Openssl Generate Rsa Key Pair C++
- Openssl Rsa Check Key
- Openssl Generate Rsa Key Pair
- Openssl Generate Rsa Private Key
- Openssl Generate Rsa Pkcs8
So, getting to your OpenSSL key question. OpenSSL will want the kernel to keep entropy as full as possible. However, OpenSSL will seed from /dev/urandom by default. This device will still exhaust entropy, but rather than block when there is no entropy estimate, it will use a PRNG to generate the rest of the data. Apr 07, 2012 Public Key Encryption and Digital Signatures using OpenSSL. I recently gave students a homework task to get familiar with OpenSSL as well as understand the use of public/private keys in public key cryptography (last year I gave same different tasks using certificates - see the steps.The tasks for the student (sender in the notes below) were to.
I am trying to generate RSA keypair using openssl library and then read the same keys later. However, it fails. Sometimes it gives me this error. C openssl rsa key-pair. Share improve this question. Edited May 16 '18 at 6:37. 1,156 1 1 gold badge 7 7 silver badges 20 20 bronze badges. Apr 28, 2012 And so I opened up the OpenSSL documentation to figure out how to encrypt and decrypt simple messages with RSA in C. Here’s a quick summary First up, to do anything with RSA we need a public/private key pair. I assume the reader knows the basic theory behind RSA so I won’t go into the math inside a key pair. Openssl genrsa -out rsa.key 1024 openssl req -new -key rsa.key -out output.csr -config config.txt where config.txt contains the distinguished name to use in the certificate. I would like to do something similar under Windows using C#. However, the method createPKCS10 does not require you to supply an RSA key. I want to generate a private and public key using C to use them in an authentication process. Using openssl to generate a DSA key pair. Making OpenSSL. I am new to openssl. I am trying to implement the program to generate CSR using openssl and c. I need to implement the following commands using C. Openssl req -new -newkey rsa:1024 -nodes -keyout key.pem -out x509Req.pem.
Hey you! This post is outdated!
Take a look at a more correct, detailed, and useful one. What’s the advantage? The EVP functions do implicit symmetric encryption for you so you don’t get hung up on the max length limitations of RSA. Plus, it has an AES implementation.
Disclaimer: I am NOT a crypto expert. Don’t take the information here as 100% correct; you should verify it yourself. You are dangerously bad at crypto.
Last month I wrapped up my Alsa Volume Control server project. To test it, I exposed the server to my public Internet connection and within a few hours, my friend was using the lack of authentication to change the volume on my computer from his apartment. It may not be a serious security hole, and funny as it may be, it would certainly be annoying if someone had malicious intentions in mind. The simple solution is just disable the port forward so the server is only accessible via my LAN, but what fun is that? What if I feel like changing my volume from anywhere for whatever stupid reason I may have?! Thus, I needed to add authentication to the server, which means I also a needed a way to encrypt credentials as they went over the network. And so I opened up the OpenSSL documentation to figure out how to encrypt and decrypt simple messages with RSA in C. Here’s a quick summary…
First up, to do anything with RSA we need a public/private key pair. I assume the reader knows the basic theory behind RSA so I won’t go into the math inside a key pair. If you’re interested, here’s a good write-up on the math behind RSA.
Here we’re using the RSA_generate_key function to generate an RSA public and private key which is stored in an RSA struct. The key length is the first parameter; in this case, a pretty secure 2048 bit key (don’t go lower than 1024, or 4096 for the paranoid), and the public exponent (again, not I’m not going into the math here), is the second parameter.
So we have our key pair. Cool. So how do we encrypt something with it?
The first thing you’ll notice is that the message length is limited to 2048 bits or 256 bytes, which is also our key size. A limitation of RSA is that you cannot encrypt anything longer than the key size, which is 2048 bits in this case. Since we’re reading in chars, which are 1 byte and 2048bits translates to 256 bytes, the theoretical max length of our message is 256 characters long including the null terminator. In practice, this number is going to be slightly less because of the padding the encrypt function tacks on at the end. Through trial and error, I found this number to be around 214 characters for a 2048 bit key.
