home-services/alexa-bridge/internal/alexa/signature_test.go

package alexa

import (
	"bytes"
	"context"
	"crypto"
	"crypto/rand"
	"crypto/rsa"
	"crypto/sha1" //nolint:gosec // matches Alexa's fixed signing scheme, see signature.go
	"crypto/x509"
	"crypto/x509/pkix"
	"encoding/base64"
	"encoding/pem"
	"math/big"
	"net/http"
	"net/http/httptest"
	"net/url"
	"testing"
	"time"
)

// generateTestChain builds a two-certificate PEM chain (leaf signed by a
// throwaway root) for exercising verifyChain/verifySignature without
// needing real Alexa-issued certificates.
func generateTestChain(t *testing.T, sans []string, notBefore, notAfter time.Time) ([]byte, *rsa.PrivateKey) {
	t.Helper()

	rootKey, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		t.Fatalf("generate root key: %v", err)
	}
	rootTemplate := &x509.Certificate{
		SerialNumber:          big.NewInt(1),
		Subject:               pkix.Name{CommonName: "Test Root CA"},
		NotBefore:             notBefore,
		NotAfter:              notAfter,
		IsCA:                  true,
		KeyUsage:              x509.KeyUsageCertSign | x509.KeyUsageDigitalSignature,
		BasicConstraintsValid: true,
	}
	rootDER, err := x509.CreateCertificate(rand.Reader, rootTemplate, rootTemplate, &rootKey.PublicKey, rootKey)
	if err != nil {
		t.Fatalf("create root cert: %v", err)
	}
	rootCert, err := x509.ParseCertificate(rootDER)
	if err != nil {
		t.Fatalf("parse root cert: %v", err)
	}

	leafKey, err := rsa.GenerateKey(rand.Reader, 2048)
	if err != nil {
		t.Fatalf("generate leaf key: %v", err)
	}
	leafTemplate := &x509.Certificate{
		SerialNumber: big.NewInt(2),
		Subject:      pkix.Name{CommonName: "echo-api.amazon.com"},
		DNSNames:     sans,
		NotBefore:    notBefore,
		NotAfter:     notAfter,
		KeyUsage:     x509.KeyUsageDigitalSignature,
		ExtKeyUsage:  []x509.ExtKeyUsage{x509.ExtKeyUsageAny},
	}
	leafDER, err := x509.CreateCertificate(rand.Reader, leafTemplate, rootCert, &leafKey.PublicKey, rootKey)
	if err != nil {
		t.Fatalf("create leaf cert: %v", err)
	}

	var buf bytes.Buffer
	if err := pem.Encode(&buf, &pem.Block{Type: "CERTIFICATE", Bytes: leafDER}); err != nil {
		t.Fatalf("encode leaf pem: %v", err)
	}
	if err := pem.Encode(&buf, &pem.Block{Type: "CERTIFICATE", Bytes: rootDER}); err != nil {
		t.Fatalf("encode root pem: %v", err)
	}
	return buf.Bytes(), leafKey
}

func parseChain(t *testing.T, chainPEM []byte) []*x509.Certificate {
	t.Helper()
	var chain []*x509.Certificate
	data := chainPEM
	for {
		var block *pem.Block
		block, data = pem.Decode(data)
		if block == nil {
			break
		}
		cert, err := x509.ParseCertificate(block.Bytes)
		if err != nil {
			t.Fatalf("parse certificate: %v", err)
		}
		chain = append(chain, cert)
	}
	return chain
}

func TestValidateCertChainURL(t *testing.T) {
	tests := []struct {
		name    string
		url     string
		wantErr bool
	}{
		{name: "valid", url: "https://s3.amazonaws.com/echo.api/echo-api-cert.pem", wantErr: false},
		{name: "valid with explicit 443 port", url: "https://s3.amazonaws.com:443/echo.api/echo-api-cert.pem", wantErr: false},
		{name: "wrong scheme", url: "http://s3.amazonaws.com/echo.api/echo-api-cert.pem", wantErr: true},
		{name: "wrong host", url: "https://evil.example.com/echo.api/echo-api-cert.pem", wantErr: true},
		{name: "host looks similar but differs", url: "https://s3.amazonaws.com.evil.com/echo.api/echo-api-cert.pem", wantErr: true},
		{name: "wrong port", url: "https://s3.amazonaws.com:8443/echo.api/echo-api-cert.pem", wantErr: true},
		{name: "wrong path prefix", url: "https://s3.amazonaws.com/not-echo-api/echo-api-cert.pem", wantErr: true},
		{name: "unparsable", url: "://bad", wantErr: true},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			err := validateCertChainURL(tt.url)
			if (err != nil) != tt.wantErr {
				t.Fatalf("validateCertChainURL(%q) error = %v, wantErr %v", tt.url, err, tt.wantErr)
			}
		})
	}
}

