Cryptography — The science of secret writing…..
Cryptography provides for secure communication in the presence of malicious third parties — known as adversaries. Encryption uses an algorithm and a key to transform an input (i.e., plaintext) into an encrypted output (i.e., ciphertext). A given algorithm will always transform the same plaintext into the same ciphertext if the same key is used.
Algorithms are considered secure if an attacker cannot determine any properties of the plaintext or key, given the ciphertext. An attacker should not be able to determine anything about a key given a large number of plaintext/ciphertext combinations that used the key.
Cryptography techniques
Cryptography is closely related to the disciplines of cryptology and cryptanalysis. It includes techniques such as microdots, merging words with images, and other ways to hide information in storage or transit. However, in today’s computer-centric world, cryptography is most often associated with scrambling plaintext(ordinary text, sometimes referred to as cleartext) into ciphertext(a process called encryption), then back again (known as decryption). Individuals who practice this field are known as cryptographers.
Modern cryptography concerns itself with the following four objectives:
- Confidentiality. The information cannot be understood by anyone for whom it was unintended.
- Integrity. The information cannot be altered in storage or transit between sender and intended receiver without the alteration being detected.
- Non-repudiation. The creator/sender of the information cannot deny at a later stage their intentions in the creation or transmission of the information.
- Authentication. The sender and receiver can confirm each other’s identity and the origin/destination of the information.
- Modern cryptography concerns itself with the following four objectives:
Symmetric cryptography
This encryption method is more classic and easier to understand. You have a secret key that is just a string of random bytes. You use the same key for data encryption and decryption. It is fast and super secure, so secure that most symmetric encryption algorithms are impossible to break even by quantum computers. This method plays a crucial role in technologies like TLS/HTTPS or SSH but not that much in the blockchain. There is one inconvenient drawback of this type of encryption — it is challenging to securely convey a secret key between parties.
Asymmetric cryptography
With help comes asymmetric cryptography, where we have a pair of keys (called the public key and private key). The public key is used for encrypting the data, and the private key is used to decrypt the data. The first one does not need to be kept private because it cannot be used to decrypt the data. So it is common to distribute it to anyone that you wish to establish an encrypted channel with.
That person can send you a message encrypted with your public key, and only you can decrypt it using your private key. This helps a lot with the security issues of symmetric encryption. If someone gets your public key, it gives them nothing — even if they somehow get an encrypted message, they won’t be able to decrypt it.
The biggest disadvantage of this cryptographic technique is that it is less secure and much slower. Often we combine both encryptions. For example in HTTPS, the browser generates a one-time symmetric secret key, encrypts it using its public key, and then sends it to the server. Then all following data is encrypted using that symmetric secret key.
Asymmetric encryption also enabled the concept of digital signatures. … If, instead of using a private key for encryption it is instead used for message authentication, one can sign a message. To sign a message one first hashes (hashes are described next) a message and then encrypts the hash.
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