Understanding Common SSL/TLS Misconfigurations

What is transport security?

Transport layer security (TLS) is a series of protocols for encrypted network communications. This is the foundation for being able to trust the security of services like HTTPS.

At its core, TLS relies upon three components:

• A protocol – a set of rules for establishing a connection
• A cipher suite – A set of algorithms to encrypt and decipher data
• A certificate – A means for verifying the integrity of an SSL connection

The three components all interact together to form transport security and are all essential in providing the confidentiality and integrity that many network services require.

Protocols

The protocols used for transport security have evolved over the years, driven in part by new technology and ways of doing things, but mostly led by research identifying fundamental flaws in the way that the protocols function.

SSLv2 (Deprecated)

The first protocol to discuss, and a precursor to TLS, is SSLv2. It would seem a strange place to start, but SSLv1 was never publicly released and so this was the first consumer protocol for transport security.

The protocol stood from 1995 until 2011, where its deprecation was announced in RFC 6176 (https://tools.ietf.org/html/rfc6176). This document detailed four core deficiencies of the protocol:

• Message authentication uses MD5 [MD5]. Most security-aware users have already moved away from any use of MD5 [RFC6151]
• Handshake messages are not protected. This permits a man-in-the-middle to trick the client into picking a weaker cipher suite than it would normally choose
• Message integrity and message encryption use the same key, which is a problem if the client and server negotiate a weak encryption algorithm
• Sessions can be easily terminated. A man-in-the-middle can easily insert a TCP FIN to close the session, and the peer is unable to determine whether or not it was a legitimate end of the session

SSLv3 (Deprecated)

SSLv3 ran concurrent to SSLv2 from 1995 until the deprecation of SSLv2. After which, SSLv3 stood until its deprecation was announced in 2015 in RFC 7568 (https://tools.ietf.org/html/rfc7568).

The catalyst for the deprecation was a weakness identified by Google engineers, and dubbed POODLE (Padding Oracle on Downgrade Legacy Encryption). This was the first main-stream issue to reach major news headlines

TLSv1.0 and TLSv1.1 (Soon Deprecated)

TLSv1.0 was announced in January of 1999, followed by TLSv1.1 in August 2006. A draft RFC was released in June 2018 and, if approved, formally announces their deprecation.

In the interim, these protocols are considered legacy as the cipher suites offered suffer many of the same weaknesses as they did under the SSLv3 implementation.

TLSv1.2 and TLSv1.3 (Best Practice)

TLSv1.2 was formally announced in RFC 5246 (https://tools.ietf.org/html/rfc5246) and was released in August of 2008. A decade later this was followed by TLSv1.3 which was released under RFC 8446 (https://tools.ietf.org/html/rfc8446).

Best practice for security dictates that these are the only protocols which should be supported, as they are the only protocols supporting cryptographically secure cipher suites .

Cipher Suites

There is a large range of cipher suites available when performing transport security negotiations. These will dictate the algorithms used, and which cipher is appropriate will depend heavily on the context in which they are used.

Cipher suites can be broken down into four components:

• Key exchange – The way that the client and the server agree upon an encryption key
• Encryption algorithm – The math responsible for converting plaintext and ciphertext
• Mode – Instructions for block ciphers on how to interact with each other
• Hashed Message authentication code – A signature used to validate the integrity of the ciphertext

Cipher suites are generally written in shorthand. For example, the following denotes a cipher suite supported by https://secarma.com:

Key Exchange

During the initial negotiation “handshake” a key exchange will take place to provide the client and the server with the secrets required to encrypt and decipher data using the selected algorithm.

The following describe the various common key exchange algorithms:

The two main considerations when implementing a key exchange are as follows:

Key size

The length of the secret used to encrypt data is fundamental to the confidentiality of the data; Larger keys exponentially increase the difficulty of brute-forcing the plaintext.

There is a debate with regards to key length and ability to provide assurance, and the ability to brute-force the key is still heavily dependant on the algorithm used, however there is a consensus that a key should be a minimum of 128-bits.

Some key exchange modes afford better security with a smaller key, for instance EC mode relies upon a mathematic property of elliptic curves to provide an equivalent level of security as a key twice their size.

In 2015 a weakness, referred to as LogJam, was identified in implementations of the Diffie-Hellman key exchange algorithm which relied upon a modulus of 1024-bits or less when generating secrets. The mitigations to this were to use a modulus >1024-bits, or to use EC mode.

