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Weak padding for RSA algorithm

Context used in encryption or signing operation is associated with insecure padding type

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Description

This defect occurs when you perform RSA encryption or signature by using a context object that was previously associated with a weak padding scheme.

For instance, you perform encryption by using a context object that is associated with the PKCS#1v1.5 padding scheme. The scheme is considered insecure and has already been broken.

ret = EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PADDING);
...
ret = EVP_PKEY_encrypt(ctx, out, &out_len, in, in_len)

Risk

Padding schemes remove determinism from the RSA algorithm and protect RSA operations from certain kinds of attacks. Padding schemes such as PKCS#1v1.5, ANSI X9.31, and SSLv23 are known to be vulnerable. Do not use these padding schemes for encryption or signature operations.

Fix

Before performing an RSA operation, associate the context object with a strong padding scheme.

  • Encryption: Use the OAEP padding scheme.

    For instance, use the EVP_PKEY_CTX_set_rsa_padding function with the argument RSA_PKCS1_OAEP_PADDING or the RSA_padding_add_PKCS1_OAEP function.

    ret = EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_OAEP_PADDING);

    You can then use functions such as EVP_PKEY_encrypt / EVP_PKEY_decrypt or RSA_public_encrypt / RSA_private_decrypt on the context.

  • Signature: Use the RSA-PSS padding scheme.

    For instance, use the EVP_PKEY_CTX_set_rsa_padding function with the argument RSA_PKCS1_PSS_PADDING.

    ret = EVP_PKEY_CTX_set_rsa_padding(ctx, RSA_PKCS1_PSS_PADDING);

    You can then use functions such as the EVP_PKEY_sign-EVP_PKEY_verify pair or the RSA_private_encrypt-RSA_public_decrypt pair on the context.

Examples

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#include <stddef.h>
#include <openssl/rsa.h>
#include <openssl/evp.h>

#define fatal_error() exit(-1)

int ret;
unsigned char *out_buf;

int func(unsigned char *src, size_t len, RSA* rsa){
  if (rsa == NULL) fatal_error(); 

  return RSA_public_encrypt(len, src, out_buf, rsa, RSA_PKCS1_PADDING); 
}

In this example, the PKCS#1v1.5 padding scheme is used in the encryption step.

Correction — Use OAEP Padding Scheme

Use the OAEP padding scheme for stronger encryption.

#include <stddef.h>
#include <openssl/rsa.h>
#include <openssl/evp.h>

#define fatal_error() exit(-1)

int ret;
unsigned char *out_buf;

int func(unsigned char *src, size_t len, RSA* rsa){
  if (rsa == NULL) fatal_error(); 

  return RSA_public_encrypt(len, src, out_buf, rsa, RSA_PKCS1_OAEP_PADDING); 
}

Result Information

Group: Cryptography
Language: C | C++
Default: Off
Command-Line Syntax: CRYPTO_RSA_WEAK_PADDING
Impact: Medium

Version History

Introduced in R2018a

See Also

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