Signing and authenticating REST requests (AWS signature version 2)

Authentication is the process of proving your identity to the system. Identity is an important factor in Amazon S3 access control decisions. Requests are allowed or denied in part based on the identity of the requester. For example, the right to create buckets is reserved for registered developers and (by default) the right to create objects in a bucket is reserved for the owner of the bucket in question. As a developer, you'll be making requests that invoke these privileges, so you'll need to prove your identity to the system by authenticating your requests. This section shows you how.

The Amazon S3 REST API uses a custom HTTP scheme based on a keyed-HMAC (Hash Message Authentication Code) for authentication. To authenticate a request, you first concatenate selected elements of the request to form a string. You then use your AWS secret access key to calculate the HMAC of that string. Informally, we call this process "signing the request," and we call the output of the HMAC algorithm the signature, because it simulates the security properties of a real signature. Finally, you add this signature as a parameter of the request by using the syntax described in this section.

When the system receives an authenticated request, it fetches the AWS secret access key that you claim to have and uses it in the same way to compute a signature for the message it received. It then compares the signature it calculated against the signature presented by the requester. If the two signatures match, the system concludes that the requester must have access to the AWS secret access key and therefore acts with the authority of the principal to whom the key was issued. If the two signatures do not match, the request is dropped and the system responds with an error message.

GET /photos/puppy.jpg HTTP/1.1
Host: awsexamplebucket1.us-west-1.s3.amazonaws.com
Date: Tue, 27 Mar 2007 19:36:42 +0000

Authorization: AWS AKIAIOSFODNN7EXAMPLE:
qgk2+6Sv9/oM7G3qLEjTH1a1l1g=

Using temporary security credentials

If you are signing your request using temporary security credentials (see Making requests), you must include the corresponding security token in your request by adding the x-amz-security-token header.

When you obtain temporary security credentials using the AWS Security Token Service API, the response includes temporary security credentials and a session token. You provide the session token value in the x-amz-security-token header when you send requests to Amazon S3. For information about the AWS Security Token Service API provided by IAM, go to Action in the AWS Security Token Service API Reference Guide .

The authentication header

The Amazon S3 REST API uses the standard HTTP Authorization header to pass authentication information. (The name of the standard header is unfortunate because it carries authentication information, not authorization.) Under the Amazon S3 authentication scheme, the Authorization header has the following form:

Authorization: AWS AWSAccessKeyId:Signature

Developers are issued an AWS access key ID and AWS secret access key when they register. For request authentication, the AWSAccessKeyId element identifies the access key ID that was used to compute the signature and, indirectly, the developer making the request.

The Signature element is the RFC 2104 HMAC-SHA1 of selected elements from the request, and so the Signature part of the Authorization header will vary from request to request. If the request signature calculated by the system matches the Signature included with the request, the requester will have demonstrated possession of the AWS secret access key. The request will then be processed under the identity, and with the authority, of the developer to whom the key was issued.

Following is pseudogrammar that illustrates the construction of the Authorization request header. (In the example, \n means the Unicode code point U+000A, commonly called newline).

Authorization = "AWS" + " " + AWSAccessKeyId + ":" + Signature;

Signature = Base64( HMAC-SHA1( UTF-8-Encoding-Of(YourSecretAccessKey), UTF-8-Encoding-Of( StringToSign ) ) );

StringToSign = HTTP-Verb + "\n" +
	Content-MD5 + "\n" +
	Content-Type + "\n" +
	Date + "\n" +
	CanonicalizedAmzHeaders +
	CanonicalizedResource;

CanonicalizedResource = [ "/" + Bucket ] +
	<HTTP-Request-URI, from the protocol name up to the query string> +
	[ subresource, if present. For example "?acl", "?location", or "?logging"];

CanonicalizedAmzHeaders = <described below>

HMAC-SHA1 is an algorithm defined by RFC 2104 - Keyed-Hashing for Message Authentication . The algorithm takes as input two byte-strings, a key and a message. For Amazon S3 request authentication, use your AWS secret access key (YourSecretAccessKey) as the key, and the UTF-8 encoding of the StringToSign as the message. The output of HMAC-SHA1 is also a byte string, called the digest. The Signature request parameter is constructed by Base64 encoding this digest.

Request canonicalization for signing

Recall that when the system receives an authenticated request, it compares the computed request signature with the signature provided in the request in StringToSign. For that reason, you must compute the signature by using the same method used by Amazon S3. We call the process of putting a request in an agreed-upon form for signing canonicalization.

