TLS kex fix for curves with size not multiple of 8 (#4827)

Some EC curves have size not a multiple of 8, such as secp521r1.
Because of this, it will crash during the Key Exchange with a 50% chance.

Added a unit test that checks for the correct signature size, or will throw
an exception before due to an "odd-length" hexadecimal string.

Author: dmillescamps

scapy/layers/tls/keyexchange.py

@@ -840,8 +840,8 @@ def fill_missing(self):
             x = pubkey.public_numbers().x
             y = pubkey.public_numbers().y
             self.ecdh_Yc = (b"\x04" +
-                            pkcs_i2osp(x, pubkey.key_size // 8) +
-                            pkcs_i2osp(y, pubkey.key_size // 8))
+                            pkcs_i2osp(x, (pubkey.key_size + 7) // 8) +
+                            pkcs_i2osp(y, (pubkey.key_size + 7) // 8))
             if s.client_kx_privkey and s.server_kx_pubkey:
                 pms = s.client_kx_privkey.exchange(ec.ECDH(),
                                                    s.server_kx_pubkey)

test/scapy/layers/tls/tls.uts

@@ -1002,6 +1002,27 @@ bytes(pkt)
 pkt.exchkeys.fill_missing()
 assert len(pkt.exchkeys.ecdh_Yc) == 32
 
+= Building secp521r1 ecdh_Yc
+~ libressl
+
+from scapy.layers.tls.record import TLS
+from scapy.layers.tls.handshake import TLSClientKeyExchange
+
+cli_hello = bytes.fromhex('160303008f0100008b0303000027104268d53e923ce05aa04cb21b8fe33aed93266c00bd1f13ea6a6dad24000018c02cc02bc030c02fc024c023c028c027c00ac009c014c0130100004a00000013001100000e7777772e676f6f676c652e636f6d000500050100000000000a00080006001d00170019000b00020100000d00140012040105010201040305030203020206010603')
+ser_hello = bytes.fromhex('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')
+ser_cert = bytes.fromhex('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')
+
+r1 = TLS(cli_hello)
+r2 = TLS(ser_hello, tls_session=r1.tls_session.mirror())
+r3 = TLS(ser_cert, tls_session=r2.tls_session)
+
+s = r3.tls_session.mirror()
+s.client_kx_ecdh_params = 25
+pkt = TLSClientKeyExchange(tls_session=s)
+bytes(pkt)
+pkt.exchkeys.fill_missing()
+assert len(pkt.exchkeys.ecdh_Yc) == 133 # len(b'\x04') + ceil(521/8) * 2
+
 = Reading TLS test session - Extended master secret
 ~ libressl