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DESCRIPTION

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

The password callback 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 password callback 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 the callback 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 callback only returns the password already stored.

When asking for the password interactively, the callback 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 which would make decryption impossible.

Other items in PEM formatting (certificates) can also be encrypted; it is however atypical, as certificate information is considered public.

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