Authenticating Requests: Using Query Parameters (AWS Signature Version 4) (original) (raw)
As described in the authentication overview (see Authentication Methods), you can provide authentication information using query string parameters. Using query parameters to authenticate requests is useful when you want to express a request entirely in a URL. This method is also referred as presigning a URL.
A use case scenario for presigned URLs is that you can grant temporary access to your Amazon S3 resources. For example, you can embed a presigned URL on your website or alternatively use it in command line client (such as Curl) to download objects.
Note
You can also use the AWS CLI to create presigned URLs. For more information, see presign in the AWS CLI Command Reference.
The following is an example presigned URL.
https://examplebucket.s3.amazonaws.com/test.txt
?X-Amz-Algorithm=AWS4-HMAC-SHA256
&X-Amz-Credential=<your-access-key-id>/20130721/us-east-1/s3/aws4_request
&X-Amz-Date=20130721T201207Z
&X-Amz-Expires=86400
&X-Amz-SignedHeaders=host
&X-Amz-Signature=<signature-value> In the example URL, note the following:
- The line feeds are added for readability.
- The
X-Amz-Credentialvalue in the URL shows the "/" character only for readability. In practice, it should be encoded as%2F. For example:
&X-Amz-Credential=<your-access-key-id>%2F20130721%2Fus-east-1%2Fs3%2Faws4_request The following table describes the query parameters in the URL that provide authentication information.
| Query String Parameter Name | Example Value |
|---|---|
| X-Amz-Algorithm | Identifies the version of AWS Signature and the algorithm that you used to calculate the signature. For AWS Signature Version 4, you set this parameter value toAWS4-HMAC-SHA256. This string identifies AWS Signature Version 4 (AWS4) and the HMAC-SHA256 algorithm (HMAC-SHA256). |
| X-Amz-Credential | In addition to your access key ID, this parameter also provides scope (AWS Region and service) for which the signature is valid. This value must match the scope you use in signature calculations, discussed in the following section. The general form for this parameter value is as follows: ////aws4_request For example: AKIAIOSFODNN7EXAMPLE/20130721/us-east-1/s3/aws4_request For Amazon S3, the AWS-service string iss3. For a list of S3 AWS-region strings, seeRegions and Endpoints in the AWS General Reference. |
| X-Amz-Date | The date and time format must follow the ISO 8601 standard, and must be formatted with the "yyyyMMddTHHmmssZ" format. For example if the date and time was "08/01/2016 15:32:41.982-700" then it must first be converted to UTC (Coordinated Universal Time) and then submitted as "20160801T223241Z". |
| X-Amz-Expires | Provides the time period, in seconds, for which the generated presigned URL is valid. For example, 86400 (24 hours). This value is an integer. The minimum value you can set is 1, and the maximum is 604800 (seven days). A presigned URL can be valid for a maximum of seven days because the signing key you use in signature calculation is valid for up to seven days. |
| X-Amz-SignedHeaders | Lists the headers that you used to calculate the signature. The following headers are required in the signature calculations: The HTTP host header. Any x-amz-* headers that you plan to add to the request. Note For added security, you should sign all the request headers that you plan to include in your request. |
| X-Amz-Signature | Provides the signature to authenticate your request. This signature must match the signature Amazon S3 calculates; otherwise, Amazon S3 denies the request. For example, 733255ef022bec3f2a8701cd61d4b371f3f28c9f193a1f02279211d48d5193d7 Signature calculations are described in the following section. |
| X-Amz-Security-Token | Optional credential parameter if using credentials sourced from the STS service. |
Calculating a Signature
The following diagram illustrates the signature calculation process.
The following table describes the functions that are shown in the diagram. You need to implement code for these functions.
| Function | Description |
|---|---|
| Lowercase() | Convert the string to lowercase. |
| Hex() | Lowercase base 16 encoding. |
| SHA256Hash() | Secure Hash Algorithm (SHA) cryptographic hash function. |
| HMAC-SHA256() | Computes HMAC by using the SHA256 algorithm with the signing key provided. This is the final signature. |
| Trim() | Remove any leading or trailing whitespace. |
| UriEncode() | URI encode every byte. UriEncode() must enforce the following rules: URI encode every byte except the unreserved characters: 'A'-'Z', 'a'-'z', '0'-'9', '-', '.', '_', and '~'. The space character is a reserved character and must be encoded as "%20" (and not as "+"). Each URI encoded byte is formed by a '%' and the two-digit hexadecimal value of the byte. Letters in the hexadecimal value must be uppercase, for example "%1A". Encode the forward slash character, '/', everywhere except in the object key name. For example, if the object key name isphotos/Jan/sample.jpg, the forward slash in the key name is not encoded. ImportantThe standard UriEncode functions provided by your development platform may not work because of differences in implementation and related ambiguity in the underlying RFCs. We recommend that you write your own custom UriEncode function to ensure that your encoding will work. To see an example of a UriEncode function in Java, seeJava Utilities on the GitHub website. |
For more information about the signing process (details of creating a canonical request, string to sign, and signature calculations), see Signature Calculations for the Authorization Header: Transferring Payload in a Single Chunk (AWS Signature Version 4). The process is generally the same except that the creation ofCanonicalRequest in a presigned URL differs as follows:
- You don't include a payload hash in the Canonical Request, because when you create a presigned URL, you don't know the payload content because the URL is used to upload an arbitrary payload. Instead, you use a constant string
UNSIGNED-PAYLOAD. - The Canonical Query String must include all the query parameters from the preceding table except for
X-Amz-Signature. - For S3, you must include the
X-Amz-Security-Tokenquery parameter in the URL if using credentials sourced from the STS service. - Canonical Headers must include the HTTP
hostheader. If you plan to include any of thex-amz-*headers, these headers must also be added for signature calculation. You can optionally add all other headers that you plan to include in your request. For added security, you should sign as many headers as possible. If you add a signed header that is also a signed query parameter, and they differ in value, you will receive anInvalidRequesterror as the input is conflicting.
