diff --git a/src/config/config.d.ts b/src/config/config.d.ts
index 240f99ba..e063fb4d 100644
--- a/src/config/config.d.ts
+++ b/src/config/config.d.ts
@@ -18,7 +18,7 @@ export interface Config {
   ignoreUnsupportedPackets: boolean;
   ignoreMalformedPackets: boolean;
   enforceGrammar: boolean;
-  additionalAllowedPackets: Array<{ new(): any }>;
+  additionalAllowedPackets: Array<{ new(): any, tag: enums.packet }>;
   versionString: string;
   commentString: string;
   allowInsecureDecryptionWithSigningKeys: boolean;
diff --git a/src/config/config.js b/src/config/config.js
index dc53a12f..135a02ec 100644
--- a/src/config/config.js
+++ b/src/config/config.js
@@ -228,7 +228,6 @@ export default {
    * Parsing of packets is normally restricted to a predefined set of packets. For example a Sym. Encrypted Integrity Protected Data Packet can only
    * contain a certain set of packets including LiteralDataPacket. With this setting we can allow additional packets, which is probably not advisable
    * as a global config setting, but can be used for specific function calls (e.g. decrypt method of Message).
-   * NB: `config.enforceGrammar` may need to be disabled as well.
    * @memberof module:config
    * @property {Array} additionalAllowedPackets Allow additional packets on parsing. Defined as array of packet classes, e.g. [PublicKeyPacket]
    */
diff --git a/src/message.js b/src/message.js
index f21ac597..68dda0a3 100644
--- a/src/message.js
+++ b/src/message.js
@@ -36,7 +36,7 @@ import {
   OnePassSignaturePacket,
   SignaturePacket
 } from './packet';
-import { getMessageGrammarValidator } from './packet/grammar';
+import { MessageGrammarValidator } from './packet/grammar';
 
 // A Message can contain the following packets
 const allowedMessagePackets = /*#__PURE__*/ util.constructAllowedPackets([
@@ -879,7 +879,7 @@ export async function readMessage({ armoredMessage, binaryMessage, config, ...re
     }
     input = data;
   }
-  const packetlist = await PacketList.fromBinary(input, allowedMessagePackets, config, getMessageGrammarValidator({ delayReporting: false }));
+  const packetlist = await PacketList.fromBinary(input, allowedMessagePackets, config, new MessageGrammarValidator());
   const message = new Message(packetlist);
   message.fromStream = streamType;
   return message;
diff --git a/src/packet/aead_encrypted_data.js b/src/packet/aead_encrypted_data.js
index e2129fdb..d184c64c 100644
--- a/src/packet/aead_encrypted_data.js
+++ b/src/packet/aead_encrypted_data.js
@@ -28,7 +28,7 @@ import CompressedDataPacket from './compressed_data';
 import OnePassSignaturePacket from './one_pass_signature';
 import SignaturePacket from './signature';
 import PacketList from './packetlist';
-import { getMessageGrammarValidator } from './grammar';
+import { MessageGrammarValidator } from './grammar';
 
 // An AEAD-encrypted Data packet can contain the following packet types
 const allowedPackets = /*#__PURE__*/ util.constructAllowedPackets([
@@ -106,7 +106,7 @@ class AEADEncryptedDataPacket {
       await runAEAD(this, 'decrypt', key, streamClone(this.encrypted)),
       allowedPackets,
       config,
-      getMessageGrammarValidator({ enforceDelay: false })
+      new MessageGrammarValidator()
     );
   }
 
diff --git a/src/packet/compressed_data.js b/src/packet/compressed_data.js
index e3dfa237..cdeddea0 100644
--- a/src/packet/compressed_data.js
+++ b/src/packet/compressed_data.js
@@ -25,7 +25,7 @@ import LiteralDataPacket from './literal_data';
 import OnePassSignaturePacket from './one_pass_signature';
 import SignaturePacket from './signature';
 import PacketList from './packetlist';
-import { getMessageGrammarValidator } from './grammar';
+import { MessageGrammarValidator } from './grammar';
 
 // A Compressed Data packet can contain the following packet types
 const allowedPackets = /*#__PURE__*/ util.constructAllowedPackets([
@@ -114,7 +114,7 @@ class CompressedDataPacket {
     }
 
     // Decompressing a Compressed Data packet MUST also yield a valid OpenPGP Message
-    this.packets = await PacketList.fromBinary(await decompressionFn(this.compressed), allowedPackets, config, getMessageGrammarValidator({ enforceDelay: false }));
+    this.packets = await PacketList.fromBinary(await decompressionFn(this.compressed), allowedPackets, config, new MessageGrammarValidator());
   }
 
   /**
diff --git a/src/packet/grammar.ts b/src/packet/grammar.ts
index bc578343..67cfe3f7 100644
--- a/src/packet/grammar.ts
+++ b/src/packet/grammar.ts
@@ -1,6 +1,4 @@
-import { type Config } from '../config';
 import enums from '../enums';
-import util from '../util';
 
 export class GrammarError extends Error {
   constructor(...params: any[]) {
@@ -14,109 +12,131 @@ export class GrammarError extends Error {
   }
 }
 
-
-const isValidLiteralMessage = (tagList: enums.packet[], _acceptPartial: boolean) => tagList.length === 1 && tagList[0] === enums.packet.literalData;
-const isValidCompressedMessage = (tagList: enums.packet[], _acceptPartial: boolean) => tagList.length === 1 && tagList[0] === enums.packet.compressedData;
-const isValidEncryptedMessage = (tagList: enums.packet[], acceptPartial: boolean) => {
-  // Encrypted Message: Encrypted Data | ESK Sequence, Encrypted Data.
-  const isValidESKSequence = (tagList: enums.packet[], _acceptPartial: boolean) => (
-    tagList.every(packetTag => new Set([enums.packet.publicKeyEncryptedSessionKey, enums.packet.symEncryptedSessionKey]).has(packetTag))
-  );
-  const encryptedDataPacketIndex = tagList.findIndex(tag => new Set([enums.packet.aeadEncryptedData, enums.packet.symmetricallyEncryptedData, enums.packet.symEncryptedIntegrityProtectedData]).has(tag));
-  if (encryptedDataPacketIndex < 0) {
-    return isValidESKSequence(tagList, acceptPartial);
-  }
-
-  return (encryptedDataPacketIndex === tagList.length - 1) &&
-    isValidESKSequence(tagList.slice(0, encryptedDataPacketIndex), acceptPartial);
-};
-
-const isValidSignedMessage = (tagList: enums.packet[], acceptPartial: boolean) => {
-  // Signature Packet, OpenPGP Message | One-Pass Signed Message.
-  if (tagList.findIndex(tag => tag === enums.packet.signature) === 0) {
-    return isValidOpenPGPMessage(tagList.slice(1), acceptPartial);
-  }
-
-  // One-Pass Signed Message:
-  //    One-Pass Signature Packet, OpenPGP Message, Corresponding Signature Packet.
-  if (tagList.findIndex(tag => tag === enums.packet.onePassSignature) === 0) {
-    const correspondingSigPacketIndex = util.findLastIndex(tagList, tag => tag === enums.packet.signature);
-    if (correspondingSigPacketIndex !== tagList.length - 1 && !acceptPartial) {
-      return false;
-    }
-    return isValidOpenPGPMessage(tagList.slice(1, correspondingSigPacketIndex < 0 ? undefined : correspondingSigPacketIndex), acceptPartial);
-  }
-
-  return false;
-};
-
-const isUnknownPacketTag = (tag: number): tag is enums.packet => {
-  try {
-    enums.read(enums.packet, tag);
-    return false;
-  } catch (e) {
-    return true;
-  }
-};
+enum MessageType {
+  EmptyMessage, // incl. empty signed message
+  PlaintextOrEncryptedData,
+  EncryptedSessionKeys,
+  StandaloneAdditionalAllowedData
+}
 
