diff --git a/keyserver/src/utils/aes-crypto-utils.js b/keyserver/src/utils/aes-crypto-utils.js
new file mode 100644
--- /dev/null
+++ b/keyserver/src/utils/aes-crypto-utils.js
@@ -0,0 +1,33 @@
+// @flow
+
+import crypto from 'crypto';
+
+import {
+  generateKeyCommon,
+  encryptCommon,
+  decryptCommon,
+} from 'lib/media/aes-crypto-utils-common.js';
+
+// crypto.webcrypto was introduced in Node 15.10.0.
+// It is not defined in Flow so we need a cast
+const commonCrypto: Crypto = (crypto: any).webcrypto;
+
+function generateKey(): Promise<Uint8Array> {
+  return generateKeyCommon(commonCrypto);
+}
+
+function encrypt(
+  keyBytes: Uint8Array,
+  plaintext: Uint8Array,
+): Promise<Uint8Array> {
+  return encryptCommon(commonCrypto, keyBytes, plaintext);
+}
+
+function decrypt(
+  keyBytes: Uint8Array,
+  sealedData: Uint8Array,
+): Promise<Uint8Array> {
+  return decryptCommon(commonCrypto, keyBytes, sealedData);
+}
+
+export { generateKey, encrypt, decrypt };
diff --git a/web/flow-typed/web-crypto.js b/lib/flow-typed/web-crypto-common.js
copy from web/flow-typed/web-crypto.js
copy to lib/flow-typed/web-crypto-common.js
--- a/web/flow-typed/web-crypto.js
+++ b/lib/flow-typed/web-crypto-common.js
@@ -279,7 +279,3 @@
       | 'AES-KW',
   ): Promise<ArrayBuffer>;
 }
-
-declare var crypto: Crypto;
-declare var msCrypto: Crypto;
-declare var webkitCrypto: Crypto;
diff --git a/lib/media/aes-crypto-utils-common.js b/lib/media/aes-crypto-utils-common.js
new file mode 100644
--- /dev/null
+++ b/lib/media/aes-crypto-utils-common.js
@@ -0,0 +1,84 @@
+// @flow
+
+const KEY_SIZE = 32; // bytes
+const IV_LENGTH = 12; // bytes - unique Initialization Vector (nonce)
+const TAG_LENGTH = 16; // bytes - GCM auth tag
+
+async function generateKeyCommon(commonCrypto: Crypto): Promise<Uint8Array> {
+  const algorithm = { name: 'AES-GCM', length: 256 };
+  const key = await commonCrypto.subtle.generateKey(algorithm, true, [
+    'encrypt',
+    'decrypt',
+  ]);
+  const keyData = await commonCrypto.subtle.exportKey('raw', key);
+  return new Uint8Array(keyData);
+}
+
+async function encryptCommon(
+  commonCrypto: Crypto,
+  keyBytes: Uint8Array,
+  plaintext: Uint8Array,
+): Promise<Uint8Array> {
+  if (keyBytes.length !== KEY_SIZE) {
+    throw new Error('Invalid AES key size');
+  }
+
+  // we're creating the buffer now so we can avoid reallocating it later
+  const outputBuffer = new ArrayBuffer(
+    plaintext.length + IV_LENGTH + TAG_LENGTH,
+  );
+  const ivBytes = new Uint8Array(outputBuffer, 0, IV_LENGTH);
+  const iv = commonCrypto.getRandomValues(ivBytes);
+
+  const algorithm = { name: 'AES-GCM', iv: iv, tagLength: TAG_LENGTH * 8 };
+  const key = await commonCrypto.subtle.importKey(
+    'raw',
+    keyBytes,
+    'AES-GCM',
+    false,
+    ['encrypt'],
+  );
