Siccome il cifrario è difficile, lo decifriamo insieme.

Here, siccome means since / because.

It introduces the reason:

• Siccome il cifrario è difficile = Since the cipher is difficult
• lo decifriamo insieme = we decipher it together

Compared with perché:

• siccome is very often placed at the beginning of the sentence
• perché is more flexible and is especially common after the main clause

So both of these are natural:

• Siccome il cifrario è difficile, lo decifriamo insieme.
• Lo decifriamo insieme perché il cifrario è difficile.

A small nuance: siccome often sounds like the reason is already known or being presented as background.

Because siccome very commonly introduces a reason clause at the beginning of the sentence.

So this structure is very natural:

• Siccome + reason, main clause

That gives:

• Siccome il cifrario è difficile, lo decifriamo insieme.

If you want to put the reason after the main clause, Italian more often uses perché, dato che, or poiché rather than siccome.

In Italian, a singular countable noun usually needs an article.

So:

• il cifrario = the cipher
• bare cifrario by itself would usually sound incomplete or unnatural here

English often allows zero article more easily than Italian, but Italian usually does not.

The accent distinguishes è = is from e = and.

So:

• è = third-person singular of essere = is
• e = and

This accent is very important in writing.

Lo is a direct object pronoun meaning it.

It refers back to il cifrario.

So instead of repeating the noun:

• Siccome il cifrario è difficile, decifriamo il cifrario insieme.

Italian uses the pronoun:

• Siccome il cifrario è difficile, lo decifriamo insieme.

Because cifrario is masculine singular, the pronoun is lo.

Because this is a normal statement, and in normal statements object pronouns usually go before the conjugated verb:

• lo decifriamo = we decipher it

Attached pronouns like decifriamolo are used in other structures, especially:

• infinitives: decifrarlo
• positive imperatives: decifralo!
• some exhortative forms: decifriamolo! = let’s decipher it!

So:

• lo decifriamo insieme = statement: we decipher it together
• decifriamolo insieme! = exhortation: let’s decipher it together!

Decifriamo is the first-person plural present indicative of decifrare.

So it means:

• we decipher
• we are deciphering

In context, it can also sound like a present decision or plan:

• Since the cipher is difficult, we’ll decipher it together

Italian often uses the present tense where English might use either present or near-future wording.

Because Italian usually drops subject pronouns when the verb ending already makes the subject clear.

Here, decifriamo already tells you the subject is we.

So:

• (Noi) lo decifriamo insieme

Both are grammatically possible, but noi is often omitted unless you want emphasis or contrast.

For example:

• Noi lo decifriamo insieme, loro no.
  = We decode it together, they don’t.

Insieme means together.

It adds the idea of doing the action collaboratively, not just that the subject is we.

Compare:

• Lo decifriamo. = We decipher it.
• Lo decifriamo insieme. = We decipher it together.

So insieme is not redundant. It emphasizes cooperation.

That is a very natural position for it.

Adverbs like insieme often come after the verb phrase:

• lo decifriamo insieme

That said, word order can move a bit for emphasis:

• Insieme lo decifriamo
• Lo decifriamo insieme

The version in your sentence is the most neutral and natural.

Because the sentence begins with a subordinate clause introduced by siccome, and it is very common to separate that opening clause from the main clause with a comma.

So the structure is:

• Siccome il cifrario è difficile, = dependent clause
• lo decifriamo insieme. = main clause

This comma helps readability and makes the sentence structure clear.

By itself, the form decifriamo can sometimes look like either:

• indicative we decipher / we are deciphering
• exhortative let’s decipher

But context usually makes it clear.

In your sentence, it is understood as a statement, not a command or suggestion:

• Since the cipher is difficult, we decipher it together

If you wanted a clear exhortation, you would more naturally write:

• Siccome il cifrario è difficile, decifriamolo insieme!
  = Since the cipher is difficult, let’s decipher it together!

A rough pronunciation guide is:

seek-KOH-meh eel chee-FRAH-ryoh eh dee-FEE-chee-leh, loh deh-chee-FRYAH-moh een-SYEH-meh

A few useful sound notes:

• ci in cifrario sounds like chee
• è is an open eh
• gli does not appear here, so there is nothing especially tricky beyond the normal Italian vowels
• stress falls roughly on:
  • siccóme
  • cifràrio
  • difficìle
  • decifriàmo
  • insième

If you want, I can also break the whole sentence down word by word with grammar labels.