diff --git a/src/rootfilespec/rntuple/envelope.py b/src/rootfilespec/rntuple/envelope.py
index eee162c..b5bdcb0 100644
--- a/src/rootfilespec/rntuple/envelope.py
+++ b/src/rootfilespec/rntuple/envelope.py
@@ -2,6 +2,7 @@
 from dataclasses import dataclass, field
 from typing import Annotated, Generic, TypeVar, cast
 
+import xxhash  # type: ignore[import-not-found]
 from typing_extensions import Self
 
 from rootfilespec.bootstrap.compression import decompress
@@ -74,12 +75,8 @@ class REnvelope(ROOTSerializable):
     @classmethod
     def read(cls, buffer: ReadBuffer) -> tuple[Self, ReadBuffer]:
         """Reads an REnvelope from the given buffer."""
-        #### Save initial buffer position (for checking unknown bytes)
-        payload_start_pos = buffer.relpos
-
         #### Get the first 64bit integer (lengthType) which contains the length and type of the envelope
-        # lengthType, buffer = buffer.consume(8)
-        (lengthType,), buffer = buffer.unpack("<Q")
+        (lengthType,), _ = buffer.unpack("<Q")
 
         # Envelope type, encoded in the 16 least significant bits
         typeID = lengthType & 0xFFFF
@@ -88,30 +85,44 @@ def read(cls, buffer: ReadBuffer) -> tuple[Self, ReadBuffer]:
             msg = f"Envelope type {typeID} read does not match passed class {cls.__name__}"
             raise ValueError(msg)
 
-        # Envelope size (uncompressed), encoded in the 48 most significant bits
+        # Envelope size (uncompressed), encoded in the 48 most significant bits.
+        # It includes the preamble and the checksum, so it is at least 16 bytes
         length = lengthType >> 16
-        # Ensure that the length of the envelope matches the buffer length
-        if length - 8 != len(buffer):
-            msg = f"Length of envelope ({length} minus 8) of type {typeID} does not match buffer length ({len(buffer)})"
+        if length < 16:
+            msg = f"Length of envelope ({length}) of type {typeID} is shorter than 16 bytes"
+            raise ValueError(msg)
+        if length != len(buffer):
+            msg = f"Length of envelope ({length}) of type {typeID} does not match buffer length ({len(buffer)})"
+            raise ValueError(msg)
+
+        #### Split the envelope once: the bytes the checksum covers, then the checksum
+        # The checksum covers [0, length - 8): the preamble, the payload and any
+        # unknown trailing bytes (root-io-spec ERRATA 5; "Checksum verification
+        # ... must include both known and unknown contents")
+        covered, trailer = buffer[: length - 8], buffer[length - 8 :]
+        (checksum,), rest = trailer.unpack("<Q")
+
+        #### Verify it before trusting the payload, as ROOT does
+        # (RNTupleSerialize.cxx:909-939)
+        computed = xxhash.xxh3_64_intdigest(covered.data)
+        if computed != checksum:
+            msg = (
+                f"{cls.__name__} checksum mismatch: "
+                f"stored {checksum:#018x}, computed {computed:#018x}"
+            )
             raise ValueError(msg)
 
-        members = {"typeID": typeID, "length": length}
-        #### Get the payload
-        members, buffer = cls.update_members(members, buffer)
+        #### Get the payload, after the 8-byte preamble
+        _, payload = covered.consume(8)
+        members = {"typeID": typeID, "length": length, "checksum": checksum}
+        members, payload = cls.update_members(members, payload)
 
-        #### Consume any unknown trailing information in the envelope
-        _unknown, buffer = buffer.consume(
-            length - (buffer.relpos - payload_start_pos) - 8
-        )
-        # Unknown Bytes = Envelope Size - Envelope Bytes Read - Checksum (8 bytes)
-        #   Envelope Bytes Read  = buffer.relpos - payload_start_pos
+        #### Keep any unknown trailing information in the envelope
+        _unknown, _ = payload.consume(len(payload))
 
-        #### Get the checksum (appended to envelope when writing to disk)
-        (checksum,), buffer = buffer.unpack("<Q")  # Last 8 bytes of the envelope
-        members["checksum"] = checksum
         envelope = cls(**members)
         envelope._unknown = _unknown
-        return envelope, buffer
+        return envelope, rest
 
