  BlockChain (BC) immutability ensures BC resilience against modification or
removal of the stored data. In large scale networks like the Internet of Things
(IoT), however, this feature significantly increases BC storage size and raises
privacy challenges. In this paper, we propose a Memory Optimized and Flexible
BC (MOF-BC) that enables the IoT users and service providers to remove or
summarize their transactions and age their data and to exercise the "right to
be forgotten". To increase privacy, a user may employ multiple keys for
different transactions. To allow for the removal of stored transactions, all
keys would need to be stored which complicates key management and storage.
MOF-BC introduces the notion of a Generator Verifier (GV) which is a signed
hash of a Generator Verifier Secret (GVS). The GV changes for each transaction
to provide privacy yet is signed by a unique key, thus minimizing the
information that needs to be stored. A flexible transaction fee model and a
reward mechanism is proposed to incentivize users to participate in optimizing
memory consumption. Qualitative security and privacy analysis demonstrates that
MOF-BC is resilient against several security attacks. Evaluation results show
that MOF-BC decreases BC memory consumption by up to 25\% and the user cost by
more than two orders of magnitude compared to conventional BC instantiations.
