SSL_CTX_USE_CERTIFICATE(3)

💖 SSL_CTX_USE_CERTIFICATE — the "load your certs and keys" page 💖

okay so bestie, this is from the Library Functions Manual, and it's literally the page about loading certificate and key data into your TLS setup. It's giving "the most important page you'll ever skim" energy. Let's get into it 🎀

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✨ NAME ✨

no cap, there are SO many functions here, so buckle up. All of these live on this one page:

SSL_CTX_use_certificate, SSL_CTX_use_certificate_ASN1, SSL_CTX_use_certificate_file, SSL_use_certificate, SSL_use_certificate_ASN1, SSL_use_certificate_chain_file, SSL_use_certificate_file, SSL_CTX_use_certificate_chain_file, SSL_CTX_use_certificate_chain_mem, SSL_CTX_use_PrivateKey, SSL_CTX_use_PrivateKey_ASN1, SSL_CTX_use_PrivateKey_file, SSL_CTX_use_RSAPrivateKey, SSL_CTX_use_RSAPrivateKey_ASN1, SSL_CTX_use_RSAPrivateKey_file, SSL_use_PrivateKey_file, SSL_use_PrivateKey_ASN1, SSL_use_PrivateKey, SSL_use_RSAPrivateKey, SSL_use_RSAPrivateKey_ASN1, SSL_use_RSAPrivateKey_file, SSL_CTX_check_private_key, SSL_check_private_key

and their whole collective vibe is: load certificate and key data. 💅

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🌸 SYNOPSIS 🌸

first things first, the library situation:

library “libssl” libcrypto #include <openssl/ssl.h>

and now, the entire lineup, mwah 💋:

int
SSL_CTX_use_certificate(SSL_CTX *ctx, X509 *x);

int
SSL_CTX_use_certificate_ASN1(SSL_CTX *ctx, int len, unsigned char *d);

int
SSL_CTX_use_certificate_file(SSL_CTX *ctx, const char *file, int type);

int
SSL_use_certificate(SSL *ssl, X509 *x);

int
SSL_use_certificate_ASN1(SSL *ssl, unsigned char *d, int len);

int
SSL_use_certificate_chain_file(SSL *ssl, const char *file);

int
SSL_use_certificate_file(SSL *ssl, const char *file, int type);

int
SSL_CTX_use_certificate_chain_file(SSL_CTX *ctx, const char *file);

int
SSL_CTX_use_certificate_chain_mem(SSL_CTX *ctx, void *buf, int len);

int
SSL_CTX_use_PrivateKey(SSL_CTX *ctx, EVP_PKEY *pkey);

int
SSL_CTX_use_PrivateKey_ASN1(int pk, SSL_CTX *ctx, unsigned char *d,
    long len);

int
SSL_CTX_use_PrivateKey_file(SSL_CTX *ctx, const char *file, int type);

int
SSL_CTX_use_RSAPrivateKey(SSL_CTX *ctx, RSA *rsa);

int
SSL_CTX_use_RSAPrivateKey_ASN1(SSL_CTX *ctx, unsigned char *d,
    long len);

int
SSL_CTX_use_RSAPrivateKey_file(SSL_CTX *ctx, const char *file,
    int type);

int
SSL_use_PrivateKey(SSL *ssl, EVP_PKEY *pkey);

int
SSL_use_PrivateKey_ASN1(int pk, SSL *ssl, unsigned char *d, long len);

int
SSL_use_PrivateKey_file(SSL *ssl, const char *file, int type);

int
SSL_use_RSAPrivateKey(SSL *ssl, RSA *rsa);

int
SSL_use_RSAPrivateKey_ASN1(SSL *ssl, const unsigned char *d, long len);

int
SSL_use_RSAPrivateKey_file(SSL *ssl, const char *file, int type);

int
SSL_CTX_check_private_key(const SSL_CTX *ctx);

int
SSL_check_private_key(const SSL *ssl);

💀 yes, SSL_use_RSAPrivateKey_ASN1 really does take const unsigned char *d while SSL_CTX_use_RSAPrivateKey_ASN1 takes a plain unsigned char *d. not me noticing that. it's in the source, don't @ me.

