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Alexa request signature verification

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Alexa request signature verification
« on: December 13, 2016, 07:07:16 am »
I've been hacking together a server-based skill & doing verification in PHP. The Alexa request comes in with HTTP headers including a signature such as:

Converting that to binary with base64_decode() and decrypting with openssl_public_decrypt()  gives this, after bin2hex():


The SHA-1 hash of the request body (JSON data) in this case is:


So it appears that the decrypted signature is the 40-character SHA-1 hash of the JSON data prepended with 30 characters of something else:

33518d943e1851830b2e596a1c025b8f9be55d74  matches the SHA-1 hash of the JSON data

So what is the 30-character first part?