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Yes, this is secure, even though scrypt uses PBKDF2 inside. PBKDF2 has the issue that it the work factor is required $n$ times where $n$ is the number hash outputs concatenated to create the final PBKDF2 output. That means that if you can check the validity of PBKDF2 using only the initial bits (in your case used for the key if the hash was SHA-256, for ...


3

No the IV doesn't get encrypted. The IV is a random vector to make sure that the ciphertext is not identical for identical plaintext. This would leak information to any eavesdropper. It needs to be unique - and in the case of CBC, indistinguishable from random to the eavesdropper ("unpredictable") - but not confidential. As the IV is separate from the ...


1

The authentication tag in GCM is generated by XORing a block cipher output with the Galois field hash (and truncating it for shorter lengths). It is thus assumed to look PRF. So it is effectively just a random nonce that should not collide until a birthday bound of $2^{t/2}$. With a tag length of 96 or more bits, it should be secure. Shorter random IV ...


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Using the same key and IV for different, independently encrypted cells is a bad idea, regardless of which encryption algorithm you use. It would allow attackers to find either XORs of cells (with CTR) or at least equality of prefixes (with CBC). If you are going to use authenticated encryption, you need to choose whether the MAC applies to each cell ...



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