Cipher Suites: Best Practices and Pitfalls...label 3DES as 168-bit, but it is really only equivalent...

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Cipher Suite Practices and Pitfalls: An Overview of Cipher Suite Configuration and Pitfalls on BIG-IP

https://en.wikipedia.org/wiki/Cipher_suite

“A cipher suite is a named combination of authentication, encryption, message authentication code (MAC) and key exchange algorithms used to negotiate the security settings for a network connection using the Transport Layer Security (TLS) / Secure Sockets Layer (SSL) network protocol.” – Wikipedia

When we talk about configuring ciphers on BIG-IP we’re really talking about configuring cipher suites. More specifically the configured list of cipher suites is a menu of options available to be negotiated. Each cipher suite specifies the key exchange algorithm, authentication algorithm, cipher, cipher mode, and MAC that will be used.

I recommend reading K15194: Overview of the BIG-IP SSL/TLS cipher suite for more information.

https://en.wikipedia.org/wiki/Cipher_suitehttps://support.f5.com/csp/article/K15194

Let’s consider this cipher suite:

ECDHE-RSA-AES256-GCM-SHA384Key Exchange Algorithm: ECDHE (Elliptic Curve Diffie-Hellman Ephemeral)

Authentication Algorithm: RSA

Cipher: AES256 (aka AES with a 256-bit key)

Cipher Mode: GCM (Galois/Counter Mode)

MAC: SHA384 (aka SHA-2 (Secure Hash Algorithm 2) with 384-bit hash)

This is arguably the strongest cipher suite we have on BIG-IP at this time.

What about this cipher suite:

AES128-SHAKey Exchange Algorithm: RSA (Implied) – When it isn’t specified, presume RSA.

Authentication Algorithm: RSA (Implied) – When it isn’t specified, presume RSA.

Cipher: AES128 (aka AES with a 128-bit key)

Cipher Mode: CBC (Cipher Block Chaining) (Implied) – When it isn’t specified, presume CBC.

MAC: SHA1 (Secure Hash Algorithm 1; SHA-1 always produces a 160-bit hash.)

This is to illustrate that the cipher suite may not always explicitly specify every parameter, but

they’re still there. There are ‘default’ values that are fairly safe to presume when not otherwise

specified. If an algorithm isn’t specified, it is RSA. That’s a safe bet. And if a cipher mode isn’t

specified it is CBC. Always CBC. Note that all ciphers supported on BIG-IP are CBC mode

except for AES-GCM and RC4. ALL. I stress this as it has been a recurring source of

confusion.

This presentation is really about cipher suite configuration and ciphers, and not the SSL/TLS

protocol, so I won’t dive too deeply here. As a quick review of the difference between asymmetric

key (aka public key) cryptography and symmetric key cryptography:

With the asymmetric key you have two keys – Kpublic and Kprivate. Since you can openly share the

public key there is no need to pre-share keys with anyone. The downside is these algorithms are

computationally expensive. Key lengths for a common algorithm such as RSA are at least 1024-bit,

and 2048-bit is really the minimally acceptable these days.

Symmetric key has only Kprivate. Both ends use the same key, which poses the problem of key

distribution. The advantage is higher performance; common key sizes are 128-bit or 256-bit.

SSL/TLS uses both systems – the Key Exchange Algorithm is the public key system used to

exchange the symmetric key. Examples: ECDHE, DHE, RSA, ECDH, ADH

This is sometimes grouped in with the Key Exchange Algorithm – written as ‘ECDHE_RSA’ for

example. But we’ll consider it as a separate component.

This is the algorithm used in the SSL/TLS handshake for the server to sign (using the server’s

private key) elements sent to the client in the negotiation. The client can authenticate them

using the server’s public key.

Examples include: RSA, ECDSA, DSS (aka DSA), and Anonymous.

Anonymous means no authentication; this is generally bad. Using an ‘ADH’ cipher suite will

cause this. More on this later when we talk about pitfalls.

Note that when RSA is used for the key exchange authentication is inherent to the scheme so

there really isn’t a separate authentication step.

