I am planning to create an application that has to be able to securely (that is, encrypted) send messages between clients. These messages may include images and videos (up to 50MB in size). Due to how public-key cryptography works, the encryption process has to be run once for each public key.
For these reasons, I am looking for an algorithm that can encrypt the media fairly quickly on modern devices (eg. mid-level smartphones).
I am asking the question because all algorithms I came across (Blowfish and RSA for example) have a relatively small payload limit, which means that even an image will not fit into it.
Don't roll your own protocol. You're thinking about this at a far too low level. People who know exactly what they're doing have a hard time writing secure code. You don't even know where to start, so you don't stand a chance of getting it right.
If you can establish a direct connection between the two endpoints, use TLS. If you can't establish a direct connection, consider using TLS and relaying the encrypted packets; if that's impractical, use Signal. TLS is ubiquitous; your operating system(s) probably come with an implementation in their default installation. Signal is less ubiquitous, so you'll probably have to embed a library. Make sure to keep up with updates to this library. And once again, don't implement your own library.
Under the hood, all systems that use public-key cryptography to store or transmit more than a few bytes of data use hybrid cryptography. Public-key cryptography is used to establish a symmetric key and authenticate the data, and symmetric cryptography does the heavy lifting.
For example, RSA can be used to encrypt a symmetric key. It's pretty difficult to get it right in practice, however. It's also slower than more modern methods that use elliptic-curve cryptography. TLS can use RSA encryption, but this is deprecated. TLS preferably uses an (elliptic curve) Diffie-Hellman key exchange and an RSA or (EC)DSA signature to set up the secure connection. If you'd like more explanations about how TLS works, read an overview of the protocol, a history of problems and how they were resolved, and an explanation of why RSA encryption is problematic.
Note that Blowfish has been obsolete for a while. If you ever need to choose a specific symmetric cryptosystem — which usually means you're doing it wrong — pick AES-GCM, AES-CCM or ChaCha20-Poly1305.
You are correct about the input size for RSA. In general, for any public key cryptography, the input size of limited and as opposed to your needs, it's quite a bit slower than what you need. Public key cryptography is usually used to share small pieces of data.
For example, in the TLS implementation using RSA (TLS_RSA_WITH_AES_128_GCM_SHA256), the RSA step is used to share a symmetric key, generally, AES, between 2 parties and then all the heavy lifting is done by AES.
What you need is a symmetric encryption algorithm. You can use AES (key sizes of 128, 192 or even 256) to encrypt your images which can be of any size. AES is a block cipher and, using a suitable block cipher mode, theoretically does not have an input limit to it. AES-128 is a fairly standard NIST approved (FIPS 197) symmetric encryption scheme, so it should be safe, but you can always go for a higher key size, say 256.
Since, you have the security of the algorithm defined, we can now talk about the speed.
RSA:
$ openssl speed rsa2048
Doing 2048 bit private rsa's for 10s: 296 2048 bit private RSA's in 10.00s
Doing 2048 bit public rsa's for 10s: 6171 2048 bit public RSA's in 9.99s
LibreSSL 2.6.5
built on: date not available
options:bn(64,64) rc4(ptr,int) des(idx,cisc,16,int) aes(partial) blowfish(idx)
compiler: information not available
sign verify sign/s verify/s
rsa 2048 bits 0.033784s 0.001619s 29.6 617.7
AES-128 in GCM mode
$ openssl speed aes-128-gcm
Doing aes-128 gcm for 3s on 16 size blocks: 3778792 aes-128 gcm's in 2.99s
Doing aes-128 gcm for 3s on 64 size blocks: 1611239 aes-128 gcm's in 3.00s
Doing aes-128 gcm for 3s on 256 size blocks: 485243 aes-128 gcm's in 2.99s
Doing aes-128 gcm for 3s on 1024 size blocks: 125054 aes-128 gcm's in 2.99s
Doing aes-128 gcm for 3s on 8192 size blocks: 15366 aes-128 gcm's in 2.96s
LibreSSL 2.6.5
built on: date not available
options:bn(64,64) rc4(ptr,int) des(idx,cisc,16,int) aes(partial) blowfish(idx)
compiler: information not available
The 'numbers' are in 1000s of bytes per second processed.