So we have the message. Let’s encrypt it! We allocate memory for a buffer to store our encrypted message in (encrypt). We can determine the max length of the encrypted message via the RSA_size function. We also allocate some memory for an error buffer, in case there’s a problem encrypting the message like if the message is over the practical max length of a message (~214 bytes). From here, all we have to do is call the RSA_public_encrypt function and let it do it’s magic. We supply the number of bytes to encrypt, the message to encrypt, the buffer to put the encrypted message, they keypair to encrypt with, and finally, the type of padding to use for the message. The padding is where the discrepancy between the theoretical length and practical length comes from. The different types can be found on the documentation page for the RSA_public_encrypt function, but the one used above is the one that should be used for new implementations of RSA.
RSA_public_encrypt will return the number of bytes encrypted, or -1 on failure. If -1 we use the OpenSSL error functions to get a more descriptive error, and print it. The error functions are pretty self-explanatory if you read their documentation, so I won’t go into them here. Another sanity check that I didn’t check for would be to ensure that the number of bytes encrypted returned by RSA_public_encrypt is the key size divided by 8, or 256 in this case. If it isn’t, something isn’t right.
Now let’s decrypt the message! Good news is that if you understood the encryption, decryption is very similar.
We allocate the length of our encrypted message to store the decrypted message in. The decrypted message may only be a few characters long, but we don’t know how it’s exact length prior to decryption, so we allocate the upper bound of its length to avoid any length issues. From here, decryption is a simple call to RSA_private_decrypt with the encrypted length, the encrypted message, the buffer to store the decrypted message in, the key to perform decryption with, and the padding type–all very similar to the encrypt function. RSA_public_decrypt returns -1 on error and we check for errors the same way as the encrypt function.
And that’s it! You can now encrypt and decrypt messages with RSA!
But let’s get a little closer to having something that’s actually useful. Let’s see if we can write our encrypted message to a file, read it back, and then decrypt it.
Writing to a file is actually pretty easy. The one caveat to remember is that we aren’t dealing with plain text anymore–we’re working with binary data now so the usual ways to write to a file like fputs aren’t going to work here. Instead, we utilize fwrite which is going to write the encrypted message buffer to the file verbatim. We should check for errors here, but this is just a quick proof-of-concept.
Reading it back is also just as trivial.
We free’d our encrypted message buffer after writing it to the file above as a proof-of-concept above so we need to allocate memory for it again. After that, remember that this data isn’t plain text so the usual fgets isn’t going to work. We need to use fread which will put the encrypted message back into the encrypt buffer which we can then use to send to the decrypt function above.
Let’s also make sure that the data we wrote the file is really there by firing up a terminal and looking at an od dump of the file we wrote.
Here we can see why the file can’t be read as a regular text file. Some of the values are outside of the range of regular characters! Compare this to the plain text of the message that’s encrypted above (hint: it’s “hello”):
Another thing we can do is separate the key pair into a public key and a private key, because what good does sending both the private and public key to decrypt a message to someone do? Let’s revisit the original code we used to generate the key pair.
We generate the key pair as before (this time with a generalized key length and public exponent), but now we used BIO structs to separate the public and private key. BIO’s are just an OpenSSL abstraction to make our lives easier. We use the PEM_write_bio_RSAPrivateKey function and it’s public key counterpart to copy the private and public keys into the newly created BIO structs. We then use the BIO_pending function to get how long our plain text character strings need to be to store the keys and allocate that amount of memory. From there, BIO_read copies the keys from the BIO structs into the character strings. Finally, let’s print them out for fun. Here’s an example of a key pair I generated via this method:
So that’s a lot of code! Let’s put it all together into one complete example:
To compile it (with debug symbols in case you want to debug it), make sure you have the OpenSSL library installed (libcrypto), and then run:
And there you have it, simple RSA encryption and decryption. I’ll be writing more posts as I further implement this into my Alsa server project on the topics on sending the public key over the network, sending arbitrary size messages with the help of a symmetric cipher (probably AES), doing authentication with Unix users, and doing all this on Android.
Disclaimer: I am NOT a crypto expert. Don’t take the information here as 100% correct; you should verify it yourself. You are dangerously bad at crypto.
This post details the EVP functions for RSA. If you’re looking for a pure RSA implementation or want something in C rather than C++, see my other post on this.
In my seemingly endless side project to implement RSA and AES encryption to my Alsa Server project, I wrote a while ago about doing simple RSA encryption with OpenSSL. Now, I’m here to say that I was doing it all wrong. In my first post about RSA encryption and OpenSSL my code was using the low level RSA functions when I should have been using the high level EVP (envelope) functions, which are much nicer to work with once you get the hang of them.