“In cryptography, a cipher (or cypher) is an algorithm for performing encryption

or decryption—a series of well-defined steps that can be followed as a

procedure. An alternative, less common term is encipherment. To encipher or

encode is to convert information into cipher or code. In common parlance,

'cipher' is synonymous with 'code', as they are both a set of steps that encrypt a

message; however, the concepts are distinct in cryptography, especially

classical cryptography.” – Wikipedia

Examples: DES (Data Encryption Standard), 3DES (Triple DES), AES

(Advanced Encryption Standard), RC4 (Rivest Cipher 4), Camellia, RC6, RC2,

Blowfish, Twofish, IDEA, SEED, GOST, Rijndael, Serpent, MARS, etc.

For a little cryptography humor see:

RFC2410: The NULL Encryption Algorithm and Its Use With IPsechttps://en.wikipedia.org/wiki/Cipher

https://tools.ietf.org/html/rfc2410https://en.wikipedia.org/wiki/Cipher

All of the secrecy in encryption comes from the key that is used. Obtain the key and you

can unlock the ciphertext. The cipher itself – the algorithm, source code, etc. – not only

can be, but should be, openly available. History is full of examples of private

cryptosystems failing due to weaknesses missed by their creators, while the most trusted

ciphers were created via open processes (AES for example).

Keys are of varying lengths and, generally speaking, the longer the key the more secure

the encryption. DES only had 56-bits of key data, and thus is considered insecure. We

label 3DES as 168-bit, but it is really only equivalent to 112-bit strength. (More on this at

the end if there is time.) Newer ciphers, such as AES, often offer options – 128-bits, 192-

bits, or 256-bits of key.

Remember, a 256-bit key is far more than twice as strong as a 128-bit key. It is 2128 vs.

2256.

Cipher mode is the mode of operation used by the cipher when encrypting

plaintext into ciphertext, or decrypting ciphertext into plaintext. The most

common mode is CBC – Cipher Block Chaining. In cipher block chaining the

ciphertext from block n feeds into the process for block n+1 – the blocks are

chained together. To steal an image from Wikipedia:

https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher_Block_Chaining_.28CBC.29

https://en.wikipedia.org/wiki/Block_cipher_mode_of_operation#Cipher_Block_Chaining_.28CBC.29

As I said earlier, all ciphers on BIG-IP are CBC mode except for RC4 (the lone

stream cipher, disabled by default starting in 11.6.0) and AES-GCM. AES-GCM

was first introduced in 11.5.0, and it is only available for TLSv1.2 connections.

GCM stands for Galois/Counter Mode, a more advanced mode of operation than

CBC. In GCM the blocks are not chained together. GCM runs in an Authenticated

Encryption with Associated Data (AEAD) mode which eliminates the separate per-

message hashing step, therefore it can achieve much higher performance on a

given HW platform than CBC. It is also immune to classes of attack that have

harried CBC, such as padding (BEAST, Lucky 13, etc.)

The main drawback to AES-GCM is that it was only added in the TLSv1.2 revision,

so any older clients which don’t support TLSv1.2 cannot use it.

Pronouncing Galois: https://youtu.be/bjHuJyGf-vE

https://youtu.be/bjHuJyGf-vE

“A hash function is any function that can be used to map data of arbitrary size to data of

fixed size. The values returned by a hash function are called hash values, hash codes, hash

sums, or simply hashes. [snip] They are also useful in cryptography. A cryptographic hash

function allows one to easily verify that some input data maps to a given hash value, but if

the input data is unknown, it is deliberately difficult to reconstruct it (or equivalent

alternatives) by knowing the stored hash value. This is used for assuring integrity of

transmitted data, and is the building block for HMACs, which provide message

authentication.” – Wikipedia

In short, the MAC provides message integrity. Hash functions include MD5, SHA-1 (aka

SHA), SHA-2 (aka SHA128, SHA256, & SHA384), and AEAD (Authenticated Encryption

with Associated Data). MD5 has long since been rendered completely insecure and is

deprecated. SHA-1 is now being ‘shamed’ by browsers as it is falling victim to advances in

cryptographic attacks. It is encouraged to migrate to SHA-2 ASAP.

https://en.wikipedia.org/wiki/Hash_function

https://en.wikipedia.org/wiki/Hash_function

Now that we’ve covered what cipher suites are, lets look at where we use them.

