Question

What format I should use in struct.unpack to decode data packed in perl using w format character (as doc says ‘BER compressed integer’)?

Answer 1

I don’t believe python’s struct module has support for that format, it mainly supports only the encodings that are commonly found in C structs. BER & DER encodings are generally only encountered within ASN.1 encoded streams… one of the Python ASN.1 modules might be helpful in that case (I should note they are not that user-friendly).

If not, you may have to implement a decoder yourself. The following bit of code will read off an int, and return where in the string that unpacking should pick up at…

def decode_ber_int(data, offset):
    value = 0
    while True:
        tmp = ord(data[offset])
        value = (value<<7) | (tmp&0x7f)
        offset += 1
        if tmp & 0x80 == 0:
            break
    return value, offset

Sadly, this will probably require breaking up your unpack call into unpack, decode_ber_int, and unpack the rest.