install callback to specify TLS 1.3 record padding


 #include <openssl/ssl.h>

 void SSL_CTX_set_record_padding_callback(SSL_CTX *ctx, size_t (*cb)(SSL *s, int type, size_t len, void *arg));
 int SSL_set_record_padding_callback(SSL *ssl, size_t (*cb)(SSL *s, int type, size_t len, void *arg));

 void SSL_CTX_set_record_padding_callback_arg(SSL_CTX *ctx, void *arg);
 void *SSL_CTX_get_record_padding_callback_arg(const SSL_CTX *ctx);

 void SSL_set_record_padding_callback_arg(SSL *ssl, void *arg);
 void *SSL_get_record_padding_callback_arg(const SSL *ssl);

 int SSL_CTX_set_block_padding(SSL_CTX *ctx, size_t block_size);
 int SSL_set_block_padding(SSL *ssl, size_t block_size);


SSL_CTX_set_record_padding_callback() or SSL_set_record_padding_callback() can be used to assign a callback function cb to specify the padding for TLS 1.3 records. The value set in ctx is copied to a new SSL by SSL_new(). Kernel TLS is not possible if the record padding callback is set, and the callback function cannot be set if Kernel TLS is already configured for the current SSL object.

SSL_CTX_set_record_padding_callback_arg() and SSL_set_record_padding_callback_arg() assign a value arg that is passed to the callback when it is invoked. The value set in ctx is copied to a new SSL by SSL_new().

SSL_CTX_get_record_padding_callback_arg() and SSL_get_record_padding_callback_arg() retrieve the arg value that is passed to the callback.

SSL_CTX_set_block_padding() and SSL_set_block_padding() pads the record to a multiple of the block_size. A block_size of 0 or 1 disables block padding. The limit of block_size is SSL3_RT_MAX_PLAIN_LENGTH.

The callback is invoked for every record before encryption. The type parameter is the TLS record type that is being processed; may be one of SSL3_RT_APPLICATION_DATA, SSL3_RT_HANDSHAKE, or SSL3_RT_ALERT. The len parameter is the current plaintext length of the record before encryption. The arg parameter is the value set via SSL_CTX_set_record_padding_callback_arg() or SSL_set_record_padding_callback_arg().

Return Values

The SSL_CTX_get_record_padding_callback_arg() and SSL_get_record_padding_callback_arg() functions return the arg value assigned in the corresponding set functions.

The SSL_CTX_set_block_padding() and SSL_set_block_padding() functions return 1 on success or 0 if block_size is too large.

The cb returns the number of padding bytes to add to the record. A return of 0 indicates no padding will be added. A return value that causes the record to exceed the maximum record size (SSL3_RT_MAX_PLAIN_LENGTH) will pad out to the maximum record size.

The SSL_CTX_get_record_padding_callback_arg() function returns 1 on success or 0 if the callback function is not set because Kernel TLS is configured for the SSL object.


The default behavior is to add no padding to the record.

A user-supplied padding callback function will override the behavior set by SSL_set_block_padding() or SSL_CTX_set_block_padding(). Setting the user-supplied callback to NULL will restore the configured block padding behavior.

These functions only apply to TLS 1.3 records being written.

Padding bytes are not added in constant-time.

See Also

ssl(7), SSL_new(3)


The record padding API was added for TLS 1.3 support in OpenSSL 1.1.1.

The return type of SSL_CTX_set_record_padding_callback() function was changed to int in OpenSSL 3.0.

Referenced By

The man pages SSL_CTX_get_record_padding_callback_arg.3ossl(3), SSL_CTX_set_block_padding.3ossl(3), SSL_CTX_set_record_padding_callback_arg.3ossl(3), SSL_get_record_padding_callback_arg.3ossl(3), SSL_set_block_padding.3ossl(3), SSL_set_record_padding_callback.3ossl(3) and SSL_set_record_padding_callback_arg.3ossl(3) are aliases of SSL_CTX_set_record_padding_callback.3ossl(3).

2023-08-31 3.1.1 OpenSSL