Random things #9

Written on 22 March 2015, 10:48am

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1. Aspect oriented programming (AOP)

In the wikipedia example about AOP, transactions, security and logging represent cross-cutting concerns. If we need to change one of these (ex. security) – then it will be a major effort, since the concerns are tangled and the related methods appear scattered around all the code.

AOP attempts to solve this problem by allowing to express cross-cutting concerns in stand-alone modules called aspects. Aspects can contain
advice – code joined to specified points in the program and
inter-type declarations – structural members added to other classes.

Drawbacks: If a programmer makes a logical mistake in expressing crosscutting, it can lead to widespread program failure.
Conversely, another programmer may change the join points in a program in ways that the aspect writer did not anticipate, with unforeseen consequences.

2. HTTPS and MTU Path discovery

I recently encountered this interesting problem with HTTPS and MTU. It is explained entirely by Mark Maunder – ‘Routers treat HTTPS and HTTP traffic differently‘. I will just summarize it:
– HTTPS servers set the ‘Do not fragment’ IP flag
– if a server sends a big HTTPS packet and a router does not allow that packet size, then the router will not break that packet (see previous point).
– so the router will simply drop the packet and send back an ICMP (Internet Control Message Protocol) message telling the host to reduce the MTU size and resend the packet
– but if the network administrator decided to block all the ICMP traffic, then the host will never see the problem
– the solution in my case was to decrease the MTU size (1400)

The same issue described also here.

3. Information security standards

(more…)

TLS handshake

Written on 12 February 2015, 09:59pm

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The initial communication in a HTTPS connection relies on a traditional D-H key exchange – which will serve as symmetric encryption key for the rest of the HTTPS conversation.
The outline of the handshake is:
– client/server hello: list the available encryption algorithms
– certificate exchange
– certificate validation
– key exchange
– finished

Here is the process explained in layman words:

1. Client sends a Client hello message to the server with some metadata (TLS version, cipher algorithms, compression methods)
2. The server replies with a Server hello message to the client with the corresponding metadata + the Server public certificate signed by a CA.
3. The client verifies the server digital certificate and cipher a symmetric cryptography key using an asymmetric cryptography algorithm, attaching the server public key and an encrypted message for verification purposes.
4. The server decrypts the key using its private key and decrypts the verification message with it, then replies with the verification message decrypted and signed with its private key
5. The client confirm the server identity, cipher the agreed key and sends a finished message to the server, attaching the encrypted agreed key.
6. The server sends a finished message to the client, encrypted with the agreed key.
From now on the TLS session communicates information encrypted with the agreed key
https://github.com/alex/what-happens-when

The same process – explained in full details.

Note: Excepting the initial TLS handshake, the other HTTPS content (headers + payload) is encrypted with the key agreed during the TLS handshake.

Understanding more about hashing

Written on 11 February 2015, 11:21pm

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Brute force vs dictionary attack vs rainbow tables

Brute force: least efficient, tries all the possible values.
Dictionary attack: tries a predefined list of values
Rainbow tables: contain pre-computed hashed values. If a single salt per db is used, then the pre-computed hashed values have to be updated (to include the salt). If a single salt per password is used, then the pre-computed hashed values have to be updated for each password.
More here.
Note: rainbow table can only be used if the hashed values are available.

Hashing properties

1. Collision resistance: it should not be realistically possible to find two messages giving the same hash
2. Pre-image resistance: given the hash, it should not be realistically possible to obtain the message (we ask an adversary, given only H(m), to find m or some m′ such that H(m′) = H(m) )
3. Second pre-image resistance: given both the hash and the message, it should not be realistically possible to obtain another message with the same hash.
More here and here

Remember:
Hashing is not compression (you can’t go back to the original string)
Hashing is not encryption (for encryption you need a key)

What are you hashing

– not sensible data: use MD5/SHA family of hashing functions
– sensible data (like passwords): PBKDF2 (*), bcrypt or scrypt

Key derivation

This is about transforming a password into a key that can be used for encryption. Think about encrypting a file with a password.

The output of a password hashing function is acceptable as a symmetric key, after possible truncation to the required size.
link
LastPass utilizes the PBKDF2 function implemented with SHA-256 to turn your master password into your encryption key.
LastPass user manual

(*) PBKDF2 is in fact a Key derivation function, not a hashing function.