fix: update hash format validation to -01 spec (plain base64url, no prefix)

Go ValidateHashFormat was still validating the old -00 format
(algorithm:base64url with sha-256/sha-384/sha-512 prefix). Updated to
validate plain base64url without prefix per -01 spec and RFC 9449.
Python was already updated but uncommitted. Both refimpls now match.

refimpl/IMPROVEMENTS.md

@@ -50,7 +50,7 @@ Suggestions that could make the implementations more robust, spec-strict, or pro
 ## 5. **Nice-to-have** ✅
 
 - **inp_hash / out_hash format**  
-  **Done.** Optional check in create and verify: `algorithm:base64url` with algorithm in allowlist (sha-256, sha-384, sha-512). Helpers: `ValidateHashFormat` / `validate_hash_format`.
+  **Done.** Optional check in create and verify: plain base64url without algorithm prefix, per -01 spec and RFC 9449. Helpers: `ValidateHashFormat` / `validate_hash_format`.
 
 - **Constant-time comparison**  
   **Done.** **Go:** `crypto/subtle.ConstantTimeCompare` for `typ` in verify. **Python:** `hmac.compare_digest` for `typ`.
@@ -72,4 +72,4 @@ The refimpl was built against draft-nennemann-wimse-ect-00. The -01 draft introd
 - **Update `MaxParLength` naming**: ✅ **Done.** Renamed to `MaxPredLength` / `max_pred_length` everywhere.
 - **Add L1 support**: The -01 draft introduces unsigned JSON ECTs (Level 1). The refimpl currently only supports L2 (signed JWS).
 - **Add L3 support**: The -01 draft introduces audit ledger requirements for Level 3. The existing in-memory ledger needs hash chain and receipt support.
-- **Update hash format**: The -01 draft specifies SHA-256 base64url without algorithm prefix (no `sha-256:` prefix), consistent with RFC 9449.
+- **Update hash format**: ✅ **Done.** Both Go and Python validate plain base64url without algorithm prefix, consistent with -01 spec and RFC 9449.

refimpl/go-lang/ect/errors.go

@@ -23,5 +23,5 @@ var (
 	ErrInvalidJTI          = errors.New("ect: jti must be UUID format")
 	ErrInvalidWID          = errors.New("ect: wid must be UUID format when set")
 	ErrPredLength          = errors.New("ect: pred exceeds max length")
-	ErrHashFormat          = errors.New("ect: inp_hash/out_hash must be algorithm:base64url (e.g. sha-256:...)")
+	ErrHashFormat          = errors.New("ect: inp_hash/out_hash must be plain base64url (no prefix)")
 )

refimpl/go-lang/ect/validate.go

@@ -4,7 +4,6 @@ import (
 	"encoding/base64"
 	"encoding/json"
 	"regexp"
-	"strings"
 )
 
 // ExtMaxSize is the recommended max serialized size of ext (Section 4.2.7).
@@ -19,8 +18,8 @@ const DefaultMaxPredLength = 100
 // uuidRegex matches RFC 9562 UUID: 8-4-4-4-12 hex.
 var uuidRegex = regexp.MustCompile(`^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$`)
 
-// allowedHashAlgs are the spec-recommended hash algorithm prefixes for inp_hash/out_hash.
-var allowedHashAlgs = map[string]bool{"sha-256": true, "sha-384": true, "sha-512": true}
+// base64urlRegex matches a non-empty base64url string without padding.
+var base64urlRegex = regexp.MustCompile(`^[A-Za-z0-9_-]+$`)
 
 // ValidateExt returns an error if ext exceeds ExtMaxSize or ExtMaxDepth.
 func ValidateExt(ext map[string]interface{}) error {
@@ -61,23 +60,18 @@ func ValidUUID(s string) bool {
 	return uuidRegex.MatchString(s)
 }
 
-// ValidateHashFormat returns nil if s is empty or matches "algorithm:base64url" (sha-256, sha-384, sha-512).
+// ValidateHashFormat returns nil if s is empty or is plain base64url (no padding)
+// per draft-nennemann-wimse-ect-01 and RFC 9449 (no algorithm prefix).
 func ValidateHashFormat(s string) error {
 	if s == "" {
 		return nil
 	}
-	idx := strings.Index(s, ":")
-	if idx <= 0 {
+	if !base64urlRegex.MatchString(s) {
 		return ErrHashFormat
 	}
-	alg := strings.ToLower(s[:idx])
-	if !allowedHashAlgs[alg] {
+	_, err := base64.RawURLEncoding.DecodeString(s)
+	if err != nil {
 		return ErrHashFormat
 	}
-	encoded := s[idx+1:]
-	if encoded == "" {
-		return ErrHashFormat
-	}
-	_, err := base64.RawURLEncoding.DecodeString(encoded)
-	return err
+	return nil
 }

