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5

GCM Personally, I would go for GCM (Galois Counter Mode) since it is efficient – meaning: it handles pretty much everything you’ld expect from it, while other modes sometimes tend to lack a specific feature here and there (see image below for a comparison that shows what I’m hinting at). Also, GCM has a pretty good performance (assuming non-flawed ...


4

From the sound of your questions, it almost appears that you have some confusion between the CBC-MAC key and the CBC-MAC tag. The CBC-MAC algorithm takes the message (in this case, most likely the ciphertext) and a secret key; it outputs a tag (which can be public). The security property of CBC-MAC is that someone who does not know the key cannot generate ...


3

I would pick EAX as it is by far the simplest to implement and therefore to understand and audit. It is reasonably fast if based on AES. GCM seems quite popular, but I personally see a number of issues with it: it is very difficult to implement in software (which is not surprising, since it was developed with hardware in mind). it is slower than it seems ...


3

In general signature creation contains the hashing part within the algorithm. A signature algorithm may also contain a padding mechanism such as PKCS#1 v1.5 or PSS for RSA. Finally it contains a one-way trap door function (modular exponentiation within RSA). Encryption has other requirements, and uses a different padding mechanism. Basically you are ...


2

encrypt it with the message author's private key This statement makes me uncomfortable. Normally, in asymmetric cryptography, one encrypts with the public key and signs with the private key. Did you mean “sign it with the message author's private key”? Otherwise, I would not accept your protocol without a clear, detailed explanation of what encryption ...


2

A symmetrically encrypted hash is not a secure MAC. You should use either an authenticated encryption scheme or a secure MAC in encrypt-then-MAC. With asymmetric encryption, it may be secure – "encrypting" with the author's private key means you are actually signing the message which is fine. However, you need to use the actual asymmetric primitive, not ...


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CMAC (or OMAC1) is the underlying MAC algorithm that provides authentication and integrity for EAX. Is stated in NIST SP 800-38B: Because CMAC is based on an approved symmetric key block cipher, such as the Advanced Encryption Standard (AES) algorithm that is specified in Federal Information Processing Standard (FIPS) Pub. 197 [3], CMAC can be ...


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As additional detail, while the two keys need to be distinct and secret, you can derive the CBC-MAC key and the CBC encryption key from the same master key. Generate a random master key, then use any key derivation algorithm with two different salts to derive the authentication and encryption keys. For example, $K(m, \text{'auth'})$ and $K(m, \text{'enc'})$ ...



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