CA5387: Do not use weak key derivation function with insufficient iteration count

#Cause

Using System.Security.Cryptography.Rfc2898DeriveBytes.GetBytes with the default iteration count or specifying an iteration count of less than 100,000.

By default, this rule analyzes the entire codebase, but this is configurable.

#Rule description

This rule checks if a cryptographic key was generated by System.Security.Cryptography.Rfc2898DeriveBytes with an iteration count of less than 100,000. A higher iteration count can help mitigate against dictionary attacks that try to guess the generated cryptographic key.

This rule is similar to CA5388, but analysis determines that the iteration count is less than 100,000.

#How to fix violations

Set the iteration count greater than or equal with 100,000 before calling System.Security.Cryptography.Rfc2898DeriveBytes.GetBytes.

#When to suppress warnings

It's safe to suppress a warning if you need to use a smaller iteration count for compatibility with existing data.

#Suppress a warning

If you just want to suppress a single violation, add preprocessor directives to your source file to disable and then re-enable the rule.

#pragma warning disable CA5387
// The code that's violating the rule is on this line.
#pragma warning restore CA5387

To disable the rule for a file, folder, or project, set its severity to none in the configuration file.

[*.{cs,vb}]
dotnet_diagnostic.CA5387.severity = none

For more information, see How to suppress code analysis warnings.

#Configure code to analyze

Use the following options to configure which parts of your codebase to run this rule on.

• Exclude specific symbols
• Exclude specific types and their derived types

You can configure these options for just this rule, for all rules they apply to, or for all rules in this category (Security) that they apply to. For more information, see Code quality rule configuration options.

#Exclude specific symbols

You can exclude specific symbols, such as types and methods, from analysis by setting the excluded_symbol_names option. For example, to specify that the rule should not run on any code within types named MyType, add the following key-value pair to an .editorconfig file in your project:

dotnet_code_quality.CAXXXX.excluded_symbol_names = MyType

Allowed symbol name formats in the option value (separated by |):

• Symbol name only (includes all symbols with the name, regardless of the containing type or namespace).
• Fully qualified names in the symbol's documentation ID format. Each symbol name requires a symbol-kind prefix, such as M: for methods, T: for types, and N: for namespaces.
• .ctor for constructors and .cctor for static constructors.

Examples:

#Exclude specific types and their derived types

You can exclude specific types and their derived types from analysis by setting the excluded_type_names_with_derived_types option. For example, to specify that the rule should not run on any methods within types named MyType and their derived types, add the following key-value pair to an .editorconfig file in your project:

dotnet_code_quality.CAXXXX.excluded_type_names_with_derived_types = MyType

Allowed symbol name formats in the option value (separated by |):

• Type name only (includes all types with the name, regardless of the containing type or namespace).
• Fully qualified names in the symbol's documentation ID format, with an optional T: prefix.

Examples:

#Pseudo-code examples

#Default iteration count violation

using System.Security.Cryptography;

class ExampleClass
{
    public void ExampleMethod(string password, byte[] salt, int cb)
    {
        var rfc2898DeriveBytes = new Rfc2898DeriveBytes(password, salt);
        rfc2898DeriveBytes.GetBytes(cb);
    }
}

#Specify iteration count in constructor violation

using System.Security.Cryptography;

class ExampleClass
{
    public void ExampleMethod(string password, byte[] salt, int cb)
    {
        var rfc2898DeriveBytes = new Rfc2898DeriveBytes(password, salt, 100);
        rfc2898DeriveBytes.GetBytes(cb);
    }
}

#Specify iteration count by property assignment violation

using System.Security.Cryptography;

class ExampleClass
{
    public void ExampleMethod(string password, byte[] salt, int cb)
    {
        var rfc2898DeriveBytes = new Rfc2898DeriveBytes(password, salt);
        rfc2898DeriveBytes.IterationCount = 100;
        rfc2898DeriveBytes.GetBytes(cb);
    }
}

#Solution

using System.Security.Cryptography;

class ExampleClass
{
    public void ExampleMethod(string password, byte[] salt, int cb)
    {
        var rfc2898DeriveBytes = new Rfc2898DeriveBytes(password, salt);
        rfc2898DeriveBytes.IterationCount = 100000;
        rfc2898DeriveBytes.GetBytes(cb);
    }
}