Table 7.

Comparison between schemes.

Scheme	Offline Phase	Cryptographic Scheme	Key Size (bits)	Communication Type	Number of Storage Keys	Memory Space Used by the Scheme	Re-Keying Strategy
Neighborhood Scheme [12]	Certificate stored in each node	Hybrid (RSA and symmetric)	128	Multicast/Unicast	Each node stores: own public/private keys; own secret neighborhood key; neighbors secret key(s), and source key (used when the node is data source)	Not Given	Local operation, where each node updates keys with its current neighbor.
Hierarchical Key Management Scheme [13]	No pre-loaded keys	Hybrid (D–H and Symmetric)	Not Given	Broadcast (2-hop adjacent node)	Level 1 head stores its secret key and secret key of Level 2 head. Level 2 head stores its secret key and secret key of Level 1 head. Ordinary node stores the secret key of the Level 2 head, secret communication key, and DH keys.	Not Given	Complicated operation where each level head node must regenerate a number of keys.
Time-Based Key Management Scheme [15]	Keys pre-loaded	Symmetric	64	Broadcast	500 keys in center node. 100 keys for sensor nodes (to ensure sharing of a key with at least one of 10 neighbors)	4 KB for center node 0.8 KB for sensor node	Only the nodes deployed at the same time when compromised node is revoked have to regenerate keys.
Presented Scheme	Keys pre-loaded	Symmetric AES-128 in ECB mode with zeros padding. Key generation -MD5 and RTC.	128	Unicast	(5 keys) Each ordinary node stores; own adjacent key; neighbor shared adjacent key; an individual key that is shared with leader node; Leader node master key, and a source key (used when the node is data source).	Program size 57,230 bytes Ram required 4987 bytes.	Re-keying in this scheme is a local operation, where only one node must update its adjacent key, in addition to the leader node key.