SSL_CTX_SET_VERIFY(3)

📦 version 4.4.0, in alpine 3.24, arch rolling

💅 SSL_CTX_SET_VERIFY(3) — like, the whole peer cert verification situation

okay so real talk, this is a page from the Library Functions Manual, and it lives in libssl / libcrypto. Bestie. Sit down. We're learning how TLS decides whether to trust the other side. 🎀

✨ NAME ✨

SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth, SSL_set_verify_depth — the four functions that set your peer certificate verification parameters. That's the vibe. 🦋

🌸 SYNOPSIS 🌸

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

void
SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
    int (*verify_callback)(int, X509_STORE_CTX *));

void
SSL_set_verify(SSL *s, int mode,
    int (*verify_callback)(int, X509_STORE_CTX *));

void
SSL_CTX_set_verify_depth(SSL_CTX *ctx, int depth);

void
SSL_set_verify_depth(SSL *s, int depth);

int
verify_callback(int preverify_ok, X509_STORE_CTX *x509_ctx);

💖 DESCRIPTION 💖

🫶 the two "set_verify" girlies

SSL_CTX_set_verify() sets the verification flags for ctx to be mode, and tells it which verify_callback function to use. If you literally do not want a callback function, you can pass the NULL pointer as verify_callback. Slay, no callback era. ✨

SSL_set_verify() does the same thing but for ssl — sets the verification flags for ssl to be mode and picks the verify_callback function to be used. Same deal: if no callback function shall be specified, the NULL pointer can be used for verify_callback. BUT — plot twist — in this case, the last verify_callback set specifically for this ssl remains. 👀 And if no special callback was set before, then the default callback for the underlying ctx gets used — the one that was valid at the time ssl was created with SSL_new(3). Within the callback function, you can call SSL_get_ex_data_X509_STORE_CTX_idx(3) to get the data index of the current SSL object that's doing the verification. iykyk. 💅

🎀 the two "set_verify_depth" girlies

SSL_CTX_set_verify_depth() sets the maximum depth for the certificate chain verification that shall be allowed for ctx. (Go look at the BUGS section, bestie, it's got lore.)

SSL_set_verify_depth() sets the maximum depth for the certificate chain verification that shall be allowed for ssl. (Yes. Also BUGS section. Same lore.) 🔥

🌈 the mode flags, aka the main characters

The verification of certificates can be controlled by a set of bitwise ORed mode flags. Bitwise ORed. Say it with me. 🫶

  • SSL_VERIFY_NONE — okay so:
    • Server mode: the server will not send a client certificate request to the client, so the client will not send a certificate. Nothing happens, it's giving peaceful.
    • Client mode: if you're not using an anonymous cipher (which is by default disabled), the server will send a certificate which will be checked. The result of the certificate verification process can be checked after the TLS/SSL handshake using the SSL_get_verify_result(3) function. And the handshake will be continued regardless of the verification result. It's giving "we move." 😭
  • SSL_VERIFY_PEER — the important one, no cap:
    • Server mode: the server sends a client certificate request to the client. The certificate returned (if any) is checked. If the verification process fails, the TLS/SSL handshake is immediately terminated with an alert message containing the reason for the verification failure. The behaviour can be controlled by the additional SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE flags. 💀
    • Client mode: the server certificate is verified. If the verification process fails, the TLS/SSL handshake is immediately terminated with an alert message containing the reason for the verification failure. If no server certificate is sent, because an anonymous cipher is used, SSL_VERIFY_PEER is ignored. Rude but fair. 🙈
  • SSL_VERIFY_FAIL_IF_NO_PEER_CERT —
    • Server mode: if the client did not return a certificate, the TLS/SSL handshake is immediately terminated with a “handshake failure” alert. This flag must be used together with SSL_VERIFY_PEER. Periodt.
    • Client mode: ignored. 🫥
  • SSL_VERIFY_CLIENT_ONCE —
    • Server mode: only request a client certificate on the initial TLS/SSL handshake. Do not ask for a client certificate again in case of a renegotiation. This flag must be used together with SSL_VERIFY_PEER. 👑
    • Client mode: ignored. Again. 🫥
Exactly one of the mode flags SSL_VERIFY_NONE and SSL_VERIFY_PEER must be set at any time. One. Not zero, not both. Exactly one. 💅