Constructing the CanonicalizedResource element

CanonicalizedResource represents the Amazon S3 resource targeted by the request. Construct it for a REST request as follows:

Elements of the CanonicalizedResource that come from the HTTP Request-URI should be signed literally as they appear in the HTTP request, including URL-Encoding meta characters.

The CanonicalizedResource might be different than the HTTP Request-URI. In particular, if your request uses the HTTP Host header to specify a bucket, the bucket does not appear in the HTTP Request-URI. However, the CanonicalizedResource continues to include the bucket. Query string parameters might also appear in the Request-URI but are not included in CanonicalizedResource. For more information, see Virtual hosting of buckets .

Constructing the CanonicalizedAmzHeaders element

To construct the CanonicalizedAmzHeaders part of StringToSign, select all HTTP request headers that start with 'x-amz-' (using a case-insensitive comparison), and use the following process.

Positional versus named HTTP header StringToSign elements

The first few header elements of StringToSign (Content-Type, Date, and Content-MD5) are positional in nature. StringToSign does not include the names of these headers, only their values from the request. In contrast, the 'x-amz-' elements are named. Both the header names and the header values appear in StringToSign.

If a positional header called for in the definition of StringToSign is not present in your request (for example, Content-Type or Content-MD5 are optional for PUT requests and meaningless for GET requests), substitute the empty string ("") for that position.

Time stamp requirement

A valid time stamp (using either the HTTP Date header or an x-amz-date alternative) is mandatory for authenticated requests. Furthermore, the client timestamp included with an authenticated request must be within 15 minutes of the Amazon S3 system time when the request is received. If not, the request will fail with the RequestTimeTooSkewed error code. The intention of these restrictions is to limit the possibility that intercepted requests could be replayed by an adversary. For stronger protection against eavesdropping, use the HTTPS transport for authenticated requests.

Some HTTP client libraries do not expose the ability to set the Date header for a request. If you have trouble including the value of the 'Date' header in the canonicalized headers, you can set the timestamp for the request by using an 'x-amz-date' header instead. The value of the x-amz-date header must be in one of the RFC 2616 formats (http://www.ietf.org/rfc/rfc2616.txt). When an x-amz-date header is present in a request, the system will ignore any Date header when computing the request signature. Therefore, if you include the x-amz-date header, use the empty string for the Date when constructing the StringToSign. See the next section for an example.

Authentication examples

The examples in this section use the (non-working) credentials in the following table.

In the example StringToSigns, formatting is not significant, and \n means the Unicode code point U+000A, commonly called newline. Also, the examples use "+0000" to designate the time zone. You can use "GMT" to designate timezone instead, but the signatures shown in the examples will be different.

Object GET

This example gets an object from the awsexamplebucket1 bucket.

GET /photos/puppy.jpg HTTP/1.1
Host: awsexamplebucket1.us-west-1.s3.amazonaws.com
Date: Tue, 27 Mar 2007 19:36:42 +0000

Authorization: AWS AKIAIOSFODNN7EXAMPLE:
qgk2+6Sv9/oM7G3qLEjTH1a1l1g=

GET\n
\n
\n
Tue, 27 Mar 2007 19:36:42 +0000\n
/awsexamplebucket1/photos/puppy.jpg

Note that the CanonicalizedResource includes the bucket name, but the HTTP Request-URI does not. (The bucket is specified by the Host header.)

import base64
import hmac
from hashlib import sha1

access_key = 'AKIAIOSFODNN7EXAMPLE'.encode("UTF-8")
secret_key = 'wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY'.encode("UTF-8")

string_to_sign = 'GET\n\n\nTue, 27 Mar 2007 19:36:42 +0000\n/awsexamplebucket1/photos/puppy.jpg'.encode("UTF-8")
signature = base64.b64encode(
                                hmac.new(
                                         secret_key, string_to_sign, sha1
                                         ).digest()
                                ).strip()


print(f"AWS {access_key.decode()}:{signature.decode()}")

Object PUT

This example puts an object into the awsexamplebucket1 bucket.

PUT /photos/puppy.jpg HTTP/1.1
Content-Type: image/jpeg
Content-Length: 94328
Host: awsexamplebucket1.s3.us-west-1.amazonaws.com
Date: Tue, 27 Mar 2007 21:15:45 +0000

Authorization: AWS AKIAIOSFODNN7EXAMPLE:
iqRzw+ileNPu1fhspnRs8nOjjIA=

PUT\n
\n
image/jpeg\n
Tue, 27 Mar 2007 21:15:45 +0000\n
/awsexamplebucket1/photos/puppy.jpg

Note the Content-Type header in the request and in the StringToSign. Also note that the Content-MD5 is left blank in the StringToSign, because it is not present in the request.

