Hi
I know a basic rules on https!
I know there is private & public key, and public key is for encryption and private key is for decryption!
Now I have questions:
* - IF I know the public key why I cannot decrypt data, Surely it's related to private key!!!
*- And does https protocol encrypt all data or only data that sent by client??
for emxample, If i go to gmail.com, html codes are encrypted or not?
now if answer is yes(and HTML codes are encrypted) how my browser can decrypt it and others can't??
If no, why we should use it for example for downloading backup of important data?
Okay, a couple of points of confusion here.
First, HTTPS isn't actually encrypted with a public/private key scheme -- technically, "asymmetric encryption." It's instead encrypted using a symmetric encryption -- one of several, actually -- with a session key that's established through an algorithm like Diffie-Hellman key exchange.
The result is that the encryption is carried out through a one-use key that's computed as part of the handshake setting up the SSL connection.
The Wikipedia article on Transport Layer Security (SSL was really a proprietary term from Netscape) is reasonably decent.
If you could get that key, you could indeed decrypt the data, but since the usual key now is 128 bits long, you have roughly 1 chance in 2128 of getting it right -- or, in another way of looking at it, you can expect to take about 2127 (170141183460469231731687303715884105728) tries before you'd find the key.
But second, asymmetric encryption does come in one way, however. When you're establishing an SSL connection, the host provides an X509 certificate to identify itself; that's so someone can't hijack DNS and make themselves appear to be paypal.com instead of Vlad's Cut Rate Hacking. The X509 certificate is signed using a public/private key pair: the signature is hashed using the private side of a trusted providers key -- say VeriSign. They provide the public side, which allows you to confirm that the certificate was indeed encrypted by VeriSign. That confirms the authenticity of the cert.
Public Key encryption systems are based on One Way Functions; functions that are far easier to compute in one direction than in the other. There are two common choices of one-way functions for public-key cryptography systems: Large integer factorization and Discrete Logarithms.
There are no mathematical proofs that large integer factorization doesn't have easy solutions: however, several decades of intense research hasn't found any polynomial-time algorithms. (Not that one would necessarily be fast, just that finding one has been a good long-term goal.) The RSA cryptosystem safety is based on the difficulty of factoring large primes.
There are mathematical proofs that solving discrete logarithms is very difficult. The El Gamal and Diffie-Hellman algorithms rely on discrete logarithms for their safety.
The public key mechanisms are only part of actual deployed solutions though. Public Key systems are usually used for digital signatures and for negotiating a session key that is used with a symmetric cipher. Symmetric ciphers are far faster, much safer to use on plain text with patterns, and are an integral portion of modern communications privacy and integrity.
Now, to directly address your questions :)
IF I know the public key why I cannot decrypt data, Surely it's related to private key!!!
They are related. And you could find one given the other. But the computational complexity of finding one is currently so much worse than generating new public / private key pairs, the key itself should have no value by the time you have cracked it. (Years for 'smaller' keys, probably millennium for 'larger' keys. Trouble is, the definition moves around. :)
And does https protocol encrypt all data or only data that sent by client?? for emxample, If i go to gmail.com, html codes are encrypted or not?
HTTPS itself encrypts everything in both directions. HOWEVER, some web sites will use unencrypted http for images, css, javascript, and https for the HTML that actually contains the user data. This is because serving unencrypted content is much faster than serving encrypted content. It is also very unsafe, because most of those types of contents can be replaced while in-flight, allowing intruders to modify the browser's DOM or inject other new code, that lets them get access to the private data. Most browsers complain about mixed SSL/TLS and unencrypted content, so hopefully not many sites do this.
how my browser can decrypt it and others can't?
During the SSL/TLS handshake at the start of the session, the server and browser negotiate a new session key that will be used for the session. All the traffic between browser and client are encrypted with the session key, and as a result of the way the SSL/TLS session is created, only the client and server know the key:
https://www.rfc-editor.org/rfc/rfc5246#page-64
8.1.1. RSA
When RSA is used for server authentication and key exchange, a 48-
byte pre_master_secret is generated by the client, encrypted under
the server's public key, and sent to the server. The server uses its
private key to decrypt the pre_master_secret. Both parties then
convert the pre_master_secret into the master_secret, as specified
above.