+  const ciphertextWithTag = await commonCrypto.subtle.encrypt(
+    algorithm,
+    key,
+    plaintext,
+  );
+
+  const result = new Uint8Array(outputBuffer);
+  result.set(new Uint8Array(ciphertextWithTag), iv.length);
+  return result;
+}
+
+async function decryptCommon(
+  commonCrypto: Crypto,
+  keyBytes: Uint8Array,
+  sealedData: Uint8Array,
+): Promise<Uint8Array> {
+  if (keyBytes.length !== KEY_SIZE) {
+    throw new Error('Invalid AES key size');
+  }
+  if (sealedData.length < IV_LENGTH + TAG_LENGTH) {
+    throw new Error('Invalid ciphertext size');
+  }
+
+  const iv = sealedData.subarray(0, IV_LENGTH);
+  const ciphertextWithTag = sealedData.subarray(IV_LENGTH);
+
+  const algorithm = { name: 'AES-GCM', iv, tagLength: TAG_LENGTH * 8 };
+  const key = await commonCrypto.subtle.importKey(
+    'raw',
+    keyBytes,
+    'AES-GCM',
+    false,
+    ['decrypt'],
+  );
+
+  const plaintextBuffer = await commonCrypto.subtle.decrypt(
+    algorithm,
+    key,
+    ciphertextWithTag,
+  );
+  return new Uint8Array(plaintextBuffer);
+}
+
+export { generateKeyCommon, encryptCommon, decryptCommon };
diff --git a/web/flow-typed/web-crypto.js b/web/flow-typed/web-crypto.js
--- a/web/flow-typed/web-crypto.js
+++ b/web/flow-typed/web-crypto.js
@@ -1,285 +1,5 @@
 // @flow
 
-declare interface RandomSource {
-  getRandomValues<T: $TypedArray>(typedArray: T): T;
-  randomUUID(): string;
-}
-
-declare interface Crypto extends RandomSource {
-  +subtle: SubtleCrypto;
-}
-
-type CryptoKey$Type = 'secret' | 'public' | 'private';
-type CryptoKey$Usages =
-  | 'encrypt'
-  | 'decrypt'
-  | 'sign'
-  | 'verify'
-  | 'deriveKey'
-  | 'deriveBits'
-  | 'wrapKey'
-  | 'unwrapKey';
-
-declare type CryptoKey = {
-  +algorithm:
-    | SubtleCrypto$AesKeyGenParams
-    | SubtleCrypto$RsaHashedKeyGenParams
-    | SubtleCrypto$EcKeyGenParams
-    | SubtleCrypto$HmacKeyGenParams,
-  +extractable: boolean,
-  +type: CryptoKey$Type,
-  +usages: $ReadOnlyArray<CryptoKey$Usages>,
-};
-
-type SubtleCrypto$KeyFormatWithoutJwk = 'pkcs8' | 'raw' | 'spki';
-type SubtleCrypto$KeyFormat = 'jwk' | SubtleCrypto$KeyFormatWithoutJwk;
-type SubtleCrypto$HashAlgo = 'SHA-1' | 'SHA-256' | 'SHA-384' | 'SHA-512';
-type SubtleCrypto$AesAlgo = 'AES-CBC' | 'AES-CTR' | 'AES-GCM' | 'AES-KW';
-
-type SubtleCrypto$RsaOaepParams = {
-  +name: 'RSA-OAEP',
-  +label?: BufferSource,
-};
-
-type SubtleCrypto$AesCtrParams = {
-  +name: 'AES-CTR',
-  +counter: BufferSource,
-  +length: number,
-};
-
-type SubtleCrypto$AesCbcParams = {
-  +name: 'AES-CBC',
-  +iv: BufferSource,
-};
-
-type SubtleCrypto$AesGcmParams = {
-  +name: 'AES-GCM',
-  +iv: BufferSource,
-  +additionalData?: BufferSource,
-  +tagLength?: number,
-};
-
-type SubtleCrypto$EcdhKeyDeriveParams = {
-  +name: 'ECDH',
-  +public: CryptoKey,
-};
-
-type SubtleCrypto$HkdfParams = {
