Add documentation of PKCS12_newpass()

PR#4478
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NStephen Henson <steve@openssl.org>

doc/crypto/PKCS12_newpass.pod

+=pod
+
+=head1 NAME
+
+PKCS12_newpass - change the password of a PKCS12 structure
+
+=head1 SYNOPSIS
+
+ #include <openssl/pkcs12.h>
+
+ int PKCS12_newpass(PKCS12 *p12, char *oldpass, char *newpass);
+
+=head1 DESCRIPTION
+
+PKCS12_newpass() changes the password of a PKCS12 structure.
+
+B<p12> is a pointer to a PKCS12 structure. B<oldpass> is the existing password
+and B<newpass> is the new password.
+
+=head1 RETURN VALUES
+
+PKCS12_newpass() returns 1 on success or 0 on failure. Applications can
+retrieve the most recent error from PKCS12_newpass() with ERR_get_error().
+
+=head1 EXAMPLE
+
+This example loads a PKCS#12 file, changes its password and writes out
+the result to a new file.
+
+ #include <stdio.h>
+ #include <stdlib.h>
+ #include <openssl/pem.h>
+ #include <openssl/err.h>
+ #include <openssl/pkcs12.h>
+
+ int main(int argc, char **argv)
+ {
+    FILE *fp;
+    PKCS12 *p12;
+    if (argc != 5) {
+        fprintf(stderr, "Usage: pkread p12file password newpass opfile\n");
+        return 1;
+    }
+    if ((fp = fopen(argv[1], "rb")) == NULL) {
+        fprintf(stderr, "Error opening file %s\n", argv[1]);
+        return 1;
+    }
+    p12 = d2i_PKCS12_fp(fp, NULL);
+    fclose(fp);
+    if (p12 == NULL) {
+        fprintf(stderr, "Error reading PKCS#12 file\n");
+        ERR_print_errors_fp(stderr);
+        return 1;
+    }
+    if (PKCS12_newpass(p12, argv[2], argv[3]) == 0) {
+        fprintf(stderr, "Error changing password\n");
+        ERR_print_errors_fp(stderr);
+        PKCS12_free(p12);
+        return 1;
+    }
+    if ((fp = fopen(argv[4], "wb")) == NULL) {
+        fprintf(stderr, "Error opening file %s\n", argv[4]);
+        PKCS12_free(p12);
+        return 1;
+    }
+    i2d_PKCS12_fp(fp, p12);
+    PKCS12_free(p12);
+    fclose(fp);
+    return 0;
+ }
+
+
+=head1 NOTES
+
+If the PKCS#12 structure does not have a password, then you must use the empty
+string "" for B<oldpass>. Using NULL for B<oldpass> will result in a
+PKCS12_newpass() failure.
+
+If the wrong password is used for B<oldpass> then the function will fail,
+with a MAC verification error. In rare cases the PKCS12 structure does not
+contain a MAC: in this case it will usually fail with a decryption padding
+error.
+
+=head1 BUGS
+
+The password format is a NULL terminated ASCII string which is converted to
+Unicode form internally. As a result some passwords cannot be supplied to
+this function.
+
+=head1 SEE ALSO
+
+L<PKCS12_create(3)>, L<ERR_get_error(3)>
+
+=cut