Why pack_into doesn't write correctly some bytes?

[Rafael Justo]

When I try to write a specific byte (letter 'p') in a writable buffer using pack_into the result is a corrupted data:

>>> from ctypes import create_string_buffer
>>> from struct import pack_into
>>> test = create_string_buffer(1)
>>> pack_into("B", test, 0, 48)
>>> print repr(test.raw)
'0'

The answer should be '\x30'. What I'm doing wrong?

[Glenton]

What are you trying to do? I've not really come across those commands before, and wonder if they are still relevant to modern versions of python?

[Glenton]

Oh, by the way:

chr(48) = "0"
chr(112) = "p"

[Rafael Justo]

I use pack_into to build a buffer of bytes that are going to be the modulus of a public key. I think that the problem isn't what I suspected, the \x30 when wrote in a terminal is replaced by 'zero' as you said.

The main problem is to convert a DNSKEY (dns record) into a public key structure (RSA) of M2crypto. Today I'm doing the following:

    # Exponent and modulus must be in a string in OpenSSL's MPINT
    # format - 4-byte big-endian bit-count followed by the appropriate
    # number of bits

    # Exponent

    exponentFinal = create_string_buffer(4 + exponentSize)
    exponentOffset = 0

    pack_into(">L", exponentFinal, exponentOffset, exponentSize)
    exponentOffset += 4

    for i in range(0, exponentSize):
      pack_into("B", exponentFinal, exponentOffset, ord(exponent[i]))
      exponentOffset += 1

    # Modulus

    modulusSize = len(modulus)
    modulusFinal = create_string_buffer(4 + modulusSize)
    modulusOffset = 0

    pack_into(">L", modulusFinal, modulusOffset, modulusSize)
    modulusOffset += 4

    for i in range(0, modulusSize):
      pack_into("B", modulusFinal, modulusOffset, ord(modulus[i]))
      modulusOffset += 1

    try:
      rsa = RSA.new_pub_key((exponentFinal.raw, modulusFinal.raw))
    except RSA.RSAError, e:
      return None

But something is wrong, because when I try to verify a signature with the public key a got an error telling that the modulus isn't compatible with data size.

Best regards,
Rafael