diff --git a/src/crypto/internal/boring/boring.go b/src/crypto/internal/boring/boring.go
index 98aa851..9ccad7eb 100644
--- a/src/crypto/internal/boring/boring.go
+++ b/src/crypto/internal/boring/boring.go
@@ -11,7 +11,10 @@
 
 // #include "goboringcrypto.h"
 import "C"
-import "math/big"
+import (
+	"crypto/internal/boring/sig"
+	"math/big"
+)
 
 const available = true
 
@@ -20,6 +23,7 @@
 	if C._goboringcrypto_FIPS_mode() != 1 {
 		panic("boringcrypto: not in FIPS mode")
 	}
+	sig.BoringCrypto()
 }
 
 // Unreachable marks code that should be unreachable
diff --git a/src/crypto/internal/boring/fipstls/dummy.s b/src/crypto/internal/boring/fipstls/dummy.s
new file mode 100644
index 0000000..53bb7d9
--- /dev/null
+++ b/src/crypto/internal/boring/fipstls/dummy.s
@@ -0,0 +1,10 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// runtime_arg0 is declared in tls.go without a body.
+// It's provided by package runtime,
+// but the go command doesn't know that.
+// Having this assembly file keeps the go command
+// from complaining about the missing body
+// (because the implementation might be here).
diff --git a/src/crypto/internal/boring/fipstls/tls.go b/src/crypto/internal/boring/fipstls/tls.go
new file mode 100644
index 0000000..4127533
--- /dev/null
+++ b/src/crypto/internal/boring/fipstls/tls.go
@@ -0,0 +1,49 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fipstls allows control over whether crypto/tls requires FIPS-approved settings.
+// This package's effects are independent of the use of the BoringCrypto implementation.
+package fipstls
+
+import "sync/atomic"
+
+var required uint32
+
+// Force forces crypto/tls to restrict TLS configurations to FIPS-approved settings.
+// By design, this call is impossible to undo (except in tests).
+//
+// Note that this call has an effect even in programs using
+// standard crypto (that is, even when Enabled = false).
+func Force() {
+	atomic.StoreUint32(&required, 1)
+}
+
+// Abandon allows non-FIPS-approved settings.
+// If called from a non-test binary, it panics.
+func Abandon() {
+	// Note: Not using boring.UnreachableExceptTests because we want
+	// this test to happen even when boring.Enabled = false.
+	name := runtime_arg0()
+	// Allow _test for Go command, .test for Bazel,
+	// NaClMain for NaCl (where all binaries run as NaClMain),
+	// and empty string for Windows (where runtime_arg0 can't easily find the name).
+	// Since this is an internal package, testing that this isn't used on the
+	// other operating systems should suffice to catch any mistakes.
+	if !hasSuffix(name, "_test") && !hasSuffix(name, ".test") && name != "NaClMain" && name != "" {
+		panic("fipstls: invalid use of Abandon in " + name)
+	}
+	atomic.StoreUint32(&required, 0)
+}
+
+// provided by runtime
+func runtime_arg0() string
+
+func hasSuffix(s, t string) bool {
+	return len(s) > len(t) && s[len(s)-len(t):] == t
+}
+
+// Required reports whether FIPS-approved settings are required.
+func Required() bool {
+	return atomic.LoadUint32(&required) != 0
+}
diff --git a/src/crypto/internal/boring/notboring.go b/src/crypto/internal/boring/notboring.go
index 257aa3b..c21cb3c 100644
--- a/src/crypto/internal/boring/notboring.go
+++ b/src/crypto/internal/boring/notboring.go
@@ -9,6 +9,7 @@
 import (
 	"crypto"
 	"crypto/cipher"
+	"crypto/internal/boring/sig"
 	"hash"
 	"math/big"
 )
@@ -17,7 +18,12 @@
 
 // Unreachable marks code that should be unreachable
 // when BoringCrypto is in use. It is a no-op without BoringCrypto.
-func Unreachable() {}
+func Unreachable() {
+	// Code that's unreachable when using BoringCrypto
+	// is exactly the code we want to detect for reporting
+	// standard Go crypto.
+	sig.StandardCrypto()
+}
 
