Add jwt.Keyfunc utilities and partial JWK support

keyfunc/README.md

@@ -0,0 +1,146 @@
+# keyfunc
+
+The purpose of this package is to provide a
+[`jwt.Keyfunc`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4#Keyfunc). This function is for parsing and validating
+JWTs.
+
+This package can consume a JSON Web Key Set (JWKs) and or a given set of key ID, `kid` to cryptographic key mappings.
+This package can additionally use a given mapping of key IDs, `kid`, to cryptographic keys parse and validate JWTs.
+
+It's common for an identity provider, such as [Keycloak](https://www.keycloak.org/)
+or [Amazon Cognito (AWS)](https://aws.amazon.com/cognito/) to expose a JWKs via an HTTPS endpoint. It is important that
+a JWKs endpoint is using HTTPS to ensure the keys are from the correct trusted source.
+
+## Supported Algorithms
+
+Currently, this package supports JWTs signed with an `alg` that matches one of the following:
+* `ES256`
+* `ES384`
+* `ES512`
+* `HS256`
+* `HS384`
+* `HS512`
+* `PS256`
+* `PS384`
+* `PS512`
+* `RS256`
+* `RS384`
+* `RS512`
+
+Additionally, the supported elliptical curve types are below:
+* `P-256`
+* `P-384`
+* `P-521`
+
+This _does_ include HMAC keys. For an example using HMAC keys, please see the `examples/hmac` directory. Do _not_ expose
+HMAC keys in public facing JWKs, as HMAC keys are secret keys that do not use public key cryptography.
+
+If there are cryptographic algorithms, curve types, or something else already standardized that you'd like supported in
+this Go package, please open an issue or pull request.
+
+## Basic usage
+
+For complete examples, please see the `examples` directory.
+
+```go
+import "github.com/golang-jwt/jwt/v4/keyfunc"
+```
+
+### Preconditions: Acquire the JWKs URL, JSON, or gather cryptographic keys (given keys)
+
+A JWKs URL is not required, one can be created directly from JSON with the
+[`keyfunc.New`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4/keyfunc#NewJSON) function.
+
+```go
+// Get the JWKs URL from an environment variable.
+jwksURL := os.Getenv("JWKS_URL")
+
+// Confirm the environment variable is not empty.
+if jwksURL == "" {
+	log.Fatalln("JWKS_URL environment variable must be populated.")
+}
+```
+
+### Step 1: Create the JWKs
+
+Via HTTP:
+```go
+// Create the JWKs from the resource at the given URL.
+jwks, err := keyfunc.Get(jwksURL)
+if err != nil {
+	log.Fatalf("Failed to get the JWKs from the given URL.\nError:%s\n", err.Error())
+}
+```
+Via JSON:
+```go
+// Get the JWKs as JSON.
+var jwksJSON = json.RawMessage(`{"keys":[{"kid":"zXew0UJ1h6Q4CCcd_9wxMzvcp5cEBifH0KWrCz2Kyxc","kty":"RSA","alg":"PS256","use":"sig","n":"wqS81x6fItPUdh1OWCT8p3AuLYgFlpmg61WXp6sp1pVijoyF29GOSaD9xE-vLtegX-5h0BnP7va0bwsOAPdh6SdeVslEifNGHCtID0xNFqHNWcXSt4eLfQKAPFUq0TsEO-8P1QHRq6yeG8JAFaxakkaagLFuV8Vd_21PGJFWhvJodJLhX_-Ym9L8XUpIPps_mQriMUOWDe-5DWjHnDtfV7mgaOxbBvVo3wj8V2Lmo5Li4HabT4MEzeJ6e9IdFo2kj_44Yy9osX-PMPtu8BQz_onPgf0wjrVWt349Rj6OkS8RxlNGYeuIxYZr0TOhP5F-yEPhSXDsKdVTwPf7zAAaKQ","e":"AQAB","x5c":["