# Tag Info

80

It (or rather, the software running on it) will use arbitrary-precision ("bignum") arithmetic. The way this works is basically the same way in which you (probably) learned to do arithmetic on paper at school. The arithmetic taught to us humans at school is base-10 arithmetic — that is, we represent numbers as strings made up of ten different digits, ...

31

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 ...

29

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 ...

17

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 ...

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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 ...

16

Contrary to the other answer, I'll be assuming the hash function is of the password-oriented kind; and my answer will be: input size has almost no influence on speed in good practice, even for much longer input than in the question. Password-oriented (or entropy-stretching, key-stretching) hash functions are, for example, suitable to transform a (password, ...

15

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 ...

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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 ...

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I believe that it is for two reasons: Nontable based implementations of AES are possible, but (assuming you don't have AES-NI or something similar) are significantly slower than table based implementations (perhaps $10\times$ to $20\times$ slower) For a lot of uses, timing attacks aren't particularly relevant (as either the attacker can't get the start/...

11

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 ...

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

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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 ...

11

Bitslicing is a technique where a computation is Reduced to elementary operations (called gates) with two bit inputs (typically NOR, XOR, and similar like OR AND NAND NXOR), rather than operations on words or integers spanning several bits. Executed in parallel, with as many simultaneous instances (on a single CPU) as there are bits in some register kind, ...

10

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 ...

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

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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 ...

9

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.

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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)

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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 $1 within a binary order of magnitude, when ordered in 6-decimal-figures quantities. They come with efficient hardware for TRNG, TDES, and ... 9 I really don't have an answer (other than saying that storing a hash of the password is good as any other way of solving your immediate problem; there are other ways, but they all allow an attacker to run a dictionary attack on the database). On the other hand, I do have these comments on what you're doing: If getting decrypted gibberish will really crash ... 9 Yes, AES could be implemented on a 4-bit micro-controller such as this EM6626, and that would not be rocket science or stupidly slow. This application note illustrates that all kind of 8-bit operations are simple, and table lookups are possible. In fact, tables are not even indispensable if performance is non-critical; see this minimalist AES source code in ... 9 This is not a limitation of the cryptographic functions, like SHA or PBKDF, since the zero byte isn't processed any differently. Since the purpose of a salt is generally to travel alongside a human password, libraries that handle the password as a zero-terminated string might also handle the salt as such a string. Obviously, a 0x00 in the salt would ... 9 If I understand your question correctly, you are essentially asking if points in Edwards and Montgomery curves can be represented in Weierstrass coordinates. This is true; in fact, any elliptic curve over a prime field can be represented in Weierstrass form$\mathcal{E}_{w}^{a, b} : y^2 = x^3 + ax + b$, and by extension its points can too. The question, ... 9 What makes crypto code vulnerable to timing attacks is data dependent timing variations. Branching according to a round counter, or to the key size, does not create a vulnerability. Most implementations of AES make no branch according to key or data value, and supressing other branches won't help. The main source of data-dependent timing variations in AES ... 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 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. 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 http:/... 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 ...

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A video camera can obtain entropy, but only at a fairly low rate and only if allowed to see "unusual" scenes… like someone making funny faces, unusual movements, etc. Of course, this only works in a room with no video bugs. Theoretical explanations… Depending on your knowledge-range, the following sources may be able to explain ways webcams can be used ...

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