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With RSA (or any asymmetric cryptography, for that matter), the key question is "how do you know you can trust the other peer's public key?". Without authentication, you could be sending your message using the attacker's public key. Example 1: the first time you SSH to a host, you are prompted to confirm that you trust the public key. That's because if an ...

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The attack is even more simple with RSA than with symmetric keys, because the asymmetric encryption key is assumed to be public. Let me tell you a story involving Alice, Bob and Mallory :). Alice wants to send a message to Bob using RSA. Alice encrypts the message using Bob's public key and sends it Mallory performs a Man-In-The-Middle attack, and ...

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From RFC 4226: 7.4. Resynchronization of the Counter Although the server's counter value is only incremented after a successful HOTP authentication, the counter on the token is incremented every time a new HOTP is requested by the user. Because of this, the counter values on the server and on the token might be out of synchronization. ...

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I am wondering if using Skein or the Keccak hash algorithm in this construction (as a stream cipher) is secure: In the case of Skein and Keccak it should be secure. However, both of those have defined their own cipher modes which you should IMO prefer. (For compatibility, if not security.) The Skein one is defined in section 4.10 of the paper. It uses ...

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If you replace $H$ with a MAC that is build using $H$, i.e. NMAC, then it will be provably secure. Encryption will be $C_i = MAC_k(IV | i) \oplus P_i$. You are guaranteed by the MAC security property (existential forgery) that an adversary cannot generate any of the key stream on his own and so you are left with a secure stream cipher. Just using $H$ by ...

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If you need a MAC, use a MAC. For example, HMAC which uses a hash function. Don't try to use a random hash function in your own scheme, because some such schemes are not secure. Reason to use/not use this data as input to the hash? (In the context of authenticity of data…) If you do have a secure MAC, any constant data will not affect the authenticity. ...

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