Translating "computationally secure" directly into german is not easily possible, because it seems to be lacking a word, that something is possible, but not feasible.
However, you could use "secure against all polynomially bounded adversaries" as an equivalent intermediate step for "computationally secure" and thereby construct the phrase
sicher gegen (alle) effiziente(n) Angriffe
where "effizient" is equivalent, by convention, to "polynomially bounded" and "Angriffe" is used to catch the more (in this case) algorithmically used "adversaries".
As your comments suggest you may be looking for something that says, that a substitution cipher is secure against brute-force, then you need a different qualifier for "Angriffe", such as "naive" or "trivial offensichtlichen", resulting in the phrase:
sicher gegen (alle) naive(n) Angriffe
sicher gegen (alle) trivial offensichtliche(n) Angriffe
with the latter having a rather neutral connotation and not excluding anything that would need more than five seconds thought.
Note further, that using the term "computationally secure" paired with "subsitution cipher" is not a good practice, because those ciphers are by themself very often not computationally secure in the standard definition.