# Tag Info

28

You are correct that it is a "bad hash". In fact it is not a hash at all. I've worked at a company that used a slightly different scheme for obfuscating database keys/numbers in URLs. And I also worked for another company that used a scheme that looked surprisingly similar for unlock codes for electronic devices. The formula for converting "hashes" back ...

19

This article is a nice introduction to the concept of white-box cryptography. It can be viewed as the devious cousin of code obfuscation. In simpler words: usually, security of a cryptographic algorithm is studied in the "black-box" model: e.g., for symmetric encryption, the attacker is given access to a "device" which runs the encryption algorithm with a ...

15

Generally speaking, a lookup-table can be implemented in constant time by doing it as if it was a hardware circuit. Consider a multiplexer: this is a circuit which accepts three inputs a, b and c, and yields one output d which is equal to a if c = 0, to b otherwise (I am talking about single-bit values here). A multiplexer can be used to implement a 1→1 ...

14

Usually, when the user registers, you will generate a random value to become the salt. Then, in the user database, you store the user's name, salt, and hash generated using the password and salt (and whatever else is relevant for a user table). Note that doing it this way allows each user to have a unique salt. Each user having a unique salt greatly ...

14

The book Cryptography Engineering devotes part of a chapter to this topic. Overwriting sensitive data with zeroes is a good start, but there are lots of other considerations. If you rely on a language's default object destruction behavior to zero the memory, it's possible for an unexpected error to prematurely halt the program's execution without it ...

11

Public-key encryption on microcontrollers Erik-oliver Blaß , Martina Zitterbart. "Towards Acceptable Public-Key Encryption in Sensor Networks". 2005. "dsPIC DSC Asymmetric Key Embedded Encryption Library" an implementation of RSA, Diffie-Hellman, DSA, SHA-1, MD5. (Are such 16-bit microcontrollers in your range of interest?) "Links to Embedded Crypto ...

11

The paper Enabling Standardized Cryptography on Ultra-Constrained 4-bit Microcontrollers (page 255) describes such an implementation.

10

Very small platforms usually have very little RAM, because RAM uses quite a lot of space (SRAM is 6 transistors per bit, i.e. 12 gates per byte -- counting 4 transistors for a "gate"). Among asymmetric algorithms, your best bet for software with very strict memory constraints is elliptic curves (ECDH for key exchange, ECDSA for signatures -- for asymmetric ...

9

This is not a limitation of the cryptographic functions, like SHA or PBKDF, since the zero byte isn't processed any differently. Since the purpose of a salt is generally to travel alongside a human password, libraries that handle the password as a zero-terminated string might also handle the salt as such a string. Obviously, a 0x00 in the salt would ...

9

Just to complement Thomas's reply, here are a couple of papers that do not rely on SIMD registers to implement bitsliced AES: How Far Can We Go on the x64 Processors? (source in appendix) A Fast and Cache-Timing Resistant Implementation of the AES (source code)

9

Yes, that omission weakens the cipher: the output $\mathtt K$ has a short cycle (at most 65280 bytes) for a sizable class of keys (one in 65536). The following details why. Because earlier code leaves $\mathtt i=256$ and the first execution of i := (i + 1) mod 256 makes that equivalent to $\mathtt i=0$, not initializing $\mathtt i$ makes no difference in ...

9

I don't think you can do any better than the best random number you can generate.

8

I really don't have an answer (other than saying that storing a hash of the password is good as any other way of solving your immediate problem; there are other ways, but they all allow an attacker to run a dictionary attack on the database). On the other hand, I do have these comments on what you're doing: If getting decrypted gibberish will really crash ...

8

Yes, you need to use the same salt each time you use a hash created with that salt. Typically a pseudorandom salt is generated for each user, and stored alongside the hash. Many hashing libraries (for example, bcrypt) create hashes with the salt embedded in them.

8

That's because you can do ECDH by exchanging only the X coordinates of your public value; as long as the shared secret depends only on the x coordinate, everything works out. Here's the fundamental property of elliptic curves that makes this work, the x coordinate of $nP$ is only a function of the x coordinate of $P$ (and $n$); it does not depend on the y ...

8

Yes, AES could be implemented on a 4-bit micro-controller such as this EM6626, and that would not be rocket science or stupidly slow. This application note illustrates that all kind of 8-bit operations are simple, and table lookups are possible. In fact, tables are not even indispensable if performance is non-critical; see this minimalist AES source code in ...