 /**
- * Implements grammar checks based on https://www.rfc-editor.org/rfc/rfc9580.html#section-10.3 .
- * @param notNormalizedList - list of packet tags to validate
- * @param acceptPartial - whether the list of tags corresponds to a partially-parsed message
- * @returns whether the list of tags is valid
+ * Implement OpenPGP message grammar based on: https://www.rfc-editor.org/rfc/rfc9580.html#section-10.3 .
+ * It is slightly more lenient as it also allows standalone ESK sequences, as well as empty (signed) messages.
+ * This latter case is needed to allow unknown packets.
+ * A new `MessageGrammarValidator` instance must be created for each packet sequence, as the instance is stateful:
+ * - `recordPacket` must be called for each packet in the sequence; the function will throw as soon as
+ *  an invalid packet is detected.
+ * - `recordEnd` must be called at the end of the packet sequence to confirm its validity.
  */
-const isValidOpenPGPMessage = (
-  notNormalizedList: number[] /** might have unknown tags */,
-  acceptPartial: boolean
-): boolean => {
-  // Take care of packet tags that can appear anywhere in the sequence:
-  // 1. A Marker packet (Section 5.8) can appear anywhere in the sequence.
-  // 2. An implementation MUST be able to process Padding packets anywhere else in an OpenPGP stream so that future revisions of this document may specify further locations for padding.
-  // 3. An unknown non-critical packet MUST be ignored (criticality is enforced on parsing).
-  const normalizedList: enums.packet[] = notNormalizedList.filter(tag => (
-    tag !== enums.packet.marker &&
-    tag !== enums.packet.padding &&
-      !isUnknownPacketTag(tag)
-  ));
-
-  return isValidLiteralMessage(normalizedList, acceptPartial) ||
-    isValidCompressedMessage(normalizedList, acceptPartial) ||
-    isValidEncryptedMessage(normalizedList, acceptPartial) ||
-    isValidSignedMessage(normalizedList, acceptPartial);
-};
-
-/**
- * If `delayReporting === false`, the grammar validator throws as soon as an invalid packet sequence is detected during parsing.
- * This setting MUST NOT be used when parsing unauthenticated decrypted data, to avoid instantiating decryption oracles.
- *  Passing `delayReporting === true` allows checking the grammar validity in an async manner, by
- * only reporting the validity status after parsing is done (i.e. and authentication is expected to
- * have been enstablished)
- */
-export const getMessageGrammarValidator = ({ delayReporting }: { delayReporting: boolean }) => {
-  let logged = false;
-
+export class MessageGrammarValidator {
+  // PDA validator inspired by https://blog.jabberhead.tk/2022/10/26/implementing-packet-sequence-validation-using-pushdown-automata/ .
+  private state: MessageType = MessageType.EmptyMessage;
+  private leadingOnePassSignatureCounter: number = 0;
 
   /**
-  * @returns `true` on successful grammar validation; if `delayReporting` is set, `null` is returned
-  *   if validation is still pending (partial parsing, waiting for authentication to be confirmed).
-  * @throws on grammar error, provided `config.enforceGrammar` is enabled.
-  */
-  return (list: number[], isPartial: boolean, config: Config): true | null => {
-    if (delayReporting && isPartial) return null; // delay until the full message has been parsed (i.e. authenticated)
-
-    if (!isValidOpenPGPMessage(list, isPartial)) {
-      const error = new GrammarError(`Data does not respect OpenPGP grammar [${list}]`);
-      if (!logged) {
-        util.printDebugError(error);
-        logged = true;
-      }
-      if (config.enforceGrammar) {
-        throw error;
-      } else {
-        return true;
-      }
+   * Determine validity of the next packet in the sequence.
+   * NB: padding, marker and unknown packets are expected to already be filtered out on parsing,
+   * and are not accepted by `recordPacket`.
+   * @param packet - packet to validate
+   * @param additionalAllowedPackets - object containing packets which are allowed anywhere in the sequence, except they cannot precede a OPS packet
+   * @throws {GrammarError} on invalid `packet` input
+   */
+  recordPacket(packet: enums.packet, additionalAllowedPackets?: { [key in enums.packet]: any }) {
+    switch (this.state) {
+      case MessageType.EmptyMessage:
+      case MessageType.StandaloneAdditionalAllowedData:
+        switch (packet) {
+          case enums.packet.literalData:
+          case enums.packet.compressedData:
+          case enums.packet.aeadEncryptedData:
+          case enums.packet.symEncryptedIntegrityProtectedData:
+          case enums.packet.symmetricallyEncryptedData:
+            this.state = MessageType.PlaintextOrEncryptedData;
+            return;
+          case enums.packet.signature:
+            // Signature | <AdditionalAllowedPacketsOnly> and
+            // OPS | Signature | <AdditionalAllowedPacketsOnly> | Signature and
+            // OPS | <AdditionalAllowedPacketsOnly> | Signature are allowed
+            if (this.state === MessageType.StandaloneAdditionalAllowedData) {
+              if (--this.leadingOnePassSignatureCounter < 0) {
+                throw new GrammarError('Trailing signature packet without OPS');
+              }
+            }
+            // this.state remains EmptyMessage or StandaloneAdditionalAllowedData
+            return;
+          case enums.packet.onePassSignature:
+            if (this.state === MessageType.StandaloneAdditionalAllowedData) {
+              // we do not allow this case, for simplicity
+              throw new GrammarError('OPS following StandaloneAdditionalAllowedData');
+            }
+            this.leadingOnePassSignatureCounter++;
+            // this.state remains EmptyMessage
+            return;
+          case enums.packet.publicKeyEncryptedSessionKey:
+          case enums.packet.symEncryptedSessionKey:
+            this.state = MessageType.EncryptedSessionKeys;
+            return;
+          default:
+            if (!additionalAllowedPackets?.[packet]) {
+              throw new GrammarError(`Unexpected packet ${packet} in state ${this.state}`);
+            }
+            this.state = MessageType.StandaloneAdditionalAllowedData;
+            return;
+        }
+      case MessageType.PlaintextOrEncryptedData:
+        switch (packet) {
+          case enums.packet.signature:
+            if (--this.leadingOnePassSignatureCounter < 0) {
+              throw new GrammarError('Trailing signature packet without OPS');
+            }
+            this.state = MessageType.PlaintextOrEncryptedData;
+            return;
+          default:
+            if (!additionalAllowedPackets?.[packet]) {
+              throw new GrammarError(`Unexpected packet ${packet} in state ${this.state}`);
+            }
+            this.state = MessageType.PlaintextOrEncryptedData;
+            return;
+        }
+      case MessageType.EncryptedSessionKeys:
+        switch (packet) {
+          case enums.packet.publicKeyEncryptedSessionKey:
+          case enums.packet.symEncryptedSessionKey:
+            this.state = MessageType.EncryptedSessionKeys;
+            return;
+          case enums.packet.symEncryptedIntegrityProtectedData:
+          case enums.packet.aeadEncryptedData:
+          case enums.packet.symmetricallyEncryptedData:
+            this.state = MessageType.PlaintextOrEncryptedData;
+            return;
+          case enums.packet.signature:
+            if (--this.leadingOnePassSignatureCounter < 0) {
+              throw new GrammarError('Trailing signature packet without OPS');
+            }
+            this.state = MessageType.PlaintextOrEncryptedData;
+            return;
+          default:
+            if (!additionalAllowedPackets?.[packet]) {
+              throw new GrammarError(`Unexpected packet ${packet} in state ${this.state}`);
+            }
+            this.state = MessageType.EncryptedSessionKeys;
+        }
     }
+  }
 
