Random API improvements (#12119) (#12143)

* In the random APIs, add a 'prng' param that causes a DRBG seeded from the selected source(s) to be the source of the returned bytes

* fixes, unit test next

* unit tests

* changelog

* memory ramifications

* switch to using a string called drbg

* Update helper/random/random_api.go



* wrong changelog

---------

Co-authored-by: Scott Miller <smiller@hashicorp.com>
Co-authored-by: Steven Clark <steven.clark@hashicorp.com>

builtin/logical/transit/path_random.go

@@ -44,6 +44,12 @@ func (b *backend) pathRandom() *framework.Path {
 				Default:     "platform",
 				Description: `Which system to source random data from, ether "platform", "seal", or "all".`,
 			},
+			"drbg": {
+				Type:    framework.TypeString,
+				Default: "",
+				Description: "If set, seed a secure DRBG from the source and use it to generate the bytes.  This can be more performant when using the seal source." +
+					" Possible values are unset (don't use a DRBG), \"auto\" and \"hmacdrbg\" which are equivalent.",
+			},
 		},
 
 		Callbacks: map[logical.Operation]framework.OperationFunc{

changelog/_12119.txt

@@ -0,0 +1,6 @@
+```release-note:improvement
+secrets/transit, core: Boost the limit of random bytes retrievable via random byte
+APIs.  And add the option to get PRNG random bytes seeded by random sources. 
+Note that requests for large numbers of bytes will increase Vault memory
+usage accordingly.
+```

helper/random/random_api.go

@@ -11,13 +11,17 @@ import (
 	"io"
 	"strconv"
 
+	"github.com/hashicorp/go-hmac-drbg/hmacdrbg"
 	"github.com/hashicorp/go-uuid"
 	"github.com/hashicorp/vault/sdk/framework"
 	"github.com/hashicorp/vault/sdk/helper/xor"
 	"github.com/hashicorp/vault/sdk/logical"
 )
 
-const APIMaxBytes = 128 * 1024
+const (
+	APIMaxBytes  = 10 * 1024 * 1024
+	SealMaxBytes = 128 * 1024
+)
 
 func HandleRandomAPI(d *framework.FieldData, additionalSource io.Reader) (*logical.Response, error) {
 	bytes := 0
@@ -43,6 +47,16 @@ func HandleRandomAPI(d *framework.FieldData, additionalSource io.Reader) (*logic
 			return logical.ErrorResponse(fmt.Sprintf("error parsing url-set byte count: %s", err)), nil
 		}
 	}
+	maybeDrbg, ok := d.GetOk("drbg")
+	var drbg string
+	if ok {
+		drbg = maybeDrbg.(string)
+		switch drbg {
+		case "", "auto", "hmacdrbg":
+		default:
+			return logical.ErrorResponse("invalid setting for drbg, must absent, auto, or hmacdrbg"), nil
+		}
+	}
 	format := d.Get("format").(string)
 
 	if bytes < 1 {
@@ -50,7 +64,20 @@ func HandleRandomAPI(d *framework.FieldData, additionalSource io.Reader) (*logic
 	}
 
 	if bytes > APIMaxBytes {
-		return logical.ErrorResponse(`"bytes" should be less than %d`, APIMaxBytes), nil
+		return logical.ErrorResponse(`"bytes" must be less than %d`, APIMaxBytes), nil
+	} else if (source == "seal" || source == "all") && bytes > SealMaxBytes && len(drbg) == 0 {
+		return logical.ErrorResponse("bytes cannot be more than %d if sourced from seal.  Use drbg mode instead", SealMaxBytes), nil
+	}
+
+	var warning string
+	switch source {
+	case "seal":
+		if rand.Reader == additionalSource {
+			warning = "no seal/entropy augmentation available, using platform entropy source"
+		}
+	case "platform", "all", "":
+	default:
+		return logical.ErrorResponse("unsupported entropy source %q; must be \"platform\" or \"seal\", or \"all\"", source), nil
 	}
 
 	switch format {
@@ -61,33 +88,30 @@ func HandleRandomAPI(d *framework.FieldData, additionalSource io.Reader) (*logic
 	}
 
