ALTER USER vasia PASSWORD EXPIRE HTTP DIGEST ENABLE;

SQL*Plus: Release 12.2.0.1.0 Production on Wed Aug 15 17:31:30 2018

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Enter user-name: scott@pdb1sol122
Enter password:
Last Successful login time: Wed Aug 15 2018 13:35:06 -04:00

Connected to:
Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production

SQL> create user u1 identified by u1 default tablespace users quota unlimited on users;

User created.

SQL> grant create session to u1;

Grant succeeded.

SQL> select  *
  2    from  dba_digest_verifiers
  3    where username = 'U1'
  4  /

USERNAME   HAS DIG
---------- --- ---
U1         NO

SQL> alter user u1 password expire http digest enable;

User altered.

SQL> select  *
  2    from  dba_digest_verifiers
  3    where username = 'U1'
  4  /

USERNAME   HAS DIG
---------- --- ---
U1         NO

SQL> connect u1@pdb1sol122/u1
ERROR:
ORA-28001: the password has expired


Changing password for u1
New password:
Retype new password:
Password changed
Connected.
SQL> connect scott@pdb1sol122/tiger
Connected.
SQL> select  *
  2    from  dba_digest_verifiers
  3    where username = 'U1'
  4  /

USERNAME   HAS DIG
---------- --- ---
U1         YES MD5

SQL>