update upload validation to use shared AES-GCM size constants on the backend.

backend/src/config/index.ts

@@ -1,3 +1,4 @@
+import { AES_GCM_AUTH_TAG_BYTE_LENGTH } from '../consts/crypto/aesGcmAuthTagByteLength';
 import { AltchaConfig } from '../types/config/AltchaConfig';
 import { AppConfig } from '../types/config/AppConfig';
 import { NodeEnv } from '../types/NodeEnv';
@@ -34,7 +35,6 @@ export const getAltchaConfig = (): AltchaConfig => {
 };
 
 export const getAppConfig = (): AppConfig => {
-    const aesGcmCiphertextOverheadBytes = 16;
     const jsonBase64ByteRatioNumerator = 4;
     const jsonBase64ByteRatioDenominator = 3;
 
@@ -61,7 +61,7 @@ export const getAppConfig = (): AppConfig => {
             upload: {
                 maxViewsMax: envInt('VALIDATION_UPLOAD_MAX_VIEWS_MAX'),
                 maxImageBytes,
-                maxCiphertextBytes: maxImageBytes + aesGcmCiphertextOverheadBytes,
+                maxCiphertextBytes: maxImageBytes + AES_GCM_AUTH_TAG_BYTE_LENGTH,
                 maxRequestBodyBytes:
                     Math.ceil((maxImageBytes * jsonBase64ByteRatioNumerator) / jsonBase64ByteRatioDenominator) +
                     jsonBodyPaddingBytes