seaweedfs/weed/s3api/auth_signature_v4.go

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/*
* The following code tries to reverse engineer the Amazon S3 APIs,
* and is mostly copied from minio implementation.
*/
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or
// implied. See the License for the specific language governing
// permissions and limitations under the License.
package s3api
import (
"bytes"
"crypto/hmac"
"crypto/sha256"
"crypto/subtle"
"encoding/hex"
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"github.com/chrislusf/seaweedfs/weed/s3api/s3err"
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"net/http"
"net/url"
"regexp"
"sort"
"strconv"
"strings"
"time"
"unicode/utf8"
)
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func (iam *IdentityAccessManagement) reqSignatureV4Verify(r *http.Request) (*Identity, s3err.ErrorCode) {
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sha256sum := getContentSha256Cksum(r)
switch {
case isRequestSignatureV4(r):
return iam.doesSignatureMatch(sha256sum, r)
case isRequestPresignedSignatureV4(r):
return iam.doesPresignedSignatureMatch(sha256sum, r)
}
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return nil, s3err.ErrAccessDenied
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}
// Streaming AWS Signature Version '4' constants.
const (
emptySHA256 = "e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855"
streamingContentSHA256 = "STREAMING-AWS4-HMAC-SHA256-PAYLOAD"
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signV4ChunkedAlgorithm = "AWS4-HMAC-SHA256-PAYLOAD"
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// http Header "x-amz-content-sha256" == "UNSIGNED-PAYLOAD" indicates that the
// client did not calculate sha256 of the payload.
unsignedPayload = "UNSIGNED-PAYLOAD"
)
// Returns SHA256 for calculating canonical-request.
func getContentSha256Cksum(r *http.Request) string {
var (
defaultSha256Cksum string
v []string
ok bool
)
// For a presigned request we look at the query param for sha256.
if isRequestPresignedSignatureV4(r) {
// X-Amz-Content-Sha256, if not set in presigned requests, checksum
// will default to 'UNSIGNED-PAYLOAD'.
defaultSha256Cksum = unsignedPayload
v, ok = r.URL.Query()["X-Amz-Content-Sha256"]
if !ok {
v, ok = r.Header["X-Amz-Content-Sha256"]
}
} else {
// X-Amz-Content-Sha256, if not set in signed requests, checksum
// will default to sha256([]byte("")).
defaultSha256Cksum = emptySHA256
v, ok = r.Header["X-Amz-Content-Sha256"]
}
// We found 'X-Amz-Content-Sha256' return the captured value.
if ok {
return v[0]
}
// We couldn't find 'X-Amz-Content-Sha256'.
return defaultSha256Cksum
}
// Verify authorization header - http://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-authenticating-requests.html
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func (iam *IdentityAccessManagement) doesSignatureMatch(hashedPayload string, r *http.Request) (*Identity, s3err.ErrorCode) {
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// Copy request.
req := *r
// Save authorization header.
v4Auth := req.Header.Get("Authorization")
// Parse signature version '4' header.
signV4Values, err := parseSignV4(v4Auth)
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if err != s3err.ErrNone {
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return nil, err
}
// Extract all the signed headers along with its values.
extractedSignedHeaders, errCode := extractSignedHeaders(signV4Values.SignedHeaders, r)
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if errCode != s3err.ErrNone {
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return nil, errCode
}
// Verify if the access key id matches.
identity, cred, found := iam.lookupByAccessKey(signV4Values.Credential.accessKey)
if !found {
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return nil, s3err.ErrInvalidAccessKeyID
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}
// Extract date, if not present throw error.
var date string
if date = req.Header.Get(http.CanonicalHeaderKey("X-Amz-Date")); date == "" {
if date = r.Header.Get("Date"); date == "" {
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return nil, s3err.ErrMissingDateHeader
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}
}
// Parse date header.
t, e := time.Parse(iso8601Format, date)
if e != nil {
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return nil, s3err.ErrMalformedDate
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}
// Query string.
queryStr := req.URL.Query().Encode()
// Get canonical request.
canonicalRequest := getCanonicalRequest(extractedSignedHeaders, hashedPayload, queryStr, req.URL.Path, req.Method)
// Get string to sign from canonical request.
stringToSign := getStringToSign(canonicalRequest, t, signV4Values.Credential.getScope())
// Get hmac signing key.
signingKey := getSigningKey(cred.SecretKey, signV4Values.Credential.scope.date, signV4Values.Credential.scope.region)
// Calculate signature.
newSignature := getSignature(signingKey, stringToSign)
// Verify if signature match.
if !compareSignatureV4(newSignature, signV4Values.Signature) {
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return nil, s3err.ErrSignatureDoesNotMatch
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}
// Return error none.
