PyCryptoDome - ValueError: MAC check failed

Question

I'm trying to create an ECC cipher in my program... the encryption works perfectly but with the decryption process, it gives the following error:

ValueError: MAC check failed.

I've checked and the only issue I could find was the input that was in .lower() making the input of the base64 ciphertext give me an error.

After changing that it still doesn't work.

The functions clear_terminal() and title() are just prints or trivial.

def generar_ECC_keys():
    ECC_key = ECC.generate(curve="P-256")
    ECC_private_key = ECC_key.export_key(format="PEM")
    ECC_public_key = ECC_key.public_key().export_key(format="PEM")
    return ECC_key, ECC_private_key, ECC_public_key

def encriptar_ECC(plaintext, ECC_public_key, ECC_key):
    receptor_key = ECC.import_key(ECC_public_key)
    shared_key = ECC_key.d * receptor_key.pointQ
    key_derivada = SHA256.new(str(shared_key.x).encode()).digest()[:16]
    AES_cipher = AES.new(key_derivada, AES.MODE_EAX)
    ciphertext, tag = AES_cipher.encrypt_and_digest(pad(plaintext.encode(), AES.block_size))
    
    return ECC_key.public_key().export_key(format='PEM'), AES_cipher.nonce, tag, ciphertext

def display_datos_encriptados(ECC_public_key, ECC_private_key, nonce, tag, ciphertext):
    nonce_b64 = base64.b64encode(nonce).decode('utf-8')
    tag_b64 = base64.b64encode(tag).decode('utf-8')
    ciphertext_b64 = base64.b64encode(ciphertext).decode('utf-8')
    
    print("===== Guarda esta información para desencriptar el mensaje posteriormente =====\n")
    print(f"Llave pública ECC (PEM):\n{ECC_public_key}\n")
    print(f"Llave privada ECC (PEM):\n{ECC_private_key}\n")
    print(f"Nonce (Base64):\n{nonce_b64}\n")
    print(f"Tag (Base64):\n{tag_b64}\n")
    print(f"Texto encriptado (Base64):\n{ciphertext_b64}\n")
    print("=============================================================")

def desencriptar_ECC(ECC_public_key, nonce, tag, ciphertext, ECC_private_key):

    compartido = ECC_private_key.d * ECC_public_key.pointQ
    llave_derivada = SHA256.new(str(compartido.x).encode()).digest()[:16]
    
    AES_cipher = AES.new(llave_derivada, AES.MODE_EAX, nonce)
    plaintext = unpad(AES_cipher.decrypt_and_verify(ciphertext, tag), AES.block_size)
    
    return plaintext

def ECC_cipher():
    print("===== Cifrado ECC =====")                                     # Printea un menú por estética
    modo = input("Elige encriptar o desencriptar (E/D): ").upper()       # Pide un input de encriptar/desencriptar
    while modo not in "ED" or modo == "" or modo in " " or modo == "ED": # Mira que la respuesta al input sea válida
        print("Error")
        modo = input("Elige encriptar o desencriptar (E/D): ").upper()                       
    
    ECC_key, ECC_private_key, ECC_public_key = generar_ECC_keys()
    if modo == "E":
        plaintext = input("Introduce un texto: ") 
        while len(plaintext) < 1:                                            
            print("Error. Introduce un texto válido.")
            plaintext = input("Introduce un texto: ")
        ECC_public_key, nonce, tag, ciphertext = encriptar_ECC(plaintext, ECC_public_key, ECC_key)
        display_datos_encriptados(ECC_public_key, ECC_private_key, nonce, tag, ciphertext)

    else:
        ciphertext_b64 = input("Introduce el texto encriptado: ")
        while len(ciphertext_b64) < 1:                                            
            print("Error. Introduce un texto válido.")
            ciphertext_b64 = input("Introduce el texto encriptado: ")
        ECC_public_key_encoded = (input("Introduce la llave pública usada durante la encripción: "))
        ECC_private_key_encoded = (input("Introduce la llave privada usada durante la encripción: "))
        nonce_b64 = (input("Introduce el nonce usado durante la encripción: "))
        tag_b64 = (input("Introduce el tag usado durante la encripción: "))
        ECC_public_key = ECC.import_key(base64.b64decode(ECC_public_key_encoded))
        ECC_private_key = ECC.import_key(base64.b64decode(ECC_private_key_encoded))
        nonce = base64.b64decode(nonce_b64)
        tag = base64.b64decode(tag_b64)
        ciphertext = base64.b64decode(ciphertext_b64)
        decrypted_message = desencriptar_ECC(ECC_public_key, nonce, tag, ciphertext, ECC_private_key)
        print(f"\nEl texto desencriptado es: \n{decrypted_message.decode()}\n\nCifrado ECC\nLlave privada: \n\n{ECC_private_key}\n")
    input("Pulsa enter para continuar.")
    clear_terminal()
    title()