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I'm trying to "manually" compute how the premaster_secret is computed using the RSA key exchange in TLS 1.2, but can't manage to make numbers meet. I am using Scapy SSL to generate a TLS session:

  • I retrieve the e and n parameters from the first certificate's pubKey (Certificate message)
  • I retrieve the premaster_secret from Scapy
  • I compute (premaster_secrete mod n)
  • I compare this result to the Encrypted Premaster sent in the Client Key Exchange message, but it is never the same

Am I missing something?

  • 2
    I think the problem is that you are using unpadded "textbook RSA" vs. actual padded RSA. – StackzOfZtuff May 10 '16 at 6:32
  • Yes, pkcs#1 v1.5 – Z.T. May 10 '16 at 10:20
  • That's indeed what I was missing. I'm almost there. It works for a site such as www.wikipedia.org but not www.google.com - even though I am using its certificate to get the RSA public key. – user7094 May 11 '16 at 1:49
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Thanks to the responses I was able to get it working. Here is some Python pseudo-code which explains the algorithm actually used by TLS 1.2:

# Come up with a random premaster_secret
# It needs to start with the TLS version (0x0303)
premaster_secret = '\x03\x03' + os.urandom(46)

# Retrieve the RSA parameters from the certificate public key
RSA_n = ...
RSA_e = ...

# Encodes premaster_secret using PKCS#1 v1.5
premaster_secret = '\x00\x02' + '\x12' * (256 - 3 - len(premaster_secret)) + '\x00' + premaster_secret

# Converts the result into a number and encrypts it
premaster_secret_nb = int(binascii.hexlify(premaster_secret), 16)
encrypted_premaster_secret = pow(premaster_secret_nb, RSA_e, RSA_n)

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