Add basic auth middleware

auth/basic/README.md

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+This package provides a Basic Authentication middleware.
+
+It'll try to compare credentials from Authentication request header to a username/password pair in middleware constructor.
+
+More details about this type of authentication can be found in [Mozilla article](https://developer.mozilla.org/en-US/docs/Web/HTTP/Authentication).
+
+## Usage
+
+```go
+import httptransport "github.com/go-kit/kit/transport/http"
+
+httptransport.NewServer(
+		AuthMiddleware(cfg.auth.user, cfg.auth.password, "Example Realm")(makeUppercaseEndpoint()),
+		decodeMappingsRequest,
+		httptransport.EncodeJSONResponse,
+		httptransport.ServerBefore(httptransport.PopulateRequestContext),
+	)
+```
+
+For AuthMiddleware to be able to pick up the Authentication header from an HTTP request we need to pass it through the context with something like ```httptransport.ServerBefore(httptransport.PopulateRequestContext)```.

auth/basic/middleware.go

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+package basic
+
+import (
+	"bytes"
+	"context"
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/base64"
+	"fmt"
+	"net/http"
+	"strings"
+
+	"github.com/go-kit/kit/endpoint"
+	httptransport "github.com/go-kit/kit/transport/http"
+)
+
+// AuthError represents an authorization error.
+type AuthError struct {
+	Realm string
+}
+
+// StatusCode is an implementation of the StatusCoder interface in go-kit/http.
+func (AuthError) StatusCode() int {
+	return http.StatusUnauthorized
+}
+
+// Error is an implementation of the Error interface.
+func (AuthError) Error() string {
+	return http.StatusText(http.StatusUnauthorized)
+}
+
+// Headers is an implementation of the Headerer interface in go-kit/http.
+func (e AuthError) Headers() http.Header {
+	return http.Header{
+		"Content-Type":           []string{"text/plain; charset=utf-8"},
+		"X-Content-Type-Options": []string{"nosniff"},
+		"WWW-Authenticate":       []string{fmt.Sprintf(`Basic realm=%q`, e.Realm)},
+	}
+}
+
+// parseBasicAuth parses an HTTP Basic Authentication string.
+// "Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==" returns ([]byte("Aladdin"), []byte("open sesame"), true).
+func parseBasicAuth(auth string) (username, password []byte, ok bool) {
+	const prefix = "Basic "
+	if !strings.HasPrefix(auth, prefix) {
+		return
+	}
+	c, err := base64.StdEncoding.DecodeString(auth[len(prefix):])
+	if err != nil {
+		return
+	}
+
+	s := bytes.IndexByte(c, ':')
+	if s < 0 {
+		return
+	}
+	return c[:s], c[s+1:], true
+}
+
+// Returns a hash of a given slice.
+func toHashSlice(s []byte) []byte {
+	hash := sha256.Sum256(s)
+	return hash[:]
+}
+
+// AuthMiddleware returns a Basic Authentication middleware for a particular user and password.
+func AuthMiddleware(requiredUser, requiredPassword, realm string) endpoint.Middleware {
+	requiredUserBytes := toHashSlice([]byte(requiredUser))
+	requiredPasswordBytes := toHashSlice([]byte(requiredPassword))
+
+	return func(next endpoint.Endpoint) endpoint.Endpoint {
+		return func(ctx context.Context, request interface{}) (interface{}, error) {
+			auth, ok := ctx.Value(httptransport.ContextKeyRequestAuthorization).(string)
+			if !ok {
+				return nil, AuthError{realm}
+			}
+
+			givenUser, givenPassword, ok := parseBasicAuth(auth)
+			if !ok {
+				return nil, AuthError{realm}
+			}
+
+			givenUserBytes := toHashSlice(givenUser)
+			givenPasswordBytes := toHashSlice(givenPassword)
+
+			if subtle.ConstantTimeCompare(givenUserBytes, requiredUserBytes) == 0 ||
+				subtle.ConstantTimeCompare(givenPasswordBytes, requiredPasswordBytes) == 0 {
+				return nil, AuthError{realm}
+			}
+
+			return next(ctx, request)
+		}
+	}
+}

auth/basic/middleware_test.go

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+package basic
+
+import (
+	"context"
+	"encoding/base64"
+	"fmt"
+	"testing"
+
+	httptransport "github.com/go-kit/kit/transport/http"
+)
+
+func TestWithBasicAuth(t *testing.T) {
+	requiredUser := "test-user"
+	requiredPassword := "test-pass"
+	realm := "test realm"
+
+	type want struct {
+		result interface{}
+		err    error
+	}
+	tests := []struct {
+		name       string
+		authHeader interface{}
+		want       want
+	}{
+		{"Isn't valid with nil header", nil, want{nil, AuthError{realm}}},
+		{"Isn't valid with non-string header", 42, want{nil, AuthError{realm}}},
+		{"Isn't valid without authHeader", "", want{nil, AuthError{realm}}},
+		{"Isn't valid for wrong user", makeAuthString("wrong-user", requiredPassword), want{nil, AuthError{realm}}},
+		{"Isn't valid for wrong password", makeAuthString(requiredUser, "wrong-password"), want{nil, AuthError{realm}}},
+		{"Is valid for correct creds", makeAuthString(requiredUser, requiredPassword), want{true, nil}},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			ctx := context.WithValue(context.TODO(), httptransport.ContextKeyRequestAuthorization, tt.authHeader)
+
+			result, err := AuthMiddleware(requiredUser, requiredPassword, realm)(passedValidation)(ctx, nil)
+			if result != tt.want.result || err != tt.want.err {
+				t.Errorf("WithBasicAuth() = result: %v, err: %v, want result: %v, want error: %v", result, err, tt.want.result, tt.want.err)
+			}
+		})
+	}
+}
+
+func makeAuthString(user string, password string) string {
+	data := []byte(fmt.Sprintf("%s:%s", user, password))
+	return fmt.Sprintf("Basic %s", base64.StdEncoding.EncodeToString(data))
+}
+
+func passedValidation(ctx context.Context, request interface{}) (response interface{}, err error) {
+	return true, nil
+}