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return identity, s3err.ErrNone
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}
// credentialHeader data type represents structured form of Credential
// string from authorization header.
type credentialHeader struct {
accessKey string
scope struct {
date time.Time
region string
service string
request string
}
}
// signValues data type represents structured form of AWS Signature V4 header.
type signValues struct {
Credential credentialHeader
SignedHeaders []string
Signature string
}
// Return scope string.
func (c credentialHeader) getScope() string {
return strings.Join([]string{
c.scope.date.Format(yyyymmdd),
c.scope.region,
c.scope.service,
c.scope.request,
}, "/")
}
// Authorization: algorithm Credential=accessKeyID/credScope, \
// SignedHeaders=signedHeaders, Signature=signature
//
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func parseSignV4(v4Auth string) (sv signValues, aec s3err.ErrorCode) {
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// Replace all spaced strings, some clients can send spaced
// parameters and some won't. So we pro-actively remove any spaces
// to make parsing easier.
v4Auth = strings.Replace(v4Auth, " ", "", -1)
if v4Auth == "" {
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return sv, s3err.ErrAuthHeaderEmpty
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}
// Verify if the header algorithm is supported or not.
if !strings.HasPrefix(v4Auth, signV4Algorithm) {
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return sv, s3err.ErrSignatureVersionNotSupported
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}
// Strip off the Algorithm prefix.
v4Auth = strings.TrimPrefix(v4Auth, signV4Algorithm)
authFields := strings.Split(strings.TrimSpace(v4Auth), ",")
if len(authFields) != 3 {
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return sv, s3err.ErrMissingFields
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}
// Initialize signature version '4' structured header.
signV4Values := signValues{}
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var err s3err.ErrorCode
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// Save credentail values.
signV4Values.Credential, err = parseCredentialHeader(authFields[0])
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if err != s3err.ErrNone {
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return sv, err
}
// Save signed headers.
signV4Values.SignedHeaders, err = parseSignedHeader(authFields[1])
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if err != s3err.ErrNone {
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return sv, err
}
// Save signature.
signV4Values.Signature, err = parseSignature(authFields[2])
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if err != s3err.ErrNone {
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return sv, err
}
// Return the structure here.
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return signV4Values, s3err.ErrNone
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}
// parse credentialHeader string into its structured form.
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func parseCredentialHeader(credElement string) (ch credentialHeader, aec s3err.ErrorCode) {
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creds := strings.Split(strings.TrimSpace(credElement), "=")
if len(creds) != 2 {
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return ch, s3err.ErrMissingFields
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}
if creds[0] != "Credential" {
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return ch, s3err.ErrMissingCredTag
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}
credElements := strings.Split(strings.TrimSpace(creds[1]), "/")
if len(credElements) != 5 {
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return ch, s3err.ErrCredMalformed
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}
// Save access key id.
cred := credentialHeader{
accessKey: credElements[0],
}
var e error
cred.scope.date, e = time.Parse(yyyymmdd, credElements[1])
if e != nil {
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return ch, s3err.ErrMalformedCredentialDate
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}
cred.scope.region = credElements[2]
cred.scope.service = credElements[3] // "s3"
cred.scope.request = credElements[4] // "aws4_request"
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return cred, s3err.ErrNone
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}
// Parse slice of signed headers from signed headers tag.
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func parseSignedHeader(signedHdrElement string) ([]string, s3err.ErrorCode) {
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signedHdrFields := strings.Split(strings.TrimSpace(signedHdrElement), "=")
if len(signedHdrFields) != 2 {
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return nil, s3err.ErrMissingFields
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}
if signedHdrFields[0] != "SignedHeaders" {
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return nil, s3err.ErrMissingSignHeadersTag
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}
if signedHdrFields[1] == "" {
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return nil, s3err.ErrMissingFields
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}
signedHeaders := strings.Split(signedHdrFields[1], ";")
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return signedHeaders, s3err.ErrNone
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}
// Parse signature from signature tag.