There are two distinct and separate areas where cipher suites are used – the

host, or control plane, and tmm, or the data plane. On the control plane

SSL/TLS is handled by OpenSSL and the configuration follows the standard

OpenSSL configuration options.

In tmm the cipher suites are configured in the Ciphers field of the Client SSL or

Server SSL profiles. See K14783 & K14806, respectively.

It is important to keep in mind that these are two different worlds with their own

requirements and quirks.

https://support.f5.com/csp/article/K14783https://support.f5.com/csp/article/K14806

SSL/TLS on the control plane basically means httpd. This is the default in 12.0.0:

tmsh list sys http ssl-ciphersuite

sys httpd {

ssl-ciphersuite DEFAULT:!aNULL:!eNULL:!LOW:!RC4:!MD5:!EXP

}

You can configure the cipher suite with ’tmsh modify sys http ssl-ciphersuite ’.

Note that the default is not just ‘DEFAULT’, as it is in tmm. So what ciphers are actually

supported? For that you want to run:

openssl ciphers -v 'DEFAULT:!aNULL:!eNULL:!LOW:!RC4:!MD5:!EXP‘

That will list out all of the ciphers enabled by this configuration. Let’s see that on the next

slide.

openssl ciphers -v 'DEFAULT:!aNULL:!eNULL:!LOW:!RC4:!MD5:!EXP'

ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=RSA Enc=AESGCM(256) Mac=AEAD

ECDHE-ECDSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AESGCM(256) Mac=AEAD

ECDHE-RSA-AES256-SHA384 TLSv1.2 Kx=ECDH Au=RSA Enc=AES(256) Mac=SHA384

ECDHE-ECDSA-AES256-SHA384 TLSv1.2 Kx=ECDH Au=ECDSA Enc=AES(256) Mac=SHA384

ECDHE-RSA-AES256-SHA SSLv3 Kx=ECDH Au=RSA Enc=AES(256) Mac=SHA1

ECDHE-ECDSA-AES256-SHA SSLv3 Kx=ECDH Au=ECDSA Enc=AES(256) Mac=SHA1

DHE-DSS-AES256-GCM-SHA384 TLSv1.2 Kx=DH Au=DSS Enc=AESGCM(256) Mac=AEAD

DHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=DH Au=RSA Enc=AESGCM(256) Mac=AEAD

DHE-RSA-AES256-SHA256 TLSv1.2 Kx=DH Au=RSA Enc=AES(256) Mac=SHA256

DHE-DSS-AES256-SHA256 TLSv1.2 Kx=DH Au=DSS Enc=AES(256) Mac=SHA256

DHE-RSA-AES256-SHA SSLv3 Kx=DH Au=RSA Enc=AES(256) Mac=SHA1

DHE-DSS-AES256-SHA SSLv3 Kx=DH Au=DSS Enc=AES(256) Mac=SHA1

DHE-RSA-CAMELLIA256-SHA SSLv3 Kx=DH Au=RSA Enc=Camellia(256) Mac=SHA1

[snipped for length]

Let’s look at one of those:

ECDHE-RSA-AES256-GCM-SHA384 TLSv1.2 Kx=ECDH Au=RSA

Enc=AESGCM(256) Mac=AEAD

The columns are:

- Cipher Suite

- Protocol

- Key Exchange Algorithm (Kx)

- Authentication Algorithm (Au)

- Cipher/Encryption Algorithm (Enc)

- MAC (Mac)

Since the control plane is OpenSSL you can use the standard OpenSSL

documentation: https://www.openssl.org/docs/man1.0.1/apps/ciphers.html

https://www.openssl.org/docs/man1.0.1/apps/ciphers.html

In TMM Client SSL and Server SSL are just about identical, so I’ll just use Client SSL

examples. I created a Client SSL profile creatively named ‘test’.

tmsh list ltm profile client-ssl test ciphers

ltm profile client-ssl test {

ciphers DEFAULT

}

You can configure it with ‘tmsh modify ltm profile client-ssl ciphers ’.

