5.1 Evaluation of Asymmetric Key Cryptography
Asymmetric cryptography is based on the use of two keys
that are mathematically related, one for encryption and the
other for decryption. The key pair is comprised of the
private and the public key. Each user has both keys, but
the private key remains a secret while the public key is
revealed to all other users. Asymmetric cryptography
incurs more computational overheads than symmetric
cryptography, but however simplifies the process of key
distribution and management as compared to symmetric
cryptography. This implies that asymmetric crypto-
systems are best suited for authentication and key
exchange services. According to , it costs 42mJ of
energy to encrypt a 1024-bit block on a MC68328
DragonBall processor using RSA, while the encryption
process of a 128-bit AES block is estimated to consume
much less at 0.104mJ.
In work , authors investigated and compared the
energy cost implications of authentication and key
exchange for two asymmetric algorithms RSA and ECC.
Experiments were performed on an 8-bit Atmel ATmega
128L low power microcontroller. Results show that ECC
has much better energy costs than RSA in both
authentication and key exchange processes. A similar
experiment was simulated in  to compare the energy
efficiency of ECC and RSA on MICA2DOT motes, and
the same results were obtained showing ECC as the better
choice of asymmetric cryptography for resource
There exists no generic security solution for all WSNs.
Appropriate security architectures for WSNs greatly
depend on the security requirements of particular WSN
applications and hardware limitations of the type nodes
Among the reviewed symmetric block ciphers, Rijndael
appears to be the most energy efficient and sufficiently
secure algorithm. Among the reviewed asymmetric block
ciphers, ECC has shown much better energy performance
while offering the same level of security as the most
commonly employed asymmetric algorithm RSA.
Selecting the right algorithm highly depends on
determining the most efficient, in terms of computation,
memory and energy, and sufficiently secure for a given
This paper provides an overview of the energy
requirements of cryptographic algorithms when employed
in WSNs. This involved performing an evaluation of the
most commonly used symmetric and asymmetric
algorithms based on published literature, and developing
ranking model based on computational, memory and
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