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func parseSignature(signElement string) (string, s3err.ErrorCode) {
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signFields := strings.Split(strings.TrimSpace(signElement), "=")
if len(signFields) != 2 {
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return "", s3err.ErrMissingFields
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}
if signFields[0] != "Signature" {
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return "", s3err.ErrMissingSignTag
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}
if signFields[1] == "" {
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return "", s3err.ErrMissingFields
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}
signature := signFields[1]
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return signature, s3err.ErrNone
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}
// doesPolicySignatureMatch - Verify query headers with post policy
// - http://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-HTTPPOSTConstructPolicy.html
// returns ErrNone if the signature matches.
func (iam *IdentityAccessManagement) doesPolicySignatureV4Match(formValues http.Header) s3err.ErrorCode {
// Parse credential tag.
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credHeader, err := parseCredentialHeader("Credential=" + formValues.Get("X-Amz-Credential"))
if err != s3err.ErrNone {
return s3err.ErrMissingFields
}
_, cred, found := iam.lookupByAccessKey(credHeader.accessKey)
if !found {
return s3err.ErrInvalidAccessKeyID
}
// Get signing key.
signingKey := getSigningKey(cred.SecretKey, credHeader.scope.date, credHeader.scope.region)
// Get signature.
newSignature := getSignature(signingKey, formValues.Get("Policy"))
// Verify signature.
if !compareSignatureV4(newSignature, formValues.Get("X-Amz-Signature")) {
return s3err.ErrSignatureDoesNotMatch
}
// Success.
return s3err.ErrNone
}
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// check query headers with presigned signature
// - http://docs.aws.amazon.com/AmazonS3/latest/API/sigv4-query-string-auth.html
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func (iam *IdentityAccessManagement) doesPresignedSignatureMatch(hashedPayload string, r *http.Request) (*Identity, s3err.ErrorCode) {
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// Copy request
req := *r
// Parse request query string.
pSignValues, err := parsePreSignV4(req.URL.Query())
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if err != s3err.ErrNone {
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return nil, err
}
// Verify if the access key id matches.
identity, cred, found := iam.lookupByAccessKey(pSignValues.Credential.accessKey)
if !found {
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return nil, s3err.ErrInvalidAccessKeyID
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}
// Extract all the signed headers along with its values.
extractedSignedHeaders, errCode := extractSignedHeaders(pSignValues.SignedHeaders, r)
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if errCode != s3err.ErrNone {
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return nil, errCode
}
// Construct new query.
query := make(url.Values)
if req.URL.Query().Get("X-Amz-Content-Sha256") != "" {
query.Set("X-Amz-Content-Sha256", hashedPayload)
}
query.Set("X-Amz-Algorithm", signV4Algorithm)
now := time.Now().UTC()
// If the host which signed the request is slightly ahead in time (by less than globalMaxSkewTime) the
// request should still be allowed.
if pSignValues.Date.After(now.Add(15 * time.Minute)) {
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return nil, s3err.ErrRequestNotReadyYet
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}
if now.Sub(pSignValues.Date) > pSignValues.Expires {
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return nil, s3err.ErrExpiredPresignRequest
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}
// Save the date and expires.
t := pSignValues.Date
expireSeconds := int(pSignValues.Expires / time.Second)
// Construct the query.
query.Set("X-Amz-Date", t.Format(iso8601Format))
query.Set("X-Amz-Expires", strconv.Itoa(expireSeconds))
query.Set("X-Amz-SignedHeaders", getSignedHeaders(extractedSignedHeaders))
query.Set("X-Amz-Credential", cred.AccessKey+"/"+getScope(t, pSignValues.Credential.scope.region))
// Save other headers available in the request parameters.
for k, v := range req.URL.Query() {
// Handle the metadata in presigned put query string
if strings.Contains(strings.ToLower(k), "x-amz-meta-") {
query.Set(k, v[0])
}
if strings.HasPrefix(strings.ToLower(k), "x-amz") {
continue
}
query[k] = v
}
// Get the encoded query.
encodedQuery := query.Encode()
// Verify if date query is same.