Forward Secrecy

Forward secrecy is a means of assuring that, if a key is compromised, future communications are not at risk. This relies on the ability to generate new keys during each negotiation, and for it to be impossible to derive one key from another. For this reason, support for non-ephemeral key exchanges is being dropped by major vendors.

Encryption Algorithm

The encryption algorithm is the heart of the cryptographic process; this is the mathematics that converts plaintext into ciphertext. There are a vast variety of different options available, however some of the most common have been explored below:

RC4 (Deprecated)

RC4 is a stream cipher that has been deprecated as of 2015, as was announced in RFC 7465 (https://tools.ietf.org/html/rfc7465).  This cipher has known biases which undermine the assurance it can provide, which have led to practical (useable) attacks when used for HTTPS.

DES (Deprecated)

DES is a block cipher which operates using 64-bit blocks. The maximum effective key size in DES was therefore smaller than that; and was in fact 54 bits. This is significantly below the minimum required for cryptographic security and so DES has been deprecated.

Triple DES (Deprecated)

Triple-DES (3DES) describes a ciphers suite that performs multiple DES encryption operations on a piece of plaintext, to increase the effective key size of the ciphertext without needing to revise the core operation of the cipher suite.

DES originally operated using a 56-bit key, and therefore by performing multiple DES operations the effective key size would be increased sufficiently to avoid deprecation of the algorithm. However, DES suffers from two core weaknesses, weakening the level of assurance it can provide.

The second weakness identified is in the use of 64-bit block sizes when operating in cipher-block chaining (CBC) mode. This block size makes it statistically probable that, when encrypting a large volume of blocks using the same key (e.g. in TLS) there will be a ciphertext collision. This collision can be used to infer the XOR of two blocks of plaintext. This may then be subjected to a range of attacks to recover the plaintext. This is commonly referred to as the Sweet32 weakness.

AES

The consensus is that AES is the standard for modern cryptography. It describes an algorithm that chunks plaintext into blocks and encrypts it in one of several modes.

The number suffixing AES determines the size of the block. Generally, the larger the block-size, the greater the effective strength of the encryption. For example, AES258 uses 258-bit blocks and is considerably stronger than AES128.

Mode

Block ciphers can operate in one of four modes, which determine how each block will be encrypted. The modes are as follows:

ECB (Deprecated)

This mode enables the direct translation between any given block of plaintext and its ciphertext counterpart, meaning that two identical blocks of plaintext would result in identical blocks of ciphertext.

The biggest issue with this, often referred to as the Tux problem, is that an attacker with a large enough volume of ciphertext may be able to use the similarities to infer details about the plaintext.

CBC (Soon Deprecated)

CBC describes a cipher that uses the last encrypted block to act as the initialisation vector (IV) for the next block. It was developed to mitigate weaknesses in ECB ciphers.

However, where the first IV is of insufficient entropy (as is commonly seen when using cryptographic libraries), this cascades through the ciphertext, effectively mimicking the Tux problem.

Furthermore, CBC-mode block ciphers in MAC-then-Encrypt (MtE) mode, as is most common, suffer from a padding oracle weakness. This occurs because the algorithm fails to perform integrity checking of the entire block, instead only validating the integrity of the plaintext portion. Therefore, an attacker can freely modify any other portion of the block, such as the block padding.

Padding is suffixed to a block to ensure it is of appropriate length, which is essential for CBC-mode to operate. The value of the padding within CBC-Mode is determined as bytes of equal length and value. In the example below, the padding has been highlighted in red:

*****ciphertext****1


*****ciphertext***22

*****ciphertext**333

The decryption routine deciphers the ciphertext, validates and remove the padding, and then validates the integrity of the plaintext.

Where an attacker can modify the cipher block, it is possible to inject an arbitrary padding. This will subsequently cause the decryption routine to either fail to validate the padding or fail to validate the integrity of the plaintext.

Where the error returned is that the integrity of the plaintext could not be validated, the last byte of plaintext can be inferred as equivalent to the value of the padding.

Finally, when operating using block sizes of 64-bits or less, CBC mode is affected by the Sweet32 attack, as previously described.

GCM and CCM

GCM and CCM are considered the best practice modes of operation. Currently supported under TLSv1.2 and above, they afford the best cryptographic security for block ciphers.

CCM has not been as widely adopted as CCM, in part because it cannot be pipelined or parallelized and is therefore not suitable for higher-bandwidth operations.

