2 cissp ® Official Study Guide Eighth Edition

Name
Hash value length
Hash of Variable Length (HAVAL)—an MD5 variant
128, 160, 192, 224, and 256 bits
Hash Message Authenticating Code (HMAC)
Variable
Message Digest 2 (MD2)
128
Message Digest 4 (MD4)
128
Message Digest 5 (MD5)
128
Secure Hash Algorithm (SHA-1)
160
SHA2-224/SHA3-224
224
SHA2-256/SHA3-256
256
SHA2-384/SHA3-384
384
SHA2-512/SHA3-512
512
Digital Signatures
Once you have chosen a cryptographically sound hashing algorithm, you can use it to 
implement a 
digital signature
system. Digital signature infrastructures have two distinct 
goals:
■
Digitally signed messages assure the recipient that the message truly came from the 
claimed sender. They enforce nonrepudiation (that is, they preclude the sender from 
later claiming that the message is a forgery).
■
Digitally signed messages assure the recipient that the message was not altered while 
in transit between the sender and recipient. This protects against both malicious 
modification (a third party altering the meaning of the message) and unintentional 
modification (because of faults in the communications process, such as electrical 
interference).

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Digital Signatures 
247
Digital signature algorithms rely on a combination of the two major concepts already 
covered in this chapter—public key cryptography and hashing functions. 
If Alice wants to digitally sign a message she’s sending to Bob, she performs the follow-
ing actions: 
1.
Alice generates a message digest of the original plaintext message using one of the 
cryptographically sound hashing algorithms, such as SHA3-512. 
2.
Alice then encrypts only the message digest using her private key. This encrypted mes-
sage digest is the digital signature.