublicKey>` |

## threshold |
specifies the minimum total weight required (in the simple case – the minimum number of child signatures required) to satisfy the sub-tree rooted at this node. `val threshold: Int` |

## checkValidity |
This method will detect graph cycles in the full composite key structure to protect against infinite loops when traversing the graph and key duplicates in the each layer. It also checks if the threshold and weight constraint requirements are met, while it tests for aggregated-weight integer overflow. In practice, this method should be always invoked on the root CompositeKey, as it inherently validates the child nodes (all the way till the leaves). `fun checkValidity(): Unit` |

## equals |
`fun equals(other: Any?): Boolean` |

## getAlgorithm |
`fun getAlgorithm(): String` |

## getEncoded |
`fun getEncoded(): ByteArray` |

## getFormat |
`fun getFormat(): String` |

## hashCode |
`fun hashCode(): Int` |

## isFulfilledBy |
Takes single PublicKey and checks if CompositeKey requirements hold for that key. `fun isFulfilledBy(key: PublicKey): Boolean`
Function checks if the public keys corresponding to the signatures are matched against the leaves of the composite key tree in question, and the total combined weight of all children is calculated for every intermediary node. If all thresholds are satisfied, the composite key requirement is considered to be met. `fun isFulfilledBy(keysToCheck: Iterable<PublicKey>): Boolean` |

## toString |
`fun toString(): String` |

## KEY_ALGORITHM |
`const val KEY_ALGORITHM: String` |

## getInstance |
Build a composite key from a DER encoded form. `fun getInstance(encoded: ByteArray): PublicKey` `fun getInstance(asn1: ASN1Primitive): PublicKey` |