Hashed Message Authentication Code

Hashed Message authentication codes are the digital signatures used to validate that a portion of ciphertext has not been tampered with. This is essential in mitigating attacks such as padding oracles, as well as ensuring that TLS sessions are not hijacked.

Message authentications may operate in one of two ways:

• MAC then Encrypt – A hash of the plaintext is created and appended to the cipher block
• Encrypt then MAC (EtM) – A hash of the ciphertext is created and appended to the cipher block

The advantage of the former is that it does not require the exchange of a separate key to perform the HMAC function, however it does open the algorithm up to cryptographic oracle attacks.

For an HMAC to be effective, it must not be susceptible to collisions within the parameters of the block size to be hashed. For instance, MD5 is not an effective HMAC algorithm for AES256, because it suffers from collisions within the 256-bit space.

Certificates

x.509 Certificates afford clients the ability to initiate a secure connection without a pre-existing secure channel to negotiate secrets. They do this by providing the connecting client with a public key which can be used to encrypt the initial handshake that the client performs.

A certificate is made up of several key pieces of information, including:

For demonstration purposes, the following certificate will be used:

SHA1 Fingerprint:                  7f48c1576cc8e0502860fff7f76990dfb51fd0aa


Common Name:                       secarma.co.uk

Issuer:                            Sectigo RSA Domain Validation Secure Server CA

Serial Number:                     8B7B793391007A9B95E7C68255C199FD

Not Before:                        Apr 30 00:00:00 2019 GMT

Not After:                         Apr 30 23:59:59 2020 GMT

Signature Algorithm:               sha256WithRSAEncryption

Public Key Algorithm:              rsaEncryption

Key Size:                          2048 bit

Exponent:                          65537 (0x10001)

X509v3 Subject Alternative Name:   {'DNS': ['secarma.co.uk', 'www.secarma.co.uk']}

Subject

The Subject determines that the certificate is permitted to identify any hostname defined in either the “Common Name” or “Subject Alternative Name” fields.

Certificates may also be configured with wildcard characters in their subject fields. These enable arbitrary subdomains to be defined. For example, the following would identify any host under the secarma.co.uk domain:

SHA1 Fingerprint:                  7f48c1576cc8e0502860fff7f76990dfb51fd0aa


Common Name:                       *.secarma.co.uk

Issuer:                            Sectigo RSA Domain Validation Secure Server CA

Serial Number:                     8B7B793391007A9B95E7C68255C199FD

Not Before:                        Apr 30 00:00:00 2019 GMT

Not After:                         Apr 30 23:59:59 2020 GMT

Signature Algorithm:               sha256WithRSAEncryption

Public Key Algorithm:              rsaEncryption

Key Size:                          2048 bit

Exponent:                          65537 (0x10001)

X509v3 Subject Alternative Name:   {'DNS': ['secarma.co.uk', 'www.secarma.co.uk']}

The risk with using wildcard characters is that, if the certificate keypair is compromised on one host, this compromises the transport security of all other hosts that use the certificate.

Issuer

The issuer is the authority that assures the validity of the certificate. Several trusted public certificate authority bodies exist for this purpose, although a private issuing authority may also be implemented in a context where clients may be expected to install their own certificate authorities (for instance on an internal domain).

The integrity of all transport security depends on implicit trust of these certificate authorities; if a trusted authority was compromised then this would undermine the ability to trust transport security.

Signature

The signature is a HMAC generated by the issuer to confirm that the public/private keypair is the same as has been installed on the service being connected to.

The issuer does this by encrypting a hash of the service’s public key with the issuers private signing key. A client may then decipher this encrypted value using a local copy of the issuers public signing key, and if the decrypted value is the same as the public key hash then that indicates that the certificate is valid.

For a signature to offer cryptographic assurance, the keys used must be of sufficient length (2048-bit or above) and the hashing algorithm used for the signature must be considered cryptographically strong. Weaker algorithms, such as MD5 and SHA1, should be avoided because of their susceptibility to collisions.

Expiry

The expiry date is the date at which the certificate should no longer be recognised as assuring the identity of the host. This is a protection to limit the impact of compromised certificates.

Certificates should not be issued with excessive expiry dates (e.g. 10+ years) and a process should be implemented to ensure that new certificates are issued when required.

Key

The server’s public key is used to encrypt the client’s first handshake when negotiating a secure connection. As such, this should be afforded the same cryptographic expectation as other keys, with an appropriate algorithm and key size selected.

The current best practice is to use RSA with a key length of 2048 bits.

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