type 16 bytes 64 bytes 256 bytes 1024 bytes 8192 bytes
aes-128 gcm 20220.96k 34373.10k 41545.89k 42827.86k 42526.44k
You can see the difference, how AES can blow away RSA in the dust with respect to speed. Symmetric Encryption algorithms are inherently faster due to its design. Asymmetric algorithms use math operations on huge numbers and primes which are slower.
Since AES is so popular, CPU manufacturers started implementing special instructions to make AES computations more and more native (AES-NI). So AES will be faster on almost all popular CPUs.
A small note on security between RSA and AES, although it's like comparing apples to oranges.
RSA2048 offers only 112 bits of security whereas AES128 offers 128 bits of security (ref: aes-vs-rsa)
Another challenge in the above problem would be key management. How would you safely manage the keys? If the keys are to be ephemeral, i.e. session based or one key per use, then you can use RSA to exchange symmetric keys and then perform the encryption. This would guarantee perfect forward secrecy. If you want persistent keys, then obviously there are much more elaborate methods to do that which is out of scope of this question.
Related
I'm trying to understand the mechanism of encryption used on the web.
One thing is not clear to me at this point:
If I check, for example, google.com certificate, I can see that the public key is 4320 bits long, but Chrome shows that the connection is encrypted using AES_128_GCM, that I would expect to work with 128 bit keys.
What am I missing here?
The 4320 bits in the public key algorithm are used to encrypt the 128 bits in the symmetric key algorithm. The public key establishes a secure channel of communication between two parties who initially don't have any shared key. Think of this as a low bandwidth channel, which isn't very useful due to the computationally expensive nature of the public key algorithm. In practice -- the sole use of this public-key is to communicate a shared key to establish a high bandwidth channel which uses something like AES (which requires both parties to have the same key). The overall process is an example of a hybrid cryptosystem.
The whole process is more involved but here are the basis to answer the question.
The data is encrypted with a symmetric algorythm such as AES, it is fast, secure and can handle any length of data.
The symmetric key is encrypted by an asymetric algorythm sucb as RSA or EC, they are slow and the data length is limited by the key length.
There are many articles on the Internet about HTTPS, it can get quite complicated.
I am trying to figure out if it is worth using differential privacy on some data that I need to secure.
So what is less CPU intensive? Encrypt the whole data in AES-256 or encrypt parts of it in AES-256 and other parts in AES-128.
Is the difference significant? Mixing schemes will require the generation of more keys and ivs for each different level of encryption.
Any references? Comments?
Encrypt everything with AES-128 if you care about performance. AES-256 is not much more secure in practice, since 128-bit keys cannot be brute forced anywhere in the foreseeable future. Also see this blog post, where Bruce Schneier recommends against using AES-256.
Introduction:
For my personal webserver I have setup apache with a self signed certificate to enable TLS security to learn and test.
I have this line in virtualhost:
SSLProtocol -all -SSLv3 +TLSv1
SSLCipherSuite TLSv1:+HIGH:!MEDIUM
With firefox, I get Camellia-256 encrypted connection, and with opera I get TLS v1.0 256 bit AES (1024 bit DHE_RSA/SHA) with the same config in same server.
That leads me to question, which is stronger, AES, or Camellia?
I noticed that if I disable camellia with SSLCipherSuite TLSv1:+HIGH:!MEDIUM:!CAMELLIA then, firefox takes the same suite than opera.
In my config, I also try to disable all SSL versions to enable only TLS (advise needed if I didn't do so correctly), but the original question still stands: Which one should be stronger?
