Divide stack usage by 255 (keystore EIP 2333)

[mratsim]

See #60

This implements keystore primitives for EIP-2333 (draft)
ethereum/EIPs#2333

This will unblock status-im/nim-beacon-chain#1093

cc @zah

Ready:

Note that the spec requires over 16KB of stack for parent_SK_to_Lamport_PK

the lamport_0 and lamport_1 are of size 255 * 32 bytes, in the code we reorder the spec and reuse the same lamport buffer.

nim-blscurve/blscurve/eth2_keygen.nim

Lines 74 to 103 in 96f2033

	# 2. lamport_0 = IKM_to_lamport_SK(IKM, salt)
	# TODO: this uses 8KB and has a high stack-overflow potential
	var lamport {.noInit.}: array[255, array[32, byte]]
	ikm.ikm_to_lamport_SK(salt, lamport)
	# TODO: unclear inclusive/exclusive ranges in spec
	# assuming exclusive:
	# https://github.com/ethereum/EIPs/issues/2337#issuecomment-637521421
	# 6. for i = 0 to 255
	# lamport_PK = lamport_PK | SHA256(lamport_0[i])
	for i in 0 ..< 255:
	ctx.update(sha256.digest(lamport[i]).data)
	# 3. not_IKM = flip_bits(parent_SK)
	# We can flip the bit of the IKM instead
	# as flipping bits of milagro representation (Montgomery)
	# doesn't make sense
	var not_ikm {.noInit.}: array[32, byte]
	for i in 0 ..< 32:
	not_ikm[i] = not ikm[i]
	# 4. lamport_1 = IKM_to_lamport_SK(not_IKM, salt)
	# We reuse the previous buffer to limit stack usage
	not_ikm.ikm_to_lamport_SK(salt, lamport)
	# TODO: inclusive/exclusive range?
	# 7. for i = 0 to 255
	# lamport_PK = lamport_PK | SHA256(lamport_1[i])
	for i in 0 ..< 255:
	ctx.update(sha256.digest(lamport[i]).data)

We could further divide stack usage by 255 (!) if we have an alternative HKDF Expand iterator:

nim-blscurve/blscurve/hkdf.nim

Lines 139 to 152 in 96f2033

	let oArray = cast[ptr UncheckedArray[byte]](output)
	for i in 0 .. N:
	ctx.init(prk.data)
	# T(0) = empty string
	if i != 0:
	ctx.update(t.data)
	ctx.update(info)
	ctx.update([uint8(i+1)])
	discard ctx.finish(t.data)
	let iStart = i * HashLen
	let size = min(HashLen, output.len - iStart)
	copyMem(oArray[iStart].addr, t.data.addr, size)

Note: the tree might change to 32 instead of 255 as per ethereum/EIPs#2337