-    return true;
-  };
-};
+  /**
+   * Signal end of the packet sequence for final validity check
+   * @throws {GrammarError} on invalid sequence
+   */
+  recordEnd() {
+    switch (this.state) {
+      case MessageType.EmptyMessage: // needs to be allowed for PacketLists that only include unknown packets
+      case MessageType.PlaintextOrEncryptedData:
+      case MessageType.EncryptedSessionKeys:
+      case MessageType.StandaloneAdditionalAllowedData:
+        if (this.leadingOnePassSignatureCounter > 0) {
+          throw new GrammarError('Missing trailing signature packets');
+        }
+    }
+  }
+}
diff --git a/src/packet/packet.js b/src/packet/packet.js
index e703fa54..4dfcfa9f 100644
--- a/src/packet/packet.js
+++ b/src/packet/packet.js
@@ -20,7 +20,7 @@
  * @module packet/packet
  */
 
-import { ArrayStream, getWriter as streamGetWriter, getReader as streamGetReader } from '@openpgp/web-stream-tools';
+import { ArrayStream, getWriter as streamGetWriter } from '@openpgp/web-stream-tools';
 import enums from '../enums';
 import util from '../util';
 
@@ -114,8 +114,7 @@ export function supportsStreaming(tag) {
  * @param {Function} callback - Function to call with the parsed packet
  * @returns {Boolean} Returns false if the stream was empty and parsing is done, and true otherwise.
  */
-export async function readPackets(input, callback) {
-  const reader = streamGetReader(input);
+export async function readPacket(reader, useStreamType, callback) {
   let writer;
   let callbackReturned;
   try {
@@ -146,8 +145,8 @@ export async function readPackets(input, callback) {
 
     const packetSupportsStreaming = supportsStreaming(tag);
     let packet = null;
-    if (packetSupportsStreaming) {
-      if (util.isStream(input) === 'array') {
+    if (useStreamType && packetSupportsStreaming) {
+      if (useStreamType === 'array') {
         const arrayStream = new ArrayStream();
         writer = streamGetWriter(arrayStream);
         packet = arrayStream;
@@ -240,38 +239,6 @@ export async function readPackets(input, callback) {
       }
     } while (wasPartialLength);
 
-    // If this was not a packet that "supports streaming", we peek to check
-    // whether it is the last packet in the message. We peek 2 bytes instead
-    // of 1 because the beginning of this function also peeks 2 bytes, and we
-    // want to cut a `subarray` of the correct length into `web-stream-tools`'
-    // `externalBuffer` as a tiny optimization here.
-    //
-    // If it *was* a streaming packet (i.e. the data packets), we peek at the
-    // entire remainder of the stream, in order to forward errors in the
-    // remainder of the stream to the packet data. (Note that this means we
-    // read/peek at all signature packets before closing the literal data
-    // packet, for example.) This forwards MDC errors to the literal data
-    // stream, for example, so that they don't get lost / forgotten on
-    // decryptedMessage.packets.stream, which we never look at.
-    //
-    // An example of what we do when stream-parsing a message containing
-    // [ one-pass signature packet, literal data packet, signature packet ]:
-    // 1. Read the one-pass signature packet
-    // 2. Peek 2 bytes of the literal data packet
-    // 3. Parse the one-pass signature packet
-    //
-    // 4. Read the literal data packet, simultaneously stream-parsing it
-    // 5. Peek until the end of the message
-    // 6. Finish parsing the literal data packet
-    //
-    // 7. Read the signature packet again (we already peeked at it in step 5)
-    // 8. Peek at the end of the stream again (`peekBytes` returns undefined)
-    // 9. Parse the signature packet
-    //
-    // Note that this means that if there's an error in the very end of the
-    // stream, such as an MDC error, we throw in step 5 instead of in step 8
-    // (or never), which is the point of this exercise.
-    const nextPacket = await reader.peekBytes(packetSupportsStreaming ? Infinity : 2);
     if (writer) {
       await writer.ready;
       await writer.close();
@@ -280,7 +247,6 @@ export async function readPackets(input, callback) {
       // eslint-disable-next-line callback-return
       await callback({ tag, packet });
     }
-    return !nextPacket || !nextPacket.length;
   } catch (e) {
     if (writer) {
       await writer.abort(e);
@@ -292,7 +258,6 @@ export async function readPackets(input, callback) {
     if (writer) {
       await callbackReturned;
     }
-    reader.releaseLock();
   }
 }
 
@@ -321,6 +286,18 @@ export class UnknownPacketError extends UnsupportedError {
   }
 }
 