 // UnreachableExceptTests marks code that should be unreachable
 // when BoringCrypto is in use. It is a no-op without BoringCrypto.
diff --git a/src/crypto/internal/boring/sig/sig.go b/src/crypto/internal/boring/sig/sig.go
new file mode 100644
index 0000000..716c03c
--- /dev/null
+++ b/src/crypto/internal/boring/sig/sig.go
@@ -0,0 +1,17 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package sig holds “code signatures” that can be called
+// and will result in certain code sequences being linked into
+// the final binary. The functions themselves are no-ops.
+package sig
+
+// BoringCrypto indicates that the BoringCrypto module is present.
+func BoringCrypto()
+
+// FIPSOnly indicates that package crypto/tls/fipsonly is present.
+func FIPSOnly()
+
+// StandardCrypto indicates that standard Go crypto is present.
+func StandardCrypto()
diff --git a/src/crypto/internal/boring/sig/sig_amd64.s b/src/crypto/internal/boring/sig/sig_amd64.s
new file mode 100644
index 0000000..64e3462
--- /dev/null
+++ b/src/crypto/internal/boring/sig/sig_amd64.s
@@ -0,0 +1,54 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+#include "textflag.h"
+
+// These functions are no-ops, but you can search for their implementations
+// to find out whether they are linked into a particular binary.
+//
+// Each function consists of a two-byte jump over the next 29-bytes,
+// then a 5-byte indicator sequence unlikely to occur in real x86 instructions,
+// then a randomly-chosen 24-byte sequence, and finally a return instruction
+// (the target of the jump).
+//
+// These sequences are known to rsc.io/goversion.
+
+#define START \
+	BYTE $0xEB; BYTE $0x1D; BYTE $0xF4; BYTE $0x48; BYTE $0xF4; BYTE $0x4B; BYTE $0xF4
+
+#define END \
+	BYTE $0xC3
+
+// BoringCrypto indicates that BoringCrypto (in particular, its func init) is present.
+TEXT ·BoringCrypto(SB),NOSPLIT,$0
+	START
+	BYTE $0xB3; BYTE $0x32; BYTE $0xF5; BYTE $0x28;
+	BYTE $0x13; BYTE $0xA3; BYTE $0xB4; BYTE $0x50;
+	BYTE $0xD4; BYTE $0x41; BYTE $0xCC; BYTE $0x24;
+	BYTE $0x85; BYTE $0xF0; BYTE $0x01; BYTE $0x45;
+	BYTE $0x4E; BYTE $0x92; BYTE $0x10; BYTE $0x1B;
+	BYTE $0x1D; BYTE $0x2F; BYTE $0x19; BYTE $0x50;
+	END
+
+// StandardCrypto indicates that standard Go crypto is present.
+TEXT ·StandardCrypto(SB),NOSPLIT,$0
+	START
+	BYTE $0xba; BYTE $0xee; BYTE $0x4d; BYTE $0xfa;
+	BYTE $0x98; BYTE $0x51; BYTE $0xca; BYTE $0x56;
+	BYTE $0xa9; BYTE $0x11; BYTE $0x45; BYTE $0xe8;
+	BYTE $0x3e; BYTE $0x99; BYTE $0xc5; BYTE $0x9c;
+	BYTE $0xf9; BYTE $0x11; BYTE $0xcb; BYTE $0x8e;
+	BYTE $0x80; BYTE $0xda;  BYTE $0xf1; BYTE $0x2f;
+	END
+
+// FIPSOnly indicates that crypto/tls/fipsonly is present.
+TEXT ·FIPSOnly(SB),NOSPLIT,$0
+	START
+	BYTE $0x36; BYTE $0x3C; BYTE $0xB9; BYTE $0xCE;
+	BYTE $0x9D; BYTE $0x68; BYTE $0x04; BYTE $0x7D;
+	BYTE $0x31; BYTE $0xF2; BYTE $0x8D; BYTE $0x32;
+	BYTE $0x5D; BYTE $0x5C; BYTE $0xA5; BYTE $0x87;
+	BYTE $0x3F; BYTE $0x5D; BYTE $0x80; BYTE $0xCA;
+	BYTE $0xF6; BYTE $0xD6; BYTE $0x15; BYTE $0x1B;
+	END
diff --git a/src/crypto/internal/boring/sig/sig_other.s b/src/crypto/internal/boring/sig/sig_other.s
new file mode 100644
index 0000000..2eb3173
--- /dev/null
+++ b/src/crypto/internal/boring/sig/sig_other.s
@@ -0,0 +1,19 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// These functions are no-ops.
+// On amd64 they have recognizable implementations, so that you can
+// search a particular binary to see if they are present.
+// On other platforms (those using this source file), they don't.
+
+// +build !amd64
+
+TEXT ·BoringCrypto(SB),$0
+	RET
+
+TEXT ·FIPSOnly(SB),$0
+	RET
+
+TEXT ·StandardCrypto(SB),$0
+	RET
diff --git a/src/crypto/tls/boring.go b/src/crypto/tls/boring.go
new file mode 100644
index 0000000..791049f
--- /dev/null
+++ b/src/crypto/tls/boring.go
@@ -0,0 +1,121 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tls
+
+import (
+	"crypto/ecdsa"
+	"crypto/internal/boring/fipstls"
+	"crypto/rsa"
+	"crypto/x509"
+)
+
+// needFIPS returns fipstls.Required(); it avoids a new import in common.go.
+func needFIPS() bool {
+	return fipstls.Required()
+}
+
+// fipsMinVersion replaces c.minVersion in FIPS-only mode.
+func fipsMinVersion(c *Config) uint16 {
+	// FIPS requires TLS 1.2.
+	return VersionTLS12
+}
+
+// fipsMaxVersion replaces c.maxVersion in FIPS-only mode.
+func fipsMaxVersion(c *Config) uint16 {
+	// FIPS requires TLS 1.2.
+	return VersionTLS12
+}
+
+// default defaultFIPSCurvePreferences is the FIPS-allowed curves,
+// in preference order (most preferable first).
+var defaultFIPSCurvePreferences = []CurveID{CurveP256, CurveP384, CurveP521}
+
+// fipsCurvePreferences replaces c.curvePreferences in FIPS-only mode.
+func fipsCurvePreferences(c *Config) []CurveID {
+	if c == nil || len(c.CurvePreferences) == 0 {
+		return defaultFIPSCurvePreferences
+	}
+	var list []CurveID
+	for _, id := range c.CurvePreferences {
+		for _, allowed := range defaultFIPSCurvePreferences {
+			if id == allowed {
+				list = append(list, id)
+				break
+			}
+		}
+	}
+	return list
+}
+
+// default FIPSCipherSuites is the FIPS-allowed cipher suites,
+// in preference order (most preferable first).
+var defaultFIPSCipherSuites = []uint16{
+	TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+	TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+	TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
+	TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
+	TLS_RSA_WITH_AES_128_GCM_SHA256,
+	TLS_RSA_WITH_AES_256_GCM_SHA384,
+}
+
+// fipsCipherSuites replaces c.cipherSuites in FIPS-only mode.
+func fipsCipherSuites(c *Config) []uint16 {
+	if c == nil || c.CipherSuites == nil {
+		return defaultFIPSCipherSuites
+	}
+	var list []uint16
+	for _, id := range c.CipherSuites {
+		for _, allowed := range defaultFIPSCipherSuites {
+			if id == allowed {
+				list = append(list, id)
+				break
+			}
+		}
+	}
+	return list
+}
+
+// isBoringCertificate reports whether a certificate may be used
+// when constructing a verified chain.
+// It is called for each leaf, intermediate, and root certificate.
+func isBoringCertificate(c *x509.Certificate) bool {
+	if !needFIPS() {
+		// Everything is OK if we haven't forced FIPS-only mode.
