How to use the rsa._compat.b function in rsa

# so keep adding data until we're at the correct length.
    while len(padding) < padding_length:
        needed_bytes = padding_length - len(padding)
        
        # Always read at least 8 bytes more than we need, and trim off the rest
        # after removing the 0-bytes. This increases the chance of getting
        # enough bytes, especially when needed_bytes is small
        new_padding = os.urandom(needed_bytes + 5)
        new_padding = new_padding.replace(b('\x00'), b(''))
        padding = padding + new_padding[:needed_bytes]
    
    assert len(padding) == padding_length
    
    return b('').join([b('\x00\x02'),
                    padding,
                    b('\x00'),
                    message])

b'\x00hello'
    >>> block[2:-6]
    b'\xff\xff\xff\xff\xff\xff\xff\xff'

    """

    max_msglength = target_length - 11
    msglength = len(message)

    if msglength > max_msglength:
        raise OverflowError('%i bytes needed for message, but there is only'
                            ' space for %i' % (msglength, max_msglength))

    padding_length = target_length - msglength - 3

    return b('').join([b('\x00\x01'),
                       padding_length * b('\xff'),
                       b('\x00'),
                       message])

that are raised contain the Python traceback information, which can be used to
deduce where in the process the failure occurred. DO NOT PASS SUCH INFORMATION
to your users.
"""

import hashlib
import os

from rsa._compat import b
from rsa import common, transform, core

# ASN.1 codes that describe the hash algorithm used.
HASH_ASN1 = {
    'MD5': b('\x30\x20\x30\x0c\x06\x08\x2a\x86\x48\x86\xf7\x0d\x02\x05\x05\x00\x04\x10'),
    'SHA-1': b('\x30\x21\x30\x09\x06\x05\x2b\x0e\x03\x02\x1a\x05\x00\x04\x14'),
    'SHA-256': b('\x30\x31\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x01\x05\x00\x04\x20'),
    'SHA-384': b('\x30\x41\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x02\x05\x00\x04\x30'),
    'SHA-512': b('\x30\x51\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x03\x05\x00\x04\x40'),
}

HASH_METHODS = {
    'MD5': hashlib.md5,
    'SHA-1': hashlib.sha1,
    'SHA-256': hashlib.sha256,
    'SHA-384': hashlib.sha384,
    'SHA-512': hashlib.sha512,
}


class CryptoError(Exception):
    """Base class for all exceptions in this module."""

>>> crypto = encrypt('hello', pub_key)
    >>> crypto = crypto[0:5] + 'X' + crypto[6:] # change a byte
    >>> decrypt(crypto, priv_key)
    Traceback (most recent call last):
    ...
    DecryptionError: Decryption failed

    '''
    
    blocksize = common.byte_size(priv_key.n)
    encrypted = transform.bytes2int(crypto)
    decrypted = core.decrypt_int(encrypted, priv_key.d, priv_key.n)
    cleartext = transform.int2bytes(decrypted, blocksize)

    # If we can't find the cleartext marker, decryption failed.
    if cleartext[0:2] != b('\x00\x02'):
        raise DecryptionError('Decryption failed')
    
    # Find the 00 separator between the padding and the message
    try:
        sep_idx = cleartext.index(b('\x00'), 2)
    except ValueError:
        raise DecryptionError('Decryption failed')
    
    return cleartext[sep_idx+1:]

WARNING: this module leaks information when decryption or verification fails.
The exceptions that are raised contain the Python traceback information, which
can be used to deduce where in the process the failure occurred. DO NOT PASS
SUCH INFORMATION to your users.
'''

import hashlib
import os

from rsa._compat import b
from rsa import common, transform, core, varblock

# ASN.1 codes that describe the hash algorithm used.
HASH_ASN1 = {
    'MD5': b('\x30\x20\x30\x0c\x06\x08\x2a\x86\x48\x86\xf7\x0d\x02\x05\x05\x00\x04\x10'),
    'SHA-1': b('\x30\x21\x30\x09\x06\x05\x2b\x0e\x03\x02\x1a\x05\x00\x04\x14'),
    'SHA-256': b('\x30\x31\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x01\x05\x00\x04\x20'),
    'SHA-384': b('\x30\x41\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x02\x05\x00\x04\x30'),
    'SHA-512': b('\x30\x51\x30\x0d\x06\x09\x60\x86\x48\x01\x65\x03\x04\x02\x03\x05\x00\x04\x40'),
}