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🦋 DESCRIPTION 🦋

so what do all these do? they load the certificates and private keys into the SSL_CTX or SSL object — respectively. that's the whole thing, that's the assignment. 🫶

🌈 the two families, iykyk

The SSL_CTX_*() class of functions loads the certificates and keys into the SSL_CTX object ctx. The info gets passed to SSL objects ssl created from ctx with SSL_new(3) by copying, so any changes you apply to ctx do not propagate to SSL objects that already exist. lowkey important. 📋

The SSL_*() class of functions only loads certificates and keys into one specific SSL object. and good news: that specific info is kept when SSL_clear(3) is called for this SSL object. slay. ✨

🌸 certificates, one by one

• SSL_CTX_use_certificate() loads the certificate x into ctx; SSL_use_certificate() loads x into ssl. the rest of the certificates needed to form the complete certificate chain can be specified using the SSL_CTX_add_extra_chain_cert(3) function. 🎀
• SSL_CTX_use_certificate_ASN1() loads the ASN1 encoded certificate from the memory location d (with length len) into ctx; SSL_use_certificate_ASN1() loads the ASN1 encoded certificate into ssl.
• SSL_CTX_use_certificate_file() loads the first certificate stored in file into ctx. the formatting type of the certificate must be specified from the known types SSL_FILETYPE_PEM and SSL_FILETYPE_ASN1. SSL_use_certificate_file() loads the certificate from file into ssl. and bestie, pls check the NOTES section on why SSL_CTX_use_certificate_chain_file() should be preferred. 👀

🧚 chains, aka the main character energy

The SSL_CTX_use_certificate_chain*() functions load a certificate chain into ctx. the certificates must be in PEM format and must be sorted starting with the subject's certificate (the actual client or server certificate), followed by intermediate CA certificates if applicable, and ending at the highest level (root) CA. order matters, don't play with it. 💅

with the exception of SSL_use_certificate_chain_file(), there is no corresponding function working on a single SSL object. yeah. that's just how it is. 😭

🔥 private keys, aka the secret part

• SSL_CTX_use_PrivateKey() adds pkey as private key to ctx.
• SSL_CTX_use_RSAPrivateKey() adds the private key rsa of type RSA to ctx.
• SSL_use_PrivateKey() adds pkey as private key to ssl; SSL_use_RSAPrivateKey() adds rsa as private key of type RSA to ssl.

⚠️ if a certificate has already been set and the private [key] does not belong to the certificate, an error is returned. to change a certificate private key pair, the new certificate needs to be set with SSL_use_certificate() or SSL_CTX_use_certificate() before setting the private key with SSL_CTX_use_PrivateKey() or SSL_use_PrivateKey(). order of operations, bestie. 🔑

🍓 ASN1 key loading

• SSL_CTX_use_PrivateKey_ASN1() adds the private key of type pk stored at memory location d (length len) to ctx.
• SSL_CTX_use_RSAPrivateKey_ASN1() adds the private key of type RSA stored at memory location d (length len) to ctx.
• SSL_use_PrivateKey_ASN1() and SSL_use_RSAPrivateKey_ASN1() add the private key to ssl.

🎀 key files

• SSL_CTX_use_PrivateKey_file() adds the first private key found in file to ctx. the formatting type of the private key must be specified from the known types SSL_FILETYPE_PEM and SSL_FILETYPE_ASN1.
• SSL_CTX_use_RSAPrivateKey_file() adds the first private RSA key found in file to ctx.
• SSL_use_PrivateKey_file() adds the first private key found in file to ssl; SSL_use_RSAPrivateKey_file() adds the first private RSA key found to ssl.

💀 the "check_private_key" moment

okay this is my favorite part because the manpage is genuinely pressed about it:

The SSL_CTX_check_private_key() function is seriously misnamed.

it compares the public key components and parameters of an OpenSSL private key with the corresponding certificate loaded into ctx. if more than one key/certificate pair (RSA/ECDSA) is installed, the last item installed will be compared. if, e.g., the last item was an RSA certificate or key, the RSA key/certificate pair will be checked. SSL_check_private_key() performs the same public key comparison for ssl; if no key/certificate was explicitly added for this ssl, the last item added into ctx will be checked. 🙈

and despite the name, neither SSL_CTX_check_private_key() nor SSL_check_private_key() checks whether the private key component is indeed a private key, nor whether it matches the public key component. they merely compare the public materials (e.g. exponent and modulus of an RSA key) and/or key parameters (e.g. EC params of an EC key) of a key pair. it's giving "the name is a lie but the function is fine." 😭

