コード

s3upload_signature_v2.rb

#
# POST with signature v2
#
require "base64"
require "openssl"
require "digest/sha1"

aws_secret_key = ENV["AWS_SECRET_ACCESS_KEY"]
aws_access_key_id = ENV["AWS_ACCESS_KEY_ID"]
s3_bucket = "mybucket"
target_file = "test.txt"

policy_document = <<EOS
{"expiration": "2016-12-31T00:00:00Z",
  "conditions": [
    {"bucket": "#{s3_bucket}"},
    {"acl": "private"},
    ["starts-with", "$key", ""],
    {"x-amz-server-side-encryption": "AES256"}
  ]
}
EOS

policy = Base64.encode64(policy_document).delete("\n")

signature = Base64.encode64(
  OpenSSL::HMAC.digest(
    OpenSSL::Digest::SHA1.new,
    aws_secret_key,
    policy
  )
).delete("\n")

cmd = <<__CURL_COMMAND__
curl -v -X POST \\
     -F x-amz-server-side-encryption="AES256" \\
     -F key=#{target_file} \\
     -F AWSAccessKeyId="#{aws_access_key_id}" \\
     -F acl="private" \\
     -F policy="#{policy}" \\
     -F signature="#{signature}" \\
     -F "file=@#{target_file}" \\
     https://#{s3_bucket}.s3.amazonaws.com/
__CURL_COMMAND__
puts cmd
puts `#{cmd}`

補足

上記コードを動作するためには以下も必要

IAMユーザのポリシーで s3:PutObject の権限が付与されている必要あり
acl を "private" 以外("public-read"など)に設定する場合は s3:PutObjectAcl の権限も必要

参考

HTML Forms (AWS Signature Version 2) - Amazon Simple Storage Service