in the following case, if privateKeyOfA is leaked, what's the security problem? Can someone decrypt the message without privateKeyOfB??
Aes.encrypt(privateKeyOfA, publicKeyOfB, message)
Aes.decrypt(publicKeyOfA, privateKeyOfB)
If not, I guess why we need privateKeyOfA here is for A's signature?
The signature is needed since the receiver must know that the message is coming from someone that he can identify. If he cannot verify the signature, this means that he doesn't know the person.
If the private key of A is compromised by a hacker, he can send messages to everybody with signature impersonating the A.
A key exchange (e.g. using DH or ECDH) would be used by A to convert privateKeyOfA + publicKeyOfB into an AES key. This same AES key can also be generated identically by B using privateKeyOfB + publicKeyOfA. All traffic between A and B would be encrypted using the same AES key.
Assuming that all public keys are known (they are public after all), then anyone who has access to privateKeyOfA can regenerate all AES keys that were generated by A to communicate with anyone. This means all traffic involving this key (messages sent or received by A, with B or anyone else) would be compromised.
But if an ephemeral version was used (like in some modes of TLS), then a new key is generated for each session, so that if 1 key is ever compromised, only this session is compromised. You can read more about forward secrecy.
If the keys are used in the way you describe, then they are not used for signature.
Related
Suppose I have a fixed message pool of 1000 messages, person A is sending message from this fixed message pool to person B using RSA.
If an interceptor also have the message pool he can precompute all the encrypted messages using B's public key. Now if he intercept A's message can he surely tell which message A has sent to B?
In this case should we use RSA only for a symmetric key exchange and then messages should be encrypted using a symmetric algorithm?
The text-book RSA encryption algorithm is deterministic. But the official RSA specifications (and also all implementations used in practice) include some (partly random) padding, so we don't actually encrypt plaintext, but pad(plaintext). So the above mentioned problem will not occur.
More details can be found in this answer https://stackoverflow.com/a/7933071/10690480
I've been experimenting with RSA encryption in python (cryptography.hazmat.primitives.asymmetric). I have the following setup: On one end is the client with the public key sending encrypted data back to the server, which holds the private key. Right now I've got one-directional encryption working, but I'm wondering how you would (or if you should) securely decrypt a message client-side. I thought about just encrypting the private key and storing it, but then the password would appear in the code and expose the key to compromise. Is there a way to securely implement this with a key exchange? Or--the most likely alternative--is this a misuse of the protocol?
EDIT: Wanted to clarify that the possible concerns here would be that using RSA in this way might expose the private key on the file system or between the server and the client.
The normal way is for the server to encrypt the reply with the client's public key and client decrypt with its private key. This requires TWO RSA keypairs -- one for the client and one for the server, and requires each end to know the other's public key.
This need (along with high cost of PKE compared to symmetric encryption) is why PKE is normally only used for authentication and/or key exchange, and a symmetric cipher is used to actually encrypt traffic.
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Why don't client and server just exchange the encryption keys directly using public key encryption or DH key exchange protocol? What the rationale behind that or what the problem it is to solve?
Its helpful to understand how keys are derived in modern SSL/TLS. Things were a bit different in early SSL (like SSLv2).
The master_secret is a common secret shared by the client and server. It is used to derive session specific keys. The master_secret derived form other parameters (discussed below).
There are 6 each secrets derived from the master_secret:
Client encryption key
Server encryption key
Client MAC key
Server MAC key
Client IV
Server IV
Assuming that neither eNULL nor aNULL is used, both the client and server use an encryption key for confidentiality and a HMAC key for authenticity. Each (client and server) has its own key.
While IVs are usually considered public, SSL/TLS treats them as secret parameters.
From RFC 5246, the master_secret is derived as:
master_secret = PRF(pre_master_secret, "master secret",
ClientHello.random + ServerHello.random)
[0..47];
The pre_master_secret comes from key agreement or key transport. If it comes from key agreement, then the pre_master_secret is the result of Diffie-Hellman key agreement. In an agreement scheme, both parties contribute to the derived secret.
If the pre_master_secret comes from a key transport scheme, then the client encrypts a random value under the server's public key. In this scheme, only the client provides keying material. When only one party provides the key, its called a key transport scheme.
What the rationale behind that or what the problem it is to solve?
The first stage, where the pre_master_secret is used, provides a "pluggable" architecture for key agreement or key transport.
The second stage, where the master_secret is derived, ensures both the client and server contribute to the keying material.
In addition, there's a label - "master secret" - that helps ensure derivation is unique even if the same parameters are used for something else (assuming a different derivation uses a different label). Use of labels are discussed in SP800-56 and SP800-57 (among other places).
The hash used in the second stage, where the master_secret is derived, performs two functions. First, it performs a mixing function. Second, it maps elements in the group used by key exchange or key agreement into random bit patterns.
The final stage is the derivation of the 6 keys from master_secret. According to 6.3. Key Calculation, the derivation does not provide key independence. It just ensures interoperability:
To generate the key material, compute
key_block = PRF(SecurityParameters.master_secret,
"key expansion",
SecurityParameters.server_random +
SecurityParameters.client_random);
until enough output has been generated. Then, the key_block is
partitioned as follows:
client_write_MAC_key[SecurityParameters.mac_key_length]
server_write_MAC_key[SecurityParameters.mac_key_length]
client_write_key[SecurityParameters.enc_key_length]
server_write_key[SecurityParameters.enc_key_length]
client_write_IV[SecurityParameters.fixed_iv_length]
server_write_IV[SecurityParameters.fixed_iv_length]
The steps above are a solid design. However, when used in SSL/TLS, there are lots of devils running around. For example, the above is not enough when a feature like renegotiation is added (triple handshake attack ftw!).
