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There are two ways to attack encryption that uses a derived key: You can attack the encryption algorithm. In the case of correctly used* 128-bit AES, that essentially amounts to a brute force attack on the 128-bit keyspace. This would succeed after on average $2^{127}$ tries (if it were practical). If you knew that two files had used the same password ...


6

PBKDF2 (as defined by RFC 2898) is a function of the form $$DK = \text{PBKDF2}(\text{PRF}, Password, Salt, c, dkLen)$$ In most practical use cases, the $\text{PRF}$ is $\text{HMAC}$ instantiated with a Merkle-Damgård hash function such as $\text{SHA-1}$. The time to compute $\text{PBKDF2}$ is roughly linear with the iteration parameter $c$, all other ...


5

Firstly, How much time will it take to crack PBKDF2 while using a 9 character password? and how do I calculate the cost? I'm not specifying any specific system or platform. If a brute force attack is made using the best ever super computer around how much time will it take to crack it? Unless the underlying PRF is broken, brute force and dictionary ...


3

When using PBKDF2, is there a practical upper limit to the iteration count above which we lose security? No. There is a limit above which you gain no security, but it isn't practical. It's on the order of $2^{128}$ iterations for PBKDF2-HMAC-SHA-2, or $2^{80}$ if you use SHA-1 as the HMAC hash. For an explanation, see the questions mikeazo linked in ...


5

I agree with the comments that SHA-256 should be fine here. However, if you already use HMAC-SHA-256 for PBKDF2, you could use HKDF Expand, which despite its name is defined even for output lengths shorter than input. In your case the output would be simply: $$\operatorname{HMAC-SHA-256}(\text{key}, \text{info} || \text{0x01}),$$ where 'info' is an ...


3

Your description of how RFC 5959 works isn't quite right. It is not quite correct to state that RFC 5959 encrypts using AES in ECB mode. A correct statement is: if the plaintext is exactly 128 bits, then use ECB mode, otherwise use a non-trivial mode of operation found in RFC 3394. In the former case, ECB mode is fine, since it's just a single block of ...



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