Introduction

The Elliptic Curve Discrete Logarithm Problem (ECDLP) is a mathematical problem that forms the basis of the security of elliptic curve cryptography (ECC). ECC is a cryptography system that relies on the difficulty of solving the ECDLP for its security.

ECDLP involves finding the exponent k in the equation Q = k * P, where P is a point on an elliptic curve, Q is the result of multiplying P by k.

The difficulty lies in finding the private key k when only the public key Q and the base point P are known.

For traditional discrete logarithm problems, such as those in finite field-based systems like RSA or Diffie-Hellman, the best-known algorithms have exponential time complexity, making them computationally infeasible for large key sizes. Similarly, the ECDLP is believed to be hard, but it is computationally more efficient compared to its finite field counterparts for equivalent security levels. This efficiency makes elliptic curve cryptography attractive for resource-constrained environments, such as mobile devices and IoT devices.

Fast ECDLP

The algorithm implemented is BSGS (Baby Step Giant Step) that allows us to solve the ECDLP problem in a reasonable time.

Implementation details are based on Solving Small Exponential ECDLP in EC-based Additively Homomorphic Encryption and Applications (opens in a new tab).

Only ~330 MB of precomputed tables with L1 = 26 are needed to solve the ECDLP for any point in the curve in a few milliseconds to decode an integer up to 2^48.

The gist of BSGS goes as follows:

• Our target point, which we want to decode, represents an integer in the range [0, 2^(L1 + L2)].
• We have a T1 hash table, where the key is the curve point and value is the decoded point. T1 = <i * G => i | i in [1, 2^L1]>
• We have a T2 linear table (an array), where T2 = [j * 2^L1 * G | j in [1, 2^L2]]
• For each j in 0..2^L2
  • Compute the difference between T2[j] and the target point.
    
  • If we have an occurrence in T1 table, the decoded integer is j * 2^L1 + i.

On top of this regular BSGS algorithm, we add the following optimizations:

• Batching. The paper uses a tree-based Montgomery trick - instead, we use the batched inversion which is implemented in FieldElement.
• T1 only contains the truncated x coordinates.
  • The table uses Cuckoo hashing, and the hash of a point is directly just a subset of the bytes of the point.
• We need a canonical encoding of a point before any hashmap lookup: this means that we must work with affine coordinates.
  • Addition of affine Montgomery points requires less inversions than Edwards points, so we use that instead.
• Using the fact -(x, y) = (x, -y) on the Montgomery curve, we can shift the inputs so that we only need half of T1 and T2 and half of the modular inversions.
• The L2 constant has been fixed here, because we can just shift the input after every batch.
  • This means that L2 has a constant size of about 16Ko, which is preferable to >100Mo when L2 = 22, for example.
  • This results in slightly more modular inversions, however this has no visible impact on performance.
  • Shifting the inputs like this also means that we support arbitrary decoding ranges for a given constant tables file.

NOTE: We are dealing with a curve which has cofactors; as such, we need to multiply by the cofactor before running ECDLP to clear it and guarantee a canonical encoding of our points. The tables also need to be based on num * cofactor to match.

Usage

The ECDLP is used in the wallet only to decode quickly a Ristretto Point to an integer, which is done in the case of a newly scanned transaction or on a new account final balance.

The point to decode is obtained by decrypting a Ciphertext using the Private Key by doing the following:

• P = CC - (SK * C) where P is the Ristretto Point, CC is the Ciphertext (Pedersen) commitment, SK is the Secret Key, and C is a Curve Point.

Using Fast ECDLP, we are able to find integer values of our ciphertexts in the most efficient way.