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"""
RFC 6979:
Deterministic Usage of the Digital Signature Algorithm (DSA) and
Elliptic Curve Digital Signature Algorithm (ECDSA)
http://tools.ietf.org/html/rfc6979
Many thanks to Coda Hale for his implementation in Go language:
https://github.com/codahale/rfc6979
"""
import hmac
from binascii import hexlify
from .util import number_to_string, number_to_string_crop, bit_length
from ._compat import hmac_compat
# bit_length was defined in this module previously so keep it for backwards
# compatibility, will need to deprecate and remove it later
__all__ = ["bit_length", "bits2int", "bits2octets", "generate_k"]
def bits2int(data, qlen):
x = int(hexlify(data), 16)
l = len(data) * 8
if l > qlen:
return x >> (l - qlen)
return x
def bits2octets(data, order):
z1 = bits2int(data, bit_length(order))
z2 = z1 - order
if z2 < 0:
z2 = z1
return number_to_string_crop(z2, order)
# https://tools.ietf.org/html/rfc6979#section-3.2
def generate_k(order, secexp, hash_func, data, retry_gen=0, extra_entropy=b""):
"""
order - order of the DSA generator used in the signature
secexp - secure exponent (private key) in numeric form
hash_func - reference to the same hash function used for generating
hash
data - hash in binary form of the signing data
retry_gen - int - how many good 'k' values to skip before returning
extra_entropy - extra added data in binary form as per section-3.6 of
rfc6979
"""
qlen = bit_length(order)
holen = hash_func().digest_size
rolen = (qlen + 7) // 8
bx = (
hmac_compat(number_to_string(secexp, order)),
hmac_compat(bits2octets(data, order)),
hmac_compat(extra_entropy),
)
# Step B
v = b"\x01" * holen
# Step C
k = b"\x00" * holen
# Step D
k = hmac.new(k, digestmod=hash_func)
k.update(v + b"\x00")
for i in bx:
k.update(i)
k = k.digest()
# Step E
v = hmac.new(k, v, hash_func).digest()
# Step F
k = hmac.new(k, digestmod=hash_func)
k.update(v + b"\x01")
for i in bx:
k.update(i)
k = k.digest()
# Step G
v = hmac.new(k, v, hash_func).digest()
# Step H
while True:
# Step H1
t = b""
# Step H2
while len(t) < rolen:
v = hmac.new(k, v, hash_func).digest()
t += v
# Step H3
secret = bits2int(t, qlen)
if 1 <= secret < order:
if retry_gen <= 0:
return secret
retry_gen -= 1
k = hmac.new(k, v + b"\x00", hash_func).digest()
v = hmac.new(k, v, hash_func).digest()
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