refimpl/python/ect/validate.py

@@ -14,9 +14,6 @@ DEFAULT_MAX_PRED_LENGTH = 100
 _UUID_RE = re.compile(
     r"^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$"
 )
-_ALLOWED_HASH_ALGS = frozenset(("sha-256", "sha-384", "sha-512"))
-
-
 def _json_depth(obj: Any, depth: int = 0) -> int:
     if depth > EXT_MAX_DEPTH:
         return depth
@@ -44,22 +41,22 @@ def valid_uuid(s: str) -> bool:
 
 
 def validate_hash_format(s: str) -> None:
-    """Raise ValueError if s is non-empty and not algorithm:base64url (sha-256, sha-384, sha-512)."""
+    """Raise ValueError if s is non-empty and not plain base64url per RFC 9449 / ECT spec.
+
+    The ECT spec (draft-nennemann-wimse-ect-01) and RFC 9449 specify
+    ``base64url(SHA-256(data))`` — a plain base64url string without any
+    algorithm prefix.  This matches how ACT handles hashes.
+    """
     if not s:
         return
-    idx = s.find(":")
-    if idx <= 0:
-        raise ValueError("ect: inp_hash/out_hash must be algorithm:base64url (e.g. sha-256:...)")
-    alg = s[:idx].lower()
-    if alg not in _ALLOWED_HASH_ALGS:
-        raise ValueError("ect: inp_hash/out_hash must be algorithm:base64url (e.g. sha-256:...)")
-    encoded = s[idx + 1:]
-    if not encoded:
-        raise ValueError("ect: inp_hash/out_hash must be algorithm:base64url (e.g. sha-256:...)")
-    pad = 4 - len(encoded) % 4
-    if pad != 4:
-        encoded += "=" * pad
+    # Reject strings containing non-base64url characters.
+    # base64url alphabet: A-Z a-z 0-9 - _  (no padding '=' expected)
+    if not re.fullmatch(r"[A-Za-z0-9_-]+", s):
+        raise ValueError("ect: inp_hash/out_hash must be plain base64url (no prefix)")
+    # Verify it actually decodes.
+    pad = 4 - len(s) % 4
+    padded = s + "=" * pad if pad != 4 else s
     try:
-        base64.urlsafe_b64decode(encoded)
+        base64.urlsafe_b64decode(padded)
     except Exception:
-        raise ValueError("ect: inp_hash/out_hash must be algorithm:base64url (e.g. sha-256:...)") from None
+        raise ValueError("ect: inp_hash/out_hash must be plain base64url (no prefix)") from None

refimpl/python/tests/test_validate.py

@@ -47,17 +47,18 @@ def test_validate_hash_format_empty():
 
 
 def test_validate_hash_format_ok():
-    # sha-256:base64url (minimal valid)
-    validate_hash_format("sha-256:YQ")
-    validate_hash_format("sha-384:YQ")
-    validate_hash_format("sha-512:YQ")
+    # Plain base64url per RFC 9449 / ECT spec (no algorithm prefix)
+    validate_hash_format("YQ")
+    validate_hash_format("dBjftJeZ4CVP-mB92K27uhbUJU1p1r_wW1gFWFOEjXk")
+    validate_hash_format("abc123-_XYZ")
 
 
 def test_validate_hash_format_bad():
-    with pytest.raises(ValueError, match="algorithm:base64url|inp_hash"):
-        validate_hash_format("md5:abc")
-    with pytest.raises(ValueError, match="algorithm:base64url|inp_hash"):
-        validate_hash_format("no-colon")
-    # Invalid base64 that triggers decode error (e.g. binary)
-    with pytest.raises(ValueError, match="algorithm:base64url|inp_hash"):
-        validate_hash_format("sha-256:YQ\x00")  # null in payload
+    # Colon is not valid base64url — rejects old prefixed format
+    with pytest.raises(ValueError, match="plain base64url"):
+        validate_hash_format("sha-256:YQ")
+    with pytest.raises(ValueError, match="plain base64url"):
+        validate_hash_format("not valid!!")
+    # Null byte in payload
+    with pytest.raises(ValueError, match="plain base64url"):
+        validate_hash_format("YQ\x00")