8.1.2. Diffie-Hellman
A conventional Diffie-Hellman computation is performed. The
negotiated key (Z) is used as the pre_master_secret, and is converted
into the master_secret, as specified above. Leading bytes of Z that
contain all zero bits are stripped before it is used as the
pre_master_secret.
Note: Diffie-Hellman parameters are specified by the server and may
be either ephemeral or contained within the server's certificate.
IF I know the public key why I cannot
decrypt data, Surely it's related to
private key!!!
Yes the are related, but to determine the private key from the public one would require solving a computational hard problem that is factorize a prime large number.
For telling it in simple words, you can do it, but the time it would require with the actual technologies is too long.
Related
From what I can tell TLS works using both symmetric and assymmetric encryption.
The assymmetric schemes are used to exchange keys but when and what symmetric schemes are used?
The asymmetric schemes are used to exchange keys
and digital-signatures.
The symmetric schemes are used to data transfer with the agreed symnetric key during the key-exchange.
This is called Hybrid cryptosystem.
Yes you are right. Asymmetric algorithms are usually slower than the symmetric algorithms. However, symmetric algorithms require a shared secret key to encrypt and decrypt messages. Therefore, TLS allows the client and the server exchange a shared secret key using the asymmetric mechanism. Without an asymmetric algorithm, there is no way the shared secret can be exchanged between the two parties in a secured way. Once both the parties have the shared secret key, all subsequent communication between the client and the server are encrypted using the symmetric algorithm which is much faster than the asymmetric algorithm.
At a very high level, the steps in establishing a TLS connection looks like this:
Client -> Requests for secured session
Server -> Sends certificate & chain certificates
Client -> Verifies certificate
Client -> Generate random key for symmetric encryption
Client -> Encrypts the generated key with the server public key and sends the encrypted value to the server
Server -> Decrypt the client sent key with its own private key
Here onwards all subsequent communications between the server and the client will be encrypted using a symmetric algorithm.
Which specific algorithm will be used is determined by the cipher suites supported by the server and the client. During the connection setup, the cipher suite to be used is determined by the client preference.
A typical cipher suite name looks like this:
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256
Here
ECDHE - Key exchange algorithm
ECDSA - Digital Signature algorithm used for signing the key
AES_128_GCM - Block cipher and mode with 128 bit key
ECDHE stands for Elliptic Curve Diffie Hellman Ephemeral. The Elliptic variant (the first E) is used for performance, whereas the Ephemeral variant (the last E) is for forward secrecy. Forward secrecy means that if an attacker keeps recording all the communications over TLS and at a later point of time somehow gets hold of the private key, he/she cannot decrypt the past recorded communications.
ECDSA is used for authenticating (verifying the integrity of) the shared secret. ECDSA is weaker and slower than the other authentication algorithms like HMAC. Yet it is used for shared key authentication because it does not need the verifier know the secret key used to create the authentication tag. The server can very well use its private key to verify the integrity of the message.
AES_128_GCM - Once a common secret key is shared between both the parties (usually a browser and a web server), a symmetric block cipher algorithm is used to encrypt the message exchanges between the parties. In this particular case, the block cipher AES with 128 bit key and GCM authentication mode is used.
If you open a HTTPS website in a browser, you can see the cipher suite used using the browser utilities. For e,g, in Firefox you can see the details under the Security tab in the Page Info, as shown below:
I've implemented RSA encryption algorithm to encrypt the symmetric key used in data encryption, but the key size and the ciphertext size of RSA created a memory issue, so I searched other methods of public key cryptography for the solution. I found elliptic curve integrated encryption scheme (ECIES) and understand the theory behind it, however, I am a bit unclear that how this method be used as public/asymmetric encryption algorithm. The method computes the symmetric encryption with the key derived from the shared secret for both encryption and decryption (using the same key).