+export class MalformedPacketError extends UnsupportedError {
+  constructor(...params) {
+    super(...params);
+
+    if (Error.captureStackTrace) {
+      Error.captureStackTrace(this, UnsupportedError);
+    }
+
+    this.name = 'MalformedPacketError';
+  }
+}
+
 export class UnparseablePacket {
   constructor(tag, rawContent) {
     this.tag = tag;
diff --git a/src/packet/packetlist.js b/src/packet/packetlist.js
index cf034861..41be1c16 100644
--- a/src/packet/packetlist.js
+++ b/src/packet/packetlist.js
@@ -1,16 +1,16 @@
 import { transformPair as streamTransformPair, transform as streamTransform, getWriter as streamGetWriter, getReader as streamGetReader, clone as streamClone } from '@openpgp/web-stream-tools';
 import {
-  readPackets, supportsStreaming,
+  readPacket, supportsStreaming,
   writeTag, writeHeader,
   writePartialLength, writeSimpleLength,
   UnparseablePacket,
   UnsupportedError,
-  UnknownPacketError
+  UnknownPacketError,
+  MalformedPacketError
 } from './packet';
 import util from '../util';
 import enums from '../enums';
 import defaultConfig from '../config';
-import { GrammarError } from './grammar';
 
 /**
  * Instantiate a new packet given its tag
@@ -47,13 +47,15 @@ class PacketList extends Array {
    * @param {Uint8Array | ReadableStream<Uint8Array>} bytes - binary data to parse
    * @param {Object} allowedPackets - mapping where keys are allowed packet tags, pointing to their Packet class
    * @param {Object} [config] - full configuration, defaults to openpgp.config
+   * @param {function(enums.packet[], boolean, Object): void} [grammarValidator]
+   * @param {Boolean} [delayErrors] - delay errors until the input stream has been read completely
    * @returns {PacketList} parsed list of packets
    * @throws on parsing errors
    * @async
    */
-  static async fromBinary(bytes, allowedPackets, config = defaultConfig, grammarValidator = null) {
+  static async fromBinary(bytes, allowedPackets, config = defaultConfig, grammarValidator = null, delayErrors = false) {
     const packets = new PacketList();
-    await packets.read(bytes, allowedPackets, config, grammarValidator);
+    await packets.read(bytes, allowedPackets, config, grammarValidator, delayErrors);
     return packets;
   }
 
@@ -63,72 +65,138 @@ class PacketList extends Array {
    * @param {Object} allowedPackets - mapping where keys are allowed packet tags, pointing to their Packet class
    * @param {Object} [config] - full configuration, defaults to openpgp.config
    * @param {function(enums.packet[], boolean, Object): void} [grammarValidator]
+   * @param {Boolean} [delayErrors] - delay errors until the input stream has been read completely
    * @throws on parsing errors
    * @async
    */
-  async read(bytes, allowedPackets, config = defaultConfig, grammarValidator = null) {
+  async read(bytes, allowedPackets, config = defaultConfig, grammarValidator = null, delayErrors = false) {
+    let additionalAllowedPackets;
     if (config.additionalAllowedPackets.length) {
-      allowedPackets = { ...allowedPackets, ...util.constructAllowedPackets(config.additionalAllowedPackets) };
+      additionalAllowedPackets = util.constructAllowedPackets(config.additionalAllowedPackets);
+      allowedPackets = { ...allowedPackets, ...additionalAllowedPackets };
     }
     this.stream = streamTransformPair(bytes, async (readable, writable) => {
+      const reader = streamGetReader(readable);
       const writer = streamGetWriter(writable);
-      const writtenTags = [];
       try {
+        let useStreamType = util.isStream(readable);
         while (true) {
           await writer.ready;
-          const done = await readPackets(readable, async parsed => {
+          let unauthenticatedError;
+          let wasStream;
+          await readPacket(reader, useStreamType, async parsed => {
             try {
               if (parsed.tag === enums.packet.marker || parsed.tag === enums.packet.trust || parsed.tag === enums.packet.padding) {
-                // According to the spec, these packet types should be ignored and not cause parsing errors, even if not esplicitly allowed:
+                // According to the spec, these packet types should be ignored and not cause parsing errors, even if not explicitly allowed:
                 // - Marker packets MUST be ignored when received: https://github.com/openpgpjs/openpgpjs/issues/1145
                 // - Trust packets SHOULD be ignored outside of keyrings (unsupported): https://datatracker.ietf.org/doc/html/rfc4880#section-5.10
                 // - [Padding Packets] MUST be ignored when received: https://datatracker.ietf.org/doc/html/draft-ietf-openpgp-crypto-refresh#name-padding-packet-tag-21
                 return;
               }
               const packet = newPacketFromTag(parsed.tag, allowedPackets);
+              // Unknown packets throw in the call above, we ignore them
+              // in the grammar checker.
+              try {
+                grammarValidator?.recordPacket(parsed.tag, additionalAllowedPackets);
+              } catch (e) {
+                if (config.enforceGrammar) {
+                  throw e;
+                } else {
+                  util.printDebugError(e);
+                }
+              }
               packet.packets = new PacketList();
               packet.fromStream = util.isStream(parsed.packet);
-              await packet.read(parsed.packet, config);
+              wasStream = packet.fromStream;
+              try {
+                await packet.read(parsed.packet, config);
+              } catch (e) {
+                if (!(e instanceof UnsupportedError)) {
+                  throw util.wrapError(new MalformedPacketError(`Parsing ${packet.constructor.name} failed`), e);
+                }
+                throw e;
+              }
               await writer.write(packet);
-              writtenTags.push(parsed.tag);
-              // The `writtenTags` are only sensitive if we are parsing an _unauthenticated_ decrypted stream,
-              // since they can enable an decryption oracle.
-              // It's responsibility of the caller to pass a `grammarValidator` that takes care of
-              // postponing error reporting until the data has been authenticated.
-              grammarValidator?.(writtenTags, true, config);
             } catch (e) {
               // If an implementation encounters a critical packet where the packet type is unknown in a packet sequence,
               // it MUST reject the whole packet sequence. On the other hand, an unknown non-critical packet MUST be ignored.
               // Packet Tags from 0 to 39 are critical. Packet Tags from 40 to 63 are non-critical.
-              if (e instanceof UnknownPacketError) {
-                if (parsed.tag <= 39) {
-                  await writer.abort(e);
+              const throwUnknownPacketError =
+                e instanceof UnknownPacketError &&
+                parsed.tag <= 39;
+              // In case of unsupported packet versions/algorithms/etc, we ignore the error by default
+              // (unless the packet is a data packet, see below).
+              const throwUnsupportedError =
+                e instanceof UnsupportedError &&
+                !(e instanceof UnknownPacketError) &&
+                !config.ignoreUnsupportedPackets;
+              // In case of packet parsing errors, e.name was set to 'MalformedPacketError' above.
+              // By default, we throw for these errors.
+              const throwMalformedPacketError =
+                e instanceof MalformedPacketError &&
+                !config.ignoreMalformedPackets;
+              // The packets that support streaming are the ones that contain message data.
+              // Those are also the ones we want to be more strict about and throw on all errors
+              // (since we likely cannot process the message without these packets anyway).
+              const throwDataPacketError = supportsStreaming(parsed.tag);
+              // Throw all other errors, including `GrammarError`s, disallowed packet errors, and unexpected errors.
+              const throwOtherError = !(
+                e instanceof UnknownPacketError ||
+                e instanceof UnsupportedError ||
+                e instanceof MalformedPacketError
+              );
+              if (
+                throwUnknownPacketError ||
+                throwUnsupportedError ||
+                throwMalformedPacketError ||
+                throwDataPacketError ||
+                throwOtherError
+              ) {
+                if (delayErrors) {
+                  unauthenticatedError = e;
                 } else {
-                  return;
+                  await writer.abort(e);
                 }
-              }
-
-              const throwUnsupportedError = !config.ignoreUnsupportedPackets && e instanceof UnsupportedError;
-              const throwMalformedError = !config.ignoreMalformedPackets && !(e instanceof UnsupportedError);
-              const throwGrammarError = e instanceof GrammarError;
-              if (throwUnsupportedError || throwMalformedError || throwGrammarError || supportsStreaming(parsed.tag)) {
-                // The packets that support streaming are the ones that contain message data.
-                // Those are also the ones we want to be more strict about and throw on parse errors
-                // (since we likely cannot process the message without these packets anyway).
-                await writer.abort(e);
               } else {
                 const unparsedPacket = new UnparseablePacket(parsed.tag, parsed.packet);
                 await writer.write(unparsedPacket);
-                writtenTags.push(parsed.tag);
-                grammarValidator?.(writtenTags, true, config);
               }
               util.printDebugError(e);
             }
           });
+          if (wasStream) {
+            // Don't allow more than one streaming packet, as read errors
+            // may get lost in the second packet's data stream.
+            useStreamType = null;
+          }
+
+          // If there was a parse error, read the entire input first
+          // in case there's an MDC error, which should take precedence.
+          if (unauthenticatedError) {
+            await reader.readToEnd();
+            // eslint-disable-next-line @typescript-eslint/no-throw-literal
+            throw unauthenticatedError;
+          }
+
+          // We peek to check whether this was the last packet.
+          // We peek 2 bytes instead of 1 because `readPacket` also
+          // peeks 2 bytes, and we want to cut a `subarray` of the
+          // correct length into `web-stream-tools`' `externalBuffer`
+          // as a tiny optimization here.
+          const nextPacket = await reader.peekBytes(2);
+          const done = !nextPacket || !nextPacket.length;
           if (done) {
             // Here we are past the MDC check for SEIPDv1 data, hence
             // the data is always authenticated at this point.
-            grammarValidator?.(writtenTags, false, config);
+            try {
+              grammarValidator?.recordEnd();
+            } catch (e) {
+              if (config.enforceGrammar) {
+                throw e;
+              } else {
+                util.printDebugError(e);
+              }
+            }
             await writer.ready;
             await writer.close();
             return;
diff --git a/src/packet/sym_encrypted_integrity_protected_data.js b/src/packet/sym_encrypted_integrity_protected_data.js
index 7941c7c5..9fbfdb49 100644
--- a/src/packet/sym_encrypted_integrity_protected_data.js
+++ b/src/packet/sym_encrypted_integrity_protected_data.js
@@ -28,7 +28,7 @@ import OnePassSignaturePacket from './one_pass_signature';
 import SignaturePacket from './signature';
 import PacketList from './packetlist';
 import { UnsupportedError } from './packet';
-import { getMessageGrammarValidator } from './grammar';
+import { MessageGrammarValidator } from './grammar';
 