+		return true
+	}
+
+	// Otherwise the key must be RSA 2048, RSA 3072, or ECDSA P-256.
+	switch k := c.PublicKey.(type) {
+	default:
+		return false
+	case *rsa.PublicKey:
+		if size := k.N.BitLen(); size != 2048 && size != 3072 {
+			return false
+		}
+	case *ecdsa.PublicKey:
+		if name := k.Curve.Params().Name; name != "P-256" && name != "P-384" {
+			return false
+		}
+	}
+
+	return true
+}
+
+// supportedSignatureAlgorithms returns the supported signature algorithms.
+// It knows that the FIPS-allowed ones are all at the beginning of
+// defaultSupportedSignatureAlgorithms.
+func supportedSignatureAlgorithms() []signatureAndHash {
+	all := defaultSupportedSignatureAlgorithms
+	if !needFIPS() {
+		return all
+	}
+	i := 0
+	for i < len(all) && all[i].hash != hashSHA1 {
+		i++
+	}
+	return all[:i]
+}
+
+var testingOnlyForceClientHelloSignatureAndHashes []signatureAndHash
diff --git a/src/crypto/tls/boring_test.go b/src/crypto/tls/boring_test.go
new file mode 100644
index 0000000..15422f8
--- /dev/null
+++ b/src/crypto/tls/boring_test.go
@@ -0,0 +1,579 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package tls
+
+import (
+	"crypto/ecdsa"
+	"crypto/elliptic"
+	"crypto/internal/boring/fipstls"
+	"crypto/rand"
+	"crypto/rsa"
+	"crypto/x509"
+	"crypto/x509/pkix"
+	"fmt"
+	"math/big"
+	"net"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+)
+
+func TestBoringServerProtocolVersion(t *testing.T) {
+	test := func(name string, v uint16, msg string) {
+		t.Run(name, func(t *testing.T) {
+			serverConfig := testConfig.Clone()
+			serverConfig.MinVersion = VersionSSL30
+			clientHello := &clientHelloMsg{
+				vers:               v,
+				cipherSuites:       allCipherSuites(),
+				compressionMethods: []uint8{compressionNone},
+			}
+			testClientHelloFailure(t, serverConfig, clientHello, msg)
+		})
+	}
+
+	test("VersionSSL30", VersionSSL30, "")
+	test("VersionTLS10", VersionTLS10, "")
+	test("VersionTLS11", VersionTLS11, "")
+	test("VersionTLS12", VersionTLS12, "")
+
+	fipstls.Force()
+	defer fipstls.Abandon()
+	test("VersionSSL30", VersionSSL30, "unsupported, maximum protocol version")
+	test("VersionTLS10", VersionTLS10, "unsupported, maximum protocol version")
+	test("VersionTLS11", VersionTLS11, "unsupported, maximum protocol version")
+	test("VersionTLS12", VersionTLS12, "")
+}
+
+func isBoringCipherSuite(id uint16) bool {
+	switch id {
+	case TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256,
+		TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384,
+		TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256,
+		TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384,
+		TLS_RSA_WITH_AES_128_GCM_SHA256,
+		TLS_RSA_WITH_AES_256_GCM_SHA384:
+		return true
+	}
+	return false
+}
+
+func isBoringCurve(id CurveID) bool {
+	switch id {
+	case CurveP256, CurveP384, CurveP521:
+		return true
+	}
+	return false
+}
+
+func isECDSA(id uint16) bool {
+	for _, suite := range cipherSuites {
+		if suite.id == id {
+			return suite.flags&suiteECDSA == suiteECDSA
+		}
+	}
+	panic(fmt.Sprintf("unknown cipher suite %#x", id))
+}
+
+func isBoringSignatureAndHash(sigHash signatureAndHash) bool {
+	switch sigHash.signature {
+	default:
+		return false
+	case signatureRSA,
+		signatureECDSA:
+		// ok
+	}
+	switch sigHash.hash {
+	default:
+		return false
+	case hashSHA256,
+		hashSHA384:
+		// ok
+	}
+	return true
+}
+
+func TestBoringServerCipherSuites(t *testing.T) {
+	serverConfig := testConfig.Clone()
+	serverConfig.CipherSuites = allCipherSuites()
+	serverConfig.Certificates = make([]Certificate, 1)
+
+	for _, id := range allCipherSuites() {
+		if isECDSA(id) {
+			serverConfig.Certificates[0].Certificate = [][]byte{testECDSACertificate}
+			serverConfig.Certificates[0].PrivateKey = testECDSAPrivateKey
+		} else {
+			serverConfig.Certificates[0].Certificate = [][]byte{testRSACertificate}
+			serverConfig.Certificates[0].PrivateKey = testRSAPrivateKey
+		}
+		serverConfig.BuildNameToCertificate()
+		t.Run(fmt.Sprintf("suite=%#x", id), func(t *testing.T) {
+			clientHello := &clientHelloMsg{
+				vers:               VersionTLS12,
+				cipherSuites:       []uint16{id},
+				compressionMethods: []uint8{compressionNone},
+				supportedCurves:    defaultCurvePreferences,
+				supportedPoints:    []uint8{pointFormatUncompressed},
+			}
+
+			testClientHello(t, serverConfig, clientHello)
+			t.Run("fipstls", func(t *testing.T) {
+				fipstls.Force()
+				defer fipstls.Abandon()
+				msg := ""
+				if !isBoringCipherSuite(id) {
+					msg = "no cipher suite supported by both client and server"
+				}
+				testClientHelloFailure(t, serverConfig, clientHello, msg)
+			})
+		})
+	}
+}
+
+func TestBoringServerCurves(t *testing.T) {
+	serverConfig := testConfig.Clone()
+	serverConfig.Certificates = make([]Certificate, 1)
+	serverConfig.Certificates[0].Certificate = [][]byte{testECDSACertificate}
+	serverConfig.Certificates[0].PrivateKey = testECDSAPrivateKey
+	serverConfig.BuildNameToCertificate()
+
+	for _, curveid := range defaultCurvePreferences {
+		t.Run(fmt.Sprintf("curve=%d", curveid), func(t *testing.T) {
+			clientHello := &clientHelloMsg{
+				vers:               VersionTLS12,
+				cipherSuites:       []uint16{TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256},
+				compressionMethods: []uint8{compressionNone},
+				supportedCurves:    []CurveID{curveid},
+				supportedPoints:    []uint8{pointFormatUncompressed},
+			}
+
+			testClientHello(t, serverConfig, clientHello)
+
+			// With fipstls forced, bad curves should be rejected.
+			t.Run("fipstls", func(t *testing.T) {
+				fipstls.Force()
+				defer fipstls.Abandon()
+				msg := ""
+				if !isBoringCurve(curveid) {
+					msg = "no cipher suite supported by both client and server"
+				}
+				testClientHelloFailure(t, serverConfig, clientHello, msg)
+			})
+		})
+	}
+}
+
+func boringHandshake(t *testing.T, clientConfig, serverConfig *Config) (clientErr, serverErr error) {
+	c, s := realNetPipe(t)
+	client := Client(c, clientConfig)
+	server := Server(s, serverConfig)
+	done := make(chan error, 1)
+	go func() {
+		done <- client.Handshake()
+		c.Close()
+	}()
+	serverErr = server.Handshake()
+	s.Close()
+	clientErr = <-done
