STS を使用した完全なバージョン 4 署名プロセスの例 (Python)

この記事は2017年11月26日に書かれたものです。
現在は内容が古い可能性が高いのでご注意ください。

どこかで需要があるかもしれないので、STS から取得した一時的な認証情報を使用した AWS のバージョン 4 署名プロセスの例を作ってみました。
GET と Authorization ヘッダーの使用 (Python) の一部を変更したものです。

この例では x-amz-security-token を末尾に追加しているだけですが、タスク 1: 署名バージョン 4 の正規リクエストを作成する にある通り “正規ヘッダーリストを作成するには、すべてのヘッダー名を小文字に変換し、先頭および末尾のスペースを削除”、”文字コードによってソート” など細かい部分を意識する必要があります。

以下のサンプルコードを実行できる状態にするためには、実際に AssumeRole の対象になるロール (<account id>, <role name>) の部分を環境に合わせて変更する必要があります。

# AWS Version 4 signing example (with STS)

# EC2 API (DescribeRegions)

# See: http://docs.aws.amazon.com/general/latest/gr/sigv4_signing.html
# This version makes a GET request and passes the signature
# in the Authorization header.
import sys, os, base64, datetime, hashlib, hmac
import requests # pip install requests

# ************* TASK 0: GET A CREDENTIAL FROM STS *************
import boto3

IAM_ROLE_ARN = 'arn:aws:iam:::role/'

client = boto3.client('sts')
response = client.assume_role(
RoleArn=IAM_ROLE_ARN,
RoleSessionName='sample'
)
access_key = response['Credentials']['AccessKeyId']
secret_key = response['Credentials']['SecretAccessKey']
session_token = response['Credentials']['SessionToken']

# ************* REQUEST VALUES *************
method = 'GET'
service = 'ec2'
host = 'ec2.amazonaws.com'
region = 'us-east-1'
endpoint = 'https://ec2.amazonaws.com'
request_parameters = 'Action=DescribeRegions&amp;Version=2013-10-15'

# Key derivation functions. See:
# http://docs.aws.amazon.com/general/latest/gr/signature-v4-examples.html#signature-v4-examples-python
def sign(key, msg):
return hmac.new(key, msg.encode('utf-8'), hashlib.sha256).digest()

def getSignatureKey(key, dateStamp, regionName, serviceName):
kDate = sign(('AWS4' + key).encode('utf-8'), dateStamp)
kRegion = sign(kDate, regionName)
kService = sign(kRegion, serviceName)
kSigning = sign(kService, 'aws4_request')
return kSigning

# Read AWS access key from env. variables or configuration file. Best practice is NOT
# to embed credentials in code.
#access_key = os.environ.get('AWS_ACCESS_KEY_ID')
#secret_key = os.environ.get('AWS_SECRET_ACCESS_KEY')
if access_key is None or secret_key is None:
print 'No access key is available.'
sys.exit()

# Create a date for headers and the credential string
t = datetime.datetime.utcnow()
amzdate = t.strftime('%Y%m%dT%H%M%SZ')
datestamp = t.strftime('%Y%m%d') # Date w/o time, used in credential scope

# ************* TASK 1: CREATE A CANONICAL REQUEST *************
# http://docs.aws.amazon.com/general/latest/gr/sigv4-create-canonical-request.html

# Step 1 is to define the verb (GET, POST, etc.)--already done.

# Step 2: Create canonical URI--the part of the URI from domain to query
# string (use '/' if no path)
canonical_uri = '/'

# Step 3: Create the canonical query string. In this example (a GET request),
# request parameters are in the query string. Query string values must
# be URL-encoded (space=%20). The parameters must be sorted by name.
# For this example, the query string is pre-formatted in the request_parameters variable.
canonical_querystring = request_parameters

# Step 4: Create the canonical headers and signed headers. Header names
# must be trimmed and lowercase, and sorted in code point order from
# low to high. Note that there is a trailing \n.
canonical_headers = 'host:' + host + '\n' + 'x-amz-date:' + amzdate + '\n' + 'x-amz-security-token:' + session_token + '\n'

# Step 5: Create the list of signed headers. This lists the headers
# in the canonical_headers list, delimited with ";" and in alpha order.
# Note: The request can include any headers; canonical_headers and
# signed_headers lists those that you want to be included in the
# hash of the request. "Host" and "x-amz-date" are always required.
signed_headers = 'host;x-amz-date;x-amz-security-token'

# Step 6: Create payload hash (hash of the request body content). For GET
# requests, the payload is an empty string ("").
payload_hash = hashlib.sha256('').hexdigest()

# Step 7: Combine elements to create create canonical request
canonical_request = method + '\n' + canonical_uri + '\n' + canonical_querystring + '\n' + canonical_headers + '\n' + signed_headers + '\n' + payload_hash

