test.rb

#!/usr/bin/env ruby

require 'aws-sdk-core'
require 'uri'

uri = URI('https://rds.amazonaws.com/v13/downloadCompleteLogFile/some_server/general/mysql-general.log.0')
credentials = Aws::Credentials.new('secret', 'secret')
signer = Aws::Signers::V4.new(credentials, 'rds', 'us-east-1')
req = Seahorse::Client::Http::Request.new(endpoint: uri)
signed_req = signer.sign(req)
authorization = signed_req.headers['authorization']
x_amz_date = signed_req.headers['x-amz-date']

puts `curl -v -H 'Authorization: #{authorization}' -H 'X-Amz-Date: #{x_amz_date}' https://rds.amazonaws.com/v13/downloadCompleteLogFile/some_server/general/mysql-general.log.0`

The request signature we calculated does not match the signature you provided. Check your AWS Secret Access Key and signing method. Consult the service documentation for details.

The Canonical String for this request should have been
'GET
/v13/downloadCompleteLogFile/some_server/general/mysql-general.log.0

host:rds.us-east-1.amazonaws.com
x-amz-content-sha256:
x-amz-date:20160411T144251Z

host;x-amz-content-sha256;x-amz-date
some_signature'

The String-to-Sign should have been
'AWS4-HMAC-SHA256
20160411T144251Z
20160411/us-east-1/rds/aws4_request
some_signature'