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"],"x5t":"IYIeevIT57t8ppUejM42Bqx6f3I","x5t#S256":"TuOrBy2NcTlFSWuZ8Kh8W8AjQagb4fnfP1SlKMO8-So"},{"kid":"ebJxnm9B3QDBljB5XJWEu72qx6BawDaMAhwz4aKPkQ0","kty":"EC","alg":"ES512","use":"sig","crv":"P-521","x":"YQ95Xj8MTzcHytbU1h8YkCN2kdEQA7ThuZ1ctB9Ekiw6tlM9RwL62eQvzEt4Rz8qN69uRqgU9RzxQOkSU5xVvyo","y":"SMMuP3QnAPHtx7Go2ARsG3NBaySWBLmVvS8s2Ss7Vm_ISWenNbdjKOsY1XvtiQz5scGzWDCEUoZzgV8Ve1mLOV0"},{"kid":"TVAAet63O3xy_KK6_bxVIu7Ra3_z1wlB543Fbwi5VaU","kty":"EC","alg":"ES384","use":"sig","crv":"P-384","x":"Pik2o5as-evijFABH5p6YLXHnWw8iQ_N1ummPY1c_UgG6NO0za-gNOhTz2-tsd_w","y":"e98VSff71k19SY_mHgp3707lgQVrhfVpiGa-sGaKxOWVpxd2jWMhB0Q4RpSRuCp5"},{"kid":"arlUxX4hh56rNO-XdIPhDT7bqBMqcBwNQuP_TnZJNGs","kty":"RSA","alg":"RS512","use":"sig","n":"hhtifu8LL3ICE3BAX5l1KZv6Lni0lhlhBusSfepnpxcb4C_z2U71cQTnLY27kt8WB4bNG6e5_KMx9K3xUdd3euj9MCq8vytwEPieeHE1KXQuhJfLv017lhpK_dRMOHyc-9-50YNdgs_8KWRkrzjjuYrCiO9Iu76n5319e-SC8OPvNUglqxp2N0Sp2ltne2ZrpN8T3OEEXT62TSGmLAVopRGw5gllNVrJfmEyZJCRrBM6s5CQcz8un0FjkAAC4DI6QD-eBL0qG3_NR0hQvR1he2o4BLwjOKH45Pk_jj-eArp-DD6Xq6ABQVb5SNOSdaxl5lnmuotRoY3G5d9YSl-K3w","e":"AQAB","x5c":["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"],"x5t":"geiCPGtT_10T8xGLUK1LA0_YQEE","x5t#S256":"dLp3_QNGwMbYll5VecnR8Q9NSeFVfqJPBTa2_8qf48I"},{"kid":"tW6ae7TomE6_2jooM-sf9N_6lWg7HNtaQXrDsElBzM4","kty":"RSA","alg":"PS512","use":"sig","n":"p32N7jqKfMUB6_dKY1uZ3wizzPlBAXg9XrntfUcwNLRPfTBnshpt4uQBf3T8fexkbzhtR18oHvim-YvcWfC5eLGQmWHYiVwACa_C7oGqx51ijK2LRbUg4TKhnZX2X3Ld9xvr3HsosKh2UXn_Ay8nuvdfH-U6S7btT6a-AIFlt3BpqZP0EOl7rY-ie8nXoA13xX6BoyzYiNcugdYCU6czQcmTIJ1JLS0zohi4aTNehRt-1VMRpIMx7q7Ouq3Zhbi7RcDo-_D8FPRhWc2eEKd-h8ebFTIxEOrkguBIomjEFTf3SfYbOB_h-14v9Q2yz-NzyId3-ujRCQGC0hn-cixe2w","e":"AQAB","x5c":["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"],"x5t":"f1l1fxICz1fe9mI-sSrtc19EDhU","x5t#S256":"NUJWRA4ADpLEg_SMkSoE4FKQN0H1Tlz85L-i7puVcqQ"},{"kid":"Lx1FmayP2YBtxaqS1SKJRJGiXRKnw2ov5WmYIMG-BLE","kty":"RSA","alg":"PS384","use":"sig","n":"q7WM4SnrdzlFSo_A1DRhc-8Ho-pBsfs49kGRbw3O_OKFIUyZrzHaRuovW_QaEAyiO3HX8CNcGPcpHdmpl4DhTGEBLcd6xXtCaa65ct00Mq7ZHCRRCrKLh6lJ0rY9fP8vCV0RBigpkNoRfrqLQQN4VeVFTbGSrDaS0LzPbap0-q5FKXUR-OQmQEtOupXhKFQtbB73tL83YnG6Swl7nXsx54ulEoDzcCCYt7pjCVVp7L9fzI2_ucTdtQclAJVQZGKpsx7vabOJuiMUwuAIz56lOJyXRMePsW8UogwC4FA2A52STsYlhOPsDEW4iIExFVNqs-CGoDGhYLIavaCkZhXM0w","e":"AQAB","x5c":["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"],"x5t":"fjRYR1986VCLzbaZaw5r25UKahw","x5t#S256":"ZHNHpizlsjD3qSZh7gJQQBu8W9jBL2HR0y7-3u2Wb-g"},{"kid":"gnmAfvmlsi3kKH3VlM1AJ85P2hekQ8ON_XvJqs3xPD8","kty":"RSA","alg":"RS384","use":"sig","n":"qUNQewKl3APQcbpACMNJ2XphPpupt395z6OZvj5CW9tiRXY3J7dqi8U0bWoIhtmmc7Js6hjp-A5W_FVStuXlT1hLyjJsHeu9ZVPnfIl2MnYN83zQBKw8E4mFsVv0UXNvkVPBF_k0yXrz-ABleWLOgFGnkNU9csc3Z5aihHcwRmC_oS7PZ9Vc-l0xBCyF3YRHI-al8ppSHwFreOweF3-JP3poNAXd906_tjX2KlHSJmNqcUNiSfEluyCp02ALlRFKXUQ1HlfSupHcHySDlanfUyIzZgM9ysCvC1vfNdAuwZ44oUBMul_XPxxhzlewL2Y8PtSDLUDWGTIou8M8049D8Q","e":"AQAB","x5c":["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"],"x5t":"CmRnQVduZWtEsdOC4mauUUsSWxA","x5t#S256":"BvC0LmuM8ZIApN3TQQZWWbGO-d082Ah5d3D6vPvahGw"},{"kid":"CGt0ZWS4Lc5faiKSdi0tU0fjCAdvGROQRGU9iR7tV0A","kty":"EC","alg":"ES256","use":"sig","crv":"P-256","x":"DPW7n9yjfE6Rt-VvVmEdeu4QdW44qifocAPPDxACDDY","y":"-ejsVw8222-hg2dJWx3QV0hE4-I0Ujp7ZsWebE68JE0"},{"kid":"C65q0EKQyhpd1m4fr7SKO2He_nAxgCtAdws64d2BLt8","kty":"RSA","alg":"RS256","use":"sig","n":"ja99ybDrLvw11Z4CvNlDI-kkqJEBpSnvDf0pZF2DvBlvYmeVYL_ChqIe8E9GyHUmLMdtO_jifSgOqE5b8vILwi1kZnJR7N857uEnbWM9YTeevi_RZ-E_hr4frW2NKJ78YGvCzwLKG2GgtSjj0zuTLnSaK8fCGzqXgy6paXNhgHUSZgGwvO0YItpMlyJeqEj1wGTWz1IyA1sguF1cC7K0fojPbPoBwrhvaAeoGRPLraE0rrBsQv8iiLwnRBIez9B1j0NiUG8Iad953Y7UzaKOAw8crIEK45NIK_yxHUpxqcHLjPIcRyIyJGioRyGK7cp-_7iPLOCutQc-u46mom1_ZQ","e":"AQAB","x5c":["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"],"x5t":"5GNr3LeRXHWI4YR8-QTSsF98oTI","x5t#S256":"Dgd0_wZZqvRuf4GEISPNHREX-1ixTMIsrPeGzk0bCxs"}]}`)
+
+// Create the JWKs from the resource at the given URL.
+jwks, err := keyfunc.NewJSON(jwksJSON)
+if err != nil {
+	log.Fatalf("Failed to create JWKs from JSON.\nError:%s\n", err.Error())
+}
+```
+Via a given key:
+```go
+// Get an HMAC key.
+key := []byte("example secret")
+uniqueKeyID := "myKeyID"
+
+// Create the JWKs from the HMAC key.
+jwks := keyfunc.NewGiven(map[string]keyfunc.GivenKey{
+	uniqueKeyID: keyfunc.NewGivenHMAC(key),
+})
+```
+
+Additional options can be passed to the [`keyfunc.Get`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4/keyfunc#Get)
+function via variadic arguments. See
+[`keyfunc.Options`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4/keyfunc#Options) and the additional features
+mentioned at the bottom of this `README.md`.
+
+### Step 2: Use the [`JWKs.Keyfunc`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4/keyfunc#JWKs.Keyfunc) method as the [`jwt.Keyfunc`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4#Keyfunc) when parsing tokens
+
+```go
+// Parse the JWT.
+token, err := jwt.Parse(jwtB64, jwks.Keyfunc)
+if err != nil {
+	return nil, fmt.Errorf("failed to parse token: %w", err)
+}
+```
+
+The [`JWKs.Keyfunc`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4/keyfunc#JWKs.Keyfunc) method will automatically
+select the key with the matching `kid` (if present) and return its public key as the correct Go type to its caller.
+
+## Test coverage
+
+Test coverage is currently `>90%`.
+
+Testing could be improved by signing all JWTs during the tests themselves. Alternatively, using JWTs that do not expire
+would accomplish the same purpose. There are some hard-coded JWTs which are expired. This means the tests with hard
+coded JWTs cannot check for parsing and validation errors, just errors within the `jwt.Keyfunc` itself.
+
+## Additional features
+
+* A background refresh of the JWKs keys can be performed. This is possible by passing
+  [`keyfunc.Options`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4/keyfunc#Options) via a variadic argument to the
+  [`keyfunc.Get`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4/keyfunc#Get) function.
