Private Key C

Private Key C

SSL & PKI

CSR Private Key: How to Generate Your Private Key from a Certificate
By
Savvy Security

June 22, 2022

7 Mins Read

Are you planning to add a secure socket layer/transport layer security (SSL/TLS) certificate to your cybersecurity arsenal but you don’t know how to get the required certificate signing request (CSR) and a private key? No worries generate private key from a certificate
Don’t be yet another victim to be added to the 85% of organizations that suffered a cyberattack in 2021. Learn how to generate a private key from a certificate using a free CSR generator tool.
It’s fast, painless, and easy — so much so that we’re going to show you how to do it right now.
An SSL/TLS certificate won’t only encrypt all communications between your server and the client, but it’ll also avoid your website being marked as “not secure” by the most used browsers.
Before you can receive the SSL/TLS certificate from a trusted certificate authority (CA), you’ll require two important digital files:
All this information will be used to create the SSL/TLS certificate that will be issued to you by the CA. A CSR generator tool will create something that looks like this:
Image caption: This is an example of CSR obtained with a generator tool
How do you get them? Let’s find it out!
Time is money. Why should you spend time and effort to manually generate your private key from a CSR or with the Windows MMC console when you can do it using a free CSR generator tool in seconds? How? By using our browser-based CSR generation tool .
The CSR generator will enable you to keep your private key really private. Why? Because instead of being server-based like many other CSR generators, it’s browser-based. This means that your private key will be generated and displayed only inside your browser. This is fine so long as you’re using a secure, encrypted website like CheapSSLsecurity.com. Your key will never be saved, transmitted, or shared to our server.
OK, enough talking, let’s get down to business and discover how to get a private key from a CSR using the CSR generation tool.
A screenshot of the form that you’ll need to complete can be seen below:
2. Enter your hostname (i.e., common name) .
Type the fully qualified domain name (FQDN) of your server that customers use to access your website.
Tip : Do you need the key for a wildcard SSL certificate? Then make sure you enter your common name using this format: *.mydomain.com
Ensure you use the exact legal name (e.g., Google, Inc.). If you don’t have one and are a private site owner, enter your full name.
Indicate the department that’ll handle the certificate (e.g., IT department, Security team). Tip : This field isn’t mandatory and can be left blank. In fact, the OU field is being deprecated across the industry as a whole because it’s a non-validated field that threat actors could misuse.
5. Enter the city where your company is located .
Make sure you enter your legal location. I’ve entered Philadelphia for demonstration purposes.
6. Add the state or province where your company is located .
This should match the information you entered in the previous step. If you typed Philadelphia, for example, you’ll then indicate Pennsylvania as your state.
Based on the information entered till now, choose from the drop-down menu the country your organization is located in.
8. Select your preferred key generation algorithm .
You can choose between the most used Rivest-Shamir-Aldman (RSA) algorithm and the newer elliptical curve cryptography (ECC). Don’t know which one to pick? Check out our SSL/TLS algorithms comparison article to make an informed choice. I’ve selected ECC here for this example as it offers the same level of encryption as RSA but it’s faster and more secure.
If you selected the RSA key algorithm in the previous step, you may want to stick to the National Institute of Standards and Technology ’s (NIST) recommendation and choose the approved, pre-filled high strength 2048 key size as a minimum. If you’re like me and opted for the ECC key algorithm, you can go with 256 as your key size.
Tip : The tool only lists highly secure key sizes. Smaller sizes aren’t included in the drop-down menu to avoid you getting a too-insecure private key.
Review all the information you entered. Is it correct? Well done! Now, all you have to do is to click on the Generate CSR button at the bottom of the form.
11. Get ready to order your SSL/TLS certificate .
There we go! Now you have your private key ready to download as a text file! Download it and save it on your device. Copy your CSR and send it to the CA to get your brand new SSL/TLS certificate. And remember, when you need to renew it, you’ll just have to follow the same process again.
If you scroll down the page, in addition to the CSR certificate and the CSR private key, the tool will also provide you with ready-made commands to manually generate a CSR directly on your server. (We list multiple server options to choose from.) Cool, huh?
Hungry for more free tools? Check out the other SSL tools and ready-to-use commands available on our SSL tools and guides page !
As highlighted in the article, a certificate authority (CA) won’t be able to issue an SSL/TLS certificate until you’ll send them the CSR for it. This also means that you won’t be able to get a hold of your private key until you complete the certificate signing request.
Remember, you can always generate your private key and CSR manually using the MMC console . But is it worth it? Do you have the time and the skills required?
Don’t put the security of your website, organization, and customers at risk of a data breach. Get your CSR and private key using a CSR generator tool like the one shown in the example. It’ll save you the hassle of a lengthy manual process and keep your data safe and sound.
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Use OpenSSL RSA key with .Net

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How to generate RSA private key using OpenSSL?

