If a secret is created with letters within a word the decryption fails because it can't find the end of the block

example:

test
encrypt
es
generating
tsecretlens:7ba21ffa39c96e7c31443741c9418550c2bb5ef12900bfaee48c7404e7499cddt

Should I add a ending string to identify the end?
Same as startsWith?

Then the block would be

tsecretlens:7ba21ffa39c96e7c31443741c9418550c2bb5ef12900bfaee48c7404e7499cdd:secretlenst

Wich would be easier to find