+	* A custom background refresh interval can be specified. For an example, please see the `examples/interval`
+	  directory.
+	* A custom background refresh request context timeout can be specified. Defaults to one minute. For an example,
+	  please see the `examples/ctx` directory.
+	* A custom background refresh error handling function can be specified. If none is specified, errors go unhandled
+	  silently. For an example, please see the `examples/recommended_options` directory.
+	* A custom rate limit can be specified to prevent too many requests for a JWKs refresh. For an example, please see
+	  the `examples/recommended_options` directory.
+	* JWTs with a previously unseen `kid` can prompt an automatic refresh of the remote JWKs resource. This should be
+	  paired with `RefreshRateLimit` to prevent abuse. For an example, please see the `examples/recommended_options`
+	  directory.
+* A custom HTTP client can be used. This is possible by passing
+  [`keyfunc.Options`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4/keyfunc#Options) via a variadic argument to the
+  [`keyfunc.Get`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4/keyfunc#Get) function.
+* A map of JWT key IDs (`kid`) to keys can be given and used for the `jwt.Keyfunc`. For an example, see
+  the `examples/given` directory.
+* Custom cryptographic algorithms can be used. Make sure to
+  use [`jwt.RegisterSigningMethod`](https://pkg.go.dev/github.com/golang-jwt/jwt/v4#RegisterSigningMethod) before
+  parsing JWTs. For an example, see the `examples/custom` directory.

keyfunc/ecdsa.go

@@ -0,0 +1,94 @@
+package keyfunc
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"encoding/base64"
+	"fmt"
+	"math/big"
+)
+
+const (
+
+	// es256 represents a public cryptography key generated by an SHA-256 hash and an ECDSA algorithm.
+	es256 = "ES256"
+
+	// es384 represents a public cryptography key generated by an SHA-384 hash and an ECDSA algorithm.
+	es384 = "ES384"
+
+	// es512 represents a public cryptography key generated by an SHA-512 hash and an ECDSA algorithm.
+	es512 = "ES512"
+
+	// p256 represents a 256-bit cryptographic elliptical curve type.
+	p256 = "P-256"
+
+	// p384 represents a 384-bit cryptographic elliptical curve type.
+	p384 = "P-384"
+
+	// p521 represents a 521-bit cryptographic elliptical curve type.
+	p521 = "P-521"
+)
+
+// ECDSA parses a jsonKey and turns it into an ECDSA public key.
+func (j *jsonKey) ECDSA() (publicKey *ecdsa.PublicKey, err error) {
+
+	// Check if the key has already been computed.
+	j.precomputedMux.RLock()
+	if j.precomputed != nil {
+		var ok bool
+		if publicKey, ok = j.precomputed.(*ecdsa.PublicKey); ok {
+			j.precomputedMux.RUnlock()
+			return publicKey, nil
+		}
+	}
+	j.precomputedMux.RUnlock()
+
+	// Confirm everything needed is present.
+	if j.X == "" || j.Y == "" || j.Curve == "" {
+		return nil, fmt.Errorf("%w: ecdsa", ErrMissingAssets)
+	}
+
+	// Decode the X coordinate from Base64.
+	//
+	// According to RFC 7518, this is a Base64 URL unsigned integer.
+	// https://tools.ietf.org/html/rfc7518#section-6.3
+	var xCoordinate []byte
+	if xCoordinate, err = base64.RawURLEncoding.DecodeString(j.X); err != nil {
+		return nil, err
+	}
+
+	// Decode the Y coordinate from Base64.
+	var yCoordinate []byte
+	if yCoordinate, err = base64.RawURLEncoding.DecodeString(j.Y); err != nil {
+		return nil, err
+	}
+
+	// Create the ECDSA public key.
+	publicKey = &ecdsa.PublicKey{}
+
+	// Set the curve type.
+	switch j.Curve {
+	case p256:
+		publicKey.Curve = elliptic.P256()
+	case p384:
+		publicKey.Curve = elliptic.P384()
+	case p521:
+		publicKey.Curve = elliptic.P521()
+	}
+
+	// Turn the X coordinate into *big.Int.
+	//
+	// According to RFC 7517, these numbers are in big-endian format.
+	// https://tools.ietf.org/html/rfc7517#appendix-A.1
+	publicKey.X = big.NewInt(0).SetBytes(xCoordinate)
+
+	// Turn the Y coordinate into a *big.Int.
+	publicKey.Y = big.NewInt(0).SetBytes(yCoordinate)
+
+	// Keep the public key so it won't have to be computed every time.
+	j.precomputedMux.Lock()
+	j.precomputed = publicKey
+	j.precomputedMux.Unlock()
+
+	return publicKey, nil
+}