I believe the reason is that if the client simply selected a random number to use as the symmetric key and encrypted it using the server's public key to send to the server, there would potentially be a vulnerability if common clients used an imperfect random number generator, leading to predictable symmetric keys and making the communications much easier to break.
The actual key exchange protocol ensures that the symmetric key contains randomized elements from both the client and the server. This means that even if the client has an imperfect random number generator, the communications are still protected if the server's random number generator is cryptographically strong. Even if both the client's and the server's random number generators have weaknesses, the attack against the combination of the two is likely to be more expensive than if only the client's random number generator were used.
The rationale is that if the secret key is never exchanged it can never be detected. Key negotation algiorithms are known to be secure. An encryption is only as secure as its key.
pre master key to master key:
one side random is not really random, but 2 side 3 times random number could be really random..
master key to 6 key pairs:
2 for encryption, 2 for message integration check, and 2 for preventing CBC attack
I have a Software that Encrypts message using AES , the random generated AES key is Encrypted by the receiver's public RSA key. now when I send the message to multiple users...
Sender Side :
Message is Encrypted by Random hashed (sha256) AES KEY
The AES key is then Encrypted many time and appended to the encrypted message using each receiver's public key.
the message has [ number for receivers, [list of encrypted keys], Encrypted message]
Receiver Side:
get the number of receivers
loop thru the appended encrypted keys and decrypt using your Private RSA. until you find the one intended for you. such that when he/she decrypt the key they get the AES Key.
3.decrypt the message using AES key.
Knowing that the key is of base 64 string which means it ends with '=', and of the length 256 because of the sha
the Question IS :
How Do i know (if I'm the receiver) that the Decrypted key using my Private RSA is correct Automatically ?
thank you in advance.
Two questions: Is the protocol you describe fixed, or might it be modified in any way? If it is fixed, which padding scheme do you use for RSA? PKCS#1 v1.5, OAEP or none at all?
If the protocol might be modified, you could use a cipher mode with authentication, such as EAX, CCM or GCM. If RSA key transport decryption fails silently, so will the authenticated AES decryption.
Use a variation of RSA-OAEP for the key transport that provides "plain text awareness" as described here: http://www.rsa.com/rsalabs/node.asp?id=2346.
There is no way to find this encrypted message belongs to which receiver.
But you can do is try to decrypt the message if the decrypt is successful then that is the Receiver
I'm wondering why ssl encrypted data can't be cracked easily once the packets are intercepted. As i understand it when you connect to a site like facebook the browser and site agree on a cipher, what stops the sniffer from seeing what cipher they agreed to?
SSL uses asymmetric encryption, meaning the decryption key is different than the encryption key. So if you as a client encrypt your packets with the server's public key, it can only be decrypted by the private key, which remains on the server. Of course, this is a simplification of everything that happens in an SSL transaction, but that's the basis of the concept.
Imagine sending a box with an open padlock to the other side - when the other side wants to send a message, they put it inside the box, lock the padlock and send it back to you, where you use your (private) key to unlock it. Even if the intercepting party has sees the padlock, they still don't have the key.
There's a lot of ways to describe it. For me, my ah-hah moment was when I figured out that, after information is encrypted multiple times, it can be decrypted in any order.
A encrypts first, passes to B a single encrypted message [A encryption].
B encrypts the message a second time, and passes to A a double encrypted message [A encryption and B encryption]
A removes [A encryption] from the message, leaving only [B encryption], and sends the message to B.
B now has a [B encrypted] message, and knows how to decrypt it.
The sniffer sees the message encrypted three different ways: [A], [AB], and [B].
That's three message passes to exchange one message, but once it's passed and both sides have the unique information to decrypt further communication, future messages only need one trip.
If you want a simple example of how a message could be decrypted in any order, you can use XOR as a sample encryption method. For keys A and B, message M, and XOR is ^, then
M ^ A ^ A = M
M ^ A ^ B ^ A ^ B = M
Facebook signs it's package with a certificate that Facebook got from an certificate authority such as RapidSLL.
As long as you trust the certificate authorities that all certificate they issue for facebook.com do really belong to facebook.com the connection is safe.
Facebook then sends you via a signed message it's public encyrption key which you can use to encrypt your messages to be read by facebook.
Yes, the cipher is public. However, the client asymmetrically encrypts a random session key (or rather a precursor) using Facebook's public key (they verify it's really Facebook's key by checking that it is signed by someone trusted). So only Facebook (who has a private key) should be able to derive the actual symmetric keys that are used to exchange website data.
See this detailed walk-through. In that example, an eavesdropper can tell that the connection uses RSA, RC4, and MD5. But they don't have Amazon's private key, so they can't derive the session keys.
Like Derek H said, there are fundamental differences between symmetric and asymmetric encryption. Look up stuff like DH key exchange protocol and RSA cipher, they are fundamental in SSL/TLS. On the other hand, it's relatively easy to decrypt sniffed data (ROBOT attack).
If you just need to be sure your communication is secure, you can simply use SSL/TLS Server Test, there you can see if you're not using recommended algorithms or see if your SSL/TLS configuration is PCI-DSS/HIPAA/NIST compliant.