Did Leia know what it was she was carrying?

Question

At the end of Rogue One, we see that

Jyn Erso is able to successfully transmit the plans and they end up in the possession of Princess Leia.

When asked by one of her crew members about the data card she's holding, her reply is ambiguous:

Crew Member: Your Highness, the transmission we received. (hands her the card) What is it they've sent us?
Leia: Hope.

Similarly, at the beginning of A New Hope, in her message to Obi-Wan Kenobi, her message is similarly ambiguous:

Leia: I have placed information vital to the survival of the Rebellion into the memory systems of this R2 unit.

So, at this time, did Leia know that she was carrying the Death Star plans and not some other intelligence?

She definitely knows by the end of A New Hope, but did she know beforehand and just not tell?

Answer 1

Sort of

We've known since 1977 that Leia knew she was carrying plans for something; if she didn't know before the film began, she certainly knew by the time she first runs into Vader:

Vader: Several transmissions were beamed to this ship by rebel spies. I want to know what happened to the plans they sent you.

Leia: I don't know what you're talking about. I'm a member of the Imperial Senate on a diplomatic mission to Alderaan.

^{A New Hope (1977)}

But a more specific answer comes from the new (2015) junior novelization of A New Hope; Leia knew she was carrying the Death Star plans, and is aware of rumours about a battle station, but doesn't connect the Death Star with that station:

All they needed to do was intercept a transmission about a new battle station the Empire was rumored to be building.

But the system had been swarming with Imperials. They'd caught on to Leia's bogus tale about her ship's breaking down, but the cover had helped her stall long enough to download the technical plans for something code-named "Death Star."

^{The Princess, the Scoundrel, and the Farm Boy Chapter 1}

It's unclear whether or not Leia was aware of the "Planet-Killer" weapon that the Rogue One rebels keep banging on about, or whether she would have connected those rumours to the more general "battle station" rumours. Regardless, she eventually pieces it all together when she's first brought onto the Death Star:

"Death Star..." The words came out in a horrified whisper.

A second too late, Leia realized her grave mistake.

Vader turned on his heel. "Yes, the Death Star. The secret weapon you claim to know nothing about. The plans you claim you do not possess.

^{The Princess, the Scoundrel, and the Farm Boy Chapter 4}

Answer 2

Leia knew what she had.

From the script of Episode IV, after our heroes find Leia and escape the Death Star:

LEIA - At least the information in Artoo is still intact.

HAN - What's so important? What's he carrying?

LEIA - The technical readouts of that battle station. I only hope that when the data is analyzed, a weakness can be found. It's not over yet!

Answer 3

It's worth noting that Leia is, at the very least, on the command cruiser when the Death Star fires at the planet below in Rogue One (we don't know if she watched it happen). It's also unlikely that Bail Organa would not have explained what she was doing there in the first place. He directly states he would trust Leia with his life, so that was be a contrary action to that statement.

Answer 4

From the onset it is clear what the"rebellious rebellion faction" aka the group around Jin was after. The weakness that would let them use the reactor weakness her father had built into the deathstar.

It is clear thus that nobody would expect the transmission to have something else than either the "weakness" they are searching for or plans where the weakness could be found on (if it exists at all).

Answer 5

From the canon book, "From a Certain Point of View", from the short story, Raymus, it is stated that Leia most likely knows that she holds the Death Star plans when it was transmitted from Scarif; however, she doesn't say anything to her crew or the ship's captain to protect them and herself:

p 3 -4

"What is it they've sent us?"

Captain Raymus Antilles watched as Princess Leia Organa of Alderaan turned away from him, holding the data card he had handed to her....

"Hope," she replied as she looked ahead, through the forward viewport of the Tantive IV's cockpit, to the limitless ocean of stars beyond.

Always so damn enigmatic, thought Raymus. Leia never told you more than you needed to know. That was for the protection of others as much as her own. She had learned that lesson well, the princess had become a galactic senator, the senator who had secretly risked her life countless times to help nurture a fledging Rebellion from a handful of squabbling, disgruntled star systems into the organized and dedicated Alliance it had become..."

p 7

All eyes were on Leia as the full gravity of their situation began to sink in. Raymus had seen this look on her face before; she was concerned, worried even, but it showed in a way that only those few who knew her best, had served with her longest, could detect. To all else she projected only firm resolve in the face of crushing adversity..."

This could also explain why in her message to Kenobi, Leia only described the plans as "information vital to the survival of the Rebellion" (probably to protect R2-D2 if any other unfriendly eyes saw the message).