# ************* TASK 2: CREATE THE STRING TO SIGN*************
# Match the algorithm to the hashing algorithm you use, either SHA-1 or
# SHA-256 (recommended)
algorithm = 'AWS4-HMAC-SHA256'
credential_scope = datestamp + '/' + region + '/' + service + '/' + 'aws4_request'
string_to_sign = algorithm + '\n' + amzdate + '\n' + credential_scope + '\n' + hashlib.sha256(canonical_request).hexdigest()

# ************* TASK 3: CALCULATE THE SIGNATURE *************
# Create the signing key using the function defined above.
signing_key = getSignatureKey(secret_key, datestamp, region, service)

# Sign the string_to_sign using the signing_key
signature = hmac.new(signing_key, (string_to_sign).encode('utf-8'), hashlib.sha256).hexdigest()

# ************* TASK 4: ADD SIGNING INFORMATION TO THE REQUEST *************
# The signing information can be either in a query string value or in
# a header named Authorization. This code shows how to use a header.
# Create authorization header and add to request headers
authorization_header = algorithm + ' ' + 'Credential=' + access_key + '/' + credential_scope + ', ' + 'SignedHeaders=' + signed_headers + ', ' + 'Signature=' + signature

# The request can include any headers, but MUST include "host", "x-amz-date",
# and (for this scenario) "Authorization". "host" and "x-amz-date" must
# be included in the canonical_headers and signed_headers, as noted
# earlier. Order here is not significant.
# Python note: The 'host' header is added automatically by the Python 'requests' library.
headers = {'x-amz-date':amzdate, 'x-amz-security-token':session_token, 'Authorization':authorization_header}

# ************* SEND THE REQUEST *************
request_url = endpoint + '?' + canonical_querystring

print '\nBEGIN REQUEST++++++++++++++++++++++++++++++++++++'
print 'Request URL = ' + request_url
r = requests.get(request_url, headers=headers)

print '\nRESPONSE++++++++++++++++++++++++++++++++++++'
print 'Response code: %d\n' % r.status_code
print r.text

以下、参考のための変更差分

@@ -8,6 +8,23 @@
import sys, os, base64, datetime, hashlib, hmac
import requests # pip install requests

+
+# ************* TASK 0: GET A CREDENTIAL FROM STS *************
+import boto3
+
+IAM_ROLE_ARN = 'arn:aws:iam:::role/'
+
+client = boto3.client('sts')
+response = client.assume_role(
+ RoleArn=IAM_ROLE_ARN,
+ RoleSessionName='test'
+)
+access_key = response['Credentials']['AccessKeyId']
+secret_key = response['Credentials']['SecretAccessKey']
+session_token = response['Credentials']['SessionToken']
+print(response['Credentials'])
+
+
# ************* REQUEST VALUES *************
method = 'GET'
service = 'ec2'
@@ -30,8 +47,8 @@

# Read AWS access key from env. variables or configuration file. Best practice is NOT
# to embed credentials in code.
-access_key = os.environ.get('AWS_ACCESS_KEY_ID')
-secret_key = os.environ.get('AWS_SECRET_ACCESS_KEY')
+#access_key = os.environ.get('AWS_ACCESS_KEY_ID')
+#secret_key = os.environ.get('AWS_SECRET_ACCESS_KEY')
if access_key is None or secret_key is None:
print 'No access key is available.'
sys.exit()
@@ -60,14 +77,14 @@
# Step 4: Create the canonical headers and signed headers. Header names
# must be trimmed and lowercase, and sorted in code point order from
# low to high. Note that there is a trailing \n.
-canonical_headers = 'host:' + host + '\n' + 'x-amz-date:' + amzdate + '\n'
+canonical_headers = 'host:' + host + '\n' + 'x-amz-date:' + amzdate + '\n' + 'x-amz-security-token:' + session_token + '\n'

# Step 5: Create the list of signed headers. This lists the headers
# in the canonical_headers list, delimited with ";" and in alpha order.
# Note: The request can include any headers; canonical_headers and
# signed_headers lists those that you want to be included in the
# hash of the request. "Host" and "x-amz-date" are always required.
-signed_headers = 'host;x-amz-date'
+signed_headers = 'host;x-amz-date;x-amz-security-token'

# Step 6: Create payload hash (hash of the request body content). For GET
# requests, the payload is an empty string ("").
@@ -104,7 +121,7 @@
# be included in the canonical_headers and signed_headers, as noted
# earlier. Order here is not significant.
# Python note: The 'host' header is added automatically by the Python 'requests' library.
-headers = {'x-amz-date':amzdate, 'Authorization':authorization_header}
+headers = {'x-amz-date':amzdate, 'x-amz-security-token':session_token, 'Authorization':authorization_header}

# ************* SEND THE REQUEST *************

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