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⭐ NOTES ⭐

The internal certificate store of OpenSSL can hold several private key/certificate pairs at a time. the certificate used depends on the cipher selected. see also SSL_CTX_set_cipher_list(3). 🔐

When reading certificates and private keys from file, files of type SSL_FILETYPE_ASN1 (also known as DER, binary encoding) can only contain one certificate or private key; consequently, SSL_CTX_use_certificate_chain_file() is only applicable to PEM formatting. files of type SSL_FILETYPE_PEM can contain more than one item. 🗂️

SSL_CTX_use_certificate_chain_file() adds the first certificate found in the file to the certificate store. the other certificates are added to the store of chain certificates using SSL_CTX_add1_chain_cert(3). it is recommended to use the SSL_CTX_use_certificate_chain_file() instead of the SSL_CTX_use_certificate_file() function in order to allow the use of complete certificate chains even when no trusted CA storage is used or when the CA issuing the certificate shall not be added to the trusted CA storage. 👑

If additional certificates are needed to complete the chain during the TLS negotiation, CA certificates are additionally looked up in the locations of trusted CA certificates (see SSL_CTX_load_verify_locations(3)). 🦋

The private keys loaded from file can be encrypted. in order to successfully load encrypted keys, a function returning the passphrase must have been supplied (see SSL_CTX_set_default_passwd_cb(3)). (certificate files might be encrypted as well from the technical point of view, it however does not make sense as the data in the certificate is considered public anyway.) 🔓

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🫶 RETURN VALUES 🫶

on success, the functions return 1. otherwise check out the error stack to find out the reason. that's it. that's the whole section. 💅

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🌈 SEE ALSO 🌈

go read these next, bestie, they're all linked:

ssl(3), SSL_clear(3), SSL_CTX_add1_chain_cert(3), SSL_CTX_add_extra_chain_cert(3), SSL_CTX_load_verify_locations(3), SSL_CTX_set_cipher_list(3), SSL_CTX_set_client_CA_list(3), SSL_CTX_set_client_cert_cb(3), SSL_CTX_set_default_passwd_cb(3), SSL_new(3), X509_check_private_key(3)

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👑 HISTORY 👑

the lore is deep, so here's the timeline:

• SSL_use_certificate(), SSL_use_certificate_file(), SSL_use_RSAPrivateKey(), and SSL_use_RSAPrivateKey_file() appeared in SSLeay 0.4 or earlier.
• SSL_use_certificate_ASN1() and SSL_use_RSAPrivateKey_ASN1() first appeared in SSLeay 0.5.1.
• SSL_use_PrivateKey_file(), SSL_use_PrivateKey_ASN1(), and SSL_use_PrivateKey() first appeared in SSLeay 0.6.0.
• SSL_CTX_use_certificate(), SSL_CTX_use_certificate_ASN1(), SSL_CTX_use_certificate_file(), SSL_CTX_use_PrivateKey(), SSL_CTX_use_PrivateKey_ASN1(), SSL_CTX_use_PrivateKey_file(), SSL_CTX_use_RSAPrivateKey(), SSL_CTX_use_RSAPrivateKey_ASN1(), and SSL_CTX_use_RSAPrivateKey_file() first appeared in SSLeay 0.6.1.
• SSL_CTX_check_private_key() and SSL_check_private_key() first appeared in SSLeay 0.6.5.
• All these functions have been available since OpenBSD 2.4. 🧚
• SSL_CTX_use_certificate_chain_file() first appeared in OpenSSL 0.9.4 and has been available since OpenBSD 2.6.
• SSL_use_certificate_chain_file() first appeared in OpenSSL 1.1.0 and has been available since OpenBSD 6.9. ✨
• Support for DER encoded private keys (SSL_FILETYPE_ASN1) in SSL_CTX_use_PrivateKey_file() and SSL_use_PrivateKey_file() was added in 0.9.8.
• SSL_CTX_use_certificate_chain_mem() first appeared in OpenBSD 5.7.

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and that's the whole page, bestie — go forth and load those certs responsibly 🎀💖

Debian · June 8, 2025 · SSL_CTX_USE_CERTIFICATE(3) — mwah, love u, bye 👋💅