# What does the〈 〉notation mean in this situation?

I was looking at solutions for a problem but I noticed it said $$\langle2\rangle$$ and was unsure of what that meant.

From Homework 5 (PDF)

• That's a very strange notation to be used here. But the instructor as noted at the top corner of the paper, @chris-peikert who happens to be a member here, maybe we should ask him? – DannyNiu Mar 13 '18 at 7:32
• I've started a chat room with him, I'm not sure if you could join. – DannyNiu Mar 13 '18 at 7:40
• It appears from context that $\langle x \rangle$ is a canonical representation of the integer $x$ as an $n$- bit bitstring. Otherwise those inputs and outputs are not in the correct donation/range. – Maeher Mar 13 '18 at 7:44
• @Maeher So basically the solution demonstrated an attack on a artifical hash compression function that is collision resistant but not pseudo-random? – DannyNiu Mar 13 '18 at 7:51
• You also see this notation in quantum mechanics, as in part 1 of Why should one model an entropy source in order to build a TRNG?. Don't ask me, I just copied it but you see if often in photonic TRNG papers. It may have multiple meanings subject to domain. – Paul Uszak Mar 22 at 16:59

## 1 Answer

From the context of the exercise, it appears that $$\langle x\rangle$$ is meant to denote a canonical representation of the integer $$x$$ as a bitstring of length $$n$$.

If this were not the case, those inputs and outputs would not be in the correct domain and range respectively.

Likely this notation was defined somewhere in the lecture notes, but I cannot find them at the moment.

• Sounds like a very logical answer, in that case it is likely defined as I2OSP as defined in RSA (a statically sized, unsigned big endian / network order integer representation. – Maarten Bodewes Mar 22 at 22:04