+	return
+}
+
+func TestBoringServerSignatureAndHash(t *testing.T) {
+	serverConfig := testConfig.Clone()
+	serverConfig.Certificates = make([]Certificate, 1)
+
+	defer func() {
+		testingOnlyForceClientHelloSignatureAndHashes = nil
+	}()
+
+	for _, sigHash := range defaultSupportedSignatureAlgorithms {
+		testingOnlyForceClientHelloSignatureAndHashes = []signatureAndHash{sigHash}
+
+		t.Run(fmt.Sprintf("%v", sigHash), func(t *testing.T) {
+			if sigHash.signature == signatureRSA {
+				serverConfig.CipherSuites = []uint16{TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256}
+				serverConfig.Certificates[0].Certificate = [][]byte{testRSACertificate}
+				serverConfig.Certificates[0].PrivateKey = testRSAPrivateKey
+			} else {
+				serverConfig.CipherSuites = []uint16{TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256}
+				serverConfig.Certificates = make([]Certificate, 1)
+				serverConfig.Certificates[0].Certificate = [][]byte{testECDSACertificate}
+				serverConfig.Certificates[0].PrivateKey = testECDSAPrivateKey
+			}
+			serverConfig.BuildNameToCertificate()
+
+			clientErr, _ := boringHandshake(t, testConfig, serverConfig)
+			if clientErr != nil {
+				t.Fatalf("expected handshake with %v to succeed; err=%v", sigHash, clientErr)
+			}
+
+			// With fipstls forced, bad curves should be rejected.
+			t.Run("fipstls", func(t *testing.T) {
+				fipstls.Force()
+				defer fipstls.Abandon()
+				clientErr, _ := boringHandshake(t, testConfig, serverConfig)
+				if isBoringSignatureAndHash(sigHash) {
+					if clientErr != nil {
+						t.Fatalf("expected handshake with %v to succeed; err=%v", sigHash, clientErr)
+					}
+				} else {
+					if clientErr == nil {
+						t.Fatalf("expected handshake with %v to fail, but it succeeded", sigHash)
+					}
+				}
+			})
+		})
+	}
+}
+
+func TestBoringClientHello(t *testing.T) {
+	// Test that no matter what we put in the client config,
+	// the client does not offer non-FIPS configurations.
+	fipstls.Force()
+	defer fipstls.Abandon()
+
+	c, s := net.Pipe()
+	defer c.Close()
+	defer s.Close()
+
+	clientConfig := testConfig.Clone()
+	// All sorts of traps for the client to avoid.
+	clientConfig.MinVersion = VersionSSL30
+	clientConfig.CipherSuites = allCipherSuites()
+	clientConfig.CurvePreferences = defaultCurvePreferences
+
+	go Client(c, testConfig).Handshake()
+	srv := Server(s, testConfig)
+	msg, err := srv.readHandshake()
+	if err != nil {
+		t.Fatal(err)
+	}
+	hello, ok := msg.(*clientHelloMsg)
+	if !ok {
+		t.Fatalf("unexpected message type %T", msg)
+	}
+
+	if hello.vers != VersionTLS12 {
+		t.Errorf("client vers=%#x, want %#x (TLS 1.2)", hello.vers, VersionTLS12)
+	}
+	for _, id := range hello.cipherSuites {
+		if !isBoringCipherSuite(id) {
+			t.Errorf("client offered disallowed suite %#x", id)
+		}
+	}
+	for _, id := range hello.supportedCurves {
+		if !isBoringCurve(id) {
+			t.Errorf("client offered disallowed curve %d", id)
+		}
+	}
+	for _, sigHash := range hello.signatureAndHashes {
+		if !isBoringSignatureAndHash(sigHash) {
+			t.Errorf("client offered disallowed signature-and-hash %v", sigHash)
+		}
+	}
+}
+
+func TestBoringCertAlgs(t *testing.T) {
+	// NaCl and arm time out generating keys. Nothing in this test is architecture-specific, so just don't bother on those.
+	if runtime.GOOS == "nacl" || runtime.GOARCH == "arm" {
+		t.Skipf("skipping on %s/%s because key generation takes too long", runtime.GOOS, runtime.GOARCH)
+	}
+
+	// Set up some roots, intermediate CAs, and leaf certs with various algorithms.
+	// X_Y is X signed by Y.
+	R1 := boringCert(t, "R1", boringRSAKey(t, 2048), nil, boringCertCA|boringCertFIPSOK)
+	R2 := boringCert(t, "R2", boringRSAKey(t, 4096), nil, boringCertCA)
+
+	M1_R1 := boringCert(t, "M1_R1", boringECDSAKey(t, elliptic.P256()), R1, boringCertCA|boringCertFIPSOK)
+	M2_R1 := boringCert(t, "M2_R1", boringECDSAKey(t, elliptic.P224()), R1, boringCertCA)
+
+	I_R1 := boringCert(t, "I_R1", boringRSAKey(t, 3072), R1, boringCertCA|boringCertFIPSOK)
+	I_R2 := boringCert(t, "I_R2", I_R1.key, R2, boringCertCA|boringCertFIPSOK)
+	I_M1 := boringCert(t, "I_M1", I_R1.key, M1_R1, boringCertCA|boringCertFIPSOK)
+	I_M2 := boringCert(t, "I_M2", I_R1.key, M2_R1, boringCertCA|boringCertFIPSOK)
+
+	L1_I := boringCert(t, "L1_I", boringECDSAKey(t, elliptic.P384()), I_R1, boringCertLeaf|boringCertFIPSOK)
+	L2_I := boringCert(t, "L2_I", boringRSAKey(t, 1024), I_R1, boringCertLeaf)
+
+	// boringCert checked that isBoringCertificate matches the caller's boringCertFIPSOK bit.
+	// If not, no point in building bigger end-to-end tests.
+	if t.Failed() {
+		t.Fatalf("isBoringCertificate failures; not continuing")
+	}
+
+	// client verifying server cert
+	testServerCert := func(t *testing.T, desc string, pool *x509.CertPool, key interface{}, list [][]byte, ok bool) {
+		clientConfig := testConfig.Clone()
+		clientConfig.RootCAs = pool
+		clientConfig.InsecureSkipVerify = false
+		clientConfig.ServerName = "example.com"
+
+		serverConfig := testConfig.Clone()
+		serverConfig.Certificates = []Certificate{{Certificate: list, PrivateKey: key}}
+		serverConfig.BuildNameToCertificate()
+
+		clientErr, _ := boringHandshake(t, clientConfig, serverConfig)
+
+		if (clientErr == nil) == ok {
+			if ok {
+				t.Logf("%s: accept", desc)
+			} else {
+				t.Logf("%s: reject", desc)
+			}
+		} else {
+			if ok {
+				t.Errorf("%s: BAD reject (%v)", desc, clientErr)
+			} else {
+				t.Errorf("%s: BAD accept", desc)
+			}
+		}
+	}
+
+	// server verifying client cert
+	testClientCert := func(t *testing.T, desc string, pool *x509.CertPool, key interface{}, list [][]byte, ok bool) {
+		clientConfig := testConfig.Clone()
+		clientConfig.ServerName = "example.com"
+		clientConfig.Certificates = []Certificate{{Certificate: list, PrivateKey: key}}
+
+		serverConfig := testConfig.Clone()
+		serverConfig.ClientCAs = pool
+		serverConfig.ClientAuth = RequireAndVerifyClientCert
+
+		_, serverErr := boringHandshake(t, clientConfig, serverConfig)
+
+		if (serverErr == nil) == ok {
+			if ok {
+				t.Logf("%s: accept", desc)
+			} else {
+				t.Logf("%s: reject", desc)
+			}
+		} else {
+			if ok {
+				t.Errorf("%s: BAD reject (%v)", desc, serverErr)
