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Figure 25 Summary of cryptographic primitives
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that will work with meaningful messages has also not been done yet Thus, in summary, we should not use SHA-1 in any new protocols but rather use other possibilities such as SHA-256 or SHA-3842 We next look at MAC functions These functions take an arbitrarily sized message and also a secret key and produce a xed size output The function is designed such that it is infeasible to produce the same output without knowledge of the key This allows ef cient authentication of the source of the messages while also guaranteeing data integrity MAC functions have similar properties as a hash function In addition they also need to have the property whereby, given one or more pairs of text and the corresponding MAC value, it is computationally infeasible to compute a MAC value for any new input The widely used MAC algorithm is hashed message authentication code (HMAC) This is speci ed as an internet standard HMAC has been designed such that it can use any of the several hash functions, such as SHA-1, MD5 etc The security of HMAC therefore depends on the security of the underlying hashing algorithm used We would like to remark here that authentication as provided by use of hashing plus signature or MAC on the message can also be provided by either symmetric encryption or asymmetric encryption This is because one can argue that message encryption by itself also provides a measure of authentication For example, if symmetric encryption is used then the receiver would know that the sender must have created it, since only the sender and the receiver have access to the key used Note that the receiver would have to depend on information such as the knowledge of message structure in order to detect if the content has been altered This also assumes that the key has not been compromised On the other hand, when using asymmetric algorithms, encryption provides no con dence in the identity of the sender since any entity that knows the public key of the entity of interest can send an encrypted message to it However, if the sender signs the message using its private key, then authentication can be achieved As opposed to these methods of achieving authentication, MAC algorithms can achieve authenticity while also ensuring
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Recommendation of a panel at the NIST First Cryptographic Hash Workshop, October 2005
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faster processing In addition, MAC algorithms provide a separation of authentication and con dentiality that offers an extra degree of exibility The cryptographic primitives described earlier help achieve the basic cryptographic attributes described earlier in Section 221 It is clear that both symmetric key and asymmetric key algorithms do indeed provide con dentiality Integrity and authentication can be achieved by the use of message digests or cryptographic hash functions along with digital signatures on the hashed values Nonrepudiation requires the use of public key algorithms to provide digital signatures Availability is ensured by use of noncryptographic means such redundancy, physical protection, and the use of robust protocols The various cryptographic primitives are shown in Figure 25
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We have earlier seen examples of both symmetric ciphers as well as asymmetric ciphers Symmetric ciphers like AES operate on xed size input messages in blocks of 128 bits, while asymmetric ciphers like RSA also require the size of the input message to be smaller than the system modulus (the product of two large primes as discussed earlier) Other cryptographic algorithms have similar constraints Given this, a question arises as to the mode of operation of these ciphers when messages which have arbitrary length (larger than the size of an input message) have to be encrypted There are ve modes of operation which are typically used in such cases and are applicable to any xed-length encryption scheme These are: 1 2 3 4 5 Electronic code book (ECB) Cipher block chaining (CBC) K-bit cipher feedback (CFB) K-bit output feedback (OFB) Counter mode (CTR)
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We next explain these modes of operation starting with ECB Note that some of these modes of operation also allow one to de ne stream ciphers from a block cipher The ECB mode is the simplest of these modes of operation and is fairly straightforward A message is broken into independent blocks, with each block being encrypted with the secret key independently of the other blocks The receiver gets encrypted blocks and decrypts each block in turn As the name of this mode implies, each block is a value which is substituted like a codebook We show the operation of the ECB mode in Figure 26 This mode of operation ensures that bit errors in a ciphertext block affect the decryption of only that block This reduces the complexity of the implementation of this scheme, but there are two serious shortcomings with this mode of operation Repetitions in message being encrypted will be obvious in ciphertext if the repetitions are aligned with the message block An adversary with access to the ciphertext can thereby gain some information about the content of the message by observing the repeated blocks Thus, this mode does not offer semantic security Another weakness is due to the fact that adversaries can rearrange the blocks or modify the blocks without being detected This weakness follows on account of the encrypted message blocks being independent On account of this, the ECB mode is not recommended for messages longer than one block
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