194 reputation
6
bio website blog.tkassembled.com
location Los Angeles, CA
age
visits member for 1 year, 9 months
seen Sep 16 at 18:18

Aug
13
asked How does Diffie-Hellman key exchange work?
Jul
16
revised Does bad IV generation weaken CBC?
added 199 characters in body
Jul
16
asked Does bad IV generation weaken CBC?
Jul
2
awarded  Curious
May
25
asked Why would splitting a password output be better than separate HMACs for encryption/authentication key derivation?
May
23
comment Encryption and HMAC key derivation
What advantages are gained by just splitting the output from scrypt in key derivation? Why is not using a HMAC as a PRNG a better option?
May
23
asked Encryption and HMAC key derivation
May
20
asked Common password derivation function for different encryption methods
Jun
23
accepted Implementing PKCS#7 Padding on a Stream of Unknown Length?
May
9
awarded  Scholar
May
9
accepted Does knowledge of original file size provide a cryptographic weakness?
May
9
comment Does knowledge of original file size provide a cryptographic weakness?
Right, and I need to be able to check passphrase validity really quickly without having to read through an entire file of 16GB in length ;)
May
9
comment Does knowledge of original file size provide a cryptographic weakness?
When I start receiving data, I'm not sure of its ultimate size. Therefore, I do a cumulative HMAC-SHA256 on each chunk just to be paranoid. It adds 16 bytes every $c$ bytes, where $c$ is the chunk size, so it's not the most efficient thing in the world, but it should work for my purposes.
May
9
asked Does knowledge of original file size provide a cryptographic weakness?
Feb
27
comment Deriving HMAC key and cipher key from passphrase?
$HMAC$ is basically the RFC-2104 implementation in Python.
Feb
27
comment Validating successful decryption in AES
I'm storing the IV among other data in a JSON string in the header to the file, terminated by a null byte. When computing the MAC, should I essentially compute $HMAC(iv + c, k\_{m})$ where $c$ is the cipher text and $k\_{m}$ is the MAC key? Is that accurate?
Feb
27
comment Deriving HMAC key and cipher key from passphrase?
Quick question on your answer: is the HMAC secure if the appended label is known? In your example above, if someone discovers that the input $label\_{n}$ is "a", is the system still secure? I'd prefer that if my code was open-sourced, no advantage would be gained by a potential attacker.
Feb
26
asked What is the difference between a HMAC and a hash of data?
Feb
26
awarded  Commentator
Feb
26
comment Deriving HMAC key and cipher key from passphrase?
I guess my main problem was in understanding how HMAC(input, salt) is different from HASH(input + salt), but if it works, it works :)