jws: Add expected algorithms for signatures

[jsha]

Following the example of #69

[valters]

Could you provide a migration guide when upgrading from v3, I have a call like

	parsedJwt, err := jwt.ParseSigned(encodedJwt)
	if err != nil {
		return "", err
	}

Now I need to know what algorithm the encodedJwt is using, but I don't necessarily know what it is or what to expect.
I need to provide a list SignatureAlgorithm to ParseSigned() call, a []SignatureAlgorithm, but is there a simple way to get common ones?

[mcpherrinm]

The full list of algorithims supported is:

go-jose/shared.go

Lines 106 to 118 in 696da50

	EdDSA = SignatureAlgorithm("EdDSA")
	HS256 = SignatureAlgorithm("HS256") // HMAC using SHA-256
	HS384 = SignatureAlgorithm("HS384") // HMAC using SHA-384
	HS512 = SignatureAlgorithm("HS512") // HMAC using SHA-512
	RS256 = SignatureAlgorithm("RS256") // RSASSA-PKCS-v1.5 using SHA-256
	RS384 = SignatureAlgorithm("RS384") // RSASSA-PKCS-v1.5 using SHA-384
	RS512 = SignatureAlgorithm("RS512") // RSASSA-PKCS-v1.5 using SHA-512
	ES256 = SignatureAlgorithm("ES256") // ECDSA using P-256 and SHA-256
	ES384 = SignatureAlgorithm("ES384") // ECDSA using P-384 and SHA-384
	ES512 = SignatureAlgorithm("ES512") // ECDSA using P-521 and SHA-512
	PS256 = SignatureAlgorithm("PS256") // RSASSA-PSS using SHA256 and MGF1-SHA256
	PS384 = SignatureAlgorithm("PS384") // RSASSA-PSS using SHA384 and MGF1-SHA384
	PS512 = SignatureAlgorithm("PS512") // RSASSA-PSS using SHA512 and MGF1-SHA512

What algorithms you want to support depends on your use-case. This parameter was added because it is often unsafe to blindly accept any algorithm.

[valters]

Would it be possible to add a helper function all() or any() that just returns all of them so I don't have to make this list on my side?
It would be helpful to log the algorithm that was detected, so I can figure out what subset I can narrow down to.

[mcpherrinm]

We'd rather not, as that's generally not a safe behaviour if we added new unexpected algorithms, and would potentially be a silent breaking change if we ever removed algorithms.

[jsha]

What is the system that produces the JWTs you're parsing? If it's a system you control, you can look at the source code and see what algorithm it uses to sign its output. If it's not a system you control, it should document the algorithms it uses, and you can use that documentation. And if there's no documentation of that, there's a good reason to file an enhancement request. 😄