 
 EnvType = TypeVar("EnvType", bound=REnvelope)
@@ -149,8 +160,24 @@ def read_from(self, buffer: ReadBuffer) -> EnvType:
 
         Envelopes are compressed, so this decompresses and deserializes.
         """
+        if len(buffer) != self.size:
+            msg = (
+                f"{self.envtype.__name__} at {self.locator}: expected {self.size} "
+                f"bytes, got {len(buffer)}"
+            )
+            raise ValueError(msg)
+
         #### Decompress the buffer if necessary
-        if len(buffer) != self.length:
+        # RNTuple decompression tests equality of the stored size (the locator's)
+        # and the length: equal means stored raw, smaller compressed, and larger
+        # is an error (root-io-spec NOTES 2; RNTupleZip.hxx:106-113)
+        if self.size > self.length:
+            msg = (
+                f"{self.envtype.__name__} at {self.locator}: stored size "
+                f"{self.size} is larger than its uncompressed length {self.length}"
+            )
+            raise ValueError(msg)
+        if self.size < self.length:
             buffer = decompress(buffer, self.length)
 
         #### Now read the envelope
diff --git a/tests/test_envelope_checksums.py b/tests/test_envelope_checksums.py
new file mode 100644
index 0000000..5179881
--- /dev/null
+++ b/tests/test_envelope_checksums.py
@@ -0,0 +1,161 @@
+from pathlib import Path
+
+import pytest
+
+from rootfilespec.bootstrap import BOOTSTRAP_CONTEXT, ROOT3a3aRNTuple
+from rootfilespec.reader import open_path
+from rootfilespec.rntuple.envelope import REnvelopeLocator
+from rootfilespec.rntuple.header import HeaderEnvelope
+from rootfilespec.rntuple.RLocator import StandardLocator
+from rootfilespec.serializable import BufferContext, ReadBuffer
+
+DATA = Path(__file__).parent.parent / "reference" / "root-io-spec" / "data" / "rntuple"
+pytestmark = pytest.mark.skipif(
+    not DATA.exists(), reason="reference/root-io-spec not checked out"
+)
+
+
+def _anchors(path: Path) -> list[ROOT3a3aRNTuple]:
+    with open_path(path) as reader:
+        keylist = reader.keylist()
+        return [
+            reader.fetch(keylist[name])
+            for name in keylist
+            if keylist[name].fClassName == b"ROOT::RNTuple"
+        ]
+
+
+def _buffer(raw: bytes, offset: int, size: int) -> ReadBuffer:
+    return ReadBuffer(
+        memoryview(raw[offset : offset + size]),
+        0,
+        BOOTSTRAP_CONTEXT,
+        BufferContext(abspos=offset),
+    )
+
+
+def _read(raw: bytes, loc):
+    return loc.read_from(_buffer(raw, loc.offset, loc.size))
+
+
+@pytest.mark.parametrize("name", sorted(p.name for p in DATA.glob("*.root")))
+def test_every_envelope_verifies(name: str):
+    """Issue #117: every envelope of every root-io-spec RNTuple fixture is checked
+
+    Including rntuple/compressed.root, whose envelopes are compressed: the
+    checksum is over the uncompressed envelope.
+    """
+    path = DATA / name
+    raw = path.read_bytes()
+    for anchor in _anchors(path):
+        header = _read(raw, anchor.header_locator)
+        footer = _read(raw, anchor.footer_locator)
+        assert footer.headerChecksum == header.checksum
+        for loc in footer.pagelist_locators:
+            assert _read(raw, loc).headerChecksum == header.checksum
+
+
+def test_header_checksum_is_the_pinned_value():
+    """rntuple/anchor.root's case.toml pins the header envelope's checksum at 500
+    and the footer's copy of it at 854"""
+    path = DATA / "anchor.root"
+    raw = path.read_bytes()
+    (anchor,) = _anchors(path)
+    pinned = bytes([0x2A, 0x13, 0xA2, 0x84, 0xB3, 0x59, 0xD3, 0xDD])
+    assert raw[500:508] == raw[854:862] == pinned
+    header = _read(raw, anchor.header_locator)
+    footer = _read(raw, anchor.footer_locator)
+    assert header.checksum == footer.headerChecksum == int.from_bytes(pinned, "little")
+
+
+def test_corrupted_header_envelope_raises():
