feat: define DWH installation credentials

tools/dwh-auth/internal/credential/credential.go

@@ -0,0 +1,78 @@
+// Package credential generates and verifies DWH installation credentials.
+package credential
+
+import (
+	"crypto/sha256"
+	"crypto/subtle"
+	"encoding/base64"
+	"io"
+	"strings"
+
+	"github.com/aritmolab/thothii/tools/dwh-auth/internal/record"
+)
+
+const Prefix = "thtdwh_v1"
+const KeyIDEncodedLength = 16
+const SecretEncodedLength = 43
+const MaxHeaderBytes = 128
+
+type Material struct {
+	Value  []byte
+	KeyID  string
+	Digest record.Digest
+}
+
+// Generate creates one canonical version 1 credential from random.
+func Generate(random io.Reader) (Material, error) {
+	keyIDBytes := make([]byte, 12)
+	secretBytes := make([]byte, 32)
+	if _, err := io.ReadFull(random, keyIDBytes); err != nil {
+		return Material{}, err
+	}
+	if _, err := io.ReadFull(random, secretBytes); err != nil {
+		return Material{}, err
+	}
+	keyID := base64.RawURLEncoding.EncodeToString(keyIDBytes)
+	secret := base64.RawURLEncoding.EncodeToString(secretBytes)
+	value := []byte(Prefix + "." + keyID + "." + secret)
+	sum := sha256.Sum256(value)
+	return Material{Value: value, KeyID: keyID, Digest: record.Digest(sum)}, nil
+}
+
+// VerifyV1 authenticates one canonical version 1 credential and returns its key ID.
+func VerifyV1(value []byte, expected record.Digest) (string, bool) {
+	if len(value) > MaxHeaderBytes || len(value) != len(Prefix)+1+KeyIDEncodedLength+1+SecretEncodedLength {
+		return "", false
+	}
+	parts := strings.Split(string(value), ".")
+	if len(parts) != 3 || parts[0] != Prefix || !canonicalBase64(parts[1], 12, KeyIDEncodedLength) || !canonicalBase64(parts[2], 32, SecretEncodedLength) {
+		return "", false
+	}
+	sum := sha256.Sum256(value)
+	if subtle.ConstantTimeCompare(sum[:], expected[:]) != 1 {
+		return "", false
+	}
+	return parts[1], true
+}
+
+// VerifyLegacy authenticates an opaque legacy credential without interpreting its syntax.
+func VerifyLegacy(value []byte, expected record.Digest) bool {
+	if len(value) < 1 || len(value) > MaxHeaderBytes {
+		return false
+	}
+	for _, b := range value {
+		if b <= 0x1f || b == 0x7f {
+			return false
+		}
+	}
+	sum := sha256.Sum256(value)
+	return subtle.ConstantTimeCompare(sum[:], expected[:]) == 1
+}
+
+func canonicalBase64(value string, decodedLength, encodedLength int) bool {
+	if len(value) != encodedLength {
+		return false
+	}
+	decoded, err := base64.RawURLEncoding.DecodeString(value)
+	return err == nil && len(decoded) == decodedLength && base64.RawURLEncoding.EncodeToString(decoded) == value
+}