Dart "encrypt" library gives unreadable string after encryption
In the Dart code the ciphertext must be Base64 decoded and not UTF8 encoded:
final decrypted = encrypter.decrypt(Encrypted.from64(cipher), iv: IV.fromUtf8(iv)); // fromBase64() works alsoThis is not necessary in the JavaScript code, where CryptoJS.AES.decrypt() implicitly converts the Base64 encoded ciphertext into a CipherParams object.
If this is fixed, the decryption works!
Note the following vulnerability: In both codes, the first 16 bytes of the 32 bytes key are also used as IV. If the same key is applied several times, this automatically results in the repetition of key/IV pairs. Especially for CRT this is insecure, see Why must IV/key-pairs not be reused in CTR mode?
Usually a random IV is generated for each encryption. The IV is not secret and is sent along with the ciphertext (typically concatenated). On the decrypting side, the IV is stripped and used for decryption.