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Minor fix in sqrt #383

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Minor fix in sqrt #383

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1612eea
Minor fix in sqrt
hecmas Apr 25, 2024
9dfcfd4
suggested renaming
hecmas Apr 27, 2024
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12 changes: 8 additions & 4 deletions main/ecrecover/FPSECP256K1/sqrtFpSecp256k1.zkasm
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Original file line number Diff line number Diff line change
Expand Up @@ -10,29 +10,33 @@
;;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

VAR GLOBAL _input

; RESOURCES:
; with sqrt: 1 arith + 1 binary + 7 steps
; without sqrt: 1 binary + 4 steps
; TOTAL (worst case): 1 arith + 1 binary + 7 steps

sqrtFpSecp256k1:
C => _input

; start by free-inputing the square root of the input C
; taking the positive one if A = 1, and the negative one if A = 0
${var _sqrtFpSecp256k1_sqrt = sqrtFpEcParity(C,A)} => A
${sqrtFpEcParity(C,A)} => A

; In this point we check whether A is LT than SECP256K1_P since, otherwise, either:
; a) A doesn't have square root. 1 is returned in C.
; b) A contains an alias, it's a MAP. 1 is returned in C. In this case, the proof cannot
; be generated because we check in assertNQRFpSecp256k1 if the root actually exists.
%SECP256K1_P => B
$ :LT, JMPNC(sqrtFpSecp256k1_NoRoot)
$ :LT, JMPNC(sqrtFpSecp256k1_NoRoot)
; From here, A,C < SECP256K1_P, which means that the root exists and it's alias-free

; √C·√C + 0 = C (mod SECP256K1_P)
A => B
0 => C
%SECP256K1_P => D
${(A*B) % D} => E :ARITH_MOD, RETURN
${(A*B) % D} :ARITH_MOD, MLOAD(_input), RETURN

sqrtFpSecp256k1_NoRoot:
1 => C :RETURN
1 => C :RETURN
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