Being that this code is eventually going to be merged in my Alsa server project, I went ahead and also implemented AES encryption/decryption and put everything in an easy to use C++ class.
I assume that readers are familiar with encryption and OpenSSL terminology (things like IV, key lengths, public vs private keys, etc.). If not, look it up since there are much better explanations out there than I could write.
Why use the EVP (envelope) functions for RSA encryption rather than the actual RSA functions? Becasue the EVP functions don’t actually encrypt your data with RSA. Rather, they encrypted a symmetric key with RSA and then encrypt your data with the symmetric key. There’s a few reasons for this. The main one being that RSA has a max length limit to how much data can be encrypted at once. Symmetric ciphers do not have this limit. If you want pure RSA, see my post about that, otherwise, you probably want to be using the EVP functions.
Moving on. First up, since all the code presented is in various functions from a class (full listing is at the end), let’s look at the class members, and constructors first to understand where some of these variables are coming from. Below are all the class members. I know, not exactly intuitive, but bear with me.
- The
EVP_PKEYvariables store the public and private keys for the server, or just the public key for the client. - The
EVP_CIPHER_CTXvariables keep track of the RSA and AES encryption/decryption processes and do all the hard, behind the scenes work. aesKeyandaesIVare the symmetric key and IV used for the AES functions.
Openssl Rsa Check Key
So now we need to initialize all these guys. In the class there are two constructors, each with different arguments so let’s look at the default constructor for simplicity’s sake.
The RSA keys are just set to NULL because their values will be initialized later when the RSA/AES functions are called. The #ifdef line certainly peaks some interest. Since this class is eventually going to be dropped in a server, it will be using the client’s public key to encrypt data, but we don’t have a client yet, so we define a fake client and generate another RSA key pair to simulate a client. The process of generating this key pair is identical to how we’re about to generate the key pair for the server so let’s look at this. This is all contained in the init() function.
There’s a lot of strange function calls in here. Most of this function deals with the OpenSSL API and how to generate keys and initialize EVP contexts. I’ll give a high level overview here, but the best way to understand this process is to read up on documentation. The first thing we do is allocate the proper amount of memory for the EVP contexts and then called the EVP_CIPHER_CTX_init() function which does some magic to initialize them. Then we use a few RSA functions to generate the RSA keys for the server. Again, the documentation for these functions will help you understand better than my explanation can, but you’ll see a pattern emerge, initialize the context, pass it along with some options and an output argument to a function and you’ll get what you want. It’s the same way for RSA. We initialize the key context with EVP_PKEY_CTX_new_id() and EVP_PKEY_keygen_init(), set the key length to use with EVP_PKEY_CTX_set_rsa_keygen_bits() (see the full listing for the actual length if you really need to–2056 bits is sufficient for most cases; use 4096 if you’re paranoid) and then actually generate the keys with EVP_PKEY_keygen().
The AES key is much simpler; it’s just random data so we call RAND_bytes() to get the number of random bytes needed for the AES encrypted key and IV. There is also the option to use the EVP_BytesToKey() function which is a PBKDF. This function, as I called it, will generate a 256 bit key in CBC mode, with a salt and passphrase that are random data (the password being random data is just for demonstration purposes). The number of rounds (or count as the documentation calls it) is the strength of randomness to use. Higher numbers are better, but slower.
I mentioned above that we generate a separate client key pair for testing. I won’t do a write-up on it since it’s the same process as generating the server key pair, only we store the keys in their own variables of course, but you can see the genTestClientKey() function in the full code listing below for how this is done.
On to the fun part, the actual encryption. Let’s start with AES since it’s a little easier to understand. The AES encryption function:
The arguments are probably as you would expect, the message to be encrypted, the length of that message, and an output string for the encrypted message (which should just be a NULL pointer unless you like memory leaks).
The first step is allocate memory for the encrypted message. Because the message will be padded for extra space left over in the last block, we need to allocate the length of the unencrypted message plus the max size of an AES block to make sure there’s enough room in the buffer.