 // A SEIP packet can contain the following packet types
 const allowedPackets = /*#__PURE__*/ util.constructAllowedPackets([
@@ -184,23 +184,17 @@ class SymEncryptedIntegrityProtectedDataPacket {
     if (isArrayStream(encrypted)) encrypted = await streamReadToEnd(encrypted);
 
     let packetbytes;
-    let grammarValidator;
+    let delayErrors = false;
     if (this.version === 2) {
       if (this.cipherAlgorithm !== sessionKeyAlgorithm) {
         // sanity check
         throw new Error('Unexpected session key algorithm');
       }
       packetbytes = await runAEAD(this, 'decrypt', key, encrypted);
-      grammarValidator = getMessageGrammarValidator({ delayReporting: false });
     } else {
       const { blockSize } = getCipherParams(sessionKeyAlgorithm);
       const decrypted = await cipherMode.cfb.decrypt(sessionKeyAlgorithm, key, encrypted, new Uint8Array(blockSize));
 
-      // Grammar validation cannot be run before message integrity has been enstablished,
-      // to avoid leaking info about the unauthenticated message structure.
-      const releaseUnauthenticatedStream = util.isStream(encrypted) && config.allowUnauthenticatedStream;
-      grammarValidator = getMessageGrammarValidator({ delayReporting: releaseUnauthenticatedStream });
-
       // there must be a modification detection code packet as the
       // last packet and everything gets hashed except the hash itself
       const realHash = streamSlice(streamPassiveClone(decrypted), -20);
@@ -219,7 +213,9 @@ class SymEncryptedIntegrityProtectedDataPacket {
       const bytes = streamSlice(tohash, blockSize + 2); // Remove random prefix
       packetbytes = streamSlice(bytes, 0, -2); // Remove MDC packet
       packetbytes = streamConcat([packetbytes, streamFromAsync(() => verifyHash)]);
-      if (!releaseUnauthenticatedStream) {
+      if (util.isStream(encrypted) && config.allowUnauthenticatedStream) {
+        delayErrors = true;
+      } else {
         packetbytes = await streamReadToEnd(packetbytes);
       }
     }
@@ -228,7 +224,7 @@ class SymEncryptedIntegrityProtectedDataPacket {
     // MUST yield a valid OpenPGP Message.
     // - Decrypting a version 2 Symmetrically Encrypted and Integrity Protected Data packet
     // MUST yield a valid Optionally Padded Message.
-    this.packets = await PacketList.fromBinary(packetbytes, allowedPackets, config, grammarValidator);
+    this.packets = await PacketList.fromBinary(packetbytes, allowedPackets, config, new MessageGrammarValidator(), delayErrors);
     return true;
   }
 }
diff --git a/src/util.js b/src/util.js
index c57ea431..3c6bf707 100644
--- a/src/util.js
+++ b/src/util.js
@@ -593,17 +593,23 @@ const util = {
     }).join('\n');
   },
 