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I need to write a C program that generates an RSA key, and saves an X.509 public key in DER format and a PKCS#8 private key in DER format. I've used Google, but haven't really found much. What I have so far is this:
This is obviously writing the keys in PEM format. I also need to be able to actually have the data in memory in the code, not just write it directly to a file, as there's some other stuff I need to do with the public key.
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Your question is a little ambiguous in what you actually mean by "saves an X.509 public key in DER format". Assuming you actually mean "save it as a SubjectPublicKeyInfo structure" (which is the bit of an X.509 certificate that holds public keys) then you should use i2d_RSA_PUBKEY (or i2d_RSA_PUBKEY_fp or i2d_RSA_PUBKEY_bio) to write it out (no need to convert it to an EVP_PKEY first).
For the PKCS#8 private key in DER format, your current method is incorrect for PEM format. The PEM_write_bio_RSAPrivateKey() function will write this out in traditional format (not PKCS#8).
I assume you don't want to do anything complicated like encrypting the key first. For this one you will need to convert it to an EVP_PKEY (using EVP_PKEY_assign_RSA() as mentioned by @JawguyChooser). Next you obtain a PKCS8_PRIV_KEY_INFO structure using the (sadly undocumented) function EVP_PKEY2PKCS8.
You need to free this structure when your done using PKCS8_PRIV_KEY_INFO_free(). Next write out the PKCS8 DER using i2d_PKCS8_PRIV_KEY_INFO() (or i2d_PKCS8_PRIV_KEY_INFO_fp() or i2d_PKCS8_PRIV_KEY_INFO_bio).
See the man page for info on various of these functions:
I think you're going to need to convert your key to an EVP_PKEY using EVP_PKEY_assign_RSA . Then you can use i2d_PUBKEY_bio to write out to a bio.
The following modification of your code works for me:
You can now find the DER of the public key in public.key . And you should be able to do the same thing for the private.

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Note  CryptoAPI does not support the CNG Diffie-Hellman or DSA asymmetric algorithms. CryptoAPI only supports Diffie-Hellman and DSA public keys through the legacy CSPs. If this flag is set for a certificate that contains a Diffie-Hellman or DSA public key, this function will implicitly change this flag to CRYPT_ACQUIRE_ALLOW_NCRYPT_KEY_FLAG to first attempt to use CryptoAPI to obtain the key.

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The CryptAcquireCertificatePrivateKey function obtains the private key for a certificate. This function is used to obtain access to a user's private key when the user's certificate is available, but the handle of the user's key container is not available. This function can only be used by the owner of a private key and not by any other user.
If a CSP handle and the key container containing a user's private key are available, the
CryptGetUserKey function should be used instead.
The address of a
CERT_CONTEXT structure that contains the certificate context for which a private key will be obtained.
A set of flags that modify the behavior of this function. This can be zero or a combination of one or more of the following values.
When this flag is set, the pfCallerFreeProvOrNCryptKey parameter receives FALSE and the calling application must not release the handle. The handle is freed when the certificate context is freed; however, you must retain the certificate context referenced by the pCert parameter as long as the key is in use, otherwise operations that rely on the key will fail.
This function will only use caching if during a previous call, the dwFlags member of the
CRYPT_KEY_PROV_INFO structure contained CERT_SET_KEY_CONTEXT_PROP .
Do not use this flag with CRYPT_ACQUIRE_SILENT_FLAG .
The following flags determine which technology is used to obtain the key. If none of these flags is present, this function will only attempt to obtain the key by using CryptoAPI.
Windows Server 2003 and Windows XP:  These flags are not supported.
The pdwKeySpec variable receives the CERT_NCRYPT_KEY_SPEC flag if CNG is used to obtain the key.
The pdwKeySpec variable receives the CERT_NCRYPT_KEY_SPEC flag if CNG is used to obtain the key.
The pdwKeySpec variable receives the CERT_NCRYPT_KEY_SPEC flag if CNG is used to obtain the key.
If the CRYPT_ACQUIRE_WINDOW_HANDLE_FLAG is set, then this is the address of an HWND . If the CRYPT_ACQUIRE_WINDOW_HANDLE_FLAG is not set, then this parameter must be NULL .
Windows Server 2008 R2, Windows 7, Windows Server 2008, Windows Vista, Windows Server 2003 and Windows XP:  This parameter was named pvReserved and reserved for future use and must be NULL .
The address of an HCRYPTPROV_OR_NCRYPT_KEY_HANDLE variable that receives the handle of either the CryptoAPI provider or the CNG key. If the pdwKeySpec variable receives the CERT_NCRYPT_KEY_SPEC flag, this is a CNG key handle of type NCRYPT_KEY_HANDLE ; otherwise, this is a CryptoAPI provider handle of type HCRYPTPROV .
For more information about when and how to release this handle, see the description of the pfCallerFreeProvOrNCryptKey parameter.
The address of a DWORD variable that receives additional information about the key. This can be one of the following values.
Windows Server 2003 and Windows XP:  This value is not supported.
The address of a BOOL variable that receives a value that indicates whether the caller must free the handle returned in the phCryptProvOrNCryptKey variable. This receives FALSE if any of the following is true:
If this variable receives TRUE , the caller is responsible for releasing the handle returned in the phCryptProvOrNCryptKey variable. If the pdwKeySpec variable receives the CERT_NCRYPT_KEY_SPEC value, the handle must be released by passing it to the NCryptFreeObject function; otherwise, the handle is released by passing it to the CryptReleaseContext function.
If the function succeeds, the return value is nonzero ( TRUE ).
If the function fails, the return value is zero ( FALSE ). For extended error information, call
GetLastError . One possible error code is the following.
When CRYPT_ACQUIRE_WINDOW_HANDLE_FLAG is set, the caller must ensure the HWND is valid. If the HWND is no longer valid, for CSP the caller should call CryptSetProvParam using flag PP_CLIENT_HWND with NULL for the HWND and NULL for the HCRYPTPROV. For KSP, the caller should set the NCRYPT_WINDOW_HANDLE_PROPERTY of the ncrypt key to be NULL . When CRYPT_ACQUIRE_WINDOW_HANDLE_FLAG flag is set for KSP, the NCRYPT_WINDOW_HANDLE_PROPERTY is se
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