lib: add Verifier interface

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/verifier.go

@@ -0,0 +1,70 @@
+package lib
+
+import (
+	"context"
+	"crypto/sha256"
+	"crypto/subtle"
+	"errors"
+	"fmt"
+)
+
+var (
+	ErrChallengeFailed          = errors.New("libanubis: challenge failed, hash does not match what the server calculated")
+	ErrWrongChallengeDifficulty = errors.New("libanubis: wrong challenge difficulty")
+)
+
+type Verifier interface {
+	Verify(ctx context.Context, challenge, verify []byte, nonce, difficulty uint32) (bool, error)
+}
+
+type VerifierFunc func(ctx context.Context, challenge, verify []byte, nonce, difficulty uint32) (bool, error)
+
+func (vf VerifierFunc) Verify(ctx context.Context, challenge, verify []byte, nonce, difficulty uint32) (bool, error) {
+	return vf(ctx, challenge, verify, nonce, difficulty)
+}
+
+func BasicSHA256Verify(ctx context.Context, challenge, verify []byte, nonce, difficulty uint32) (bool, error) {
+	h := sha256.New()
+	fmt.Fprintf(h, "%x%d", challenge, nonce)
+	data := h.Sum(nil)
+
+	if subtle.ConstantTimeCompare(data, verify) != 1 {
+		return false, fmt.Errorf("%w: wanted %x, got: %x", ErrChallengeFailed, verify, data)
+	}
+
+	if !hasLeadingZeroNibbles(data, difficulty) {
+		return false, fmt.Errorf("%w: wanted %d leading zeroes in calculated data %x, but did not get it", ErrWrongChallengeDifficulty, data, difficulty)
+	}
+
+	if !hasLeadingZeroNibbles(verify, difficulty) {
+		return false, fmt.Errorf("%w: wanted %d leading zeroes in verification data %x, but did not get it", ErrWrongChallengeDifficulty, verify, difficulty)
+	}
+
+	return true, nil
+}
+
+// hasLeadingZeroNibbles checks if the first `n` nibbles (in order) are zero.
+// Nibbles are read from high to low for each byte (e.g., 0x12 -> nibbles [0x1, 0x2]).
+func hasLeadingZeroNibbles(data []byte, n uint32) bool {
+	count := uint32(0)
+	for _, b := range data {
+		// Check high nibble (first 4 bits)
+		if (b >> 4) != 0 {
+			break // Non-zero found in leading nibbles
+		}
+		count++
+		if count >= n {
+			return true
+		}
+
+		// Check low nibble (last 4 bits)
+		if (b & 0x0F) != 0 {
+			break // Non-zero found in leading nibbles
+		}
+		count++
+		if count >= n {
+			return true
+		}
+	}
+	return count >= n
+}