# Hash to Curve

One important use of Elligator is mapping inputs to elliptic curve points. Ideally we want a procedure that works in a way that matches the random oracle model, though in practice this is not always strictly necessary. The Hash to Curve RFC draft recommends two procedures. The simpler one is done in three steps:

``````encode_to_curve(msg)
u = hash_to_field(msg, 1)
Q = map_to_curve(u)
P = clear_cofactor(Q)
return P
``````

This procedure hits only half of the curve, so it is not like a random oracle. For most use cases, however, this is enough. When we do need a proper instantiation matching the random oracle model, the procedure is a bit more complex:

``````hash_to_curve(msg)
u0, u1 = hash_to_field(msg, 2)
Q0     = map_to_curve(u0)
Q1     = map_to_curve(u1)
R      = Q0 + Q1
P      = clear_cofactor(R)
return P
``````

## Hash to Field

This function hashes an input into one or more field elements. It MUST be match the expectations for a random oracle as closely as possible.

When we use a prime field GF(p) where p is very close to a power of two (as is the case for Curve25519 and Curve448), we can use a regular key derivation function like HKDF then interpret the result directly as a field element. The resulting distribution will be indistinguishable from a uniform random distribution in practice.

If however p is not close to a power of two, it is best to first derive hashes that are twice the size of field elements, interpret those hashes as numbers, which we then reduce modulo p.

## Map to Curve

Just use Elligator 2's direct map.

For protocols requiring more than scalar multiplication we generally convert the Montgomery curve point given to us by Elligator 2 to an equivalent (twisted) Edwards point.

## Clear cofactor

This steps project the point mapped by Elligator2 to the prime order subgroup of the curve. In practice we can just multiply the point by the cofactor. For Curve25519 this requires only 3 point doublings (23 = 8). For Curve448 we need only two (22 = 4).

This step can be skipped when the `map_to_curve()` step maps to the Ristretto group.

## Example

Libsodium provides the `crypto_core_ed25519_from_uniform()` function, which fuses the `map_to_curve()` and `clear_cofactor()` steps above. It starts from 32 random bytes interpreted as a little endian number:

``````def map_to_curve(random):
y_sign  = random / 2^255    # Most significant bit
r       = random % 2^255    # Representative (field element)
u       = elligator2_map(r) # We ignore the v coordinate

# Convert u to an Edwards point
# There are 2 possibilities: (x, y), and (-x, y)
y = (u - 1) / (u + 1)
x = sqrt((y^2 - 1) / ((-121665 / 121666) * y^2 + 1))

# Set the sign of the x coordinate (negative if y_sign == 1)
# Here we consider that odd field elements are negative
if x % 2 != y_sign:
x = -x

# Multiply the resulting point by the cofactor (8)
P       = x, y
P       = point_add(P, P)
P       = point_add(P, P)
P       = point_add(P, P)
return P
``````

We provide test vectors for this function.