func TestVerifyChain(t *testing.T) {
	now := time.Now()

	t.Run("valid chain with correct SAN", func(t *testing.T) {
		chainPEM, _ := generateTestChain(t, []string{certSANRequired}, now.Add(-time.Hour), now.Add(time.Hour))
		leaf, err := verifyChain(parseChain(t, chainPEM))
		if err != nil {
			t.Fatalf("verifyChain() error = %v", err)
		}
		if leaf == nil {
			t.Fatal("verifyChain() leaf = nil")
		}
	})

	t.Run("missing required SAN", func(t *testing.T) {
		chainPEM, _ := generateTestChain(t, []string{"not-echo-api.amazon.com"}, now.Add(-time.Hour), now.Add(time.Hour))
		if _, err := verifyChain(parseChain(t, chainPEM)); err == nil {
			t.Fatal("verifyChain() error = nil, want error for missing SAN")
		}
	})

	t.Run("expired certificate", func(t *testing.T) {
		chainPEM, _ := generateTestChain(t, []string{certSANRequired}, now.Add(-2*time.Hour), now.Add(-time.Hour))
		if _, err := verifyChain(parseChain(t, chainPEM)); err == nil {
			t.Fatal("verifyChain() error = nil, want error for expired cert")
		}
	})

	t.Run("not yet valid certificate", func(t *testing.T) {
		chainPEM, _ := generateTestChain(t, []string{certSANRequired}, now.Add(time.Hour), now.Add(2*time.Hour))
		if _, err := verifyChain(parseChain(t, chainPEM)); err == nil {
			t.Fatal("verifyChain() error = nil, want error for not-yet-valid cert")
		}
	})

	t.Run("leaf not actually signed by the presented root", func(t *testing.T) {
		chainA, _ := generateTestChain(t, []string{certSANRequired}, now.Add(-time.Hour), now.Add(time.Hour))
		chainB, _ := generateTestChain(t, []string{certSANRequired}, now.Add(-time.Hour), now.Add(time.Hour))
		mismatched := []*x509.Certificate{parseChain(t, chainA)[0], parseChain(t, chainB)[1]}
		if _, err := verifyChain(mismatched); err == nil {
			t.Fatal("verifyChain() error = nil, want error for mismatched leaf/root")
		}
	})
}

func TestVerifySignature(t *testing.T) {
	now := time.Now()
	chainPEM, leafKey := generateTestChain(t, []string{certSANRequired}, now.Add(-time.Hour), now.Add(time.Hour))
	leaf := parseChain(t, chainPEM)[0]

	body := []byte(`{"request":{"type":"LaunchRequest"}}`)
	hash := sha1.Sum(body) //nolint:gosec
	sig, err := rsa.SignPKCS1v15(rand.Reader, leafKey, crypto.SHA1, hash[:])
	if err != nil {
		t.Fatalf("sign body: %v", err)
	}

	t.Run("valid signature", func(t *testing.T) {
		if err := verifySignature(leaf, body, sig); err != nil {
			t.Fatalf("verifySignature() error = %v", err)
		}
	})

	t.Run("tampered body fails", func(t *testing.T) {
		if err := verifySignature(leaf, []byte(`{"request":{"type":"IntentRequest"}}`), sig); err == nil {
			t.Fatal("verifySignature() error = nil, want error for tampered body")
		}
	})

	t.Run("garbage signature fails", func(t *testing.T) {
		if err := verifySignature(leaf, body, []byte("not a real signature")); err == nil {
			t.Fatal("verifySignature() error = nil, want error for garbage signature")
		}
	})
}