if req.URL.Query().Get("X-Amz-Date") != query.Get("X-Amz-Date") {
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return nil, s3err.ErrSignatureDoesNotMatch
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}
// Verify if expires query is same.
if req.URL.Query().Get("X-Amz-Expires") != query.Get("X-Amz-Expires") {
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return nil, s3err.ErrSignatureDoesNotMatch
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}
// Verify if signed headers query is same.
if req.URL.Query().Get("X-Amz-SignedHeaders") != query.Get("X-Amz-SignedHeaders") {
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return nil, s3err.ErrSignatureDoesNotMatch
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}
// Verify if credential query is same.
if req.URL.Query().Get("X-Amz-Credential") != query.Get("X-Amz-Credential") {
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return nil, s3err.ErrSignatureDoesNotMatch
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}
// Verify if sha256 payload query is same.
if req.URL.Query().Get("X-Amz-Content-Sha256") != "" {
if req.URL.Query().Get("X-Amz-Content-Sha256") != query.Get("X-Amz-Content-Sha256") {
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return nil, s3err.ErrContentSHA256Mismatch
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}
}
/// Verify finally if signature is same.
// Get canonical request.
presignedCanonicalReq := getCanonicalRequest(extractedSignedHeaders, hashedPayload, encodedQuery, req.URL.Path, req.Method)
// Get string to sign from canonical request.
presignedStringToSign := getStringToSign(presignedCanonicalReq, t, pSignValues.Credential.getScope())
// Get hmac presigned signing key.
presignedSigningKey := getSigningKey(cred.SecretKey, pSignValues.Credential.scope.date, pSignValues.Credential.scope.region)
// Get new signature.
newSignature := getSignature(presignedSigningKey, presignedStringToSign)
// Verify signature.
if !compareSignatureV4(req.URL.Query().Get("X-Amz-Signature"), newSignature) {
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return nil, s3err.ErrSignatureDoesNotMatch
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}
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return identity, s3err.ErrNone
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}
func contains(list []string, elem string) bool {
for _, t := range list {
if t == elem {
return true
}
}
return false
}
// preSignValues data type represents structued form of AWS Signature V4 query string.
type preSignValues struct {
signValues
Date time.Time
Expires time.Duration
}
// Parses signature version '4' query string of the following form.
//
// querystring = X-Amz-Algorithm=algorithm
// querystring += &X-Amz-Credential= urlencode(accessKey + '/' + credential_scope)
// querystring += &X-Amz-Date=date
// querystring += &X-Amz-Expires=timeout interval
// querystring += &X-Amz-SignedHeaders=signed_headers
// querystring += &X-Amz-Signature=signature
//
// verifies if any of the necessary query params are missing in the presigned request.
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func doesV4PresignParamsExist(query url.Values) s3err.ErrorCode {
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v4PresignQueryParams := []string{"X-Amz-Algorithm", "X-Amz-Credential", "X-Amz-Signature", "X-Amz-Date", "X-Amz-SignedHeaders", "X-Amz-Expires"}
for _, v4PresignQueryParam := range v4PresignQueryParams {
if _, ok := query[v4PresignQueryParam]; !ok {
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return s3err.ErrInvalidQueryParams
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}
}
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return s3err.ErrNone
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}
// Parses all the presigned signature values into separate elements.
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func parsePreSignV4(query url.Values) (psv preSignValues, aec s3err.ErrorCode) {
var err s3err.ErrorCode
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// verify whether the required query params exist.
err = doesV4PresignParamsExist(query)
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if err != s3err.ErrNone {
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return psv, err
}
// Verify if the query algorithm is supported or not.
if query.Get("X-Amz-Algorithm") != signV4Algorithm {
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return psv, s3err.ErrInvalidQuerySignatureAlgo
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}
// Initialize signature version '4' structured header.
preSignV4Values := preSignValues{}
// Save credential.
preSignV4Values.Credential, err = parseCredentialHeader("Credential=" + query.Get("X-Amz-Credential"))
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if err != s3err.ErrNone {
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return psv, err
}
var e error
// Save date in native time.Time.
preSignV4Values.Date, e = time.Parse(iso8601Format, query.Get("X-Amz-Date"))
if e != nil {
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return psv, s3err.ErrMalformedPresignedDate
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}
// Save expires in native time.Duration.
