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Question

Cryptology splits into two complementary disciplines — what are they, and what does each one do?

Answer

Cryptography builds the protection (encryption); cryptanalysis tries to break it (extracting information from ciphertext).

How cryptology divides into cryptography and cryptanalysis, and where symmetric and asymmetric sit

* Cryptology splits into building protection (cryptography) and breaking it (cryptanalysis); cryptography further splits into symmetric and asymmetric. *

Kryptologie (cryptology) is the umbrella term, divided into:

Discipline Goal
Kryptografie (cryptography) Verschlüsselung — encrypting/protecting information
Kryptoanalyse (cryptanalysis) Informationsgewinn — recovering information from encrypted data

The two are in a permanent arms race: cryptographers design schemes, cryptanalysts attack them, and schemes that survive sustained cryptanalysis are the ones we trust.

Tip: "-grafie" = writing (building secrets), "-analyse" = breaking (taking them apart). Both together = cryptology.

Go deeper:

First page of Al-Kindi's 9th century Manuscript on Deciphering Cryptographic Messages.
First page of Al-Kindi's 9th century Manuscript on Deciphering Cryptographic Messages.
Public domain · Wikimedia Commons
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Question

What is the fundamental difference between symmetric and asymmetric cryptography?

Answer

Symmetric uses the same key to encrypt and decrypt; asymmetric uses different keys (a public/private pair).

Symmetric Asymmetric
Keys one shared secret key a key pair: public + private
Encrypt / decrypt same key for both different keys for each
Also called secret-key crypto public-key crypto
Key challenge distributing the shared secret safely binding a public key to the right identity

Why both exist: symmetric crypto (AES) is fast and great for bulk data, but needs a secure way to share the key. Asymmetric crypto solves key distribution but is slow. Real systems (like TLS) combine them: asymmetric to exchange a key, symmetric for the actual data.

Tip: Symmetric = one key both ways. Asymmetric = two keys, mathematically linked, that undo each other.

Go deeper:

In the Diffie–Hellman key exchange scheme, each party generates a public/private key pair and distributes the public key of the pair. After obtaining an authentic (n.b., this is critical) copy of each other's public keys, Alice and Bob can compute a shared secret offline. The shared secret can be used, for instance, as the key for a symmetric cipher.
In the Diffie–Hellman key exchange scheme, each party generates a public/private key pair and distributes the public key of the pair. After obtaining an authentic (n.b., this is critical) copy of each other's public keys, Alice and Bob can compute a shared secret offline. The shared secret can be used, for instance, as the key for a symmetric cipher.
Davidgothberg · Public domain · Wikimedia Commons
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