Note here the default is really just DEFAULT. What that actually expands to will vary

depending on the version of TMOS. You can check K13156, or use:

tmm --clientciphers DEFAULT

Let’s take a look on the next slide.

https://support.f5.com/csp/article/K13156

tmm --clientciphers DEFAULT

ID SUITE BITS PROT METHOD CIPHER MAC KEYX

0: 159 DHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 EDH/RSA


2: 107 DHE-RSA-AES256-SHA256 256 TLS1.2 Native AES SHA256 EDH/RSA

3: 57 DHE-RSA-AES256-SHA 256 TLS1 Native AES SHA EDH/RSA

4: 57 DHE-RSA-AES256-SHA 256 TLS1.1 Native AES SHA EDH/RSA


6: 57 DHE-RSA-AES256-SHA 256 DTLS1 Native AES SHA EDH/RSA

7: 103 DHE-RSA-AES128-SHA256 128 TLS1.2 Native AES SHA256 EDH/RSA

8: 51 DHE-RSA-AES128-SHA 128 TLS1 Native AES SHA EDH/RSA



11: 51 DHE-RSA-AES128-SHA 128 DTLS1 Native AES SHA EDH/RSA

[snipped for length]

Some differences are readily apparent. For starters tmm kindly includes a column label header, and

actually aligns the columns. The first column is simply a 0-ordinal numeric index.

ID: The official SSL/TLS ID assigned to that cipher suite.

SUITE: The cipher suite.

BITS: The size of the key in bits.

PROT: The protocol supported.

METHOD: NATIVE (in TMM) vs. COMPAT (using OpenSSL code).

CIPHER: The cipher.

MAC: The hash function.

KEYX: The Key Exchange and Authentication Algorithms

Note that the MAC is a little misleading for AES-GCM cipher suites. There is no separate MAC as

they’re AEAD. But the hashing algorithm is used in the Pseudo-Random Function (PRF) and a few

other handshake related places. We are planning to correct this display to be ‘AEAD’.

Let’s say you have determined that you need to support only AES & AES-GCM, 128-

bit or 256-bit, and only ECDHE key exchange. Any MAC or Authentication is fine.

On 12.0.0 there are six cipher suites that fit those criteria. We could list them all

explicitly:

tmm --clientciphers 'ECDHE-RSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-

GCM-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES256-CBC-

SHA:ECDHE-RSA-AES128-SHA256:ECDHE-RSA-AES128-CBC-SHA'

That will work, but it gets unwieldy fast. Fortunately there is an alternative –

keywords! This results in the same list of cipher suites:

tmm --clientciphers 'ECDHE+AES-GCM:ECDHE+AES‘

That specifies the ECDHE key exchange with AES-GCM ciphers, and ECDHE with

AES ciphers.

F5 specific:

NATIVE – Cipher suites implemented natively in tmm

COMPAT – Cipher suites using OpenSSL code; removed as of 12.0.0

@SPEED: Re-orders the list to put ‘faster’ (based on TMOS implementation

performance) ciphers first.

Sorting:

@SPEED: Re-orders the list to put ‘faster’ (based on TMOS implementation performance)

ciphers first. Actually not that reliable due to many platform variations, not recommended.

@STRENGTH: Re-orders the list to put ‘stronger‘ (larger keys) ciphers first.

Protocol:

TLSv1_3 – cipher suites available under TLSv1.3



TLSv1 – cipher suites available under TLSv1.0

SSLv3 – cipher suites available under SSLv3

Note the ‘Protocol’ keywords in the cipher configuration control the ciphers associated with

that protocol, and not the protocol itself!

