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4

See NIST SP 800-107, section 5.3.4: The effective security strength of the HMAC key is the minimum of the security strength of $K$ and the value of $2C$. That is, security strength = min(security strength of $K$, $2C$). For example, if the security strength of $K$ is 128 bits, and SHA-1 is used, then the effective security strength of the HMAC key is 128 ...


4

Short answer: 32 bytes of full-entropy key is enough. Assuming full-entropy key (that is, each bit of key is chosen independently of the others by an equivalent of fair coin toss), the security of HMAC-SHA-256 against brute force key search is defined by the key size up to 64 bytes (512 bits) of key, then abruptly drops to 32 bytes (256 bits) for larger ...


3

The strength here depends on the collision resistance of $H$. If $H$ is not collision resistant, like MD5, then the attacker can find $H(m) = H(m')$, ask for the MAC of one message and forge it for the other. So for many secure hashes you lose half the security bits. E.g. SHA-256 should give you a 256-bit secure HMAC, but would be at most 128-bit secure in ...


3

Should be comparable in strength to $H(m||k)$. The weakness is that a collision in the inner hash breaks the MAC. Using strong hashes the strength in bits is $\min(2^{n_O},2^{{n_I}/2})$ where $n_O$ is the output size of the outer hash and $n_I$ the output size of the inner hash. But since cryptoanalysis usually breaks collision resistance long before it ...


3

"Given the above assumptions and limitations, is the encryption scheme still secure?" No; the attacker can remove blocks of [IV + rest_of_ciphertext] from either end to remove corresponding plaintext blocks without affecting any other part of what it decrypts to change the IV to change the initial plaintext block in the same way as for the OTP, without ...


3

Would these steps result in a suitable pair of keys for AES-encrypt-then-HMAC-authenticate? Yes. That would be fine. It almost is HKDF-Expand, in fact. However, as you note, by deriving the two 256-bit keys from a 160-bit key your effective security will "only" be 160 bits, since an attacker could brute force the intermediate key. That is not at all a ...


3

PBKDF2 alone would be bad for step 4, since the blocks of its output can trivially be computed independently. ‚Äč If the PBKDF you use can't directly produce enough output, then you should compose with a fast key-based KDF. Make sure that the IV is part of step 9's ciphertext and step 5 of decryption is constant-time .



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