// rewriteTransport redirects requests to a local httptest.Server while
// leaving the request's URL (as seen by application code, e.g.
// validateCertChainURL's host check) untouched.
type rewriteTransport struct {
	target *url.URL
	base   http.RoundTripper
}

func (t *rewriteTransport) RoundTrip(req *http.Request) (*http.Response, error) {
	req = req.Clone(req.Context())
	req.URL.Scheme = t.target.Scheme
	req.URL.Host = t.target.Host
	return t.base.RoundTrip(req)
}

func TestSignatureValidatorValidate(t *testing.T) {
	now := time.Now()
	chainPEM, leafKey := generateTestChain(t, []string{certSANRequired}, now.Add(-time.Hour), now.Add(time.Hour))

	newTestValidator := func(t *testing.T, handler http.HandlerFunc) (*SignatureValidator, func()) {
		t.Helper()
		ts := httptest.NewServer(handler)
		targetURL, err := url.Parse(ts.URL)
		if err != nil {
			t.Fatalf("parse test server url: %v", err)
		}
		httpClient := &http.Client{Transport: &rewriteTransport{target: targetURL, base: http.DefaultTransport}}
		return NewSignatureValidator(httpClient), ts.Close
	}

	signBody := func(t *testing.T, key *rsa.PrivateKey, body []byte) string {
		t.Helper()
		hash := sha1.Sum(body) //nolint:gosec
		sig, err := rsa.SignPKCS1v15(rand.Reader, key, crypto.SHA1, hash[:])
		if err != nil {
			t.Fatalf("sign body: %v", err)
		}
		return base64.StdEncoding.EncodeToString(sig)
	}

	t.Run("valid request end to end", func(t *testing.T) {
		v, closeServer := newTestValidator(t, func(w http.ResponseWriter, r *http.Request) {
			_, _ = w.Write(chainPEM)
		})
		defer closeServer()

		body := []byte(`{"request":{"type":"LaunchRequest"}}`)
		req := httptest.NewRequest(http.MethodPost, "/", bytes.NewReader(body))
		req.Header.Set("SignatureCertChainUrl", "https://s3.amazonaws.com/echo.api/echo-api-cert.pem")
		req.Header.Set("Signature", signBody(t, leafKey, body))

		if err := v.Validate(context.Background(), req, body); err != nil {
			t.Fatalf("Validate() error = %v", err)
		}
	})

	t.Run("caches the leaf certificate across calls", func(t *testing.T) {
		fetches := 0
		v, closeServer := newTestValidator(t, func(w http.ResponseWriter, r *http.Request) {
			fetches++
			_, _ = w.Write(chainPEM)
		})
		defer closeServer()

		body := []byte(`{"request":{"type":"LaunchRequest"}}`)
		req := httptest.NewRequest(http.MethodPost, "/", bytes.NewReader(body))
		req.Header.Set("SignatureCertChainUrl", "https://s3.amazonaws.com/echo.api/echo-api-cert.pem")
		req.Header.Set("Signature", signBody(t, leafKey, body))

		if err := v.Validate(context.Background(), req, body); err != nil {
			t.Fatalf("Validate() #1 error = %v", err)
		}
		if err := v.Validate(context.Background(), req, body); err != nil {
			t.Fatalf("Validate() #2 error = %v", err)
		}
		if fetches != 1 {
			t.Fatalf("fetches = %d, want 1 (second Validate should hit the cache)", fetches)
		}
	})

	t.Run("missing headers", func(t *testing.T) {
		v, closeServer := newTestValidator(t, func(w http.ResponseWriter, r *http.Request) {
			_, _ = w.Write(chainPEM)
		})
		defer closeServer()

		body := []byte(`{}`)
		req := httptest.NewRequest(http.MethodPost, "/", bytes.NewReader(body))
		if err := v.Validate(context.Background(), req, body); err == nil {
			t.Fatal("Validate() error = nil, want error for missing headers")
		}
	})

	t.Run("cert chain url pointing at the wrong host is rejected before any fetch", func(t *testing.T) {
		fetched := false
		v, closeServer := newTestValidator(t, func(w http.ResponseWriter, r *http.Request) {
			fetched = true
			_, _ = w.Write(chainPEM)
		})
		defer closeServer()

		body := []byte(`{}`)
		req := httptest.NewRequest(http.MethodPost, "/", bytes.NewReader(body))
		req.Header.Set("SignatureCertChainUrl", "https://evil.example.com/echo.api/echo-api-cert.pem")
		req.Header.Set("Signature", signBody(t, leafKey, body))