Key Exchange Algorithms (sometimes with Authentication specified):

ECDHE or ECDHE_RSA: Elliptic Curve Diffie-Hellman Ephemeral (with RSA)

ECDHE_ECDSA: ECDHE with Elliptic Curve Digital Signature Algorithm

DHE or EDH: Diffie-Hellman Ephemeral (aka Ephemeral Diffie-Hellman) (with RSA)

DHE_DSS: DHE with Digital Signature Standard (aka DSA – Digital Signature

Algorithm)

ECDH_RSA: Elliptic Curve Diffie-Hellman with RSA

ECDH_ECDSA: ECDH with ECDSA

RSA: RSA, obviously

ADH: Anonymous Diffie-Hellman.

Note the Authentication Algorithms don’t work as standalone keywords in tmm. You

can’t use ‘ECDSA’ or ‘DSS’ for example. And you might think ECDHE or DHE includes

all such cipher suites – note that they don’t if you read carefully.

General cipher groupings:

DEFAULT: The default cipher suite for that version; see K13156

ALL: All NATIVE cipher suites; does not include COMPAT on 11.x

HIGH: ‘High’ security cipher suites; >128-bit

MEDIUM: ‘Medium’ security cipher suites; effectively 128-bit suites (Also 3DES)

LOW: ‘Low’ security cipher suites;

Cipher families:

AES-GCM: AES in GCM mode; 128-bit or 256-bit

AES: AES in CBC mode; 128-bit or 256-bit

CAMELLIA: Camellia in CBC mode; 128-bit or 256-bit

3DES: Triple DES in CBC mode; 168-bit (well, 112-bit really)

DES: Single DES in CBC mode, includes EXPORT! 40-bit & 56-bit.

RC4: RC4 stream cipher

NULL: NULL cipher; just what it sounds like, it does nothing – no encryption

MAC aka Hash Function:

SHA384: SHA-2 384-bit hash

SHA256: SHA-2 256-bit hash

SHA1 or SHA: SHA-1 160-bit hash

MD5: MD5 128-bit hash

Other:

On older versions, when using COMPAT, it also enables two additional keywords:

SSLv2: Ciphers supported on the SSLv2 protocol

RC2: RC2 ciphers.

So back to our example:

tmm --clientciphers 'ECDHE+AES-GCM:ECDHE+AES‘

Note that you can combine keywords using ‘+’. And multiple entries in the ciphers

configuration line are separated with ‘:’.

We can also exclude suites or keywords. There are two ways to do that:

‘!’ is a hard exclusion. Anything excluded this way cannot be implicitly or explicitly re-

enabled. It is disabled, period.

‘-’ is a soft exclusion. Anything excluded this way can be explicitly re-enabled later in

the configuration string.

Let’s look at the different in hard and soft exclusions on the next slide.

tmm --clientciphers 'ECDHE+AES-GCM:DHE+AES-GCM'


0: 49200 ECDHE-RSA-AES256-GCM-SHA384 256 TLS1.2 Native AES-GCM SHA384 ECDHE_RSA




tmm --clientciphers 'ECDHE+AES-GCM:!DHE:DHE+AES-GCM'




tmm --clientciphers 'ECDHE+AES-GCM:-DHE:DHE+AES-GCM'






Note that in the second example we used ‘!DHE’ and even though we then explicitly

added ‘DHE+AES-GCM’ those ciphers were not enabled. While in the third example

we used ‘-DHE’ and then ‘DHE+AES-GCM’ did enable those ciphers.

You might be wondering what soft disabling is useful for – reordering the ciphers is a

common use case. As an example, DEFAULT orders ciphers differently in different

versions, but mainly based on strength – bit size. Say I know 3DES is really 112-bit and

not 168-bit, and while I need it enabled, I want it to be last on the list.

