diff --git a/spec/util.spec.ts b/spec/util.spec.ts
new file mode 100644
index 0000000..34636e0
--- /dev/null
+++ b/spec/util.spec.ts
@@ -0,0 +1,34 @@
+import { areAttachmentFieldIdsValid } from '../src/util/crypto'
+describe('utils', () => {
+  describe('areAttachmentFieldIdsValid', () => {
+    it('should return true when all the fieldIds are within the filenames', () => {
+      // Arrange
+      const MOCK_FILENAMES = {
+        mock: 'file',
+        alsomock: 'file2',
+      }
+      const MOCK_FIELD_IDS = Object.keys(MOCK_FILENAMES)
+
+      // Act
+      const actual = areAttachmentFieldIdsValid(MOCK_FIELD_IDS, MOCK_FILENAMES)
+
+      // Assert
+      expect(actual).toBe(true)
+    })
+
+    it('should return false when some fieldIds are not within the filenames', () => {
+      // Arrange
+      const MOCK_FILENAMES = {
+        mock: 'file',
+        alsomock: 'file2',
+      }
+      const MOCK_FIELD_IDS = Object.keys(MOCK_FILENAMES).concat('missingField')
+
+      // Act
+      const actual = areAttachmentFieldIdsValid(MOCK_FIELD_IDS, MOCK_FILENAMES)
+
+      // Assert
+      expect(actual).toBe(false)
+    })
+  })
+})
diff --git a/src/crypto.ts b/src/crypto.ts
index 00955e9..7b0d4dc 100644
--- a/src/crypto.ts
+++ b/src/crypto.ts
@@ -12,9 +12,11 @@ import {
   encryptMessage,
   generateKeypair,
   verifySignedMessage,
+  areAttachmentFieldIdsValid,
+  convertEncryptedAttachmentToFileContent,
 } from './util/crypto'
 import { determineIsFormFields } from './util/validate'
-import { MissingPublicKeyError } from './errors'
+import { MissingPublicKeyError, AttachmentDecryptionError } from './errors'
 import {
   DecryptedAttachments,
   DecryptedContent,
@@ -227,40 +229,42 @@ export default class Crypto {
       }
     })
 
-    const downloadPromises: Array<Promise<void>> = []
-    for (const fieldId in attachmentRecords) {
-      // Original name for the file is not found
-      if (filenames[fieldId] === undefined) return null
+    const fieldIds = Object.keys(attachmentRecords)
+    // Check if all fieldIds are within filenames
+    if (!areAttachmentFieldIdsValid(fieldIds, filenames)) {
+      return null
+    }
 
-      downloadPromises.push(
+    const downloadPromises = fieldIds.map((fieldId) => {
+      return (
         axios
-          .get(attachmentRecords[fieldId], { responseType: 'json' })
-          .then((downloadResponse) => {
-            const encryptedAttachment: EncryptedAttachmentContent =
-              downloadResponse.data
-            const encryptedFile: EncryptedFileContent = {
-              submissionPublicKey:
-                encryptedAttachment.encryptedFile.submissionPublicKey,
-              nonce: encryptedAttachment.encryptedFile.nonce,
-              binary: decodeBase64(encryptedAttachment.encryptedFile.binary),
-            }
-
+          // Retrieve all the attachments as JSON
+          .get<EncryptedAttachmentContent>(attachmentRecords[fieldId], {
+            responseType: 'json',
+          })
+          // Decrypt all the attachments
+          .then(({ data: downloadResponse }) => {
+            const encryptedFile =
+              convertEncryptedAttachmentToFileContent(downloadResponse)
             return this.decryptFile(formSecretKey, encryptedFile)
           })
           .then((decryptedFile) => {
-            if (decryptedFile === null)
-              throw new Error('Attachment decryption failed')
-            decryptedRecords[fieldId] = {
-              filename: filenames[fieldId],
-              content: decryptedFile,
+            // Check if the file exists and set the filename accordingly; otherwise, throw an error
+            if (decryptedFile) {
+              decryptedRecords[fieldId] = {
+                filename: filenames[fieldId],
+                content: decryptedFile,
+              }
+            } else {
+              throw new AttachmentDecryptionError()
             }
           })
       )
-    }
+    })
 
     try {
       await Promise.all(downloadPromises)
-    } catch (e) {
+    } catch {
       return null
     }
 
diff --git a/src/errors.ts b/src/errors.ts
index 518c65c..69632df 100644
--- a/src/errors.ts
+++ b/src/errors.ts
@@ -33,8 +33,16 @@ class WebhookAuthenticateError extends Error {
   }
 }
 
+class AttachmentDecryptionError extends Error {
+  constructor(message = 'Attachment decryption with the given nonce failed.') {
+    super(message)
+    this.name = this.constructor.name
+  }
+}
+
 export {
   MissingSecretKeyError,
   MissingPublicKeyError,
   WebhookAuthenticateError,
+  AttachmentDecryptionError,
 }
diff --git a/src/util/crypto.ts b/src/util/crypto.ts
index 21d6346..a569408 100644
--- a/src/util/crypto.ts
+++ b/src/util/crypto.ts
@@ -1,7 +1,12 @@
 import nacl from 'tweetnacl'
 import { decodeBase64, encodeBase64, encodeUTF8 } from 'tweetnacl-util'
 
-import { EncryptedContent, Keypair } from '../types'
+import {
+  Keypair,
+  EncryptedContent,
+  EncryptedAttachmentContent,
+  EncryptedFileContent,
+} from '../types'
 
 /**
  * Helper method to generate a new keypair for encryption.
@@ -79,3 +84,29 @@ export const verifySignedMessage = (
     throw new Error('Failed to open signed message with given public key')
   return JSON.parse(encodeUTF8(openedMessage))
 }
+
+/**
+ * Helper method to check if all the field IDs given are within the filenames
+ * @param fieldIds the list of fieldIds to check
+ * @param filenames the filenames that should contain the fields
+ * @returns boolean indicating whether the fields are valid
+ */
+export const areAttachmentFieldIdsValid = (
+  fieldIds: string[],
+  filenames: Record<string, string>
+): boolean => {
+  return fieldIds.every((fieldId) => filenames[fieldId])
+}
+
+/**
+ * Converts an encrypted attachment to encrypted file content
+ * @param encryptedAttachment The encrypted attachment
+ * @returns EncryptedFileContent The encrypted file content
+ */
+export const convertEncryptedAttachmentToFileContent = (
+  encryptedAttachment: EncryptedAttachmentContent
+): EncryptedFileContent => ({
+  submissionPublicKey: encryptedAttachment.encryptedFile.submissionPublicKey,
+  nonce: encryptedAttachment.encryptedFile.nonce,
+  binary: decodeBase64(encryptedAttachment.encryptedFile.binary),
+})