+			} else {
+				t.Errorf("%s: BAD accept", desc)
+			}
+		}
+	}
+
+	// Run simple basic test with known answers before proceeding to
+	// exhaustive test with computed answers.
+	r1pool := x509.NewCertPool()
+	r1pool.AddCert(R1.cert)
+	testServerCert(t, "basic", r1pool, L2_I.key, [][]byte{L2_I.der, I_R1.der}, true)
+	testClientCert(t, "basic (client cert)", r1pool, L2_I.key, [][]byte{L2_I.der, I_R1.der}, true)
+	fipstls.Force()
+	testServerCert(t, "basic (fips)", r1pool, L2_I.key, [][]byte{L2_I.der, I_R1.der}, false)
+	testClientCert(t, "basic (fips, client cert)", r1pool, L2_I.key, [][]byte{L2_I.der, I_R1.der}, false)
+	fipstls.Abandon()
+
+	if t.Failed() {
+		t.Fatal("basic test failed, skipping exhaustive test")
+	}
+
+	if testing.Short() {
+		t.Logf("basic test passed; skipping exhaustive test in -short mode")
+		return
+	}
+
+	for l := 1; l <= 2; l++ {
+		leaf := L1_I
+		if l == 2 {
+			leaf = L2_I
+		}
+		for i := 0; i < 64; i++ {
+			reachable := map[string]bool{leaf.parentOrg: true}
+			reachableFIPS := map[string]bool{leaf.parentOrg: leaf.fipsOK}
+			list := [][]byte{leaf.der}
+			listName := leaf.name
+			addList := func(cond int, c *boringCertificate) {
+				if cond != 0 {
+					list = append(list, c.der)
+					listName += "," + c.name
+					if reachable[c.org] {
+						reachable[c.parentOrg] = true
+					}
+					if reachableFIPS[c.org] && c.fipsOK {
+						reachableFIPS[c.parentOrg] = true
+					}
+				}
+			}
+			addList(i&1, I_R1)
+			addList(i&2, I_R2)
+			addList(i&4, I_M1)
+			addList(i&8, I_M2)
+			addList(i&16, M1_R1)
+			addList(i&32, M2_R1)
+
+			for r := 1; r <= 3; r++ {
+				pool := x509.NewCertPool()
+				rootName := ","
+				shouldVerify := false
+				shouldVerifyFIPS := false
+				addRoot := func(cond int, c *boringCertificate) {
+					if cond != 0 {
+						rootName += "," + c.name
+						pool.AddCert(c.cert)
+						if reachable[c.org] {
+							shouldVerify = true
+						}
+						if reachableFIPS[c.org] && c.fipsOK {
+							shouldVerifyFIPS = true
+						}
+					}
+				}
+				addRoot(r&1, R1)
+				addRoot(r&2, R2)
+				rootName = rootName[1:] // strip leading comma
+				testServerCert(t, listName+"->"+rootName[1:], pool, leaf.key, list, shouldVerify)
+				testClientCert(t, listName+"->"+rootName[1:]+"(client cert)", pool, leaf.key, list, shouldVerify)
+				fipstls.Force()
+				testServerCert(t, listName+"->"+rootName[1:]+" (fips)", pool, leaf.key, list, shouldVerifyFIPS)
+				testClientCert(t, listName+"->"+rootName[1:]+" (fips, client cert)", pool, leaf.key, list, shouldVerifyFIPS)
+				fipstls.Abandon()
+			}
+		}
+	}
+}
+
+const (
+	boringCertCA = iota
+	boringCertLeaf
+	boringCertFIPSOK = 0x80
+)
+
+func boringRSAKey(t *testing.T, size int) *rsa.PrivateKey {
+	k, err := rsa.GenerateKey(rand.Reader, size)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return k
+}
+
+func boringECDSAKey(t *testing.T, curve elliptic.Curve) *ecdsa.PrivateKey {
+	k, err := ecdsa.GenerateKey(curve, rand.Reader)
+	if err != nil {
+		t.Fatal(err)
+	}
+	return k
+}
+
+type boringCertificate struct {
+	name      string
+	org       string
+	parentOrg string
+	der       []byte
+	cert      *x509.Certificate
+	key       interface{}
+	fipsOK    bool
+}
+
+func boringCert(t *testing.T, name string, key interface{}, parent *boringCertificate, mode int) *boringCertificate {
+	org := name
+	parentOrg := ""
+	if i := strings.Index(org, "_"); i >= 0 {
+		org = org[:i]
+		parentOrg = name[i+1:]
+	}
+	tmpl := &x509.Certificate{
+		SerialNumber: big.NewInt(1),
+		Subject: pkix.Name{
+			Organization: []string{org},
+		},
+		NotBefore: time.Unix(0, 0),
+		NotAfter:  time.Unix(0, 0),
+
+		KeyUsage:              x509.KeyUsageKeyEncipherment | x509.KeyUsageDigitalSignature,
+		ExtKeyUsage:           []x509.ExtKeyUsage{x509.ExtKeyUsageServerAuth, x509.ExtKeyUsageClientAuth},
+		BasicConstraintsValid: true,
+	}
+	if mode&^boringCertFIPSOK == boringCertLeaf {
+		tmpl.DNSNames = []string{"example.com"}
+	} else {
+		tmpl.IsCA = true
+		tmpl.KeyUsage |= x509.KeyUsageCertSign
+	}
+
+	var pcert *x509.Certificate
+	var pkey interface{}
+	if parent != nil {
+		pcert = parent.cert
+		pkey = parent.key
+	} else {
+		pcert = tmpl
+		pkey = key
+	}
+
+	var pub interface{}
+	var desc string
+	switch k := key.(type) {
+	case *rsa.PrivateKey:
+		pub = &k.PublicKey
+		desc = fmt.Sprintf("RSA-%d", k.N.BitLen())
+	case *ecdsa.PrivateKey:
+		pub = &k.PublicKey
+		desc = "ECDSA-" + k.Curve.Params().Name
+	default:
+		t.Fatalf("invalid key %T", key)
+	}
+
+	der, err := x509.CreateCertificate(rand.Reader, tmpl, pcert, pub, pkey)
+	if err != nil {
+		t.Fatal(err)
+	}
+	cert, err := x509.ParseCertificate(der)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Tell isBoringCertificate to enforce FIPS restrictions for this check.
+	fipstls.Force()
+	defer fipstls.Abandon()
+
+	fipsOK := mode&boringCertFIPSOK != 0
+	if isBoringCertificate(cert) != fipsOK {
+		t.Errorf("isBoringCertificate(cert with %s key) = %v, want %v", desc, !fipsOK, fipsOK)
+	}
+	return &boringCertificate{name, org, parentOrg, der, cert, key, fipsOK}
+}
+
+func boringPool(t *testing.T, list ...*boringCertificate) *x509.CertPool {
+	pool := x509.NewCertPool()
+	for _, c := range list {
+		cert, err := x509.ParseCertificate(c.der)
+		if err != nil {
+			t.Fatal(err)
+		}
+		pool.AddCert(cert)
+	}
+	return pool
+}
+
+func boringList(t *testing.T, list ...*boringCertificate) [][]byte {
+	var all [][]byte
+	for _, c := range list {
+		all = append(all, c.der)
+	}
+	return all
+}
+
+// realNetPipe is like net.Pipe but returns an actual network socket pair,
+// which has buffering that avoids various deadlocks if both sides
+// try to speak at the same time.
+func realNetPipe(t *testing.T) (net.Conn, net.Conn) {
+	l := newLocalListener(t)
+	defer l.Close()
+	c, err := net.Dial("tcp", l.Addr().String())
+	if err != nil {
+		t.Fatal(err)
+	}
+	s, err := l.Accept()
+	if err != nil {
+		c.Close()
+		t.Fatal(err)
+	}
+	return c, s
+}
diff --git a/src/crypto/tls/common.go b/src/crypto/tls/common.go
index de833a9..ac3176d 100644
--- a/src/crypto/tls/common.go
+++ b/src/crypto/tls/common.go
@@ -145,10 +145,10 @@
 	hash, signature uint8
 }
 
