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75 lines
3 KiB
Markdown
75 lines
3 KiB
Markdown
# Ed25519 curve
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!!! note
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Author is nowhere close to being a cryptographer. Be sceptical on accuracy.
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!!! note
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This article is only about the underlying curve. Public key derivation and signing algorithm will be treated separately.
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Monero employs Ed25519 elliptic curve as a basis for its key pair generation.
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However, Monero does not exactly follow EdDSA reference signature scheme.
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## Definition
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This is the standard Ed25519 curve definition, no Monero specific stuff here,
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except the naming convention. The convention comes from the CryptoNote
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whitepaper and is widely used in Monero literature.
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### Curve equation
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−x^2 + y^2 = 1 − (121665/121666) * x^2 * y^2
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Note:
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* curve is in two dimensions (nothing fancy, like all the curves is high school)
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* curve is mirrored below y axis due to `y^2` part of the equation (not a polynomial)
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### Base point `G`
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The base point is a specific point on the curve. It is used
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as a basis for further calculations. It is an arbitrary choice
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by the curve authors, just to standardize the scheme.
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Note that it is enough to specify the y value and the sign of the x value.
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That's because the specific x can be calculated from the curve equation.
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G = (x, 4/5) # take the point with the positive x
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# The hex representation of the base point
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5866666666666666666666666666666666666666666666666666666666666666
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### Prime order of the base point `l`
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In layment terms, the "canvas" where the curve is drawn is assumed
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to have a finite "resolution", so point coordinates must "wrap around"
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at some point. This is achieved by modulo the `l` value (lowercase L).
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In other words, the `l` defines the maximum scalar we can use.
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l = 2^252 + 27742317777372353535851937790883648493
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The `l` is a prime number specified by the curve authors.
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In practice this is the private key's strength.
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### Total number of points on the curve
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The total number of points on the curve is also a prime number:
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q = 2^255 - 19
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In practice not all points are "useful" and so the private key strength is limited to `l` describe above.
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## Implementation
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Monero uses (apparently modified) Ref10 implementation by Daniel J. Bernstein.
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## Reference
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* [Understanding Monero Cryptography](https://steemit.com/monero/@luigi1111/understanding-monero-cryptography-privacy-introduction) - excellent writeup by Luigi
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* [StackOverflow answer](https://monero.stackexchange.com/questions/2290/why-how-does-monero-generate-public-ed25519-keys-without-using-the-standard-publ)
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* [Python implementation](https://github.com/monero-project/mininero/blob/master/ed25519.py) - not the reference one but easier to understand
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* [Encoding point to hex](https://monero.stackexchange.com/questions/6050/what-is-the-base-point-g-from-the-whitepaper-and-how-is-it-represented-as-a)
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* [Ed25519 on Wikipedia](https://en.wikipedia.org/wiki/EdDSA#Ed25519)
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* [A (Relatively Easy To Understand) Primer on Elliptic Curve Cryptography](https://blog.cloudflare.com/a-relatively-easy-to-understand-primer-on-elliptic-curve-cryptography/)
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