Add HMAC template function for httpjson input

CHANGELOG.next.asciidoc

@@ -867,6 +867,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 - Add new template functions and `value_type` parameter to `httpjson` transforms. {pull}26847[26847]
 - Add support to merge registry updates in the filestream input across multiple ACKed batches in case of backpressure in the registry or disk. {pull}25976[25976]
 - Update Elasticsearch module's ingest pipeline for parsing new deprecation logs {issue}26857[26857] {pull}26880[26880]
+- Add new `hmac` template function for httpjson input {pull}27168[27168]
 
 *Heartbeat*
 

x-pack/filebeat/docs/inputs/input-httpjson.asciidoc

@@ -203,6 +203,7 @@ Some built-in helper functions are provided to work with the input state inside
 - `add`: adds a list of integers and returns their sum.
 - `mul`: multiplies two integers.
 - `div`: does the integer division of two integer values.
+- `hmac`: calculates the hmac signature of a list of strings concatenated together. Supports sha1 or sha256. Example `[[hmac "sha256" "secret" "string1" "string2" (formatDate (now) "RFC1123")]]`
 
 In addition to the provided functions, any of the native functions for https://golang.org/pkg/time/#Time[`time.Time`], https://golang.org/pkg/net/http/#Header[`http.Header`], and https://golang.org/pkg/net/url/#Values[`url.Values`] types can be used on the corresponding objects. Examples: `[[(now).Day]]`, `[[.last_response.header.Get "key"]]`
 

x-pack/filebeat/input/httpjson/internal/v2/value_tpl.go

@@ -6,7 +6,12 @@ package v2
 
 import (
 	"bytes"
+	"crypto/hmac"
+	"crypto/sha1"
+	"crypto/sha256"
+	"encoding/hex"
 	"errors"
+	"hash"
 	"reflect"
 	"regexp"
 	"strconv"
@@ -48,6 +53,7 @@ func (t *valueTpl) Unpack(in string) error {
 			"add":                 add,
 			"mul":                 mul,
 			"div":                 div,
+			"hmac":                hmac_string,
 		}).
 		Delims(leftDelim, rightDelim).
 		Parse(in)
@@ -236,3 +242,26 @@ func mul(a, b int64) int64 {
 func div(a, b int64) int64 {
 	return a / b
 }
+
+func hmac_string(hmac_type string, hmac_key string, values ...string) string {
+	data := strings.Join(values[:], "")
+	if data == "" {
+		return ""
+	}
+	// Create a new HMAC by defining the hash type and the key (as byte array)
+	var mac hash.Hash
+	switch hmac_type {
+	case "sha256":
+		mac = hmac.New(sha256.New, []byte(hmac_key))
+	case "sha1":
+		mac = hmac.New(sha1.New, []byte(hmac_key))
+	default:
+		// Upstream config validation prevents this from happening.
+		return ""
+	}
+	// Write Data to it
+	mac.Write([]byte(data))
+
+	// Get result and encode as hexadecimal string
+	return hex.EncodeToString(mac.Sum(nil))
+}

x-pack/filebeat/input/httpjson/internal/v2/value_tpl_test.go

@@ -252,6 +252,30 @@ func TestValueTpl(t *testing.T) {
 			paramTr:     transformable{},
 			expectedVal: "4",
 		},
+		{
+			name:        "func sha1 hmac",
+			value:       `[[hmac "sha1" "secret" "string1" "string2"]]`,
+			paramCtx:    emptyTransformContext(),
+			paramTr:     transformable{},
+			expectedVal: "87eca1e7cba012b2dd4a907c2ad4345a252a38f4",
+		},
+		{
+			name:        "func sha256 hmac",
+			setup:       func() { timeNow = func() time.Time { return time.Unix(1627697597, 0).UTC() } },
+			teardown:    func() { timeNow = time.Now },
+			value:       `[[hmac "sha256" "secret" "string1" "string2" (formatDate (now) "RFC1123")]]`,
+			paramCtx:    emptyTransformContext(),
+			paramTr:     transformable{},
+			expectedVal: "adc61cd206e146f2d1337504e760ea70f3d2e34bedf28d07802e0e776568a06b",
+		},
+		{
+			name:          "func invalid hmac",
+			value:         `[[hmac "md5" "secret" "string1" "string2"]]`,
+			paramCtx:      emptyTransformContext(),
+			paramTr:       transformable{},
+			expectedVal:   "",
+			expectedError: errEmptyTemplateResult.Error(),
+		},
 	}
 
 	for _, tc := range cases {