-// supportedSignatureAlgorithms contains the signature and hash algorithms that
+// defaultSupportedSignatureAlgorithms contains the signature and hash algorithms that
 // the code advertises as supported in a TLS 1.2 ClientHello and in a TLS 1.2
 // CertificateRequest.
-var supportedSignatureAlgorithms = []signatureAndHash{
+var defaultSupportedSignatureAlgorithms = []signatureAndHash{
 	{hashSHA256, signatureRSA},
 	{hashSHA256, signatureECDSA},
 	{hashSHA384, signatureRSA},
@@ -670,6 +670,9 @@
 }
 
 func (c *Config) cipherSuites() []uint16 {
+	if needFIPS() {
+		return fipsCipherSuites(c)
+	}
 	s := c.CipherSuites
 	if s == nil {
 		s = defaultCipherSuites()
@@ -678,6 +681,9 @@
 }
 
 func (c *Config) minVersion() uint16 {
+	if needFIPS() {
+		return fipsMinVersion(c)
+	}
 	if c == nil || c.MinVersion == 0 {
 		return minVersion
 	}
@@ -685,6 +691,9 @@
 }
 
 func (c *Config) maxVersion() uint16 {
+	if needFIPS() {
+		return fipsMaxVersion(c)
+	}
 	if c == nil || c.MaxVersion == 0 {
 		return maxVersion
 	}
@@ -694,6 +703,9 @@
 var defaultCurvePreferences = []CurveID{X25519, CurveP256, CurveP384, CurveP521}
 
