Synnefo

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# ===================================================================

from Crypto.Util.number import long_to_bytes, bytes_to_long

__all__ = [ 'DerObject', 'DerInteger', 'DerSequence' ]

class DerObject:

        typeTags = { 'SEQUENCE':'\x30', 'BIT STRING':'\x03', 'INTEGER':'\x02' }

        def __init__(self, ASN1Type=None):

                self.typeTag = self.typeTags.get(ASN1Type, ASN1Type)

                self.payload = ''

        def _lengthOctets(self, payloadLen):

                '''

                Return an octet string that is suitable for the BER/DER

                length element if the relevant payload is of the given

                size (in bytes).

                '''

                if payloadLen>127:

                        encoding = long_to_bytes(payloadLen)

                        return chr(len(encoding)+128) + encoding

                return chr(payloadLen)

        def encode(self):

                return self.typeTag + self._lengthOctets(len(self.payload)) + self.payload

        def _decodeLen(self, idx, str):

                '''

                Given a string and an index to a DER LV,

                this function returns a tuple with the length of V

                and an index to the first byte of it.

                '''

                length = ord(str[idx])

                if length<=127:

                        return (length,idx+1)

                else:

                        payloadLength = bytes_to_long(str[idx+1:idx+1+(length & 0x7F)])

                        if payloadLength<=127:

                                raise ValueError("Not a DER length tag.")

                        return (payloadLength, idx+1+(length & 0x7F))

        def decode(self, input, noLeftOvers=0):

                try:

                        self.typeTag = input[0]

                        if (ord(self.typeTag) & 0x1F)==0x1F:

                                raise ValueError("Unsupported DER tag")

                        (length,idx) = self._decodeLen(1,input)

                        if noLeftOvers and len(input) != (idx+length):

                                raise ValueError("Not a DER structure")

                        self.payload = input[idx:idx+length]

                except IndexError:

                        raise ValueError("Not a valid DER SEQUENCE.")

                return idx+length

class DerInteger(DerObject):

        def __init__(self, value = 0):

                DerObject.__init__(self, 'INTEGER')

                self.value = value

        def encode(self):

                self.payload = long_to_bytes(self.value)

                if ord(self.payload[0])>127:

                        self.payload = '\x00' + self.payload

                return DerObject.encode(self)

        def decode(self, input, noLeftOvers=0):

                tlvLength = DerObject.decode(self, input,noLeftOvers)

                if ord(self.payload[0])>127:

                        raise ValueError ("Negative INTEGER.")

                self.value = bytes_to_long(self.payload)

                return tlvLength

class DerSequence(DerObject):

        def __init__(self):

                DerObject.__init__(self, 'SEQUENCE')

                self._seq = []

        def __delitem__(self, n):

                del self._seq[n]

        def __getitem__(self, n):

                return self._seq[n]

        def __setitem__(self, key, value):

                self._seq[key] = value

        def __setslice__(self,i,j,sequence):

                self._seq[i:j] = sequence

        def __delslice__(self,i,j):

                del self._seq[i:j]

        def __getslice__(self, i, j):

                return self._seq[max(0, i):max(0, j)]

        def __len__(self):

                return len(self._seq)

        def append(self, item):

                return self._seq.append(item)

        def hasOnlyInts(self):

                if not self._seq: return 0

                test = 0

                for item in self._seq:

                        try:

                                test += item

                        except TypeError:

                                return 0

                return 1

        def encode(self):

                '''

                Return the DER encoding for the ASN.1 SEQUENCE containing

                the non-negative integers and longs added to this object.

                '''

                self.payload = ''

                for item in self._seq:

                        try:

                                self.payload += item

                        except:

                                try:

                                        self.payload += DerInteger(item).encode()

                                except:

                                        raise ValueError("Trying to DER encode an unknown object")

                return DerObject.encode(self)

        def decode(self, input,noLeftOvers=0):

                '''

                This function decodes the given string into a sequence of

                ASN.1 objects. Yet, we only know about unsigned INTEGERs.

                Any other type is stored as its rough TLV. In the latter

                case, the correctectness of the TLV is not checked.

                '''

                self._seq = []

                try:

                        tlvLength = DerObject.decode(self, input,noLeftOvers)

                        if self.typeTag!=self.typeTags['SEQUENCE']:

                                raise ValueError("Not a DER SEQUENCE.")

                        # Scan one TLV at once

                        idx = 0

                        while idx<len(self.payload):

                                typeTag = self.payload[idx]

                                if typeTag==self.typeTags['INTEGER']:

                                        newInteger = DerInteger()

                                        idx += newInteger.decode(self.payload[idx:])

                                        self._seq.append(newInteger.value)

                                else:

                                        itemLen,itemIdx = self._decodeLen(idx+1,self.payload)

                                        self._seq.append(self.payload[idx:itemIdx+itemLen])

                                        idx = itemIdx + itemLen

                except IndexError:

                        raise ValueError("Not a valid DER SEQUENCE.")

                return tlvLength