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Used to work until very recently, but now it doesn't. I have looked through the source of the Niconico page and indeed it doesn't contain a string like 'thumbPlayKey'.

I can also confirm, I was just about to open a ticket myself. Although it seems to work with a few videos. E.g I was able to download this .swf
http://www.nicovideo.jp/watch/nm3601701

But couldn't download other videos:
http://www.nicovideo.jp/watch/sm8628149

Many nicovideos seem to have began requiring login account to download.
I succeeded to download most of nicovideos by adding following options to youtube-dl.
--username {username} --password {password}
Of course, {username},{password} must be registered on nicovideo site.

Niconico requires login for viewing many (most?) videos since a long time. However:

• They do work without login when viewed via an embed player on other sites
• This never prevented youtube-dl from working without login on all of them.

Same here. Stopped working for me.

I have an imcomplete fix. Those who are interested can check https://github.com/yan12125/youtube-dl/tree/wip-niconico

I can confirm that if you provide your niconico password and username with -u and -p options, then the download works fine. Still would be nice to fix downloading without that, as not everyone wants to sign up on every site.

They do work without login when viewed via an embed player on other sites

@romanrm When I was implementing the incomplete fix 2 months ago, there are indeed some videos available via embedded players. Unfortunately, apparently all such ways are blocked now. So I just remove the old codes and add a clearer error message.

If anyone knows how to play Niconico videos without an account, feel free to open a new issue.

Can't download NicoNico videos even when credentials are provided

http://www.nicovideo.jp/watch/sm27643036 works fine here:

Are you using Twitter/Facebook/Nintendo accounts? Currently only NicoNico accounts are supported.

Nope this is a totally new account directly registered on niconico.

Also putting my credentials in ~/.netrc works and I can download from niconico(I just checked after seeing the netrc argument in your command), but if I pass the username and password then youtube-dl gives the error

Sorry for the weird censoring, I am just hiding my password. As you can see in the screenshot the login credentials from the ~/.netrc file work properly but give out an error when passed as arguments.

--username and --password works for me, too

I guess there are some special characters in your password and your shell does not pass it correctly?

nope my password is a mix of small/capital numbers/alphabets no symbols at all.

Edit: Your 2017.02.24.1 version seems to be newer than mine 2017.02.22

Edit 2: checked the diff between both our versions and I can't see anything that might cause the issue I am having and you are not

That's strange. --netrc works for you but --username and --password not?

Try to change the line 901 of /usr/lib/python3.6/site-packages/youtube_dl/options.py, replace _hide_login_info(conf) with simply conf and see whether the passwords are identical or not for --netrc and --username/--password.

yep the password being passed through the argument is correct (same as the one in netrc).

Can't imagine why... Fortunately .netrc works.

yep fortunately it works, bugs they sure are weird.

P.S upgraded to the youtube-dl-git version from AUR and the issue is still there.

Windows user; unable to use '.netrc workaround.' Have tested with currently available version (just upgraded to be sure); have niconico account; -u / -p command line options don't work.

[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['--ignore-errors', '-v']
[debug] Encodings: locale cp1252, fs mbcs, out cp437, pref cp1252
[debug] youtube-dl version 2017.05.23
[debug] Python version 3.4.4 - Windows-Vista-6.0.6002-SP2
[debug] exe versions: avconv v13_dev0-753-gba6a49e, avprobe v13_dev0-753-gba6a49e, ffmpeg
1.2
[debug] Proxy map: {}
Usage: YouTube-dl.exe [OPTIONS] URL [URL...]

youtube-dl.exe -u <user> -p <pass> http://www.nicovideo.jp/watch/nm3362855
[niconico] Logging in
[niconico] nm3362855: Downloading webpage
[niconico] nm3362855: Downloading video info page
[niconico] nm3362855: Downloading flv info
ERROR: Niconico videos now require logging in

FWIW I used the e-mail address for the username field and it works.