I am interested in the theoretical consideration of the bit strength of an encryption key and its precursor.
Assume a given environment as follows:
a. My encryption algorithm is AES-256
b. My key derivation function is PBKDF2 with HMAC-SHA1 with
b1. Salt strength of 128 bit (32 chars of a truely random hex string)
b2. Iteration count is 4096
My objective is to achieve 256 bit "encryption strength" based on the above environment
Evidently, I can not achieve a "true 256-bit total strength" with the environment as stated above, since the randomness provided by the SHA1 in the PBKDF2 standard is at most 160 bit.
From RSA Document "PKCS #5 v2.0: Password-Based Cryptography Standard", March 25, 1999:
"The length of the derived key is essentially unbounded. (However, the maximum effective search space for the derived key may be limited by the structure of the underlying pseudorandom function"
"....Thus, even if a long derived key consisting of several pseudorandom function outputs is produced from a key, the effective search space for the derived key will be at most 160 bits. Although the specific limitation for other key sizes depends on details of the HMAC construction, one should assume, to be conservative, that the effective search space is limited to 160 bits for other key sizes as well."
My questions is: Since the most I can expect is "160-bit encryption strength" from my existing environment, if I feed the result of the PBKDF2 output to a Skein 512-512 hash function, and use the left-most 61 hex (=244 bit) characters of its output as an encryption key, will I then have my sought-after 256-bit total encryption strength (I have added the 12 bit extra "strength", due to iteration count, to my calculations)?