Verify signature from iOS transactionReceipt

Question

I'm attempting to verify the deprecated iOS transactionReceipt; and I'm stuck on the verifying the signature.

I've base64 decoded the receipt and parsed the initial plist, resulting with:

payload = {
  "signature"=>"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",
  "purchase-info"=>"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",
  "environment"=>"Sandbox",
  "pod"=>"100",
  "signing-status"=>"0"
}

So now I want to verify the signature, here is what I have so far:

version, sig, cert_length, cert = payload.fetch('signature')
                                  .unpack('m').first
                                  .unpack('c a128 N a*')
return false unless
  version == 2 &&
  sig.size == 128 &&
  cert.size == cert_length

cert = OpenSSL::X509::Certificate.new cert

digest = OpenSSL::Digest::SHA1.new
digest << version.to_s
digest << payload.fetch('purchase-info').unpack('m').first

cert.public_key.verify OpenSSL::Digest::SHA1.new, sig, digest.digest

The result from cert.public_key.verify is always false which is not what I want since I'm working from a real receipt.

I'm using this Quora post as a guide which has given me insight into how to unpack the signature, and I have successfully extracted all the parts from the blob.

Perhaps encoding is an issue? sig and data are both ASCII-8BIT and version is an integer.

I appreciate your help on this.

Answer 1

Hereby the answer according to https://gist.github.com/lxcid/4441003

message = [version, payload.fetch('purchase-info').unpack('m').first].pack('CA*') 

cert.public_key.verify OpenSSL::Digest::SHA1.new, sig, message