+    """rntuple/anchor.root's header envelope is 268..508 (length 240, checksum at
+    500..508). Changing any byte before the checksum must be caught."""
+    path = DATA / "anchor.root"
+    raw = bytearray(path.read_bytes())
+    (anchor,) = _anchors(path)
+    loc = anchor.header_locator
+    assert (loc.offset, loc.size, loc.length) == (268, 240, 240)
+    # The "n" of the ntuple's name "ntpl" at 288: flipping its case keeps the
+    # envelope parseable, so only the checksum can catch it
+    assert raw[288:292] == b"ntpl"
+    raw[288] ^= 0x20
+    with pytest.raises(ValueError, match="HeaderEnvelope checksum mismatch"):
+        _read(bytes(raw), loc)
+
+
+def test_corrupted_checksum_raises():
+    path = DATA / "anchor.root"
+    raw = bytearray(path.read_bytes())
+    (anchor,) = _anchors(path)
+    loc = anchor.header_locator
+    raw[loc.offset + loc.length - 1] ^= 0x01
+    with pytest.raises(ValueError, match="HeaderEnvelope checksum mismatch"):
+        _read(bytes(raw), loc)
+
+
+def test_stored_size_larger_than_length_raises():
+    """root-io-spec NOTES 2: RNTuple decompression tests equality; a stored size
+    larger than the uncompressed length is an error, not a raw payload"""
+    loc = REnvelopeLocator(
+        length=16, locator=StandardLocator(size=24, offset=0), envtype=HeaderEnvelope
+    )
+    with pytest.raises(ValueError, match="larger than its uncompressed length"):
+        loc.read_from(_buffer(bytes(24), 0, 24))
+
+
+def test_envelope_shorter_than_16_bytes_raises():
+    """The length counts the 8-byte preamble and the 8-byte checksum, so it is
+    at least 16, as ROOT's DeserializeEnvelope requires"""
+    loc = REnvelopeLocator(
+        length=8, locator=StandardLocator(size=8, offset=0), envtype=HeaderEnvelope
+    )
+    # The preamble alone: type 1 in the low 16 bits, length 8 in the upper 48
+    preamble = (8 << 16 | 0x01).to_bytes(8, "little")
+    with pytest.raises(ValueError, match="shorter than 16 bytes"):
+        loc.read_from(_buffer(preamble, 0, 8))
+
+
+def test_envelope_length_not_matching_the_locator_raises():
+    """rntuple/anchor.root's header envelope with the length in its preamble
+    (bytes 270..276) changed from 240 to 248: the anchor says 240 were stored"""
+    path = DATA / "anchor.root"
+    raw = bytearray(path.read_bytes())
+    (anchor,) = _anchors(path)
+    loc = anchor.header_locator
+    assert raw[268:276] == (240 << 16 | 0x01).to_bytes(8, "little")
+    raw[268:276] = (248 << 16 | 0x01).to_bytes(8, "little")
+    with pytest.raises(
+        ValueError, match=r"Length of envelope \(248\) .* buffer length \(240\)"
+    ):
+        _read(bytes(raw), loc)
+
+
+def test_every_corrupted_byte_is_a_checksum_mismatch():
+    """The checksum is verified before the payload is parsed, as ROOT does, so
+    corrupting any byte the checksum covers reports the checksum, not whatever
+    the parser trips over first (an out-of-range slice, unknown feature flags,
+    ...)."""
+    path = DATA / "anchor.root"
+    raw = path.read_bytes()
+    (anchor,) = _anchors(path)
+    loc = anchor.header_locator
+    # The 8-byte preamble is checked first (type, length); everything after it,
+    # up to the checksum, is covered only by the checksum
+    for pos in range(loc.offset + 8, loc.offset + loc.length - 8):
+        corrupted = bytearray(raw)
+        corrupted[pos] ^= 0x80
+        with pytest.raises(ValueError, match="HeaderEnvelope checksum mismatch"):
+            _read(bytes(corrupted), loc)
+
+
+@pytest.mark.parametrize("size", [0, 200, 239])
+def test_short_read_raises(size: int):
+    """Fewer bytes than the locator's size (a truncated file) are reported as
+    such, not taken for a compressed envelope"""
+    path = DATA / "anchor.root"
+    raw = path.read_bytes()
+    (anchor,) = _anchors(path)
+    loc = anchor.header_locator
+    with pytest.raises(ValueError, match=f"expected 240 bytes, got {size}"):
+        loc.read_from(_buffer(raw, loc.offset, size))