 	var randBytes []byte
-	var warning string
-	switch source {
-	case "", "platform":
-		randBytes, err = uuid.GenerateRandomBytes(bytes)
+	if len(drbg) > 0 {
+		// right now only one value for drbg besides unset is possible, so we assume it was validated
+		// above
+
+		// Seed the drbg from source, but use it to generate byes
+		var seed []byte
+		// HMACDRBG wants a seed at least 3x the security level
+		seed, err = getEntropy(source, (256*3)/8, additionalSource)
 		if err != nil {
 			return nil, err
 		}
-	case "seal":
-		if rand.Reader == additionalSource {
-			warning = "no seal/entropy augmentation available, using platform entropy source"
-		}
-		randBytes, err = uuid.GenerateRandomBytesWithReader(bytes, additionalSource)
-	case "all":
-		randBytes, err = uuid.GenerateRandomBytes(bytes)
-		if err == nil && rand.Reader != additionalSource {
-			var sealBytes []byte
-			sealBytes, err = uuid.GenerateRandomBytesWithReader(bytes, additionalSource)
-			if err == nil {
-				randBytes, err = xor.XORBytes(sealBytes, randBytes)
-			}
-		}
-	default:
-		return logical.ErrorResponse("unsupported entropy source %q; must be \"platform\" or \"seal\", or \"all\"", source), nil
-	}
+		drbg := hmacdrbg.NewHmacDrbg(256, seed, []byte("api-random-with-hmac-drbg"))
+		reader := hmacdrbg.NewHmacDrbgReader(drbg)
+		randBytes = make([]byte, bytes)
+		_, err = io.ReadFull(reader, randBytes)
 		if err != nil {
 			return nil, err
 		}
+	} else {
+		randBytes, err = getEntropy(source, bytes, additionalSource)
+		if err != nil {
+			return nil, err
+		}
+	}
 
 	var retStr string
 	switch format {
@@ -109,6 +133,32 @@ func HandleRandomAPI(d *framework.FieldData, additionalSource io.Reader) (*logic
 	return resp, nil
 }
 
+func getEntropy(source string, bytes int, additionalSource io.Reader) ([]byte, error) {
+	var randBytes []byte
+	var err error
+	switch source {
+	case "", "platform":
+		randBytes, err = uuid.GenerateRandomBytes(bytes)
+		if err != nil {
+			return nil, err
+		}
+	case "seal":
+		randBytes, err = uuid.GenerateRandomBytesWithReader(bytes, additionalSource)
+	case "all":
+		randBytes, err = uuid.GenerateRandomBytes(bytes)
+		if err == nil && rand.Reader != additionalSource {
+			var sealBytes []byte
+			sealBytes, err = uuid.GenerateRandomBytesWithReader(bytes, additionalSource)
+			if err == nil {
+				randBytes, err = xor.XORBytes(sealBytes, randBytes)
+			}
+		}
+	default:
+		return nil, fmt.Errorf("unsupported entropy source %q; must be \"platform\" or \"seal\", or \"all\"", source)
+	}
+	return randBytes, err
+}
+
 func isValidSource(s string) bool {
 	switch s {
 	case "", "platform", "seal", "all":

vault/logical_system_paths.go

@@ -2586,6 +2586,12 @@ func (b *SystemBackend) toolsPaths() []*framework.Path {
 					Default:     "platform",
 					Description: `Which system to source random data from, ether "platform", "seal", or "all".`,
 				},
+				"drbg": {
+					Type:    framework.TypeString,
+					Default: "",
+					Description: "If set, seed a secure DRBG from the source and use it to generate the bytes.  This can be more performant when using the seal source." +
+						" Possible values are unset (don't use a DRBG), \"auto\" and \"hmacdrbg\" which are equivalent.",
+				},
 			},
 
 			Operations: map[logical.Operation]framework.OperationHandler{

vault/logical_system_test.go

@@ -4377,7 +4377,7 @@ func TestSystemBackend_ToolsRandom(t *testing.T) {
 			}
 			if errExpected {
 				if !resp.IsError() {
-					t.Fatalf("bad: got error response: %#v", *resp)
+					t.Fatalf("bad: got no error response: %#v", *resp)
 				}
 				return nil
 			} else {
@@ -4435,6 +4435,13 @@ func TestSystemBackend_ToolsRandom(t *testing.T) {
 	req.Data["format"] = "hex"
 	doRequest(req, false, "hex", 24)
 
+	// Test PRNG with large output
+	req.Path = "tools/random/1024768"
+	req.Data["format"] = "hex"
+	req.Data["drbg"] = "hmacdrbg"
+	doRequest(req, false, "hex", 1024768)
+	delete(req.Data, "drbg")
+
 	// Test bad input/format
 	req.Path = "tools/random"
 	req.Data["format"] = "base92"
@@ -4445,7 +4452,12 @@ func TestSystemBackend_ToolsRandom(t *testing.T) {
 	doRequest(req, true, "", 0)
 
 	req.Data["format"] = "hex"
-	req.Data["bytes"] = maxBytes + 1
+	req.Data["bytes"] = random.APIMaxBytes + 1
+	doRequest(req, true, "", 0)
+
+	req.Path = "tools/random/seal"
+	req.Data["format"] = "hex"
+	req.Data["bytes"] = random.SealMaxBytes + 1
 	doRequest(req, true, "", 0)
 }