Hi,
I have a doubt here regarding the length in bits for shared secret key
generated using Diffie-Hellman, hope someone could kindly clear my doubt
:)
In "DH *DH_generate_parameters(int prime_len, int generator, void
*callback, char *cb_arg)", the first argument is the length in bits for
the prime number to be generated in order for subsequent calculation of
the shared secret key. I have a doubt here. Let say I want to generate
a 128 bits (16 bytes) shared secret key, so I pass the value 128 to the
first argument which is prime_len. Then after the "numofbyte =
DH_compute_key(a_shared_secret_key, b->pub_key, a)" function is called,
the return value, numofbyte in this case holds the number of bytes for
the shared secret key generated. I noticed that numofbyte is sometimes
15 bytes and sometimes 16 bytes. Does this mean that prime_len is not
the place to specify the number of bits for the shared secret key
required? Then where to specify? How about if I want a shared secret
key of 126 bits? If you specify prime_len = 126, most probably
numofbyte returned from DH_compute_key equals 16, but not all the bits
constitute the bits for the shared secret key right? I mean only 126
bits out of the 128 bits (16 bytes). Or is it that the length of the
shared secret key in bits must be in multiple of 8 (prime_len must be
set to mutiple of 8 in the very first place)?
Thank you in advance!
Regards,
Wayne
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