Challenge 15: PKCS#7 padding validation

Problem

PKCS#7 padding validation
Write a function that takes a plaintext, determines if it has valid PKCS#7 padding, and strips the padding off.

The string:

"ICE ICE BABY\x04\x04\x04\x04"
... has valid padding, and produces the result "ICE ICE BABY".

The string:

"ICE ICE BABY\x05\x05\x05\x05"
... does not have valid padding, nor does:

"ICE ICE BABY\x01\x02\x03\x04"
If you are writing in a language with exceptions, like Python or Ruby, make your function throw an exception on bad padding.

Crypto nerds know where we're going with this. Bear with us.

package challenge15

import (
  "errors"
  "fmt"
)

func Challenge15() {
  goodString := []byte("ICE ICE BABY\x04\x04\x04\x04")
  getGood, err := Unpad(goodString)
  if err != nil {
    fmt.Println(err)
    return
  }
  fmt.Printf("Good string: %q\nUnpadded:%q", goodString, getGood)

  badString1 := []byte("ICE ICE BABY\x05\x05\x05\x05")
  badString2 := []byte("ICE ICE BABY\x01\x02\x03\x04")
  getBad1, err := Unpad(badString1)
  if err != nil {
    fmt.Println(err)
    return
  }
  getBad2, err := Unpad(badString2)
  if err != nil {
    fmt.Println(err)
    return
  }
  fmt.Printf("Bad string: %q\nUnpadded:%q", badString1, getBad1)
  fmt.Printf("Bad string: %q\nUnpadded:%q", badString2, getBad2)

}

func Unpad(input []byte) ([]byte, error) {
  if input == nil || len(input) == 0 {
    return nil, nil
  }

  pc := input[len(input)-1]
  padLen := int(pc)

  if exists := checkPaddingIsValid(input, padLen); !exists {
    return nil, errors.New("Invalid padding, Cannot unpad.")
  }
  return input[:len(input)-padLen], nil
}
func checkPaddingIsValid(input []byte, paddingLength int) bool {
  if len(input) < paddingLength {
    return false
  }
  p := input[len(input)-(paddingLength):]
  for _, pc := range p {
    if uint(pc) != uint(len(p)) {
      return false
    }
  }
  return true
}