Basic string encryption with Python

####################
# Basic String Encryption

test = False

input_raw = 'string to encrypt'

offset = 2 # needs to be <26. 26 returns input_raw

index = {
    'a': 1,
    'b': 2,
    'c': 3,
    'd': 4,
    'e': 5,
    'f': 6,
    'g': 7,
    'h': 8,
    'i': 9,
    'j': 10,
    'k': 11,
    'l': 12,
    'm': 13,
    'n': 14,
    'o': 15,
    'p': 16,
    'q': 17,
    'r': 18,
    's': 19,
    't': 20,
    'u': 21,
    'v': 22,
    'w': 23,
    'x': 24,
    'y': 25,
    'z': 26,
}

def find_key(input_dict, value):
    for k, v in input_dict.items():
        if v == value:
            return k

def encrypt_with_offset(input,offset=4, test=False):
    global index
    input = input.lower().strip()
    if test:
        print(f"\nencrypt_with_offset {input=}\n")
    output = ''
    for letter in input:
        if letter.isalpha(): # check if character is alphabet, else return raw character
            letter_index = index[letter]
            if test:
                print(f"{letter_index=}")
            if (letter_index + offset) > len(index):
                letter_index = letter_index - 26
            encrypted_letter = find_key(index, letter_index + offset)
            if test:
                print(letter, encrypted_letter)
            output = output + encrypted_letter
        else:
            output = output + letter

    if test:
        print(f"\n{output=}\n-----------\n")
    return output

def decrypt_with_offset(input,offset=4, test=False):
    global index
    input = input.lower().strip()
    if test: 
        print(f"\ndecrypt_with_offset {input=}\n")
    output = ''
    for letter in input:
        if letter.isalpha(): # check if character is alphabet, else return raw character
            letter_index = index[letter]
            if test:
                print(f"{letter_index=}")
            if (letter_index - offset) < 0:
                letter_index = letter_index + 26
            encrypted_letter = find_key(index, letter_index - offset)
            if test:
                print(letter, encrypted_letter)
            output = output + encrypted_letter
        else:
            output = output + letter
    if test:
        print(f"\n{output=}\n-----------\n")
    return output

encrypt_output = encrypt_with_offset(input_raw, offset)

print(f"Encrypt: {repr(input_raw)}\toutput with offset {offset}: {repr(encrypt_output)}")
print()
print(f"Decrypt with offset {offset}: {repr(encrypt_output)}\toutput: {repr(decrypt_with_offset(encrypt_output, offset))}")

Encrypt: 'string to encrypt'    output with offset 2: 'uvtkpi vq gpetarv'

Decrypt with offset 2: 'uvtkpi vq gpetarv'      output: 'string to encrypt'