The procedures for encrypting and decrypting the session key encrypt_local_key and decrypt_remote_key are called in the get_common_key module. Both of these operations are performed in Extra Systems Cypher Net via the same long_power_mod function (modulo exponentiation), but they use different RSA keys: encryption is performed with one's own private key, and decryption is performed with the partner's public key.
typedef struct session_key_item { unsigned char packed[PACKED_VALUE_LENTH]; unsigned char unpacked[VALUE_LENTH]; } SESSIONKEYITEM; SESSIONKEYITEM local_key, remote_key, common_key; unsigned char crypted_local_key[PACKED_VALUE_LENTH], crypted_remote_key[PACKED_VALUE_LENTH]; void encrypt_local_key(void) { unsigned char crypted_value[VALUE_LENTH]; long_power_mod(local_key.unpacked, encrypt_rsa_key.exponent, encrypt_rsa_key.modulus, crypted_value); pack_value(crypted_value, crypted_local_key); } void decrypt_remote_key(void) { unsigned char crypted_value[VALUE_LENTH]; unpack_value(crypted_remote_key, crypted_value); long_power_mod(crypted_value, decrypt_rsa_key.exponent, decrypt_rsa_key.modulus, remote_key.unpacked); pack_value(remote_key.unpacked, remote_key.packed); }
The algorithm for the pack_value and unpack_value procedures can be found here.
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