 func (c *Config) curvePreferences() []CurveID {
+	if needFIPS() {
+		return fipsCurvePreferences(c)
+	}
 	if c == nil || len(c.CurvePreferences) == 0 {
 		return defaultCurvePreferences
 	}
diff --git a/src/crypto/tls/fipsonly/fipsonly.go b/src/crypto/tls/fipsonly/fipsonly.go
new file mode 100644
index 0000000..85b3532
--- /dev/null
+++ b/src/crypto/tls/fipsonly/fipsonly.go
@@ -0,0 +1,27 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package fipsonly restricts all TLS configuration to FIPS-approved settings.
+//
+// The effect is triggered by importing the package anywhere in a program, as in:
+//
+//	import _ "crypto/tls/fipsonly"
+//
+// This package only exists in the dev.boringcrypto branch of Go.
+package fipsonly
+
+// This functionality is provided as a side effect of an import to make
+// it trivial to add to an existing program. It requires only a single line
+// added to an existing source file, or it can be done by adding a whole
+// new source file and not modifying any existing source files.
+
+import (
+	"crypto/internal/boring/fipstls"
+	"crypto/internal/boring/sig"
+)
+
+func init() {
+	fipstls.Force()
+	sig.FIPSOnly()
+}
diff --git a/src/crypto/tls/fipsonly/fipsonly_test.go b/src/crypto/tls/fipsonly/fipsonly_test.go
new file mode 100644
index 0000000..facd248
--- /dev/null
+++ b/src/crypto/tls/fipsonly/fipsonly_test.go
@@ -0,0 +1,16 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package fipsonly
+
+import (
+	"crypto/internal/boring/fipstls"
+	"testing"
+)
+
+func Test(t *testing.T) {
+	if !fipstls.Required() {
+		t.Fatal("fipstls.Required() = false, must be true")
+	}
+}
diff --git a/src/crypto/tls/handshake_client.go b/src/crypto/tls/handshake_client.go
index 6eda18d..1ff8bd2 100644
--- a/src/crypto/tls/handshake_client.go
+++ b/src/crypto/tls/handshake_client.go
@@ -99,7 +99,11 @@
 	}
 
 	if hello.vers >= VersionTLS12 {
-		hello.signatureAndHashes = supportedSignatureAlgorithms
+		hello.signatureAndHashes = supportedSignatureAlgorithms()
+	}
+
+	if testingOnlyForceClientHelloSignatureAndHashes != nil {
+		hello.signatureAndHashes = testingOnlyForceClientHelloSignatureAndHashes
 	}
 
 	var session *ClientSessionState
@@ -285,6 +289,8 @@
 
 		if !c.config.InsecureSkipVerify {
 			opts := x509.VerifyOptions{
+				IsBoring: isBoringCertificate,
+
 				Roots:         c.config.RootCAs,
 				CurrentTime:   c.config.time(),
 				DNSName:       c.config.ServerName,
diff --git a/src/crypto/tls/handshake_messages_test.go b/src/crypto/tls/handshake_messages_test.go
index f1154d4..3235821 100644
--- a/src/crypto/tls/handshake_messages_test.go
+++ b/src/crypto/tls/handshake_messages_test.go
@@ -137,7 +137,7 @@
 		}
 	}
 	if rand.Intn(10) > 5 {
-		m.signatureAndHashes = supportedSignatureAlgorithms
+		m.signatureAndHashes = supportedSignatureAlgorithms()
 	}
 	m.alpnProtocols = make([]string, rand.Intn(5))
 	for i := range m.alpnProtocols {
diff --git a/src/crypto/tls/handshake_server.go b/src/crypto/tls/handshake_server.go
index b786c30..8a9434b 100644
--- a/src/crypto/tls/handshake_server.go
+++ b/src/crypto/tls/handshake_server.go
@@ -422,7 +422,7 @@
 		}
 		if c.vers >= VersionTLS12 {
 			certReq.hasSignatureAndHash = true
-			certReq.signatureAndHashes = supportedSignatureAlgorithms
+			certReq.signatureAndHashes = supportedSignatureAlgorithms()
 		}
 