-  wrapError: function(message, error) {
-    if (!error) {
-      return new Error(message);
+  wrapError: function(error, cause) {
+    if (!cause) {
+      if (error instanceof Error) {
+        return error;
+      }
+      return new Error(error);
     }
 
-    // update error message
-    try {
-      error.message = message + ': ' + error.message;
-    } catch (e) {}
-
-    return error;
+    if (error instanceof Error) {
+      // update error message
+      try {
+        error.message += ': ' + cause.message;
+        error.cause = cause;
+      } catch (e) {}
+      return error;
+    }
+    return new Error(error + ': ' + cause.message, { cause });
   },
 
   /**
diff --git a/test/general/config.js b/test/general/config.js
index 98e22af5..89c6a969 100644
--- a/test/general/config.js
+++ b/test/general/config.js
@@ -35,7 +35,7 @@ habAyxd1AGKaNp1wbGFpbnRleHQgbWVzc2FnZQ==
 
     await expect(
       openpgp.readMessage({ armoredMessage: skeskPlusLiteralData, config: { enforceGrammar: true } })
-    ).to.be.rejectedWith(/Data does not respect OpenPGP grammar/);
+    ).to.be.rejectedWith(/Unexpected packet/);
   });
 
   it('openpgp.readSignature', async function() {
diff --git a/test/general/packet.js b/test/general/packet.js
index 76982462..c998ef4a 100644
--- a/test/general/packet.js
+++ b/test/general/packet.js
@@ -13,7 +13,7 @@ import * as random from '../../src/crypto/random';
 
 import * as input from './testInputs.js';
 import { mockCryptoRandomGenerator, restoreCryptoRandomGenerator } from '../mockRandom.ts';
-import { getMessageGrammarValidator } from '../../src/packet/grammar.js';
+import { MessageGrammarValidator } from '../../src/packet/grammar.js';
 
 function stringify(array) {
   if (stream.isStream(array)) {
@@ -1335,8 +1335,8 @@ kePFjAnu9cpynKXu3usf8+FuBw2zLsg1Id1n7ttxoAte416KjBN9lFBt8mcu
     it('Ignores non-critical packet even with tolerant mode disabled', async function() {
       const unknownPacketTag63 = util.hexToUint8Array('ff0a750064bf943d6e756c6c'); // non-critical tag
 
-      await expect(openpgp.PacketList.fromBinary(unknownPacketTag63, allAllowedPackets, { ...openpgp.config, ignoreUnsupportedPackets: false, ignoreMalformedPackets: false })).to.eventually.have.length(0);
-      await expect(openpgp.PacketList.fromBinary(unknownPacketTag63, allAllowedPackets, { ...openpgp.config, ignoreUnsupportedPackets: true, ignoreMalformedPackets: true })).to.eventually.have.length(0);
+      await expect(openpgp.PacketList.fromBinary(unknownPacketTag63, allAllowedPackets, { ...openpgp.config, ignoreUnsupportedPackets: false, ignoreMalformedPackets: false })).to.eventually.have.length(1);
+      await expect(openpgp.PacketList.fromBinary(unknownPacketTag63, allAllowedPackets, { ...openpgp.config, ignoreUnsupportedPackets: true, ignoreMalformedPackets: true })).to.eventually.have.length(1);
     });
 
     it('Throws on disallowed packet even with tolerant mode enabled', async function() {
@@ -1375,11 +1375,237 @@ kePFjAnu9cpynKXu3usf8+FuBw2zLsg1Id1n7ttxoAte416KjBN9lFBt8mcu
     });
 
