angou wo tukatte messeezi wo okutta.

Because there are effectively two verb units:

• 暗号を使って = using a code / cipher
• メッセージを送った = sent a message

Each を marks the direct object of its own verb:

• 暗号を goes with 使って because 使う means to use
• メッセージを goes with 送った because 送る means to send

So the structure is:

• 暗号を使って = using a code
• メッセージを送った = sent a message

This is completely normal in Japanese.

使って is the て-form of 使う (to use).

Here, the て-form connects 使う to the main verb 送った. In this sentence, it gives the sense of using X to do Y.

So:

• 暗号を使って = using a code
• メッセージを送った = sent a message

Together:

• 暗号を使ってメッセージを送った。
• (Someone) sent a message using a code.

In this kind of sentence, the て-form often works like and, by doing, or using depending on context.

送った is the plain past form of 送る (to send), so it means sent.

That tells you the action already happened.

• 送る = send / sends / will send
• 送った = sent

So this sentence describes a completed action:

• Sent a message using a code.

Japanese does not always need words like did or already if the past form makes the time clear.

The subject is omitted.

This is very common in Japanese. If the subject is obvious from context, it is often left unstated.

So the sentence could mean:

• I sent a message using a code.
• He sent a message using a code.
• She sent a message using a code.
• They sent a message using a code.

Japanese often leaves this to context instead of explicitly saying I, he, she, etc.

Because the sentence tells you what was sent, but not to whom.

With 送る, a common pattern is:

• Aは Bに Cを送る
• A sends C to B

Example:

• 彼にメッセージを送った。
• I sent a message to him.

In your sentence, the receiver is simply omitted:

• 暗号を使ってメッセージを送った。
• (Someone) sent a message using a code.

So the sentence is complete even without に, because to whom is not necessary here.

Yes, often you could say:

• 暗号でメッセージを送った。

That also means something like sent a message using a code.

The nuance is a little different:

• 暗号を使って explicitly says using a code
• 暗号で marks the means/method more compactly

So:

• 暗号を使ってメッセージを送った。 = slightly more explicit about the act of using
• 暗号でメッセージを送った。 = more compact, focusing on the method

Both are natural in many contexts.

Yes, to some extent.

Because particles show the role of each word, Japanese word order is more flexible than English word order.

For example, these are close in meaning:

• 暗号を使ってメッセージを送った。
• メッセージを暗号を使って送った。

Both mean basically sent a message using a code.

However, the original order sounds very natural because it presents the method first and then the main action.

A good beginner rule is:

• keep the main verb at the end
• rely on particles to understand each noun’s role

Here are the readings:

• 暗号 = あんごう
• を = を (pronounced o)
• 使って = つかって
• メッセージ = めっせーじ
• を = を (pronounced o)
• 送った = おくった

Full reading:

• あんごう を つかって メッセージ を おくった。

Because メッセージ is a loanword, originally from English message.

Katakana is commonly used for:

• foreign loanwords
• some onomatopoeia
• emphasis
• certain technical terms

So メッセージ is written in katakana because it is not a native Japanese word.

You might also see native-Japanese or Sino-Japanese alternatives in other contexts, but メッセージ is very common and natural.

Not exactly.

暗号 usually means:

• code
• cipher
• encryption
• something encoded or cryptographic

A password is more often:

• パスワード

So in this sentence, 暗号 suggests a coded or encrypted method, not just a login password.

A useful way to see it is:

• [tool/method] を使って [object] を [verb]
• use [tool/method] and do [verb] to [object]

Applied here:

• 暗号を使って = using a code
• メッセージを送った = sent a message

So the full pattern is:

• 暗号を使ってメッセージを送った。
• (Someone) sent a message using a code.

This is a very common pattern in Japanese:

• スマホを使って調べた。 = looked it up using a smartphone
• ナイフを使って切った。 = cut it using a knife

So this sentence is a good example of Xを使って YをVた.