-  +name: 'HKDF',
-  +hash: 'SHA-1' | 'SHA-256' | 'SHA-384' | 'SHA-512',
-  +info: BufferSource,
-  +salt: BufferSource,
-};
-
-type SubtleCrypto$Pbkdf2Params = {
-  +name: 'PBKDF2',
-  +hash: 'SHA-1' | 'SHA-256' | 'SHA-384' | 'SHA-512',
-  +iterations: number,
-  +salt: BufferSource,
-};
-
-type SubtleCrypto$HmacImportParams = {
-  +name: 'HMAC',
-  +hash: 'SHA-256' | 'SHA-384' | 'SHA-512',
-  +length?: number,
-};
-
-type SubtleCrypto$RsaHashedKeyGenParams = {
-  +name: 'RSASSA-PKCS1-v1_5' | 'RSA-PSS' | 'RSA-OAEP',
-  +modulusLength: number,
-  +publicExponent: Uint8Array,
-  +hash: 'SHA-256' | 'SHA-384' | 'SHA-512',
-};
-
-type SubtleCrypto$HmacKeyGenParams = {
-  +name: 'HMAC',
-  +hash: SubtleCrypto$HashAlgo,
-  +length?: number,
-};
-
-type SubtleCrypto$RsaHashedImportParams = {
-  +name: 'RSASSA-PKCS1-v1_5' | 'RSA-PSS' | 'RSA-OAEP',
-  +hash: 'SHA-256' | 'SHA-384' | 'SHA-512',
-};
-
-type SubtleCrypto$EcKeyImportParams = {
-  +name: 'ECDSA' | 'ECDH',
-  +namedCurve: 'P-256' | 'P-384' | 'P-521',
-};
-
-type SubtleCrypto$EcKeyGenParams = {
-  +name: 'ECDSA' | 'ECDH',
-  +namedCurve: 'P-256' | 'P-384' | 'P-521',
-};
-
-type SubtleCrypto$RsaPssParams = {
-  +name: 'RSA-PSS',
-  +saltLength: number,
-};
-
-type SubtleCrypto$EcdsaParams = {
-  +name: 'ECDSA',
-  +hash: 'SHA-256' | 'SHA-384' | 'SHA-512',
-};
-
-type SubtleCrypto$AesKeyGenParams = {
-  +name: 'AES-CBC' | 'AES-CTR' | 'AES-GCM' | 'AES-KW',
-  +length: 128 | 192 | 256,
-};
-
-type SubtleCrypto$ImportKeyAlgo =
-  | SubtleCrypto$RsaHashedImportParams
-  | SubtleCrypto$EcKeyImportParams
-  | SubtleCrypto$HmacImportParams
-  | SubtleCrypto$AesAlgo
-  | 'PBKDF2'
-  | 'HKDF';
-
-type SubtleCrypto$RsaOtherPrimesInfo = {
-  +d?: string,
-  +r?: string,
-  +t?: string,
-};
-
-type SubtleCrypto$JsonWebKey = {
-  +alg?: string,
-  +crv?: string,
-  +d?: string,
-  +dp?: string,
-  +dq?: string,
-  +e?: string,
-  +ext?: boolean,
-  +k?: string,
-  +key_ops?: $ReadOnlyArray<string>,
-  +kty?: string,
-  +n?: string,
-  +oth?: $ReadOnlyArray<SubtleCrypto$RsaOtherPrimesInfo>,
-  +p?: string,
-  +q?: string,
-  +qi?: string,
-  +use?: string,
-  +x?: string,
-  +y?: string,
-};
-
-declare interface SubtleCrypto {
-  decrypt(
-    algorithm:
-      | SubtleCrypto$RsaOaepParams
-      | SubtleCrypto$AesCtrParams
-      | SubtleCrypto$AesCbcParams
-      | SubtleCrypto$AesGcmParams,
-    key: CryptoKey,
-    data: BufferSource,
-  ): Promise<ArrayBuffer>;
-  deriveBits(
-    algorithm:
-      | SubtleCrypto$EcdhKeyDeriveParams
-      | SubtleCrypto$HkdfParams
-      | SubtleCrypto$Pbkdf2Params,
-    baseKey: CryptoKey,
-    length: number,
-  ): Promise<ArrayBuffer>;
-  deriveKey(
-    algorithm:
-      | SubtleCrypto$EcdhKeyDeriveParams
-      | SubtleCrypto$HkdfParams
-      | SubtleCrypto$Pbkdf2Params,
-    baseKey: CryptoKey,
