Add asymmetric JWT signing

[KN4CK3R]

Close #15912

Added asymmetric JWT signing. Supports HS256 (already implemented), HS384, HS512, RS256, RS384, RS512, ES256, ES384 and ES512.

The default signing algorithm is changed from HS256 (symmetric encryption) to RS256 (asymmetric encryption).

⚠️ BREAKING ⚠️

• Asymmetric algorithms require a secret asymmetric key pair to be in JWT_SIGNING_PRIVATE_KEY_FILE (by default APP_DATA_PATH/jwt), a pair will be generated if it is not present. (NB: this was originally in CUSTOM_PATH but was changed by Fix mkdir jwt - permission denied #16227)
• The original symmetric JWT_SECRET will only be used if JWT_SIGNING_ALGORITHM is set to HS256 (previous default), HS384 or HS512.
• As a result of the change of algorithm gitea OAuth2 tokens (and potentially client secrets) will need to be regenerated unless you change your JWT_SIGNING_ALGORITHM back to HS256.
• Legacy installations of Drone assume a short key length, however as no automated migrations are run by Drone you'll need to do them manually if the column type for user_oauth_token and user_oauth_refresh are limited to 500 characters:

-- an example manual migration for Drone database connected to postgresql
alter table users alter column user_oauth_token type bytea using convert_to(user_oauth_token, 'LATIN1');
alter table users alter column user_oauth_refresh type bytea using convert_to(user_oauth_refresh, 'LATIN1');

or see: #16010 (comment)

[codecov-commenter]

Codecov Report

Merging #16010 (3cfaecc) into main (1ec9e90) will decrease coverage by 0.06%.
The diff coverage is 32.69%.

@@            Coverage Diff             @@
##             main   #16010      +/-   ##
==========================================
- Coverage   44.57%   44.50%   -0.07%     
==========================================
  Files         700      701       +1     
  Lines       82867    83092     +225     
==========================================
+ Hits        36940    36984      +44     
- Misses      39936    40105     +169     
- Partials     5991     6003      +12     

Impacted Files	Coverage Δ
cmd/generate.go	0.00% <0.00%> (ø)
models/oauth2.go	28.00% <0.00%> (-1.17%)	⬇️
modules/generate/generate.go	0.00% <0.00%> (-16.00%)	⬇️
modules/setting/lfs.go	52.94% <0.00%> (ø)
routers/install/install.go	0.00% <0.00%> (ø)
routers/web/user/oauth.go	36.53% <26.47%> (-2.43%)	⬇️
modules/auth/oauth2/jwtsigningkey.go	32.50% <32.50%> (ø)
models/oauth2_application.go	69.39% <88.88%> (+0.21%)	⬆️
modules/setting/setting.go	50.28% <100.00%> (-1.23%)	⬇️
routers/web/web.go	91.65% <100.00%> (+0.01%)	⬆️
... and 7 more

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[techknowlogick]

per discussion in chat, these changed lines are fine due to the fact that it cleans up line endings (shakes fist at windows)

[techknowlogick]

Thank you so much so far on your work on this PR!!

I was able to sign into Sourcegraph using this PR, however testing this PR more I found that 1. sourcegraph may not be using the cert to validate the signature on this token, 2. possibly the upstream jwt library may not be signing the token correctly.

[KN4CK3R]

Currently only 32 bytes are allowed. Should we really enforce this? Smaller keys are valid and larger keys are hashed by the internal hashing algorithm to create a secret with the correct length.
https://github.com/golang/go/blob/2ebe77a2fda1ee9ff6fd9a3e08933ad1ebaea039/src/crypto/hmac/hmac.go#L148-L152

[proton-ab]

Friendly warning to anyone out there using Drone - it fails to support new tokens which are too long
"msg": "cannot update user: pq: value too long for type character varying(500)",

[markeissler]

TL;DR: For Drone compatibility... Set the gitea environment variable to:

GITEA__OAUTH2__JWT_SIGNING_ALGORITHM=HS256

And restart gitea - OR - alter your database to accommodate the longer value that needs to be stored. Something like the following might work, or perhaps set the column type to TEXT instead of BYTEA.

ALTER TABLE users ALTER COLUMN user_oauth_token BYTEA;

I would suggest modifying the column in the drone db for production environments since the intention is to improve security with asymmetric tokens. By default, the column is setup as a varchar(500) type.

[KN4CK3R]

A shorter RSA key may help too. The default key size is 4096 bit.
The drone problem was fixed in this commit two months ago:
harness/harness@eaadd82

[6543]

thanks for the hint, for more issues etc please open a new issues