List

This example lists the content of the awsexamplebucket1 bucket.

GET /?prefix=photos&max-keys=50&marker=puppy HTTP/1.1
User-Agent: Mozilla/5.0
Host: awsexamplebucket1.s3.us-west-1.amazonaws.com
Date: Tue, 27 Mar 2007 19:42:41 +0000

Authorization: AWS AKIAIOSFODNN7EXAMPLE:
m0WP8eCtspQl5Ahe6L1SozdX9YA=

GET\n
\n
\n
Tue, 27 Mar 2007 19:42:41 +0000\n
/awsexamplebucket1/

Note the trailing slash on the CanonicalizedResource and the absence of query string parameters.

Fetch

This example fetches the access control policy subresource for the 'awsexamplebucket1' bucket.

GET /?acl HTTP/1.1
Host: awsexamplebucket1.s3.us-west-1.amazonaws.com
Date: Tue, 27 Mar 2007 19:44:46 +0000

Authorization: AWS AKIAIOSFODNN7EXAMPLE:
82ZHiFIjc+WbcwFKGUVEQspPn+0=

GET\n
\n
\n
Tue, 27 Mar 2007 19:44:46 +0000\n
/awsexamplebucket1/?acl

Notice how the subresource query string parameter is included in the CanonicalizedResource.

Delete

This example deletes an object from the 'awsexamplebucket1' bucket using the path-style and Date alternative.

DELETE /awsexamplebucket1/photos/puppy.jpg HTTP/1.1
User-Agent: dotnet
Host: s3.us-west-1.amazonaws.com
Date: Tue, 27 Mar 2007 21:20:27 +0000

x-amz-date: Tue, 27 Mar 2007 21:20:26 +0000
Authorization: AWS AKIAIOSFODNN7EXAMPLE:XbyTlbQdu9Xw5o8P4iMwPktxQd8=

DELETE\n
\n
\n
Tue, 27 Mar 2007 21:20:26 +0000\n
/awsexamplebucket1/photos/puppy.jpg

Note how we used the alternate 'x-amz-date' method of specifying the date (because our client library prevented us from setting the date, say). In this case, the x-amz-date takes precedence over the Date header. Therefore, date entry in the signature must contain the value of the x-amz-date header.

Upload

This example uploads an object to a CNAME style virtual hosted bucket with metadata.

PUT /db-backup.dat.gz HTTP/1.1
User-Agent: curl/7.15.5
Host: static.example.com:8080
Date: Tue, 27 Mar 2007 21:06:08 +0000

x-amz-acl: public-read
content-type: application/x-download
Content-MD5: 4gJE4saaMU4BqNR0kLY+lw==
X-Amz-Meta-ReviewedBy: joe@example.com
X-Amz-Meta-ReviewedBy: jane@example.com
X-Amz-Meta-FileChecksum: 0x02661779
X-Amz-Meta-ChecksumAlgorithm: crc32
Content-Disposition: attachment; filename=database.dat
Content-Encoding: gzip
Content-Length: 5913339

Authorization: AWS AKIAIOSFODNN7EXAMPLE:
jtBQa0Aq+DkULFI8qrpwIjGEx0E=

PUT\n
4gJE4saaMU4BqNR0kLY+lw==\n
application/x-download\n
Tue, 27 Mar 2007 21:06:08 +0000\n

x-amz-acl:public-read\n
x-amz-meta-checksumalgorithm:crc32\n
x-amz-meta-filechecksum:0x02661779\n
x-amz-meta-reviewedby:
joe@example.com,jane@example.com\n
/static.example.com/db-backup.dat.gz

Notice how the 'x-amz-' headers are sorted, trimmed of extra spaces, and converted to lowercase. Note also that multiple headers with the same name have been joined using commas to separate values.

Note how only the Content-Type and Content-MD5 HTTP entity headers appear in the StringToSign. The other Content-* entity headers do not.

Again, note that the CanonicalizedResource includes the bucket name, but the HTTP Request-URI does not. (The bucket is specified by the Host header.)

