Most symmetric-key algorithms currently in use are block ciphers: this means that they encrypt data a block at a time. The size of each block is fixed and determined by the algorithm: for example AES uses 16-byte blocks. Block ciphers are always used with a mode , which specifies how to securely encrypt messages that are longer than the block size. For example, AES is a cipher, while CTR, CBC, and GCM are all modes. Using an inappropriate mode, or using a mode incorrectly, can completely undermine the security provided by the underlying cipher.