I would be more worried about the fact that your SSL encryption is not secure because you are only using 1024 bit asymmetric encryption to protect your keys.
Adi Shamir (the 'S' in RSA) recommended moving to 2048 bit keys back in 2006, even the american standards institute (NIST) have made 2048 bit a required minimum strength since January 2011 (see NIST SP800-57 for recommended minumum key strengths -- this states 2048 bit for both RSA and DH/el-gamal).
In short, make sure your RSA encryption is strong enough first, as it is used to protect the symmetric keys (AES/Camellia). Never rely on a key which is protected by a weaker key (this is like using a secure 256 bit random WPA 2 key on a wireless access point and then trusting it to WPS which will reveal in in a few hours!)
Even if this is a test system, learn to use crypto the way you intend to go forward; do not compromise on certificate key strength (all CAs these days should reject 1024 bit requests or CSRs using MD5 on sight, if not don't use them; create your own test certs as you would a real request, and don't use default key sizes).
Difficult to compare strengths, both have received cryptographic analysis (AES more publicly) and are adequate for securing data.
At the risk of repeating myself, I’d be more worried about the 1024 bits used to secure the key negotiation.
It's hard to judge the strength of these algorithms. Camellia is considered roughly equivalent to AES in security (source). In any case, the difference probably won't matter. Either algorithm is secure enough to make your data channel no longer be the weakest link in your system, so you don't need to bother modifying any configuration.
The OpenSSL cipherlist TLSv1:+HIGH is a really bad choice. The "+something" notation means move all the ciphers that match "something" to the end of the list.
Therefore, you're using HIGH only as a last resort, with anything that's not HIGH preferred.
A much better choice is "DEFAULT:!MEDIUM:!LOW:!EXPORT:+3DES", which starts with sensible defaults, removes MEDIUM, LOW and EXPORT, and uses 3DES last (which it probably is anyway, but on some systems it comes before AES128 because it may be considered to be 168-bit strong).
I have a pair of transceivers connected to the micro-controller from Port A, and a MM232R connected to Port B on a separate PCB. Each transceiver will send encrypted data, while MM232R will receive a decrypted data. I need write encryption algorithm and decryption code. Can anyone give me idea on how to go about it? I am new to programming and encryption algorithm.
I'm not sure about the ROM/RAM limits of the PIC18F1320, that is the main constraint.
This page contains an implementation of AES on a PIC18F4620. It uses 2K words of ROM and 240 bytes of RAM.
This other page seems to have an implementation of RSA on a PIC18F4550, but it warns that it is very slow (which does't surprise me, since RSA encryption requires modular arithmetic on large integers).
I just installed a SSL certificate. This certificate is encrypted with 2048 bit encryption.
However, the cypher is 128 bit encryption(or 40, or some other variation depending on the browser.)
It seems that there are two different types of encryption here. The "handshake" encryption of 2048 and the "over the wire" encryption of some magnitude smaller.
Do I have this right in theory? Can anyone explain it better?
I have been all over the Google and cannot find a clear explanation of the difference between the two.
There is a good entry in Wikipedia.
You are right, there are two kinds of encryption going on. The first one is asymmetric encryption or public key encryption - this is the one with the larger key. The second type is symmetric encryption with the smaller key.
The first type of encryption (asymmetric - larger key) is used to negotiate what type of symmetric encryption the client and the server will use. They'll also exchange the session key that they'll use. This is the handshake process and this is encrpyted using the asymmetric encryption
The session key is basically the key that they'll use when sending the real data, encrypted by whatever type they've decided on the handshake process. This is the symmetric encryption part.
It is true that symmetric encryption typically uses much fewer bits for its key length. The reason is because symmetric encryption is much stronger at a given number of bits.
Asymmetric encryption (where each side has a different key) is much harder to pull off. It is more computationally intensive and therefore only used for the handshake portion or for encrypting a symmetric key that the rest of the message uses.