using Obscur.Core.Cryptography.Entropy;

namespace Obscur.Core.Cryptography.Ciphers.Block.Padding
{
    /**
    * A padder that adds Pkcs7/Pkcs5 padding to a block.
    */
    public class Pkcs7Padding
		: IBlockCipherPadding
    {
        /**
        * Initialise the padder.
        *
        * @param random - a CsRng if available.
        */
        public void Init(
			CsRng random)
        {
            // nothing to do.
        }

        /**
        * Return the name of the algorithm the cipher implements.
        *
        * @return the name of the algorithm the cipher implements.
        */
        public string PaddingName
        {
            get { return "PKCS7"; }
        }

		/**
        * add the pad bytes to the passed in block, returning the
        * number of bytes added.
        */
        public int AddPadding(
            byte[]  input,
            int     inOff)
        {
            byte code = (byte)(input.Length - inOff);

            while (inOff < input.Length)
            {
                input[inOff] = code;
                inOff++;
            }

            return code;
        }

        /**
        * return the number of pad bytes present in the block.
        */
        public int PadCount(
			byte[] input)
        {
            int count = (int) input[input.Length - 1];

			if (count < 1 || count > input.Length)
            {
                throw new InvalidCipherTextException("pad block corrupted");
            }

			for (int i = 1; i <= count; i++)
            {
                if (input[input.Length - i] != count)
                {
                    throw new InvalidCipherTextException("pad block corrupted");
                }
            }

            return count;
        }
    }

}
