[SalesForce] SFMC Ampscript subject line in automation from content builder

There are several ways to do this, but most of them will still charge you for the send once email generation is underway. The BEST way is you need to contact support and/or your account representative and get Audience Exclusion scripts enabled under your account (not standard). This will show up when you create a User Initiated Send. A new text area will show up at the bottom, where you can write a one line ampscript expression that evaluates to boolean. If its true, the email doesn't go out. Otherwise, it does.

I would start with a simple use-case to make sure the decryption is the true issue. Something like this perhaps:

%%[

var @first_name_enc, @first_name_plain, @first_name_dec, @null
set @first_name_plain = "John"
/* or if the first name is a personalization string in the sending audience */
/* set @first_name_plain = [first_name] */
set @password = "A4DF6AF2"
set @first_name_enc = EncryptSymmetric(@first_name_plain, "des;mode=cbc", @null, @password, @null, @null, @null, @null)

]%%
<br>@first_name_plain: %%=v(@first_name_plain)=%%
<br>@first_name_enc: %%=v(@first_name_enc)=%%
%%[

set @first_name_dec = DecryptSymmetric(@first_name_enc, "des;mode=cbc", @null, @password, @null, @null, @null, @null)

]%%
<br>@first_name_dec: %%=v(@first_name_dec)=%%

Results:

@first_name_plain: John 
@first_name_enc: Du5zgS1AF3w= 
@first_name_dec: John 

You can un-comment the second @first_name_plain declaration if the first name is a personalization string and not a variable set somewhere else in the AMPScript block.