Disadvantages of the Vernam Cipher

• Key Management: The main practical challenge of Vernam ciphers is the management of truly random private keys. The key must be the same length as the message, and a new key must be generated for each message. Distributing keys securely is not a trivial problem.

• Key Reuse Compromises Security: If keys are reused, the security of the Vernum cipher is compromised. If an attacker intercepts two ciphertexts encrypted with the same key, he can use XOR to cancel the key stream and reveal the XOR of the two plaintexts. This could potentially leak information about the original message.

• Key Storage and Transmission: Transmitting and storing keys securely is a major challenge. Once an adversary has access to the key, he or she can decrypt the ciphertext.

• Practicality and Efficiency: Vernam ciphers are often impractical for large-scale use due to the challenges of generating and securely distributing one-time pads. It is typically used only in scenarios where complete confidentiality is most important.

• Key Length Equal to Message Length: The key must be at least as long as the message, which can be inefficient for long messages. This is especially true when compared to modern symmetric key cryptography, which uses shorter keys.

For the implementation please refer to this article: Implementation of Vernam Cipher or One Time Pad Algorithm

Vernam Cipher is a method of encrypting alphabetic text. It is one of the Substitution techniques for converting plain text into cipher text. In this mechanism, we assign a number to each character of the Plain-Text, like (a = 0, b = 1, c = 2, … z = 25). 
Method to take key: In the Vernam cipher algorithm, we take a key to encrypt the plain text whose length should be equal to the length of the plain text.