🧚 who actually does the verifying

The actual verification procedure is performed either using the built-in verification procedure, or using another application provided verification function set with SSL_CTX_set_cert_verify_callback(3). The descriptions below apply in the case of the built-in procedure. An application provided procedure also has access to the verify depth information and the verify_callback() function, but the way this information is used may be different. So like, don't @ me if your custom one vibes differently. 🌸

🦋 depth limits, explained

SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set the limit up to which depth certificates in a chain are used during the verification procedure. If the certificate chain is longer than allowed, the certificates above the limit are ignored. Error messages are generated as if these certificates would not be present — most likely a X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY will be issued. 😭

The depth count goes like this:

  • “level 0: peer certificate”
  • “level 1: CA certificate”
  • “level 2: higher level CA certificate”
  • and so on. ⭐

Setting the maximum depth to 2 allows the levels 0, 1, and 2. The default depth limit is 100, allowing for the peer certificate and an additional 100 CA certificates. generous queen. 👑

💜 the verify_callback function, the moment

The verify_callback function is used to control the behaviour when the SSL_VERIFY_PEER flag is set. It must be supplied by the application and receives two arguments:

  • preverify_ok — indicates whether the verification of the certificate in question was passed (preverify_ok=1) or not (preverify_ok=0).
  • x509_ctx — a pointer to the complete context used for the certificate chain verification.

Here's the little journey it goes on. 🎢 The certificate chain is checked starting with the deepest nesting level (the root CA certificate) and worked upward to the peer's certificate. At each level, signatures and issuer attributes are checked. Whenever a verification error is found, the error number is stored in x509_ctx and verify_callback is called with preverify_ok equal to 0. By applying X509_CTX_store_*() functions, verify_callback can locate the certificate in question and perform additional steps (see EXAMPLES). And if no error is found for a certificate, verify_callback is called with preverify_ok equal to 1 before advancing to the next level. ✨

🔥 what the return value does

The return value of verify_callback controls the strategy of the further verification process:

  • If verify_callback returns 0, the verification process is immediately stopped with “verification failed” state. If SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and the TLS/SSL handshake is terminated. that's a wrap. 💀
  • If verify_callback returns 1, the verification process is continued. If verify_callback always returns 1, the TLS/SSL handshake will not be terminated with respect to verification failures and the connection will be established. The calling process can however retrieve the error code of the last verification error using SSL_get_verify_result(3), or by maintaining its own error storage managed by verify_callback. you're so real for that. 🫶

🍓 and if you didn't specify one

If no verify_callback is specified, the default callback will be used. Its return value is identical to preverify_ok, so that any verification failure will lead to a termination of the TLS/SSL handshake with an alert message, if SSL_VERIFY_PEER is set. no notes. 🌟

okay that's the whole vibe so far — part 2 is loading, don't go anywhere 👀

✨ EXAMPLES ✨

okay bestie, this is the part you actually came for 💖 The code below shows a verify_callback function that will always keep the TLS/SSL handshake going even if verification fails, if that's what you want. It also gives you a verification depth limit with extra informational output on top. Slay.

All verification errors get printed. Info about the certificate chain gets printed on request. The example is written for a server that does allow client certificates but does not require them.

The example uses the ex_data technique to store application data into / retrieve application data from the SSL structure (peep SSL_get_ex_new_index(3) and SSL_get_ex_data_X509_STORE_CTX_idx(3), iykyk).

