[jose] RFC7520 5.5 Clarification

[Tommy Wang <[e-mail]>]

Looking for some guidance on how the ECDH-ES Key Agreement (5.5.2)
derived the CEK:

hzHdlfQIAEehb8Hrd_mFRhKsKLEzPfshfXs9l6areCc

No apv/apu values were provided leading me to believe that it was not
derived using ConcatKDF.

I tried to implement via D-H Key Agreement (RFC2631) with no
partyAInfo but was not able to arrive at the same CEK.

I used the following OIDS:

OIDS = {
    'A128CBC-HS256': '2.16.840.1.101.3.4.1.2',
    'A192CBC-HS384': '2.16.840.1.101.3.4.1.22',
    'A256CBC-HS512': '2.16.840.1.101.3.4.1.42',
    'A128GCM': '2.16.840.1.101.3.4.1.6',
    'A192GCM': '2.16.840.1.101.3.4.1.26',
    'A256GCM': '2.16.840.1.101.3.4.1.46',
}

And the following pyasn1:

from pyasn1.type import univ, namedtype, tag, constraint
from pyasn1.codec.der import encoder
import hashlib

class Counter(univ.OctetString):
    subtypeSpec = constraint.ValueSizeConstraint(4, 4)

class KeySpecificInfo(univ.Sequence):
    componentType = namedtype.NamedTypes(
        namedtype.NamedType('algorithm', univ.ObjectIdentifier()),
        namedtype.NamedType('counter', Counter())
    )

class OtherInfo(univ.Sequence):
    componentType = namedtype.NamedTypes(
        namedtype.NamedType('keyInfo', KeySpecificInfo()),
        namedtype.OptionalNamedType('partyAInfo', univ.OctetString().subtype(
            explicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 0)
        )),
        namedtype.NamedType('suppPubInfo', univ.OctetString().subtype(
            explicitTag=tag.Tag(tag.tagClassContext, tag.tagFormatSimple, 2)
        ))
    )

def km(alg, zz, n):
    oid = OIDS[alg]
    ainfo = None
    pinfo = 128
    k = KeySpecificInfo()
    k.setComponentByName('algorithm', oid)
    k.setComponentByName('counter', struct.pack('>I', n))
    o = OtherInfo()
    o.setComponentByName('keyInfo', k)
    o.setComponentByName('suppPubInfo', struct.pack('>I', pinfo))
    o = encoder.encode(o)
    return hashlib.sha1(zz + o).digest()

zz was derived using cryptography's EC key exchange.