-    derivedKeyType:
-      | SubtleCrypto$HmacKeyGenParams
-      | SubtleCrypto$AesKeyGenParams,
-    extractable: boolean,
-    keyUsages: $ReadOnlyArray<CryptoKey$Usages>,
-  ): Promise<CryptoKey>;
-  digest(
-    algorithm: SubtleCrypto$HashAlgo | { +name: SubtleCrypto$HashAlgo },
-    data: BufferSource,
-  ): Promise<ArrayBuffer>;
-  encrypt(
-    algorithm:
-      | SubtleCrypto$RsaOaepParams
-      | SubtleCrypto$AesCtrParams
-      | SubtleCrypto$AesCbcParams
-      | SubtleCrypto$AesGcmParams,
-    key: CryptoKey,
-    data: BufferSource,
-  ): Promise<ArrayBuffer>;
-  exportKey(format: 'jwk', key: CryptoKey): Promise<SubtleCrypto$JsonWebKey>;
-  exportKey(
-    format: SubtleCrypto$KeyFormatWithoutJwk,
-    key: CryptoKey,
-  ): Promise<ArrayBuffer>;
-  generateKey(
-    algorithm:
-      | SubtleCrypto$RsaHashedKeyGenParams
-      | SubtleCrypto$EcKeyGenParams
-      | SubtleCrypto$HmacKeyGenParams
-      | SubtleCrypto$AesKeyGenParams,
-    extractable: boolean,
-    keyUsages: $ReadOnlyArray<CryptoKey$Usages>,
-  ): Promise<CryptoKey>;
-  importKey(
-    format: SubtleCrypto$KeyFormatWithoutJwk,
-    keyData: BufferSource,
-    algorithm: SubtleCrypto$ImportKeyAlgo,
-    extractable: boolean,
-    keyUsages: $ReadOnlyArray<CryptoKey$Usages>,
-  ): Promise<CryptoKey>;
-  importKey(
-    format: 'jwk',
-    keyData: SubtleCrypto$JsonWebKey,
-    algorithm: SubtleCrypto$ImportKeyAlgo,
-    extractable: boolean,
-    keyUsages: $ReadOnlyArray<CryptoKey$Usages>,
-  ): Promise<CryptoKey>;
-
-  sign(
-    algorithm:
-      | 'RSASSA-PKCS1-v1_5'
-      | 'HMAC'
-      | SubtleCrypto$RsaPssParams
-      | SubtleCrypto$EcdsaParams,
-    key: CryptoKey,
-    data: BufferSource,
-  ): Promise<ArrayBuffer>;
-  unwrapKey(
-    format: SubtleCrypto$KeyFormat,
-    wrappedKey: ArrayBuffer,
-    unwrappingKey: CryptoKey,
-    unwrapAlgorithm:
-      | SubtleCrypto$RsaOaepParams
-      | SubtleCrypto$AesCtrParams
-      | SubtleCrypto$AesCbcParams
-      | SubtleCrypto$AesGcmParams
-      | 'AES-KW',
-    unwrappedKeyAlgorithm: SubtleCrypto$ImportKeyAlgo,
-    extractable: boolean,
-    keyUsages: $ReadOnlyArray<CryptoKey$Usages>,
-  ): Promise<CryptoKey>;
-  verify(
-    algorithm:
-      | SubtleCrypto$RsaPssParams
-      | SubtleCrypto$EcdsaParams
-      | 'RSASSA-PKCS1-v1_5'
-      | 'HMAC',
-    key: CryptoKey,
-    signature: ArrayBuffer,
-    data: ArrayBuffer,
-  ): Promise<boolean>;
-  wrapKey(
-    format: SubtleCrypto$KeyFormat,
-    key: CryptoKey,
-    wrappingKey: CryptoKey,
-    wrapAlgorithm:
-      | SubtleCrypto$RsaOaepParams
-      | SubtleCrypto$AesCtrParams
-      | SubtleCrypto$AesCbcParams
-      | SubtleCrypto$AesGcmParams
-      | 'AES-KW',
-  ): Promise<ArrayBuffer>;
-}
-
 declare var crypto: Crypto;
 declare var msCrypto: Crypto;
 declare var webkitCrypto: Crypto;