Standard way to generate pseudorandom bytes on demand

Myath

6/7/23, 7:57 PM

I considered Extendable-Output Function (XOF) with a random seed but it seems I would have to specify the output length at the start and store the entire output. I don't know how many bytes I will need in advance, and I don't want to store a very long string.

I also considered some ad-hoc stateful construction using XOF that maintains a running counter. I wonder what is the "standard" and efficient way to do this.

0 + 0

hash

symmetric

pseudo-random-generator

Paul Uszak

6/7/23, 9:37 PM

Is there some nuance that's not apparent from the question's title? Why not just do the standard thing: CSPRNG?

Maarten Bodewes

6/7/23, 9:59 PM

@PaulUszak One thing that might be an issue is that many CSPRNG *implementations* are seeded by the system, may be reseeded etc. So they are not reliable to generate the same bits all the time. Some CSPRNG's may also specify a specific speed size, so there's that. But yeah, if this is just about getting random bytes or integers: use a CSPRNG.

Score:0

Crypto

poncho

6/7/23, 8:15 PM

I considered Extendable-Output Function (XOF) with a random seed but it seems I would have to specify the output length at the start and store the entire output.

No; the standard SHAKE XOF doesn't need to know up front how long of an output it can generate; there's no reason why you can't keep on 'squeezing' to get as much output as you need.

Now, it's possible that the API for the crypto library that you are using has such a limitation (that is, it'll always generate all its output at once, and has no provision to generate any more). On the other hand, I wouldn't naively expect that from a library - have you looked to ensure that your library doesn't have a 'generate more output' function/API available (possibly by simply calling the 'generate output API' with the SHAKE state)?

0 + 0

Myath

6/7/23, 11:39 PM

I'm looking at Python hashlib https://docs.python.org/3/library/hashlib.html and it doesn't seem to have such an API.

poncho

6/8/23, 3:17 AM

@Myath: I suspect that calling shake.digest(length) multiple times will generate consecutive parts of the shake output.

JamesTheAwesomeDude

6/9/23, 12:07 PM

@poncho Sadly, that is not the case; I just tested it. It looks like @Myath will have to use a 3rd-party library such as [`pycryptodome`](https://pycryptodome.readthedocs.io/en/stable/src/hash/shake256.html)—neither the standard library nor [`pyca/cryptography`](https://cryptography.io/en/latest/hazmat/primitives/cryptographic-hashes/#cryptography.hazmat.primitives.hashes.SHAKE256) supports that functionality, strangely.

Elon Musk

I sit in a Tesla and translated this thread with Ai:

EN: Standard way to generate pseudorandom bytes on demand

TH: วิธีมาตรฐานในการสร้างไบต์เทียมเทียมตามความต้องการ

RO: Mod standard de a genera octeți pseudoaleatori la cerere

RU: Стандартный способ генерации псевдослучайных байтов по запросу

VI: Cách tiêu chuẩn để tạo byte giả ngẫu nhiên theo yêu cầu

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