     describe('Grammar validation', async function () {
+      describe('MessageGrammarValidator - unit tests', () => {
+        it('valid nested signed messages should be valid', () => {
+          // Sig | OPS | Literal | Sig
+          const m1 = new MessageGrammarValidator();
+          m1.recordPacket(openpgp.enums.packet.signature);
+          m1.recordPacket(openpgp.enums.packet.onePassSignature);
+          m1.recordPacket(openpgp.enums.packet.literalData);
+          m1.recordPacket(openpgp.enums.packet.signature);
+          expect(() => m1.recordEnd()).to.not.throw();
+
+          // OPS | Sig | Literal | Sig
+          const m2 = new MessageGrammarValidator();
+          m2.recordPacket(openpgp.enums.packet.onePassSignature);
+          m2.recordPacket(openpgp.enums.packet.signature);
+          m2.recordPacket(openpgp.enums.packet.literalData);
+          m2.recordPacket(openpgp.enums.packet.signature);
+          expect(() => m2.recordEnd()).to.not.throw();
+
+          // OPS | Sig | Literal -  should throw due to missing trailing signature
+          const m3 = new MessageGrammarValidator();
+          m3.recordPacket(openpgp.enums.packet.onePassSignature);
+          m3.recordPacket(openpgp.enums.packet.signature);
+          m3.recordPacket(openpgp.enums.packet.literalData);
+          expect(() => m3.recordEnd()).to.throw();
+
+          // Sig -  should throw due to standalone signature packet
+          const m4 = new MessageGrammarValidator();
+          m4.recordPacket(openpgp.enums.packet.signature);
+          expect(() => m3.recordEnd()).to.throw();
+
+          // ESK | Sig | SEIPD -  should throw
+          const m5 = new MessageGrammarValidator();
+          m5.recordPacket(openpgp.enums.packet.publicKeyEncryptedSessionKey);
+          expect(() => m5.recordPacket(openpgp.enums.packet.signature)).to.throw();
+        });
+
+        it('standalone additional allowed packets should be valid', () => {
+          const additionalAllowedPackets = { [openpgp.PublicKeyPacket.tag]: openpgp.PublicKeyPacket };
+          // Sig | OPS | PublicKeyPacket | Sig
+          const m1 = new MessageGrammarValidator();
+          m1.recordPacket(openpgp.enums.packet.signature, additionalAllowedPackets);
+          m1.recordPacket(openpgp.enums.packet.onePassSignature, additionalAllowedPackets);
+          m1.recordPacket(openpgp.enums.packet.publicKey, additionalAllowedPackets);
+          m1.recordPacket(openpgp.enums.packet.signature, additionalAllowedPackets);
+          expect(() => m1.recordEnd()).to.not.throw();
+
+          // OPS | Sig | PublicKeyPacket | Sig
+          const m2 = new MessageGrammarValidator();
+          m2.recordPacket(openpgp.enums.packet.onePassSignature, additionalAllowedPackets);
+          m2.recordPacket(openpgp.enums.packet.signature, additionalAllowedPackets);
+          m2.recordPacket(openpgp.enums.packet.publicKey, additionalAllowedPackets);
+          m2.recordPacket(openpgp.enums.packet.signature, additionalAllowedPackets);
+          expect(() => m2.recordEnd()).to.not.throw();
+
+          // standalone PublicKeyPacket
+          const m3 = new MessageGrammarValidator();
+          m3.recordPacket(openpgp.enums.packet.publicKey, additionalAllowedPackets);
+          expect(() => m3.recordEnd()).to.not.throw();
+
+          // OPS | Sig | PublicKey -  should throw due to missing trailing signature
+          const m4 = new MessageGrammarValidator();
+          m4.recordPacket(openpgp.enums.packet.onePassSignature, additionalAllowedPackets);
+          m4.recordPacket(openpgp.enums.packet.signature, additionalAllowedPackets);
+          m4.recordPacket(openpgp.enums.packet.publicKey, additionalAllowedPackets);
+          expect(() => m4.recordEnd()).to.throw();
+
+          // Sig | PublicKey | Sig | PublicKey -  should throw
+          const m5 = new MessageGrammarValidator();
+          m5.recordPacket(openpgp.enums.packet.signature, additionalAllowedPackets);
+          m5.recordPacket(openpgp.enums.packet.publicKey, additionalAllowedPackets);
+          expect(() => m5.recordPacket(openpgp.enums.packet.signature, additionalAllowedPackets)).to.throw();
+
+          // Sig | PublicKey | OPS | PublicKey | Sig -  should throw
+          const m6 = new MessageGrammarValidator();
+          m6.recordPacket(openpgp.enums.packet.signature, additionalAllowedPackets);
+          m6.recordPacket(openpgp.enums.packet.publicKey, additionalAllowedPackets);
+          expect(() => m6.recordPacket(openpgp.enums.packet.onePassSignature, additionalAllowedPackets)).to.throw();
+        });
+
+        it('standalone disallowed packets should not be valid', () => {
+          // standalone PublicKeyPacket
+          const m1 = new MessageGrammarValidator();
+          expect(() => m1.recordPacket(openpgp.enums.packet.publicKey)).to.throw();
+        });
+      });
+
       it('reject duplicate literal packet', async () => {
         const packets = new openpgp.PacketList();
         packets.push(new openpgp.LiteralDataPacket());
         packets.push(new openpgp.LiteralDataPacket());
-        await expect(openpgp.PacketList.fromBinary(packets.write(), allAllowedPackets, openpgp.config, getMessageGrammarValidator({ delayReporting: false }))).to.be.rejectedWith(/Data does not respect OpenPGP grammar/);
+        await expect(openpgp.PacketList.fromBinary(packets.write(), allAllowedPackets, openpgp.config, new MessageGrammarValidator())).to.be.rejectedWith(/Unexpected packet/);
+      });
+
+      it('reject duplicate literal packet inside encrypted data', async () => {
+        const literalPackets = new openpgp.PacketList();
+        literalPackets.push(new openpgp.LiteralDataPacket());
+        literalPackets.push(new openpgp.LiteralDataPacket());
+        const encrypted = new openpgp.SymEncryptedIntegrityProtectedDataPacket();
+        encrypted.version = 1;
+        encrypted.packets = literalPackets;
+        const packets = new openpgp.PacketList();
+        packets.push(encrypted);
+        await encrypted.encrypt(openpgp.enums.symmetric.aes128, new Uint8Array(16));
+        await expect(openpgp.decrypt({
+          message: await openpgp.readMessage({
+            binaryMessage: packets.write()
+          }),
+          sessionKeys: [{ algorithm: 'aes128', data: new Uint8Array(16) }]
+        })).to.be.rejectedWith(/Unexpected packet/);
+      });
+
+      it('reject duplicate literal packet inside encrypted data (streaming)', async () => {
+        const literalPackets = new openpgp.PacketList();
+        literalPackets.push(new openpgp.LiteralDataPacket());
+        literalPackets.push(new openpgp.LiteralDataPacket());
+        const encrypted = new openpgp.SymEncryptedIntegrityProtectedDataPacket();
+        encrypted.version = 1;
+        encrypted.packets = literalPackets;
+        const packets = new openpgp.PacketList();
+        packets.push(encrypted);
+        await encrypted.encrypt(openpgp.enums.symmetric.aes128, new Uint8Array(16));
+        const decrypted = await openpgp.decrypt({
+          message: await openpgp.readMessage({
+            binaryMessage: new ReadableStream({
+              start(controller) {
+                controller.enqueue(packets.write());
+                controller.close();
+              }
+            })
+          }),
+          sessionKeys: [{ algorithm: 'aes128', data: new Uint8Array(16) }],
+          config: {
+            allowUnauthenticatedStream: true
+          }
+        });
+        await expect(stream.readToEnd(decrypted.data)).to.be.rejectedWith(/Unexpected packet/);
+      });
+
+      it('reject duplicate literal packet inside encrypted data (MDC error gets precedence)', async () => {
+        const literalPackets = new openpgp.PacketList();
+        literalPackets.push(new openpgp.LiteralDataPacket());
+        const literal = new openpgp.LiteralDataPacket();
+        literal.data = new Uint8Array(1000);
+        literalPackets.push(literal);
+        const encrypted = new openpgp.SymEncryptedIntegrityProtectedDataPacket();
+        encrypted.version = 1;
+        encrypted.packets = literalPackets;
+        const packets = new openpgp.PacketList();
+        packets.push(encrypted);
+        await encrypted.encrypt(openpgp.enums.symmetric.aes128, new Uint8Array(16));
+        const encryptedData = packets.write();
+        encryptedData[encryptedData.length - 5] ^= 1;
+        const decrypted = await openpgp.decrypt({
+          message: await openpgp.readMessage({
+            binaryMessage: new ReadableStream({
+              start(controller) {
+                controller.enqueue(encryptedData.subarray(0, 500));
+                controller.enqueue(encryptedData.subarray(500));
+                controller.close();
+              }
+            })
+          }),
+          sessionKeys: [{ algorithm: 'aes128', data: new Uint8Array(16) }],
+          config: {
+            allowUnauthenticatedStream: true
+          }
+        });
+        await expect(stream.readToEnd(decrypted.data)).to.be.rejectedWith(/Modification detected/);
+      });
+
+      it('reject malformed packet inside encrypted data', async () => {
+        const literalPackets = new openpgp.PacketList();
+        const signature = new openpgp.SignaturePacket();
+        signature.signatureData = signature.signedHashValue = new Uint8Array([4, 4]);
+        signature.params = {};
+        literalPackets.push(signature);
+        literalPackets.push(new openpgp.LiteralDataPacket());
+        const encrypted = new openpgp.SymEncryptedIntegrityProtectedDataPacket();
+        encrypted.version = 1;
+        encrypted.packets = literalPackets;
+        const packets = new openpgp.PacketList();
+        packets.push(encrypted);
+        await encrypted.encrypt(openpgp.enums.symmetric.aes128, new Uint8Array(16));
+        const encryptedData = packets.write();
+        await expect(openpgp.decrypt({
+          message: await openpgp.readMessage({
+            binaryMessage: new ReadableStream({
+              start(controller) {
+                controller.enqueue(encryptedData.subarray(0, 500));
+                controller.enqueue(encryptedData.subarray(500));
+                controller.close();
+              }
+            })
+          }),
+          sessionKeys: [{ algorithm: 'aes128', data: new Uint8Array(16) }],
+          config: {
+            allowUnauthenticatedStream: true
+          }
+        })).to.be.rejectedWith(/Missing signature creation time subpacket/);
+      });
+
+      it('reject malformed packet inside encrypted data (MDC error gets precedence)', async () => {
+        const literalPackets = new openpgp.PacketList();
+        const signature = new openpgp.SignaturePacket();
+        signature.signatureData = signature.signedHashValue = new Uint8Array([4, 4]);
+        signature.params = {};
+        literalPackets.push(signature);
+        literalPackets.push(new openpgp.LiteralDataPacket());
+        const encrypted = new openpgp.SymEncryptedIntegrityProtectedDataPacket();
+        encrypted.version = 1;
+        encrypted.packets = literalPackets;
+        const packets = new openpgp.PacketList();
+        packets.push(encrypted);
+        await encrypted.encrypt(openpgp.enums.symmetric.aes128, new Uint8Array(16));
+        const encryptedData = packets.write();
+        encryptedData[encryptedData.length - 5] ^= 1;
+        await expect(openpgp.decrypt({
+          message: await openpgp.readMessage({
+            binaryMessage: new ReadableStream({
+              start(controller) {
+                controller.enqueue(encryptedData.subarray(0, 500));
+                controller.enqueue(encryptedData.subarray(500));
+                controller.close();
+              }
+            })
+          }),
+          sessionKeys: [{ algorithm: 'aes128', data: new Uint8Array(16) }],
+          config: {
+            allowUnauthenticatedStream: true
+          }
+        })).to.be.rejectedWith(/Modification detected/);
       });
 
