I created an symmetric encryption to add a layer of security or obfuscation to my projects.
Can you help me and find out if it`s easily broken, even when the key is secret?

You can take a look at the algorithm, also as highlighted version.

To prove that it`s easily broken, you have to decipher the following message.
The whole plaintext is readable and in the current language.
There are no line breaks, but correct punctuation.
There is one word that does not exist: your random solution, which is 12 random chars in uppercase HEX.

64 74 7A 59 6B 3A 36 5B 7E 37 72 49 73 69 65 2A
6D 45 5E 73 37 55 6B 7F 0C 77 73 49 36 4D 70 40
6F 0C 63 43 21 5E 74 77 51 24 62 7E 7B 79 37 75
49 6E 70 38 2A 55 44 5C 65 37 45 77 2A 4D 7B 60
56 7F 42 75 0D 2A 65 63 0C 76 43 64 7A 51 24 74
73 32 69 78 6E 40 36 6D 70 2A 78 43 40 36 74 4D
6A 2A 40 73 60 5A 73 0C 73 0C 67 49 64 5F 60 4B
74 36 5C 6A 36 62 7A 6F 37 72 43 7A 71 62 63 6E
42 15 70 78 5E 71 67 5F 3A 21 58 73 40 7E 45 64
4B 37 59 72 0C 79 79 42 24 6F 79 67 2A 64 6E 40
60 61 72 2A 68 58 1B 36 58 44 28 2A 65 36 60 40
7B 43 61 58 2A 4A 78 5E 66 43 65 36 41 6B 36 62
77 66 7B 21 55 79 71 36 7E 69 49 15 66 76 5F 77
7D 43 64 65 16 36 1F 51 19 3B 69 24 1E 38 18 25
50 73 2A 36 5F 7C 2A 74 60 5F 73 24 6F 65 74 0C
54 64 72 0C 77 7E 45 7A 6D 0C 62 5E 6B 45 64 4B
37 4D 6F 48 31 7E 54 72 73 36 7C 65 37 6D 59 75
6F 3A 2A 78 43 40 36 7A 4D 7D 68 49 36 6F 49 73
48 32 5F 65 41 72 0C 6C 43 63 73 15 67 7F 66 7A
6F 65 75 49 6E 70 38 2A 55 44 5C 65 37 45 77 2A
58 7E 64 0C 73 42 76 0C 65 4A 37 58 69 49 31 7B
50 77 65 77 75 6F 39 

You can also download the ciphertext as a file.

Additional hint: The secret key is 11 chars long, and contains only a-zA-Z.
Thanks go out to Edge and Stack for their feedback and testing the challenge.