TLS and Ciphers Suites
Transport Layer Security (TLS), the successor to Secure Sockets Layer (SSL), is a protocol for secure network communication and is most frequently known trough its use in HTTPS. The goal of TLS is to achieve both integrity and confidentiality of the transmitted information between a client and a server.
TLS is initiated with a handshake, which after complete, the connection is considered secure for information exchange. Due to this TLS establishes a stateful connection.
The handshake utilizes both asymmetric and symmetric cryptography. The asymmetric keys are the public-private key-pair of the X.509v3 certificate which the server presents to the client.
By using this certificate, the client can authenticate the server. By verifying whether the certificate has been signed by a trusted Certificate Authority (CA), the trust of the CA is extended to the server. If the client also has a certificate, the server may verify the identity of the client as well in a similar manner.
Cipher suites are made up of several components:
- Key Exchange Algorithm: How the symmetric keys will be exchanged
- Authentication Algorithm: How the authentication of the server and optionally the client will be performed
- Data Encryption Algorithm: How the symmetric key will be used to encrypt the data
- Message Authentication Algorithm: How the connection will perform integrity checks
- Symmetric Block Ciphers: DES, AES
- Symmetric Stream Ciphers: ChaCha20, RC4
- Asymmetric Ciphers: RSA, DSA, DH, ECDH
AES encrypts blocks of 128-bits using a key of length 128, 192 or 256 bits while DES encrypts blocks of 64-bits. Stream ciphers on the other hand encrypt bit by bit.
$ cat /dev/urandom \ rngtest -c 100000
Copyright (c) 2004 by Henrique de Moraes Holschuh
This is free software; see the source for copying conditions. There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
rngtest: starting FIPS tests...
rngtest: bits received from input: 2000000032
rngtest: FIPS 140-2 successes: 99927
rngtest: FIPS 140-2 failures: 73
rngtest: FIPS 140-2(2001-10-10) Monobit: 13
rngtest: FIPS 140-2(2001-10-10) Poker: 9
rngtest: FIPS 140-2(2001-10-10) Runs: 26
rngtest: FIPS 140-2(2001-10-10) Long run: 25
rngtest: FIPS 140-2(2001-10-10) Continuous run: 0
rngtest: input channel speed: (min=5.989; avg=707.717; max=19073.486)Mibits/s
rngtest: FIPS tests speed: (min=18.393; avg=180.330; max=200.774)Mibits/s
rngtest: Program run time: 13286865 microseconds
"Concretely, this work illustrates the inability of the FIPS 140 family of tests to detect bias in three obviously flawed PRNGs." - On the unbearable lightness of FIPS 140-2 randomness tests, DOI:10.1109/TIFS.2020.2988505
Mode of operations are general
Examples of modes:
In contrast to confidentiality-only modes, Authenticated Encryption (AE) schemes ensure both confidentiality and data authenticity.
By utilizing AE, the algorithm can recognize improperly-constructed ciphertexts and refuse to decrypt them. This prevents an attacker from requesting the decryption of any ciphertext unless it was generated using the encryption algorithm with knowledge of the plaintext and the key.
AE with associated data (AEAD) is a variant of AE that allows a recipient to check the integrity of both encrypted and unencrypted information in a message.
Examples of modes:
- GCM - widely used in TLS.
- Reusing IVs https://en.wikipedia.org/wiki/Initialization_vector
- "Reusing an IV with the same key in CTR, GCM or OFB mode results in XORing the same keystream with two or more plaintexts, a clear misuse of a stream, with a catastrophic loss of security."