def get_public_key_sha1()

in src/watchdog/__init__.py [0:0]

• 26 lines of code
• 7 McCabe index (conditional complexity)

def get_public_key_sha1(public_key):
    # truncating public key to remove the header and footer '-----(BEGIN|END) PUBLIC KEY-----'
    with open(public_key, "r") as f:
        lines = f.readlines()
        lines = lines[1:-1]

    key = "".join(lines)
    key = bytearray(base64.b64decode(key))

    # Parse the public key to pull out the actual key material by looking for the key BIT STRING
    # Example:
    #     0:d=0  hl=4 l= 418 cons: SEQUENCE
    #     4:d=1  hl=2 l=  13 cons: SEQUENCE
    #     6:d=2  hl=2 l=   9 prim: OBJECT            :rsaEncryption
    #    17:d=2  hl=2 l=   0 prim: NULL
    #    19:d=1  hl=4 l= 399 prim: BIT STRING
    cmd = "openssl asn1parse -inform PEM -in %s" % public_key
    output, err = subprocess_call(
        cmd, "Unable to ASN1 parse public key file, %s, correctly" % public_key
    )

    key_line = ""
    for line in output.splitlines():
        if "BIT STRING" in line.decode("utf-8"):
            key_line = line.decode("utf-8")

    if not key_line:
        logging.error("Public key file, %s, is incorrectly formatted", public_key)
        return None

    key_line = key_line.replace(" ", "")

    # DER encoding TLV (Tag, Length, Value)
    # - the first octet (byte) is the tag (type)
    # - the next octets are the length - "definite form"
    #   - the first octet always has the high order bit (8) set to 1
    #   - the remaining 127 bits are used to encode the number of octets that follow
    #   - the following octets encode, as big-endian, the length (which may be 0) as a number of octets
    # - the remaining octets are the "value" aka content
    #
    # For a BIT STRING, the first octet of the value is used to signify the number of unused bits that exist in the last
    # content byte. Note that this is explicitly excluded from the SubjectKeyIdentifier hash, per
    # https://tools.ietf.org/html/rfc5280#section-4.2.1.2
    #
    # Example:
    #   0382018f00...<subjectPublicKey>
    #   - 03 - BIT STRING tag
    #   - 82 - 2 length octets to follow (ignore high order bit)
    #   - 018f - length of 399
    #   - 00 - no unused bits in the last content byte
    offset = int(key_line.split(":")[0])
    key = key[offset:]

    num_length_octets = key[1] & 0b01111111

    # Exclude the tag (1), length (1 + num_length_octets), and number of unused bits (1)
    offset = 1 + 1 + num_length_octets + 1
    key = key[offset:]

    sha1 = hashlib.sha1()
    sha1.update(key)

    return sha1.hexdigest()