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You can get what you want programmatically. No special ciphers or modes needed. You say there is a single, continuous subset that needs to be encrypted. Thus, you could have a function where the programmer specifies the start of the portion of the plaintext that needs to be encrypted and the number of bytes to encrypt. The function could pull that part out, ...


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A key derivation function lets you derive keys from others. In this case I would use HKDF, which means using HMAC in a predefined way. Your key material is the keys $X$ and $Y$, so you can concatenate those to get the PRK for HKDF-Expand. An output key would then be $\operatorname{HMAC}(X||Y, \text{info} || \text{0x01})$, if the size of the HMAC is long ...


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GPG is an implementation of OpenPGP, which is a higher level protocol than e.g. mcrypt. So use GPG for PGP compatibility and mcrypt or related libraries for more direct - lower level - access to algorithms. AES is Rijndael for a block size of 128 bits and the 128, 192 or 256 bit key sizes. So you are OK there. Learn about modes of operation and something ...


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HMAC is considered the most secure way of combining two keys, as compared to a single round of SHA256. hmac is designed to fold in the key material in 2 hash operations, which helps resist chosen plaintext attacks on sha-256, although SHA256 has no known chosen plaintext attacks at this time. Symmetric ciphers are considered less reliable than hashes for ...


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As block ciphers are invertible, and since XOR is too - the main operation on the key stream for many stream ciphers - the resulting encryption/decryption modes of operation are often invertible. For stream ciphers that create a key stream that is XOR'ed with the plaintext, it is even true that $E = E^{-1}$. Some block cipher modes of operation however ...


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I think my definition of 'simple' is different from yours. : ) Are there any evident attacks on it? It's vulnerable to man-in-the-middle attacks, like: Step 9: M replaces $\langle b_{U}, v'_{U}, h'_{U} \rangle$ with $\langle b_{U}, v_{M}, h_{M} \rangle$, where they've calculated their own random bit string and used their own key to encrypt it. Now M ...



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