So how could it be taken as an asymmetric encryption algorithm?
Or Is there any method to implement it as asymmetric encryption?
Meta: this isn't really a programming or development question or problem. It probably belongs on crypto.SX; you might ask for migration.
To be exact, ECIES is a hybrid public-key encryption scheme, but so are most others. For example RSA is commonly used, just as you said, to encrypt a working (per-message) symmetric key, not to directly encrypt data.
Paraphrasing the wikipedia description:
(Usually in advance) Bob generates a (static) keypair and publishes the publickey authentically (for example using a certificate)
2-5. Alice generates an ephemeral keypair, derives the shared DEK, and encrypts the data, and sends it with her ephemeral publickey
(edit) and destroys the ephemeral privatekey
Bob uses his privatekey to derive the DEK and decrypts the data
ADDED, and expanded below, per comments: Yes the DEK is the same at both ends (notice I used 'the' meaning one and not several) and that's why this scheme works; and the part of ECIES that uses DEK for data encryption and decryption is symmetric, but all the other operations (which securely create the ephemeral shared DEK) are not.
It is vital no one besides Alice (or Bob) learns her ephemeral privatekey; if they do they can decrypt. But she doesn't need to explicitly keep it secret because she destroys it immediately after using it to send a message; that's what ephemeral means.
Let's see:
the recipient's publickey is public and anyone can encrypt
the recipient has the (static) privatekey and can decrypt
nobody else has Bob's (static) privatekey or Alice's ephemeral privatekey, and nobody else can decrypt
the recipient needs only one keypair; if there are multiple senders they can all use the same publickey but can't decrypt each other's traffic, and don't need to get the publickey secretly; for a thousand or a million senders this costs the same as or very little more than one sender
Consider the properties of a standard/traditional symmetric scheme instead:
the two parties must have a key (only one, not a pair) shared in advance; both must keep it secret and not share with anybody else
this typically requires the parties meet in advance, or use a physically secure means such as a courier to carry the key from one to the other or perhaps from a central authority to both
each key can only be used by one pair of parties; for multiple senders, Bob must have and manage that many different keys, and each sender (Alice, Abby, Anne, etc) must have a different key. Each sender must separately meet Bob, or they must each have a separate courier (or two), before they communicate with him. For a thousand or a million senders this becomes immensely costly
ECIES has none of these properties of a conventional or symmetric system, and all of the properties of a publickey or asymmetric system above, although it does also use some symmetric operations along with its asymmetric operations.
And that's why it sounds like (hybrid) public-key encryption to me!
#dave_thompson_085 has explained the concept well. However, I'd like to add an example to make it clear.
Eg:
Alice generates Public "qA" and private key "dA".
Alice sends over her public key to Bob.
Using this public key, Bob generates a random pair of symmetric keys (R and S).
Bob encrypts the message with key "S" and sends over this ciphertext along with key "R" over to Alice.
With this "R" key, Alice can multiply her private key "dA" and generate the symmetric key "S" to decrypt the ciphertext.
So the message is encrypted using a symmetric key, but over the network it is asymmetric as only the public key is exchanged over the network which is used to generate the symmetric key for the receiver and the private key is used to generate the same symmetric key on the sender's side.
I'm trying to use the Qt Cryptographic Architecture to generate public/private key pairs for nodes, and want to have a protocol in which nodes confirm that a message came from a given source (public key) by issuing a challenge to ensure they are the owners of the corresponding private key. I'm a little stuck on the actual syntax for QCA though ...
This should get me a private key, from which I can also get the public key:
QCA::KeyGenerator generator;
QCA::PrivateKey privateKey = generator.createRSA(1024);
Does QCA already have some sort of challenge protocol built in? If so, how can I use it? If not, how can I apply some of the standard cryptographic challenge protocols using these keys? I'm really just trying to demo a simple security concept - I'm not looking for anything complicated or even particularly robust.
Also, this is a side question, but since I don't need encryption, should I be opting for DSA as opposed to RSA?