preSignV4Values.Expires, e = time.ParseDuration(query.Get("X-Amz-Expires") + "s")
if e != nil {
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return psv, s3err.ErrMalformedExpires
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}
if preSignV4Values.Expires < 0 {
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return psv, s3err.ErrNegativeExpires
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}
// Check if Expiry time is less than 7 days (value in seconds).
if preSignV4Values.Expires.Seconds() > 604800 {
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return psv, s3err.ErrMaximumExpires
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}
// Save signed headers.
preSignV4Values.SignedHeaders, err = parseSignedHeader("SignedHeaders=" + query.Get("X-Amz-SignedHeaders"))
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if err != s3err.ErrNone {
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return psv, err
}
// Save signature.
preSignV4Values.Signature, err = parseSignature("Signature=" + query.Get("X-Amz-Signature"))
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if err != s3err.ErrNone {
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return psv, err
}
// Return structed form of signature query string.
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return preSignV4Values, s3err.ErrNone
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}
// extractSignedHeaders extract signed headers from Authorization header
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func extractSignedHeaders(signedHeaders []string, r *http.Request) (http.Header, s3err.ErrorCode) {
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reqHeaders := r.Header
// find whether "host" is part of list of signed headers.
// if not return ErrUnsignedHeaders. "host" is mandatory.
if !contains(signedHeaders, "host") {
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return nil, s3err.ErrUnsignedHeaders
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}
extractedSignedHeaders := make(http.Header)
for _, header := range signedHeaders {
// `host` will not be found in the headers, can be found in r.Host.
// but its alway necessary that the list of signed headers containing host in it.
val, ok := reqHeaders[http.CanonicalHeaderKey(header)]
if ok {
for _, enc := range val {
extractedSignedHeaders.Add(header, enc)
}
continue
}
switch header {
case "expect":
// Golang http server strips off 'Expect' header, if the
// client sent this as part of signed headers we need to
// handle otherwise we would see a signature mismatch.
// `aws-cli` sets this as part of signed headers.
//
// According to
// http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.20
// Expect header is always of form:
//
// Expect = "Expect" ":" 1#expectation
// expectation = "100-continue" | expectation-extension
//
// So it safe to assume that '100-continue' is what would
// be sent, for the time being keep this work around.
// Adding a *TODO* to remove this later when Golang server
// doesn't filter out the 'Expect' header.
extractedSignedHeaders.Set(header, "100-continue")
case "host":
// Go http server removes "host" from Request.Header
extractedSignedHeaders.Set(header, r.Host)
case "transfer-encoding":
for _, enc := range r.TransferEncoding {
extractedSignedHeaders.Add(header, enc)
}
case "content-length":
// Signature-V4 spec excludes Content-Length from signed headers list for signature calculation.
// But some clients deviate from this rule. Hence we consider Content-Length for signature
// calculation to be compatible with such clients.
extractedSignedHeaders.Set(header, strconv.FormatInt(r.ContentLength, 10))
default:
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return nil, s3err.ErrUnsignedHeaders
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}
}
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return extractedSignedHeaders, s3err.ErrNone
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}
// getSignedHeaders generate a string i.e alphabetically sorted, semicolon-separated list of lowercase request header names
func getSignedHeaders(signedHeaders http.Header) string {
var headers []string
for k := range signedHeaders {
headers = append(headers, strings.ToLower(k))
}
sort.Strings(headers)
return strings.Join(headers, ";")
}
// getScope generate a string of a specific date, an AWS region, and a service.
func getScope(t time.Time, region string) string {
scope := strings.Join([]string{
t.Format(yyyymmdd),
region,
"s3",
"aws4_request",
}, "/")
return scope
}
// getCanonicalRequest generate a canonical request of style
//
// canonicalRequest =
// <HTTPMethod>\n
// <CanonicalURI>\n
// <CanonicalQueryString>\n
// <CanonicalHeaders>\n
// <SignedHeaders>\n
// <HashedPayload>
//
func getCanonicalRequest(extractedSignedHeaders http.Header, payload, queryStr, urlPath, method string) string {
rawQuery := strings.Replace(queryStr, "+", "%20", -1)
encodedPath := encodePath(urlPath)
canonicalRequest := strings.Join([]string{
method,
encodedPath,
rawQuery,
getCanonicalHeaders(extractedSignedHeaders),
getSignedHeaders(extractedSignedHeaders),
payload,
}, "\n")
return canonicalRequest
}
// getStringToSign a string based on selected query values.