		if err := v.Validate(context.Background(), req, body); err == nil {
			t.Fatal("Validate() error = nil, want error for wrong cert chain host")
		}
		if fetched {
			t.Fatal("cert chain should never be fetched when the URL itself is rejected")
		}
	})

	t.Run("signature does not match body", func(t *testing.T) {
		v, closeServer := newTestValidator(t, func(w http.ResponseWriter, r *http.Request) {
			_, _ = w.Write(chainPEM)
		})
		defer closeServer()

		signedBody := []byte(`{"request":{"type":"LaunchRequest"}}`)
		req := httptest.NewRequest(http.MethodPost, "/", bytes.NewReader(signedBody))
		req.Header.Set("SignatureCertChainUrl", "https://s3.amazonaws.com/echo.api/echo-api-cert.pem")
		req.Header.Set("Signature", signBody(t, leafKey, signedBody))

		tamperedBody := []byte(`{"request":{"type":"IntentRequest"}}`)
		if err := v.Validate(context.Background(), req, tamperedBody); err == nil {
			t.Fatal("Validate() error = nil, want error for body/signature mismatch")
		}
	})

	t.Run("fetch failure surfaces as an error", func(t *testing.T) {
		v, closeServer := newTestValidator(t, func(w http.ResponseWriter, r *http.Request) {
			w.WriteHeader(http.StatusInternalServerError)
		})
		defer closeServer()

		body := []byte(`{}`)
		req := httptest.NewRequest(http.MethodPost, "/", bytes.NewReader(body))
		req.Header.Set("SignatureCertChainUrl", "https://s3.amazonaws.com/echo.api/echo-api-cert.pem")
		req.Header.Set("Signature", signBody(t, leafKey, body))

		if err := v.Validate(context.Background(), req, body); err == nil {
			t.Fatal("Validate() error = nil, want error when cert fetch fails")
		}
	})
}

func TestValidateTimestamp(t *testing.T) {
	tests := []struct {
		name    string
		ts      string
		wantErr bool
	}{
		{name: "now", ts: time.Now().Format(time.RFC3339), wantErr: false},
		{name: "within tolerance", ts: time.Now().Add(-100 * time.Second).Format(time.RFC3339), wantErr: false},
		{name: "too old", ts: time.Now().Add(-200 * time.Second).Format(time.RFC3339), wantErr: true},
		{name: "too far in the future", ts: time.Now().Add(200 * time.Second).Format(time.RFC3339), wantErr: true},
		{name: "unparsable", ts: "not-a-timestamp", wantErr: true},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			err := ValidateTimestamp(tt.ts)
			if (err != nil) != tt.wantErr {
				t.Fatalf("ValidateTimestamp(%q) error = %v, wantErr %v", tt.ts, err, tt.wantErr)
			}
		})
	}
}

func TestValidateApplicationID(t *testing.T) {
	const skillID = "amzn1.ask.skill.test"

	t.Run("matches via context", func(t *testing.T) {
		req := &Request{Context: &Context{System: System{Application: Application{ApplicationID: skillID}}}}
		if err := ValidateApplicationID(req, skillID); err != nil {
			t.Fatalf("ValidateApplicationID() error = %v", err)
		}
	})

	t.Run("matches via session fallback when context absent", func(t *testing.T) {
		req := &Request{Session: &Session{Application: Application{ApplicationID: skillID}}}
		if err := ValidateApplicationID(req, skillID); err != nil {
			t.Fatalf("ValidateApplicationID() error = %v", err)
		}
	})

	t.Run("context takes precedence over session", func(t *testing.T) {
		req := &Request{
			Context: &Context{System: System{Application: Application{ApplicationID: skillID}}},
			Session: &Session{Application: Application{ApplicationID: "some-other-skill"}},
		}
		if err := ValidateApplicationID(req, skillID); err != nil {
			t.Fatalf("ValidateApplicationID() error = %v", err)
		}
	})

	t.Run("mismatch errors", func(t *testing.T) {
		req := &Request{Context: &Context{System: System{Application: Application{ApplicationID: "wrong-skill"}}}}
		if err := ValidateApplicationID(req, skillID); err == nil {
			t.Fatal("ValidateApplicationID() error = nil, want error")
		}
	})

	t.Run("neither context nor session present errors", func(t *testing.T) {
		req := &Request{}
		if err := ValidateApplicationID(req, skillID); err == nil {
			t.Fatal("ValidateApplicationID() error = nil, want error")
		}
	})
}