Gets the IV from Salesforce encryptWithManagedIV

RetrieveIv.cs

        /// <summary>
        /// Returns the IV needed for decryption. 16 Bytes
        /// </summary>
        /// <param name="base64_cipher_text"></param>
        /// <returns></returns>
        public static byte[] RetrieveIv(string base64_cipher_text)
        {
            // Just 16 the first bytes
            // 16 not divisable by 3 (4 characters per 3 bytes)
            // 4 -> 3
            //   ?? 18
            // (4 * 18) / 3 = 24
            // This will be 18 bytes long
            base64_cipher_text = base64_cipher_text.Substring(0, 24);

            // 18 bytes long 
            byte[] cipher_binary = Convert.FromBase64String(base64_cipher_text);
            byte[] iv = new byte[16];
            // Take first 16 only and discard the left
            Array.Copy(cipher_binary, 0, iv, 0, 16);
            return iv;
        }