List all my buckets

GET / HTTP/1.1
Host: s3.us-west-1.amazonaws.com
Date: Wed, 28 Mar 2007 01:29:59 +0000

Authorization: AWS AKIAIOSFODNN7EXAMPLE:qGdzdERIC03wnaRNKh6OqZehG9s=

GET\n
\n
\n
Wed, 28 Mar 2007 01:29:59 +0000\n
/

Unicode keys

GET /dictionary/fran%C3%A7ais/pr%c3%a9f%c3%a8re HTTP/1.1
Host: s3.us-west-1.amazonaws.com
Date: Wed, 28 Mar 2007 01:49:49 +0000
Authorization: AWS AKIAIOSFODNN7EXAMPLE:DNEZGsoieTZ92F3bUfSPQcbGmlM=

GET\n
\n
\n
Wed, 28 Mar 2007 01:49:49 +0000\n
/dictionary/fran%C3%A7ais/pr%c3%a9f%c3%a8re

REST request signing problems

When REST request authentication fails, the system responds to the request with an XML error document. The information contained in this error document is meant to help developers diagnose the problem. In particular, the StringToSign element of the SignatureDoesNotMatch error document tells you exactly what request canonicalization the system is using.

Some toolkits silently insert headers that you do not know about beforehand, such as adding the header Content-Type during a PUT. In most of these cases, the value of the inserted header remains constant, allowing you to discover the missing headers by using tools such as Ethereal or tcpmon.

Query string request authentication alternative

You can authenticate certain types of requests by passing the required information as query-string parameters instead of using the Authorization HTTP header. This is useful for enabling direct third-party browser access to your private Amazon S3 data without proxying the request. The idea is to construct a "presigned" request and encode it as a URL that an end-user's browser can retrieve. Additionally, you can limit a presigned request by specifying an expiration time.

For more information on using query parameters to authenticate requests , see Authenticating Requests: Using Query Parameters (AWS Signature Version 4) in the Amazon Simple Storage Service API Reference. For examples of using the AWS SDKs to generating presigned URLs, see Sharing objects with presigned URLs .

Creating a signature

Following is an example query string authenticated Amazon S3 REST request.

GET /photos/puppy.jpg
?AWSAccessKeyId=AKIAIOSFODNN7EXAMPLE&Expires=1141889120&Signature=vjbyPxybdZaNmGa%2ByT272YEAiv4%3D HTTP/1.1
Host: awsexamplebucket1.s3.us-west-1.amazonaws.com
Date: Mon, 26 Mar 2007 19:37:58 +0000

The query string request authentication method doesn't require any special HTTP headers. Instead, the required authentication elements are specified as query string parameters:

The query string request authentication method differs slightly from the ordinary method but only in the format of the Signature request parameter and the StringToSign element. Following is pseudo-grammar that illustrates the query string request authentication method.

Signature = URL-Encode( Base64( HMAC-SHA1( YourSecretAccessKey, UTF-8-Encoding-Of( StringToSign ) ) ) );

StringToSign = HTTP-VERB + "\n" +
    Content-MD5 + "\n" +
    Content-Type + "\n" +
    Expires + "\n" +
    CanonicalizedAmzHeaders +
    CanonicalizedResource;    

YourSecretAccessKey is the AWS secret access key ID that Amazon assigns to you when you sign up to be an Amazon Web Service developer. Notice how the Signature is URL-Encoded to make it suitable for placement in the query string. Note also that in StringToSign, the HTTP Date positional element has been replaced with Expires. The CanonicalizedAmzHeaders and CanonicalizedResource are the same.

Query string request authentication

GET /photos/puppy.jpg?AWSAccessKeyId=AKIAIOSFODNN7EXAMPLE&
    Signature=NpgCjnDzrM%2BWFzoENXmpNDUsSn8%3D&
    Expires=1175139620 HTTP/1.1

Host: awsexamplebucket1.s3.us-west-1.amazonaws.com

GET\n
\n
\n
1175139620\n

/awsexamplebucket1/photos/puppy.jpg

We assume that when a browser makes the GET request, it won't provide a Content-MD5 or a Content-Type header, nor will it set any x-amz- headers, so those parts of the StringToSign are left blank.

Using Base64 encoding

HMAC request signatures must be Base64 encoded. Base64 encoding converts the signature into a simple ASCII string that can be attached to the request. Characters that could appear in the signature string like plus (+), forward slash (/), and equals (=) must be encoded if used in a URI. For example, if the authentication code includes a plus (+) sign, encode it as %2B in the request. Encode a forward slash as %2F and equals as %3D.

For examples of Base64 encoding, refer to the Amazon S3 Authentication examples.