A pattern quickly emerges when doing encryption/decryption with both AES and RSA. First there’s a call to an init function, EVP_EncryptInit_ex() in this case, then a call to an update function, EVP_EncryptUpdate() here and finally a call to a finalize function, or EVP_EncryptFinal_ex(). Each of these functions has two versions, a “regular” one and one suffixed with _ex. The _ex versions just provide a few extra parameters. See the documentation for the exact differences; I only used the _ex functions where necessary. Ubuntu generate ssh key git.
So, a little more info on the encryption process. First, we call the init function to initialize the context for encryption. Then the update function actually starts encrypting the message. This may involve multiple calls to the update function so if you were say, encrypting an entire file, you could read the file line by line, calling the update function as you read each line to encrypt it in a loop. In this case, we’re only encrypting a single message so we only need to call it once. The update function will return the number of bytes that were encrypted. It is our responsibility to keep track of the total number of bytes encrypted and to advance the encrypted message pointer for each subsequent call to the update and finalize function, otherwise you will be overwriting previously encrypted data. I bold this because the documentation does not make reference to this and was the source of much confusion when I was originally writing these functions. Thankfully, one of the smart people over at Stack Overflow helped me out. Now, the last step is to finalize the encryption. This is will pad extra space in the buffer to fill up the last block and then that completes the encryption! In my function I update the number of bytes encrypted and return that to the caller. It is important to return the number of bytes encrypted so the caller can keep track of the encrypted message length because it is now a binary string and calling strlen() on it won’t give the correct length. Though it’s possible to convert it to base64.
Lastly, clean up the context to avoid memory leaks.
On to decryption!
The AES decryption function looks very similar to the encryption function. The arguments are the encrypted message, its length, and a double pointer to an output buffer for the decrypted string (should be NULL as well else memory leaks). The decryption process is very similar to encryption so I won’t go into as much detail as I did with encryption here. First we allocate memory for the decrypted string, init our decryption context, call the update function on the encrypted message (note we have to keep track of the number of decrypted bytes here too), and then finalize the decryption. That’s all there is to it.
Next up, RSA.
Again, RSA is very similar to AES. The major difference being that we’re now using the “Seal” functions. The OpenSSL API differentiates between Seal & Open for encrypting with public keys and decrypting with private keys and Sign & Verify for encrypting with private keys and decrypting with public keys. Since I will eventually be dropping this in a server, I want to be encrypting with public keys so I’m using the Seal functions here.
Openssl Generate Rsa Key Pair
As before, we first allocate memory for the encrypted message. A limitation of RSA is that the max length of the encrypted message is roughly equal to the length of the key it’s being encrypted with. However, a key point of the EVP functions is that they don’t acutally encrypt your data with RSA, but rather encrypt a symmetric key with RSA and then use the symmetric key to encrypt your data since there is no max length on symmetric key encryption. This is also what the ek argument is. It will returned the encrypted symmetric key that the data is actually encrypted with. To decrypt it, we’ll need the symmetric key and the IV. This is a lot of data to keep track of, but that’s part of the challenge of using encryption.
From here we init the context again, call the update function (only once since we only have one message) and then finalize it all while keeping track of the number of bytes that were encrypted.
Decryption time.
Once again, the decryption is similar to AES decryption so I won’t go into much detail. One interesting note, however, is the #ifdef. This is where the client key pair I made in the init function way above comes into place. Because this is all just a test of the crypto functions, I don’t have a client to give me a public key from yet. So we generated an extra key pair at the beginning to simulate the client’s keypair key with. In the future, decryption will be done with the server’s private key and we will, of course, not have access to the client’s private key so that’s where the #ifdef comes from.
Other than that, the typical init, update, finalize process is in effect again. After decryption we have a regular char string again so don’t forget about the null terminator.
Okay, so we have all these functions now. How about seeing them in action? First, let’s look at how to compile this guy.
Openssl Generate Rsa Private Key
So here we have debugging and warnings turned on. class_test.cpp is my main file and Crypto.cpp is my class with all the encryption/decryption functions in it. The key part is linking with the OpenSSL library, called the crypto library at the end of the command. Note that like all libraries with g++, it must be after the source files. Also, make sure you have a recent version of OpenSSL. I wrote this with version 1.0.1. YMMV with other versions.
Finally, let’s run this guy with a test program (available on GitHub at the link below):
Arrested Development references aside, you can see that it, at the very least, does encrypt and decrypt our messages. You’ll notice that the output has been converted to base64 so it can be printed out as a regular ASCII string.