constant-time-select ( flag a b -- a/b )
Cryptographic utilities

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constant-time-zero? ( seq -- ? )

Vocabulary
crypto.utils

Inputs

flag	0 or 1
a	integer
b	integer

Outputs

a/b	a if flag=1, b if flag=0

Word description
Selects between two integers in constant time without branching. If flag is 1, returns a. If flag is 0, returns b. Uses arithmetic masking to avoid conditional branches.

Warning
Flag MUST be exactly 0 or 1. Other values produce undefined results.

Examples

USING: crypto.utils ; 1 42 99 constant-time-select
! => 42

USING: crypto.utils ; 0 42 99 constant-time-select
! => 99

Definition

USING: math ;

IN: crypto.utils

:: constant-time-select ( flag a b -- a/b )
    flag neg :> mask a mask bitand b mask bitnot bitand bitor ;