Table 2.

Limitations of existing security measures/tools

Measures/tools	Limitations
Firewall and VPN	Only protect the information up to the point of the internal networks [7]
	Provide a certain level of isolation between the intra-net and internet but are easily bypassed by hackers [7]
Encryption	Probably an efficient tool for secure storage and transmission, but once the sensitive data is decrypted, the information is not protected anymore [7, 13, 33]
	Simply using encryption is no guarantee of confidentiality or secrecy [39].
	The randomness of the data for encrypted files stored on media can be used to distinguish the files from other stored data [39].
File-header	Can be easily usurped by a pirate in the plaintext format
	If encrypted, can be very sensitive to bit errors occurring during storage and transmission [7, 32]
Cryptographic hash function and its derivatives (e.g., DS, MAC, MDC, etc.)	Hash function cannot locate where the images have been tampered [31, 32, 47].
	The security of DS largely depends on the strength of the hash functions used to validate the signatures [30].
	It is possible to generate two datasets with different content but having the same message-digest algorithm 5 (MD5) hash [34].
	Cryptographic hash function is extremely bit sensitive to the input [32, 47].
Perceptual hashing	Perceptual hashing usually requires searching for match and access to a central database, where a large amount of pre-computed perceptual hashes are stored [7].
	Most randomization methods in perceptual hashing are linear, which introduces security flaws as known input/hash pairs can be used to recover a secret key [46].
	Their quantization and encoding stages require the learning of appropriate quantization thresholds.
	The quantizer training as well as the storage of thresholds introduces additional security weaknesses.