A method of implementing security in a modular exponentiation function for cryptographic operations is provided. A key is obtained as a parameter when the modular exponentiation function is invoked. The key may be one of either a public key or a private key of a cryptographic key pair. Within the modular exponentiation function, the method ascertains whether the key is greater than L bits long, where L is a positive integer. A countermeasure against an attack is implemented if the key is greater than L bits long. The countermeasure may include one or more techniques (e.g., hardware and/or software techniques) that inhibit or prevent information about the key from being ascertained through analysis. One or more exponentiation operations may then be performed using the key. The same modular exponentiation function may be used to perform encryption and decryption operations but with different keys.