First configure the DEFAULT list, then remove all of the 3DES ciphers. But then add

the 3DES ciphers back explicitly – at the end of the list. Try it – compare

tmm --clientciphers 'DEFAULT‘

tmm --clientciphers 'DEFAULT:-3DES:!SSLv3:3DES+ECDHE:3DES+DHE:3DES+RSA'

Let’s say a new CVE is announced which exposes a very serious vulnerability in

SSLv3 & TLSv1.0. There is no way to mitigate it, and the only solution is to limit

connections to only TLSv1.1 & TLSv1.2.

This seems straight forward – just configure it to use only ciphers on TLSv1.1 &

TLSv1.2. Right? Let’s try that:

tmsh modify ltm profile client-ssl ciphers ‘TLSv1_2:TLSv1_1’

Congratulations, you’ve solved the problem. You are no longer vulnerable to

this CVE.

You know there is a but coming, right?

What’s wrong? Well, you just enabled all TLSv1.2 & TLSv1.1 ciphers. That includes such

gems as RC4-MD5, RC4-SHA, DES, and a few ADH (Anonymous Diffie-Hellman) suites

which have no authentication. As recently as 11.3.0 you’d even be enabling some 40-bit

EXPORT ciphers. (We pulled them out of NATIVE in 11.4.0.) You just leapt out of the

frying pan and into the fire.

Always, always, always check the configuration before using it. Running that through tmm -

-clientciphers 'TLSv1_2:TLSv1_1‘ should’ve raised red flags.

Instead, this configuration would work without causing all of those problems:

tmsh modify ltm profile client-ssl ciphers ‘DEFAULT:!TLSv1:!SSLv3’

Another option is to disable the SSLv3 and TLSv1.0 protocols on the VIP.

In SSL/TLS there are two main models to the cipher suite negotiation – Server Cipher

Preference or Client Cipher Preference. What does this mean? In SSL/TLS the client

sends the list of cipher suites it is willing and able to support in the Client Hello. The

server also has its list of cipher suites that it is willing and able to support.

In Client Cipher Preference the server will select the first cipher on the client’s list that

is also in the server’s list. Effectively this gives the client influence over which cipher is

selected based on the order of the list it sends.

In Server Cipher Preference the server will select the first server on its own list that is

also on the client’s list. So the server gives the order of its list precedence.

BIG-IP always operates in Server Cipher Preference, so be very careful in how you

order your cipher suites. Preferred suites should go at the top of the list.

We’ve seen a lot of confusion over the cipher suite vs. the protocol on the SSL/TLS

connection. Many have the mistaken belief that using “!TLSv1” in the ciphers configuration

will disable the TLSv1.0 protocol, for example. That is incorrect.

The protocol is configured on the VIP independently of the ciphers. !TLSv1 would disable

all ciphers supported under the TLSv1.0 protocol, but not the protocol itself. Note that the

protocol negotiation and the cipher negotiation in the SSL/TLS handshake are

independent. What happens if the VIP only supports TLSv1.0/v1.1/v1.2 and the client only

supports SSLv3 & TLSv1.0? Well, they’d agree on TLSv1.0 as the common protocol.

The cipher list the client sends in the Client Hello is independent of the protocol that is

eventually negotiated. Say the client sends AES128-SHA and the server has that in its list,

so it is selected. OK, we’ve agreed on a protocol and a cipher suite – only the server won’t

do any ciphers on TLSv1.0 because of ‘!TLSv1’ in the ciphers configuration. Oops.

So how do you disable the specific SSL/TLS protocols, if not in the cipher suite?

Under ‘Options’ in the SSL Profile! You’ll find a few options:

No SSL – disable all SSL

No SSLv1 – disable the SSLv1 protocol



No TLS – disable all TLS

No TLSv1 – disable the TLSv1.0 protocol

No TLSv1.1 – disable the TLSv1.1 protocol



Note that this is for TMM profiles, for httpd on the host see SOL17491: Unable to limit

Configuration utility access to clients using only TLSv1.1 or TLSv1.2

https://support.f5.com/kb/en-us/solutions/public/17000/400/sol17491.html

In versions up to 11.5.0 the cipher suite configuration string was truncated at 256

bytes. Starting in 11.5.0 that was increased to 768 bytes (K11481, K63241636), but

that can still silently truncate long configurations. 13.0.0 resolved this along with the

introduction of Cipher Groups and Cipher Rules.