There are more than one of doing that. But since all clients already have their asymmetric key pairs you may deploy two-way TLS in establishing connection.
In this scheme both client and server allow establishing connection only, if they trust certificates (public keys) of each other. This guarantees client authentication and non-repudiation.
From your side you will need to provision server certificate to all clients and all client certificates to the server. To avoid dealing with certificate authorities, certificates could be self-signed.
QCA has examples for TLS client and server.
I am having an application where I have to send several small data per second through the network using UDP. The application needs to send the data in real-time (no waiting). I want to encrypt these data and ensure that what I am doing is as secure as possible.
Since I am using UDP, there is no way to use SSL/TLS, so I have to encrypt each packet alone since the protocol is connectionless/unreliable/unregulated.
Right now, I am using a 128-bit key derived from a passphrase from the user, and AES in CBC mode (PBE using AES-CBC). I decided to use a random salt with the passphrase to derive the 128-bit key (prevent dictionary attack on the passphrase), and of course use IVs (to prevent statistical analysis for packets).
However I am concerned about few things:
Each packet contains small amount of data (like a couple of integer values per packet) which will make the encrypted packets vulnerable to known-plaintext attacks (which will result in making it easier to crack the key). Also, since the encryption key is derived from a passphrase, this will make the key space way smaller (I know the salt will help, but I have to send the salt through the network once and anyone can get it). Given these two things, anyone can sniff and store the sent data, and try to crack the key. Although this process might take some time, once the key is cracked all the stored data will be decrypted, which will be a real problem for my application.
So my question is, what are the best practices for sending/encrypting continuous small data using a connectionless protocol (UDP)?
Is my way the best way to do it? ...flowed? ...Overkill?
Please note that I am not asking for a 100% secure solution, as there is no such thing.
You have several choices. You can use DTLS, which is a version of TLS adapated for datagrams. It is specified in an RFC and implemented in the openssl library. You can also use the IKE/IPsec protocol and use a UDP encapsulation of the IPsec portion. Usually IPsec is available at the OS level. You can also use OpenVPN, which looks to be a hybrid of TLS for key exchange and a proprietary UDP-based packet encryption protocol.
If your problem is that the data is too small, how about extending the data with random bytes? This will make the plaintext much harder to guess.
This question is a little old, but what about using a One Time Pad type approach? You could use a secure reliable transport mechanism (like HTTPS) to transmit the one time keys from the server to your client. There could be two sets of keys -- one for client to sever, and one for server to client. Each datagram would then include a sequence number (used to identify the one time key) and then the encrypted message. Because each key is used for only one datagram, you shouldn't be exposed to the small data problem. That said, I'm not an expert at this stuff, so definitely check this idea out before using it...
Use Ecdh key exchange (use a password to encrypt the client private key; left on the client) instead of a password. This is a very strong key.
Aes cbc does not help you; the messages are too short and you want to prevent replay attacks. Pad your 64 bit message (two integers) with a counter (starting with 0) 64 bits means 2^64 messages can be sent. Encrypt the block twice (aes ecb) and send e(k;m|count)|e(k;e(k;m|count)). Receiver only accepts monotonically increasing counts where the second block is the encryption of the first. These are 32 byte messages that fit fine in a udp packet.
if 2^64 messages is too small; see if your message could be smaller (3 byte integers means the counter can be 80 bits); or go back to step 1 (new private keys for at least one side) once you are close (say 2^64-2^32) to the limit.
You could always generate a fresh pair of IVs and send them alongside the packet.
These days a good streaming cipher is the way to go. ChaCha20 uses AES for a key stream. Block ciphers are the ones that need padding.
Still that's only part of the picture. Don't roll your own crypto. DTLS is probably a mature option. Also consider QUIC which is emerging now for general availability on the web.
Consider using ECIES Stateless Encryption https://cryptopp.com/wiki/Elliptic_Curve_Integrated_Encryption_Scheme where you sending devices use the public key of the central system and an ephemeral key to generate a symmetric key pair, then a KDF, then AES-256-GCM. You end up with modest size packets which are stateless and complete. No need for an out-of-band key agreement protocol.
