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// Package nist implements helpers to generate NIST's Known Answer Tests (KATs).
package nist
import (
"crypto/aes"
)
// See NIST's PQCgenKAT.c.
type DRBG struct {
key [32]byte
v [16]byte
}
func (g *DRBG) incV() {
for j := 15; j >= 0; j-- {
if g.v[j] == 255 {
g.v[j] = 0
} else {
g.v[j]++
break
}
}
}
// AES256_CTR_DRBG_Update(pd, &g.key, &g.v).
func (g *DRBG) update(pd *[48]byte) {
var buf [48]byte
b, _ := aes.NewCipher(g.key[:])
for i := 0; i < 3; i++ {
g.incV()
b.Encrypt(buf[i*16:(i+1)*16], g.v[:])
}
if pd != nil {
for i := 0; i < 48; i++ {
buf[i] ^= pd[i]
}
}
copy(g.key[:], buf[:32])
copy(g.v[:], buf[32:])
}
// randombyte_init(seed, NULL, 256).
func NewDRBG(seed *[48]byte) (g DRBG) {
g.update(seed)
return
}
// randombytes.
func (g *DRBG) Fill(x []byte) {
var block [16]byte
b, _ := aes.NewCipher(g.key[:])
for len(x) > 0 {
g.incV()
b.Encrypt(block[:], g.v[:])
if len(x) < 16 {
copy(x[:], block[:len(x)])
break
}
copy(x[:], block[:])
x = x[16:]
}
g.update(nil)
}
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