Comment 42 for bug 1273261

I had this problem too and while it still exists for me I have identified a workaround that enables my system to boot. My issue was to do with cryptsetup not accepting my password. This prevents me for booting and fixing system issues.

Turns out that my password included a special character '@' as in P@ssword. At the prompt pressing RIGHT SHIFT and key 2 does not generate a '@' instead a '2' is generated. This can be confirmed by switching over to another TTY and typing in the password (CTRL + ALT + F1). I noted that using LEFT SHIFT and key 2 generated the '@' control character. Thus if LEFT SHIFT is used in complex password it will be typed correctly and cryptsetup will be able to decrypt the volume.

Note the Ubuntu install program rates user password with Poor, Fair, or Good thus encouraging the use of special characters leading to this scenario in the ubuntu recovery mode.