# -*- coding: utf-8 -*-
#
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# The contents of this file are dedicated to the public domain.  To
# the extent that dedication to the public domain is not available,
# everyone is granted a worldwide, perpetual, royalty-free,
# non-exclusive license to exercise all rights associated with the
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# No rights are reserved.
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# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
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# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
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"""Public-key encryption and signature algorithms.

Public-key encryption uses two different keys, one for encryption and
one for decryption.  The encryption key can be made public, and the
decryption key is kept private.  Many public-key algorithms can also
be used to sign messages, and some can *only* be used for signatures.
"""

from Crypto.Util.asn1 import (DerSequence, DerInteger, DerBitString,
                             DerObjectId, DerNull)


def _expand_subject_public_key_info(encoded):
    """Parse a SubjectPublicKeyInfo structure.

    It returns a triple with:
        * OID (string)
        * encoded public key (bytes)
        * Algorithm parameters (bytes or None)
    """

    #
    # SubjectPublicKeyInfo  ::=  SEQUENCE  {
    #   algorithm         AlgorithmIdentifier,
    #   subjectPublicKey  BIT STRING
    # }
    #
    # AlgorithmIdentifier  ::=  SEQUENCE  {
    #   algorithm   OBJECT IDENTIFIER,
    #   parameters  ANY DEFINED BY algorithm OPTIONAL
    # }
    #

    spki = DerSequence().decode(encoded, nr_elements=2)
    algo = DerSequence().decode(spki[0], nr_elements=(1,2))
    algo_oid = DerObjectId().decode(algo[0])
    spk = DerBitString().decode(spki[1]).value

    if len(algo) == 1:
        algo_params = None
    else:
        try:
            DerNull().decode(algo[1])
            algo_params = None
        except:
            algo_params = algo[1]

    return algo_oid.value, spk, algo_params


def _create_subject_public_key_info(algo_oid, secret_key, params=None):

    if params is None:
        params = DerNull()

    spki = DerSequence([
                DerSequence([
                    DerObjectId(algo_oid),
                    params]),
                DerBitString(secret_key)
                ])
    return spki.encode()


def _extract_subject_public_key_info(x509_certificate):
    """Extract subjectPublicKeyInfo from a DER X.509 certificate."""

    certificate = DerSequence().decode(x509_certificate, nr_elements=3)
    tbs_certificate = DerSequence().decode(certificate[0],
                                           nr_elements=list(range(6, 11)))

    index = 5
    try:
        tbs_certificate[0] + 1
        # Version not present
        version = 1
    except TypeError:
        version = DerInteger(explicit=0).decode(tbs_certificate[0]).value
        if version not in (2, 3):
            raise ValueError("Incorrect X.509 certificate version")
        index = 6

    return tbs_certificate[index]