...

typedef struct {
        int     verbose_mode;
        int     verify_depth;
        int     always_continue;
} mydata_t;
int mydata_index;
...

and here's the callback itself, no cap, this is the whole thing 🫶

static int
verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
{
        char buf[256];
        X509 *err_cert;
        int err, depth;
        SSL *ssl;
        mydata_t *mydata;

        err_cert = X509_STORE_CTX_get_current_cert(ctx);
        err = X509_STORE_CTX_get_error(ctx);
        depth = X509_STORE_CTX_get_error_depth(ctx);

        /*
         * Retrieve the pointer to the SSL of the connection currently
         * treated * and the application specific data stored into the
         * SSL object.
         */
        ssl = X509_STORE_CTX_get_ex_data(ctx,
            SSL_get_ex_data_X509_STORE_CTX_idx());
        mydata = SSL_get_ex_data(ssl, mydata_index);

        X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);

        /*
         * Catch a too long certificate chain. The depth limit set using
         * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so
         * that whenever the "depth>verify_depth" condition is met, we
         * have violated the limit and want to log this error condition.
         * We must do it here, because the CHAIN_TOO_LONG error would not
         * be found explicitly; only errors introduced by cutting off the
         * additional certificates would be logged.
         */
        if (depth > mydata->verify_depth) {
                preverify_ok = 0;
                err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
                X509_STORE_CTX_set_error(ctx, err);
        }
        if (!preverify_ok) {
                printf("verify error:num=%d:%s:depth=%d:%s\n", err,
                    X509_verify_cert_error_string(err), depth, buf);
        } else if (mydata->verbose_mode) {
                printf("depth=%d:%s\n", depth, buf);
        }

        /*
         * At this point, err contains the last verification error.
         * We can use it for something special
         */
        if (!preverify_ok && (err ==
            X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)) {
                X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert),
                    buf, 256);
                printf("issuer= %s\n", buf);
        }

        if (mydata->always_continue)
                return 1;
        else
                return preverify_ok;
}
...

and then the setup side of things, the glue that makes it all work ✨

mydata_t mydata;

...

mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL);

...

SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE,
    verify_callback);

/*
 * Let the verify_callback catch the verify_depth error so that we get
 * an appropriate error in the logfile.
 */
SSL_CTX_set_verify_depth(verify_depth + 1);

/*
 * Set up the SSL specific data into "mydata" and store it into the SSL
 * structure.
 */
mydata.verify_depth = verify_depth; ...
SSL_set_ex_data(ssl, mydata_index, &mydata);

...

SSL_accept(ssl); /* check of success left out for clarity */
if (peer = SSL_get_peer_certificate(ssl)) {
        if (SSL_get_verify_result(ssl) == X509_V_OK) {
                /* The client sent a certificate which verified OK */
        }
}

🌈 SEE ALSO 🌈

go read these next, bestie, they're literally all connected 👑

🕰️ HISTORY 🕰️

a lil ancestry moment, we love a legacy 💅

SSL_set_verify() showed up in SSLeay 0.4 or even earlier. SSL_CTX_set_verify() first appeared in SSLeay 0.6.4. Both functions have been available since OpenBSD 2.4.

SSL_CTX_set_verify_depth() and SSL_set_verify_depth() first appeared in OpenSSL 0.9.3 and have been available since OpenBSD 2.6.

💀 BUGS 💀

okay so these are the messy bits, pay attention fr fr 🙈

In client mode, it is not checked whether the SSL_VERIFY_PEER flag is set, but whether SSL_VERIFY_NONE is not set. This can lead to unexpected behaviour, if the SSL_VERIFY_PEER and SSL_VERIFY_NONE are not used as required (exactly one must be set at any time).
The certificate verification depth set with SSL[_CTX]_verify_depth() stops the verification at a certain depth. The error message produced will be that of an incomplete certificate chain and not X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected.

and that's the whole page, you ate that 🍓 this version came from Debian, dated June 8, 2025 — go off, icon, see you in the next manpage 👑✨💜

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