hkdf 0.0.3

Functional interface

To use the functional interface, pass the pseudorandom key generated by hmac_extract([salt], [input key material]) to hmac_expand(…). salt should be a random, non-secret, site-specific string, but may be set to None. See section 3.1 of the HKDF draft for more details.

In addition to the PRK output by hmac_extract, hmac_expand takes an info argument, which permits generating multiple keys based on the same PRK, and a length argument, which defines the number of bytes of output key material to generate. length must be less than or equal to 255 time the block size, in bytes, of the hash function being used. See section 3.2 of the HKDF draft for more information on using the info argument.

The hash function to use can be specified for both hmac_extract and hmac_expand as the hash kw argument, and defaults to SHA-256 as implemented by the hashlib module. It must be the same for both extracting and expanding.

Example:

prk = hkdf_extract("8e94ef805b93e683ff18".decode("hex"), "asecretpassword")
key = hkdf_expand(prk, "context1", 16)

Hkdf wrapper class

To use the wrapper class, instantiate the Hkdf() class with a salt, input key material, and optionally, a hash function. You may then call expand([info], [length]) on the Hkdf instance to generate output key material:

kdf = Hkdf("8e94ef805b93e683ff18".decode("hex"), "asecretpassword")
key = kdf.expand("context1", 16)

HKDF-Extract and HKDF-Expand definitions from the draft RFC

> Step 1: Extract > > PRK = HKDF-Extract(salt, IKM) > > Options: > Hash a hash function; HashLen denotes the length of the > hash function output in octets > Inputs: > salt optional salt value (a non-secret random value); > if not provided, it is set to a string of HashLen zeros. > IKM input keying material > Output: > PRK a pseudo-random key (of HashLen octets) > > The output PRK is calculated as follows: > > PRK = HMAC-Hash(salt, IKM) > > Step 2: Expand > > OKM = HKDF-Expand(PRK, info, L) > > Options: > Hash a hash function; HashLen denotes the length of the > hash function output in octets > Inputs: > PRK a pseudo-random key of at least HashLen octets > (usually, the output from the Extract step) > info optional context and application specific information > (can be a zero-length string) > L length of output keying material in octets > (<= 255*HashLen) > Output: > OKM output keying material (of L octets) > > The output OKM is calculated as follows: > > N = ceil(L/HashLen) > T = T(1) | T(2) | T(3) | … | T(N) > OKM = first L octets of T > > where: > T(0) = empty string (zero length) > T(1) = HMAC-Hash(PRK, T(0) | info | 0x01) > T(2) = HMAC-Hash(PRK, T(1) | info | 0x02) > T(3) = HMAC-Hash(PRK, T(2) | info | 0x03) > … > > (where the constant concatenated to the end of each T(n) is a > single octet.)

Changelog

• 0.0.3 – Move documentation from module docstring to README.rst

• 0.0.2 – Python 3.3, 3.4 support

• 0.0.1 – Initial release

Please report any bugs at

https://www.github.com/casebeer/python-hkdf