The Fletcher checksum and scodec

The Fletcher checksum could be computed using the scodec infrastructure as follows:

object FletcherChecksum {

  def apply(bv:BitVector): BitVector = {
    val csum = bv.toByteArray.foldLeft((0, 0)) { (p, b) =>
        val lsb = (p._2 + (0xff & b)) % 255
        ((p._1 + lsb) % 255, lsb)
    }
    bv ++BitVector(csum._1.asInstanceOf[Byte], csum._2.asInstanceOf[Byte])
  }
}

Note the checksum is appended to the input BitVector.

With a test

@Test
def wikipedia {
   assertEquals(BitVector(0x01, 0x02, 0x04, 0x03), 
      FletcherChecksum(BitVector(0x01, 0x02)))
}

This design is not consistent with the scodec cryptographic signing functionality. Unfortunately that code is built around the Java security API and the core java.security.Signature class is not appropriate for very much simpler checksum functionality.

Richard Searle

The Fletcher checksum and scodec