* number of transactions included in the block (around 1-2 bytes; subject to varint representation)
See [get_block_hashing_blob()](https://github.com/monero-project/monero/blob/master/src/cryptonote_basic/cryptonote_format_utils.cpp#L1078) function to dig further.
## Algorithm
!!! warning
The article attempts to give reader a high-level understanding of the CryptoNight algorithm.
For implementation details refer to CryptoNote Standard and Monero source code.
See references at the bottom.
### Overview
CryptoNight attempts to make memory access a bottleneck for performance ("memory hardness"). It has three steps:
1. Initialize large area of memory with pseudo-random data. This memory is known as the scratchpad.
See the [source code diff](https://github.com/monero-project/monero/pull/3253/files) for CryptoNight v1 modifications.
### CryptoNight v2
The v2 changes were more involved.
See the [rationale](https://github.com/SChernykh/xmr-stak-cpu/blob/master/README.md) and the [source code diff](https://github.com/monero-project/monero/commit/5fd83c13fbf8dc304909345e60a853c15b0de1e5#diff-7000dc02c792439471da62856f839d62).
## Critique
* CryptoNight hash is relatively expensive to verify. This poses a risk of DoS-ing nodes with incorrect proofs to process. See [strong asymmetry](/proof-of-work/what-is-pow/#strong-asymmetry) requirement.
* The hash function was designed from scratch with limited peer review. While CryptoNight is composed of proven and peer-reviewed primitives, combining secure primitives doesn't necessarily result in a secure cryptosystem.
* CryptoNight ultimately failed to prevent ASIC-s.
* Complexity of CryptoNight kills competition in ASIC manufacturing.
CryptoNight proof of work remains one of the most controversial aspect of Monero.
## Reference
* [CryptoNight hash function](https://cryptonote.org/cns/cns008.txt) description in the CryptoNote Standard
* The entry point is `cn_slow_hash()` function. Manually removing support and optimizations for multiple architectures should help you understand the actual code.
* "Egalitarian Proof of Work" chapter in [CryptoNote whitepaper](https://downloads.getmonero.org/whitepaper_annotated.pdf)
* [First days of Monero mining](https://da-data.blogspot.com/2014/08/minting-money-with-monero-and-cpu.html) by dr David Andersen
* Some [test vectors](https://github.com/monero-project/monero/tree/master/tests/hash) in Monero source code