Performing a constant time cyclic redundancy check (CRC) over an entire packet to obtain a constant time CRC value. A first CRC is performed on an original header of the packet and a second CRC is performed on a modified header of the packet. The size of the payload of the packet is obtained. An XOR operation is performed on the results of the first and second CRC to calculate a third result. An intermediate CRC value is obtained by performing a CRC on a number of zero values corresponding to the size of the payload using the third result as an initial value. The intermediate CRC value may be employed with other packets having a same size and same header as the packet. The constant time CRC value is obtained by performing an XOR operation on the intermediate CRC value and the original CRC value contained in the packet.