Cryptographic Storage Cheat Sheet

= WORK IN PROGRESS =

= Introduction =

This article provides a simple model to follow when implementing solutions for data at rest.

Architectural Decision
An architectural decision must be made to determine the appropriate method to protect data at rest. There are such wide variety of products, methods and mechanisms for cryptographic storage that this cheat sheet will only focus on low-level guidelines for developers and architects who are implementing cryptographic solutions. We will not address specific vendor solutions, nor will we address the design of cryptographic algorithms.

= Providing Cryptographic Functionality =

Rule - Only Use Strong Cryptographic Algorithms
Only use approved public algorithms such as AES, RSA public key cryptography, and SHA-256 or better for hashing. Do not use weak algorithms, such as MD5 / SHA1/ Favor safer. The definition of a "strong" cryptographic algorithms change over time.

http://csrc.nist.gov/groups/STM/cavp/index.html

Rule - Encrypt sensitive data with AES + CBC
This is a good default if one doesn't have AES and one of the authenticated encryption modes that provide confidentiality and authenticity (i.e., data origin authentication) such as CCM, EAX, CMAC, etc. For Java, if you are using SunJCE that will be the case. The cipher modes supported in JDK 1.5 and later are CBC, CFB, CFBx, CTR, CTS, ECB, OFB, OFBx, PCBC, None of these cipher modes are authentication encryption modes. (That's why I added it explicitly.) If you are using an alternate JCE provider such as Bouncy Castle, RSA JSafe, IAIK, etc then some of these authentication encryption modes probably should be preferred.

Rule - Check integrity with SHA-256 and a random “salt”
Should note that integrity should be use a MAC (i.e., a keyed hash) and not a MIC (message integrity code). A MIC can be used as a keyed hash via

MIC(msg) = Secure_Hash( salt + msg )

but that is not as good as using something like HMAC-SHA1 or even better, HMAC-SHA256.

Rule - Only Use Widely Accepted Implementations of Cryptographic Algorithms
Do not implement an existing cryptographic algorithm on your own, no matter how easy it appears. Use widely accepted algorithms and widely accepted implementations only. Ensure that an implementation has, at least, had some cryptography experts involved in its creation. If possible, use an implementation that is FIPS 140-2 certified.

Rule - Only Use Strong Cryptographic Cipher Modes
Use cryptographic cipher modes that offer both authenticity and confidentiality

ECB should almost unequivocally be avoided. If you use a cipher mode that treats a block cipher as a streaming mode, such as OFB or CFB, be careful you don't repeat the IV for the same encryption key. In general CBC is a decent cipher mode to use if message authenticity is not an issue and it a cipher mode that is almost always available. Better cipher modes are ones that offer both authenticity and confidentiality such as those mentioned under the Wikipedia "Authenticated Encryption" article. See the "see also" section of  for a list of such cipher modes.

http://www.owasp.org/index.php/Transport_Layer_Protection_Cheat_Sheet#Rule_-_Only_Support_Strong_Cryptographic_Ciphers

Rule - Ensure That Random Numbers are Cryptographically Strong
Ensure that all random numbers, random file names, random GUIDs, and random strings are generated in a cryptographically strong fashion.

TODO: This probably could use further explanation especially how entropy used to seed PRNGs plays an important role as well.

Rule - Ensure That Proper Key Management Policy is Enforced
This includes key generation, distribution, revocation, expiration.

TODO: NIST has some good stuff written up on this.

Rule - Ensure that any secret key is protected from unauthorized access
Store any private keys with extreme care.

Ensure that encrypted data stored on disk is not easy to decrypt. For example, database encryption is worthless if the database connection pool provides unencrypted access.

Ensure that infrastructure credentials such as database credentials or MQ queue access details are properly secured (via tight file system permissions and controls), or securely encrypted and not easily decrypted by local or remote users.

Rule - Only transmit secret keys over secure channels
TODO: This also applies to private keys, and secret (aka, symmetric) keys as well.

This applies to any secret key, not just a master key. Even derived keys should be protected from unauthorized access.

Rule - Use cryptographic keys for single purposes
Do not use keys for multiple purposes and don't needlessly share them with multiple parties as well. If you are encrypting Bank Account #s and CC#s with Alice, Bob, and Carol, choose at least 2 keys-- one for encrypting the back account numbers and the other for encrypting credit card numbers. And if the parties that you are sharing them with have different trust levels, use a different key for each trust level.

For example, say Alice and Bob are applications that are highly trusted because they are applications that are written, maintained, and hosted within your company but Carol is less trusted then either Alice and Bob because it (pick one) was outsourced and not developed by your company or perhaps it is an application hosted by another company, that you would use (at least) 2 keys--one for Alice & Bob, and a different one for Carol. (Of course this gets difficult to track as your number of keys increases so you probably will want to invest in a key manager of some sort.

Rule - Ensure key revocation support
A proper PKI solution should be implemented to so that keys can be revoked and rendered immediately invalid. This is complicated, especially if you want to do it automatically w/out any downtime. Manually is a bit easier if you can afford some downtime. Either way, you need to define your key lifecycle management process.

Rule - Under PCI Data Security Standard requirement 3, you must protect cardholder data
PCI DSS compliance is mandatory by 2008 for merchants and anyone else dealing with credit cards. Good practice is to never store unnecessary data.

= Related Articles =

OWASP - Testing for SSL-TLS, and OWASP Guide to Cryptography

OWASP – Application Security Verification Standard (ASVS) – Communication Security Verification Requirements (V10)

Other Articles in the OWASP Prevention Cheat Sheet Series


 * XSS (Cross Site Scripting) Prevention Cheat Sheet
 * Cross-Site Request Forgery (CSRF) Prevention Cheat Sheet
 * SQL Injection Prevention Cheat Sheet
 * Transport Layer Protection Cheat Sheet

= Authors and Primary Editors =

Jim Manico - jim.manico[at]aspectsecurity.com

Kevin Wall - kevin.w.wall[at]gmail.com