       it('accepts padding and marker packets', async () => {
@@ -1389,33 +1615,14 @@ kePFjAnu9cpynKXu3usf8+FuBw2zLsg1Id1n7ttxoAte416KjBN9lFBt8mcu
         packets.push(padding);
         packets.push(new openpgp.MarkerPacket());
         packets.push(new openpgp.LiteralDataPacket());
-        const parsed = await openpgp.PacketList.fromBinary(packets.write(), allAllowedPackets, openpgp.config, getMessageGrammarValidator({ delayReporting: false }));
+        const parsed = await openpgp.PacketList.fromBinary(packets.write(), allAllowedPackets, openpgp.config, new MessageGrammarValidator());
         expect(parsed.length).to.equal(1); // marker and padding packets are always dropped on parsing
-
-        const messageGrammarValidatorWithLatentReporting = getMessageGrammarValidator({ delayReporting: true });
-        const parsed2 = await openpgp.PacketList.fromBinary(packets.write(), allAllowedPackets, openpgp.config, messageGrammarValidatorWithLatentReporting);
-        expect(parsed2.length).to.equal(1);
-        const sampleGrammarValidatorReturnValue = isPartial => messageGrammarValidatorWithLatentReporting([] /* valid */, isPartial, openpgp.config);
-        expect(sampleGrammarValidatorReturnValue(true)).to.be.null;
-        expect(sampleGrammarValidatorReturnValue(false)).to.be.true;
       });
 
       it('accepts unknown packets', async () => {
         const unknownPacketTag63 = util.hexToUint8Array('ff0a750064bf943d6e756c6c'); // non-critical tag
-        const parsed = await openpgp.PacketList.fromBinary(unknownPacketTag63, allAllowedPackets, openpgp.config, getMessageGrammarValidator({ delayReporting: false }));
-        expect(parsed.length).to.equal(0);
-      });
-
-      it('delay reporting', () => {
-        const messageGrammarValidatorWithLatentReporting = getMessageGrammarValidator({ delayReporting: true });
-
-        const sampleGrammarValidatorReturnValueValid = isPartial => messageGrammarValidatorWithLatentReporting([] /* valid */, isPartial, openpgp.config);
-        expect(sampleGrammarValidatorReturnValueValid(true)).to.be.null;
-        expect(sampleGrammarValidatorReturnValueValid(false)).to.be.true;
-
-        const sampleGrammarValidatorReturnValueInvalid = isPartial => messageGrammarValidatorWithLatentReporting([openpgp.enums.packet.literalData, openpgp.enums.packet.literalData] /* invalid */, isPartial, openpgp.config);
-        expect(sampleGrammarValidatorReturnValueInvalid(true)).to.be.null;
-        expect(() => sampleGrammarValidatorReturnValueInvalid(false)).to.throw(/Data does not respect OpenPGP grammar/);
+        const parsed = await openpgp.PacketList.fromBinary(unknownPacketTag63, allAllowedPackets, openpgp.config, new MessageGrammarValidator());
+        expect(parsed.length).to.equal(1);
       });
     });
   });
diff --git a/test/general/signature.js b/test/general/signature.js
index 05360536..78c23f00 100644
--- a/test/general/signature.js
+++ b/test/general/signature.js
@@ -1881,10 +1881,10 @@ hkJiXopCSWKSlQInL1devkJJUWJmTmZeugJYlpdLAagQJM0JpsCqIQZwKgAA
       expect(pubKey.getKeys(keyIDs[0])).to.not.be.empty;
 
       await openpgp.verify({ verificationKeys: [pubKey], message, config: { minRSABits: 1024 } }).then(async ({ data, signatures }) => {
-        await expect(stream.readToEnd(data)).to.be.rejectedWith('Data does not respect OpenPGP grammar');
+        await expect(stream.readToEnd(data)).to.be.rejectedWith('Missing trailing signature packets');
         expect(signatures).to.have.length(1);
-        await expect(signatures[0].verified).to.be.rejectedWith('Data does not respect OpenPGP grammar');
-        await expect(signatures[0].signature).to.be.rejectedWith('Data does not respect OpenPGP grammar');
+        await expect(signatures[0].verified).to.be.rejectedWith('Missing trailing signature packets');
+        await expect(signatures[0].signature).to.be.rejectedWith('Missing trailing signature packets');
       });
       await openpgp.verify({ verificationKeys: [pubKey], message: messageWithoutGrammar, config: { minRSABits: 1024 } }).then(async ({ data, signatures }) => {
         expect(await stream.readToEnd(data)).to.equal(plaintext);