Мы расшифровали шифр.

Sometimes yes, but in this sentence мы is useful.

In the Russian past tense, the verb shows gender/number, but not person. So расшифровали by itself just means that more than one person did the action. Depending on context, it could mean:

• we decoded
• you decoded (plural or formal)
• they decoded

So adding мы makes it clear that the subject is we.

Расшифровали is:

• past tense
• plural
• from the verb расшифровать
• perfective aspect

So it means the action was viewed as completed: we decoded / we deciphered.

The ending -ли is the normal past plural ending in Russian.

Because the subject is мы = we.

In the Russian past tense:

• masculine singular: расшифровал
• feminine singular: расшифровала
• neuter singular: расшифровало
• plural: расшифровали

Since мы refers to more than one person, Russian uses the plural past form.

It is in the accusative, but for many masculine inanimate nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: шифр
• accusative: шифр

Because шифр is:

• masculine
• singular
• inanimate

the form stays the same.

Russian has no articles like a or the.

So шифр can mean:

• a cipher
• the cipher

You understand which one is meant from context.

That is very normal in Russian.

The verb расшифровать is built from шифр (cipher/code) and means to decipher / decode.

The prefix рас- often gives a sense of unpacking, breaking apart, or undoing something hidden or compressed. In this verb, it suggests making the cipher understandable, or turning coded text into plain meaning.

So расшифровать is a standard verb meaning to decode / decipher.

Because the sentence presents the action as finished.

• расшифровали = we decoded it / we managed to decode it
• imperfective расшифровывали would mean we were decoding it, we used to decode it, or would focus on the process rather than the result

So if the point is that the cipher has now been successfully decoded, perfective is the natural choice.

The usual imperfective verb is расшифровывать.

Compare:

• Мы расшифровали шифр.
  We decoded the cipher.
  completed result

• Мы расшифровывали шифр весь вечер.
  We were decoding the cipher all evening.
  process/duration

This perfective–imperfective pairing is very important in Russian.

Yes. Russian word order is more flexible than English because case endings show grammatical roles.

The neutral order here is:

• Мы расшифровали шифр.

But you could also say:

• Шифр мы расшифровали.
  Focus on the cipher
• Расшифровали мы шифр.
  More marked; emphasis depends on context

Even when the order changes, the basic meaning stays the same, though the emphasis changes.

The stress is:

• расшифровАли
• шИфр

So the full sentence is pronounced approximately:

• мы рас-шиф-ро-ВА-ли ШИФР

A few helpful notes:

• мы is short and usually unstressed
• the stress in расшифровали is on ВА
• шифр is one syllable, so the stress is naturally there

Yes, it can be omitted in context:

• Расшифровали шифр.

But on its own, that sounds less specific, because the verb form does not tell you whether it was we, you all, or they. So in an isolated sentence, keeping мы is usually clearer.

Not exactly.

• шифр usually means a cipher or encrypted system/message
• код usually means a code

In real usage, there can be overlap, but расшифровать шифр specifically sounds like deciphering/decrypting a cipher, which is a very natural phrase.

Because расшифровать is perfective, it does not have a true present tense.

Its forms are:

• past: мы расшифровали = we decoded
• future: мы расшифруем = we will decode

If you want a real present tense meaning like we are decoding, you use the imperfective verb:

• мы расшифровываем = we are decoding / we decode

So aspect determines which tense forms are possible.