INTEGER length error in the tlv

[EKA-13]

Hi

I am trying to create the following structure

 0x30, 0x82, 0x02, 0x12,         ; SEQUENCE (530 Bytes)
    0x30, 0x82, 0x01, 0x0A,     ; SEQUENCE (266 Bytes) (public key) 
        0x02, 0x82, 0x01, 0x01  ; INTEGER  (277 Bytes)
            <257 bytes public key-modulus  >,
        0x02, 0x03,             ; INTEGER  ( 03 Bytes)
            <3 bytes PK-exponent (value: 65537)>,
    0x02, 0x82, 0x01, 0x00,     ; INTEGER  (256 Bytes)
        <256byte Signature { public key  } *singed by CA private key >_

class publicKey(Sequence):

    componentType = NamedTypes(
        NamedType('pk',Integer()),
        NamedType('exponent', Integer())
    )


class SrklHost(Sequence):
    componentType = NamedTypes(
        NamedType('hostPk',publicKey()),        
        NamedType('signature', Integer())
    )


ca_key = serialization.load_pem_private_key( ...

host_key = rsa.generate_private_key(
    public_exponent=65537,
    key_size=0x800,
    backend=default_backend()
    )

host_key_pk = host_key.public_key().public_bytes(serialization.Encoding.DER,format=serialization.PublicFormat.PKCS1)
chosen_hash = hashes.SHA256()
hasher = hashes.Hash(chosen_hash, default_backend())
hasher.update(host_key_pk)
digest = hasher.finalize()

sig = ca_key.sign(
       digest,
       padding.PKCS1v15(),
       utils.Prehashed(hashes.SHA256()))


message , _ = decode(host_key_pk)

pk , _ = decode(host_key_pk, asn1Spec= publicKey())

print(len(host_key_pk))
print(len(encode(pk)))



signedItem = SrklHost()
signedItem['hostPk'] = pk
signedItem['signature'] = int.from_bytes(sig,'big')

print(len(encode(signedItem)))

The signedItem message should be in total 534 bytes long
But sometimes it return an extra byte resulting to 535 bytes in total

I can see that in the 0x02, 0x82, 0x01, 0x00, ; INTEGER (256 Bytes) section of signature adds an extra byte
Resulting to 0x02 0x82 0x01 0x01 0x00.

Am I doing something wrong ?

I am using cryptography io for other cryptography functionality

Thank you !

[fvanderwerf]

I have seen a similar issue where an encoded integer is one byte too long. I have the following testcode:

from pyasn1.type import univ
from pyasn1.codec.der import encoder

from binascii import hexlify

val = -2 ** 63
print(val)
asn1 = encoder.encode(univ.Integer(val))
print(hexlify(asn1).decode('ascii'))

With current master (db8f1a7) this gives on Python2:

-9223372036854775808
02088000000000000000

which looks correct. With Python3 an extra byte (0xff) is prepended:

-9223372036854775808
0209ff8000000000000000

Seems like length calculation is wrong under Python3.