8

You could encrypt them using some key derived from the user's password (to your site). Of course, this assumes that you get your user's passwords in plain text (or in any form which is always the same) - thus you need to have an encrypted connection to your user. Do not allow any non-SSL login. You can use some key derivation function like PBKDF or bcrypt ...

8

You can use TLS 1.0 as guidance: it is the direct successor of SSL 3.0, so many things are quite similar, and in some respects TLS 1.0 is a bit clearer. In section 6.3 you will find the key generation process, with the exact sentence: To generate the key material, compute [...] until enough output has been generated. Then the key_block is ...

8

Here's a paper showing how to realize the BGN cryptosystem with a prime order group. You could implement the cryptosystem with PBC or one of the other paring libs. "Converting Pairing-Based Cryptosystems from Composite-Order Groups to Prime-Order Groups" David Mandell Freeman Eurocrypt 2010 http://theory.stanford.edu/~dfreeman/papers/subgroups.pdf ...

8

I don't think idea 1 can be made to work at all. The main point is that in order to generate a correct secret decryption key, the key generator must know the order of $\mathbb Z^*_n$, i.e., the totient of the modulus $n$. The generator knows that $n=p \cdot q$, where it believes that $p$ and $q$ are primes, and so it believes that the totient is ...

7

First recommendation: Don't invent your own protocol, but use an existing one. Use SSL/TLS, in the newest version possible if you don't have to provide downwards compatibility to existing clients. This will take care of most problems here, you simply put in a pair of plaintext data streams, and get a pair of encrypted streams. There are TLS implementations ...

7

You are right to be confused, because you could just as well have asked "How can I encrypt a file using a CPU that supports xor, shifts and rotates?" The answer is that of course you can, but there is obviously a lot more to it, if you are going to do it right. AES is just a standard block cipher primitive. The only thing this standard tells you, is how to ...

7

Not quite on topic, but closely related: 8 bit microcontrollers with ISO 14443 "RFID" communication hardware from e.g. Infineon, NXP, STM (in alphabetical order), and several others, now abound. Some of these ICs cost $1 within a binary order of magnitude, when ordered in 6-decimal-figures quantities. They come with efficient hardware for TRNG, TDES, and ... 7 The reason we, at the end of the compression function, add the input to the compression function, well, that's because otherwise the compression function would be invertible, and that would be bad. Without that final step, the compression function would be invertible in this sense: given a desired compression function output and a message block, we would be ... 7 I agree that reverse-engineering the binary code might be a worthwhile approach. Another option is to try a chosen-plaintext attack. e.g., try hashing 0000, 0001, 0002, ..., 0009, 0010, .., 0090, .., 1000, .., 9000, and see what you can learn from that. 7 Yes there are. The first publicly accessible McEliece implementation was this one from The Error Correcting Codes (ECC) Page, but it isn't particularly useful for reading, being quite obfuscated. There's INRIA's SECRET group implementation called HyMES that implements something quite similar. FlexiProvider (java library) contains quite a good amount of ... 7 Using a static IV isn't simply "poor form" — it introduces crippling weaknesses to the security of your ciphertexts. Likewise, using correctly-generated IVs (the requirements differ from mode-to-mode, but cryptographically random IVs almost always meet those requirements) isn't "better"; it's absolutely necessary. That said, there is absolutely no ... 7 This is an implementation of AES written completely in Python. It's not written as a library, it's a program to encrypt files, but looking at the source (which is a single file) the entire AES workings are implemented within it. There are publicly available test vectors from CMVP for validating that an AES implementation conforms to the spec properly. ... 7 Does matching all the test vectors mean my implementations are valid mathematically? Basically the comments got it, but test vectors are designed to attempt to hit lots of cases, but with high probability will not catch every single mistake. Should you do it? Definitely. Does it mean everything is perfect? No. Is implementing mathematics correctly ... 7 For your application: "I need the (underpowered 8-bit) slave to be able to tell if a command issued is really trustable", RSA signature with low public exponent ($e=3$), or Rabin (an analog with$e=2\$), is likely the most appropriate, assuming you can't trust the slaves to keep a key secret, which is the only realistic assumption unless that slave uses ...

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