 		// An empty list of certificateAuthorities signals to
@@ -526,7 +526,7 @@
 		var signatureAndHash signatureAndHash
 		if certVerify.hasSignatureAndHash {
 			signatureAndHash = certVerify.signatureAndHash
-			if !isSupportedSignatureAndHash(signatureAndHash, supportedSignatureAlgorithms) {
+			if !isSupportedSignatureAndHash(signatureAndHash, supportedSignatureAlgorithms()) {
 				return errors.New("tls: unsupported hash function for client certificate")
 			}
 		} else {
@@ -722,6 +722,8 @@
 
 	if c.config.ClientAuth >= VerifyClientCertIfGiven && len(certs) > 0 {
 		opts := x509.VerifyOptions{
+			IsBoring: isBoringCertificate,
+
 			Roots:         c.config.ClientCAs,
 			CurrentTime:   c.config.time(),
 			Intermediates: x509.NewCertPool(),
diff --git a/src/crypto/tls/key_agreement.go b/src/crypto/tls/key_agreement.go
index 1b27c04..e8a46b8 100644
--- a/src/crypto/tls/key_agreement.go
+++ b/src/crypto/tls/key_agreement.go
@@ -114,7 +114,7 @@
 // only used for >= TLS 1.2 and precisely identifies the hash function to use.
 func hashForServerKeyExchange(sigAndHash signatureAndHash, version uint16, slices ...[]byte) ([]byte, crypto.Hash, error) {
 	if version >= VersionTLS12 {
-		if !isSupportedSignatureAndHash(sigAndHash, supportedSignatureAlgorithms) {
+		if !isSupportedSignatureAndHash(sigAndHash, supportedSignatureAlgorithms()) {
 			return nil, crypto.Hash(0), errors.New("tls: unsupported hash function used by peer")
 		}
 		hashFunc, err := lookupTLSHash(sigAndHash.hash)
@@ -149,7 +149,7 @@
 		if sigAndHash.signature != sigType {
 			continue
 		}
-		if isSupportedSignatureAndHash(sigAndHash, supportedSignatureAlgorithms) {
+		if isSupportedSignatureAndHash(sigAndHash, supportedSignatureAlgorithms()) {
 			return sigAndHash.hash, nil
 		}
 	}
diff --git a/src/crypto/tls/prf.go b/src/crypto/tls/prf.go
index 5833fc1..b93ce4f 100644
--- a/src/crypto/tls/prf.go
+++ b/src/crypto/tls/prf.go
@@ -319,7 +319,7 @@
 	}
 
 	for _, v := range serverList {
-		if v.signature == sigType && isSupportedSignatureAndHash(v, supportedSignatureAlgorithms) {
+		if v.signature == sigType && isSupportedSignatureAndHash(v, supportedSignatureAlgorithms()) {
 			return v, nil
 		}
 	}
diff --git a/src/crypto/x509/verify.go b/src/crypto/x509/verify.go
index 29345a1..64feb6a 100644
--- a/src/crypto/x509/verify.go
+++ b/src/crypto/x509/verify.go
@@ -156,6 +156,11 @@
 	// constraint down the chain which mirrors Windows CryptoAPI behavior,
 	// but not the spec. To accept any key usage, include ExtKeyUsageAny.
 	KeyUsages []ExtKeyUsage
+
+	// IsBoring is a validity check for BoringCrypto.
+	// If not nil, it will be called to check whether a given certificate
+	// can be used for constructing verification chains.
+	IsBoring func(*Certificate) bool
 }
 
 const (
@@ -248,6 +253,13 @@
 		}
 	}
 
+	if opts.IsBoring != nil && !opts.IsBoring(c) {
+		// IncompatibleUsage is not quite right here,
+		// but it's also the "no chains found" error
+		// and is close enough.
+		return CertificateInvalidError{c, IncompatibleUsage}
+	}
+
 	return nil
 }
 
diff --git a/src/go/build/deps_test.go b/src/go/build/deps_test.go
index bdfa517..81657a7 100644
--- a/src/go/build/deps_test.go
+++ b/src/go/build/deps_test.go
@@ -110,14 +110,17 @@
 	"reflect":             {"L2"},
 	"sort":                {"reflect"},
 
-	"crypto/internal/boring":   {"L2", "C", "crypto", "crypto/cipher", "crypto/subtle", "encoding/asn1", "hash", "math/big"},
-	"crypto/internal/cipherhw": {"crypto/internal/boring"},
+	"crypto/internal/boring":         {"L2", "C", "crypto", "crypto/cipher", "crypto/internal/boring/sig", "crypto/subtle", "encoding/asn1", "hash", "math/big"},
+	"crypto/internal/boring/fipstls": {"sync/atomic"},
+	"crypto/internal/cipherhw":       {"crypto/internal/boring"},
+	"crypto/tls/fipsonly":            {"crypto/internal/boring/fipstls", "crypto/internal/boring/sig"},
 
 	"L3": {
 		"L2",
 		"crypto",
 		"crypto/cipher",
 		"crypto/internal/boring",
+		"crypto/internal/boring/fipstls",
 		"crypto/internal/cipherhw",
 		"crypto/subtle",
 		"encoding/base32",
diff --git a/src/runtime/runtime.go b/src/runtime/runtime.go
index 5b5e7de..33ca75d 100644
--- a/src/runtime/runtime.go
+++ b/src/runtime/runtime.go
@@ -59,4 +59,13 @@
 func os_runtime_args() []string { return append([]string{}, argslice...) }
 
 //go:linkname boring_runtime_arg0 crypto/internal/boring.runtime_arg0
-func boring_runtime_arg0() string { return argslice[0] }
+func boring_runtime_arg0() string {
+	// On Windows, argslice is not set, and it's too much work to find argv0.
+	if len(argslice) == 0 {
+		return ""
+	}
+	return argslice[0]
+}
+
+//go:linkname fipstls_runtime_arg0 crypto/internal/boring/fipstls.runtime_arg0
+func fipstls_runtime_arg0() string { return boring_runtime_arg0() }