Cryptography and SSL/TLS Toolkit



SSL_CTX_set_default_passwd_cb, SSL_CTX_set_default_passwd_cb_userdata, SSL_set_default_passwd_cb, SSL_set_default_passwd_cb_userdata - set passwd callback for encrypted PEM file handling


 #include <openssl/ssl.h>

 void SSL_CTX_set_default_passwd_cb(SSL_CTX *ctx, pem_password_cb *cb);
 void SSL_CTX_set_default_passwd_cb_userdata(SSL_CTX *ctx, void *u);
 void SSL_set_default_passwd_cb(SSL *s, pem_password_cb *cb);
 void SSL_set_default_passwd_cb_userdata(SSL *s, void *u);

 int pem_passwd_cb(char *buf, int size, int rwflag, void *userdata);


SSL_CTX_set_default_passwd_cb() sets the default password callback called when loading/storing a PEM certificate with encryption.

SSL_CTX_set_default_passwd_cb_userdata() sets a pointer to userdata which will be provided to the password callback on invocation.

SSL_set_default_passwd_cb() and SSL_set_default_passwd_cb_userdata() perform the same function as their SSL_CTX counterparts, but using an SSL object.

The pem_passwd_cb(), which must be provided by the application, hands back the password to be used during decryption. On invocation a pointer to userdata is provided. The pem_passwd_cb must write the password into the provided buffer buf which is of size size. The actual length of the password must be returned to the calling function. rwflag indicates whether the callback is used for reading/decryption (rwflag=0) or writing/encryption (rwflag=1).


When loading or storing private keys, a password might be supplied to protect the private key. The way this password can be supplied may depend on the application. If only one private key is handled, it can be practical to have pem_passwd_cb() handle the password dialog interactively. If several keys have to be handled, it can be practical to ask for the password once, then keep it in memory and use it several times. In the last case, the password could be stored into the userdata storage and the pem_passwd_cb() only returns the password already stored.

When asking for the password interactively, pem_passwd_cb() can use rwflag to check, whether an item shall be encrypted (rwflag=1). In this case the password dialog may ask for the same password twice for comparison in order to catch typos, that would make decryption impossible.

Other items in PEM formatting (certificates) can also be encrypted, it is however not usual, as certificate information is considered public.


These functions do not provide diagnostic information.


The following example returns the password provided as userdata to the calling function. The password is considered to be a '\0' terminated string. If the password does not fit into the buffer, the password is truncated.

 int pem_passwd_cb(char *buf, int size, int rwflag, void *password)
  strncpy(buf, (char *)(password), size);
  buf[size - 1] = '\0';


ssl, SSL_CTX_use_certificate