3 ms·

What is a KDF? What is this doing in layman's terms?

swordswinger12 13y ago
Key Derivation Function. It's a tool cryptographers use to derive cryptographic keys from different kinds of input material. Their most common use is to 'amplify' the entropy of a password or other low-entropy input and make it suitable for use as a key for a symmetric encryption scheme.
sneak 13y ago
When you type in your password to decrypt your ssh private key on disk, it runs the string (and optionally also some salt, stored in the clear with the encrypted key to foil rainbowtable lookup attacks) through a key derivation function, or KDF. The output is used as the encryption key for the symmetric cipher that protects the key at rest. One can use any cryptographically secure (output indistinguishable from random, infeasible to predict input given output, et c) hash as a KDF but the faster it is to run, the more per second an attacker can try when attempting to bruteforce your password/phrase after stealing your private key in its encrypted, on-disk format. bcrypt is a function that is specifically designed to be somewhat time consuming to calculate (with a variable difficulty argument to the function itself), to reduce the bruteforce rate (in normal use, it's only run once or twice, once for each password input). Its cousin scrypt was specifically designed to also require a fair bit of memory to calculate, as well, reducing effective parallelism in fpga/asic implementations as the memory circuits must be included as well. The difference between one <1ms hash and 100ms hash doesn't much matter to a user, but is the difference between a 10-char password being feasibly bruteforceable or not on dedicated hardware where you have to run 256^10 iterations. It adds up fast. My ssh private key password is 8 chars that match [a-z0-9]. This matters a lot.
- midas007 13y ago
  Ideally, a KDF should use as many resources as practical. The NIST competition process of favoring performant algorithms is not always in the interests of users because it means it's cheaper to bruteforce/crack.
  - sp332 13y ago
    The NIST competition was for a hash function, not a key derivation function.
- sneak 13y ago
  Err, more like 128^10 iterations, unless you have the unicode snowman in there.
midas007 13y ago
yourmagicnoncrappypassword -> 101010101110101111...

Key Derivation Function. It's a tool cryptographers use to derive cryptographic keys from different kinds of input material. Their most common use is to 'amplify' the entropy of a password or other low-entropy input and make it suitable for use as a key for a symmetric encryption scheme.

sneak 13y ago

When you type in your password to decrypt your ssh private key on disk, it runs the string (and optionally also some salt, stored in the clear with the encrypted key to foil rainbowtable lookup attacks) through a key derivation function, or KDF. The output is used as the encryption key for the symmetric cipher that protects the key at rest. One can use any cryptographically secure (output indistinguishable from random, infeasible to predict input given output, et c) hash as a KDF but the faster it is to run, the more per second an attacker can try when attempting to bruteforce your password/phrase after stealing your private key in its encrypted, on-disk format. bcrypt is a function that is specifically designed to be somewhat time consuming to calculate (with a variable difficulty argument to the function itself), to reduce the bruteforce rate (in normal use, it's only run once or twice, once for each password input). Its cousin scrypt was specifically designed to also require a fair bit of memory to calculate, as well, reducing effective parallelism in fpga/asic implementations as the memory circuits must be included as well. The difference between one <1ms hash and 100ms hash doesn't much matter to a user, but is the difference between a 10-char password being feasibly bruteforceable or not on dedicated hardware where you have to run 256^10 iterations. It adds up fast. My ssh private key password is 8 chars that match [a-z0-9]. This matters a lot.

midas007 13y ago
Ideally, a KDF should use as many resources as practical. The NIST competition process of favoring performant algorithms is not always in the interests of users because it means it's cheaper to bruteforce/crack.
- sp332 13y ago
  The NIST competition was for a hash function, not a key derivation function.
sneak 13y ago
Err, more like 128^10 iterations, unless you have the unicode snowman in there.

midas007 13y ago

yourmagicnoncrappypassword -> 101010101110101111...