Be aware of this; it is very sneaky. The configuration will look fine; the truncation

happens in the code when it loads the configuration. This is also one reason why I

always recommend listing your exclusions first in the configuration string. Then you

can never accidentally enable something.

https://support.f5.com/csp/article/K11481K63241636

Back in the section on ciphers I mentioned that we label 3DES as being 168-bit, but that it only

provides the equivalent of 112-bit strength. So, what did I mean by that?

DES operates on 64-bit data blocks, using 56-bits of key. So it has a strength of 256. 3DES, aka

Triple DES, was a stop-gap designed to stretch the life of DES once 56-bits was too weak to be safe,

until AES became available. 3DES uses the exact same DES cipher, it just uses it three times –

hence the name. So you might think 3x56-bits = 168-bits. 2168 strong! Right? No, not really.

The standard implementation of 3DES is known as EDE – for Encrypt, Decrypt, Encrypt. (For

reasons I can explain.) You take the 64-bit data block, run it through DES once to encrypt it with K1,

then run it through again to decrypt it using K2, then encrypt it once again using K3.

Three keys, that’s still 168-bits, right?

Well, you’d think so. But the devil is in the (implementation) details. First of all there are

three keying options for 3DES:

- Keying option 1: K1, K2, K3 – 168 unique bits (but only 112-bit strength!)

- Keying option 2: K1, K2, K1 – 112 unique bits (but only 80-bit strength!)

- Keying option 3: K1, K1, K1 – 56 unique bits, 56-bit strength (Equivalent to DES via EDE!)

We do use keying option one, so we have 168-bits of unique key. However, 3DES with

keying option one is subject to a meet-in-the-middle cryptographic attack which only has a

cost of 2112. So it does not provide the expected 168-bits of security, and is in fact weaker

than AES128.

Worse, we used to describe 3DES as being 192-bit. That’s just flat out wrong no matter how

you look at it. DES operates on 64-bit data blocks, but that’s not the same as key size. See:

K17296: The BIG-IP system incorrectly reports a 192-bit key length for cipher suites using

3DES (DES-CBC3)

https://support.f5.com/csp/article/K17296

Cipher Suite Practices and Pitfalls:

https://devcentral.f5.com/articles/cipher-suite-practices-and-pitfalls-25564

BIG-IP SSL Cipher History by David Holmes: https://devcentral.f5.com/articles/big-

ip-ssl-cipher-history

OpenSSL ciphers docs: https://www.openssl.org/docs/man1.0.1/apps/ciphers.html

K8802: Using SSL ciphers with BIG-IP Client SSL and Server SSL profiles

K15194: Overview of the BIG-IP SSL/TLS cipher suite

K13163: SSL ciphers supported on BIG-IP platforms (11.x - 13.x)

K13156: SSL ciphers used in the default SSL profiles (11.x - 13.x)

K17370: Configuring the cipher strength for SSL profiles (12.x - 13.x)

K13171: Configuring the cipher strength for SSL profiles (11.x)

K14783: Overview of the Client SSL profile (11.x - 13.x)

K14806: Overview of the Server SSL profile (11.x - 13.x)

https://devcentral.f5.com/articles/cipher-suite-practices-and-pitfalls-25564https://devcentral.f5.com/articles/big-ip-ssl-cipher-historyhttps://www.openssl.org/docs/man1.0.1/apps/ciphers.htmlhttps://support.f5.com/csp/article/K8802https://support.f5.com/csp/article/K15194https://support.f5.com/csp/article/K13163https://support.f5.com/csp/article/K13156https://support.f5.com/csp/article/K17370https://support.f5.com/csp/article/K13171https://support.f5.com/csp/article/K14783https://support.f5.com/csp/article/K14806

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Cipher Suites: Best Practices and Pitfalls...label 3DES as 168-bit, but it is really only equivalent...

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