Base class for asymmetric cipher implementation
        /// <summary>
        /// Initializes a new instance of the <see cref="CipherDigitalSignature"/> class.
        /// </summary>
        /// <param name="oid">The object identifier.</param>
        /// <param name="cipher">The cipher.</param>
        protected CipherDigitalSignature(ObjectIdentifier oid, AsymmetricCipher cipher)
        {
            if (cipher == null)
                throw new ArgumentNullException("cipher");

            _cipher = cipher;
            _oid = oid;
        }
        /// <summary>
        /// Initializes a new instance of the <see cref="CipherDigitalSignature"/> class.
        /// </summary>
        /// <param name="oid">The object identifier.</param>
        /// <param name="cipher">The cipher.</param>
        protected CipherDigitalSignature(ObjectIdentifier oid, AsymmetricCipher cipher)
        {
            if (cipher == null)
            {
                throw new ArgumentNullException("cipher");
            }

            _cipher = cipher;
            _oid    = oid;
        }