In addition to being unlockable by the separately described method, Ken's Shoryuken can be obtained by combining a password with a specific button combination. On the password entry screen, set the password to one of the following:

1172 3367 6182 3246 (all obtainables except Zero parts, plus the Shoryuken)
8377 8143 6822 7651 (all obtainables plus the Shoryuken)

In order for the Shoryuken part of the password to work, the password must be submitted by holding down-left, R, B, and Y together. First move the password cursor to the bottom line of the password (above Start), then press and hold down (which moves the cursor to Start), left, R, B, and finally Y.