Multiple encryption of part of block Is there any security risk in encrypting parts of blocks twice? I'd like to get around block sizes and make encrypted messages just as long as the unencrypted ones:


Block 1: Byte 0 - 7
Block 2: Byte 8 - 15
Block 3: Byte 16 - 23
Block 4: Byte 21 - 29


In this example blocks 3 and 4 overlap by 2 bytes. I plan on using CAST-128.

Thank you very much for helping!

Florian
This technique is known as Ciphertext stealing. Ciphertext stealing avoids padding, but only works if the total message size is bigger than one block.

Ciphertext stealing secure in principle, but as @mikeazo already pointed out using ECB and using 64 bit block ciphers is generally a bad idea.
There are fancier length preserving encryption schemes. FFX mode is one example.
I strongly recommend adding a nonce/IV and a MAC. While these increase the size of the message, the security implications of not having them can be severe.

In particular the MAC prevents attacks that modify the ciphertext and watch the reaction of the decrypted and nonces/IVs ensure that encrypting the message twice doesn't leak that it's the same.