An Example
Suppose you have an object test.txt in yourexamplebucket bucket. You want to share this object with others for a period of 24 hours (86400 seconds) by creating a presigned URL.
https://examplebucket.s3.amazonaws.com/test.txt
?X-Amz-Algorithm=AWS4-HMAC-SHA256
&X-Amz-Credential=AKIAIOSFODNN7EXAMPLE%2F20130524%2Fus-east-1%2Fs3%2Faws4_request
&X-Amz-Date=20130524T000000Z&X-Amz-Expires=86400&X-Amz-SignedHeaders=host
&X-Amz-Signature=<signature-value>The following steps illustrate first the signature calculations and then construction of the presigned URL. The example makes the following additional assumptions:
- Request timestamp is
Fri, 24 May 2013 00:00:00 GMT. - The bucket is in the US East (N. Virginia) region, and the credential
Scopeand theSigning Keycalculations useus-east-1as the region specifier. For more information, see Regions and Endpoints in the AWS General Reference.
You can use this example as a test case to verify the signature that your code calculates; however, you must use the same bucket name, object key, time stamp, and the following example credentials:
| Parameter | Value |
|---|---|
| AWSAccessKeyId | AKIAIOSFODNN7EXAMPLE |
| AWSSecretAccessKey | wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY |
StringToSign
CanonicalRequest
GET /test.txt X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIOSFODNN7EXAMPLE%2F20130524%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20130524T000000Z&X-Amz-Expires=86400&X-Amz-SignedHeaders=host host:examplebucket.s3.amazonaws.com host UNSIGNED-PAYLOADStringToSign
AWS4-HMAC-SHA256 20130524T000000Z 20130524/us-east-1/s3/aws4_request 3bfa292879f6447bbcda7001decf97f4a54dc650c8942174ae0a9121cf58ad04SigningKey
signing key = HMAC-SHA256(HMAC-SHA256(HMAC-SHA256(HMAC-SHA256("AWS4" + "<YourSecretAccessKey>","20130524"),"us-east-1"),"s3"),"aws4_request") Signature
aeeed9bbccd4d02ee5c0109b86d86835f995330da4c265957d157751f604d404 Now you have all information to construct a presigned URL. The resulting URL for this example is shown as follows (you can use this to compare your presigned URL):
https://examplebucket.s3.amazonaws.com/test.txt?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=AKIAIOSFODNN7EXAMPLE%2F20130524%2Fus-east-1%2Fs3%2Faws4_request&X-Amz-Date=20130524T000000Z&X-Amz-Expires=86400&X-Amz-SignedHeaders=host&X-Amz-Signature=aeeed9bbccd4d02ee5c0109b86d86835f995330da4c265957d157751f604d404 Example 2
The following is an example (unrelated to the previous example) showing a presigned URL with the X-Amz-Security-Token parameter.
https://examplebucket.s3.us-east-1.amazonaws.com/test.txt
?X-Amz-Algorithm=AWS4-HMAC-SHA256
&X-Amz-Credential=AKIAIOSFODNN7EXAMPLE%2F20130524%2Fus-east-1%2Fs3%2Faws4_request
&X-Amz-Date=20200524T000000Z&X-Amz-Expires=86400&X-Amz-SignedHeaders=host
&X-Amz-Security-Token=IQoJb3JpZ2luX2VjEMv%2F%2F%2F%2F%2F%2F%2F%2F%2F%2FwEaCXVzLWVhc3QtMSJGMEQCIBSUbVdj9YGs2g0HkHsOHFdkwOozjARSKHL987NhhOC8AiBPepRU1obMvIbGU0T%2BWphFPgK%2Fqpxaf5Snvm5M57XFkCqlAgjz%2F%2F%2F%2F%2F%2F%2F%2F%2F%2F8BEAAaDDQ3MjM4NTU0NDY2MCIM83pULBe5%2F%2BNm1GZBKvkBVslSaJVgwSef7SsoZCJlfJ56weYl3QCwEGr2F4BmCZZyFpmWEYzWnhNK1AnHMj5nkfKlKBx30XAT5PZGVrmq4Vkn9ewlXQy1Iu3QJRi9Tdod8Ef9%2FyajTaUGh76%2BF5u5a4O115jwultOQiKomVwO318CO4l8lv%2F3HhMOkpdanMXn%2B4PY8lvM8RgnzSu90jOUpGXEOAo%2F6G8OqlMim3%2BZmaQmasn4VYRvESEd7O72QGZ3%2BvDnDVnss0lSYjlv8PP7IujnvhZRnj0WoeOyMe1lL0wTG%2Fa9usH5hE52w%2FYUJccOn0OaZuyROuVsRV4Q70sbWQhUvYUt%2B0tUMKzm8vsFOp4BaNZFqobbjtb36Y92v%2Bx5kY6i0s8QE886jJtUWMP5ldMziClGx3p0mN5dzsYlM3GyiJ%2FO1mWkPQDwg3mtSpOA9oeeuAMPTA7qMqy9RNuTKBDSx9EW27wvPzBum3SJhEfxv48euadKgrIX3Z79ruQFSQOc9LUrDjR%2B4SoWAJqK%2BGX8Q3vPSjsLxhqhEMWd6U4TXcM7ku3gxMbzqfT8NDg%3D
&X-Amz-Signature=<signature-value>