status-im/status-go

package extkeys

import (
    "crypto/hmac"
    "crypto/sha512"
    "errors"
    "math/big"

    "github.com/btcsuite/btcd/btcec"
)

// errors
var (
    ErrInvalidSecretKey = errors.New("generated secret key cannot be used")
)

func splitHMAC(seed, salt []byte) (secretKey, chainCode []byte, err error) {
    data := hmac.New(sha512.New, salt)
    if _, err = data.Write(seed); err != nil {
        return
    }
    I := data.Sum(nil)

    // Split I into two 32-byte sequences, IL and IR.
    // IL = master secret key
    // IR = master chain code
    secretKey = I[:32]
    chainCode = I[32:]

    // IL (secretKey) is expected to be a 256-bit integer (it is used as parse256(IL)),
    // and consequently that integer must be within range for SECP256k1 private key.
    //
    // There's tiny possibility (<1 in 2^127) this invariant is violated:
    //   error is returned in that case, and simple resolution is to request another child with i incremented.
    keyBigInt := new(big.Int).SetBytes(secretKey)
    if keyBigInt.Cmp(btcec.S256().N) >= 0 || keyBigInt.Sign() == 0 {
        err = ErrInvalidSecretKey
    }

    return
}

// paddedAppend appends the src byte slice to dst, returning the new slice.
// If the length of the source is smaller than the passed size, leading zero
// bytes are appended to the dst slice before appending src.
// nolint: unparam
func paddedAppend(size uint, dst, src []byte) []byte {
    for i := 0; i < int(size)-len(src); i++ {
        dst = append(dst, 0)
    }
    return append(dst, src...)
}