/// <summary>
/// Decode USM portion of the SNMP version 3 packet.
/// </summary>
/// <param name="buffer">Received SNMP packet BER encoded</param>
/// <param name="offset">Offset within the buffer to start decoding USM information</param>
/// <returns>Buffer position after the decoded value</returns>
/// <exception cref="SnmpDecodingException">Thrown when decoding enountered invalid data type in USM information</exception>
/// <exception cref="OverflowException">Thrown when packet is too small to contain information length specified in header</exception>
public override int Decode(byte[] buffer, int offset)
{
// Grab the octet string header
byte type = ParseHeader(buffer, ref offset, out int len);
if (type != OCTETSTRING)
{
throw new SnmpDecodingException("Invalid value type found while looking for USM header.");
}
if (len > (buffer.Length - offset))
{
throw new OverflowException("Packet too small");
}
// Now grab the sequence header
type = ParseHeader(buffer, ref offset, out len);
if (type != SnmpConstants.SMI_SEQUENCE)
{
throw new SnmpDecodingException("Sequence missing from USM header.");
}
if (len > (buffer.Length - offset))
{
throw new OverflowException("Packet too small");
}
// now grab values one at the time
offset = _engineId.Decode(buffer, offset);
offset = _engineBoots.Decode(buffer, offset);
offset = _engineTime.Decode(buffer, offset);
offset = _securityName.Decode(buffer, offset);
int saveOffset = offset;
offset = AuthenticationParameters.Decode(buffer, offset);
if (AuthenticationParameters.Length > 0)
{
// walk through and set the authentication parameters to 0x00 in the packet
saveOffset += 2; // Skip BER encoded variable type and length
for (int i = 0; i < AuthenticationParameters.Length; i++)
{
buffer[saveOffset + i] = 0x00;
}
}
offset = _privacyParameters.Decode(buffer, offset);
return(offset);
}