diff --git a/src/constants.ts b/src/constants.ts
index e69de29..0bf192d 100644
--- a/src/constants.ts
+++ b/src/constants.ts
@@ -0,0 +1,11 @@
+/**
+ * Matches 64-character lowercase hexadecimal strings.
+ * Used for validating event IDs and cryptographic keys.
+ */
+export const HEX_64_PATTERN = /^[a-f0-9]{64}$/;
+
+/**
+ * Matches 128-character lowercase hexadecimal strings.
+ * Used for validating signatures.
+ */
+export const HEX_128_PATTERN = /^[a-f0-9]{128}$/;
diff --git a/src/errors.ts b/src/errors.ts
index e69de29..6818dfb 100644
--- a/src/errors.ts
+++ b/src/errors.ts
@@ -0,0 +1,79 @@
+/**
+ * Public key is not 64 lowercase hex characters.
+ */
+export class MalformedPubKeyError extends Error {
+  constructor() {
+    super("public key must be 64 lowercase hex characters");
+    this.name = "MalformedPubKeyError";
+  }
+}
+
+/**
+ * Private key is not 64 lowercase hex characters.
+ */
+export class MalformedPrivKeyError extends Error {
+  constructor() {
+    super("private key must be 64 lowercase hex characters");
+    this.name = "MalformedPrivKeyError";
+  }
+}
+
+/**
+ * Event ID is not 64 hex characters.
+ */
+export class MalformedIDError extends Error {
+  constructor() {
+    super("event id must be 64 hex characters");
+    this.name = "MalformedIDError";
+  }
+}
+
+/**
+ * Event signature is not 128 hex characters.
+ */
+export class MalformedSigError extends Error {
+  constructor() {
+    super("event signature must be 128 hex characters");
+    this.name = "MalformedSigError";
+  }
+}
+
+/**
+ * Event tag contains fewer than two elements.
+ */
+export class MalformedTagError extends Error {
+  constructor() {
+    super("tags must contain at least two elements");
+    this.name = "MalformedTagError";
+  }
+}
+
+/**
+ * Event ID could not be computed during validation.
+ */
+export class FailedIDCompError extends Error {
+  constructor() {
+    super("failed to compute event id");
+    this.name = "FailedIDCompError";
+  }
+}
+
+/**
+ * Event ID field is empty.
+ */
+export class NoEventIDError extends Error {
+  constructor() {
+    super("event id is empty");
+    this.name = "NoEventIDError";
+  }
+}
+
+/**
+ * Event signature failed cryptographic validation.
+ */
+export class InvalidSigError extends Error {
+  constructor() {
+    super("event signature is invalid");
+    this.name = "InvalidSigError";
+  }
+}
diff --git a/src/types.ts b/src/types.ts
index e69de29..95f50ed 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -0,0 +1,51 @@
+/**
+ * Tag represents a single tag within an event as an array of strings.
+ * The first element identifies the tag name, the second contains the value,
+ * and subsequent elements are optional.
+ */
+export type Tag = string[];
+
+/**
+ * Event represents a Nostr protocol event with its seven required fields.
+ * All fields must be present for a valid event.
+ */
+export interface Event {
+  id: string;
+  pubkey: string;
+  created_at: number;
+  kind: number;
+  tags: Tag[];
+  content: string;
+  sig: string;
+}
+
+/**
+ * TagFilters maps tag names to arrays of values for tag-based filtering.
+ * Keys correspond to tag names without the "#" prefix.
+ */
+export interface TagFilters {
+  [tagName: string]: string[];
+}
+
+/**
+ * FilterExtensions holds arbitrary additional filter fields as raw JSON values.
+ * Allows custom filter extensions without modifying the core Filter type.
+ */
+export interface FilterExtensions {
+  [key: string]: unknown;
+}
+
+/**
+ * Filter defines subscription criteria for events.
+ * All conditions within a filter are applied with AND logic.
+ */
+export interface Filter {
+  ids?: string[];
+  authors?: string[];
+  kinds?: number[];
+  since?: number;
+  until?: number;
+  limit?: number;
+  tags?: TagFilters;
+  extensions?: FilterExtensions;
+}