It's called a Vigenere cipher, and this variant is an especially weak variant because the length of the key is known to be 13 characters. It is somewhat more secure if the key length isn't known to the attacker.

If the key length is variable, for long ciphertexts, there are algorithms that can quickly yield the plaintext. For very short cipertexts, it's a lot more secure than you might think. If the key is as long as the message, it's basically a one-time pad.