diff --git a/web/flow-typed/WebCrypto.js b/web/flow-typed/WebCrypto.js
new file mode 100644
--- /dev/null
+++ b/web/flow-typed/WebCrypto.js
@@ -0,0 +1,109 @@
+// @flow
+
+// This code was copied from https://github.com/facebook/flow/pull/7714/files
+// and slightly modified to get rid of TODO types that broke flow checks.
+
+// This is a helper type to simplify the specification, it isn't an interface
+// and there are no objects implementing it.
+// https://developer.mozilla.org/en-US/docs/Web/API/BufferSource
+type BufferSource = ArrayBuffer | $ArrayBufferView;
+
+type DOMString = string;
+
+declare interface Crypto extends RandomSource {
+  +subtle: SubtleCrypto;
+}
+
+type CryptoKey$Type =
+| 'secret'
+| 'public'
+| 'private';
+type CryptoKey$Usages =
+| 'encrypt'
+| 'decrypt'
+| 'sign'
+| 'verify'
+| 'deriveKey'
+| 'deriveBits'
+| 'wrapKey'
+| 'unwrapKey';
+declare interface CryptoKey {
+  +algorithm: Object;
+  +extractable: boolean;
+  +type: CryptoKey$Type;
+  +usages: CryptoKey$Usages[];
+}
+
+declare interface RandomSource {
+  getRandomValues<T: $TypedArray>(typedArray: T): T;
+};
+
+type SubtleCrypto$Algorithm =
+| {name: 'AES-CBC', iv: BufferSource}
+| {name: 'AES-CTR', counter: BufferSource, length: number}
+| {name: 'AES-GCM', iv: BufferSource, additionalData?: BufferSource, tagLength?: number}
+| {name: 'RSA-OAEP', label?: string};
+type SubtleCrypto$DerivedKeyAlgorithm = {
+  name: DOMString,
+  length?: 128 | 192 | 256,
+};
+type SubtleCrypto$AesKeyGenParams = {
+  name: 'AES-CBC' | 'AES-CTR' | 'AES-GCM' | 'AES-KW',
+  length: 128 | 192 | 256,
+};
+// TODO: fix types for parameters
+type SubtleCrypto$DeriveKeyAlgorithm =
+| {name: 'ECDH', public: any}
+| {name: 'DH', public: any}
+| {name: 'PBKDF2', salt: BufferSource, iterations: number, hash: DOMString}
+| {name: 'HKDF-CTR', hash: any, label: any, context: any};
+type SubtleCrypto$GenerateKeyAlgo =
+| 'AES-CBC'
+| 'AES-CTR'
+| 'AES-GCM'
+| 'RSA-OAEP'
+| 'AES-KW'
+| 'HMAC'
+| 'RSASSA-PKCS1-v1_5'
+| 'ECDSA'
+| 'ECDH'
+| 'DH';
+type SubtleCrypto$HashAlgo =
+| 'SHA-1'
+| 'SHA-256'
+| 'SHA-384'
+| 'SHA-512';
+type SubtleCrypto$KeyFormat =
+| 'raw'
+| 'pkcs8'
+| 'spki'
+| 'jwk';
+type SubtleCrypto$SignAlgo =
+| 'HMAC'
+| 'RSASSA-PKCS1-v1_5'
+| 'ECDSA';
+type SubtleCrypto$WrapAlgo =
+| 'AES-CBC'
+| 'AES-CTR'
+| 'AES-GCM'
+| 'RSA-OAEP'
+| 'AES-KW';
+declare interface SubtleCrypto {
+  decrypt(algorithm: SubtleCrypto$Algorithm, key: CryptoKey, data: BufferSource): Promise<ArrayBuffer>;
+  // TODO: fix type for 'algorithm' param
+  deriveBits(algorithm: any, baseKey: CryptoKey, length: number): Promise<ArrayBuffer>;
+  deriveKey(algorithm: SubtleCrypto$DeriveKeyAlgorithm, masterKey: CryptoKey, derivedKeyAlgorithm: DOMString | SubtleCrypto$DerivedKeyAlgorithm, extractable: boolean, keyUsages: CryptoKey$Usages[]): Promise<CryptoKey>;
+  digest(algo: SubtleCrypto$HashAlgo, buffer: BufferSource): Promise<ArrayBuffer>;
+  encrypt(algorithm: SubtleCrypto$Algorithm, key: CryptoKey, data: BufferSource): Promise<ArrayBuffer>;
+  exportKey(format: SubtleCrypto$KeyFormat, key: CryptoKey): Promise<ArrayBuffer>;
+  generateKey(algo: SubtleCrypto$AesKeyGenParams, extractable: boolean, keyUsages: CryptoKey$Usages[]): Promise<CryptoKey>;
+  importKey(format: SubtleCrypto$KeyFormat, keyData: ArrayBuffer | $TypedArray, algo: SubtleCrypto$GenerateKeyAlgo, extractable: boolean, usages: CryptoKey$Usages[]): Promise<CryptoKey>;
+  sign(algo: SubtleCrypto$SignAlgo, key: CryptoKey, text2sign: BufferSource): Promise<ArrayBuffer>;
+  unwrapKey(format: SubtleCrypto$KeyFormat, wrappedKey: ArrayBuffer, unwrappingKey: CryptoKey, unwrapAlgo: SubtleCrypto$WrapAlgo, unwrappedKeyAlgo: SubtleCrypto$WrapAlgo, extractable: boolean, keyUsages: CryptoKey$Usages[]): Promise<CryptoKey>;
+  verify(algo: SubtleCrypto$SignAlgo, key: CryptoKey, signature: BufferSource, text2verify: BufferSource): Promise<boolean>;
+  wrapKey(format: SubtleCrypto$KeyFormat, key: CryptoKey, wrappingKey: CryptoKey, wrapAlgo: SubtleCrypto$WrapAlgo): Promise<ArrayBuffer>;
+}
+
+declare var crypto: Crypto;
+declare var msCrypto: Crypto;
+declare var webkitCrypto: Crypto;
diff --git a/web/utils/text-utils.js b/web/utils/text-utils.js
--- a/web/utils/text-utils.js
+++ b/web/utils/text-utils.js
@@ -4,11 +4,6 @@
 
 let canvas;
 
-declare var crypto: {
-  getRandomValues<NumArray: $TypedArray>(typedArray: NumArray): NumArray,
-  ...
-};
-
 function calculateMaxTextWidth(
   texts: $ReadOnlyArray<string>,
   font: string,