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.
Dynamic/Personalised Subject Line:
In Subject Line section of email:
%%=v(@Subject)=%%
In Email Body (either in HTML Template or Content Area):
%%[
VAR @Subject,@Date,@Name
SET @Date = FORMAT(DateAdd(Now(), "X","D"), "MMMM dd, yyyy")
SET @Name = Lookup("Customers","Name","SubscriberKey",_subscriberkey)
SET @Subject = Concat('Hi ',@Name,',The date is: ',@Date)
]%%
The subject line would be:
Hi JOHN, The date is: Saturday, April 1, 2017
Best Answer
Its not too hard, so you're in luck!
If you want to have a dynamic subject line:
At the top of your email, make an AMPscript block, like %%[ ]%%. Declare a variable, like VAR @subject
Within that block, do all your logic that will set this dynamic variable.
Make the subject of your email %%=v(@subject)=%% . This will populate at send time.
If you want to store your subject lines in a Data Extension and retrieve them dynamically:
Create your Data Extension to lookup the Subject Line from, it should contain Segment as the key. I named my DE "Subjectline Table":
In your email content, set your @Subject variable using the LookUp() function.
In your Email Properties, set the subject line to be
%%=v(@Subject)=%%Hope this helps, good luck!