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The efficiency of a digital envelope is best described by the use of both a private key and a public key in its encoding process. This method leverages asymmetric encryption, where a message is encrypted with a symmetric key (often referred to as a session key) and then that symmetric key is itself encrypted using the recipient's public key.

This approach is efficient for several reasons:

1. **Speed of Symmetric Encryption**: Symmetric encryption (used for the actual message content) is generally faster than asymmetric encryption, which allows for quicker processing of large amounts of data. By encrypting the message with a symmetric key, the overall time taken to encrypt the message is reduced.

2. **Safety of the Key**: The session key is protected by being encrypted with the recipient's public key, ensuring that only the recipient can decrypt it using their private key. This provides both security and efficiency, as it avoids the need to share the symmetric key in an insecure manner.

3. **Reduced Overhead**: By only transmitting a small symmetric key instead of re-encrypting large amounts of data multiple times, the digital envelope optimizes both time and resource usage in the encryption and decryption processes.

This highlights how the combination of symmetric and asymmetric