There are good examples on the internet, for example: https://github.com/insanum/ecies/blob/master/ecies_openssl.c
I am using such a system to deliver telemetry from mobile devices over an unsecure channel.
Can a Diffie-Hellman key exchange algorithm be used to encrypt client-server communication on a web-page in place of SSL? If it can, what are the disadvantages (i.e. why does the standard use SSL which requires a certificate authority)? My understanding is that Diffie-Hellman can be used to secretly establish a shared key which then can be use to encrypt any further communication.
The two aren't really comparable. DH is a key-exchange algorithm, nothing more and nothing less. SSL attempts to establish that the server you're connecting to is really who it says it is. To do that, it uses a certificate that can be traced back to somebody you (are supposed to be able to) trust.
DH, by itself, only keeps others from reading the transmitted data. SSL is intended to establish considerably more than that (but can use DH to keep others from reading the stream).
Just for an obvious example, using DH (by itself) a Man in the middle attack is fairly simple. If I can get you to connect to my server instead of the one you intended to, I can use DH to establish a "secure" session with you. I then connect to the server you originally intended to. Every packet I get from you, I decrypt, re-encrypt with a key I used to connect to that server, and send on to that server. I do the same with all its response packets. To you, everything looks like it came directly from the original server, and the purchase you made (for example) works just like normal. The only thing that changes is that I also store your credit card number, and when you try to fill your car with fuel the next day, the charge is declined, because in the meantime I've spent all your credit.
The authentication in SSL is at least intended to prevent that from happening. If your browser tried to connect to (for example) www.amazon.com, it should give you a warning if my SSL certificate doesn't specify that it was issued to www.amazon.com -- and a CA shouldn't issue such a certificate to anybody but Amazon.
DH, by itself, doesn't really even guarantee most of what I've said above. By itself, DH is just a way to exchange a key (or, perhaps it could be phrased as "exchange information necessary for both parties to create identical keys, without ever exchanging the key itself in the clear"). After both parties have the key, they can (and presumably will) use it to encrypt/decrypt data -- but that encryption is actually separate from DH itself.
Actually Diffie-Hellman is a part of SSL.
But one part does not replace others.
From here SSL Diffie-Hellman is used for:
This a Diffie-Hellman key exchange in
which the server's certificate
contains the Diffie-Hellman public
parameters signed by the certificate
authority (CA). That is, the
public-key certificate contains the
Diffie-Hellman public-key parameters.
The client provides its Diffie-Hellman
public key parameters either in a
certificate, if client authentication
is required, or in a key exchange
message. This method results in a
fixed secret key between two peers,
based on the Diffie-Hellman
calculation using the fixed public
keys.
You can use anonymous Diffie-Hellman key agreement with SSL. This provides privacy on the channel, but no authentication.
Of course, without authentication, you really can't have privacy, because your private channel could be connected to a "man-in-the-middle". That's why the anonymous DH cipher suites are discouraged.
If the lack of a certificate is stopping you from using SSL where it's really needed, get a free one from startcom.org.
Diffie-Hellman key exchange is only for keyexchange. It does not give you authenticitation (who you're talking to), you need certificates and a PKI for that.
So yes you can do encryption, but you dont know with who you're talking to
The DH key exchange cannot, of itself, do encryption. It is used to establish a session key, but not to do the encryption. So, at this level, the question is mis-stated or reveals either lack of precision or lack of understanding (I suspect precision is the problem this time).
The question is:
Do you want to encrypt data with anybody at all?
Do you want to be sure who you are talking to?
As already pointed out, SSL uses a DH key exchange to establish a session key. However, it also ensures that the program on the other end is someone you trust (directly or indirectly). If you don't need to worry about whether the other person is trustworthy, you could just use a simple DH key exchange and then send encrypted data without needing certificates. But you won't be sure who you are talking to unless you validate that - and the certificates used by SSL etc helps with that validation.