func getStringToSign(canonicalRequest string, t time.Time, scope string) string {
stringToSign := signV4Algorithm + "\n" + t.Format(iso8601Format) + "\n"
stringToSign = stringToSign + scope + "\n"
canonicalRequestBytes := sha256.Sum256([]byte(canonicalRequest))
stringToSign = stringToSign + hex.EncodeToString(canonicalRequestBytes[:])
return stringToSign
}
// sumHMAC calculate hmac between two input byte array.
func sumHMAC(key []byte, data []byte) []byte {
hash := hmac.New(sha256.New, key)
hash.Write(data)
return hash.Sum(nil)
}
// getSigningKey hmac seed to calculate final signature.
func getSigningKey(secretKey string, t time.Time, region string) []byte {
date := sumHMAC([]byte("AWS4"+secretKey), []byte(t.Format(yyyymmdd)))
regionBytes := sumHMAC(date, []byte(region))
service := sumHMAC(regionBytes, []byte("s3"))
signingKey := sumHMAC(service, []byte("aws4_request"))
return signingKey
}
// getSignature final signature in hexadecimal form.
func getSignature(signingKey []byte, stringToSign string) string {
return hex.EncodeToString(sumHMAC(signingKey, []byte(stringToSign)))
}
// getCanonicalHeaders generate a list of request headers with their values
func getCanonicalHeaders(signedHeaders http.Header) string {
var headers []string
vals := make(http.Header)
for k, vv := range signedHeaders {
headers = append(headers, strings.ToLower(k))
vals[strings.ToLower(k)] = vv
}
sort.Strings(headers)
var buf bytes.Buffer
for _, k := range headers {
buf.WriteString(k)
buf.WriteByte(':')
for idx, v := range vals[k] {
if idx > 0 {
buf.WriteByte(',')
}
buf.WriteString(signV4TrimAll(v))
}
buf.WriteByte('\n')
}
return buf.String()
}
// Trim leading and trailing spaces and replace sequential spaces with one space, following Trimall()
// in http://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html
func signV4TrimAll(input string) string {
// Compress adjacent spaces (a space is determined by
// unicode.IsSpace() internally here) to one space and return
return strings.Join(strings.Fields(input), " ")
}
// if object matches reserved string, no need to encode them
var reservedObjectNames = regexp.MustCompile("^[a-zA-Z0-9-_.~/]+$")
// EncodePath encode the strings from UTF-8 byte representations to HTML hex escape sequences
//
// This is necessary since regular url.Parse() and url.Encode() functions do not support UTF-8
// non english characters cannot be parsed due to the nature in which url.Encode() is written
//
// This function on the other hand is a direct replacement for url.Encode() technique to support
// pretty much every UTF-8 character.
func encodePath(pathName string) string {
if reservedObjectNames.MatchString(pathName) {
return pathName
}
var encodedPathname string
for _, s := range pathName {
if 'A' <= s && s <= 'Z' || 'a' <= s && s <= 'z' || '0' <= s && s <= '9' { // §2.3 Unreserved characters (mark)
encodedPathname = encodedPathname + string(s)
continue
}
switch s {
case '-', '_', '.', '~', '/': // §2.3 Unreserved characters (mark)
encodedPathname = encodedPathname + string(s)
continue
default:
len := utf8.RuneLen(s)
if len < 0 {
// if utf8 cannot convert return the same string as is
return pathName
}
u := make([]byte, len)
utf8.EncodeRune(u, s)
for _, r := range u {
hex := hex.EncodeToString([]byte{r})
encodedPathname = encodedPathname + "%" + strings.ToUpper(hex)
}
}
}
return encodedPathname
}
// compareSignatureV4 returns true if and only if both signatures
// are equal. The signatures are expected to be HEX encoded strings
// according to the AWS S3 signature V4 spec.
func compareSignatureV4(sig1, sig2 string) bool {
// The CTC using []byte(str) works because the hex encoding
// is unique for a sequence of bytes. See also compareSignatureV2.
return subtle.ConstantTimeCompare([]byte(sig1), []byte(sig2)) == 1
}