Sunday, December 13, 2015

CommonCrypto3

This is a brief note about a big project (at least for a one-day thing). I finished a Cocoa app that implements basic AES-mode encryption and decryption from the CommonCrypto library. It does not do any I/O.

For now, you can just watch it run in the debugger, using pre-formed messages. But it can handle messages that are larger than a single block by breaking the data into blocks and encrypting/decrypting each one.

One nice thing is the BinaryData class, which implements some functionality around a [UInt8] including the Indexable and CustomStringConvertible protocols, and a convenience initializer that takes a String representing binary data.



BinaryData is a class because it has a derived class Key, which implements key "stretching" from CommonCrypto.

At the very end, I ran into trouble because I did not really understand the CBC protocol.

When encrypting, the output ciphertext becomes the initialization vector for the next round. But when decrypting, the input (which is also the ciphertext) becomes the initialization vector for the next round.

Here is the output for a test:


pw: my secret
salt: 3356ec169bb6
msgText: a much longer and still really big secret
61206d756368206c6f6e67657220616e64207374696c6c207265616c6c792062696720736563726574
encryptMany
encrypt round: 1
encryptOneChunk
msgLen: 16
msg:
61206d756368206c6f6e67657220616e
keyLen: 16
iv:
39131435ae3d5bbf2e300ab5edddc8c9
status: 0
result:
55bd582296f708842e5d0833e3673a99

encrypt round: 2
encryptOneChunk
msgLen: 16
msg:
64207374696c6c207265616c6c792062
keyLen: 16
iv:
55bd582296f708842e5d0833e3673a99
status: 0
result:
ded208434eb0a4e753e6cdf1c41da54c

encrypt round: 3
encryptOneChunk
msgLen: 16
msg:
69672073656372657400000000000000
keyLen: 16
iv:
ded208434eb0a4e753e6cdf1c41da54c
status: 0
result:
66e7cb3ba1b51a0d657edad13de97b37

cipherData: 55bd582296f708842e5d0833e3673a99ded208434eb0a4e753e6cdf1c41da54c66e7cb3ba1b51a0d657edad13de97b37

decryptMany
decryptOneChunk
data:
55bd582296f708842e5d0833e3673a99
keyLen: 16
iv:
39131435ae3d5bbf2e300ab5edddc8c9
status: 0
result:
61206d756368206c6f6e67657220616e

decryptOneChunk
data:
ded208434eb0a4e753e6cdf1c41da54c
keyLen: 16
iv:
55bd582296f708842e5d0833e3673a99
status: 0
result:
64207374696c6c207265616c6c792062

decryptOneChunk
data:
66e7cb3ba1b51a0d657edad13de97b37
keyLen: 16
iv:
ded208434eb0a4e753e6cdf1c41da54c
status: 0
result:
69672073656372657400000000000000

decryptedData: 61206d756368206c6f6e67657220616e64207374696c6c207265616c6c79206269672073656372657400000000000000
a much longer and still really big secret


Whoa... there were some bytes in the cut and pasted text that did not appear in the debug console nor in the editor here on blogger,, but did appear in the Preview. They are the null bytes used to pad the plaintext. They do not appear in the final product. Curious...

The Xcode project is on github here.

It was a world of fun, but I think I am done with CommonCrypto for now.

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