HASH_METHODS = {
    'MD5': hashlib.md5,
    'SHA-1': hashlib.sha1,
    'SHA-256': hashlib.sha256,
    'SHA-384': hashlib.sha384,
    'SHA-512': hashlib.sha512,
}

class CryptoError(Exception):
    '''Base class for all exceptions in this module.'''

Never display the stack trace of a
        :py:class:`rsa.pkcs1.VerificationError` exception. It shows where in
        the code the exception occurred, and thus leaks information about the
        key. It's only a tiny bit of information, but every bit makes cracking
        the keys easier.

    '''
    
    blocksize = common.byte_size(pub_key.n)
    encrypted = transform.bytes2int(signature)
    decrypted = core.decrypt_int(encrypted, pub_key.e, pub_key.n)
    clearsig = transform.int2bytes(decrypted, blocksize)

    # If we can't find the signature  marker, verification failed.
    if clearsig[0:2] != b('\x00\x01'):
        raise VerificationError('Verification failed')
    
    # Find the 00 separator between the padding and the payload
    try:
        sep_idx = clearsig.index(b('\x00'), 2)
    except ValueError:
        raise VerificationError('Verification failed')
    
    # Get the hash and the hash method
    (method_name, signature_hash) = _find_method_hash(clearsig[sep_idx+1:])
    message_hash = _hash(message, method_name)

    # Compare the real hash to the hash in the signature
    if message_hash != signature_hash:
        raise VerificationError('Verification failed')

>>> block[2:-6]
    '\xff\xff\xff\xff\xff\xff\xff\xff'
    
    '''

    max_msglength = target_length - 11
    msglength = len(message)
    
    if msglength > max_msglength:
        raise OverflowError('%i bytes needed for message, but there is only'
            ' space for %i' % (msglength, max_msglength))
    
    padding_length = target_length - msglength - 3
    
    return b('').join([b('\x00\x01'),
                    padding_length * b('\xff'),
                    b('\x00'),
                    message])

>>> block[2:-6]
    b'\xff\xff\xff\xff\xff\xff\xff\xff'

    """

    max_msglength = target_length - 11
    msglength = len(message)

    if msglength > max_msglength:
        raise OverflowError('%i bytes needed for message, but there is only'
                            ' space for %i' % (msglength, max_msglength))

    padding_length = target_length - msglength - 3

    return b('').join([b('\x00\x01'),
                       padding_length * b('\xff'),
                       b('\x00'),
                       message])

DecryptionError: Decryption failed

    '''
    
    blocksize = common.byte_size(priv_key.n)
    encrypted = transform.bytes2int(crypto)
    decrypted = core.decrypt_int(encrypted, priv_key.d, priv_key.n)
    cleartext = transform.int2bytes(decrypted, blocksize)

    # If we can't find the cleartext marker, decryption failed.
    if cleartext[0:2] != b('\x00\x02'):
        raise DecryptionError('Decryption failed')
    
    # Find the 00 separator between the padding and the message
    try:
        sep_idx = cleartext.index(b('\x00'), 2)
    except ValueError:
        raise DecryptionError('Decryption failed')
    
    return cleartext[sep_idx+1:]

# We remove 0-bytes, so we'll end up with less padding than we've asked for,
    # so keep adding data until we're at the correct length.
    while len(padding) < padding_length:
        needed_bytes = padding_length - len(padding)
        
        # Always read at least 8 bytes more than we need, and trim off the rest
        # after removing the 0-bytes. This increases the chance of getting
        # enough bytes, especially when needed_bytes is small
        new_padding = os.urandom(needed_bytes + 5)
        new_padding = new_padding.replace(b('\x00'), b(''))
        padding = padding + new_padding[:needed_bytes]
    
    assert len(padding) == padding_length
    
    return b('').join([b('\x00\x02'),
                    padding,
                    b('\x00'),
                    message])