Score:2

Crypto

Digital signature with Big Brother

User4567

1/27/23, 4:51 PM

I have read digital signature with Big Brother but don't understand the sequence.

One approach to digital signatures is to have a central authority that knows everything and whom everyone trusts, say Big Brother $(BB).$Each user then chooses a secret key and carries it by hand to $BB$'s office. Thus, only Alice and $BB$ know Alice's secret key, $K_A$, and so on.

When Alice wants to send a signed plaintext message, $P$, to her banker, Bob, she generates $K_A(B, R_A, t, P)$, where $B$ is Bob's identity, $R_A$ is a random number chosen by Alice, $t$ is a timestamp to ensure freshness, and $K_A(B, R_A, t, P)$ is the message encrypted with her key, $K_A.$

After then I don't understand how the following sequence works because there is nothing explained. Can anyone please explain in detail?

209

0 + 0

rsa

encryption

authentication

diffie-hellman

schroeder

1/27/23, 5:20 PM

When quoting from a source, please cite the source.

Score:1

Crypto

Riccardo

1/27/23, 6:48 PM

Alice - knowing $K_A$ - sends $K_A(B, R_A, t, P)$ to $BB$.

$BB$ - knowing $K_A$ - decrypts $K_A(B, R_A, t, P)$, obtaining $(B, R_A, t, P)$.

$BB$ - knowing $K_B$ and $K_{BB}$ - sends $K_B(A, R_A, t, P, K_{BB}(A, t, P))$ to Bob.

Bob - knowing $K_B$ - decrypts $K_B(A, R_A, t, P, K_{BB}(A, t, P))$, obtaining $(A, R_A, t, P, K_{BB}(A, t, P))$.

At this point you can go on reading.

0 + 0

Alok Maity

1/27/23, 6:57 PM

could you explain little, I don't understand what is happening here..

Riccardo

1/27/23, 7:00 PM

Maybe you don't understand the notation: $K_A(M)$ means encrypting the message $M$ using the key $K_A$. I usually write it as $Enc(M,K_A)$.

Alok Maity

1/27/23, 7:39 PM

all things are understood. But in last paragraph "Trudy replaying either message "-- what does mean by replaying message?

Riccardo

1/28/23, 6:47 AM

It means trying to impersonate Alice by sending to Bob copies of messages sent by Alice to Bob.

Alok Maity

1/28/23, 8:32 AM

@Riccardo here middle in attack is impossible because everything is encrypted?

Riccardo

1/28/23, 8:39 AM

A man-in-the-middle can only forward messages that he intercepts, but he can't impersonate $BB$ since he doesn't know others' private keys.

Alok Maity

1/28/23, 12:54 PM

@Riccardo but how Alice got fooled?

Riccardo

1/28/23, 2:24 PM

If Bob does not check for freshness of a message, he could be tricked to repeat an action that he has already taken. This is why $t$ and $R_A$ are used.

Alok Maity

1/28/23, 4:40 PM

@Riccardo I want to how A got attacked by any attacker?

Riccardo

1/28/23, 4:58 PM

The only one that can attack $A$ is $BB$, but we are assuming that $BB$ is honest.

Alok Maity

1/28/23, 5:19 PM

@Riccardo when $A$ is attack by $BB$, we can't caught $BB$, inspite of using timestamp. Am I right?

Riccardo

1/28/23, 5:34 PM

Yes, this is the problem with a central authority: you have to blindly trust it.

Alok Maity

1/28/23, 5:38 PM

@Riccardo So, my question is Alice has not attacking chance except by BB, when A is attack by BB timestamp is useless, why we use timestamp?

Riccardo

1/28/23, 5:54 PM

As I said before, the timestamp is used to prove the freshness of the message to Bob.

Alok Maity

1/28/23, 6:02 PM

@Riccardo "freshness of the message" means message without repeating. Alice doesn't sent any repeating message. Trudy who may be the attacker send the replay message. My question is how Trudy may be pretend as Alice? . So Alice is fooled by Trudy. Am I right?

Riccardo

1/28/23, 7:24 PM

There is nothing in the system that identifies the participants, so Trudy can simply say to Bob: "Hi, I am $BB$ and this is a message from Alice". If Bob does not check the message freshness, he could be fooled.

Alok Maity

1/29/23, 4:50 AM

@Riccardo how Trudy can get message which is repeating message sent by Alice?

Riccardo

1/29/23, 6:30 AM

We always assume that an attacker can intercept messages, since the communication channel is insecure.

Alok Maity

1/29/23, 8:16 AM

@Riccardo intercepter could be exist between Alice and BB or BB and Bob or BB could be treated as interceptor? Am I right?

Riccardo

1/29/23, 9:19 AM

Yes, perfectly right.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Digital signature with Big Brother

TH: ลายเซ็นดิจิทัลกับพี่ใหญ่

RO: Semnătură digitală cu Big Brother

RU: Цифровая подпись с Большим Братом

VI: Chữ ký số với Big Brother

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